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thesis entitled

DEVELOPMENT OF A HIGH SPEED POWDER
PROCESS TO MAPREPPEE’RE COMPOSITE

presented by

MURTY N. VYAKARNAM

has been accepted towards fulfillment
of the requirements for

Wegree in SCIENCE

 

 

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MSU to An Afﬁnnotivo Action/Equal Opportunity Institution
Wm1

DEVELOPMENT OF A HIGH SPEED POWDER
PROCESS TO MANUFACTURE COMPOSITE
PREPREG

BY

Murty N. Vyakarnam

A THESIS

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

MASTER OF SCIENCE

Department of Chemical Engineering
Composite Materials and Structures Center

1992

ABSTRACT

DEVELOPMENT OF A HIGH SPEED POWDER PROCESS TO
MANUFACTURE COMPOSITE PREPREG

BY

Murty N. Vyakarnam

A High Spwd Powder Process has been developed to manufacture unidirectional
prepreg tapes of any desired volume fraction matrix based on the prototype Powder
Process developed at Michigan State University.

In this investigation, the limitations in operating the prototype at higher spwds
were analyzed. This led to the development of a vertical conﬁguration of the process in
which the aerosol is generated independently and transported into a novel counter current
contacting chamber from the top while spread ﬁbers enter from the bottom. Research
into the aerosolization characteristics of polymer powder matrices has resulted in
optimizing the design of the impregnation chamber, designing a gas distributor ring and
a nozzle for efﬁcient aerosol generation and entrainment, and determining the operating
curves for estimating the volume fraction of matrix in the prepreg. A tension sensor was
also incorporated in the process to minimize ﬂuctuations in the ﬁber motion.

The process has been successfully demonstrated for the A84 carbon ﬁber -

polyamide powder matrix system at economically viable speeds of 20 cm/s and greater.

Copyright by
Murty N. Vyakarnam
1992

In loving memory of my father

ACKNOWLEDGEMENTS

I would like to thank Dr Lawrence T. Drzal for his constant guidance and
leadership which encouraged and inspired me in developing the process.

The development of the process was an outcome of the fundamental work done
by Dr. Shri R. Iyer. I gratefully acknowledge his contribution in the advancement of
composite prepreg processing. I thank Sanjay for his valuable suggestions from time to
time which contributed in a big way in making the process a success. I am also indebted
to James for the contributions he made in building and operating the process.

The High Spwd Powder Process would not have shaped up in the CMSC facility
without the excellent infrastructural support and cooperation provided by Mike Rich. My
thanks are also to Brian Rook for helping me set up various equipment. I appreciate the
prompt and efﬁcient services provided by the DER Machine Shop personnel especially
Mike. Pete Walalauge of the IDC Corporation deserves special thanks for trouble
shooting the control system and keeping the downtime of the process to a minimum.

I am thankful to all my friends and family members who have been a constant
source of support and encouragement. Lastly, I would thank my mother who always

inspired me to do better things in life.

Table of Contents

List of Tables . . viii
List of Figures . . ix
1. Introduction 1
1.1 Background . . 2
1.2 MSU Powder Process . . 8
1.3 Thesis Objective . . 10
2. Aerosolization Characteristics of Polymer Powders . . 12
2.1 Introduction . . 12
2.2 Review of Cohesivity and Tribocharging of Polymer Powders . . 13
2.3 Powders Investigated . . 19
2.4 Experimental Setup . . 25
2.5 Results and Discussion . . 27
2.6 Summary . . 38
3. Process Design . . 39
3.1 Introduction . . 39
3.2 Process Design Criteria . . 39
3.3 Development of Process Scheme . . 41
3.4 Spreader . . 44
3.5 Aerosol Generator . . 45
3.5 Impregnation Chamber . . 48
3.7 Heater . . 51
3.8 Summary . . 52
4. Process Control . . 55
4.1 Introduction . . 55
4.2 Fiber Motion . . 56
4.3 Line Speed Control . . 59
4.4 Temperature Controller . . 62

4.5 Process Variables . . 65

vi

4.6 Summary

5. Prepreg Processing
5.1 Materials
5.2 Spreader Performance
5.3 Aerosol Generation
5.4 Process Runs
5.5 Sintering Operation
5.6 Summary

6. Conclusions

Bibliography

vii

66

67
67
68
70
73
76
76

79

82

Table 1.1
Table 1.2
Table 2.1
Table 2.2
Table 5.1
Table 5.2

Table 5.3

List of Tables

Typical Reinforcing Fibers ‘31

High Performance Polymers “-51

Triboelectric Series of Polymer Materials [23'

Powders Investigated for Aerosolization Characteristics
Fiber - Matrix Material Properties

Summary of High Speed Prepreg Process Runs

Typical Analysis of a Process Run

viii

Figure 1.1

Figure 1.2
Figure 1.3

Figure 2.1

Figure 2.2

Figure 2.3

Figure 2.4

Figure 2.5
Figure 2.6

Figure 2.7a

Figure 2.7b

Figure 2.8
Figure 2.9
Figure 2.10

Figure 3.1

List of Figures

Electrostatic Fluidized Bed

Forced Air Impregnation Process

MSU Powder Prepreg Process

Classiﬁcation of Powders based on Fluidization Behavior “3‘

SEM Micrographs of Polyamide Powders - Typical Distributions and
Surface Characteristics (5 u and 10 p powders)

SEM Micrographs of Polyamide Powders - Typical Distributions and
Surface Characteristics (20 a and 65 u powders)

SEM Micrographs of PEEK Powder - Typical Distribution & Surface
Characteristics and Impregnation on Carbon Fiber

Experimental Setup to Determine Aerosolization Characteristics
Momentum Balance in Aerosolization Chamber “1

Effect of Bed Weight on the Resonant Frequency of Small Aerosolizer
(Glass Spheres)

Effect of Bed Weight on the Resonant Frequency of Big Aerosolizer
(Polyamide 5 pm Powder)

Variation of Particle Entrainment with Gas Flow Rate
Gas Distributor Ring
Effect of Gas Distributor on Particle Entrainment

Economic Analysis of Powder Process‘m

ix

? F _Ilm_

-_ “a. “mi!

Figure 3.2
Figure 3.3
Figure 3.4
Figure 3.5
Figure 3.6
Figure 4.1
Figure 4.2
Figure 4.3
Figure 4.4
Figure 5.1
Figure 5.2

Figure 5.3
Figure 5.4

Schematic of the High Speed Powder Prepreg Process
Schematic of Spreader

Features of Aerosol Generator

Impregnation Chamber

Heater for Sintering Operation

Process Control Strategy of the Process

Schematic of the Tension Control System

Oven Temperature Proﬁle

Temperature Control Circuit for Oven

Effect of Speed and Amplitude on Spreading

Aerosol Generation Characteristics - Constant Nozzle Flowrate
Aerosol Generation Characteristics - Constant Distributor Flowrate

SEM Micrographs of A84 Carbon - Polyamide Prepreg

Chapter 1

 

Introduction

Composite materials are a combination of two or more physically and/ or
chemically distinct materials resulting in mechanical or other property performance
superior to the individual constituents. These materials are generally (i) Fibrous; (ii)
Laminate; or (iii) Particulate; depending upon the physical state of the reinforcing
material and the matrix [1]. The dispersion of the different components in the
composite material must be controlled to achieve optimum properties. Prepreg tape is
a percussor composite form that aids the fabrication of a continuous ﬁber composite
material. Iyer and Drzal [2] developed a novel dry powder process for inexpensively
making prepreg tape. The successful performance of a laboratory prototype led to the
development of a scaled-up version, with a prepregging rate an order of magnitude
higher than the prototype in order to validate the operating models. This dissertation
focuses on the development of the high spwd powder process prototype to manufacture
unidirectional prepreg tape.

The selection of ﬁbers and matrices for a composite material are ﬁnally dictated
by the end use properties, ﬁber matrix interface and processability. In polymer
composites the reinforcing ﬁbers are usually carbon, glass or Kevlar. Typical high

performance ﬁbers and their pr0perties are listed in Table 1.1 [3]. The matrix in a

l

r“ Led-Mm m In.- bmm
. A e-

2

prepreg tape is an unconsolidated polymer which can be either a thermoplastic or a
thermoset. Thermoset matrices such as epoxies are used because of low shrinkage,
excellent adhesion, and good chemical resistance. However, thermoset tapes have the
disadvantages of limited shelf life, low resin strength, low fracture toughness, high
degree of brittleness, tendency to degrade at high temperatures and cold storage
requirements to prevent additional curing. On the other hand, thermoplastic matrices
have certain advantages over thermoset resins. These include short molding cycle time,
inﬁnite shelf life of prepreg tape, better fracture toughness, capability of fusion bonding,
recyclability, repairability, increased moisture resistance, and reduced storage and
handling problems. Nonetheless, processing problems are encountered in the making
of thermoplastic prepreg tapes because of their high viscosities. Table 1.2 lists

common thermoplastic resins [4,5].

1.1 Background
A literature survey in the area of composite prepreg manufacture has shown that
various processes have been developed since 1962. The mode of matrix impregnation

and the salient features of each of these processes are reviewed in this chapter.

1.1.1 Sluny Process
In this process, Taylor [6] passed ﬁber tow through a slurry tank containing the

polymer in suspension in a liquid carrier. Polymer particles are. trapped within the ﬁber

Table 1.1 Typical Reinforcing Fibers ‘31

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Fiber Type Speciﬁc Young’s Strength Diameter Highest Usable
Gravity Modulus (GPa) (#111) Temperature
E (GPa) °C
E Glass 2.5 - 2.6 69 - 72 1.7 - 3.5 5 - 25 350
S Glass 2.48 85 4.8 5 - 15 300
Carbon HM 1.96 517 1.86 8 - 14 600
Carbon HS 1.8 295 5.6 5.5 500
Kevlar 49 1.45 135 3 12 250
Table 1.2 High Performance Polymers “'51
Polymer Trade Glass Transition Melt Temperature
Name Temperature, °C °C
Polyamide Atochem- 42 175 - 180
Orgasol
Poly ether ether ICI- 143 343
ketone Victrex
PEEK
Polyether ketone DuPont- 156 338
ketone PEKK
Polypheneylene Ryton PPS 90 285
sulﬁde
Thermoplastic LARC - 217 - 229 275 - 305
imide TPI
Polyamide imide Torlon C 275 None

 

 

 

 

 

 

4

tow. This impregnated tow is dried in a drying chamber before passing through a heater
to be wound on a take up spool. O’Connor [7] reported that some of the problems
encountered in this technique include ﬁnding the right concentration of slurry and
maintaining the optimum concentration in the resin tank besides the time consuming
drying step prior to consolidation to ensure there is no liquid remaining that can lead to

voids in the ﬁnal composite.

1.1.2 Solution Process

Turton et a1. [8] developed a process in which low viscosity solution of the
thermoplastic/thermoset material is made by dissolving it in a solvent and ﬁber tow is
impregnated by passing it through this solution. Complete removal of the solvent is a
difﬁcult, stringent and time consuming step. Epoxies, polyimides, polysulfones are some

of the matrices that have been solution impregnated.

1.1.3 Melt Impregnation

Prepreg tapes are produced by passing the ﬁber tow through a molten polymer.
In the case of therrnosets, processing by this method is feasible when temperature and
reaction kinetics allow for continuous impregnation. In the case of thermoplastics
viscosity of the melt is a major problem and forces exerted to pull the ﬁbers are often
so high that ﬁber damage occurs. Thermal degradation of the polymer may also become
a problem. Two approaches have been used in processing the thermoplastics, (i) a cross

head extruder feeds molten polymer into a die through which the rovings pass [9], and

5
(ii) ﬁbers pass through a molten resin bath ﬁtted with impregnation pins [10]. The

resulting prepreg tape by this method usually lacks tack and drape.

1.1.4 ﬁlm Stacking

In this process layers of the reinforcing ﬁbers either in the form of unidirectional
tows or woven fabric are stacked with thermoplastic sheets and consolidated under
pressure for long times. This process is widely used with low viscosity materials [11].
Disadvantages include high resin content, difﬁculty in impregnating the ﬁber tow (high
pressure forces the ﬁbers together) with high viscosity material besides being labor

intensive.

1.1.5 Fiber Commingling

In this technique a co-mingled hybrid yarn consisting of the thermoplastic yarn
and the reinforcing ﬁber tow is produced. These hybrid yarns are then consolidated to
form composite parts [12]. While drapeability of the hybrid yarn is a distinct advantage,
high costs are involved in producing the thermoplastic yarn and weaving it with the

reinforcing ﬁbers.

1.1.6 Foam Process
Chary et al. [13] developed a technique to continuously coat carbon ﬁbers using

aqueous foam. The process may be an attractive alternative to the slurry process, where

6
signiﬁcant drying time is required. Nonetheless, the removal of the surfactant from the

ﬁber-matrix system is a difﬁcult and time consuming task.

1.1. 7 Dry Powder Impregnation

Dry powder impregnation was ﬁrst developed by Price [14] in which glass roving
was passed through a bed of thermoplastic powder viz. polypropylene. The particles
stick to the ﬁbers due to natural electrostatic attraction. This tow is then heated and
passed through a die to obtain an impregnated tow. Spreading of tow is not achieved,
hence this technique is restricted to producing short ﬁber reinforced composites. Tight
control over the matrix volume fraction is also not possible. Later, three dry powder
processes using different approaches were developed concurrently by Muzzy et al. [15],
Allen et al. [16] and Iyer et al. [2,4]. Dry powder processes offer the potential for
creation of a high speed, low cost process with controllable matrix volume fraction.
1.1.7.a Electrostatic

Muzzy et. al. [15] developed a process to manufacture powder prepreg using an
electrostatic ﬂuidized bed of PEEK powder (Figure 1.1). The ﬁber tow passes through
a spreader and then through an electrostatic ﬂuidized bed of polymer powder of 10 - 250
microns in size. The imposed electrostatics ensure that the particles adhere to the ﬁbers
and is necessary for the large particles used in the process. However, an electrostatic
ﬂuidized bed has to solve the problem of agglomeration and channeling faced by

conventional techniques before it is applicable to the ﬂuidization of smaller particle sizes.

 

 

Figure 1.1 Electrostatic Fluidized Bed Process

 

 

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Figure 1.2 Forced Air Impregnation Process

1.1.7.1) Forced Air Impregnation

Allen et al. [16] impregnated the ﬁbers in a recirculating powder deposition
chamber with annular walls for powder recovery (Figure 1.2). The chief drawback in
this process lies in the control. Humidity of the environment, material balances in the
chamber to decide feed rates, pressure drop in the recirculating tubes to prevent clogging
and the cohesive nature of ﬁne powders make process control a very difﬁcult task,

especially at particle concentrations in excess of 10,000 mg/m’.

1.2 MSU Powder Process

Iyer et al. [2,4] developed a powder prepreg process which involves the passing
of spread ﬁbers through an aerosol of polymer particles generated by acoustic means.
This process is capable of producing prepreg tapes where each ﬁber is intimately wetted
by the polymer matrix and offers better control over the ﬁber-matrix volume fractions.
No external means is used to charge the polymer powder. Instead the process relies on
the tribocharging of the polymer particles to acquire high charge to mass ratios and to
adhere to the spread reinforcing ﬁbers. The principle in this process is that the average
size of the polymer powder used should be approximately of the same order of magnitude
in size as the reinforcing ﬁber. This principle was later conﬁrmed by Padaki [5] who
found the existence of an optimum particle size for the process in the same order of
magnitude as the reinforcing ﬁber. The process is independent of matrix viscosity and

does not use any solvents.

 

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The prototype process was developed for an operating speed of 2 curls and a tow

size of 3K. The process schematic is shown in Figure 1.3. Continuous ﬁber is unwound
from the fwd spool (A) and passed on to the spreader through a guide and nip rollers.
The spreader (B) consists of a set of highly polished rollers placed over an acoustic
speaker. The tow spreads into individual ﬁlaments and then passes into the deposition
chamber known as the aerosolizer (C). In this chamber the generation of aerosol is by
acoustic vibration and polymer particles are deposited on the spread ﬁbers. Later, the
particles deposited on the ﬁbers in the aerosolizer are sintered in the heater (D). The
prepreg produced is then wound on the take up spool (E). The prepreg tape obtained

from this process has good tack and drape qualities.

1.3 Thesis Objective

Economic studies [17] indicated that an operating speed of 20 cm/s would allow
the MSU Powder Process to be operated at a cost of less than $1/lb. The objective
therefore was to analyze the performance of the prototype and design a scaled up version
of the prototype which can operate at 20 cm/s or greater. This involved investigation
into the aerosolizing behavior of polymer powders and design of the means for
impregnating the particles on ﬁbers at the higher speed of operation. Modiﬁcation to the
process was required to incorporate a different design for the impregnation chamber
which provided the means for the deposition of particles on the ﬁbers. The ﬁber motion

control also had to be improved so that the spreading efﬁciency was not lowered at

1 1
higher spwds of operation. The performance of the high speed process was evaluated

for the carbon ﬁber-polyamide matrix system.

The thesis is divided into six chapters including the introduction. Aerosolization
characteristics of polymer powders is discussed in the second chapter. The third chapter
describes the develOpment of the high speed process, its design and construction. Next,
the process control aspects are discussed in Chapter 4. Chapter 5 deals with the
performance of the high speed process. Chapter 6 summarizes the conclusions of this

investigation with recommendations for future work.

Chapter 2

 

Aerosolization Characteristics Of Polymer
Powders

2.1 Introduction

The polymer powders used in the process are typically below 100 microns in size.
The small sizes of the particles and the low dielectric constants of these polymers make
these powders cohesive (due to the presence of strong inter-particle forces) in nature and
amenable to tribocharging. The phenomenon of tribocharging is beneﬁcial to the
impregnation process but is one of the less understood areas even today. Cohesivity of
these powders requires special modes of aerosolization and poses certain processing
problems like agglomeration during the course of operation. A brief review of the
tribocharging concepts and the ﬂuidization of cohesive powders is presented in this
chapter.

Aerosolization characteristics of various powders were evaluated in order to make
the powder process suitable for different mauix systems and to obtain design parameters
for a scaled-up version of the process. Powders differing in size, density and shape were
subjected to aerosolization studies in a small scale experimental set up. The effect of

geometry on the efﬁciency of aerosolization was also probed. These semi-quantitative

12

13

geometry on the efﬁciency of aerosolization was also probed. These semi-quantitative
studies gave the necessary understanding to design the scaled-up aerosol generator and

the impregnation chamber.

2.2 Review of Cohesivity and Tribocharging of Polymer Powders
The behavior of the polymer powders with respect to aerosolization and
impregnation on to the reinforcing ﬁbers is important to the process. The required

characteristics of the powders suitable for this process are:

i) Base in aerosolization
ii) Ease in acquiring electrostatic charge

iii) Size range about the same order of magnitude as the reinforcing ﬁbers

The size range of polymer powders desirable for this process are also quite susceptible
to acquiring a surface charge by triboelectriﬁcation thereby making them very cohesive
and difﬁcult to aerosolize. The process of aerosolization or dispersion of powders in a
gaseous medium is akin to gas-solid ﬂuidization in many respects. A literature review
on aerosolization and ﬂuidization indicated that considerable work has been done in the
area of cohesivity of ﬁne powders in recent years. Still a deﬁnite understanding of the
underlying principles is lacking, especially for polymer powders. A brief review of these

aspects is presented here.

14
2.2.1 Cohesive powders

Geldart [18] classiﬁed powders into A, B, C and D types based mainly on
empirical observations (Figure 2.1). Type A powders are the most aeratable, while type
B which are more coarse, do not ﬂuidize homogeneously and are sand-like. Type C
powders, which are of immediate interest with respect to this study, are generally ﬁner
than type A powders and are characterized by relatively strong cohesive forces. Type
D powders are the coarsest of all and free ﬂowing. Iyer and Drzal [19] classiﬁed the
polyamide powders of sizes 12.2, 14.9, 19.4 microns as types C, AC and AC
respectively.

Powder aerosolization is a balance between ﬂuid dynamics forces, gravitational
and inter particle forces (or cohesion). The dominant force of interaction between the
particles in a powder is the omnipresent van der Waals force of attraction. Other
attractive forces are capillary and electrostatic forces in the gaseous medium which are
generally smaller than the van der Waals forces [20]. The separation distance between
. particles is critical in determining the van der Waals forces, since surface asperities or
roughness can nullify this force if they increase the separation distance over 1 micron.

The inﬂuence of each force can be is related to the particle size ’r’ [20].

Gravitational o: 1-3
Van der Waals o: r
Electrostatic o: r-2
Capillary 0: r

The above relationships indicate the strong dependence of the interacting forces on the

particle size.

15

B

Sond - like

N u DUIOIV

3
pp --p‘ (g/Cm l

0.5

 

20 50 100 2 5 1,000
5‘; (pm)

Figure 2.1 Classification of Powders based on Fluidization Behavior “3'

16
2.2.2 Tribocharging

Aerosolization of powders results in frequent contact of the particles with surfaces
resulting in the generation of static charges, referred to as tribocharging. Gallo and
Lama [21] related contact charging to particle relative permitivitty e, and particle size.
These two parameters have a signiﬁcant effect on the work function, which is deﬁned
as the minimum energy required to extract the weakest bound electron from a particle
surface to inﬁnity. When particles of different work functions are brought into sliding
and/or frictional contact tribocharging is invariably involved [22]. The net effect of the
relative permitivitty and size of the particle indicates that the work function decreases
with an increase in particle size and/or with particle permittivity. Therefore, electron
transfer from a larger particle to a smaller particle is favorable which results in large
particles being positively charged and small particles being negatively charged. The
bipolar charging of many powders may be attributed to this mechanism because of the
size distribution in a given powder.

Polymeric materials can be arranged in a Triboelectric series depending upon the
sign and magnitude of charge acquired on the surface due to contact and or frictional
electriﬁcation. Gallo and Lama [21]; and Bauch [23] have separately arranged many
polymeric materials in a triboelectric series and found a strong correlation between the
macroscopic property of relative permittivity and the sign and magnitude of charge of the
material (Table 2.1). At the top of the series is polyamide with a relative permitivitty
of about 4 which charges positively and at the other end is PTFE having a relative

permitivitty of 2.1 which charges negatively. The sign and magnitude of the charge

17

Table 2.1 Triboelectric Series of Polymer Materials

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

a

Material Permittivity Sign of Charge:
Polyamide 3 - 6 +ve
PPS 3.9 +ve
Polyurethane 4 - 6 +ve
Epoxy resin 4 +ve
PEEK 3 .2 +ve
Epoxy Polyester 3 - 4 +ve
Polyester + TGIC 3 - 4 +ve
PMMA 3 - 3.5 +ve
Polycarbonate 2.9 ~ve
Polystyrene 2.6 -ve
Polyethylene 2.2 - 2.7 -ve
PVC 3 — 6 -ve
PTFE 2.1 A -ve

 

 

 

 

 

18

becomes decisive as the distance between the two materials increases in the Triboelectric
series. In the case of polymers, the electron afﬁnity of the elements or functional groups
bound to the carbon atoms and their stereometric arrangement in the macromolecule
inﬂuence the sign and magnitude of the charge [23].

The magnitude of charge on a particle is strongly dependent on particle size. The
surface charge on a spherical particle increases with the square of the diameter while the
mass increases with the cube of the diameter. Hence, the charge to mass ratio increases
with decrease in particle size. This makes the effect of triboelectriﬁcation more
pronounced in the case of smaller particles, especially for size ranges under 50 microns.

Once a particle acquires surface charge, the rate at which redistribution proceeds
depends on the electrical relaxation time of the particle. Relaxation time is a product of
particle resistivity and permitivitty [22]. Insulating materials (most polymers are
insulating) having resistivity greater than 103 ohms have relaxation times of minutes or
even hours, which makes them most susceptible to retaining surface charge. The
existence of a surface charge on a particle gives rise to an electrostatic ﬁeld which has
a maximum intensity at the particle surface and decays inversely with the square of
distance away from the surface. Bailey [22] estimates the break down ﬁeld strength of
air to be 3.0 X 10‘ V/m which corresponds to a surface charge density of 26.5 uC/mz.
Harper[24] has come up with simpliﬁed expressions for the maximum particle charge and
the charge to mass ratio for a spherical particle of radius ’r’ and density ’p’ .

Total particle charge , 1.03 X 10“ r"7 Coulombs

Charge to mass ratio (2.45 X 10") / (p r”) C/kg

19

While extensive literature has been devoted to the explanation of the cohesive behavior
of powders in general, not much research has been done with respect to polymer
powders. An attempt to understand the behavior of polymer powders during

aerosolization is made in this chapter.

2.3 Powders Investigated

Five different size ranges of Polyamide (N ylon12) powders manufactured by
ATOCHEM under the trade name of ORGASOL were used in this investigation. ICI’s
Victrex PEEK was the next candidate chosen for the study. 3M’s Scotchlite hollow glass
spheres were also chosen in view of the low density of these particles and the perfect
spherical nature of these bubbles. The size distributions of the powders were evaluated
by a Java Image Analyze and Malvem Size Analyzer. The shape and surface asperities
of the particles were examined under J EOL Scanning Electron Microscope. Table 2.2

summarizes the powders investigated.

2.3.1 Particle Size Analysis

Size distributions of the ﬁve different polyamide powders were supplied by the
manufacturer. The size distribution and the average particle size was determined for
Orgasol-2001 UD Natl powder by two different methods; i) Malvem Laser Beam Size
Analyzer and ii) Java Image Analyzer. Sizes obtained by both the methods came close
_ to the average size of 10.2 urn quoted by the manufacturer (Malvem size analyzer: 112

um and Java image analyzer: 12.1 pm). While measuring the size distribution in the

 

20

Malvem Size Analyzer a dilute surfactant was used to make a dispersion of the polymer
particles in water. The average size of all Orgasol powders in Table 2.2 is that indicated
by the manufacturer. In the case of Victrex PEEK, size distribution was not available
from the manufacturer, and the average size was determined to be 94 am by the Malvem
size analyzer and 97 am by the Image Analyzer. The size distribution for the Glass

spheres was measured using the Malvem size analyzer and the average size was 38 um.

Table 2.2 Powders Investigated

 

 

 

 

 

 

 

 

 

 

1:10. Powder Commercial name Av. Size, Density,
um g/cc
1 Polyamide Orgasol-2001UD Natl 5 1.02
2 Polyamide Orgasol-ZOOIEXD Nat 10 1.02
3 Polyamide Orgasol-2002 D Nat 20 1.02
4 Polyamide Orgasol-2002 ES4 Nat 35 1.02
5 Polyamide Orgasol-2002 E86 Nat 65 1.02
6 PEEK ICI’s Victrex 96 1.34
_L Glass Spheres 1ilM’s Scotchlite E22/4OO 38 0.20

 

 

 

 

 

21

2.3.2 Scanning Electron Microscopy

All the powders investigated were observed using a Scanning Electron Microscope
to determine the particle shapes, surface asperities, typical size distributions and presence
or absence of particle agglomeration. Low magniﬁcation micrographs (< 300X) were
taken of the particles as a typical population indicating the general shape of the particles
and the size distribution. Micrographs at magniﬁcations greater than SOOX were taken
to resolve a single particle for its surface asperities.

A close look at Figures 2.2 and 2.3 indicate the fractal nature of the polyamide
particles. The particle shape and surface asperities did not change with the change in the
particle size as it can be seen that a 5 pm particle is similar to a 65 um particle. The
surface asperities of all the polyamide particles are similar, indicating that different size
range powders were directly a result of the polymerization process and not subjected to
any size reduction techniques. The porous nature of the polyamide particles is also
evidenced from the micrographs. However, it may be noted that the 65 um powder has
a sizeable fraction of smaller particles. In the case of PEEK, (Figure 2.4 (a)&(b)) the
neat cleavage surfaces of the particles is indicative that this size range was obtained by
grinding the granules. During the course of this study, a sample was prepared which had
PEEK particles adhered on carbon ﬁber. Figure 2.4 (c) shows how the particle adhered

to the carbon ﬁber and Figure 2.4 (d) shows a particle in a completely sintered state.

22

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2.4 Experimental Setup

The experimental setup for providing a vibro-ﬂuidizing condition is as shown in
Figure 2.5. A 3 inch ID aerosolizer is used with the vibrating energy provided by a 6.5
inch acoustic speaker. The aerosolizer is a cylindrical plexiglass chamber capped with
a rubber diaphragm on the top and bottom. The aerosolizer is charged with powder on
to the rubber diaphragm at the bottom, which vibrates with the given frequency and
amplitude of the speaker. A sinusoidal wave input is given to the speaker by a B&K
function generator which is ampliﬁed by a Crown ampliﬁer. Inlet ports for dry nitrogen
gas are provided both at the bottom and at the top of the aerosolizer. Near the top of
the aerosolizer a disposable ﬁlter is connected for determination of entrainment rates.

First, the resonant frequency of the aerosolizer was determined after loading with
each of the different powders. To study the effect of a parameter, a set of experiments
was conducted at the resonant frequency of the material keeping the amplitude constant.
The height to which the powder bed expanded at a given frequency and amplitude is
termed bed height. Bed height and the entrainment rate of aerosol in a one hour run
period were the criteria used to characterize aerosolization.

The behavior of the cohesive powders with respect to the geometry of the
aerosolizer was also investigated. Two aerosolizers were chosen for the study which
differed widely in the L/D ratios 1) the 3" ID aerosolizer with a L/D ratio of 3.5 (small
aerosolizer) and ii) 17.75" ID with a L/D ratio of 1.35 (big aerosolizer). The bed

weights were determined by keeping the weight per unit area of the bed constant. Three

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different levels of weight/area were chosen. The frequency of operation varied from
0 to 30 Hz. All experiments were conducted with the same amplitude of 10.5 V. The
effect of various parameters on the aerosolization behavior of these powders are

discussed below.

2.5 Results and Discussion

The objective of the aerosolization studies in the small scale setup was to analyze
the behavior of aerosolization under various conditions which would help in designing
and operating a larger system. Seven powders differing in size, density and shape were
used in these studies (Table 2.2). The effect of amplitude and frequency, bed weight,

density of powder, size distribution of powder, and gas ﬂowrate on aerosolization was

studied.

2.5.1 Momentum Balance

A simple momentum balance around a control volume in the aerosolizer chamber
gives an understanding of the various forces at play (Figure 2.6). In equation (1) the
upward force on the particles is balanced by the hydrodynamic forces, acoustic forces,

shear forces and gravity [25].

28

 

 

 

 

 

 

 

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tion Chamber “1

Figure 2.6 Momentum Balance in Aeroso

29

du dT
ppC'a; = ‘jiyg‘png‘I'Fa‘i'Fh'l'Fc (1)
where, p, = particle density

C = Volumetric concentration of particles
u = velocity of particle

Tn= Shear stress

g = Acceleration due to gravity

F, = Acoustic force

Fh = Hydrodynamic force

Fc = Inter-particle cohesive forces

Ideally, a mathematical model based on the momentum balance describing the
phenomenon of aerosolization would have predicted the design parameters for the scale
up. However, the cohesive nature of the polymer powders in the size range of interest
and the lack of sufﬁcient literature to predict inter-particle cohesive forces resulted in
seeking empirical trends for analyzing the phenomenon of aerosolization by acoustic

energy.

2.5.2 Amplitude and Frequency

The effect of amplitude on the aerosolization for 10 um polyamide powder has
been studied by Iyer[4]. In this study the effect of aerosolization height with frequency
at constant amplitude was recorded for each of the powder tested. It may be concluded
that operation of the aerosolizer at the resonant frequency has a far greater effect on the

aerosol height than changing the amplitude. Another investigation led to the conclusion

30

that vibration of the lower diaphragm and the upper diaphragm is almost in unison
implying that the motion of the medium inside the aerosolizer is plug ﬂow. This was
conﬁrmed by a vibration detector. An eddy current probe was placed close to the top
vibrating rubber diaphragm and the frequency signal read out from an oscilloscope was
compared with the frequency counter of the speaker. Despite noise in the oscilloscope

read out, peaks matched those of the speaker’s frequency.

2.5.3 Bed Weight

The resonant frequency of the aerosolizer does not change in any signiﬁcant
manner with change in bed weight. Three different bed weights of 3.0, 7.1 and 14.2
grams were used in the case of the small aerosolizer and two different bed weights of
250 and 500 grams in the big aerosolizer. Figure 2.7(a) shows this effect for glass
spheres in the small aerosolizer and Figure 2.7(b) shows the effect with respect to 5 am
polyamide powder in the big aerosolizer.

From this experiment, it is concluded that the system should be operated at the
resonant or the natural frequency of the acoustic system. Maximum energy is transferred
to the diaphragm at the natural frequency irrespective of the powder bed weight. The
rubber diaphragm behaves like an elastic spring with the powder bed weight being the
dead weight. An increase in the bed weight can be considered as an increase in the dead
weight of the spring, with the spring constant remaining the same. Thus, it may be

concluded that during a normal run of the process the decrease in the bed weight due to

1 2.0
1 0.8
9.6
8.4
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Bed Height, cm

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0.0

 

 

 

 

 

 

 

 

 

 

Frequency, Hz

*- F I I T I l l l 4
t Bed wt. 39 ~
" Bed wt. 7.19 1
t _
0 3 6 1 2 1 5 1 8 21 24 27

30

Figure 2.7a Effect of Bed Weight on the Resonant Frequency of Small Aerosolizer

30
27
24

 

 

 

 

 

 

 

 

Frequency, Hz

(Glass Spheres)
- Bed wt. 2509
h 0 Bed wt. 5009 ‘
i _
L.
0 3 6 12 1 5 1 8 21 24 27

30

Figure 2.7b Effect of Bed Weight on the Resonant Frequency of Big Aerosolizer

(Polyamide 5 pm Powder)

32

depletion of powder is not going to signiﬁcantly change the resonant frequency of

operation.

2.5.4 Density

The difference in the densities of glass spheres (0.2 g/cc) and 35 pm polyamide
(1.02 g/cc) is substantial but are nearly of the same size. However, it is interesting to
note that the height to which they aerosolize is nearly the same in a particular aerosolizer

setup. This indicates the dominance of hydrodynamic forces over gravity forces.

2.5.5 Gas ﬂowrate

The role of gas ﬂow in the process of aerosolization had to be investigated in
terms entrainment of particles out of the aerosolizer through an opening. This gave a
comparison between the rate of entrainment due to acoustic energy alone and a
combination of acoustic energy and gas ﬂow. Dry nitrogen gas was supplied through an
entry port just above the fwd bed. The relative humidity was also noted using a sling
type RH meter. All experiments were conducted at relative humidity levels of below 50
%. The mass ﬂow rate of particles exiting from the aerosolizer was determined by
collecting the particles in a disposable ﬁlter for two half hour runs. The average mass
ﬂow rate of the two half hour runs were used for further analysis of data. It was
observed that acoustic energy alone was not sufﬁcient to entrain the particles out of the
aerosolizing chamber in any signiﬁcant way when operated at the resonant frequency and

an energy level of 10 V (Figure 2.8).

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Gas was supplied at the bottom of the aerosolizer at a rate of 51.6, 104.3 and
155.6 cc/s metered through a rotameter. The particle entrainment rate increases
exponentially with gas ﬂowrate as shown in Figure 2.8. Of the three different materials
studied only PEEK could not be entrained even at a gas ﬂowrate of 155.6 cc/s while all
sizes of polyamide and glass spheres did respond even at lower gas ﬂowrates. This may
be attributed to higher density and larger particle size of the PEEK powder. On the

other hand glass sphere entrainment is the highest because of their low density.

2.5.5.1 Gas Distributor Ring

The dry nitrogen gas provided a good method for the entrainment of particles.
However, preliminary calculations for the operation of the scaled-up aerosolizer indicated
the requirement of large gas ﬂows rates, if nitrogen were to be just passed into the
aerosolizer from the chamber wall. This led to the development of an efﬁcient gas
distributor located just above the aerosolizer powder to impart momentum to the particles
and aid in the entrainment [26]. This gas distributor is a tubular ring with small holes
drilled at regular intervals on one side of it which would provide vertical jets and is
placed in the center of the aerosolizer just above the feed bed (Figure 2.9). This
distributor proved to be a very effective means for particle entrainment. For the same
gas ﬂowrate the use of the distributor increased the entrainment rate by more than ten
times for all the three different materials tested. For a given gas ﬂowrate of 51.6 cc/s
comparison of particle entrainment with and without the gas distributor is shown in

Figure 2.10 for Polyamide powders of sizes 10, 20, 35 and 65 microns. In all the cases

Figure 2.9

35

 

 

Connecting tube

Circular tube

Hole for vertical jet

Gas Distributor Ring

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there is at least an order of magnitude increase in the entrainment rate after installing the

gas distributor.

2.5.6 Aerosolizer size effects

Differences in the behavior of the powders have been observed when

aerosolization takes place in aerosolizers with different L/D ratios.

(0

(ii)

(iii)

(iv)

There is a marked difference in the height to which the aerosol bed rises
between the two aerosolizers. The aerosol height is about 10 - 20 cm in
the case of the larger unit and 3 - 8 cm in the smaller one irrespective of
the powder type for the frequency range studied.

The agglomeration of 5 micron particles was signiﬁcantly less in the case
of the big aerosolizer. This may be due to the reduction in the wall
effects which accentuate ’balling’.

For the operating range of the frequency (0 - 30 Hz) there is only one
resonant peak in the big aerosolizer while there are distinctly two peaks
in the small one. This is also irrespective of the powder type.

Glass spheres with about one ﬁfth the bulk density of nylon still aerosolize
to about the same height as nylon particles. This clearly indicates that the
drag and buoyancy forces are more predominant than the gravitational

forces.

 

38
2.6 Summary

From the powder entrainment studies (Figure 2.8 and 2.10), it has been observed
that an increase in particle size from 10 to 35 microns follows an expected decreasing
trend in entrainment. However, there is a reversal in the trend for the 65 micron powder
and there is a dramatic increase in the entrainment rate primarily due to the aeratable
nature of the powder. This indicates that while 5 and 10 micron powders are type C
powders, the behavior of 65 micron powder alludes to a type A powder and the others
lie in-between (type AC). The effect of various parameters on aerosolization

characteristics of polymer powders provided the guidelines for designing the scale up

aerosolizer.

 

 

 

Chapter 3

 

Process Design

3.1 Introduction

The successful performance of the prototype powder prepreg process required a
technical and economic analysis to determine the prepregging spwd for an industrial
scale operation. The study conducted by Ball et. al. [17] indicated that for this process
to be viable the prepregging speed should be at least 20 cm/s (Figure 3.1) in order to
keep the non-material costs around $1 per pound (based on an annual production of
25,000 pounds). A scaled up version of the powder prepreg process was required to
demonstrate the feasibility of operating the process at spwds greater than 20 cm/s. An
impregnating system had to be designed for this speed of operation. Analysis of the
mode of powder impregnation in the prototype and investigation of the aerosolization
behavior of polymer powders (Chapter 2) gave the necessary background to design the
impregnating system for the scaled up unit. In this chapter the development of the high

speed process is discussed and the design features of each of the units is detailed.

3.2 Process Design Criteria
Based on the economic study and the operational experience gained from the

prototype the design criteria for the scaled up powder prepreg process were established.

39

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i) The prepregging rate of the scaled up version should be at least 20 cm/s
with the capability of consistently impregnating any desired polymer to
any matrix volume fraction.

ii) In order to maximize production, the scaled up version should be capable
of spreading and impregnating 12K ﬁber tows.

iii) A ﬁber motion system should be designed which incorporates a tension
controller to minimize ﬂuctuations in the motion and provides zero tension
on the ﬁber tow in the spreader unit of the process.

iv) Sintering operation should be capable of Sintering the particles on the
ﬁbers at the highest rates of prepregging.

v) A dedicated process control and data logging system should be

incorporated for minimum supervision.

3.3 Development of Process Scheme

There are principally four unit operations in this powder process. The operations
in sequence are i) spreading the ﬁber tow; ii) generating an aerosol of polymer powder;
iii) impregnating the polymer particles onto spread ﬁbers; and iv) Sintering the particles
in place. In the prototype, operations (ii) and (iii) were performed in a single stage
known as the aerosolizer. It was designed for a line speed of 2 cm/s and each stage of
Operation was sequenced in a horizontal conﬁguration. In order to maintain the same
residence time for the ﬁber tow in the aerosol at speeds greater than 20 cm/s, the scaled

up process would require a horizontal aerosolizer with dimensions nearly 7 feet long in

 

42

the direction of ﬁber travel. The mode of aerosol generation in such a situation can be
accomplished either by a large acoustic horn or by using a series of small acoustic
speakers. The design of such an impregnating system is subject to operational difﬁculties
and complications in process layout. This led to the concept of separating the operations
of aerosol generation and particle impregnation on the ﬁbers. As a result, a separate
vertical impregnation chamber coupled with the aerosolizer as an aerosol generator was
proposed-

Various design options for an impregnation chamber were considered in terms of
design feasibility and operational simplicity. The chamber had to provide an efﬁcient
and controllable means for contacting the ﬁber ﬁlaments with the polymer particles. A
vertical contact chamber with a counter current contacting means of impregnation was
chosen. In this conﬁguration, the entrained aerosol from the aerosol generator enters the
top of the impregnation chamber while the spread ﬁber tow enters from the bottom. The
scaled up process was then conﬁgured around this impregnating means as shown in the
schematic of the process in Figure 3.2.

In the scaled up version of the process, the ﬁber unwinds from the feed spool and
enters the spreader where the tow is spread into individual ﬁlaments. The spread ﬁber
tow then passes over a tension sensor roller and enters the impregnation chamber at the
bottom. Simultaneously, the aerosol of polymer particles generated by the acoustic
means is transported into the impregnation chamber from the top and are deposited onto
the ﬁbers. The particles deposited on the ﬁber ﬁlaments are sintered in place in the oven

which is positioned above the impregnation chamber. The prepreg tape leaving the oven

"-v- ‘bI-h ). .‘a.m 'I-I

 

43

 

 

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Figure 3.2 Schematic of the High Speed Powder Prepreg Process

44
is then wound on the take up drum. The ﬁber tow is driven through the process by a set

of ﬁve motors controlled by a dedicated motor and tension controller. The ﬁber motion
is discussed in greater detail in Chapter 4 along with other process control aspects. The

design of each unit of the process is detailed in the following sections.

3.4 Spreader
The concept and principle of operation of the spreader for the high speed

prepregger remains the same as that of the prototype [28]. The design is based on the

 

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Figure 3.3 Schematic of Spreader

45
use of pulsating energy provided by an acoustic Speaker (Figure 3.3). The speaker

vibrates at a constant predetermined frequency and amplitude. The acoustic pressure
produced by the speaker is directed upwards and forces the collimated ﬁber tow to spread
out as it passes over a set of polished, friction-free rollers. The function of these rollers
is to laterally hold the tow in its spread form (due to friction) while maintaining the
forward tow motion. The efﬁciency of tow spreading is at a maximum when the
frequency of operation is at the natural frequency at which the ﬁbers absorb most energy.
The ﬁber tow spread width is also a function of amplitude of the acoustic energy, tension
in the tow, and the spwd of ﬁber tow.

The spreader was designed to spread 12K AS4 carbon ﬁber to expose individual
ﬁlaments. A 3:1 ratio of speaker size to ﬁber spread width was used to determine the
speaker size. A 15" , 100 Watt poly-woofer speaker was selected and a suitable cabinet
built around it. On top of the speaker cabinet a set of highly polished case hardened
shafts are mounted in friction-less bearings and held in position by an aluminum block

mounted on top of the speaker housing and to each side of the speaker.

3.5 Aerosol Generator

In the scaled up process the role of the aerosolizer is now that of an aerosol
generator (Figure 3.4). An aerosol of polymer powder which is otherwise difﬁcult to
ﬂuidize by conventional ﬂuidization techniques is generated by acoustic means. The
principle of operation is similar to that of the prototype aerosolizer developed by Iyer

et.al.[28]. A cylindrical plexiglass chamber is charged with a bed of polymer powder.

 

46

This chamber has neoprene diaphragms at the bottom and at the top. The powder bed
is vibrated and ﬂuidized by the acoustic energy of the speaker operating at a
predetermined frequency and amplitude and transmitted to the powder through the
diaphragm. In this unit additional buoyancy forces are created by incorporation of a gas
distributor and a nozzle which assist in the generation and entrainment of the aerosol.
Acoustic energy coupled with the buoyancy forces of the gases results in entrainment
of the aerosol into the impregnation chamber.

The parameters involved in the selection of the speaker size and the dimensions
of the cylinder were based on conclusions drawn from experiments conducted on a 3"
and 17.75" aerosolizer (Chapter 2). An 18", 200 Watt speaker was selected and a 14.5"
ID column with openings for the aerosol entrainment at the top was fabricated. Inlet
ports for dry nitrogen gas are provided at the bottom of the column for the gas
distributor and at the top for the nozzle. Nitrogen gas is dried and metered using
rotameters. Provision for continuously charging the aerosol generator with polymer
particles has also been made by incorporating a screw feeder in the system. The screw

feeder nozzle is mounted through a airlock in the wall of the plexiglass chamber.

3.5.1 Gas Distributor and Nozzle

A new feature of the aerosol generator is the ability to control the rate of aerosol
generation and its rate of entrainment to the impregnation chamber. Dry nitrogen gas
is passed into the aerosol generator through a gas distributor located just above the

powder bed. This circular gas distributor provides evenly spaced vertical jets of gas

 

47

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48

(Figure 3.4). Entrainment studies indicated at least a tenfold increase in the rate of
aerosol generation and entrainment with the use of gas distributor (Chapter 2). The gas
ﬂow momentum of these jets provides upward buoyancy to the particles thereby
increasing the rate of aerosol generation. In order to direct the aerosol through the duct
connecting the aerosolization chamber and the impregnation chamber, a high speed gas

nozzle of dry nitrogen gas was added to the chamber.

3.5.2 Rapping Mechanism

Caking of polymer powder on the aerosol generator walls is a continuous
operational problem in the process. Increased calcing results in reduced bed weight and
lower aerosol generation [4]. In order to overcome this problem, impact type rappers
have been installed on the outside walls of the aerosolization chamber similar to the ones
installed by Padaki [5] in the prototype process. These rappers are spaced evenly around

the plexiglass chamber and an optimum rapping cycle is predetermined for the system.

3.6 Impregnation Chamber

3.6.1 Principle of Operation

A counter current contacting chamber has been designed for impregnating
polymer particles onto the spread ﬁber tow (Figure 3.5) [26,29,25]. This chamber offers
ﬂexibility in speed of operation and makes impregnation independent of aerosol

generation conditions. It provides the requisite residence time for the powder particles

 

Roller

49

    
   
    

l

Aerosol
Entry Duct

—
Spread Fiber Tow

Powder Collector

 

Figure 3.5 Impregnation Chamber

50

and the ﬁber ﬁlaments to come in contact. The rate of impregnation is directly
proportional to the aerosol particle concentration in the chamber and the residence time.
The rate of aerosol generation and entrainment can be controlled by either the gas
ﬂowrate to the gas distributor or the amplitude of the acoustic energy. The volumetric
ﬂowrate of the aerosol entrained into the impregnation chamber is therefore equal to the
total gas ﬂowrate used in the aerosol generator. By keeping the amplitude of the speaker
constant, the total gas ﬂowrate directly controls the concentration of polymer particles
in the impregnation chamber. Thus, at steady state the ﬁnal volume fraction of matrix

on the prepreg tape is a function of line speed and the gas ﬂowrate.

3. 6.2 Design Criteria

The underlying principles in designing the impregnation chamber are:

* Height of the chamber should provide the necessary residence time for the
ﬁber tow to be impregnated. A height of 80" was chosen for a residence
time of 10 seconds at the design speed of 20 cm/s.

* The area of cross section of the chamber should be optimized for the two
opposing factors of (a) Particle concentration and (b) Particle settling
velocity. It should be noted that an increase in the cross section decreases
both the particle concentration for a given gas ﬂowrate and also the
particle settling velocity. While a decrease in the particle settling velocity

is beneﬁcial for the impregnation process, the decrease in the particle

 

51

concentration produces an undesirable effect. The area of cross section
chosen was 2"X6".

* A conical entry duct, rectangular in cross section, is designed which
connects the aerosol generator and the impregnation chamber. A sudden
expansion in the cross section is provided to create conditions for reducing
the velocity of the aerosol that is being transported so that lower particle
settling velocities are achieved.

* The cone angle of the entry duct should be greater than the angle of
repose of the powder, so as to avoid a powder build up in the duct due the
settling of particles.

* To minimize the loss of powder and particulate emissions to the

surroundings a set of bafﬂes are provided near the top of the chamber.

3.7 Heater

In the heating stage of the process, the particles that adhere electrostatically to the
ﬁbers in the impregnation chamber are sintered in place so that handling of the prepreg
downstream does not result in loss of powder and that the prepreg acquires the right
drape. The heater should provide the necessary residence times for the particles to
sinter. The estimated time for heating polyamide-A84 prepreg tape is 0.012 secs when
the particle size is 10 um [4,30]. A Blue M gravity convection oven was selected which
provided a heating zone of nearly 20" thereby a residence time of 2.5 seconds when the

line speed is 20 cm/s. The effective heating volume inside the oven was reduced by

 

52

sealing the extra space with an insulation panel to improve the thermal efﬁciency of the
oven. The modiﬁed oven was positioned in such a way as to heat the impregnated ﬁber
tow moving vertically (Figure 3.6).

The vertical mode of prepreg heating poses certain unique problems. In this
conﬁguration, the fact that (i) forced circulation of air inside the oven is not desirable as
it might dislodge the particles impregnated on the ﬁbers, and (ii) continuous motion of
prepreg requires an entry slot through which cold air also enters, and an exit slot through
which hot gases escape by natural convection, makes it is extremely difﬁcult to maintain
a uniform temperature throughout the heating zone of the oven. Even though the opening
for the ﬁber entry and the exit are as narrow as possible steep thermal gradients are
formed inside the oven which result in very short net residence times for the prepreg at
the sintering temperature of the matrix desired. The problem of heating is further
elaborated while describing temperature control in Chapter 4.

A typical solution to such a heating problem would be to use a tubular furnace
with zonal heating, where each zone has an independent temperature controller and all
the zonal controllers would coordinate to maintain the desired sintering temperature.
Theoretically, one would expect to maintain the desired sintering temperature best with

an inﬁnite number of zones.

3.8 Summary
The design of the high speed version involved developing a new vertical scheme

for impregnation of polymer particles on the ﬁbers, resulting in the development of the

 

53

 

 

 

 

 

 

In bullt OTP Top
Thermocouple
(Set Point for Control)
Insulation
Bottom
Thermocouple Sealed Space
(Monitoring)

 

Heating Zone

Figure 3.6 Heater For Sintering Operation

 

54

impregnation chamber. The principle used in designing the spreader was the same as
that used in the prototype process. In view of the vertical conﬁguration, the heater for
sintering the prepreg tape requires a more efﬁcient design which takes into consideration

the convective currents.

 

Chapter 4

 

Process Control

4.1 Introduction

Coupling of the high speed of operation with the brittle nature of the carbon ﬁbers
as well as the heating of the impregnated tow in a vertical conﬁguration through the
heater are some of the important aspects of this new design which needed careful
consideration while designing the control system for this process. In designing the
control scheme, the strategy adopted in the prototype [4] was studied and the
shortcomings noted by Padaki [5] were taken into consideration. The overall control
strategy of this process consists of supervisory control of the system by Dow Chemical’s
Camile Data Acquisition & Control system [31] and a dedicated Speed and Tension
controller (Figure 4.1) [32]. In this chapter, the ﬁber motion system is discussed
followed by the line control strategy and ﬁnally the temperature control in the oven.
While the various control features are described in this chapter, the performance of the
process as a whole is dealt with in Chapter 5.

Camile 2000R is a data acquisition and control system acquired for this process
as part of the effort to automate the process completely. This system will act as the

interface between the operator, tension & motor controller system and the process itself.

55

1

 

in. ..__.. ..... X‘s-.- .L.l.ll "In .341

56
Camile 2000R has 8 channel multisensor and analog boards and a 32 channel input/output

digital board which operates at 1 Hz. The system is interfaced with all the variables that
are to be controlled and monitored. Camile 2000R forms the supervisory controller of
the process.

In view of the slow response time for control action to take place with Camile
2000ll (once every second) a dedicated Spwd and Tension Controller had to be designed
to overcome ﬂuctuations in the ﬁber motion. This resulted in controlling the line speed
of the process with Camile 2000R while all the drives are synchronized to run at that
speed by the Speed and Tension Controller. Upon a recommendation from Dow, the
controller has been custom designed and built by the IDC Corporation. It can control

independently all the dc drives as well as take command inputs from Camile 2000“.

4.2 Fiber Motion

Figure 4.1 shows the path through which the ﬁber traverses along with the
process scheme superimposed with the control strategy. The ﬁber spool mounted on a
cantilever shaft is mounted on a bearing pillow block. The bearing block is ﬁtted with
precision ball bearings which makes the shaft rotate freely, however, the shaft is driven
by a dc motor (motorl) when the feed spool weighs greater than 4 lbs. Presently, the
feed spool is conﬁgured to be driven by the motor. The ﬁber tow then passes through

a guide slot and is pulled by the ﬁrst set of nip rollers.

 

 

 

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57

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Master Set Point

O-I-I-I-O-.-O-O-O-0-0-0-0-0-0-0-I
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61181011

Process Control Strategy of the Process

Motor & T
Controller

 

 

 

 

Figure 4.1

58

Circular ceramic guides increase the number of twists that would form in the ﬁber
tow especially at higher speeds. It is undesirable to have twists in the tow because they
affect the spreading efﬁciency drastically and subsequently the levels of impregnation.
To overcome this, a slotted guide of a cross section 0.04” X 0.75 " was designed which
ensured that the ﬁber tow passed through it as a ﬂat tape and minimized the occurrence .
of twists.

Each set of nip rollers consists of a 1.25 " diameter roller (bottom) that is motor
driven and a pinch roller of 1" in diameter on the top. All rollers are fabricated from
case hardened stainless steel and are mounted on precision ball bearings. The pinch
roller, mounted on a precision ball bearing is also free to move in the vertical direction.
The ﬁber tow passes through the spreader and the second set of nip rollers downstream.
The bottom shafts of the ﬁrst and second nip rollers are linked by a chain so that they
rotate at the same speed to maintain a net zero tension on the ﬁber tow in the spreader.

The spread ﬁber tow goes around a "dancing" tension roller before entering the
impregnation chamber. This tension sensor roller is made of a hollow aluminum shaft
which is chrome plated and mounted on precision engineered ball bearings frxed in a
light aluminum frame which forms one arm of the cantilever frame. The "dancing”
motion of the tension sensor roller is caused by the variation in tension in the ﬁber.
The other end of the frame is coupled to two counter weight springs which balance the
ﬁber tension. The objective of the tension sensor is to synchronize the speeds of motors
2 & 3 and also to help in maintaining the spread of the ﬁber tow entering the

impregnation chamber.

 

 

59

The powder on the impregnated ﬁber tow leaving the impregnation chamber is
sintered in the oven located just above it. The ﬁber is pulled through the impregnation
chamber and the oven by a third set of nip rollers driven by motor 3. As it exits the
oven, the prepreg tape then passes over an idler roller to change direction before it is
ﬁnally wound on a take-up spool. The take—up spool driven by motor4 is similar in
design to the feed spool and is assembled on a Thomson 23D Superslide linear motion
system. The take up spool winds the prepreg tape across the entire drum as it rasters in
a linear fashion. This linear motion is provided by drive5 which reverses direction with

the help of two limit switches placed at either end of the linear slide.

4.3 Line Speed Control

A sophisticated line speed control system has been conceptualized for this process
which coordinates the operation of ﬁve dc motors to provide a line speed ranging from
5 cm/s to greater than 50 cm/s and ensure a constant minimum tension across the
spreader. This is achieved by the linking of all the ﬁve dc motor controllers to a line
speed set by Camile 2000R with ﬁne tuning provided by motor2 controller with feedback
from the tension sensor. Each motor is provided with a dc servo analog PID controller.
This PID controller has the unique feature of mixing up to four command inputs to form
a composite signal to run the motor. Two command inputs with separate speed and ramp
time; a tachometer input with its own scale adjustment; and a fourth input with separate
gain and offset can be the inputs to this controller. The command output is a +/- 10 V

dc to any power ampliﬁer. The controller provides controlled action in both directions

 

61

and can operate in the tech or torque modes. Tuning adjustments of feed forward (Ki),

overshoot (Kd) and overall gain (Kp) are provided on the controller. The control

features of the ﬁve motors of the process have been conﬁgured as indicated below.

Motor 1

Motor 2

Motor 3

Motor 4

Motor 5

Tach mode; unidirectional in rotation

Tach mode with command inputs from Tension sensor and master
set point of motor3; unidirection in rotation

Tach mode with command input from the Camile; unidirectional
in rotation. The spwd of this motor sets the line speed of the
process.

Tach mode with command input from motor3;unidirectional in
rotation

Tach mode with command input from motor3 and limit switches

to reverse direction of motion

The performance of this Spwd and Tension Controller has been satisfactory and it gives

the ﬂexibility of ﬁne tuning the tension at any line speed.

4.3.1 Tension Control

Operation under steady state requires that the spwds of motor 2 and motor 3 be

synchronized. A feedback tension control loop incorporated in the process serves this

purpose (Figure 4.2). A change in ﬁber tension due to a disturbance in the ﬁber motion

is reﬂected by the angular movement of the frame. The position of the frame is

monitored by Allen Bradley’s absolute optical position encoder [33]. The position

62

encoder is ﬁne tuned to serve as a feedback signal to control the speed of motor 2 so that
it matches the speed of motor 3. The frame tends to come to its equilibrium position due
to forces (forces of the counter weight springs) acting in the opposite direction to the
tension in the ﬁber. The output from the encoder is converted to an analog signal and

connected to the PID controller of motor 2 to complete the feedback loop.

4.4 Temperature Controller

The heating of the impregnated ﬁber tow is accomplished in a modiﬁed gravity
convection oven as described in Section 3.7. Very steep thermal gradients are created
in the oven across the entry and exit of the oven due to the rising convective ﬂow of hot
air. This can be noted from Figure 4.3 where the temperature proﬁle across the bottom
and top of the oven was plotted against time at a set point of 200°C. The situation is
aggravated because of the lack of any air circulation inside the oven. This oven is
originally ﬁtted with a direct acting thermostat which responds to the oven temperature.
The control consisted of a sensing bulb, capillary tube, diaphragm and electrical contacts.
The rise in oven temperature expands the ﬂuid in the system forcing the diaphragm
outward. The motion of the diaphragm opens or closes a set of electrical contacts which
supply power to the heating elements of the oven. This controller also has a separate
Over Temperature Protection (OTP), which shuts off the power to the oven if the
temperature exceeds a maximum of 25°C over the set point. Due to the location of the
sensing bulb near the top of the oven, frequent overheating of the bulb used to occur

resulting in the shutting off of the oven. This required a manual restart for the oven.

63

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65

An Omega temperature controller with a I type thermocouple as the sensor was
incorporated to overcome the difﬁculties encountered with the existing controller in the
oven. The external Omega controller bypasses the controller in the oven but the OTP
remains intact. In view of the high amperage of the oven, a solid state relay had to be
incorporated in the circuit as indicated in Figure 4.4. The location of the thermocouple
is critical in terms of optimum control of the temperature. The thermocouple was placed
at the exit of the oven where the temperature is expected to be the highest. With the
incorporation of this solid state controller the problem of frequent tripping was
eliminated, however, the temperature proﬁle inside the oven has to be monitored to

obtain the desired sintering levels in the prepreg tape.

4.5 Process Variables

While the variables related to the ﬁber motion are controlled by the Tension and
Motor controller there are other variables which inﬂuence the process. Both acoustic
speakers require the voltage (amplitude) and the frequency of operation to be monitored.
In the present state of operation both these parameters are set at predetermined values
at the start of a run and are monitored by the dual display Fluke 45 multimeter. The
frequencies and power levels remain constant and do not require adjustment. The
flowrates of the dry nitrogen gas to the gas distributor and the nozzle are also ﬁxed at

the start of the run and metered by Cole Parmer rotameters without adjustment.

 

66
4.6 Summary

Effective and reliable control forms the key to the success of this process. The
line spwd control strategy adopted performs well at the high speeds of operation. A
better means for heating and controlling the temperature of the prepreg is required. In
a manufacturing version of this process, the addition of an on-line feedback system to the
control system to monitor and control the impregnation level on the prepreg, would be

desirable.

Chapter 5

 

Prepreg Processing

This chapter describes the high speed processing of prepreg tape. The
performance of each unit of the process is evaluated with respect to the design of the
process and the performance of the system. The ﬁber matrix system used for
manufacturing the prepreg was A84 12 K carbon ﬁbers and the polyamide powder
matrix. The performance of the process was evaluated in terms of (i) the ability to
manufacture prepreg at speeds greater than 20 cm/s; (ii) the ability to impregnate and

control to obtain the desired volume fraction of matrix.

5.1 Materials

AS4 carbon ﬁbers from Hercules and Orgasol polyamide powder from Atochem
were the materials used to demonstrate the operation of the scaled-up version. In order
to compare the performance of the scaled-up version with that of the prototype, the ﬁber-
matrix system has been kept the same. Moreover, the availability of the polyamide
powders in different size ranges up to 100 um with tight distributions has also been a
favorable aspect of choosing this matrix. Some of the properties of the polyamide
(Orgasol powder) matrix pertinent to the process have been evaluated. The average

particle size is determined to be 10.2 um (Section 2.2). The melt temperature of the

67

 

68
powder, determined using a DSC, is 175°C. The properties of the ﬁber and matrix

important for this process are listed in Table 5.1. All the ﬁber properties are quoted

from the manufacturer’s literature.

Table 5.1 Material Properties

Orgasol 2000 Polyamide Matrix

  

J

L A84 12 K Carbon Fiber

     

 

 

 

 

 

 

 

Av. diameter,pm 8.0 Av. particle size,pm 10.2
Sp. gravity 1.8 Sp. gravity 1.02
Tensile Strength, Ksi 580 Melt Temperature 175°C
Modulus, Msi 31 Viscosity Pa.s 52.6
(0.1 sec" 197.3°C)
Speciﬁc heat cal/gm/°C 0.27 Speciﬁc heat cal/gm/°C 0.811016
@ 175°C @ 140°C
Thermal conductivity 5.73 Thermal conductivity 0.22 - 0.31
W/m/°C W/m/°C

 

 

 

5.2 Spreader Performance
The width of the spread ﬁber tow under normal operating conditions depends on
the (i) amplitude of the acoustic speaker at the natural frequency; (ii) speed of ﬁber tow;

and (iii) tension in the ﬁber tow. A series of experiments were conducted to determine

 

69

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70
the natural frequency of the unit. The natural frequency band at which the spread width

is maximum at a given amplitude is found to be 15.5 - 16.0 Hz for the system.

An experiment was conducted to determine the effect of ﬁber velocity on spread
width (measured at the exit of the spreader). The spreader was operated at two different
amplitude levels of 10 V and 15 V. The spread width was noted as the spwd of the ﬁber
tow was increased to 30 cm/ s (Figure 5.1). As expected the spread width decreases with
an increase in speed, however, when operated at a higher amplitude the rate of decrease
of the spread width is drastically reduced and the spread width is almost independent of
the speed. This may be attributed to the fact that spreading is a direct function of
amplitude or acoustic energy and at higher spwds less energy is transmitted to the ﬁber
tow compared to lower speeds resulting in smaller spread widths. While at higher
amplitudes, the rate at which energy is transmitted to the ﬁbers is so high that the effect

of tow speed is not the dominant factor.

5 .3 Aerosol Generation

The system parameters that are needed to establish the processing condition for
aerosol generation are (i) determining the natural frequency of the aerosol generator; (ii)
optimizing the gas ﬂowrates to the gas distributor and the nozzle to get the desired
aerosol entrainment into the impregnation chamber; and (iii) determination of the
conditions necessary for the aerosol generator to run the process at steady state.

The effect of bed weight on the natural frequency of the generator was ascertained

by charging the chamber with different bed weights - 250, 500, 750 and 1000 grams and

71

observing the frequency at which the maximum displacement of the powder bed occurs.

As explained earlier natural frequency is independent of bed weight.

5.3.1 Gas ﬂowrates

Dried nitrogen gas is supplied separately to the gas distributor ring and the nozzle
at predetermined flowrates. The gas distributor ring located just above the powder bed
provides an upward thrust to the particles which are expanded as aerosols due to the
vibrating diaphragm. The upward thrust provided by the momentum of the vertical gas
jets located all along the circular distributor lifts the aerosol of particles upwards. The
nozzle placed near the top of the generator directs the aerosol toward and transports it
through the opening in the generator wall into the impregnation chamber. The relative
signiﬁcance of the gas distributor and the nozzle in entraining the aerosol and effectively
controlling it forms an important criteria in deciding the gas ﬂowrates to each of these
implements. Figure 5.2 & 5.3 show the operating curves obtained from a study in which
the particle entrainment rate is plotted against the nozzle gas ﬂowrate and the distributor
ring gas ﬂowrate. It is evident from these curves that the gas distributor ring makes a
larger contribution to the total aerosol transported than the nozzle. This is in agreement
with the assumption that the function of the distributor ring is to generate the aerosol of
particles while the high speed nozzle helps in directing the aerosol through the duct into
the impregnation chamber. As more aerosol is generated, the entrainment can still be

controlled by the nozzle.

 

72

Aerosol Generation Characteristics
Constant Distributor Ring flowrate 206 cc/s; Run time 15 min

 

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108
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140 153 166 179 192 205 218 231 244 257 270
Nozzle Gas flowrate, cc/s

r
1

Weight of Powder Collected, gm

I
J

 

 

 

Figure 5.3 Aerosol Generation Characteristies - Constant Distributor Flowrate

Aerosol Generation Characteristics
Constant Nozzle flowrate 156 cc/s; Run time 15 min

 

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157.5 j
.1400
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Ring Gasflowrate, cc/s

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gm
0

I
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\

I

I
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Weight of Powder Collected

I

 

l r L

 

 

Figure 5.2 Aerosol Generation Characteristics - Constant Nozzle Flowrate

 

73

The concentration of particles in the aerosol is a important parameter which
controls the rate of powder impregnation on the ﬁbers. The gas flowrate is optimized
to get the maximum concentration of particles per unit volume of the aerosol entering the
impregnation chamber but at the same time the amount of powder that gets settled at the
bottom of the chamber and needs to be recycled is kept to a minimum. The strategy
adopted to optimize the gas ﬂowrate was to ﬁrst ﬁx the gas ﬂowrate to the distributor
which generates enough aerosol for the levels of impregnation needed and then control
the gas flowrate to the nozzle so that the desired volume fraction of matrix is achieved.

Operation of the aerosol generator for long times depletes the powder in the
chamber which reduces the particle concentration in the aerosol entrained. It has been
noted that the natural frequency of aerosolization is not affected by the bed weight and
which fortunately does not require any continuous changing of the frequency of
operation. However, the chamber needs to be charged with fresh powder on a
continuous basis. An AccuRateR screw feeder has been incorporated in the process

which has the capability of charging the chamber on a continuous basis.

5 .4 Process Runs

The task of demonstrating that the process is capable of operating at spwds
greater than 20 cm/s involved continuous reﬁning of the system’s hardware to overcome
the problems encountered in setting and ﬁne tuning of the operating variables to get the
desired prepreg quality. First the ﬁber spwd and tension control system was tested for

performance and the speeds calibrated to the dial settings. The scaled-up process has a

 

74

capability of controlling the ﬁber motion from 4.5 cm/s to over 50 cm/s. In view of the
near industrial scale of manufacture, operating problems which could have been
otherwise overlooked in a laboratory scale of operation needed immediate attention.

Eighteen runs, typically lasting 30 minutes each, were conducted on this scaled-up
version to ﬁne tune and demonstrate the versatility of the process. Table 5.2 gives an
indication of the versatility of the process for the six runs that were conducted when the

process was considered stable.

Table 5.2 Summary of High Speed Prepreg Process Runs

 

 

 

 

 

 

 

[ Aerosol Entrainment Rate
L (Total Gas Flowrate)
i 4,5 410 39.87 0.4
i 21.8 456 21.03 1.6 u
1' 21.8 460 28.69 1.7
21.8 506 43.31 1.9
21.8 554 43.75 2.0
30.0 =..=._ 554 ._—_ 45.76 2.9

 

 

 

 

 

The protocol adopted in running the process was to ﬁrst set the conditions at
which the spreader, aerosol generator and the oven are to operate for the desired line
speed and volume fraction of matrix. The process variables which had to be set were
the amplitude and frequency of the acoustic speakers of the spreader and the aerosol

generator; the gas ﬂowrates to the distributor ring and the nozzle; and the oven

Wt of bare ﬁber/ meter .80208 gm/m
Density of carbon ﬁber 1.8 gm/cc
Density of Nylon 1.02 gm/cc
S. No. Prepreg (wt) Wt. Fr. Fiber Vol fr. ﬁber
1 915 8 .8758244 .7998705
2 .9835 .8155363 7147179
3 .925 .8671135 .7871264
4 .889 .9022272 .8394628
5 .9553 .8396106 .7478823
6 .889 .9022272 .8394628
7 .9226 .8693692 .7904117
8 .9102 .8812129 .8078318
9 .983 .8159512 7152803
10 .8979 .8932843 .8258869
1 l .8975 .8936825 .8264876
12 .9514 .8430523 .7527127
13 .9275 .8647763 .7837333
14 .8976 .8935829 .8263374
15 .9246 .8674886 .7876721
Average .8683293 .7896584
Standard deviation .0276088 .0397611

Table 5.3

Line Spwd -
Gas ﬂowrate 1- 300 cc/s (Distributor Ring)

75

Typical Analysis of a Process Run

21.8 cm/s

Gas ﬂowrate 2- 156 cc/s (nozzle)

Wt.fr. of matrix
Vol. fr. of matrix .2103416

.1316707

Vol fr. m
.2001295

.2852821

.2128736
. 1605372
.2521177
. 1605372
.2095883

.1921682

.2847197

.1741131
.1735124
.2472873
.2162667
.1736626
.2123279

.2103416
.0397611

76
temperature. Prepreg tape wound on the take up spool was analyzed gravimetrically to

determine the volume fraction of the matrix deposited on the ﬁber. At least ten prepreg
samples one meter long, were cut at random from the take-up spool and weighed to
determine the average volume fraction of the matrix in the run. The statistical average
weight of one meter long bare 12 K ﬁber tow run through the process has been
determined and is used in the calculations to determine the volume fraction of matrix.

A typical analysis of a run at 21.8 cm/s is shown in Table 5 .3.

5.5 Sintering Operation

The polymer particles are impregnated in the impregnation chamber are sintered in place
in the heater in this process. As discussed earlier optimum conditions for sintering have
not been achieved in this process in view of the limitations in the oven. A close look at
the SEM micrographs in Figure 5.4 (a) and (b) indicates that the particles that have been
impregnated are slightly sintered in place. The matrix droplets can be seen in a
thermodynamic equilibrium in Figure 5.4 (c), where a prepreg sample (partially sintered
by the process) was heated in a hot cell. In the case of the line spwd 21.8 ends, the
particles can be seen starting to sinter while in the case of 30.0 cm/s the particles have

undergone less sintering.

5.6 Summary
The versatility of the process lies in (i) the capability of operating the process at

a wide range of speeds - as low as 4.5 cm/s to as high as 30 cm/s; and (ii) the volume

77

     

(a) Process Speed 21.8 cm/s
(b) Process Speed 30 0 cm/s
(c) Equilibrium Sintering

200002

IOOMM

X208
Figure 5.4 SEM Micrographs of AS4 Carbon - Polyamide Prepreg

lSKU

(a)

   

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1
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78

fraction of matrix on ﬁbers can be controlled from very low values to as high as 50% .
The motion of spread ﬁbers in an aerosol of tribo-charged polymer particles results in
a uniform deposition of particles by electrostatic forces. The success of the process also

lies in the uniform impregnation of particles on the ﬁbers as evident from the SEM

micrographs in Figure 5.4.

Chapter 6

 

Conclusions

The conclusions that can be drawn as a result of this study are summarized below.
1. A new high speed prototype of the powder prepreg process has been designed,
constructed and demonstrated successfully. This prototype operates at the economically
viable speed of 20 cm/s or greater, which is an order of magnitude greater than the
operating speed of the ﬁrst laboratory version of the process. The high spwd powder
prepreg process is capable of producing impregnated reinforcing ﬁbers with any desired
volume fraction of matrix to manufacture prepreg tapes where each ﬁber is intimately

wetted by the polymer matrix.

2. The process does not use any external means to charge the polymer particles,
instead it relies on the natural phenomenon of tribocharging of the polymer particles to
acquire high charge to mass ratios and then adhere to the reinforcing ﬁbers to produce

prepreg tape. The process is also independent of matrix viscosity and does not use any

solvents.

3. The process is composed of four unit operations: i) spreading the ﬁber tow; ii)
generating an aerosol of polymer powder; iii) impregnating the polymer particles onto

79

80
spread ﬁbers; and iv) sintering the particles in place. The original laboratory prototype

process was designed for a line speed of 2 cm/s with each stage of operation placed in
a horizontal conﬁguration. Operations (ii) & (iii) were performed in a single stage
known as the aerosolizer. Research into the behavior of the aerosolizer led to the design
of an impregnating system for the scaled-up version in which aerosol generation and

particle impregnation on the ﬁbers were separated.

4. A vertical counter current impregnation chamber has been designed for
impregnating polymer particles onto the spread ﬁber tow. This chamber offers ﬂexibility
in speed of operation and makes impregnation independent of aerosol generation. It
provides the requisite residence time for the powder particles and the ﬁber ﬁlaments to

come in contact while requiring a small area of ﬂoor space.

5. Research into the aerosolization characteristics of various polymer powders in a
small-scale setup made the powder process suitable for different matrix systems and
provide the design parameters for the scaled-up version. As a result of these studies the
concept of a gas distributor ring was developed which can control the rate of aerosol
generation and entrainment. Entrainment studies indicated at least a tenfold increase in
the rate of aerosol generation and entrainment with the use of this gas distributor.
Further, in order to direct the aerosol to the impregnation chamber a nozzle for dry

nitrogen gas was used.

 

81

6. In view of the high speed of operation, a sophisticated process control strategy
was adopted. Steady state operation required that the speeds of the motors be
synchronized. A tension controller which is very fast responding in sensing the
ﬂuctuations in the ﬁber motion and very effective in controlling the motor speeds

accordingly has been incorporated.

The following recommendations for future work are made as a result of this
investigation.

(i) An improvement in the sintering operation will be required for prepregging higher
temperature material. The existing oven should be replaced with a tubular heater with
three zone temperature controller. Alternatively, an infra red heater with proper
temperature control for heating prepreg in a vertical mode could be considered.

(ii) Maintaining the same ﬁber tow spread width from the time it leaves the spreader
to the time it enters the heater is often an operational challenge. A means to keep the
spread width constant in this vertical conﬁguration should be explored.

(iii) Investigate the physics of powder impregnation on the spread ﬁbers in the
impregnation chamber.

(iv) Incorporate an in-line sensor and feedback system which can control the volume

fraction of the matrix on the prepreg.

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