Ill/INWWWIWIWIHIIHHHHWllHlUNHHlH/W THS THESIS |||3||||||||||||||||||l||||||||l|l|||||||||||||| .3 ,' ' 31293 10467 3284 I This is to certify that the thesis entitled APPLICATION OF POLYETHYLENE HEAT SEAL CHARACTERISTICS TO SOLVE A HEAT SEAL STRENGTH PROBLEM presented by MELVIN S. HARDER has been accepted towards fulfillment of the requirements for M . S . PACKAGING degree in //0°f’/7 E 56%;?" Dr. Hugh E. Lockhart Major professor Date February 9, 1982 0-7639 MSU LIBRARIES ”n. RETURNING MATERIALS: Place in book drop to remove this checkout from your record. FINES will be charged if book is returned after the date stamped below. We? .~- K, .3533, ; worms ' .2 1mm! ‘ 5”¥/ ' 1” APR é‘emép NOV 9 3 “5512007 APPLICATION OF POLYETHYLENE HEAT SEAL CHARACTERISTICS TO SOLVE A HEAT SEAL STRENGTH PROBLEM By Melvin S. Harder A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE School of Packaging 1982 \‘7 /// //3,33’ ’3/4 ABSTRACT APPLICATION OF POLYETHYLENE HEAT SEAL CHARACTERISTICS TO SOLVE A HEAT SEAL STRENGTH PROBLEM BY Melvin S . Harder The use of flexible packaging in the food industry is expanding very rapidly. Milk, traditionally packaged in blown polyethylene bottles, is now being packaged in flexible polyethylene pouches. One problem involves forming a heat seal strong enough to endure the hardships of the distribution environment. In this study, a material characteristics approach was taken to solve a machine/material interface problem. First, the material was researched to determine the optimum values for the variables; temperature, pressure and dwell time, necessary to form the strongest heat seal possible. Next, the form-fill-seal machine adjustments for temperature and dwell time were changed to the optimum heat seal strength values. The resulting heat seals proved to be stronger than the previously formed heat seals. .....-.—.—. . ACKNOWLEDGMENTS The author wishes to express his sincere appreciation and gratitude to the following individuals: Dr. Hugh Lockhart, School of Packaging, who provided the encouragement and guidance necessary to assure completion of the thesis. G Mr. Ken Martin, Quality Dairy Company, who took time out of his busy day to assist me in identifying the heat seal adjustments to the form-fill-seal machine. Mr. Norm Asman, School of Packaging, who provided the essential technical guidance on heat sealing. Dr. Richard Gonzalez, Graduate School of Business Administration, who, in addition to being a committee member, helped determine the scope of the thesis work. Dr. Julian Lee, School of Packaging, who provided assistance in determining the necessary statistical analysis. Dr. James Goff, School of Packaging, who served as a committee member. Ms. Kimberly Carswell, Graduate Student, School of Packaging, who provided moral and.editorial support to my thesis work. Mr. Frank Paine, Consultant in Packaging Technology & Management, who provided technical and moral support. ii TABLE OF CONTENTS LIST OF TABLES . . . . . . . . . . . . . . . . . . . . . . . . v LIST OF FIGURES . . . . . . . . . . . . . . . . . . . . . . . vi INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . 1 LITERATURE REVIEW . . . . . . . . . . . . . . . . . . . . . . 4 Heat Sealing of Polyethylene is Actually a Welding of the Material . . . . . . . . . . . . . b The Three Impulse Heat Seal Variables . . . . . . . . . . . 5 The Interrelationship of the Three Heat Seal Variables . . . . . . . . . . . . . . . . . 8 Summation of the Various Literature Search Concerns . . . . . . . . . . . . . . . . . . . . . 9 STATEMENT OF THE PROBLEM . . . . . . . . . . . . . . . . . . . 11 Description of an Impulse Heat Sealer . . . . . . . . . . . 11 Quality Dairy' 5 Form— Fill- Seal Machine . . . . . . . . . . . 12 The Problem . . . . . . . . . . . . . . . . . . . . . 17 Limits of the Study . . . . . . . . . . . . . . . . . . . . 18 METHODOLOGY I I I I I I I I I I I I I I I I I I I I I I I I I 20 Basis for Procedure . . . . . . . . . . . . . . . . . . . . 20 Test Us ed I I I I I I I I I I I I I I I I I I I I I I I I 25 Test Sample Size Determination . . . . . . . . . . . . . . . 26 The Test . . . . . . . . . . . . . . . . . . . . . . . . . . 27 RESULTS/DISCUSSION/CONCLUSION . . . . . . . . . . . . . . . . 29 The Results . . . . . . . . . . . . . . . . . . . . . . . . 29 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 30 Conclusion . . . . . . . . . . . . . . . . . . . . . . . 32 Areas for Further Study . . . . . . . . . . . . . . . . . . 33 APPENDICES . . . . . . . . . . . . . . . . . . . . . . . . . . 35 A. LETTER FROM DUPONT CANADA INC. KINGSTON, ONTARIO . . . . . . . . . . . . . . . . . . . 35 B. LETTER FROM THE US ARMY NATICK RESEARCH AND DEVELOPMENT LABORATORIES NATICK, MASSACHUSETTS . C. DIAGRAM SHOWING HOW THE TEST SPECIMENS WERE CUT FROM THE SAMPLE POUCHES AND DIAGRAM SHOWING HOW THE SPECIMENS WERE POSITIONED IN THE INSTRON TENSILE TESTER D. PILOT TEST RESULTS OF HEAT SEAL STRENGTH . E. TABLE OF MATERIAL STRENGTH TEST ASTM TEST D 882-75b F. TABLE OF HEAT SEAL STRENGTH TEST RESULTS AND TABLE OF MATERIAL THICKNESS TESTED ASTM TEST D 882-75b G. STUDENT'S t - TEST FOR BREAKING FACTOR MEANS AND % EILONGATION MEANS BETWEEN CONDITION I SEALS AND CONDITION II SEALS . BIBLIOGRAPHY . iv 36 37 39 #0 L11 10. 11. LIST OF TABLES Heat Seal Strengths for Various Temperature and Pressure Conditions . . . . . . . . . . . . . Maximum Range Conditions for Temperature and Dwell Time . . . . . . . . . . . . . . . . . Summary of the Horizontal Heat Seal Variable Adjustment Ranges . . . . . . . . . . . . . . . . Summary of the Variables: Current Settings . . . Comparison of Variable Current Settings, Variable Optimum Settings and Variable Adjustment Ranges . Comparison of Current Variable Settings (Seal Condition I) to the Adjusted Variable Settings (Seal Condition II) . . . . . . . . . . . . . . . Result Averages of ASTM Test D 882-75b . . . . . Pilot Test Results of Heat Seal Strength . . . . Table of Material Strength Test AST'M TeSt D 882-75b o o o o o o o o o o o o o o 0 Table of Heat Seal Strength Test Results . . . . Table of Material Thickness Tested . . . . . . . 17 22 23 25 29 39 #0 41 43 10. LIST OF FIGURES Heat Seal Strength Versus Dwell Time for a Four Mil Polyethylene . . . . . . . . . . . . . . . . . Impulse Heat Sealer . . . . . . . . . . . . . . . . . . Quality Dairy's Form-Fill-Seal Machine Heat S eal cut 0 ff Bar I I I I I I I I I I I I I I I I I I I Timing Cam Shaft of the Horizontal H eat S eal er I I I I I I I I I I I I I I I I I I I I I I Heat Seal Strength Versus Heating Dwell Time for a Four Mil Polyethylene (Short Heating Dwell Time Machine Setting) . . . . . . . . . . . . . . Heat Seal Strength Versus Heating Dwell Time for Four Mil Polyethylene (Long Heating Dwell Time Machine Setting) . . . . . . . . . . . . . . Quality Dairy Half-Gallon Milk P011011 0 I o o o o o o 0 Seal Lines Formed Under Condition I and Condition II (Drawings of Observations Made at 48x Magnification) Diagram Showing How the Test Specimens Were Cut From the Sample Pouches . . . . . . . . . . . . . . Diagram Showing How the Specimens Were Positioned in the Instron Tensile Tester . . . . . . . . . . . . . vi 12 13 15 20 21 26 31 37 38 INTRODUCTION The use of flexible packaging in the food industry is expanding very rapidly. Milk, traditionally packaged in blown polyethylene bottles, is now being packaged in flexible polyethylene pouches. The cost savings are substantial. A blown polyethylene bottle costs about fifteen cents plus one cent for the cap. A pouch only costs three cents. With this magnitude of cost savings available, the switch from bottles to pouches has some very appealing economic advantages. Unfortunately, there are some problems with the flexible pouches. One problem involves forming a heat seal strong enough to endure the hardships of the distribution environment. A strong seal is necessary to prevent undesirable leakage, premature spoilage and unsightly appearance. To accomplish all of these objectives, an appropriate application of temperature, pressure and dwell time in forming the heat seal is necessary. Forming a heat seal in polyethylene is actually a welding of the material or fusion of the material interfaces by melting. The welding is accomplished by a combination of variables; temperature, pressure and dwell time. Each of the variables has an upper and lower limit. Also, there is an optimum value where any deviation from that optimum peak value will result in a weakening of the heat seal being formed. In addition to these limits, the variables are interrelated to each other. A lowering of one variable can be compensated by increasing 1.—,~.. another. An increase in temperature, for example, will correspond— ingly decrease the dwell time necessary to form a heat seal. If too high a temperature is used to decrease the dwell time, the polyeth- ylene being heat sealed will become degraded by the excessive heat.15 The heat seals currently being formed by a local company's form- fill-seal machine leak. These "leaking" heat seals are a problem arising from the new application of polyethylene in the food industry. In theory, the heat seal should be stronger than the material being heat sealed. When moderate pressure is applied to the heat seal formed by the local company's form-fill-seal machine, the heat seals fail before the material deforms, and milk leaks out. Leaking pouches are found throughout this company's distribution system. The failure of the heat seal before the material deforms indicates the need for an adjustment to the machinery forming the heat seal. The methodology of this study was to identify the optimum ranges of the variables; temperature, pressure and dwell time, necessary to form a strong heat seal. Next, the variable adjustments on the form- fill-seal machine were harmonized with the optimum ranges of the variables. A strength test was performed on the heat seals formed under the current variable settings and the new optimum variable settings to determine if the heat seal strength could be improved. The purpose of the study was to increase the strength of the heat seal being formed by making adjustments to the variables; temperature, pressure and dwell time. Any other changes or modifications to the machine or the packaging material were outside the scope of this thesis. The results of the strength test showed the heat seals formed 3 under the optimum variable settings were stronger than the current variable setting heat seals. LITERATURE REVIEW Heat Sealing of Polyethylene is Actually a Welding of the Material The heating and pressing of the material together causes the material to be fused or joined by melting. The softening or melting of the material must be done at a gradual rate to allow control of the welding operation. Polyethylene is well suited for welding since it does not have a definite melting point because of the random arrangement of the molecular chains. It softens gradually with an increase in temperature as the weak secondary bonds between the long chains cf molecules begin to break down.2 The gradual softening of polyethylene with an increase in temperature allows the polyethylene to be welded to itself under controlled temperature and pressure conditions. Polyethylene is welded to itself when it is heat sealed.14 F. Kohler explains the welding operation of polyethylene very well with the passage: The joining of thermoplastic materials by high frequency heat occupies the position of a true welding operation. The applied heat causes the plastics to soften and the surfaces to fuse together. This fusion takes place in a temperature range which depends on the properties of the plastics and is affected by the pressure applied. The achievement of proper welds therefore depends on the attainment of proper temperature at the interface of the material being joined, with a simultaneous application of proper pressure. In the end of the passage, F. Kohler pointed out there is a proper temperature and a proper pressure to be applied to polyethylene to 7 achieve a proper heat seal. The conditions necessary to achieve the proper heat seal involve specific temperatures and related pressures in the heat seal operation. Since the temperatures and the pressures can be adjusted in the formation of a heat seal, temperatures and pressures are called variables. The Three Impulse Heat Seal Variables In an impulse heat sealer, an additional variable, dwell time, is added to the variables of temperature and pressure. In the impulse heat seal operation, the heat seal jaws come together to apply pressure to the material being heat sealed. On one of the jaws is a resistance wire that heats up when electrical current is applied. While the jaws are applying pressure to the material, the resistance wire is heated for a specific length of time and then cooled for a specific length of time. Dwell time is the interval of time the jaws are under pressure and consists of heating dwell time and cooling dwell time. The three variables; temperature, pressure and dwell time, control the strength of the heat seal being formed.16 The temperature causes the interfaces between the two surfaces to disappear, the pressure assures intimate contact of the interfaces and the dwell time brings the interfaces to the needed sealing temperature and allows time for cooling with the surfaces held together.19 Each of the variables has upper and lower limits. The upper boundary of the temperature range is limited by damage to the material caused by heat degradation. The lower temperature boundary is limited by the amount of heat necessary to melt the interface between the two surfaces to be heat sealed.19 To form a good heat seal, the temperature must be high enough to melt the interface between the two layers of polyethylene and low enough to prevent heat degradation of the polyethylene. Pressure is the second variable in the formation of a heat seal and, like temperature, pressure also has upper and lower limits. The upper limit is present because too high a pressure in the heat seal jaws will thin out the polyethylene during formation of the heat seal. A pressure that is very high will extrude the polyethylene from between the heat sealing jaws.14 The lower limit of the pressure range is the minimum pressure necessary to assure intimate contact between the polyethylene material interfaces. David Pegazll noted the boundaries of pressure when he was studying the bonding rate of polymeric films. He stated in his thesis: It can be observed that initially the bond strength values rise with increase in pressure which indicates that some amount of pressure is necessary to overcome a kind of "bonding surface resistance"....with further increase in pressure the bond strength values tend to become lower, or the rate of bond strength growth slows down as if the process of bond formation, diffusion, were laboring against some restrictive force. The bond strength increases and then decreases as pressure is increased indicating the presence of an optimum.range. This supports the previous statement, the pressure must be high enough to assure intimate contact of the interfaces and low enough to prevent a thinning out of the polyethylene at the heat seal area. The third and last variable, dwell time, also has upper and 7 lower limits. Joseph Miltz observed that heat seal strength passed through a maximum as dwell time increased for all sets of temperature 9 Miltz's results can be illustrated on a and pressure conditions. graph (See Figure 1). The graph shows a family of curves for four mil polyethylene where the temperature and pressure have been held constant while the heated duration of the dwell time was varied from 0.50 to 1.25 seconds. The temperature is expressed in volts of electric potential (V30, V25) and the pressure is expressed in p.s.i. (P30, P20). V = Volts in Electric Potential 1 P = Pressure in .s.i. 1.95 a P 2303 Seal 1.85 - V0 30 Strength P20 (ks/cm) 1.75 -.-| - V95 333.0 1065 "‘ 1-55 ~ 1'45 ‘ l r l 1 0.00 0.50 0.75 1.00 1.25 Heating Dwell Time (sec.) SOURCE: Joseph Miltz, "Effect of Structure on Heat Sealing Properties, Seal Strength of LDPEs", Package Development and Systems 10 (March/April 1980): p. 23. Figure 1. Heat Seal Strength Versus Dwell Time for a Four Mil Polyethylene The dwell times on the horizontal axis of Figure 1 represent only the time the resistance wire is heated (heating dwell time). The cooling time was held constant at two seconds. In figure 1, dwell times of 0.50 and 1.25 seconds formed a weaker seal for each set of temperature and pressure conditions than 0.75 seconds dwell time (See Table 1). Table 1. Heat Seal Strengths for Various Temperature and Pressure Conditions Temperature Dwell Time Dwell Time Dwell Time Expressed 0.50 sec. 0.75 sec. 1.25 sec. in Pressure Strength Strength Strength Condition Voltage p.s.i. kg/cm kg/cm kg/cm i 1 30 30 1.77 1.92 1.78 2 30 20 1.71 1.85 1.76 3 25 30 1.56 1.78 1.63 SOURCE: From Figure 1, Joseph Miltz, "Effect of Structure on Heat Sealing Properties, Seal Strength of LDPEs", Package Development and Systems 10 (March/April): p. 23. Miltz's findings showed an optimum dwell time in the formation of a strong heat seal in polyethylene. Insufficient dwell time formed weak heat seals as did too much dwell time. EIELInterrelationship of the Three Heat Seal Variables The three variables; temperature, pressure and dwell time, are interrelated. The largest optimum temperature range in forming a strong heat seal occurs at high temperatures, short dwell times and low pressures. The largest dwell time range occurs at long dwell 9 times, low temperatures and low pressures (See Table 2). Table 2. Maximum Range Conditions for Temperature and Dwell Time Condition Variable Setting Largest Temperature High Short Low Range . . . . . . . . . . Temperatures Dwell Times Pressures Largest Dwell Time Low Long Low Range . . . . . . . . . . Temperatures Dwell Times Pressures SOURCE: R. M. Knight, E. E. Froste and W. U. Funk, "Polyethylene Heat-Seal Factors", Modern Packaging 31 (April 1958): p. 156. From Table 2, a direct relationship between dwell time and temperature can be seen. An increase in temperature will shorten the dwell time necessary to form a strong heat seal. Similarly, an increase in the dwell time will lower the temperature necessary to form a strong heat seal. The dwell time required at a given temperature or the temperature required at a given dwell time can be reduced by increasing the pressure.6 This means an increase in pressure can be used to reduce the temperature and dwell time necessary to form a strong heat seal when the pressure used is within the optimum range. The interrela- tionship of the three variables is very important and directly relates to heat seal strength. Summation of the Various Literature Search Concerns Heat sealing of polyethylene is actually a welding of the 1O material. The material is fused together by melting. The welding is accomplished by a combination of three variables; temperature, pressure and dwell time. The variables; temperature, pressure and dwell time, each have upper and lower limits. Too high a temperature will degrade the material and too low a temperature will not melt the interfaces of the material. Enough pressure is needed to assure intimate contact of the material interfaces but too much pressure will thin out the polyethylene in the heat seal area and cause the polyethylene to extrude out of the heat seal jaws. Dwell time has an optimum duration which can be seen from Figure 1. Any increase or decrease in the heating dwell time from the optimum duration will correspondingly decrease the heat seal strength. In addition to having limits, the three variables are also interrelated. A change in one variable can be compensated by a change in another to form a heat seal. A low temperature can be compensated by a longer dwell time and a short dwell time can be compensated by a higher temperature. An increase in pressure will decrease both the temperature and dwell time necessary to form a heat seal. STATEMENT OF THE PROBLEM Description of an Impplse Heat Sealer A heat sealing machine brings the thermoplastic material together and, with an application of heat, pressure and dwell time to the interfaces, a weld is formed, sealing the material together. In the impulse heat seal operation, a set of jaws is used to apply pressure to the material interfaces being heat sealed. Inside both of the jaws is a resilient surface to hold the material interfaces in place. On the surface of one of the resilient pads is the resistance wire. While the pressure is being applied by the jaws to the material, electric current is passed through the resistance wire causing the wire to heat up. The amount of current passed through the wire regulates the temperature or amount of heat the material interfaces receive. After the appropriate heating dwell time has passed to raise the material interfaces to welding temperature, the resistance wire is cooled, the jaws open up and release the material, leaving a welded seam between the two interfaces of the polyethylene. The impulse heat sealer is designed for continuous sealing of unsupported thermoplastic material.19 During the full pressure jaw closure time of the heat seal operation, the jaws are held apart by the unmelted portion of the material. When the material around the resistance wire is melted, the unmelted material holding the jaws apart prevents the pressure from thinning out the seal area (See 11 12 Figure 2). The lower unmelted portion of the film in the jaws also supports the end of the pouch while the resistance wire welds the interfaces together. With these techniques, the impulse heat sealer has the ability to form seals for unsupported thermoplastic materials. Unmelted.Portion of -“——-Four Mil Polyethylene Material Holding Jaws Material Two Sheets Apart 'Metal 7 of Jaw Pressure l l I 1 \\\\ i l Resistance Wire-—J/7 x-L-—-Heat Seal Area Resilient Part of Jaws Side View SOURCE: Quality Dairy Form-Fill-Seal Machine, Manufacturer - Societe Prepac, 62 Rue Pastur, Villejuif - 94 - France. Figure 2. Impulse Heat Sealer Quality Dairy's Form—Fill-Seal Machine Quality Dairy Company of Lansing, Michigan elected to switch their packaging of milk from blown polyethylene bottles to flexible polyethylene pouches. The form-fill—seal machine promised to increase productivity, reduce working space, improve packaging efficiency and 13 reduce overall packaging costs. The machine uses four mil, low density polyethylene film dis- pensed from a roll located on the back side of the machine. The film travels, from the roll, over the top of the machine where it is folded lengthwise and a vertical heat seal is applied to the loose ends forming a tube. The tube of polyethylene travels down the front of the machine where it is filled with milk. At the bottom of the machine, the polyethylene tube, encasing the milk, is pinched together at ten inch intervals, horizontally heat sealed and cut off. The mechanism that perfbrms the heat sealing and cutting off of the material is called a heat seal cut off bar. The heat seal cut off bar seals the top of the bottom pouch and the bottom of the top pouch while simultaneously separating the two pouches (See Figure 3). \\\ Milk Polyethylene-——44 Tube Top Pouch Heat Seal Heat Seal Jaw Jaw Heat Sealsdzg Bottom Pouch Milk Side View Front View Figure 3. Quality Dairy's Form-Fill-Seal Machine Heat Seal Cut Off Bar 14 Quality Dairy's form-fill-seal machine can be adjusted for all three heat seal variables; temperature, pressure and dwell time, on the horizontal heat cut off bar. The temperature is adjusted by a large dial located on the front of the machine which is calibrated from 0 - 100. The dial controls the percentage of current entering a transformer. The transformer steps down the source voltage from 220 to 24 volts and passes the current through the resistance wire in the heat seal jaw. For example, if the dial is set at 80%, this means 80% of 220 (176) volts is stepped down to a corresponding 80% of 24 (19) volts and passed through the resistance wire. The tempera- ture control is the easiest to adjust of the three variables and it is changed frequently by the operator. The pressure is controlled by a 3/16 inch thick rubber strip squeezed between the heat seal jaws of the machine. Standard practice has been to compress the rubber strip to 1/8 inch clearance between the jaws of the heat sealer. The amount of pressure applied to the heat seal depends on the force required to crush the rubber strip. The strip was compressed on a compression tester and it was found that 825 pounds was required to crush the rubber strip to a factory recommended thickness of 1/8 inch. The 825 pound force on the rubber strip equates to 137.5 p.s.i. (See Equation 1). 825 lbs. Force __=_ 137.5 p.s.i. (1) 6 inches2 Area of Rubber Strip The pressure adjustment has a locking nut capable of compressing the rubber strip 0.02 inches per half-turn. The rubber strip has a linear relation of compression versus force exerted equal to 825 15 pounds per 1/16 inch. With a surface area of approximately six square inches on the rubber strip, each half-turn on the locking nut would exert a force of 44 p.s.i. (See Equation 2). 3(0.02 inches)(825 lbs:[ 1116 inch) = 44 lbs./inch2 6 inches2 Area of Rubber Strip (p.s.i.) (2) The force of 44 p.s.i. is the minimum pressure the form-fill-seal machine can exert on the material being heat sealed. The factory recommended pressure is 137.5 p.s.i.. The dwell time is controlled by a cam mounted on a shaft. The cam shaft rotates one revolution per heat seal formed and the cam shaft controls jaw closure, dwell time and jaw opening. While rotating, the dwell time cam pushes in a button completing the resistance wire electrical circuit resulting in a flow of current through the resistance wire in the heat seal jaw (See Figure 4). \ Heating Dwell Time Shaft Actuator Button 1 I b _ h _I I 7 Rotation Figure 4. Timing Cam Shaft of the Horizontal Heat Sealer 16 The dwell time is limited by the full pressure jaw closure time. The heating dwell time is only effective when the material is being squeezed between the heat seal jaws at full pressure. The form-fill- seal machine forms thirty-three seals per minute which equates to 1.8 seconds per revolution of the timing cam shaft. The sequence starts at the 00 mark on the shaft, 1100 marks full pressure jaw closure and 2800 marks full pressure just before jaw opening. The jaws are at full pressure jaw closure for 1700 of the shaft rotation (280o - 1100 = 170°). The 1700 portion of the shaft at 1.8 seconds per revolution equates to 0.85 seconds full pressure jaw closure time (See Equation 3). (1.8 sec.) (170°) = 0.85 seconds Full Pressure Jaw (3) (360°) Closure Time Since the full pressure jaw closure time is 0.85 seconds, the heating dwell time can be adjusted from 0 to 0.85 seconds. Cooling time normally associated with an impulse heat sealer for this kind of application is unnecessary because the heat seal jaws are pinching off a column of 36°F milk encased in polyethylene. The cold temperature of the milk absorbs the heat of the molten polyethylene almost instantly as the seal is being formed. A summary of the adjustment ranges for temperature, pressure and the heating dwell time is in Table 3. 17 Table 3. Summary of the Horizontal Heat Seal Variable Adjustment Ranges Variable Temperature Pressure Heating Dwell Time % Current p.s.i. seconds Range . . . . 0 — 100 44 - 137.5 0.0 - 0.85 The Problem Five percent of the seals formed by Quality Dairy's form-fill- seal machine are defective. The "leaking" heat seals are a problem 1 arising from the new application of polyethylene in the food industry. A good heat seal is needed to prevent undesirable leakage, premature spoilage, and unsightly appearance. To achieve all these objectives, an appropriate combination of temperature, pressure and dwell time in forming the heat seal is necessary. The cost of the heat seal leaker problem can be calculated. 14,000 Pouches of Milk Formed per Day Days per Week Operation 42,000 Pouches Formed per Week 52 Weeks per Year 2,184,000 Pouches Formed per Year 3% Leakers (Conservative Estimate) 109,200 Pouches of Milk Lost Due to Leakers 64¢ Cost of Milk and Package (Estimation) $69,888 Direct Loss Attributable to the Heat Seal Problem 18 This simple calculation does not take into account the loss of future sales due to poor performance of the milk pouches used by the customer (i.e. A pouch of milk that leaks in a customers car while he/she is traveling from the store to home). The temperature adjustment mechanism on the form-fill—seal machine is easy to reach by the machine operator and can be changed while the machine is running. The other two variable adjustment mechanisms are not accessible while the machine is running and are not calibrated. The dwell time variable mechanism is particularly difficult to adjust because any change would require welding additional pieces of metal to the timing cam. With these conditions present, less than optimum settings of the heat seal variables could easily occur resulting in the formation of weak heat seals. In this study the difficult to adjust variable, dwell time, was changed to strengthen the heat seals being formed. Limits of the Study The purpose of the study is to increase the strength of the heat seal being formed by Quality Dairy's form-fill-seal machine by making adjustments to the variables; temperature, pressure and dwell time. The study will not consider variations in the pressure over the length of the heat seal, variations in the power supply, changes in the resin composition (density, molecular weight, additives etc.), effect of the film treatment for printing, variations in film thickness, changes to the resistance wire or resilient jaw pad, variations in temperature of the jaw cooling water and continuity of the heat seal. 19 The ultimate objective of the study is to identify the optimum ranges of the variables; temperature, pressure and dwell time, to form a strong heat seal and adjust the form-fill—seal machine to conform with the optimum ranges. Any other changes or modifications to the machine or the packaging material will be outside the scope of this thesis. METHODOLOGY Basis for Procedure In Joseph Miltz's article "Effect of Structure on Heat Sealing Properties, Seal Strength of LDPEs",9 he found the seal strength had an optimum heating dwell time setting for all combinations of temper- ature and pressure conditions. This relationship is very important to a heat seal machine operator. If the machine is forming weak seals, the machine operator must know which side of the seal strength versus heating dwell time curve peak the heating dwell time is currently set. If the heating dwell time is set on the left side of the curve peak, he should increase the heating dwell time to increase the heat seal strength (See Figure 5). 1.50 .. Seal Short Heating Strength Dwell Time (kg/cm) Machine Setting 1.40 —-‘h ----- ' I i : To Increase Strength I 1030 —-4 l f T“ F 0.0 0.35 0.75 1.25 Heating Dwell Time (sec.) Figure 5. Heat Seal Strength Versus Heating Dwell Time for Four Mil Polyethylene (Short Heating Dwell Time Machine Setting) 20 21 Conversely, if the heating dwell time setting is on the right side of the curve peak, the operator should decrease the heating dwell time to increase the seal strength (See Figure 6). 1.50-— Long Heating Seal Dwell Time Strength (kg/cm) Machine Setting L40_¢—-—- ------------ I I To Increase Strength : I 1030 —-JA _1 7' 1 WI“ 0.0 0.35 0.75 1.25 Heating Dwell Time (sec.) Figure 6. Heat Seal Strength Versus Heating Dwell Time for Four Mil Polyethylene (Long Heating Dwell Time Machine Setting) The first step in solving the heat seal leaker problem is to identify the current settings of the variables in the horizontal heat seal jaw. In this case the temperature is set at 80%. The amount of pressure applied to the heat seal depends on the force required to crush the rubber strip between the heat seal jaws. From Equation 1 in the preceding section, the current pressure setting is 137.5 p.s.i.. The heating dwell time is controlled by the length of the cam on the timing cam shaft (See Figure 4). The length of the horizontal heatwsealer heating dwell time cam is 80°. The 800 length equates to 0.40 seconds heating dwell time (See Eqation 5). 22 4(1.8 sec. Revolution Time)(80O Dwell Time Cam Length) = 0.40 sec. (5) (360° Full Revolution) The current settings of temperature, pressure and heating dwell time of the horizontal heat sealer are enumerated in Table 4. Table 4. Summary of the Variables: Current Settings Variables Temperature Pressure Heating Dwell Time % Current p.s.i. seconds Current Settings 80 137.5 0.40 The next step is to identify optimum settings of the variables to form a strong heat seal using four mil polyethylene. The tempera- ture should be set just high enough to melt the interfaces between the two surfaces to be heat sealed. This setting can be easily determined by running pouches through the horizontal heat sealer at different temperature settings and observing the setting at which the melting of the polyethylene begins. Joseph Miltz observed twenty p.s.i. as being the optimum pressure for heat sealing four mil polyethylene having the characteristics of Quality Dairy's film (See Appendix A). Any pressure above or below twenty p.s.i. will weaken the heat seal being formed.9 He also found the heating dwell time formed an optimum seal strength at a 0.75 sec. duration. In his experiment, he used a 0.75 sec. heating dwell time coupled with a two second cooling dwell time. In Quality Dairy's 23 application, the two second cooling dwell time is unnecessary because the cold temperature of the milk absorbs the heat from the molten polyethylene almost instantly. With these optimum settings identified and the current settings calculated, a comparison of the differences and a determination of the adjustments necessary to change the variables to the optimum settings can be made (See Table 5). Table 5. Comparison of Variable Current Settings, Variable Optimum Settings and Variable Adjustment Ranges Variables Temperature Pressure Heating Dwell Time % Current p.s.i. seconds Current Setting . . . . . . 80 137.5 0.40 Optimum setting a o o o o o -"" 20 0075 Adjustment Range 0 o o o o o o O - 100 44 - 137.5 0.0 - 0085 The final step was to make the adjustments necessary to change the current variable settings to coincide as nearly as possible with the optimum variable settings. The temperature will automatically adjust to the optimum setting when the voltage is adjusted to the setting at which the two interface surfaces begin to melt. The pressure setting could not be adjusted to the optimum value because the management of Quality Dairy was reluctant to make any time con- suming changes to the machine. For the test conditions, the pressure 24 remained constant at 137.5 p.s.i.. The heating dwell time can be increased from the current setting, 0.40 seconds, to the optimum setting, 0.75 seconds, by increasing the length of the heating dwell time cam from 800 to 1500 (See Equation 6). (6) _(o.75 sec. Optimum Dwell Time)(360° Full Revolution) = 150° Cam Length (1.8 sec. Revolution Time) Unfortunately, to increase the cam from the current length of 800 to the optimum length of 1500 required welding an additional piece of metal to the timing shaft. Again, the management of Quality Dairy was reluctant to make any permanent changes to the machine. In order to increase the heating dwell time without performing any permanent changes to the machine, the heating dwell time actuator button was activated manually during the entire heat seal cycle. Since the dwell time is only effective during the full pressure jaw closure time, this action resulted in a heating dwell time equal to the full pressure jaw closure time or 0.85 seconds. The increased heating dwell time of 0.85 seconds was longer than the optimum dwell time of 0.75 seconds. However, 0.85 seconds is closer to the optimum than the current setting of 0.40 seconds. In the adjusted settings, increasing the heating dwell time from 0.40 seconds to 0.85 seconds correspondingly lowered the temperature necessary to weld the material interfaces from 80% to 42%. Condition I was 80% current and 0.40 seconds heating dwell time. Condition II was 42% current and 0.85 seconds heating dwell time (See Table 6). 25 Table 6. Comparison of Current Variable Settings (Seal Condition I) to the Adjusted Variable Settings (Seal Condition II) Variables Temperature Pressure Heating Dwell Time % Current p.s.i. seconds Current Variable Settings (Sealing Condition I) . . . . 80 137.5 0.40 Adjusted Variable Settings (Sealing Condition II). . . . 42 137.5 0.85 Test~Used The method of test selected to determine the strength of the heat seals must be capable of detecting and quatifying the strength difference between the heat seals in the two sealing conditions. In addition, the test results and method must be applicable to other studies and furture works. A standard test capable of fulfilling the requirements of strength determination and results uniformity was ASTM test D 882—75b.1 The ASTM test was capable of detecting heat seal Strength differences and the method of test was similiar to the one used by;Joseph,Miltz in his study.9 The ASTM test was also recommended by the US Army Natick Research and Development Laboratories as the accepted standard test for determining heat seal strength (See Appendix B). For the purposes of this study, ASTM test D 882-75b was used as the test for strength determination and results uniformity. 26 Test Sample Size Determination A pilot study was done to determine the sample Size of the main study. The pilot study consisted of six seals formed under each condition. A one inch wide and four inch long strip of polyethylene was cut from the heat sealed pouches with the heat seal in the center of the strip (See Appendix C). The test samples were cut from the same place on each pouch (See Figure 7). TOP p==l===.—__=T=____—=——__qr‘— TOP Heat Seal Horizontal Heat Seal __‘_, Vertical Heat Seal . :-— Test —4 rm“. \\ ,’ Specimen N : I I I Bottom \ I I 2 Bottom Heat 3% ‘ .' Heat Seal Side View Back View Figure 7. Quality Dairy Half-Gallon Milk Pouch The test specimens were subjected to a grip separation of ten inches per minute in accordance with ASTM test D 882-75b on the Instron Tensile Tester Machine. This result was recorded as pounds per inch of heat seal length since the specimens were one inch wide. The pilot study temperature variable differed from the main study. In Condition I the pilot study current was set at 55% as compared to 80% in the ' main study. In Condition II the pilot study current was set at 30% as compared to 42% in the main study. The other two variables, 27 pressure and heating dwell time, were the same for both the pilot and main study. The results of the pilot study, with six seals under each condition, had a mean difference of 1.1 pounds per inch of heat seal length between the two sealing conditions in favor of Condition II, with a pooled standard deviation of 0.45 (See Appendix D for Results of the Pilot Study). With a 1.1 pound per inch difference between the two conditions and a pooled standard deviation of 0.45, the necessary sample size was determined for the main study. For a Student's t- test at a 99% significance level, a mean difference of 0.5 lbs. /inch could be detected with a 0.99 probability (i.e. "power of the test) by using 40 samples under each condition. During the pilot test sealing operation, a couple of the pouches were subjected to a simple squeeze test where the pouches were hand squeezed until the package ruptured. Condition I seals failed by rupture of the material at the heat seal. In Condition II, the material of the pouch deformed at some point well away from the seal area, leaving the heat seal intact. While this was qualitative only, and was not really a test, it is indicative of different failure modes for the two sealing conditions. The Test Forty samples were run off at each condition for the main study. Condition I was the heat seals formed by the variable settings that were currently being used. Condition II was the heat seals formed by adjusting the temperature and heating dwell time (See Table 6). The samples were run off in the middle of the production day at 12:30 P.M.. 28 A section of the material was also collected to perform a "material only" tensile test. The purpose of the "material only" test was to set an upper limit on the heat seal strength to be achieved. A heat seal, when formed properly, should be stronger than the material being sealed together.16 RESULTS/DISCUSSION/CONCLUSION The Results The heat seal strength test was conducted in accordance with the procedures in ASTM test D 882-75b. The test was done on the material only, Condition I seals, and Condition II seals (See Table 7 for Summary of Averaged Test Values)(See Appendix E for Results of the Material Only Test, See Appendix F for the Results of Condition I Seal Test and Condition II Seal Test). Table 7. Result Averages of ASTM Test D 882-75b Tensile Breaking Factor % Elongation Strength lbs./inch at Break p.s.i. (1) (2) (3) Material Without Heat Seal . . . . . . . 7.91 866.3 2610 Heat Seal Condition I . . . . . . 3.65 343.5 1200 Heat Seal I Condition II . . . . . 4.58, 591.4 1490 Difference Between Condition I and Condition II . . . . . 0.93 247.9 290 % Change 25 72 24 29 30 The mean results and standard errors of means from 40 seals were 3.65 1' 0.30 lbs./inch for Condition I seals and 4.58 i 0.30 lbs./ inch for Condition II seals. A pooled standard deviation was used to calculate standard errors because the variances within the two groups were not significantly different (0.0333 and.0.0391). A Student's t - Test of mean difference provided a test statistic of 21.63 (See Appendix G) versus a critical value of 3.421 at the 99.9% level of confidence. This means that the observed difference~waSrsignificant and Condition II seals were stronger than Condition I seals (In fact the minimum value for any seal under Condition II was as high as the maximum value for any seal under Condition I). Discussion Adjusting the variable settings for Condition II seals resulted in a 23% increase in strength as compared to Condition I seals. All of the seals were well below the material strength indicating the improved seals in Condition II were still not being properly formed. The increase in seal strength exhibited by Condition II seals supports Joseph Miltz's findings about optimum variable settings to form a strong heat seal. Miltz discovered: "At all voltage/pressure combinations used the seal strength passed through a maximum as sealing time was increased." When temperature and pressure were held constant and the heating dwell time was varied from 0.50 to 1.25 seconds, the strength of the heat seal passed through a maximum strength value at 0.75 seconds for a four mil polyethylene film. It was also observed that the average elongation at break was much less for Condition I seals than for Condition II seals, and the 31 standard deviation of the elongation at break was much higher (145.22% versus 33.59%) for Condition I seals than for Condition II seals. In addition, the standard deviation was a much higher percentage of the average elongation at break for Condition I (42%) than for Condition II (6%) (See Appendix F). The seal lines for Condition I seals and Condition II seals were examined to determine any physical evidence of differences. Under a 48X microscope the seal lines formed under Condition I showed a number of microscopic holes or uneven concentrations of material between the two layers of polyethylene being heat sealed. Condition II seal lines showed a lesser number of microscopic holes or uneven concentrations of material (See Figure 8). Seal Line Microscopic Holes or Uneven Concentrations of Material Condition I Condition II Seal Line Seal Line Figure 8. Seal Lines Formed Under Condition I and Condition II (Drawings of Observations Made at 48X Magnification) 32 The increased duration of the heating dwell time in Condition II seals could have allowed more time for the molten polyethylene to flow together (molecular netting of molecules) thus, reducing the number and size of microscopic holes or uneven concentrations of material in the seal lines. The reduction in number'and size of the microscopic holes or uneven concentrations of material could have increased the area of contact between the two material interfaces resulting in the formation of a stronger heat seal. The difference seen in the standard deviations of elongation at break between Condition I seals and Condition II seals tends to support this theory. The high standard deviation value for elongation at break in Condition I seals could be caused by the random number and size of the microscopic holes or uneven concentrations of material in the seal lines. The reduction of the size and number of micro- scopic or uneven concentrations of material in Condition II seals could account for the lower standard deviation and increase in seal strength. The increased area of contact between the two material interfaces in Condition II could have formed a more uniform and stronger seal. Conclusion In this study, the known properties of a material were applied to a packaging process to achieve a desired result. The known properties of the material determined the optimum settings for temper- ature, pressure and dwell time to form a strong heat seal. Two of the optimum settings were applied to a form-fill-seal machine and the heat seals formed under the two optimum settings were tested. The 33 test results supported Joseph Miltz's findings about the optimum settings of the variables. Only two of the three variables were changed but the changed variables formed a 25% stronger heat seal than the previously used variable settings. The increase in strength seen in Condition II seals could have been caused by the longer heating dwell time. The longer heating dwell time may have allowed the molten polyethylene to flow together more thoroughly thus reducing the number and size of the microscopic holes or uneven concentrations of material in the heat seal. Areas for Further Study A properly formed heat seal should be stronger than the material itself.17 In this study, the formed heat seals never reached the strength of the material (See Table 7). This fact indicates the need for further adjustments of the variables and further investigation of the form-fill-seal machine. The pressure variable was not adjusted and the presently used setting of 137.5 p.s.i. was well over the optimum value of 20 p.s.i. prescribed by Joseph Miltz.9 The high pressure setting could be part of the cause of the less than optimum strength of the heat seals being formed.18 In another study the pressure should be adjusted along with temperature and dwell time to see if the seal strength can be increased. Unfortunately, the ASTM test used does not simulate the distri- bution environment. The specified grip separation speed of ten inches per minute on the Instron Tensile Tester was not fast enough to simulate the rate of stress the heat seals will have to endure when subjected to shipment shock and vibration. A further study should 34 be conducted to design a test method capable of simulating the distri- bution environment and test the heat seals formed under various conditions. This type of study could directly relate heat seal 5 formation to performance. APPEN DI CES APPENDIX A LETTER FROM DUPONT CANADA INC. KINGSTON, ONTARIO 35 cc: D. Ball - CTC DU PONT CANADA INC. Research Centre Tel. 613-544-6400 Research Division P.0. Box 5000 Kingston, Ontario K7L 5A5 1981 January 30 Mr. Melvin S. Harder 435 M.A.C. East Lansing, Michigan. 48823 U.S.A. Dear Sir: The following information is in response to your inquiry of 1981-1-16 to Mr. D. Ball regarding our polyethylene liquid packaging film: Density: 0.919 gm/cm3 Melt Flow Index: 0.759m/10min. @ 190°C Ultimate Tensile Strength: 450kg/cm2 Mn 30,000 r 5,000 Mw 120,000 i 5,000 Mw/Mn m4.0 Newtonian Viscosity 0 190°C 250,000 poise I trust that the above information will be of assistance to your study. Yours very truly, DU PONT CANADA INC. A.N. Mollison APPENDIX B LETTER FROM THE US ARMY NATICK RESEARCH AND DEVELOPMENT LABORATORIES NATICK, MASSACHUSETTS 36 DEPARTMENT OF THE ARMY us ARMY NATICK RESEARCH and DEVELOPMENT LABORATORIES NATICK. MASSACHUSETTS 01760 REPLY TO ATTENTION OF: 19 January 1981 Mr. Mel Harder 435 N.A.C. East Lansing, MI 48823 Dear Mr. Harder: Reference is made to your phone request of 14 January 1981 regarding ASTM procedures for tensile testing heat seals on A-mil thick poly bags. The procedures described in ASTM D882 (inclosed) are applicable. Also inclosed is a sketch showing how the test specimens should be cut and mounted on the tensile tester together with a check list which can be used as a guide in preparing the test specimens, testing, and reporting test results. I believe that this information should help you in your work, but if you should have any further questions, please feel free to call on us again. Sincerely, 2 Incl TEDIO CIAVARINI As stated Physical Scientist (Packaging) Food Packaging and Processing Group Food Technology Division Food Engineering Laboratory APPENDIX C DIAGRAM SHOWING HOW THE TEST SPECIMENS WERE CUT FROM THE SAMPLE ‘POUCHES AND DIAGRAM SHOWING HOW THE SPECIMENS WERE POSITIONED IN THE INSTRON TENSILE TESTER 37 APPENDIX C DIAGRAM SHOWING HOW THE TEST SPECIMENS WERE CUT FROM THE SAMPLE POUCHES AND DIAGRAM OF HOW THE SPECIMENS WERE POSITIONED IN THE INSTRON TENSILE TESTER Pouch Test Specimen was Verticall Cut Perpendicular Heat Sea é to the Heat Seal e— WT ' l—‘~——4inches ; r-——--—--T-Sea'em‘”“fj' | - Heat Seal I ' 3. ' Test Specimen (Transverse) l 1 inch I . I L_-_____i_-__-__.J_L .900) a." Pouch Cut Down Both Sides and Laid Down Flat SOURCE: American Society for Testing Materials. Standard Test Method for Tensile Properties of Thin Plastic Sheeting, D 882-755. Figure 9. Diagram Showing How the Test Specimens Were Cut From the Sample Pouches 38 Ly- Direction of Pull ’11 1///..///..{{/ I ///:/l//1 1 1 inch L 2 inches [WW 2 inches 4 inches War 1 £/% P- Direction of Pull ’31 Side View Front View SOURCE: American Society for Testing Materials. Standarleest Method for Tensile Properties of Thin Plastic Sheeting, D 882-75b. Figure 10. Diagram Showing How the Specimens Were Positioned in the Instron Tensile Tester APPENDIX D PILOT TEST RESULTS OF HEAT SEAL STRENGTH 39 APPENDIX D PILOT TEST RESULTS OF HEAT SEAL STRENGTH Table 8. Pilot Test Results of Heat Seal Strength Condition I Condition II Temperature Pressure Dwell Time Temperature Pressure Dwell Time % Current p.s.i. seconds % Current p.s.i. seconds 55 137.5 0.40 30 137.5 0.85 Breaking Factor Breaking Factor Seal lbs ./inch Seal lbs./inch 1 , 4.9 1 6.6 2 5.45 2 7.4 3 5-5 3 5-85 4 5-75 4 6.45 5 5.3 5 5.9 6 5.4 6 6.25 Average 5.38 Average 6.41 Std Deviation 0.2805 Std Deviation 0.5687 APPENDIX E TABLE OF MATERIAL STRENGTH TEST ASTM TEST D 882-75b 40 APPENDIX E TABLE OF MATERIAL STRENGTH TEST ASTM TEST D 882-75b Table 9. Table of Material Strength Test ASTM Test D 882-75b Breaking Factor % Elongation Tensile Strength.Material Thickness Seal lbs./in. at Break p.s.i. x 103 1 7.9 880 2600 3.0 2 7.2 825 2400 3.0 3 7-5 850 2500 3.0 4 7.7 855 2600 3.0 5 8.3 875 2700 3.1 6 7.8 850 2600 3.0 7 7.9 870 2500 3.1 8 8.2 870 2700 3.0 9 8.2 860 2700 3.0 10 8.1 875 2600 3.1 11 8.0 890 2700 3.0 12 8-1 895 2700 3.0 Mean 7.91 866.3 2610 3.03 Std Dev 0.3204 19.44 99.62 0.0452 APPENDIX F TABLE OF HEAT SEAL STRENGTH TEST RESULTS AND TABLE OF MATERIAL THICKNESS TESTED ASTM TEST D 882-75b 41 APPENDIX F TABLE OF HEAT SEAL STRENGTH TEST RESULTS AND TABLE OF MATERIAL THICKNESS TESTED ASTM TEST D 882-75b Table 10. Table of Heat Seal Strength Test Results Condition I Condition II Temperature Pressure Dwell Time Temperature Pressure Dwell Time % Current p.s,i. Seconds % Current ptsli. Seconds 80 137.5 0.40 42 137.5 0.85 Breaking % Elonga— Tensile Breaking % Elonga- Tensile Factor tion at Strength Factor tion at Strength Seal Lbs./in. Break PSI Seal Lbs./in. Break PSI 1 4.0 490 1300 1 4.7 610 1500 2 3.5 90 1100 2 4.8 590 1600 3 3.9 455 1300 3 4.6 600 1500 4 3.9 465 1300 4 4.7 620 1500 5 3.6 385 1200 5 4.0 520 1300 6 3.8 465 1300 6 4.5 575 1500 7 3.8 475 1300 7 4.8 610 1500 8 3.4 125 1100 8 4.4 535 1400 9 3.6 490 1200 9 4.7 615 1600 10 3.8 500 1300 *10 5.3 675 1700 11 3.5 395 1200 11 4.6 580 1500 12 3.7 480 1200 12 4.5 615 1500 13 3.8 480 1300 13 4.8 690 1500 14 3.5 375 1100 14 4.6 570 1500 15 3.4 380 1100 15 4.5 580 1500 16 3.5 420 1100 16 4.7 615 1500 17 3.6 115 1200 17 4.2 565 1400 18 3.4 425 1100 18 4.5 575 1500 19 3.7 470 1300 19 4.7 600 1500 20 3.5 105 1100 20 4.6 530 1500 21 3.4 140 1100 21 4.6 605 1500 22 3.7 325 1200 22 4.7 620 1600 23 3.4 115 1200 23 4.5 575 1500 24 3.4 125 1100 24 4.7 585 1500 25 3.8 ' 475 1300 25 4.6 605 1500 Table 10 - Continued 42 Condition I Condition II Breaking % Elonga- Tensile Breaking % Elonga- Tensile Factor tion at Strength Factor tion at Strength Seal lbs./in. Break p.s.i. Seal lbs./in. Break p.s.i. 26 3.5 320 1200 26 4.5 580 1400 27 3.8 450 1300 27 4.6 590 1500 28 4.0 455 1300 28 4.6 600 1500 29 3.5 80 1100 29 4.4 560 1400 30 3.7 250 1200 30 4.6 605 1500 31 3.8 405 1200 31 4.6 615 1500 32 3.5 285 1100 32 4.4 535 1500 33 3.8 420 1200 33 4.4 570 1500 34 3.9 455 1200 34 4.6 615 1500 35 3.6 310 1100 35 4.5 560 1500 36 3.7 415 1200 36 4.4 565 1400 37 3.9 470 1300 37 4.5 590 1400 38 3.5 110 1200 38 4.5 600 1500 39 3.5 155 1200 39 4.5 595 1500 40 3.6 395 1200 40 4.8 615 1500 Mean 3.65 343.5 1200 Mean 4.58 591.4 1490 Std Std Dev 0.1826 145.22 74.45 Dev 0.1977 33.59 65.58 % of Mean 5 42 6 4 6 4 * A suspiciously large value for seal 10 under Condition II (5.3) was determined to be a statistical "outlier" (probability greater than 0.99 that it did not conform to the distribution of values represented by this test). Without the value of 5.3, the mean for Condition II is reduced from 4.58 to 4.56, not altering the basic conclusions at all. 43 Table 11. Table of Material Thickness Tested Condition I Condition II Material Thic ess Seal Tested Inches X 10 Material Thic ess Seal Tested Inches X 10 \O®\70\\n-F'\»)1\)H 2 1 3. 2 3 3 3 4 3. 5 3. 6 3 7 3- 8 3 9 3. 10 3 11 3. 12 3 13 3. 14 3 15 3. 16 3. 17 3. 18 3. 19 3. 20 3. 21 3. 22 3. 23 3. 24 3. 25 3 26 3. 27 3. 28 3 29 3 30 3 31 3 32 2 33 3- 34 3- 35 3 36 3 37 3 38 3 39 3 40 3 LsE>LsR>LsLsc>C>Q>E>E>LsLsc>N>Lshaesc>Esc>EsesEsesese~h>cnesnsc>hsh>e~cnesesc>es Mean 3.06 St 3. 3 1 3.1 3.1 3. 0 3. 0 3. 0 3. 0 3. 0 3. 0 3. 0 3.1 3 0 3 1 3 1 3 1 3 1 3 0 2 9 3.1 3.0 3.1 2 9 3 0 2 9 3. 0 3. 0 3. 0 3 2 3 1 3 1 3 1 3 1 3 2 3 2 3 1 3 1 3. 0 3. 0 3.1 d D ev 0.0782 Mean 3.08 Std Dev 0.0723 APPENDIX G STUDENT'S t - TEST FOR BREAKING FACTOR MEANS AND % ELONGATION MEANS BETWEEN CONDITION I SEALS AND CONDITION II SEALS 44 APPENDIX G STUDENT'S t - TEST FOR BREAKING FACTOR MEANS BETWEEN CONDITION I SEALS AND CONDITION II SEALS Condition I Condition II Xa = 3.6475 lbs./in. Xb = 4.5800 lbs./in. Sa = 0.1826 Sb = 0.1977 Na = 40 Nb = 40 (Two Tailed Test) x Sample Mean S Standard Deviation of Sample Data N = Sample Size a“ ='\NaSa2 + NbSbZ Na + Nb - 2 6': (40)(0.1826)2+ (40)(0.1977)2 = 0.19272 40 + 40 - 2 t = Xb - Xa t = 4.5800 — 3.6475 = 21.63899 7 1 + 1 6' q— — 0.19272 -1—+—i Nb Na 40 40 tC = 3.421 for a 99.9% Confidence Level t) tC 21.63) 3.421 Seals formed under Condition II are stronger than those formed under Condition I. 45 STUDENT'S t - TEST FOR % ELONGATION MEANS BETWEEN CONDITION I SEALS AND CONDITION II SEALS Condition I Condition II Xa = 343.5 Xb = 591.4 Sa = 145.22 Sb = 33.59_ Na = 40 Nb = 40 (Two Tailed Test) X S N Sample Mean Standard Deviation of Sample Data Sample Size Comparing Two Means with Unequal Variances t = Xb - Xa t = 591.4 - 343.5 = 10.52 EZ + S_b_2 “(342.522 + (591.4)2 Na Nb 40 40 Critical Value Calculation s = §_a_2 3192 s = (343.522 (591.422 = 18.695 Na Nb 40 40 Adjusted Degrees 2 2 of Freedom = (1 + g) = (19.695) = 43 ._53 + _1_ (18.695)2 + _i;. (Na - 1)(Nb - 1) 39 39 tC = 3.532 for a 99.9% Confidence Level t> tc 10.52) 3.532 ., % Elongation is Greater for Condition II BIBLIOGRAPHY 10. 11. BIBLIOGRAPHY American Society for Testing Materials. Standard Test Method for Tensile Properties of Thin Plastic Sheeting, D 882-75b (Reapproved 1975). Brown, Terry McAvoy. "One Piece Heat Sealable Folding Cartons: A Study of Characteristics and Sealing Methods." M. S. thesis, Michigan State University, 1958. Christie, Howard W. "Improved Closure of Flexible Barriers." Gill, Modern Packaging 34 (April 1961):213-221. John L. Design and.Analysis of Experiments. 3 vols. Ames: Iowa State University Press, 1978. Graft, Edward H. "Analysis of Methods to Evaluate Heat Seals." M. S. thesis, Michigan State University, 1967. Knight, R. M.: Froste, E. E.; and.Funk, W. U. "Polyethylene Heat-Seal Factors." Modern Packaging 31 (April 1958): 152-156. Kohler, F. "Heat Sealing Plastic Films." Plastics 5 (December 1946):50,52,55,91-93. Lautner, W. K. 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Ryan, William. "Sealing of Polyethylene." Modern Packaging 23 (December 1949):132-137. "Safe Sack Seals Spell Success." Packaging 49 (October 1978): 12,15. Sherman, Gale A. Flexible Packaging. Dallas: Allied Automation Inc. ’ 1974, PP. 16-200 Williams, D. "The Effect of Siloxane-Additioned Titanium Dioxide Pigments on the Heat Sealing and Printing Properties of Low Density Polyethylene Film." Paper presented at the 37th Society of Plastic Engineers Annual Technical Conference (New Orleans, Louisiana: May 1979), pp. 307— 310. Young, W. E. "Approaches to Mechanical Sealing Problems." Paper presented at the National Flexible Packaging Association Heat Seal Symposium (Atlanta, Georgia: February 1976). Young, W. E. "Heat and Sonic Sealing." Modern Packaging Encyclopedia 41 (July 1968):495-498. "11111111111411f