Thermoforming & Die Cutting of PET Sheet shrunk9-29-02 .d.burchamintl.com/papers/petpapers/34_gpec03.pdf · Thermoforming & Die Cutting of Recycled/Virgin PET Sheet (PETCO of Lavergne

Thermoforming & Die Cutting of Recycled/Virgin PET Sheet (PETCO of Lavergne Group)

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Thermoforming & Die Cutting of

Recycled/Virgin PET Sheet

by Larry Koester, Sheila Nemeth, & Mark Koester of Lavergne Group

Contents Section Page(s) Introduction 2-3 What is PET sheet? 3-6 Intro: PET in relation to other resins 3-4 PET vs. PVC 4-5 PET Sheet Properties 5-6 Extruding PET Sheet 6 Thermoforming PET Sheet 7-9 Heating PET Sheet 7-8 Forming PET sheet 8 General Do’s and Don’ts of Thermoforming PET Sheet 9 Die Cutting PET Sheet 10-14 Kiss Cut Dies (Steel Rule and Forged Dies) 10-11 Scissor-type Dies (Matched Metal Dies) 12-13 General Do’s and Don’ts of Die Cutting PET Sheet 14 Conclusions 14-16 Summary Checklist 15-16 Appendixes 17-27 I. Glossary 17-20 II. Transition Temperatures of Thermoformable Polymers 20 IIIA. 2000 Gross Recycling Rate 20 IIIB. RPET End Use Products 2000 21 IV. Plastic Bottles by Resin Type 21 V. Comparison of Thermal Conductivity and Thermal Diffusivity for Several Polymer and Mold Materials

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VI. Shrinkage Values 22 VII. Inflation Pressure Ranges 22 VIII. Drying Conditions 23 IX. Coefficients of Thermal Expansion for Thermoformable Polymers 23 X. Make-Ready Procedure for “Kiss Cut” Dies 24 XI. Physical Properties of Film Extruded of PETG and APET 25 XII. Technical Data and Property Comparison: RPET vs. PVC 26 XIII. Rockwell Hardness Scale of Abbreviations 26 XIV. Application Pictures

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Sources, Thanks and Contact Information 28

Lavergne Group Inc. 8800, 1er Croissant, Ville d’Anjou, Quebec, Canada H1J 1C8


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~ Introduction ~ Over the years, a steady increase in the use of PET has triggered a decrease in the use

of aluminum, glass, and other conventional packaging materials. Convenience stores are now stocked with chilled rows of PET bottles of soda, water, milk, and juice, and finding a glass bottle has become a rarity. Even beer bottles have seen a recent shift to plastic at sporting events. The upward trend and usage of PET bottles has trickled down to increased use of PET in other applications including thermoformed PET sheet. Local groceries and hardware stores are a gallery to the multiple uses of PET sheet from fruit containers to plastic trays to nut and bolt packages. With this perspective in mind, it is crucial to evaluate the current uses, techniques, properties, and characteristics in thermoforming and die cutting recycled and virgin PET sheet.

When a polymer is heated from a low temperature, it transforms from a glassy state to a

rubbery state. The temperature in which this transition occurs is generally termed “glass transition temperature” (abbreviated Tg), and the temperature range over which the polymer is sufficiently pliable for stretching and shaping to a desirable shape is called “thermoforming window.” Thermoforming is the general category of processes heating a polymer sheet to this rubbery state and then using one of several methods to shape the heated sheet into the desired form. After cooling and hardening, the edges are cut away through a procedure called die cutting leaving the completed product. While the process may seem simple, numerous factors dictate and manipulate the slim degree of perfection needed to create a perfect product. Not only must the physical properties of the cooled substance be considered, but the properties of the polymer when it is heated must also be calculated.

Polyethylene terephthalate or more commonly PET is a polymer made by combining

either terephthalic acid or dimethyl terephthalate acid with ethylene glycol. From this chemical combination, a vast range of thermoplastic applications and uses arise for PET and its additive offshoots. PET is an extremely versatile substance, because its properties and characteristics provide relatively easy usability and versatility. Virgin PET sheet’s compliance with Food and Drug Administration (FDA) regulations has allowed a diversity of food applications including such packaging staples as clamshells, trays, containers, and fruit and vegetable baskets. And recycled PET (RPET) through regrind and addition with virgin has allowed companies to create their needed product along with allaying many environmental concerns of tomorrow.

The industry has coined several acronyms to specify PET’s specific end use

capabilities. For example, when used in the crystalline state for ovenable trays, PET is referred to as CPET; when used as oriented film to utilize its toughness, high-temperature and chemical resistance properties, it is termed OPET; when used for the extrusion blow-molding of containers, it is called EPET; and when glycol modifiers are added to minimize brittleness and premature aging, it is called PETG. The acronym APET describes PET when it is in the form of clear, amorphous sheet for thermoformed packaging and related products. And RPET signifies recycled PET sheet, which displays similar properties as virgin PET or APET. PETE is utilized on the bottoms of bottles because of copyright infringement of “PET Carnation Milk” products. For all intense and purposes, APET, PET, RPET, Polyester and PETE are the same thing, polyethylene terephthalate.


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Thermoforming is heating the sheet, to a temperature below its melting point, to a glassy or soft state, and then stretching it to contours of a mold. The characteristics of PET sheet are similar to other amorphous sheet, and thus the thermoforming methods are comparable as well, especially with PVC. The key considerations to remember with PET sheet are to keep it very dry and to not overheat; otherwise significant changes occur in the PET weakening its properties. PET is a tough substance, which leads to the biggest challenge facing thermoformers: die cutting. Although other substances break after only cutting part way through, PET sheet has to be cut completely through for it to fracture. This puts a tremendous strain on equipment and laborers. The two techniques for die cutting are clamp cut or kiss cut, which includes steel rule and forged die, and scissor type, which includes matched-metal cutting. This will provide the basics to cutting, but if a few guidelines are followed, then your efforts will improve and benefit as well.

~ What is PET sheet? ~

As previously mentioned in the introduction, PET (also known as APET, RPET, PETE or polyester) is a plastic resin chemically constructed by combining terephthalic acid with ethylene glycol. Plastics consist of hydrocarbons, basic building blocks typically derived from natural gas or petroleum. These hydrocarbon monomers are bonded into long chains called polymers or plastic resins. Different combinations of monomers will result in resins with specialized characteristics and properties. Much like different metals like copper, silver, and aluminum displaying unique properties, which result in varying uses, plastics, are very versatile and display varying properties and characteristics resulting in a wide range of applications. No single polymer is perfect for every application. Cost as well as an individual polymer’s benefits must be considered.

The resins that constitute nearly all the plastics used in thermoforming (See Appendix

IV for percentage breakdown of top 6 resins used in plastic bottles): • Polyesters or PET (polyethylene terephthalate) is a clear, tough, stable polymer with

exceptional gas and moisture barrier properties. It is often used to contain carbon dioxide (alias carbonation) in soft drinks bottles. Its applications also include film, sheet, fiber, trays, displays, clothing, and wire insulation.

• Acrylic or PMMA (polymethyl methacrylate) is a tough polymer with good optical

clarity, weatherability, and resistance to sunlight, which make it great for outdoor items like sky domes, signs, light fixtures, and bathtubs.

• PC (polycarbonate) is a tough, high temperature transparent plastic but is difficult to

thermoform and is very susceptible to moisture. It is used in windows, helmets, cases, and glasses.

• PE (polyethylene) is the most used polymer in thermoforming with an array of

applications. It is a durable, tough, inexpensive plastic with excellent impact, moisture and chemical resistance.


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• PP (polypropylene) has great high-temperature chemical resistance and is used in manufacturing industrial parts, automotive and electrical hardware, stadium seats, and battery cases.

• PS (polystyrene) was the dominating thermoforming material 20 years ago. It has

excellent processability and good dimensional stability but limited solvent resistance. Its uses today include food and medical packaging, housewares, toys, furniture, advertising displays, and refrigerator liners.

- HIPS (high impact polystyrene)

• Vinyl or PVC (polyvinyl chloride) has very similar properties as PET displaying

excellent clarity, puncture resistance, and cling. As a film, vinyl breathes the right amount making it ideal for packaging fresh meats.

PET vs. PVC

As Table 1 shows, PET displays physical similarities as PVC. The comparable nature of their density, thermal conductivity, thermal expansion, rigidity, and shrinkage allows for machines designed and used for thermoforming PVC to be slightly altered in order to thermoform PET. As Table 1 points out key similarities, Table 2 (on the top of the next page) shows several key differences. The advantages of PET over PVC is its faster cycle times and lower oven temperatures, which leads to less energy used and economic savings. PET’s toughness of cutting and wearing out of dies gives PVC a slight advantage. But with their similar price, PET displays it’s environmental edge over PVC with its ease of recycling and regrinding. PET regrind can easily be used to return to sheet while PVC regrind is much more difficult and expensive to reuse.

Table 1: Physical Properties of Thermoformable Polymers

Polymer Density [lb/ft3]

Density [kg/m3]

Thermal Conductivity [Btu/] [x 10-3

ft.hr.°F

Thermal Conductivity [Btu/lb. kW/ m. °C]

Heat Capacity Coefficient [°F or cal/g°C]

Thermal Expansion [x10-6 °F-1]

Thermal Expansion [x10-6 °C-1]

Polystyrene 65.5 1050 0.105 0.18 0.54 40 70 ABS 65.5 1050 0.07 0.12 0.4 50 90 Polycarbonate 74.9 1200 0.121 0.207 0.49 40 70 LDPE 57.4 920 0.23 0.39 0.95 140 250 HDPE 59.9 960 0.29 0.50 1.05 110 200 PP Homo. 56.2 900 0.11 0.19 0.83 85 150 Low-density PS foam 4.0 64 0.016 0.027 0.5 110 200

Rigid PVC 84.2 1350 0.100 0.171 0.365 45 80 PET 85.5 1370 0.138 0.236 0.44 40 70

from Understanding Thermoforming by Throne, pg. 13 Table 2.2


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Table 2: Technical Comparison of PET and PVC PET Comparison PVC

Faster Lower (450°F, 230°C)

Lower = = =

Stronger Wears out faster

More pressure needed = +

Recyclable

Cycle Times Oven Temps.

Energy Rigidity

Shrinkage Molds

Steel for Cutting Wear of Die

Pressure for Cutting Price

Environmental Regrind Reuse

Slower Higher (600°F, 315°C)

Higher = = =

Not needed to be as strong Better wear

Less = -

Difficult to Reuse

PET Sheet Properties While PET packaging is predominately familiar in its application in carbonated

beverage bottles, under proper conditions, PET slowly crystallizes to give a high-temperature, semi-crystalline plastic. With certain extrusion machines and equipment, extruded PET can be cooled quickly enough to prevent substantial crystallization, and the result is clear sheet used in thermoforming. The PET properties that make it desirable include:

• Clarity and Sparkle • Toughness • Light Weight • Good Gas Barrier • Solvent/Corrosion Resistance • Good Cost/Performance Ratio • Durable, difficult to break • Durable hinge properties • Recyclable and Regrindable

While these advantageous qualities do stand out, there are disadvantages and difficulties that must also be considered.

PET is very moisture-sensitive. In other polymers the moisture emerges as bubbles, but moisture in PET directly attacks its chemical backbone, breaking it down. This is called hydrolytic degradation (or intrinsic viscosity breakdown) and tends to result in excessive sag while heating and hard-to-detect loss in properties. PET flake or resin must be dried to a “moisture level of 0.005% or less” (TRS-106B) before extrusion of sheet or injection molding of bottles, otherwise there will be a reduction in physical properties including impact strength. Impact strength is the sheet’s ability to withstand puncture. While PET can be properly dried in several ways, it is worth repeating that PET flake must not be moist before processes including extrusion and injection molding. PET sheet does not need any special drying prior to thermoforming, but should not be exposed to rain or water. See Appendix VIII for Dry Conditions of other polymers.

Roll of PET Sheet Courtesy of the Lavergne Group


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PET is tougher than other plastic polymers. This toughness is one of the positive reasons for the growth of PET sheet applications. In particular PET sheet exhibits outstanding durable hinge properties making longer life packages like PET egg cartons and nut and bolt packages possible. Polystyrene (OPS or HIPS) packages are suitable for short life packages like bakery packages, but do not have the toughness or durable hinge properties for longer life packages. Even PVC sheet do not have the durable hinge properties of PET sheet. This toughness of PET sheet allows for very durable, longer-life packages, but also creates more difficulty for cutting and trimming.

The toughness of PET requires very sharp and often heated dies, yet improper trimming

can occur resulting in fuzz, angel hair, and trim dust. Trim dust sometimes dirties the remaining trim or scrap and consequently ruins PET sheet reclaim or regrind. In order for reclaim and regrind to function appropriately, the regrind must be kept clean, free from contamination, and dry.

This paper focuses on amorphous PET sheet or simply APET, but it is worth noting this

distinction between CPET (crystallized PET) and APET. CPET is allowed during processing to form very quick crystals. These crystals allow it to withstand higher heats that normal APET cannot. While this makes thermoforming more difficult, CPET’s heat resistance enables usage in microwaves and ovens.

A special characteristic of APET is ease in recycling. Recycled PET or RPET, which

comes from reground trim or scrap of APET as well as recycling of PET beverage bottles, enables prolonged use without sapping resources or increasing waste. Recycled material is often combined with virgin PET when it is re-extruded creating a stable, usable blend. RPET’s only real restriction is usage in food packaging, but it is still highly usable in other industrial packaging applications displaying very similar properties as virgin APET sheet. Extruding PET Sheet

In extrusion as opposed to thermoforming, raw PET must be heated past its glass transition temperature of 70 °C (158°F) to above its melting temperature of 255°C (490°F). At this temperature this PET is in a liquid state where extrusion continues. (For a basic comparison of transition temperatures of thermoformable polymers, see Appendix II.)

The extrusion basic process is as follows: 1. PET pellets or flake are dried in a desiccant dryer, fed

into a hopper, and placed on top of the barrel. 2. The barrel of the extruder contains a rotating screw,

which conveys, melts, and pumps the melted resin into a flat sheet.

3. Calender rolls adjust the sheet thickness. 4. The extruded PET sheet is wound into a clear roll or

stock and cut to the appropriate width. This finish sheet can be used in various thermoform operations to create the desired product.

PET Sheet Extruder Courtesy of the Lavergne Group

PET Sheet Extruder Courtesy of the Lavergne Group


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~ Thermoforming PET sheet ~

After examining and comparing PET’s various properties and characteristics against other plastics, it is apparent that, for all practical purposes, PET sheet’s viscoelastic properties are similar to other amorphous polymers such as PVC (polyvinyl chloride) and HIPS (high impact polystyrene). And consequently, most thermoforming equipment can be adapted to properly handle PET sheet. There are only slight changes needed in the thermoforming conditions of PET as opposed to PVC or HIPS.

Basically, thermoforming uses heat, vacuum, pressure, and/or mechanical means to

force the plastic sheet against contours of a mold (or molds). The PET sheet is heated to a temperature above the glass transition but far below the melting temperature where it softens. It is stretched over the shape of its mold. Once cooled and removed from the mold, secondary actions occur on the plastic like trimming, labeling, printing, and cutting. Heating PET Sheet

When thermoforming PET sheet and most thermoformed polymers, one of the most important considerations is heating. Heating contributes a significant percentage to the final cost of a formed product. Under-heating will result in failing to forming to the contours of the mold. Overheating leads to numerous problems including poor quality and weak end products. Overheating will crystallize the PET sheet and result in excessive sag and visible haze. This increases brittleness and hinders thermoformability. With thicker sheet and its longer heating cycles, crystallinity and haze become greater concerns. Once crystalline haze appears, it can only be eliminated by re-extrusion of that material. Simply, it is crucial to maintain the PET’s proper forming temperatures (300°F or 149°C).

Table 4: Normal Forming Temperatures

Temperature Polymer °F °C Polystyrene [GPPS] 300 149 ABS 330 166 Rigid PVC 280 138 PMMA [Acrylic] 350 177 Polycarbonate [PC] 375 191 HDPE 295 146 Polypropylene 310 154 40% GR PP 400 204 APET 300 149 from Understanding Thermoforming by Throne, pg. 73 Table 5.8

There are three basic methods of heating sheet: conduction, conventions, and radiation. Conduction is heat transfer via direct contact between the sheet and the heated area. No matter what method of actual heating is used, conduction is the primary way energy moves through the plastic sheet. The speed and heat needed to transfer heat from the surface to the entire sheet is a controlling factor especially in thicker sheet. Convection is heat transfer by contact between a fluid medium and a solid. For example, the cooler sheet will meet warmer air, and an energy and heat exchange will warm the sheet. Faster and more efficient transfer also occurs in moving air compared to still or stagnant air. Radiation is heat transfer via an interchange of electromagnetic energy between cold and hot surfaces.


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Conduction is much more energy effective than convection heating, but PET’s tendency to adhere to hot metal requires Teflon-coated heating plates. Often, hybrid methods with combination of each are employed. No matter which heating method used, it is critical to maintain a uniform temperature across the sheet. Air currents and sudden shifts in surrounding temperatures should be kept to a minimum. A temperature sensing device will also vastly improve results when thermoforming.

Along with temperature, time should also be considered when heating. “To prevent

excessive sag and possible crystallization of the sheet, the heating cycle should be as short as possible, provided the proper sheet temperature is reached.” (TRS-111). Time to heat the sheet will control the machine-cycle and also dictate overall time needed to thermoform. One benefit of PET over PVC is its faster cycle time, but it should still be noted the importance of consistency with thermoforming. Each and every cycle should have the same time and temperatures. Forming PET Sheet

One of the main advantages of thermoforming PET sheet is its versatility along with its toughness, durable hinge properties, and reasonable cost. There are numerous options for thermoforming; one could use plug assist or drape forming; one could use vacuum or pressure forming; one could use matched mold; the options and adaptations are numerous. This paper will not go into specific procedures and techniques for thermoforming, but provide only some general considerations when working with PET sheet.

If one wants information on specifics of thermoforming procedures, see Eastman’s

TRS-194A (available by request from Eastman) or “Thermoforming in a Nutshell” by Empire West Inc.’s Thermoforming Tech Academy (available at www.empirewest.com/academy/)

The following chart will provide a good starting point for forming sheet, particularly if the sheet thickness is 1,250 microns (50 mils) or less:

Eastar PETG 6763 APET Mold Temperature, °C (°F) 40-60 (100-140) 25-50 (80-120) Sheet Temperature, °C (°F) 140-150 (280-300) 140-165 (280-330) Plug Temperature, °C (°F) 120-135 (250-275) 125-155 (260-310) Cycle, seconds 3-10 2.5-6 Forming Pressure, MPa (psi) 0.21-0.28 (30-40) 0.10-0.28 (15-40) Vacuum, mm (in.) of Mercury 508-660 (20-26) 508-660 (20-26)

The following criteria should also be noted (from Eastman’s TRS-111, pg. 15):

1. Mold temperatures below 27οC (80οF) may cause “freezing” of the sheet, non-uniform drawing, and stressed parts; however, mold temperature is the key to faster cycles. Above 60οC (140οF), a longer production cycle could be required, and distortion of the part may occur.

2. At temperatures slightly above the Tg of 80οC (176οF), PET can be oriented, but it takes forming forces much greater than those available in vacuum forming to do so. As the APET sheet temperature approaches 149οC (300οF), its viscosity is reduced to the point where it is very formable by pressure and vacuum forces. It should be emphasized again that stressed and brittle parts can result from sheet that is too cold.


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General Do’s and Don’ts of Thermoforming PET Sheet (from DDS-3C) To achieve controlled, consistent, and reliable results when thermoforming PET sheet, a few basic guidelines should be considered: Do

1. Use moderate heat settings on thermoforming equipment to give a sheet temperature between 140οC to 165οC (280οF to 325οF).

2. Use mold temperatures that range from 40οC to 60οC (100οF to 140οF). 3. Monitor temperatures 4. Use shorter forming cycles and lower temperatures than those used in thermoforming

other sheet such as PVC. 5. Use silicone-coated sheet for optimum denesting of blisters.

Don’t

1. Overheat sheet. Crystallization will occur if sheet is overheated, resulting in whitening and embrittlement of the sheet. Excessive sag with resultant webbing can also occur.

2. Use cold molds. Mold temperatures as low as 20οC to 25οC (70οF to 80οF) can cause “freezing” of the film and non-uniform drawing, especially with male molds.

3. Use sheet temperatures below 140οC (280οF). Due to freezing internal stresses in the part, cold forming can cause embrittlement.


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~ Die Cutting ~

As a recent study by Moskla and Barr so appropriate stated, “thermoplastic sheet such as oriented polystyrene (OPS) and polyvinyl chloride (PVC) are generally easy to cut, producing minimal wear on the die and few defects on the cut edge of the plastic,” but “amorphous polyethylene terephthalate (APET)…is notoriously difficult to cut.” Although PVC or HIPS break or facture after being cut only “approximately 75 percent through the thickness” (TRS-1111B 16), PET sheet must be cut completely through in order to separate cleanly. This puts a tremendous strain on cutting materials, time, and laborers.

While there are several ways to cut thermoformed PET sheet, this paper will cover only

the three major ones: steel rule, matched-metal, and forged dies. The steel rule die and forged die have the same principle of cutting which is a “clamp cut” or “kiss cut”, where as the matched-metal die cutting principle is more of a “scissor” type cutting action.

I. “Kiss Cut” Dies (Steel Rule and Forged Dies)

Most of the concepts and principles in steel rule dies apply to forged dies.

Steel rule dies offer the least expensive option for limited

volume cutting. This method of cutting is simply a sharpened, metal cutting edge or knife mounted on a laminated birch or maple die board. Steel rule dies can function as cut-in-place, and cut-in-line trimming, but often is best used cut-in-place because that way the PET sheet is still warm and easing wear on the cutting surface. (For Stanley Rosen’s make-ready procedure for accurately and effectively setting up steel rule dies, see Appendix X.) For cutting simple designs with a steel rule die, use a long central bevel or a double bevel die with a hardness of 50-55 Rockwell C, especially if the sheet is thicker than 0.25 mm (0.010 in.). Although harder dies wear better, softer dies of 45-50 Rockwell C may have to be used to prevent breakage during die fabrication when complex shapes and sharp bends are used. (For information on meanings of Rockwell Hardness scale, see Appendix XIII.)

Forged die can be used instead of a steel rule die in a cut-in-place trimming procedure, or also known as a “contact heat pressure forming and cutting” procedure. The forged die method is excellent for middle of the road volume applications, and has an advantage in cutting PET sheet, because the forged die is cutting hot PET sheet. In a forged die, the PET sheet is heated and thermoformed by vacuum or pressure forming to the contours of the mold. While the thermoformed part is still in the mold, the heated forged die is pressed down further

Cutting performance of PET (circles), PETG (squares) an OPS (triangles). Solid symbols are for the sheet. Open symbols for the baseline. (from Moskla-Barr Study)

Steel Rule Die


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through the PET sheet. The die edge makes “kiss cut” contact on the base plate cutting the hot PET sheet.

An important factor to consider is the

material’s shrinkage from forming to the time it gets to cutting. Although it would be best to measure shrinkage from an actual cavity, “0.005 mm per mm (.005 in per in.)” (TRS-111) or 0.3-0.6 percent is a good rule of thumb to use with PET sheet. (For comparison of shrinkage values of other thermoformable polymers, see Appendix VI.)

Dies should be installed in a press that

has enough force so that it can close smoothly as and cut completely through the sheet. As the cut progresses, the press should be capable of consistently bringing the die to the same line on the backing plate. Eastman’s rule of thumb for clamp requirements is 70 kg. per lineal cm (400 lbs. per lineal in.) of rule die. Pressing too hard will bend the cutting edge removing its edge and only pressing with its bent side. Pressing too softly will fail to cut the part at all. The preferred material for the backing plate is stainless steel. Aluminum is not recommended for backing plates because it tends to have a short life and often splinters contaminating parts. No matter what material is used, the backing plate should have a hardness less than that of the die. That way the backing plate will take the wear and the die will dull less. (There is disagreement on this point.)

Cutting hot PET sheet is easier than cutting cold PET sheet. Heating the PET sheet by applying heat to the die and/or the backing plate will improve cutting, but temperatures should not exceed the Tg of the PET sheet or 71°C (160°F). One method of heating the PET sheet only at the cutting area can be termed “Heat Assisted Die Cutting.” The first step of this die cutting method involves the die touching the PET sheet with low pressure, so that the sheet contacts the base plate and heat transfers from the base plate to the PET sheet cutting area via conduction.

Kiss Cut Die Diagram


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The base plate temperature should be controlled just below the point where the PET sheet sticks to the base plate (PET sheet at 71οC (160οF)). The base plate should be insulated in order to maintain temperature and to prevent heat loss. Only the spot where the die touches the sheet is heated. The rest of the PET part is not distorted. After a short time delay and heat transfer, the PET sheet in contact with the die becomes softer. The hot PET sheet has reduced shear strength, which requires less cutting force. The final step is high pressure cutting, which finishes with “kiss” cut contact.

Since the dies must evenly contact the backing plate during cutting to insure the PET is cutting, one must accurately “make-ready” the die to the backing plate to avoid damaging the die as well as properly cutting the PET. This “make-ready” procedure is critical to correctly cutting PET sheet and not damaging the dies. See Appendix X. for Stanley Rosen’s Make-Ready Procedure for “Kiss Cut” Dies.

It may seem obvious, but it is worth reiterating: Cut in the same place every time and

keep dies as sharp as possible to insure the best and most reliable cutting. II. Scissor-type Dies (Matched Metal Dies)

Matched Metal Die diagrams from Thermoforming: Improving Process Performance by Stanley R. Rosen, copyright 2002


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II. Scissor-type Dies (Matched Metal Dies)

A Matched Metal Die, also called a punch and die, is recommended for large volume applications. This in-line trimming method is mounted in a separate cutting press through which the continuously formed sheet passes. The basic concept and functionality of the matched metal die works with hardness or more specifically the contrasting hardness. A harder punch is used with a softer die (typical hardness is 43 Rockwell C for the die and 55 for the punch). Minimum die clearance must be maintained, and as it wears, the die can be peened (A process used on a die to recover minimum clearance by spreading the edge of the die back to its original size. Peening is done with an air-operated hammer. Any excessive spreading is sheared off by the punch on the first cycle.) to recover the proper clearance. An alternative to contrasting hardness is equal hardness for both the die and punch (at roughly 62 Rockwell C). Minimum clearance is maintained by resurfacing the die and punch, which takes more time due to resurfacing of both. There is less downtime with only one tool, the die, to fix. Coining may be used in order to extend die life. Coining is a process that occurs during thermoforming whereby the areas to be cut are thinned by as much as 50% by ridges located on the pressure box. Coining occurs as the pressure box is clamped to the mold. Coining seals the cavity, locks the sheet in place, reduces cutting tonnage, and extends cutter life. Shrinkage of the formed part must be considered when locating the area to be coined so that it will properly match the cutting die. With matched metal dies, the key concept to remember is maintaining minimum clearance; otherwise, a clean and efficient cut will not occur.

Matched Metal Die Diagram

#1

#2

#3


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General Do’s and Don’ts of Die Cutting (from DDS-3C) To achieve controlled, consistent, and reliable results when die cutting PET sheet, a few basic guidelines should be considered: Do

1. Use properly guarded roller-die, matched-die, slitters, or guillotine cutters for maximum tool life.

2. Use sharp and well-maintained cutters. 3. “Make Ready” Procedure is critical 4. Make sure that proper clearance is maintained between punch and die when matched-

die cutters are used. 5. Cut completely through the sheet to cleanly separate the parts. 6. Use a stainless surface with a hardness less than that of the die for the backing plate

when using steel-rule dies. (There is disagreement on this point.) Don’t

1. Trim formed blisters if the temperature of the sheet is above 71οC (160οF). 2. Attempt to trim formed blisters unless the equipment and die perimeter are such that

the available cutting force is at least 400 pounds per linear inch.

~ Conclusions ~

While this paper presents numerous aspects, considerations, and suggestions for thermoforming and die cutting PET sheet, the true functionality of any idea or thought is results. This paper is intended as merely a stepping-stone into greater consideration of PET as an option when thermoforming. PET has definite advantages including toughness, hinge-ability, ease of recycling, and environmentally. PET also has challenges including difficulty cutting and increased wear of dies resulting from its generally advantageous toughness. Its clarity and sparkle make it great for applications in packaging from clamshells to nut and bolt containers. The possibility for applications is vast. See Appendix XV “Application Pictures” to explore the current offerings in PET.

No guide, paper, book, or suggestion provides the exact solution to any problem. As any engineer knows, a perfect plan may not create a functional solution. Trial-and-error is a must when thermoforming and die cutting any polymer especially PET and recycled PET, but if the few suggestion is this paper are followed then experimentation will quickly provide the desired results.


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The following should provide a general overview when thermoforming PET sheet:

Summary Checklist for Thermoforming and Die Cutting PET Sheet

• Keep PET sheet dry. • Don’t assume that APET sheet will process that same as PETG sheet. • Use suggested sheet-forming temperatures of 140οC to 165οC (280οF to 325οF). • Do not overheat the PET sheet, which will cause crystallinity and brittleness in the PET

sheet. (If the PET sheet turns white and crystallizes, then it is overheated.) • Maintain uniform and consistent sheet temperature • Choice of PET sheet dies:

o Steel rule dies are the least expensive, and are for small volume applications o Forged dies are best for middle of the road volume applications o “Kiss” cut contact for steel rule and forged dies is required for PET sheet o Match-mold dies are the most expensive, are the surest with regard to cutting,

and are preferred for fast, high volume applications

• The recommended cutting force for PET sheet is at least 400 pounds per linear inch of “kiss” cut die. The force required on the corners of the part are probably 2X the linear sections. These cutting force requirements can be reduced if the PET sheet is hot.

• Hot PET sheet is easier for cutting than cold PET sheet.

o Control the base plate temperature just below the point where the PET sheet sticks to the base plate (PET sheet at 71οC (160οF)). Insulate the base plate in order to maintain temperature and to prevent heat loss.

o Steel rule dies are the most difficult to maintain heat, and forged dies are the easiest to maintain heat. (Some people feel that die temperature are not that important.)

• Heat Assisted Die Cutting: Low Pressure / Time Delay / Heat Transfer / High Pressure

o The first step of die cutting involves the die touching the PET sheet with low pressure, so that the sheet contacts the base plate and heat transfers from the base plate to the PET sheet via conduction.

o The base plate temperature is just below the PET sticking point, and only the spot where the die touches the sheet is heated. The rest of the PET part is not distorted.

o After a short time delay and heat transfer, the PET sheet in contact with the die becomes softer. The hot PET sheet has reduced shear strength, which requires less cutting force.

o The final step is high pressure cutting, which finishes with “kiss” cut contact.


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Summary Checklist for Thermoforming and Die Cutting PET Sheet (cont.)

• Precision of the cutting tools and dies is critical for proper cutting of PET sheet o HIPS, OPS, and PVC sheet will fracture or break when the die is 50-70%

through the sheet; (Some of the PVC sheet is hard and the sheet will fracture. Some of the PVC sheet is soft and the sheet will distort, which will cause the thermoformed part to break easily from the sheet.)

o The “kiss” cut die must be 90-100% of the way through the PET sheet before the thermoformed part is separated from the PET sheet;

o With matched metal dies, the key concept to remember is maintaining tolerance; otherwise, a clean and efficient cut will not occur.

• No matter what material is used, the backing plate should have hardness less than that

of the die. That way the backing plate will take the wear and the die will dull less. (There is disagreement on this point.)

• “Make Ready” Procedure is critical for proper “kiss” cutting of PET sheet

o A flat base plate, which is parallel to the die is critical. If the base plate is worn out and cannot be repaired, then the base plate must be replaced.

o Doing the proper “make ready” procedure takes time and patience. (This job should be assigned to the person with the most patience in the work force.)

o If the die cutting edge contacts the base metal too much, then the die edge will mushroom. The die edge will not be able to cut the PET sheet cleanly, and will create angel hair and fines.

o See Appendix X. for Stanley Rosen’s Make-Ready Procedure for “Kiss Cut” Dies.

www.lavergne.ca

This paper is part of a research and marketing project by the Lavergne Group. For more information about PET Sheet, its PET sheet extruding line, thermoforming and die cutting PET sheet or about the company and its writers, please contact us: Lavergne Group PETCO Division 8800, 1er Croissant Ville d’Anjou (Quebec) Canada H1J 1C8 To contact any of the presenters or writers of this paper: Larry Koester, VP Marketing & Sales, tel. 402-861-9524, fax 402-861-9527, [email protected] Sheila Nemeth, Marketing & Sales, tel. 514-354-5757 ext. 116, fax 514-354-3087, [email protected]


- 17 -

Appendix I. Glossary of Terms Acrylic Polymethyl methacrylate. It is a tough polymer with good optical clarity, weatherability, and

resistance to sunlight, which make it great for outdoor items like sky domes, signs, light fixtures, and bathtubs. Also known as PMMA.

Amorphous Refers to the physical arrangement of PET molecules. In amorphous PET, the molecules have had no interaction with each other, and remain independent and randomly oriented. The term “amorphous” implies that no crystalline structures have developed and that end uses will be limited to temperatures reasonably below the Tg. All polymers are essentially amorphous when molten. Clarity is a characteristic of amorphous polymers.

Angel Hair Fine fibers caused by improper trimming technique

APET Acronym that refers to an end use of PET that utilizes its amorphous state. Thermoforming is a prime example. Also known as PET, PETE, or Polyester.

Backing Plate The mild or stainless plate against which steel rule dies cut out the APET sheet.

Bearers Spacers placed outside the sheet area to ensure that press platens do not over-close on steel rule dies during cutting. Two to four long bears are used, usually 12 mm (½ in.) wide by the height of the die.

CPET Acronym that refers to an end use of PET that utilizes its unoriented crystalline state. Dual ovenable trays are an example of this end use.

Coining A process that occurs during thermoforming whereby the areas to be cut are thinned by as much as 50% by ridges located on the pressure box. Coining occurs as the pressure box is clamped to the mold. Coining seals the cavity, locks the sheet in place, reduces cutting tonnage, and extends cutter life. Shrinkage of the formed part must be considered when locating the area to be coined so that it will properly match the cutting die.

Conduction Heat transfer via direct contact between the sheet and the heated area. It is also the primary way energy moves through the plastic sheet

Convention Heat transfer by contact between a fluid medium and a solid. For example, the cooler sheet will meet warmer air, and an energy and heat exchange will warm the sheet.

Co-polyester Polyester modified with additional component(s) to achieve specific properties.

Copolymer A polymer modified with additional component(s) to achieve specific properties.

Crystalline Refers to the physical arrangement of molecules in a crystallizable polymer. Molecules align themselves into dense, highly ordered crystals when subjected to either a thermal treatment above their Tg or to an orientation process. The percent crystallinity can range from 0% up to approximately 50%, depending on the thermal and mechanical history of that sample. A polymer that is 50% crystalline, for example, will be made up of 50% crystals by weight, all uniformity dispersed throughout the remaining 50%, which is still amorphous.

Crystallizable Refers to any polymer capable of being crystallized with a thermal or mechanical treatment.

Crystallization Half-Time

The time required for a sample of PET to crystallize to 50% of the maximum crystallinity that could occur at a given temperature.

Denest Lugs Shapes thermoformed into a part that controls the depth to which parts will nest

Drape Forming Thermoforming in which a male mold is pushed into a hot plastic sheet or the plastic sheet is pulled over the male mold. It is similar to straight forming except that after the PET sheet is framed and heated, it is mechanically stretched, and a pressure differential is then applied to form the sheet over the male mold.

EPET Acronym that refers to the use of PET in the extrusion blow-molding process.


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Glass Transition Temperature (Tg)

The temperature at which APET sheet transitions from a glassy state to a rubbery state. As the temperature of APET sheet rises from room temperature, an abrupt softening occurs as the temperature passes through the Tg. During cooling, the formed APET part will return to its glassy state as it cools below the Tg.

HDPE High density polyethylene. It is used in milk, juice, and water containers. Its chemical resistance properties also suit it for use in containers for household chemicals and detergents.

Heat Setting A process wherein oriented PET is subjected to an additional heat treatment to increase percent crystallinity.

Heavy-gauge Commonly, a sheet with thickness greater than 120 mils (03120 in. or 3 mm)

HIPS High impact polystyrene plastic.

I.V. Intrinsic viscosity is a number obtained from a solution viscosity test that represents the average molecular weight of PET. The value is calculated by extrapolating the concentration of PET in solution back to zero. The industry commonly uses I.V. as a specification.

In-line Trimming In thin gauge, roll-fed forming, trimming that takes place in a separate machine after the thermoforming machine.

In-machine Trimming

Trimming that takes place while the formed sheet is still within the thermoforming machine

In-place Trimming Trimming that takes place while the formed sheet is still on the mold surface.

LDPE Low density polyethylene. It offers clarity and flexibility, and is used to make bottles that require flexibility. To take advantage of its strength and toughness in film form, it is used to produce grocery bags and garbage bags, shrink and stretch film, and coating for milk containers.

Make-Ready The process of setting up a steel rule die cutting operation. The prime objective of this process is to obtain clean cutting without incurring damage to the rule dies.

Matched-Mold Forming

Thermoforming technique in which the heated sheet is trapped between the male and female dies, and the male form rams the sheet so that it forms appropriately.

OPET Acronym that refers to an end use of PET that utilizes its oriented state. PET fibers and biaxially oriented film are examples.

Orientation The process of imparting a degree of molecular alignment by stretching at a temperature above its Tg. If stretching is in the machine direction only, it is considered uniaxial, whereas biaxial orientation implies stretching in the machine and transverse directions. Orientation of PET creates internal stress and rapid crystallization that work together to enhance strength properties and chemical resistance. Crystals generated by orientation (with or without heat setting) are too small to refract light and will not influence optical properties.

PC Polycarbonate. A tough, high temperature transparent plastic, which is difficult to thermoform and is very susceptible to moisture, used in windows, helmets, cases, glasses, and compact discs.

PE Polyethylene. The most used polymer in thermoforming with an array of applications. It is a durable, tough, inexpensive plastic with excellent impact, moisture and chemical resistance. See also HDPE and LDPE.

Peening A process used on a die to recover zero clearance by spreading the edge of the die back to its original size. Peening is done with an air-operated hammer. Any excessive spreading is sheared off by the punch on the first cycle.

PET Polyethylene terephthalate. A polyester homopolymer made by reacting either terephthalic acid or dimethyl terephthalate with ethylene glycol. It is a clear, tough, stable polymer with exceptional gas and moisture barrier properties, and often used to contain carbon dioxide (alias carbonation) in soft drinks bottles. Its applications also include film, fiber, trays, displays, clothing, and wire insulation. Also known as APET, PETE, or Polyester.


- 19 -

Polyester A polymer made by reacting dibasic acid with glycol. These monomers are polymerized to a molecular weight suitable for a specific end use. See PET.

Polymer A generic term used to describe any plastic, usually a homopolymer.

PP Polypropylene. It has great high-temperature chemical resistance and is used in manufacturing industrial parts, automotive and electrical hardware, stadium seats, and battery cases.

Pressure Box A chamber that is clamped to the mold(s) during thermoforming to supply air pressure to the exposed surface of the forming sheet. This air pressure, added to that from the applied vacuum, provides faster forming with improved cycles and better definition. Coining ridges can be added to the pressure box.

PVC Polyvinyl chloride plastic. It has very similar properties as PET displaying excellent clarity, puncture resistance, and cling. As a film, vinyl can breathe just the right amount, making it ideal for packaging fresh meats. Also categorized by Vinyl.

PS Polystyrene. It was the dominating thermoforming material 20 years ago. It has excellent processability and good dimensional stability but limited solvent resistance. Its uses today include food and medical packaging, housewares, toys, furniture, advertising displays, and refrigerator liners.

Radiation Heat transfer via an interchange of electromagnetic energy between cold and hot surfaces

Re-crystallization A process whereby the initial crystallization in a polymer is destroyed through melting and then allowed to reform when the product is held at a temperature above its Tg.

Rockwell C Category on Rockwell Hardness Scale includes steel, hard cast irons, pearlitic malleable iron, titanium, deep case hardened steel, and other materials. It is harder than B. See also “Rockwell Hardness Test” and Appendix XIII.

Rockwell Hardness Test

A hardness measurement based on the net increase in depth of impression as a load is applied. Hardness numbers have no units and are commonly given in the R, L, M, E and K scales. The higher the number in each of the scales means the harder the material. Hardness has been variously defined as resistance to local penetration, scratching, machining, wear or abrasion, and yielding.

RPET Recycled PET.

Shear Point Cutting Refers to the crowning of a matched metal die so that the punch first contacts the centerline and proceeds to cut with a shearing action as it enters the die.

Shrinkage Refers to the unit difference in dimension of a thermoformed APET part with respect to the mold dimension. An APET part will always be smaller than the mold. A typical shrinkage value for APET is 0.005 mm per mm (.005 in. per in.) of mold dimension.

Snap-Back Refers to the tendency of an APET sheet to try to shrink and remain horizontal, rather than sag, when being heated during thermoforming. The degree of snap-back depends on the amount of internal stress imparted to the sheet by nip polishing during extrusion.

Spherulites PET crystals whose major dimension is greater than one half the wavelength of visible light. They form when an un-oriented, crystallizable polymer is held at a temperature above its Tg. Spherulites will refract light to create haze from slight to complete opacity depending on temperature and time of exposure.

Stability PET is very stable with respect to heat and oxygen at processing temperatures. However, it is hydrolytically unstable and must be thoroughly dried before melting processing. Thermoforming temperatures are not high enough to effect hydrolytic stability.

Stripping Rubber Foamed, compressible rubber placed adjacent to a steel rule cutting die that expands after cutting to force the cut APET part off the die.

Tg See Glass Transition Temperature


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Thermal Diffusivity Ratio of thermal conductivity to the product of density and specific heat. It is important in time-dependent heat conduction

Thermoplastic Any plastic material that can be repeatedly subjected to a thermal cycling history without incurring an increase in molecular weight. This is in contrast to a thermosetting material, which polymerizes with heat and/or catalyst to achieve its final and permanent state.

Thin-gauge Commonly, sheet thickness less than 60 mils (0.060 in. or 1.5 mm)

Trim That portion of the formed sheet that is not part of the final product

Vinyl See PVC.

Appendix II. Transition Temperatures of Thermoformable Polymers

Glass Transition Temperature

Melting Temperature

Heat Distortion Temperature 66 lb/in2 or 0.46 N/mm2

Polymer

°F °C °F °C °F °C Polystyrene 200 94 - - 155-204 68-96 PMMA 212 100 - - 165-235 74-113 PMMA/PVC 221 105 - - 177 81 ABS 190 248 88-120 - - 170-235 77-113 Polycarbonate 300 150 - - 280 138 Rigid PVC 170 77 - - 135-180 57-82 LDPE -13 -25 239 115 104-112 40-44 HDPE -166 -100 273 134 175-196 79-91 Cellulose acetate 158,212 70,100 445 230 125-200 52-93 Homopolymer Polypropylene 41 5 334 168 225-250 107-121 Copolymer Polypropylene -4 -20 302-347 150-175 185-220 85-104 PETG 180 82 - - 158 70 PET 158 70 490 255 120 49

from Understanding Thermoforming by Throne, pg. 11 Table 2.1 Appendix IIIA. 2000 Gross Recycling Rate (NAPCOR 2000 Final Report)

Total U.S. Bottles Collected and Sold for Recycling ÷

Total U.S. Bottles Available for Recycling

769 million lbs

3,445 million lbs

=

22.3%

Year Total U.S. Bottles Collected (MM lbs.) Bottles on U.S. Shelves Gross Recycling Rate

1995 775 1,950 39.7% 1996 697 2,198 31.7% 1997 691 2,551 27.1% 1998 745 3,006 24.8% 1999 771 3,250 23.7% 2000 769 3,445 22.3%

http://www.napcor.com/rate00.html


- 21 -

Appendix IIIB. RPET End Use Products 2000 (NAPCOR 2000 Final Report)

http://www.napcor.com/rate00.html

Appendix IV. Plastic Bottles by Resin Type

from http://www.plasticsresource.com/resource_conservation/plastics_in_perspective/plastics.html


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Appendix V. Comparison of Thermal Conductivity and Thermal Diffusivity for Several Polymer and Mold Materials

Material Thermal Conductivity {Btu/ft.hr.°F]

Thermal Conductivity

[x10-3 kW/m°C]

Thermal Diffusivity

[x10-4 ft2/hr]

Thermal Diffusivity

[x10-4 cm2/s]

Thermal Conductivity Relative to

PS Polystyrene 0.105 0.180 29.7 7.66 1 ABS 0.07 0.12 25 6.45 0.67 Polycarbonate 0.121 0.207 33.0 8.51 1.15 Rigid PVC 0.100 0.171 32.5 8.39 0.95 LDPE 0.23 0.39 46 11.9 2.2 HDPE 0.29 0.50 55 14.2 2.75 PP homopolymer 0.11 0.19 25 6.45 0.67 PET 0.138 0.236 36.8 9.49 1.3 Low density PS foam 0.016 0.027 80 20.6 0.15 Aluminum 72.5 124 18,850 4860 690 Steel 21.3 36.4 3,930 1010 200 Maple 0.073 0.125 104 26.8 0.7 Plaster 0.174 0.298 120 31.0 1.66 Syntactic foam 0.07 0.12 40 10.3 0.67

from Understanding Thermoforming by Throne, pg. 60 Table 5.1 Appendix VI. Shrinkage Values

Polymer Shrinkage Range (%)

Recommended Value (%)

ABS 0.5-0.9 0.7 EVA 0.3-0.8 0.6 FEP fluoropolymer 1.5-4.5 3.0 Polycarbonate 0.5-0.7 0.6 LDPE 1.5-4.5 3.0 HDPE 2.0-4.5 2.5 PMMA 0.2-0.8 0.6 PP 1.0-2.5 2.0 PS 0.5-0.8 0.6 Rigid PVC 0.1-0.5 0.3 K-Resin 0.4-0.8 0.6 APET 0.3-0.6 0.5 CPET 10-18 12 from Understanding Thermoforming by Throne, pg. 116 Table 9.1

Appendix VII. Inflation Pressure Ranges Polymer Inflation Pressure

Range [lb/in2] Inflation Pressure Range [kPa]

Inflation Temperature Range [°F]

Inflation Temperature Range [°C]

PS 2-4 14-28 275-300 135-150 ABS 1.5-4 10-28 285-300 140-150 PMMA 7-10 48-70 320-355 160-180 Rigid PVC 1.5-3 10-21 240-285 110-140 Flexible PVC 1-3 7-21 240-285 110-140 PC 6-10 41-70 350-375 170-190 PET 2-4 14-28 275-320 135-160 LDPE 1-3 7-21 255-290 125-145 HDPE 1-3 7-21 265-300 130-150 PP 1-2 7-14 300-330 150-165



- 23 -

Appendix VIII. Drying Conditions

Polymer Typical Drying Temperature Typical Drying Time [hr] [°F] [°C] APET 150 65 3-4 CPET 320 160 4 ABS 175 80 2 PBT 320 160 4 PMMA 175 80 3 PC 300 150 4

from Understanding Thermoforming by Throne, pg. 125 Table 10.1 Appendix IX. Coefficients of Thermal Expansion for Thermoformable Polymers

Polymer Range (10-6/οF) Range (10-6/οC) ABS 60-130 35-70 EVA 80-200 45-110 FEP fluoropolymer 35-70 20-40 Polycarbonate 70 40 LDPE 100-220 55-120 HDPE 60-110 35-60 PMMA 50-90 30-50 PP 80-100 45-55 PS 50-80 30-45 Rigid PVC 70 40 K-Resin 65-70 35-40 APET 65 35



- 24 -

Appendix X. Stanley Rosen, Author: Make-Ready Procedure for “Kiss Cut” Dies Since the dies must contact the backing plate to get complete cut-through, great care must be taken during make-ready to ensure that the dies are not damaged. The following is an example of a make-ready procedure for a “kiss” cut die in an on-line trim press:

1. Reduce hydraulic press cutting pressure to the minimum level that will allow actuation of the lower platen.

2. Prepare a heavy Kraft paper sheet to the exact size of the platen and mark the outgoing

edge as “front”. Tape the paper facing the front to the face of the base plate, allow the press and die to close, and strike the paper, leaving a cut impression on what is now the “master sheet”. Usually 75% of the die will either cut or mark the paper. Remove the master sheet and base plate; use a pencil to complete the cut impression of cavities that are incomplete.

3. Obtain 0.002-in. thick x 0.250-in. wide (0.05-mm x 6.35-mm) stainless make-ready

shim tapes with adhesive backing from a die-maker supply house. Study the die impression on the master and apply one layer of shim tape only on the very light or penciled-in die impressions. Trim the shim tape so it never extends closer than 0.250 in. (6.35 mm) to a neighboring heavy die impression. The objective is to build up the shim pack so it never disturbs an existing cut section. Avoid installing loose shims. They may shift and disrupt the process.

4. Place the master sheet on the lower buildup in the same orientation marked “front” as

the die. Install the base plate on top of the master sheet and replace the mounting screws.

5. Tape a clean Kraft paper cut to the exact size of the striker plate on top. Mark it “No. 1

front” on the appropriate edge. Die cut the No. 1 sheet and compare the results to the master sheet under the base plate. If the die impression still is not uniform, add one thickness of shim to the master sheet on any faint cuts, including those on the top of earlier shims. When building upon an earlier shim, cut the length shorter by 0.250 in. (6.35 mm) from each end so the shimming is feathered and not abrupt at its edges.

6. Save sheet No. 1. Then cut sheet No. 2 and continue the process until the die

impression becomes uniform. Save all the trial-cutting Kraft sheets to keep a record of progress and as a guide to avoid disturbing sections that were previously cutting. If previously cutting segments stop cutting, remove the last shims placed on the adjoining segments and start the process anew.

7. When satisfied that make-ready is complete, insert a flat sheet of the same plastic to be

thermoformed and attempt to trim it at low pressure. If it appears to be a uniform impression, raise the hydraulic pressure until the die cuts through. Lock the hydraulic pressure regulator at that point and shim-up area of the impression that may not have cut through. Judgment and experience will indicate when the die impression is uniform and when additional hydraulic pressure is needed to cut through a plastic sheet without dulling the die.

Ref. Thermoforming: Improving Process Performance by Stanley R. Rosen, copyright 2002


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Appendix XI: Physical Properties of Film Extruded of PETG and APET (Eastman's Laboratories)

Property Units Test Method

(ASTM) PETG APET

Inherent Viscosity D 3763 0.70 0.74 Thickness of Film Tested Microns Mils

D374 250 10

250 10

Density, kg/m3 (g/cm3) D 1505 1,270 (1.27) (1.33) Haze, % D 1003 0.8 0.8 Gloss @ 45° D 2457 108 108 Transparency, % D 1746 85 85 Transmittance, % Regular (Specular) Total

D 1003 89 91

89 91

Water Vapor Transmission Rate g/m2 ּ 24h g/100 in.2 ּ 24h

F 372 6 0.4

6 0.4

Gas Permeability cm3ּmm/m2ּ24hּatm (cm3ּmil/100 in. 2ּ24hּatm) CO2 O2

D 1434 D 3985

49 (125) 10 (25)

28 (70) 5.1 (13)

Elmendorf Tear Strength, N (gf) M.D. T.D.

D 1922 13.7 (1,400) 16.7 (1,700)

9.8 (1,000) 12.7 (1,300)

PPT Tear Strength, N (lb-ft) M.D. T.D.

D 2582 93 (21) 93 (21)

102 (23) 120 (27)

Tear Propagation Resistance Split-Tear Method @ 254 mm/min (10 in./min) M.D., N (lb-ft) N/mm (lb-ft/in.) T.D., N (lb-ft/in.) N/mm (lb-ft/in.)

D 1938

9.1 (2.1) 36 (205) 9.1 (2.1) 36 (205)

15 (3.3) 58 (330) 16 (3.6) 63 (360)

Tear Resistance, Trouser @ 200 mm/min speed, N/mm (lb-ft/in.) M.D. T.D.

D 882

36 (205) 36 (205)

54 (310) 59 (340)

Tensile Strength @ Yield, MPa (psi) M.D. T.D.

D 882 52 (7,500) 52(7,500)

59 (8,500) 57 (8,300)

Tensile Strength@ Break. MPa (psi) M.D. T.D.

D 882 59 (8,600) 55 (8,000)

58 (8,400) 39 (5,600)

Elongation @ Yield, % M.D. T.D.

D 882 4 4

4 4

Elongation @ Break, % M.D. T.D.

D 882 400 400

300 200

Tensile Modulus of Elasticity Mpa (105 psi) M.D. T.D.

D 882

1,900 (2.8) 1,900 (2.8)

2,200 (3.2) 2,200 (3.2)

Dart Impact, 12.7-mm (½-in.) dia. head, 127-mm (5-in.) dia.clamp, 660-mm (26-in.) drop, g @ 23°C (73°F) @ -18°C (0°F)

D 1709A

400 500

400 500


- 26 -

Appendix XII. Technical Data and Property Comparison: RPET vs. PVC

Material RPET PVC Gauge, Mils 10 10 Density, g/cm3 1.33 1.35 Haze, % 0.5 1.2 Gloss at 45 deg. (Gardner) 110 93 Transparency, % 85 36 Tensile strength, psi 7,100 7,100 Tensile Modulus of Elasticity, psi 280,000 325,000 Oxygen Transmission Rate cc/sq.meter/24 hr/mil

109 174

HVTR.g/100 sq.in. /24 hr 0.40 0.19 Vicat Softening Point (°C) 80 82 Blushing No Yes Dart, Impact, ½ in. Dart, g@ 26 in. drop At 73 °F (23 °C) At -20 °F (-29 °C)

425 300

415 345

Heat Deflection (°F at 264 psi) 145 167 Sealing Temperature (°F) 275-400 315-360 Sheet Temperature (°F) 250-300 275-350

Courtesy of the Lavergne Group

Appendix XIII. Rockwell Hardness Scale of Abbreviations The ASTM (American Society for Testing & Materials) has standardized a set of scales (ranges) for Rockwell hardness testing. Each scale is designated by a letter.

• A Cemented carbides, thin steel and shallow case hardened steel • B Copper alloys, soft steels, aluminum alloys, malleable iron, etc. • C Steel, hard cast irons, pearlitic malleable iron, titanium, deep case hardened steel and other materials

harder than B 100 • D Thin steel and medium case hardened steel and pearlitic malleable iron • E Cast iron, aluminum and magnesium alloys, bearing metals • F Annealed copper alloys, thin soft sheet metals • G Phosphor bronze, beryllium copper, malleable irons • H Aluminum, zinc, lead • K, L, M, P, R, S, V Bearing metals and other very soft or thin materials, including plastics.


- 27 -

Appendix XIV. Application Pictures

PET Clamshells

PET Packaging

PET Hinge

PET Sheet

PET Snap


- 28 -

Sources

• Eastman Publications: DDS-3C, TRS-65L, TRS-106B, TRS- 111B, and TRS-194A. • Empire West Inc.’s “Thermoforming in a Nutshell” available at www.empirewest.com • Thermoforming: A Practical Guide by Adolf Illig, copyright 2001 • Understanding Thermoforming by James L. Throne, copyright 1999 • “Evaluating the Cutting Behavior of Amorphous PET Sheet Using Steel Rule Dies” by

Moskala-Barr from ANTEC 2000 • Thermoforming: Improving Process Performance by Stanley R. Rosen, copyright 2002

Special Thanks to:

• G.N. Plastics Ltd. • Ontario Die Company • American Tool & Engineering Inc. • C.R. Clarke & Co. • Future Mold Corp. • Sherwood Technologies, Inc., James L. Throne • Mold Systems Corporation, Stanley R. Rosen • Selected PETCO customers, who have reviewed this paper.

www.lavergne.ca

This paper is part of a research and marketing project by the Lavergne Group. For more information about PET Sheet, its PET sheet extruding line, thermoforming and die cutting PET sheet or about the company and its writers, please contact us: Lavergne Group PETCO Division 8800, 1er Croissant Ville d’Anjou (Quebec) Canada H1J 1C8 To contact any of the presenters or writers of this paper: Larry Koester, VP Marketing & Sales, tel. 402-861-9524, fax 402-861-9527, [email protected] Sheila Nemeth, Marketing & Sales, tel. 514-354-5757 ext. 116, fax 514-354-3087, [email protected]

3/11/2005

1

PET SheetThermoforming & Die Cutting

Presented by Larry Koester and Sheila Nemethfrom the PETCO Division of Lavergne Group

IntroductionWhat is PET Sheet?

PET Sheet PropertiesApplicationsPET vs. PVCExtruding PET Sheet

Thermoforming PET SheetHeating & Forming PET SheetGeneral Considerations

Die Cutting PET SheetKiss Dies

including Steel Rule Dies and Forged DiesScissor-type / Matched-Metal DiesGeneral Considerations

Thermoforming & Die Cutting PET Sheet

What is PET Sheet?

PET is a plastic resin chemically constructed by combining terephthalatic acid with ethylene glycol. It has a wide-range of applications.

Its other names include APET, RPET, PETE or polyester

Advantageous PropertiesClarity and SparkleToughnessDurable Hinges & Closures Light WeightGood Gas BarrierSolvent/Corrosion Resistance Good Cost/Performance RatioDurable, difficult to breakRecyclable and Regrindable


Challenges of PET Sheet

Moisture Sensitive

Toughness resulting in difficulty cutting and trimming

Patience & Precision for “Make-Ready”


Applications

PET Sheet


3/11/2005

2

Applications

Packaging


Applications

Clamshells


Applications

PET Hinge


Applications

Snap Closure


PET Sheet vs. PVCThermoforming & Die Cutting PET Sheet

SlowerHigher (600ºF, 315ºC)

Higher====

Less Difficult-

Difficult to Reuse

Cycle TimesOven Temps.

EnergyRigidity

ShrinkageToolingPrice

Die CuttingEnvironmentalRegrind Reuse

FasterLower (450ºF, 230ºC)

Lower====

More Difficult+

Recyclable

PVCComparisonPET

Extruding PET SheetIn extrusion as opposed to

thermoforming, raw PET must be heated past its glass transition temperature of 70°C (158°F) to above its melting temperature of 255°C (490°F). At this temperature the PET changes to a liquid state where extrusion continues.

Thermoforming & Die Cutting PET Sheet (a Lavergne Group presentation)

3/11/2005

3

Thermoforming & Die Cutting PET Sheet (a Lavergne Group presentation)

Extruding PET SheetPET pellets or flake are dried in a desiccant dryer, fed into a hopper, and placed on top of the barrel. The barrel of the extruder contains a rotating screw, which conveys, melts, and pumps the melted resin into a flat sheet. Calendar rolls adjust the sheet thickness. The extruded PET sheet is wound into a clear roll or stock and cut to the appropriate width.


Methodology of This PaperPETCO/Lavergne Group is PET Bottle Recycler & PET Sheet Producer

Review of Existing Literature & Publications on TF & Die Cutting of PET Sheet

Focused on Die Cutting (most challenging aspect)

Reviewed by Equipment Mfg., PETCO Customers, & TF Consultants (not necessarily endorsed)

Thanks to…


G.N. Plastics Ltd.Ontario Die CompanyAmerican Tool & Engineering Inc.C.R. Clarke & Co.Future Mold Corp.Eastman ChemicalsJames L. ThroneStanley R. RosenSelected PETCO Customers

…for their help and improvement of this presentation

Thermoforming PET Sheet


Similarity with other amorphous polymers allows thermoforming equipment to be adapted to properly handle PET sheet.

Cycle Times and Forming Temperatures must be adjusted to appropriately suit PET.

PET typically has shorter forming cycles and lower temperatures than those used in thermoforming other sheet such as PVC.

Don’t Assume PET & PETG are the Same

Thermoforming PET SheetThermoforming & Die Cutting PET Sheet

Moderate heat settings between 140οC to 165οC (280οF to 325οF).

Mold temperatures ranging from 40οC to 60οC (100οF to 140οF).

Monitor temperatures

DO NOT:Overheat sheetUse cold moldsUse sheet temperatures below 140οC (280οF)

***These can result in embrittlement, excessive sag, non-uniform drawing, and “freezing” of the sheet***

Die Cutting PET Sheet


As a Moskla and Barr study stated…

APET is notoriously difficult to cut.“”

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I. Kiss Cut Dies (Steel Rule & Forged Dies)


General Procedure:

PET sheet is heated and thermoformed by vacuum or pressure to the contours of the mold.

While still in the mold, the heated forged die is pressed down further through the PET sheet.

The die edge makes “kiss cut” contact on the base cutting the hot PET sheet.



Steel rule dies…a sharpened, metal cutting edge or knife mounted on a

sturdy piece of birch or maple die board

Cost Value: least expensive option for limited volume cutting

Usable: virtually anywhere on the line

Main Advantage: relatively easy to make allowing easier change as products change

Main Disadvantage: tends to wear the easiest



Forged die… (a.k.a “form and cut” procedure)

Cost Value: Excellent for middle of the road volume applications

Usable: In-place trimming procedure

Main Advantage: Easiest die to heat

Main Disadvantage: Its attachment to the mold requires a completely new mold if die ever breaks.



PVC or HIPS break or facture after being cut only “approximately 50-75% through the thickness”

PET sheet must be cut completely (90-100%) through in order to separate cleanly “Kiss Cut”

Make-Ready Procedure & Precision are Critical



Hot PET sheet is easier to cut than cold sheet (reduced shear strength requires less force)

Control Base Plate Temperature Just Below PET Sheet Sticking Point (Sheet <71οC or <160οF Tg)

Heat Assisted Die Cutting: 1. Low Pressure Contact (Die/Sheet/Base Plate)2. Slight Time Delay & Heat Conduction into Sheet3. High Pressure “Kiss Cut”


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Thermoforming & Die Cutting PET Sheet Thermoforming & Die Cutting PET Sheet

I. Kiss Cut Dies (Steel Rule and Forge Dies)




Matched Metal Die… (a.k.a a punch and die)

Cost Value: recommended for large applications

Usable: in-line trimming method

Main Advantage: easiest to repair as die wears

Main Disadvantage: inflexibility in adjusting to different cutting specs



Matched Metal Die… (a.k.a a punch and die)Procedure:

Mounted in a separate cutting press through which the continuously formed sheet passes…The matched metal die works with hardness. A harder punch is used with a softer die (typical hardness is 43Rockwell C for the die and 55 for the punch). Minimum clearance is maintained as it wears. The die may be peened – a process used on a die to recover proper clearance by spreading the edge of the die back to its original size.



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Conclusions:


PET Sheet offers clarity, toughness, durable hinges, good cost/performance ratio & ease of recycling

Die Cutting requires precision & patience (Make-Ready Procedure)

Heat Assist to Reduce Shear Strength

PET Sheet is a packaging opportunity that is worth the investment

Contact Us…Thermoforming & Die Cutting PET Sheet

For more information about PET Sheet, its PET sheet extruding line, thermoforming and die cutting PET sheet or about the company and its writers, please contact us:

PETCO, Division of Lavergne Group8800, 1er CroissantVille d’Anjou(Quebec) Canada H1J1C8

Larry Koester, VP Marketing & Sales, tel. 402-861-9524, fax 402-861-9527, [email protected]

Sheila Nemeth, Marketing & Sales, tel. 514-354-5757 ext. 116, fax 514-354-3087, [email protected]

Documents

Thermoforming & Die Cutting of PET Sheet shrunk9-29-02 .d.burchamintl.com/papers/petpapers/34_gpec03.pdf · Thermoforming & Die Cutting of Recycled/Virgin PET Sheet (PETCO of Lavergne