What is PET Sheet?
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.
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
• Light Weight
• Good Gas Barrier
• Solvent/Corrosion Resistance
• Good Cost/Performance Ratio
• Durable, difficult to break
• Durable hinge properties
• Recyclable and Regrindable
Wide Application of PET Sheet
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.
Basic categories of PET Sheet
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. For all intense and purposes, APET, PET, RPET, Polyester and PETE are the same thing, polyethylene terephthalate.
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.
Precautions for PET Sheet
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.
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.
Thermoforming of PET Sheet
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.
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.
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.
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).
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.
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.
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.
The following criteria should also be noted:
1. Mold temperatures below 27ο C (80ο F) may cause “freezing” of the sheet, non-uniform drawing, and stressed parts; however, mold temperature is the key to faster cycles. Above 60ο C (140ο F), a longer production cycle could be required, and distortion of the part may occur.
2. At temperatures slightly above the Tg of 80ο C (176ο F), 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 149ο C (300ο F), 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.
General Do’s and Don’ts of Thermoforming PET Sheet
To achieve controlled, consistent, and reliable results when thermoforming PET sheet, a few basic guidelines should be considered:
1. Use moderate heat settings on thermoforming equipment to give a sheet temperature between 140ο C to 165ο C (280ο F to 325ο F).
2. Use mold temperatures that range from 40ο C to 60ο C (100ο F to 140ο F).
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.
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 20ο C to 25ο C (70ο F to 80ο F) can cause “freezing” of the film and non-uniform drawing, especially with male molds.
3. Use sheet temperatures below 140ο C (280ο F). Due to freezing internal stresses in the part, cold forming can cause embrittlement.