PLA Thermoforming: Compostable Trays on Equipment You Already Have

What PLA is

Polylactic acid is a polyester made from lactic acid, which is fermented from starch — usually corn, sometimes sugarcane or cassava. It is the most commercially established bioplastic, and its thermoforming behaviour is close enough to PET that most converters can run it without new equipment.

It is clear, glossy, stiff and printable. It is also brittle, has a low heat deflection temperature in its amorphous form, and its compostability depends entirely on the conditions it meets at end of life.

Those three facts define the market. PLA wins where clarity and a sustainability claim matter and the product is used cold. It loses where the product sees heat, where toughness is required, or where industrial composting is not available.

Table 1 — PLA grades for thermoforming. The choice is decided by the service temperature, not by the forming behaviour.

The forming window

PLA forms between 105 and 145 °C at the surface. That is lower than PET, which is why a machine set up for APET needs its heaters turned down by 30 to 40 °C rather than a new oven.

The window is narrow at the bottom end. Below 105 °C the sheet is stiff and stress-whitens at every radius; the whitening in PLA is more visible than in PET because the material is clear and the craze lines scatter light. Above 145 °C the sheet sags and becomes tacky enough to stick to the heater or the mould.

PLA also has a thermal history limit. Held above 140 °C for more than a minute, the molecular weight drops through hydrolysis and the part becomes brittle. The smell of lactic acid in the workshop is the warning sign.

Table 2 — PLA forming parameters for amorphous sheet. For crystalline PLA the mould temperature rises to 90 – 110 °C and the cycle doubles.

Crystallinity and the heat resistance problem

Amorphous PLA has a heat deflection temperature of about 55 °C. A takeaway coffee at 85 °C, a car interior at 60 °C in summer, or a microwave will all deform it. This is the single limitation that decides most PLA projects.

The answer is crystallinity. PLA can be crystallised during forming by holding it against a hot mould at 90 to 110 °C for long enough for the chains to organise. The result is opaque, white, and heat resistant to 120 °C — a direct analogue of the APET to CPET transition.

The cost is cycle time. A crystalline PLA tray needs 15 to 30 seconds in the mould against 2 to 4 for an amorphous one, because crystallisation is a time-temperature process and the mould has to stay hot. That is a factor of five to ten on the cycle, and it changes the economics completely.

Drying: less critical than PET, still necessary

PLA absorbs about 0.2 percent moisture, less than PET and more than PS. At forming temperature, moisture causes hydrolysis and a loss of molecular weight, which shows as brittleness rather than as bubbles.

The target is 0.025 percent or below. At 60 °C in a desiccant dryer that takes four hours; at 80 °C it takes two but risks the pellets sticking together in the hopper. Most sheet suppliers dry before winding and ship in a sealed bag, in which case the converter only needs to keep the sheet dry.

Sheet stored in an open warehouse will pick up moisture within days. A roll that has been open for a week should be re-dried before use, and the drying must be at low temperature to avoid the pellets or the sheet sticking.

Compostability: what the claim actually requires

PLA is certified industrially compostable to EN 13432 or ASTM D6400. Both standards require disintegration within 12 weeks and biodegradation of 90 percent within 180 days under controlled conditions of 58 °C and high humidity.

Those conditions exist in an industrial composting facility. They do not exist in a home compost heap, in a landfill, or in the sea. A PLA cup in a home compost bin will still be recognisable after a year. A PLA cup in the ocean behaves like any other plastic.

This matters commercially because the claim is regulated. In the EU, describing a PLA pack as biodegradable without qualifying it as industrially compostable is a misleading claim. In several markets the pack must also be a colour that sorting equipment recognises — usually green — and must carry the certification mark.

The practical requirement is that the collection infrastructure exists. A compostable pack in a market with no industrial composting is functionally the same as a conventional plastic pack, with a better story and a worse carbon footprint.

Running PLA on PET equipment

The changeover is mostly temperature. Heaters down 30 to 40 °C, mould temperature down to 20 to 40 °C, and the vacuum and pressure settings can stay as they were because PLA's forming behaviour is similar to APET.

The differences that catch people are trimming and static. PLA is more brittle than PET, so a steel rule die that cuts PET cleanly will crack a PLA part. A matched punch or a heated blade gives a better edge. PLA also generates more static than PET, which makes parts stick together and attracts dust.

Extrusion coating and sealing are different too. PLA seals at 90 to 120 °C against a PLA-coated card, lower than PVC or PET, and the seal is more sensitive to pressure. A sealing machine set for PVC will burn through a PLA blister.