PET and rPET Thermoforming: Crystallinity, Drying and the Forming Window

Two PETs, and only one of them is clear

PET is the largest thin-gauge thermoforming material in the world, and the first thing to understand about it is that "PET sheet" describes two products that behave almost oppositely.

APET is amorphous PET. It is extruded and quenched fast enough that the polymer chains freeze in a disordered state, which makes it transparent, tough and easy to form at relatively low temperature. It is what a clear food tray, a blister and a display box are made of.

CPET is crystalline PET. It is formulated with a nucleating agent and formed against a hot mould so that the chains arrange into crystallites, which makes it opaque, white, and able to go from a freezer at −40 °C into an oven at 220 °C without deforming. It is what an airline meal tray and a ready-meal container are made of.

Table 1 — APET and CPET are the same polymer with different thermal histories. The choice is decided by whether the pack has to see heat.

The distinction matters commercially because the two are often quoted against each other by suppliers who do not say which one they mean. A quotation for "PET tray, 0.6 mm" at an attractive price is almost always APET, and it will fail the first time a customer puts it in a microwave for four minutes.

APET: a window that closes from both sides

APET forms at a lower temperature than most people expect, and the window is narrow in a way that punishes both errors.

Below about 120 °C the sheet is not soft enough. It will form, but it springs back, the corners thin unevenly, and the part often cracks at a sharp radius within a day. Above about 160 °C the material starts to crystallise on the mould, and the result is a milky white haze that appears first at the corners and then across the whole part. That haze is not a surface defect — it is the material changing phase, and it cannot be reversed.

Table 2 — APET forming parameters. The window is roughly 40 °C wide and both edges are hard failures.

The 40 °C window is why APET lines are fussy about heater control. A quartz heater that is 10 °C hotter on one side of the oven will produce parts that are clear on one edge and hazy on the other, and the operator will blame the sheet. Zone control across the oven width and length is not a luxury on an APET line, it is the process.

The draw ratio limit is the other constraint that catches designers. APET will physically form at a draw ratio of 1.5 to 1, but the side walls thin to the point where the pack has no stiffness and the barrier is compromised. A 1.2 to 1 limit is the practical ceiling for a pack that has to survive a supply chain.

CPET: crystallinity is the product, not the defect

CPET inverts the logic of the APET process. Instead of avoiding crystallisation, the process is designed to produce it, and the mould is the tool that makes it happen.

The sheet is heated to 150 to 180 °C, formed against a mould held at 140 to 170 °C, and held there long enough for the crystallites to grow. The dwell is the critical variable: too short and the part is only partly crystalline, which means it will continue to crystallise in service and change dimensions; too long and the cycle cost becomes uncompetitive.

Table 3 — CPET forming. The hot mould is not a convenience; it is the reaction vessel.

A CPET tool therefore runs hot, which changes everything downstream: the tool needs heaters and insulation, the cycle is longer, the energy cost is higher, and the trim station has to handle a part that is still warm. This is why CPET is roughly 1.5 to 2 times the cost of APET for the same geometry, and why it is only used where the ovenability is genuinely needed.

The test for adequate crystallinity is simple: put the part in an oven at 200 °C for 30 minutes and measure it. A properly crystallised CPET tray comes out within a millimetre of where it went in. An under-crystallised one shrinks, warps and curls at the rim, and the failure is visible to anyone.

Drying is not optional

PET is hygroscopic and it hydrolyses. Water absorbed into the sheet reacts with the ester bonds at melt temperature, breaking the polymer chain and dropping the intrinsic viscosity. A sheet that was 0.80 dL/g going into the extruder can be 0.68 coming out, and the difference is a material that cracks instead of drawing.

Table 4 — Drying schedules. The rPET numbers are stricter because the starting IV is already lower.

The mistake that shows up most often is not a dryer that is off — it is a dryer that is working but not delivering dry air. A desiccant bed that has stopped regenerating, a dew point sensor that has drifted, or a hopper that is open to a humid shop all produce the same result: sheet that looks and measures fine and forms badly. The symptom is a part with poor corner strength and a slightly dull surface, and the cause is invisible unless the dew point is logged.

rPET changes the process, not just the label

Recycled PET is now a requirement in many markets and a marketing asset in all of them, and it is not a drop-in substitution. Three things change when rPET enters the sheet.

The viscosity drops. Food-grade rPET typically arrives at 0.72 to 0.80 dL/g against 0.80 to 0.84 for virgin. Lower IV means lower melt strength, which means the sheet sags more in the oven and the forming window narrows from both ends. A line that runs comfortably at 145 °C on virgin sheet may find that 145 °C is now too hot, because the sheet is already stretching under its own weight before the mould closes.

The scatter widens. Virgin PET is a specification; rPET is a distribution. Two deliveries of nominally identical rPET can differ by 0.04 dL/g in IV and by a visible amount in colour, because they came from different feedstock streams. The line has to be able to absorb that variation, which usually means a wider tolerance on the finished part and more frequent first-article checks.

The colour shifts. rPET carries a yellow tint from thermal history in the recycling process, measured as an increase in b* of 1 to 4 units. For a clear tray this is visible against a white background and it is the single most common complaint about recycled content. Optical brighteners and chain extenders can compensate, but both cost money and both have to be declared for food contact.

Table 5 — What recycled content changes. Every row is a process adjustment, not a purchasing decision.

There is also a legal dimension that belongs in the same conversation. Recycled PET for food contact has to come from a process that has been shown to remove contaminants, which in the US means an FDA letter of no objection supported by a surrogate challenge test, and in the EU means an EFSA assessment and a registered recycling process. Buying rPET from a source that cannot produce that documentation is buying a compliance problem, and the problem surfaces at the customer's audit rather than at yours.

Defects and what they point at

Table 6 — PET forming defects. The first two rows are the same parameter in opposite directions.

The last-but-one row is the one that causes arguments between a converter and a customer. An APET tray that measured correctly at despatch and is 2 mm shorter after a week in a warm warehouse has crystallised, because it was formed at the top of the window. The part was not out of tolerance when it was made, and it is out of tolerance now, and both statements are true.

What to put on the specification

PET packaging specifications are usually written by the customer's packaging team and they usually name the material and the dimensions and stop. Six more items belong on the drawing.

APET or CPET, explicitly. "PET" is ambiguous and the ambiguity is expensive.

Recycled content percentage and the documentation route. If rPET is required, say how much and require the challenge test or EFSA registration with the delivery.

The IV or the sheet grade. This is the property that decides whether the part forms, and it is not visible on the finished tray.

Colour measured as b* on a clear part. A visual "clear" is not a specification for recycled material.

Service temperature, stated as a test. "Microwave safe" and "ovenable to 200 °C for 30 minutes" are different claims and only the second one is testable.

The draw ratio limit implied by the geometry. If the design needs 1.5 to 1, the material has to be CPET or the design has to change.