PP Thin-Gauge Thermoforming: Shrinkage, Warpage and Stacking

PP is the hardest thin-gauge material, and it is the one you cannot avoid

Polypropylene is the material of choice for anything that has to go in a microwave, survive a freezer and cost almost nothing. It is also the most difficult of the common thin-gauge materials to form, and the difficulty is not a matter of technique — it is built into the polymer.

PP is semi-crystalline. It melts at 160 to 165 °C, crystallises on cooling, and shrinks by 1.2 to 2.0% as it does. Its melt strength is low, so a heated sheet sags under its own weight. Its forming window is about 20 °C wide, which is half that of PS and a third of PET.

Those four facts produce every problem in this article: sagging sheet, warped parts, inconsistent stacking, and a window that closes if the oven drifts by 5 °C.

Table 1 — PP against the other two thin-gauge staples. It wins on temperature and loses on every dimensional measure.

The comparison explains the market. A microwave meal tray has to be PP because nothing else survives 120 °C in a microwave with a fatty sauce. The customer accepts the warpage, the tighter process window and the higher scrap rate because the alternative does not exist.

The shrinkage is large and it is not uniform

PP shrinks 1.2 to 2.0%, which is three times PS and comparable to PET, but the number that matters is not the average. It is the difference between the two directions.

Extruded PP sheet has oriented chains along the machine direction, because the extruder pulled it. When the sheet is heated and formed, those chains relax and the sheet shrinks more in the machine direction than across it. The difference is typically 0.3 to 0.8 percentage points, and it is what turns a square tray into a slightly rectangular one.

Table 2 — Shrinkage by PP grade. The last row is the one that gets missed in the tool design.

The practical consequence is that the tool has to be scaled by 1/(1−S), and S has to be the shrinkage in the direction being corrected. A tool scaled uniformly at 1.5% will produce parts that are correct in one direction and 0.4% long in the other, and on a 200 mm tray that is 0.8 mm of out-of-square.

The anisotropic shrinkage is also why PP sheet has a grain. A tray formed with its long axis along the machine direction will shrink differently from the same tray rotated 90° on the sheet. If the pack has to fit a lid or a sleeve, the orientation has to be specified, not left to the operator.

Melt strength: the sag problem

PP has low melt strength. A sheet heated to forming temperature is not a solid and not a liquid — it is a very viscous fluid with almost no ability to support itself. On a 0.8 mm sheet spanning a 400 mm oven, the sag at forming temperature is 10 to 25 mm, and it happens in seconds.

Sag matters because the sheet is no longer flat when it reaches the mould. The centre of the sheet is closer to the mould than the edges, so it contacts first, cools first, and forms a thinner wall than the edges. The part comes out with a wall thickness gradient that has nothing to do with the mould geometry.

Table 3 — What sag does to a PP part. Every row is fixed by shortening the time between heat and form.

The most effective countermeasure is speed, not hardware. The shorter the distance and the time between the oven and the mould, the less the sheet sags. A machine with a 300 mm gap between the oven exit and the mould will produce better PP parts than one with a 600 mm gap, and no amount of temperature tuning compensates.

The second is support. Some machines carry the sheet on a chain or a frame that supports it across its width until the mould closes. Others use an air curtain that holds the sheet up. Both work; neither is standard on a machine bought for PS.

The narrow forming window

PP's usable forming range is roughly 155 to 175 °C, and both edges are hard.

Below about 150 °C the sheet is not fully softened. It will form, but the material springs back, the corners thin, and the part often shows stress whitening along the bend. Above about 180 °C the sag becomes uncontrollable, the sheet may touch the heaters, and the part sticks to the mould.

Table 4 — PP forming settings. The mould temperature uniformity line is the one that decides whether the part is flat.

The 20 °C window is why PP lines are usually fitted with better heater control than PS lines. A zone that drifts 8 °C takes a PP part from good to scrap, where the same drift on PS would be invisible.

The plug assist threshold is lower for PP than for any other common material. A 0.6 to 1 draw that PS handles with vacuum alone needs a plug in PP, because the material will not stretch evenly without mechanical help.

Warpage, and the four things that cause it

Warpage is the defining defect of PP thin-gauge packaging and it has four independent causes. Diagnosing which one you have is most of the work.

Differential shrinkage. The material shrinks more where it is thicker and less where it is thinner, and more along the machine direction than across. A part with an uneven wall will always warp toward the thick side.

Differential cooling. The mould face and the plug face cool the part at different rates. A mould that is 45 °C on one side and 35 °C on the other produces a part that curls toward the cold side, because the cold side solidifies first and the hot side continues to shrink against it.

Frozen-in orientation. The sheet was stretched during forming, and the chains want to relax back. If the part is demoulded before it is cool enough to hold the orientation, it relaxes in the stack instead of in the mould.

Demould temperature. A part taken off the mould at 80 °C is still soft and will continue to shrink and distort on the conveyor. The warp appears five minutes later, in the stacker, which is why it is often blamed on the stacker.

Table 5 — Warp patterns against causes. The last two rows are the ones that get missed because the part looked fine at the machine.

The "warps hours later" case is the most expensive because it is invisible at inspection. A tray that measures flat off the line and is domed in the warehouse will pass every check the converter runs and fail at the customer's filling line. Cooling to below 60 °C before stacking is the fix, and it costs cycle time.

Stacking and nesting

PP packaging is almost always nested and stacked, and the stack has its own set of requirements that the formed part has to satisfy.

The nesting taper has to be steep enough that the parts separate cleanly and shallow enough that the stack does not become too tall. Three to five degrees per side is the usual range. Below 2° the parts jam; above 6° the stack is unstable and the pack volume is wasted.

Anti-blocking matters more for PP than for PS, because PP surfaces are softer and two warm parts pressed together will stick. The standard fixes are a fine embossed pattern on the inside of the base, small stacking lugs, or a matte finish on the contact surface.

Table 6 — Stacking requirements. The demould temperature row is the one that links warpage to stacking.

The stack is also where a small warpage becomes a large problem. A 0.3 mm dome on a single tray is invisible; stacked forty high with a case on top, it becomes a leaning column that the customer's depalletiser cannot handle. The tolerance on flatness has to be set with the stack in mind, not with the single part in mind.