PP Cup Thermoforming: Two Seconds a Cup and No Room for Error

The product and why it is polypropylene

A thermoformed PP cup is a thin-walled container with an integral rim, made from a roll of sheet on a form-cut-stack line. It is used for cold drinks, dairy products, ice cream and food tubs, and it is the highest-volume thermoformed product in the world after the egg tray.

Polypropylene wins for four reasons. It is heat resistant to about 100 °C, so it takes hot fill and microwave. It is chemically resistant, so it holds acidic and fatty foods. It is cheap. And it has enough melt strength to form a deep cup without a plug.

The alternative materials each lose on one of those. PS is cheaper and clearer but softens at 75 °C. PET is clearer and stronger but needs drying and is more expensive. PLA is compostable and cannot take hot fill in its amorphous form.

Table 1 — Cup materials. PP is the only one that combines hot fill, no drying and a low price.

Two seconds, and where the time goes

A roll-fed PP cup line runs at 30 to 40 strokes per minute with 20 to 40 cavities per stroke. The cycle for a 0.5 mm cup is about 2 seconds: heating 0.9, forming 0.3, cooling 0.5, cutting and stacking 0.3.

Heating dominates because PP has a narrow window and the sheet must reach temperature evenly across its thickness. Radiant heaters above and below the sheet, at 400 to 500 °C and 10 to 20 mm away, bring a 0.5 mm sheet to 155 °C in under a second. Faster than that and the surface overheats before the core is ready.

Cooling is the second-largest component and it is controlled by the mould temperature. A cold mould (15 to 25 °C) gives the shortest cycle but produces a cup with high residual stress that distorts when filled hot. A warm mould (40 to 60 °C) gives a more stable cup and a longer cycle.

Table 2 — Cycle breakdown for a PP cup on a roll-fed line. Heating is nearly half the cycle at every wall.

The 15 °C window

PP forms between 145 and 162 °C at the surface. Below 145 the sheet is too stiff and the cup has stress whitening at the base radius; above 162 it sags and the rim thins.

The window is narrow because PP is semi-crystalline with a sharp melting range, and because the sheet is thin enough that the surface and the core are within a few degrees of each other. There is no thermal gradient to buffer the process.

The practical control is heater power, not heater temperature. A heater at 450 °C that is on 60 percent of the time gives a different result from a heater at 380 °C on 90 percent of the time, even though both deliver the same average energy. The second gives a more even sheet and a wider window.

The rim, and why it decides the product

The rim of a cup is formed by rolling or folding the edge, either in the mould or in a separate station. It gives the cup its top-load strength and its sealing surface for a lid.

A rim that is too thin will not hold a lid seal and will deform under stacking load. A rim that is too thick will not roll properly and will have a visible seam. The rim geometry is specific to the lid system, and changing lid supplier usually means changing the rim.

Stacking geometry is the other hidden specification. Cups nest, and the nesting height determines how many fit in a case and how the cup behaves in a dispenser. The taper and the stacking shoulder are designed together with the rim, and a cup that is dimensionally correct but nests too deep will jam in a dispenser.

Tooling at speed

A cup tool is aluminium, water-cooled, with vacuum holes in the base and the side wall. At 30 strokes per minute the tool is opening and closing every two seconds, and the wear items are the cutting edge, the ejector and the vacuum circuit.

The cutting station is a matched punch and die, and its condition determines the rim quality. A dull punch tears the rim instead of cutting it, and the tear is the start of a crack when the cup is filled. Punch sharpening is a scheduled maintenance item, not a response to a complaint.

Vacuum must be sized for the leak rate, not for the cavity volume. At two-second cycles there is no time to recover from a vacuum drop, and a single blocked hole in one cavity produces one defective cup in every stroke — which at 30 strokes per minute is 1,800 cups an hour.

Defects at line speed

Table 3 — PP cup defects. Two of them are tooling wear, which at line speed becomes a thousand defective cups before it is noticed.