PVC Thermoforming: The Material That Still Owns Packaging

Why rigid PVC still owns the clear pack

Every few years someone announces that PVC packaging is finished. It is not, and the reason is arithmetic rather than sentiment. Rigid PVC sheet costs less per square metre than APET at the same thickness, forms at a lower temperature, accepts solvent seals and print without treatment, and gives a glass-clear pack with better stiffness than the alternatives.

The environmental argument against PVC is real and it has changed the market rather than closed it. In Western Europe, PVC blister and tray volumes have moved to PET where the brand owner has a public commitment. In Asia, the Middle East, Africa and much of the Americas, PVC remains the default because the cost difference is 20 to 35 percent and the performance is better for the specific job of holding a product against a printed card.

This article is about forming it well, not about whether you should. If you are forming PVC, the material will punish you for three specific mistakes and reward you for avoiding them.

The forming window is narrow, and it moves with thickness

Rigid PVC sheet forms between about 120 and 175 °C at the surface. Below 120 the sheet is too stiff to draw without surface marks and it springs back after demoulding. Above 175 it begins to degrade, and degradation in PVC is not a gradual loss of quality — it is a chain reaction that releases hydrogen chloride, which catalyses further degradation.

The window shifts slightly with thickness because a thick sheet has a larger thermal gradient through its section. The surface may read 170 °C while the core is at 140, and the sheet will form as though it were at 155. This is why thick PVC needs a longer soak at temperature rather than a higher set point: you are waiting for the core, not heating the skin.

The other variable is time at temperature. PVC has a finite heat history. A sheet held at 170 °C for 90 seconds is fine; the same sheet held for four minutes will show yellowing at the clamp frame and a faint HCl smell at the heater. Once you can smell it, the sheet is already damaged.

Table 1 — Starting points for rigid PVC sheet on a radiant heater. Soak time is the time at temperature, not the total heater-on time.

Heating: the step everyone gets wrong

PVC is a poor conductor and a strong absorber in the near infrared. That combination means radiant heaters heat the surface much faster than the core, and the surface is exactly where degradation starts. The correct approach is to heat from both sides where the machine allows it, and to keep the heater temperature moderate while extending the time.

A heater set to 500 °C at 120 mm will bring a 1 mm sheet to forming temperature in 25 seconds and will also scorch the surface if the sheet stops moving. A heater set to 380 °C at 100 mm takes 45 seconds and is far more forgiving. On a production line running the same part all day, the faster setting wins; on a job shop running six jobs a shift, the slower one produces less scrap.

The clamp frame is a separate heat problem. The sheet edge held in the frame cannot stretch, and it is usually the hottest part of the sheet because it is closest to the heater at the edges of the oven. It will yellow first. If you see yellowing only at the frame line, the oven is too hot at the edges, not overall.

Draw ratio and the wall you actually get

Wall loss in thermoforming is geometric before it is anything else. When a sheet is drawn into a cavity, the material that was over the cavity is stretched over the cavity surface plus the walls. A deeper draw spreads the same material further, and the base centre — the last point to touch the mould — loses the most.

For PVC the practical limit is a draw ratio of about 0.8 to 1.0, measured as depth divided by the narrowest width. At 0.6 the base retains around 55 percent of the original thickness. At 1.0 it retains around 40 percent. Beyond 1.0 the base becomes a film and the pack loses the stiffness that made PVC worth choosing.

The corner is worse than the centre. A square cavity has four corners where the material has to stretch in two directions at once, and the corner wall typically retains 70 percent of what the base retains. That is where a PVC pack tears during demoulding, and it is where the wall gauge should be measured if you are checking a supplier's claim.

Table 2 — Wall retained against draw ratio for rigid PVC, vacuum only, no plug. Percentages are of original sheet thickness.

Moulds for PVC: aluminium, temperature and venting

Almost all PVC packaging tooling is aluminium, machined or cast. Aluminium conducts heat five times faster than steel, which matters because PVC needs a mould that removes heat quickly without chilling the sheet so hard that it marks.

Mould temperature for PVC sits between 30 and 60 °C. Below 30 the sheet freezes on contact and you get chill marks and drag lines. Above 60 the part stays soft at demould and distorts. The narrow band is a consequence of the narrow forming window: the sheet arrives only 40 to 60 °C above its softening point, so the mould has little margin either way.

Venting is the part that separates a good tool from a cheap one. PVC forms at a viscosity that traps air in the last corner, and a cavity without adequate venting will show a rounded, unformed corner that looks like a short shot. Vacuum holes of 0.6 to 1.0 mm at 20 to 30 mm spacing in the base, plus dedicated vents at the corners, are the minimum.

Degradation: the mistake that costs a week

PVC degradation is autocatalytic. Heat breaks a chain, releasing hydrogen chloride; the HCl catalyses further breakdown, which releases more HCl. Once it starts, it accelerates. The visible signs are yellowing, then brown streaks, then black specks, and the smell of HCl in the workshop.

The damage is not confined to the sheet. Hydrogen chloride is corrosive to steel, and a line that has run degraded PVC will show rust on the heater frames, the chain rails and the vacuum pump internals within weeks. This is the reason PVC lines are built with stainless or coated components and the reason a degraded batch is more expensive than the scrap value of the sheet.

Recovery takes longer than prevention. After a degradation event, purge the oven, clean the heaters, check the extraction, and run a test sheet before resuming production. If the smell persists, the extraction is inadequate and it will happen again.

Table 3 — Reading PVC degradation from the part. The colour tells you where the heat came from.

Trimming, sealing and the edge that decides the pack

A thermoformed PVC pack is only as good as its edge. Steel rule trimming leaves a slightly ragged edge that is acceptable for a blister sealed to a card but not for a pack that will be handled. For high-volume work, a matched punch and die in the same tool as the forming station gives a clean edge at full cycle speed.

Solvent sealing is the other reason PVC persists. A PVC blister seals to a PVC-coated card with a solvent or a heat-activated coating, and the bond is strong enough that the pack fails by tearing the card rather than by opening. PET needs a coated card and a different seal, and the seal is more sensitive to temperature and pressure.

If you are specifying a PVC pack, specify the seal as a system: blister material, coating on the card, seal temperature, dwell and pressure. A blister that seals perfectly with one card supplier and peels with another is not a forming problem, and no amount of adjusting the former will fix it.