Series · Plastics processing
Thermoforming
heat a sheet until soft, then pull or push it onto a mould
Most trays, cups, blisters and clamshells are not injection moulded. They are formed from extruded sheet. This article walks through the cycle, the variables that matter, the difference between vacuum and pressure forming, and the defects you see at the machine.
Marketing Department, ADVANCEDCHEM SOLUTIONS CO., LTD. · 21 October 2026 · about 7 min read
The principle in one minute
Thermoforming starts from sheet, not pellets. The sheet is clamped in a frame and heated with infrared heaters until it is soft enough to stretch. It is then drawn onto a mould by vacuum, compressed air or both, freezes against the cooler mould surface and is trimmed into finished parts.
Unlike injection moulding, there is no full melting and no melt flow, only stretching. A sheet of uniform thickness therefore becomes a part with non-uniform walls by nature: wherever it stretches most, it gets thinnest. Much of a thermoformer's craft is managing that distribution.
- Vacuum forming evacuates the air between sheet and mould. The forming pressure can never exceed atmospheric pressure (about 1 bar).
- Pressure forming adds compressed air at several bar from above, pressing the sheet harder against the mould for sharper corners and crisper texture.
- Plug assist uses a plug shaped roughly like the part to pre-stretch the sheet and carry material down to the bottom and corners of a female mould.
The production cycle
- Feed and clamp Thin-gauge machines are roll-fed: chain rails grip the edges of a continuous web. Heavy-gauge machines load one cut sheet at a time into a clamp frame.
- Heat Infrared heaters above, below or on both sides, split into zones whose power is set individually. The sheet softens and starts to sag under its own weight.
- Form The mould rises (or the sheet drops), the plug assist pushes material down, then vacuum and/or compressed air press the sheet onto the mould surface. This is the shortest step in the cycle.
- Cool The sheet gives up heat to the water-cooled mould until the part is stiff enough not to distort when released.
- Release and trim Thin-gauge parts are usually cut with a steel-rule or matched-metal trim press, in the mould or in a separate station. Heavy-gauge parts are usually trimmed on a CNC router. The leftover web (skeleton) goes to the granulator.
Most of the cycle is spent heating and cooling, not forming. The thicker the sheet, the more those two steps dominate, because heat has to soak through to the core of the sheet and back out again.
Variables to control
| Variable | What it controls | What goes wrong if it is off |
|---|---|---|
| Sheet temperature and zone heater profile | How soft each area of the sheet is, which decides where it stretches most | Too hot: heavy sag, webbing, poor surface · Too cold: incomplete forming, stress whitening, moulded-in stress |
| Heating time | Whether heat has soaked through to the core | Hot skin, cold core: uneven stretching |
| Vacuum / air pressure level and timing | The force pressing the sheet onto the mould | Soft detail, unfilled corners, dimensional drift |
| Plug depth, speed, temperature and material | How much material is carried down to the bottom and corners | Too shallow: thin corners · Plug too cold: chill marks where it touches · Too fast: sheet can tear |
| Mould temperature and cooling water | Cooling rate, gloss, shrinkage | Uneven mould temperature: warpage · Too cold: poor surface, residual stress |
| Sheet quality | Thickness uniformity, moisture, regrind ratio | Uneven sheet gives uneven walls · Wet PET bubbles and hazes |
| Trim registration | Part dimensions and edge quality | Burrs, trim dust (angel hair), cracked edges in brittle materials |
Textbook reference temperatures: amorphous sheet must be heated above its Tg (PS about 100 °C, PET about 70–80 °C), while PP must be formed close to its melting point (Tm about 160–170 °C). Typical range — follow the TDS of your grade. The practical point is that PP has a much narrower forming window than PS: a little too cold and it will not form, a little too hot and it sags fast.
Parts and resins
The industry splits thermoforming into two worlds with different machines and economics.
| Item | Thin gauge | Heavy gauge |
|---|---|---|
| Sheet thickness | Thin, typically below about 1.5 mm | Thick, typically above about 3 mm |
| Feeding | Roll-fed, continuous | Cut sheet, one at a time |
| Parts | Cups, food trays, clamshells, blisters, component trays | Machine covers, vehicle parts, bathtubs, pallets, signs |
| Economic volume | Very high, many cavities per cycle | Low to medium |
The thickness boundaries differ between references; treat them as a guide only.
- PS / HIPS forms easily over a wide temperature window; widely used for cups, trays and lids.
- APET / RPET is clear and stiff, used for clamshells, blisters and fruit punnets. Moisture must be controlled, and overheating causes crystallisation haze.
- PP is heat resistant, microwavable and chemically resistant, but has a narrow forming window. Sheet grades often use a nucleating agent or clarifier for clarity and stiffness.
- PE (HDPE), ABS and PMMA are common in heavy gauge where toughness or surface appearance matters.
All of this sheet is first made by sheet extrusion, which calls for lower-MFI resin than injection grades so the sheet has enough melt strength not to sag too quickly when reheated.
Common defects
Most thermoforming problems are solved in the process and the tooling, not in the formulation. The table says plainly where an additive helps and where it does not.
| Symptom | Likely causes | How to check | Does an additive help? |
|---|---|---|---|
| Webbing (folds or ridges at corners) | Sheet overheated and sagging, high draw ratio, cavities too close together | Map where folds repeat against heater zones and cavity spacing | Not the main fix. Reduce heat in that zone, add web catchers or a plug, adjust the tool |
| Thin bottom corners | Sheet freezes on the walls first and stops stretching; small corner radius | Section the part and measure thickness down the wall into the corner | No. Use plug assist, heat profiling and larger radii |
| Uneven wall thickness | Uneven zone heating, sheet gauge variation from extrusion, failed heater elements | Infrared camera on the sheet surface; measure sheet gauge before forming | No. Fix the heaters and the sheet extrusion |
| Stress whitening at highly stretched areas | Formed too cold, especially rubber-modified grades such as HIPS and PP copolymer | Check whether whitening is only at high-stretch areas; try more heat in that zone | Mostly no. Fix sheet temperature |
| Blushing / haze | APET overheated and crystallising, wet sheet, uneven mould cooling, PP without clarifier | Correlate haze with sheet temperature and sheet moisture | In PP, yes: a clarifier/nucleating agent improves clarity, see crystallinity in PP · In APET, fix heating and drying |
| Warpage | Uneven cooling, part released too hot, stress from cold forming | Lay parts on a flat surface after 24 hours; compare with mould temperature at each point | Partly: in PP a nucleating agent makes shrinkage more uniform, but fix cooling first |
| Trays attract dust; stacked parts hard to separate | Static charge on the sheet surface, high surface friction | Watch dust pick-up after trimming; measure surface resistivity | Yes: antistatic in the sheet, see antistatic solutions · Slip helps denesting of stacked cups, see slip and antiblock |
Trim skeleton and recycling
Thermoforming generates more in-plant scrap than most processes, because the web between cavities and the clamped edges are cut away every cycle. That skeleton is ground and fed back into sheet extrusion. What matters is a constant ratio: reprocessed material has lower viscosity, shifted colour and less clarity, so if the regrind ratio swings, the sheet and the parts swing with it. If you also use bought-in recycled resin, see recycled PCR solutions.
Additives in this process
The thermoformer receives finished sheet, so every additive goes in at the sheet extruder: masterbatch is blended with the base resin at the hopper, ideally with a gravimetric doser at the set let-down ratio. The additives that matter most here:
- Antistatic for packaging and component trays. Migrating types need time to bloom to the surface after production; applications that need a stable value over a long life should consider permanent types.
- Nucleating agent / clarifier for PP sheet that needs clarity, stiffness and consistent shrinkage.
- Slip to help denest stacked cups and trays.
- UV stabiliser for heavy-gauge parts used outdoors, see UV solutions.
- Colour and stabilisers for regrind when the regrind ratio is high.
Some migrating additives can affect printing, sealing or labelling. Trial a formulation change across the whole line, not just on the sheet.
FAQ
Thermoforming questions we hear often
Vacuum forming or pressure forming: how do I choose?
Vacuum forming can never push harder than atmospheric pressure, but the tooling and machine are cheaper, so it suits parts that do not need sharp detail. Pressure forming adds compressed air from above, pressing the sheet against the mould much harder for crisp corners, textures and lettering, at the cost of a pressure box and a stronger mould. Many thin-gauge machines use both together.
Why are the bottom corners of my tray thinner than the rest?
In a female mould the sheet touches the side walls first, cools and stops stretching. The still-hot remainder has to keep stretching to reach the bottom corners, so they end up thinnest. The main fixes are process and tooling: use a plug assist to carry material down first, adjust the heater profile, increase the corner radius and reduce the draw ratio if the design allows.
Can masterbatch be added at the thermoformer?
Generally not. A thermoformer takes finished sheet, so additives and colour go in at the sheet extruder hopper. Even in in-line lines that extrude and form continuously, the dosing still happens at the extruder.
Can the trim skeleton be reused?
Yes. Most plants grind the skeleton and feed it back into sheet extrusion, but the ratio must be held constant because reprocessed material has a different viscosity, colour and clarity. PET must be dried before re-extrusion, and for food-contact packaging check customer requirements and the applicable regulations first.
Technical caution
Let-down ratios and technical data on this page are typical reference values, not a guarantee of properties. Confirm by trial in your own process before production use.
