Thermoforming Materials

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Material choice drives both how a thermoformed part forms and performs. Here’s what we run at STM Plastics, some qualities of each material, and how to pick one for your part.

What we run

High Molecular Weight Polyethylene (HMWPE) and High Density Polyethylene (HDPE) – Our most commonly run materials. Both offer high impact resistance, excellent chemical resistance, and good performance in the cold. They absorb almost no moisture. We use them for tubs, enclosures, tanks and liners, and parts that live outdoors or take abuse. Surface hardness is lower than ABS, so they scratch more easily.

Acrylonitrile Butadiene Styrene (ABS) – The most widely used industrial thermoforming material because it balances impact strength, ease of forming, paintability, and cost. It has good dimensional stability and a crisp surface finish. ABS is common for housings, machine guards, and covers. It’s not a first choice for outdoor service unless you include a UV-stable grade or a cap layer.

High Impact Polystyrene (HIPS) – Cost-effective, rigid, and dimensionally stable, with decent impact strength. HIPS is a good option when you need a formed part to hold its shape at lower material cost than ABS. It’s the poor man’s ABS.

Polyethylene Terephthalate Glycol (PETG) – Clear, tough, and chemically resistant. This is the material to specify when you need to see through the part. It’s also FDA-compliant for food contact.

Polypropylene (PP) – Good chemical resistance and light weight. PP is semi-crystalline, so it has a narrower forming window than the amorphous materials above.

Polyvinyl Chloride (PVC), Kydex, and Boltaron – Kydex and Boltaron are both PVC/acrylic alloys, and they’re direct alternatives to each other. All three offer good chemical resistance and rigidity. Kydex and Boltaron are available in grades with flame ratings and textured finishes.

Thermoplastic Polyolefin (TPO) – Good outdoor durability, scratch and mar resistant, and impact strength in the cold. TPO is often used on large, low-volume parts as a replacement for fiberglass.

We form all of these from .020″ through .400″ and heavier. If your application calls for something we haven’t listed, ask us. We can often source it or suggest a substitute.

How to pick

Start with what the part has to survive:

  • Impact, especially cold – HMWPE, HDPE, ABS, TPO
  • Chemical exposure – HMWPE, HDPE, PP, PVC
  • Outdoors and UV – HMWPE or HDPE, TPO, or a UV-stable ABS grade
  • Clarity – PETG
  • Paint or a finished surface – ABS
  • Rigidity at lower cost – HIPS
  • Flame rating – Kydex or Boltaron
  • Food contact – PETG, PP, HDPE

Then consider gauge. Does your part need to withstand impact load or is it purely aesthetic? Large parts generally need heavier gauge, both for stiffness in service and because a thin sheet sags too much under its own weight as it heats. See large part thermoforming for more on that.

Two forming behaviors worth knowing about

Amorphous vs. semi-crystalline – ABS, HIPS, PETG, PVC, Kydex, and Boltaron are amorphous. They soften gradually over a wide temperature range, which makes them forgiving to form and gives predictable shrinkage. HDPE, HMWPE, PP, and TPO are semi-crystalline. They transition from solid to formable across a narrower window and shrink more, and unevenly with the direction of the sheet.

This doesn’t mean that the polyethylenes are hard to form. It means they need tighter temperature control, and mold shrinkage has to be accounted for in the tool. We build that in.

Shrinkage – Every thermoformed part contracts as it cools, anywhere from 0.3% to 3.0% depending on the material. Semi-crystalline materials can shrink up to 10 times as much as amorphous ones, and they shrink unevenly with the direction of the sheet. We build the compensation into the tool. If your part has tight dimensional requirements, tell us during quoting so we account for it before the tool is made.

Choosing a material with us

Most projects don’t arrive with the material settled, and that’s fine. Send us a drawing, a sketch, or dimensions with a description of what the part has to do and where it will live. We’ll recommend a material and gauge or tell you if a substitute would work better than what you had in mind.

For our full range of capabilities, see custom thermoforming. For draw ratio, draft angles, and radii, see our thermoforming design guide.

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Large Part Thermoforming

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One of the greatest benefits of plastic thermoforming is the ability to economically make large parts.  At STM Plastics, we can thermoform parts up to 8′ x 10′.

Why large parts get thermoformed instead of injection molded

Tooling cost is the key driver when choosing thermoforming over other manufacturing processes for large parts.  The tooling cost gap widens as parts get bigger.

Vacuum forming shapes plastic sheet over an aluminum-cast tool with atmospheric pressure, about 15 psi. By contrast, injection molding pushes molten resin into a closed mold at 10,000 to 20,000 psi. A tool that holds that pressure needs hardened steel in matched halves, cooling channels, and an ejection system. That complexity is what drives up the tooling cost.

Injection molding clamping force also scales with the projected area of the part, so a large part needs a much bigger press, which can be difficult to find. For a thermoformed large part, expect lower tooling cost and a shorter wait for first parts.

Where large thermoformed parts show up

We can serve any industry and we’ve made large parts for equipment enclosures, machine guarding, agricultural components, vehicle and transportation panels, water features, recreational products, process tubs and trays, material handling, and more.

We also ship nationwide and internationally.

What we run

  • Part size up to 8′ x 10′
  • Gauge from .020″ through .400″ and heavier (heavier is better for large parts)
  • Materials HMWPE, HDPE, ABS, HIPS, PETG, polypropylene, PVC, and Kydex
  • In-house finishing trimming, deburring, drilling, and assembly

Large Part Design Considerations

Draw ratio, draft angles, corner radii, and material selection work the same on a 10-foot part as on a 6-inch part. Draw ratio, for example, is a proportion, not a dimension: a shallow 10-foot panel and a shallow 6-inch tray present the same forming condition (more or less).

Large shallow parts (panels, covers, trays, enclosure faces) are what thermoforming does best. See our thermoforming design guide for draw ratio, draft angles, and radii.

Is your part a fit?

Send us a drawing, a sketch, or just dimensions and a description. We’ll tell you whether thermoforming is the right process for your part.

For our full range of capabilities, see our custom thermoforming page.

 

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