Thermoforming is a versatile and cost-effective manufacturing method, but selecting the right material is critical when your parts cannot feel flimsy. For most thermoformed parts, the relevant mechanical metric is stiffness in bending, which is captured by a material’s flexural modulus measured according to ASTM D790. Instron+2Bound: R&D for Injection Molding+2
This guide looks at the relative stiffness of commonly thermoformed plastics so you can pick a material that meets your rigidity requirements without over-engineering or driving unnecessary cost.
Key Considerations for Stiffness
The stiffness of a thermoformed part is not just about the material name on the quote. It is mainly driven by:
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Flexural modulus (ASTM D790)
Flexural modulus is the slope of the stress–strain curve in bending. ASTM D790 uses a three-point bend test on bars cut from sheet and reports modulus in psi or MPa. This is the correct property for evaluating how much a panel will deflect under load. Instron+1 -
Wall thickness and section geometry
Bending stiffness scales with E × I, where E is flexural modulus and I is the second moment of area. For a flat wall, I increases with thickness cubed, which means small changes in thickness or adding ribs and flanges can rival or exceed the stiffness gain from changing resin. -
Operating temperature
Modulus drops as temperature approaches the material’s glass transition or softening range. A material that feels stiff at room temperature can soften significantly in a hot environment, so you should tie stiffness decisions back to realistic service temperatures and heat deflection temperature (HDT). MatWeb+1 -
Fillers and blends
Mineral or glass fillers can raise flexural modulus substantially, but they affect impact resistance, notch sensitivity and formability. Many thermoforming grades are unfilled or lightly filled to maintain drawability, so check the datasheet for flexural modulus rather than assuming “reinforced” always equals better. -
Orientation and forming effects
Draw thinning in deep features reduces local stiffness even if the base sheet modulus is high. Good tool design spreads draw, keeps critical areas thicker, and uses geometry to carry the load instead of relying on resin choice alone.
Stiffness Chart for Common Thermoformable Plastics
The table below compares relative stiffness at room temperature, based on typical flexural modulus values from supplier data sheets and reference databases such as MatWeb. MatWeb+1
| Material | Relative Stiffness (Flexural Modulus at ~23 C) | Properties (stiffness focused) | Typical Applications |
|---|---|---|---|
| HDPE | Low to medium-low (0.7–1.0 GPa) | Excellent toughness and chemical resistance, but very soft in bending. Will deflect significantly more than ABS, HIPS or PVC at the same thickness. Curbell Plastics+1 | Chemical tanks, bins, outdoor equipment, liners, wear surfaces. |
| PP (Polypropylene) | Medium-low (about 1.5 GPa) | More flexible than ABS and HIPS, but with acceptable rigidity in many designs. Very good chemical resistance and low density. Curbell Plastics+2Docslib+2 | Food containers, medical devices, guards, chemically exposed parts. |
| PETG | Medium (about 2.0 GPa) | Clear, reasonably stiff, good impact strength, excellent thermoformability. Stiffer than HDPE/PP but not as rigid as acrylic-family materials. Curbell Plastics+2Toolless+2 | Display cases, signage, machine guards requiring clarity. |
| ABS | Medium (2.0–2.5 GPa) | Balanced stiffness and toughness; versatile and easy to thermoform. Curbell Plastics+1 | Automotive trim, equipment covers, housings, luggage shells. |
| PC (Polycarbonate) | Medium-high (~2.3 GPa)** | Stiffer than PP and HDPE, similar to ABS but with far higher impact strength and better high-temperature performance. Curbell Plastics+2MatWeb+2 | Aircraft and medical covers, safety shields, machine guards. |
| HIPS | Medium-high (~2.5 GPa)** | Stiffer than PP and HDPE but below PVC; very easy to thermoform with good impact resistance. MatWeb | Trays, packaging, interior panels, POP displays. |
| KYDEX T | Medium-high (~2.48 GPa) | Acrylic-PVC alloy with good stiffness, fire rating, impact strength and chemical resistance. Susheng Polymer+1 | Aircraft interiors, transit panels, medical housings, equipment cases. |
| PVC (Rigid) | High (2.5–3.0 GPa) | High stiffness, good dimensional stability, moderate impact strength, low cost. MatWeb+1 | Enclosures, architectural panels, signage, applications where rigidity is required. |
Notes
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Flexural modulus values above are representative of unfilled sheet grades measured per ASTM D790. Actual datasheets should always be checked for the specific grade and thickness you plan to use. Instron+1
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Typical modulus ranges used in the “relative stiffness” column are supported by average data in MatWeb’s flexural modulus table for ABS, acrylic, polycarbonate, polypropylene, polyethylene and PET. MatWeb+1
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KYDEX T’s specific flexural modulus of about 2.48 GPa comes from the manufacturer’s technical data sheet. Susheng Polymer
Material Selection Tips
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Start with stiffness targets at the part level, not the resin name
Decide how much deflection is acceptable at a known load and support condition. From there, you can trade off between resin, wall thickness and geometry. Remember that going from low-stiffness HDPE to medium-stiffness ABS or PETG is a much bigger change than small differences within the same stiffness band. MatWeb -
Use geometry before you jump to exotic materials
If a panel in ABS is too flexible, adding a simple rib, flange, or a modest thickness increase can raise bending stiffness as much as switching to a stiffer resin, often at lower total cost. You get a cubic benefit from thickness in bending and only a linear benefit from modulus. -
Match stiffness band to application
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High stiffness required (flat panels, doors, covers that must feel solid): lean toward PVC, KYDEX T or PC.
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Balanced stiffness and toughness: ABS, HIPS and PETG cover a wide swath of “good enough” rigidity with better impact resistance and easier forming.
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Stiffness is secondary to chemical resistance or impact: PP and HDPE are usually the more robust choice, but you should plan on ribs, corrugations or heavier gauges to hit deflection targets. Curbell Plastics+2Curbell Plastics+2
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Do not ignore temperature and environment
A material that looks stiff on paper can soften in a hot environment or creep under sustained load. Connect your stiffness choice back to heat deflection temperature, operating temperature range and exposure duration so the panel that feels rigid in a lab does not sag in service. ASTM International | ASTM+1
Choosing the Right Plastic for Your Project
While cost, availability and ease of processing are always important, stiffness in bending is often what your end user notices first when they push on a cover, door, tray or guard.
If your part feels too flexible today, the levers you can pull are:
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Change the geometry: ribs, flanges or slightly thicker walls.
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Move to a stiffer resin band within the same forming window, for example from PP to ABS, or from ABS to KYDEX T or PC.
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In extreme cases, consider filled grades or a different manufacturing process once thermoforming can no longer hit the stiffness target economically. MatWeb+2Susheng Polymer+2
For project-specific guidance, your best option is to combine material data with real part geometry and loading. If you share a model and operating conditions, a quick stiffness assessment will usually tell you whether you need a new plastic, a new thickness, or just better ribbing.
