Picking the Correct Thermoforming Plastic by Impact Resistance and Durability

(Notched impact + elongation at break)

When your thermoformed part fails, what usually kills it is not a slow static load. It is a corner that cracks when the part gets dropped, a hinge that snaps after a few bends, or a stress riser around a screw boss.

That behavior is controlled by its brittleness – how likely a material is to crack suddenly instead of deforming.

For thermoforming plastics, brittleness is best understood by looking at two test metrics together:

1. Notched Izod impact – how much energy a notched specimen absorbs before it breaks.

2. Elongation at break – how far the material can stretch in a tensile test before it fractures.

Low impact + low elongation means brittle. High impact + high elongation means tough and ductile. Curbell Plastics+2SpecialChem+2

Below we compare eight common thermoforming plastics:

• PVC (rigid)

• HIPS

• PETG

• ABS

• PC

• KYDEX T

• PP (copolymer sheet grade)

• HDPE

using typical room temperature values for unfilled sheet grades.

How brittleness is measured

Notched Izod impact (ASTM D256)

In the ASTM D256 notched Izod test, a machined bar with a sharp notch is struck by a pendulum. The reported value is energy to break per unit notch width in ft·lb/in.

From Curbell’s material selection guide for common amorphous and semicrystalline plastics: Curbell Plastics+1

• Kydex: 18 ft·lb/in

• Polycarbonate: 12.0–16.0 ft·lb/in

• ABS: 7.7 ft·lb/in

• HIPS: 2.0 ft·lb/in

• PETG: 1.7 ft·lb/in

• PVC: 1.0 ft·lb/in

• Polypropylene (copolymer): 12.5 ft·lb/in

• HDPE: about 1.5 ft·lb/in typical (HDPE sheets show 1.5–14.1 ft·lb/in depending on grade and test conditions). K-Mac Plastics+1

Low Izod and a sharp notch means cracks initiate and run easily. High Izod means the material can absorb more energy before failing.

Elongation at break (ASTM D638 / ISO 527)

Elongation at break is measured in a tensile test (ASTM D638 or ISO 527). It is the total strain when the specimen fractures.

Typical values from representative sheet or stock-shape data:

• PVC (rigid) – elongation at break about 20 percent City Plastics

• ABS – elongation at break about 20–25 percent Proleantech | Custom Parts On-Demand

• HIPS (RapidMade sheet) – elongation at break 50 percent 3D Printing Service

• PETG (VIVAK sheet) – elongation at break greater than 60 percent Plaskolite.com

• PC (TECANAT PC) – elongation at break about 90 percent Ensinger+1

• KYDEX T – elongation 110 percent (ASTM D638) Mpfs

• PP (PP-C sheet) – elongation at break 650 percent S-Polytec

• HDPE sheet – elongation at break 500–1000 percent K-Mac Plastics

SpecialChem and other references are clear: elongation below roughly 5 percent is a hallmark of brittle behavior; hundreds of percent is clearly ductile. SpecialChem+1

Comparison chart – from most brittle to most durable

This ranking uses:

• Notched Izod as the primary toughness metric (same dataset where possible, from the Curbell selection guide), and

• Elongation at break as a modifier – to distinguish materials that have similar Izod but very different ductility.

It is approximate and assumes typical unfilled sheet grades at about 23 °C.

Overview of Individual Plastics

PVC (rigid) – stiff and crack prone

• Notched Izod around 1.0 ft·lb/in in the Curbell table. Curbell Plastics

• Elongation at break about 20 percent for typical rigid PVC sheet. City Plastics

It is strong and stiff, but the combination of low impact energy and only modest elongation means it will crack around sharp corners, drilled holes or cold environments if abused.

Use PVC where you want flat, rigid panels that will not see heavy impact – signage, non-abused covers, chemical tanks with conservative design.

PVC has incredible chemical properties and is the most rigid food safe option.

HIPS – relatively brittle

• Tensile strength about 4000 psi 3D Printing Service+1

• Elongation at break 50 percent 3D Printing Service+1

• Flexural modulus ~260,000 psi 3D Printing Service+1

So HIPS is Less brittle than rigid PVC in tension, but more crack prone than ABS at notches and under impact.

Use HIPS when you want low cost, easy forming and the parts are not supposed to take serious abuse – POP displays, interior panels, trays.

HIPS makes great packaging material and has a beautiful finish. It comes in food safe versions but is not great for mechanical applications.

PETG – flexible and formable, and clear

• Notched Izod: 11.5 kJ/m² (about 5–6 ft·lb/in) on that specific product. Plaskolite.com+1

• Elongation at break: greater than 60 percent. Plaskolite.com+1

PETG feels less brittle in forming and bending than its Izod number suggests, and why it is widely sold as “less brittle than acrylic” with “good impact strength” for POP and O&P work. Curbell Plastics+1

Use PETG where you want clarity, deep draws and moderate abuse resistance, and you can live with the fact that it is not as tough as ABS or PC at sharp notches.

PETG is fairly mechanical, food safe and forms beautifully. It is relatively expensive.

ABS – genuinely tougher than HIPS and PETG

• 7.7 ft·lb/in notched Izod for ABS sheet. Curbell Plastics

• 20–25 percent elongation at break. Proleantech | Custom Parts On-Demand+1

In other words, ABS trades away some elongation vs HIPS and PETG but gains a factor of 3–4 in impact toughness.

So under the failure mode you care about most – notched impact on real parts – ABS is much less brittle than your HIPS and PETG.

Use ABS whenever you need durable housings, covers and structural panels that will see knocks, drops or regular handling, and you do not need the fire performance of KYDEX or the optical clarity of PC/PETG.

Due to cost, formability and impact resistance ABS is the choice of engineers for general purpose mechanical applications.

PC – “unbreakable” reputation is earned

• Notched Izod 12–16 ft·lb/in in the Curbell selection guide. Curbell Plastics+1

• Elongation at break ~90–100 percent for TECANAT PC. Ensinger+1

This combination is why polycarbonate is the default choice for machine guards, impact shields and safety glazing. It can absorb a lot of energy at a notch and will stretch significantly before it fails.

Use PC where cracking is simply not acceptable and you can tolerate, higher resin and tooling costs, more difficulty forming complex shapes, and some chemical stress cracking risks.

PC is hard and expensive to work with but is very tough stuff. Its hard to get in low volume in anything but clear.

KYDEX T – engineered to be even less brittle than FR-ABS

The KYDEX T data sheet spells it out: Mpfs+2Curbell Plastics+2

• Elongation (ASTM D638): 110 percent Mpfs+2Curbell Plastics+2

• Notched Izod: 15 ft·lb/in at 3.18 mm Mpfs+2Curbell Plastics+2

So KYDEX T sits in the same “very tough” range as PC.

• Fire performance (UL 94 V-0, 5V)

• Easier forming than PC in many cases

• FDA USP Class I-VI for medical devices and long term skin contact

Use KYDEX T when you want low brittleness plus FR requirements for interiors, transit and medical housings.

Kydex T is a premium plastic preferred for the toughest applications. It is easy to work with, has a beautiful finish and amazing mechanical properties. It is NOT CHEAP.

PP (copolymer sheet) – very ductile with high impact for the right grade

For the PP-copolymer sheet used in many thermoforming and O&P applications, S-Polytec reports: S-Polytec

• Elongation at break: 650 percent S-Polytec

• Izod of 12.5 ft·lb/in, in the same toughness band as Kydex and PC. Curbell Plastics+1

PP-copolymer has a high impact toughness, extreme ductility, and is very unlikely to fracture at room temperature.

Use PP-C where you want live food contact, hinges, snap details, chemical resistance and parts that deform rather than crack.

HDPE – the least brittle of the group

• Notched Izod: 1.5–14.1 ft·lb/in (wide range by grade and density) K-Mac Plastics

• Elongation at break: 500–1000 percent, typical around 840 percent K-Mac Plastics

So even if you take a conservative Izod number at the low end, the enormous elongation makes HDPE very resistant to sudden catastrophic fracture. It tends to yield, neck, and whiten long before it actually breaks.

Use HDPE when you want the lowest brittleness and excellent impact/abrasion resistance – bins, liners, wear parts, rough handling environments.

HDPE will bend and deform under load so it is not suitable for extreme stress. It can be easily crushed.

How to use this in real thermoforming decisions

1. If you must avoid brittle cracking at all costs
   Prioritize: HDPE, PP-C, KYDEX T, PC. Those four combine high impact energy with high elongation.

2. If you want a stiff, cosmetic panel and can accept some brittleness
   ABS is a safer choice than HIPS or PVC. HIPS and PETG should be treated as “moderately brittle” structurally, even though PETG is forgiving during forming.

3. If you are comparing HIPS, PETG and ABS
   For the grades we just walked through, the data support the following in terms of brittleness:

   • Most brittle: HIPS and PETG

   • Less brittle: ABS

   That lines up with your experience and the Curbell Izod data.

4. Always sanity-check with the exact grade
   These are typical values. For any critical design you should always pull the actual TDS for the sheet you buy to confirm:

   • Notched impact (ASTM D256 or ISO 180)

   • Elongation at break (ASTM D638 or ISO 527)