High-Impact Polystyrene Sheet

High-impact polystyrene is the workhorse of signage, displays and thermoforming - cheap, easy to cut, easy to form, easy to print, and tough enough to handle. The impact modifier is what separates it from ordinary polystyrene: notched Izod runs 2.1 to 2.8 ft-lb/in against general-purpose polystyrene's 0.3 or so, which is the difference between a sheet you can work with and one that cracks when you look at it.

Published figures vary more by grade than most materials, so these are the defensible middle: tensile strength at yield about 3,000 psi, elongation at break around 50 percent, flexural modulus 240,000 to 310,000 psi, notched Izod 2.1 to 2.8 ft-lb/in, Rockwell M63. Specific gravity 1.04, so a 48 by 96 inch sheet runs about 5.4 lb per square foot per inch of thickness. Heat deflection is 185F at 264 psi and Vicat softening is around 210F.

Where it goes: point-of-purchase displays and signage, thermoformed packaging and trays, machine guards and equipment covers, appliance and refrigerator liners, model and prototype work, and backing or separator sheet anywhere cheap rigid plastic does the job.

Thermoforming is what most of this sheet is bought for. The forming range is about 300 to 350F, it draws deeply and faithfully, and mold shrinkage runs 0.003 to 0.006 inch per inch. One practical caveat: a gloss face loses some of its gloss through a deep draw, so if the finished part has to look glossy, plan for that or form the matte side out.

Working with it: it saws, routers and die-cuts cleanly with sharp tooling and does not need much coolant. It solvent-bonds and solvent-welds easily with styrene cements, which is a real advantage over polyethylene - but the flip side is that it is attacked by a long list of common solvents including ketones, esters, aromatic and chlorinated hydrocarbons, so be careful what cleaner goes near a finished part. It takes screen and digital ink well, better on the matte face.

The honest weakness, and it is a big one: HIPS is poor outdoors. Unprotected polystyrene yellows, chalks and embrittles under ultraviolet, and it happens in months rather than years. This is indoor material. If you need a formable printable sheet that will live outside, ask us about the alternatives rather than painting this and hoping.

On service temperature we would rather be straight with you: there is no published continuous-use rating for commodity HIPS sheet - no RTI, no UL 746B data on any datasheet we could find - and suppliers cite anywhere from 140F to 180F as a maximum. Heat deflection is 185F at 264 psi, which is a test result rather than a service rating. If your application gets warm, tell us what temperature and we will find you something with a real number behind it.

On ratings and approvals we claim nothing we cannot back. There is no UL 94 rating on this sheet; polystyrene burns, with an autoignition temperature published around 914F, and no building-code flame class applies to it as supplied. Food-grade HIPS exists as a separate specification, but this is not sold as it and we make no FDA or food-contact claim. If you need either a flame rating or food contact, that is a different material and we will quote it.