Polystyrene comes in two main forms. General-purpose polystyrene (GPPS) is clear, stiff and brittle, used for cases, cups, displays and lab ware. High-impact polystyrene (HIPS) adds rubber for toughness and is opaque, used for appliance liners, trays and printer parts.
Typical Polystyrene properties
| Property | GPPS | HIPS | Test method |
|---|---|---|---|
| Density | 1.04–1.05 g/cm³ ~0.038 lb/in³ | 1.03–1.05 g/cm³ 0.037–0.038 lb/in³ | ASTM D792 / ISO 1183 |
| Tensile strength | 35–50 MPa 5,100–7,300 psi | 18–30 MPa 2,600–4,400 psi | ASTM D638 / ISO 527 |
| Flexural modulus | 3.0–3.3 GPa 435,000–479,000 psi | 1.6–2.4 GPa 232,000–348,000 psi | ASTM D790 / ISO 178 |
| Notched Izod impact | 15–25 J/m 0.3–0.5 ft·lb/in | 60–150 J/m 1.1–2.8 ft·lb/in | ASTM D256 |
| Heat deflection, 1.8 MPa (264 psi) | 75–90°C 167–194°F | 70–85°C 158–185°F | ASTM D648 / ISO 75 |
| Water absorption, 24 h | 0.03–0.10% | 0.05–0.10% | ASTM D570 / ISO 62 |
| Mold shrinkage | 0.3–0.6% 0.003–0.006 in/in | 0.4–0.7% 0.004–0.007 in/in | ASTM D955 / ISO 294-4 |
Typical ranges for standard grades, dry as molded, from published data sheets. Your grade’s own data sheet and the lot COA are what count. Notched Izod uses 3.2 mm (1/8 in) bars.
Processing and drying
| Processing | GPPS | HIPS |
|---|---|---|
| Melt temperature | 356–500°F / 180–260°C | 356–480°F / 180–249°C / melt should not exceed 464°F / 240°C |
| Mold temperature | 50–150°F / 10–66°C | 50–150°F / 10–66°C |
| Drying | 160–167°F / 71–75°C, only if needed, 1–2 h; never >12 h | 160–167°F / 71–75°C, only if needed, 1–2 h; never >12 h |
| Target moisture | No published maximum | No published maximum |
| Max barrel residence | Not published | Not published |
| General | GPPS | HIPS |
|---|---|---|
| Clarity | Clear | Opaque |
| Chemical resistance | Poor to fair: attacked by many solvents | Poor to fair: attacked by many solvents |
Starting points only. See the full processing temperature chart and drying guide for every resin.
What changes these numbers
- Rubber is the difference. HIPS trades stiffness, clarity and some heat resistance for several times the impact strength.
- Chemicals. Oils, fats and many solvents attack polystyrene. Refrigerator liners use special stress-crack-resistant HIPS.
- Drying. Polystyrene does not absorb water. Dry only to remove surface moisture.
- Low shrinkage. About 0.3–0.8%, so parts hold tight dimensions.
Common Polystyrene grades
- GPPS by melt flow
- Medium- and high-impact HIPS
- Flame-retardant HIPS
- Stress-crack-resistant HIPS for refrigerator liners
Common questions
What is the density of polystyrene?
About 1.03–1.05 g/cm³ (0.037–0.038 lb/in³) for both GPPS and HIPS.
GPPS or HIPS?
GPPS when you need clarity and stiffness. HIPS when the part has to take a knock. See GPPS vs. HIPS.
Does polystyrene need drying?
No, only for regrind, humid storage or condensation. Then about 71–75°C for 1–2 hours.
Need Polystyrene, or something close?
A polymer chemist reviews every request, then matches your grade or finds a qualified equivalent and prices it.
What to send for an accurate quote
- The grade name, from any supplier, or a data sheet
- Color (natural, black or a custom match)
- Fill type and percentage, if any
- Melt flow, if you know it
- Quantity for this order and what you use in a year
- Ship-to city and state or province