Polystyrene Injection Moulding: GPPS and HIPS Guide
Polystyrene gets less attention than engineering polymers, but it is one of the most commonly moulded materials in the world. GPPS is water-clear, dimensionally stable, and cheap. HIPS adds rubber toughening to produce an opaque, durable material that handles drops and assembly stress that would shatter GPPS. Between them they cover a large slice of consumer electronics, medical disposables, packaging, and appliance interiors.
Mechanical and Thermal Properties
Polystyrene is amorphous—no sharp melting point, very low and isotropic shrinkage, predictable dimensions. The flip side is that it is not a high-temperature material. Most grades begin to soften above 80°C, which limits its use to room-temperature and mild-heat environments.
| Property | GPPS | HIPS |
|---|---|---|
| Tensile strength | 45–55 MPa | 20–40 MPa |
| Flexural modulus | 3,000–3,500 MPa | 1,800–2,700 MPa |
| Elongation at break | 1–4% | 15–50% |
| Notched Izod impact | 1–2 kJ/m² | 5–12 kJ/m² |
| Heat deflection temp (0.45 MPa) | 85–95°C | 75–85°C |
| Continuous service temp | 60–80°C | 60–75°C |
| Water absorption (24 h) | 0.05–0.1% | 0.05–0.1% |
| Density | 1.04–1.06 g/cm³ | 1.02–1.06 g/cm³ |
| Mould shrinkage | 0.3–0.6% | 0.3–0.6% |
The rubber content that makes HIPS tougher also reduces its tensile strength and HDT compared to GPPS. The trade-off is deliberate—HIPS is designed for housings that take knocks, not for stiffness or heat resistance.
Common Applications
GPPS (Clear):
- Medical and lab disposables—Petri dishes, test tubes, and clear tray covers.
- Consumer packaging—rigid boxes, display cases, and jewellery packaging.
- Point-of-sale—signage holders and display stands.
HIPS (Opaque):
- Electronics—printer housings, TV frames, and remote control shells.
- Appliances—refrigerator liners and interior panels, appliance bezels.
- Toys and models—figure kits and coloured plastic components where cost is the primary driver.
Moulding and Design Guidelines
PS is an easy-flowing, fast-cooling material that forgives most process parameter variations. The primary design discipline is around brittleness and stress cracking.
- Shrinkage (0.3–0.6%): Very predictable. Warping and bowing common with PP or PE are rarely a problem with PS. This makes it a good choice for flat panels that must stay flat.
- Gate design: For clear GPPS, gate placement is critical—flow lines and weld lines are visible in the finished part. Gate into non-aesthetic zones, and keep fill paths short to minimise weld-line formation.
- Wall thickness: 1.5–3.5 mm covers most applications. PS cools quickly and cycle times are generally shorter than with PC or Nylon.
- Draft angles: Minimum 1°. PS is rigid and does not "give" on ejection—insufficient draft produces white stress marks (crazing) on the pulled surface.
- Radii: Particularly important for GPPS—sharp internal corners are where craze cracks initiate. A minimum 0.5 mm radius on all internal edges is standard.
Material Grades
| Grade | Feature | Typical Use |
|---|---|---|
| GPPS Clear | High clarity, low cost | Packaging, clear lids |
| HIPS Natural | Tough, rigid, opaque | Housings, frames |
| Flame-Retardant HIPS | UL94 V-0 rated | Electrical equipment |
| Food-Grade | FDA/EU certified | Cutlery, yogurt pots |
| High-Gloss HIPS | Superior surface finish | Consumer electronics |
For parts that need chemical resistance but must remain optically clear, SAN (Styrene Acrylonitrile) is a natural next step—it sits between GPPS and ABS in both cost and performance.
Advantages and Limitations
Why choose Polystyrene:
- Among the lowest material costs of any moulded plastic.
- Excellent dimensional stability (very low, isotropic shrinkage).
- Easily pigmented for bright, consistent colours—HIPS takes colour well.
- GPPS gives water-clear transparency at lower cost than PMMA.
What to watch for:
- Brittleness: GPPS will shatter if dropped or bent sharply. HIPS is tougher but still not for high-impact applications.
- Chemical sensitivity: Fats, oils, and solvents cause stress cracking. Avoid contact with these in service and during assembly.
- Low heat resistance: Neither grade is suitable for dishwashers, hot environments, or proximity to heat sources.
- Poor UV stability: Will yellow outdoors within months.
Choosing PS vs Alternatives
- PS vs ABS: Choose HIPS if cost is the primary driver. Choose ABS if you need better heat resistance, impact toughness, or a surface that will be painted.
- PS vs PMMA: Choose GPPS for low-cost disposables or simple display parts. Choose PMMA for better scratch resistance and clarity in premium, long-life optical products.
Sustainability
Polystyrene carries resin code 6. Industrial regrind is easy to reprocess and up to 20–25% regrind can typically be incorporated in non-aesthetic parts. General consumer recycling infrastructure for PS is limited compared to PE or PP, though this varies by region.
Nordmould provides tooling from €3,000 and low-volume production starting at 100 pieces. Send us your STEP file for a free DFM review—we'll confirm the best grade for your part and return a quote typically within 24–48 hours.