Polystyrene Injection Moulding: GPPS and HIPS Material Guide
Polystyrene is a low-cost, easy-to-process amorphous thermoplastic available in two primary injection-moulding grades: general-purpose polystyrene (GPPS) and high-impact polystyrene (HIPS). GPPS is optically transparent and stiff; HIPS sacrifices clarity for significantly improved toughness. Both grades flow easily, shrink predictably, and tool well, making them a competitive choice for consumer goods, packaging, and display components. Nordmould moulds both grades for orders from 100 pieces.
What are the mechanical and thermal properties of GPPS and HIPS?
| Property | GPPS | HIPS |
|---|---|---|
| Tensile strength | 35–55 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) | 85–100 °C | 75–90 °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 |
| Shrinkage | 0.3–0.6 % | 0.3–0.6 % |
| Clarity | Water-clear | Opaque |
Both grades are amorphous, meaning they have no sharp melting point and exhibit low, isotropic shrinkage — an advantage when dimensional consistency matters. Neither grade is suited to extended service above 80 °C.
What are typical applications of polystyrene injection moulding?
PS's combination of low cost, easy flow, good dimensional stability, and (for GPPS) optical clarity drives its use in high-volume consumer and packaging applications.
GPPS applications:
- Transparent food packaging: lids, containers, display trays, and covers
- Petri dishes, test-tube racks, and laboratory disposables where optical clarity is needed at low cost
- Point-of-sale display stands, picture frames, and light diffuser panels
- Cosmetics packaging where glass-like clarity is required at a lower price point than PMMA
HIPS applications:
- Consumer electronics housings: printer covers, television back panels, appliance fascias
- Refrigerator inner liners (thermoformed HIPS) and appliance internal components
- Toys and model components where coloured, stiff, toughened parts are needed at low cost
- Point-of-sale shelving, card holders, and retail display elements
- Protective packaging inserts where EPS (expanded PS foam) and HIPS housings are combined
How is polystyrene processed in injection moulding?
PS is one of the most forgiving injection-moulding materials. Its low melt viscosity and amorphous structure mean it fills easily, cools quickly, and ejects cleanly. Cycle times are typically short.
| Processing parameter | GPPS | HIPS |
|---|---|---|
| Melt temperature | 200–260 °C | 200–250 °C |
| Mould temperature | 20–50 °C | 20–50 °C |
| Injection pressure | 70–130 MPa | 70–120 MPa |
| Shrinkage | 0.3–0.6 % | 0.3–0.6 % |
| Recommended draft angle | ≥ 1° per side | ≥ 1° per side |
Sink and warp: PS's low shrinkage means it is much less susceptible to warp than semi-crystalline plastics. Sink marks are the more common defect; they are addressed by reducing wall thickness in heavy sections (keep walls 1.5–3.5 mm) or adding gas-assist if section elimination is not possible.
Brittleness in GPPS: the principal processing concern with GPPS is residual stress. High injection speeds, cold moulds, and sharp inside corners all raise residual stress, which manifests as brittle fracture or crazing under load or solvent contact. Nordmould recommends internal radii of at least 0.5 mm and mould temperatures at the higher end of the range for stress-critical GPPS parts.
Clarity for GPPS: to achieve water-clear parts, the tooling surface must be polished and the melt temperature must be sufficient to eliminate flow lines. Weld lines remain visible in transparent GPPS and require careful gate placement to push them to non-visible areas.
Colouring: both grades accept a wide range of masterbatch colours and can be produced in virtually any colour. PS is easy to pigment uniformly, making it a favourite for branded consumer products.
What grades and variants of polystyrene are available?
| Grade | Key feature | Typical use case |
|---|---|---|
| GPPS natural/clear | Optical clarity, low cost | Transparent packaging, display items |
| GPPS food-contact | FDA/EU 10/2011 certified | Food containers, cutlery |
| HIPS natural | Tough, opaque | Consumer electronics, toys |
| HIPS fire-retardant (UL94 V-0) | Flame retardant | Electrical housings, IT equipment |
| HIPS food-contact | FDA/EU 10/2011 certified | Refrigerator liners, food trays |
| SAN (styrene-acrylonitrile) | Higher chemical & heat resistance vs GPPS | Housewares, battery cases |
| ABS | Higher impact & heat vs HIPS | See ABS material guide |
SAN is a closely related styrene copolymer offering better chemical resistance and slightly higher service temperature than GPPS, at the cost of clarity — SAN is slightly hazy. It is positioned between GPPS and ABS.
ABS is often the natural upgrade path from HIPS when higher impact strength, better heat resistance, or easier painting and bonding is required.
What are the advantages and limitations of polystyrene?
Advantages:
- Very low material cost — PS is among the cheapest injection-moulding materials
- Low, isotropic shrinkage (0.3–0.6 %) supports good dimensional consistency
- Easy, forgiving processing with short cycle times
- GPPS is water-clear with very good light transmission (~90 %)
- Excellent colourability with standard masterbatch systems
- Dries quickly; no pre-drying required in most cases
Limitations:
- GPPS is brittle — very low notched impact strength (1–2 kJ/m²)
- Poor resistance to solvents, oils, and fats — stress cracking is a real service risk
- Low heat resistance — not suitable above 80 °C in sustained service
- No UV stability in standard grades; yellows and embrittles outdoors
- Not suitable where strong acids, aromatic solvents, or ketones are present
- PS recycling infrastructure is poor compared with PP or HDPE; EPS is better recycled than injection-moulded PS in practice
When should you choose polystyrene over alternative materials?
Choose GPPS over PMMA when optical clarity is needed but cost is constrained and scratch resistance is not critical. PMMA is harder, more scratch-resistant, and has better UV stability; GPPS is cheaper and easier to process. For premium outdoor displays, PMMA wins. For indoor disposables, GPPS wins on cost.
Choose HIPS over ABS when cost is the primary driver and the part is not exposed to high temperatures, impacts, or solvents. ABS outperforms HIPS on every mechanical metric but costs more per kilogram and per tool.
Choose PS over PP when low shrinkage and tight dimensions are required on a flat or thin-walled part. PP's higher shrinkage (1.0–2.0 %) makes it harder to hold to close tolerances on amorphous geometries; PS is more predictable.
Avoid PS in applications where solvent or fat exposure is possible, where the part must operate above 80 °C, or where long-term outdoor weathering is required.
Recyclability and sustainability
Polystyrene carries resin code 6 and is technically recyclable, though real-world collection and reprocessing infrastructure for PS is poor compared with PP or HDPE. Some EU member states do collect and recycle rigid PS packaging. In the moulding process, PS sprues and runners can be reground and reblended at up to 20–25 % without significant property loss in non-optical applications. Chemical recycling of PS back to styrene monomer (depolymerisation) is an emerging technology. If sustainability is a key project requirement, Nordmould can discuss material alternatives including bio-based PS grades or recyclability-by-design approaches.
Submit your part design to Nordmould for a free DFM review — we'll confirm whether GPPS or HIPS is the right grade, review wall thickness, and return an indicative quote within one business day.
Frequently asked questions
What is the difference between GPPS and HIPS in injection moulding?
GPPS (general-purpose polystyrene) is water-clear and rigid but brittle — notched Izod impact strength is only 1–2 kJ/m². HIPS (high-impact polystyrene) blends polybutadiene rubber into the PS matrix, raising impact strength to 5–12 kJ/m² at the cost of clarity. Nordmould recommends GPPS for transparent aesthetic parts and HIPS for opaque housings needing toughness.
Is polystyrene food-safe for injection moulded parts?
Both GPPS and HIPS are approved for food contact in specific EU and FDA grades — cutlery, yoghurt containers, and disposable cups are among the most common food-contact PS applications. Confirm food-contact requirements with Nordmould during the DFM review so the correct certified resin is sourced.
Why does polystyrene crack or craze in service?
PS is sensitive to stress cracking caused by contact with fats, oils, and many solvents (including acetone, esters, and aromatic hydrocarbons), especially under residual moulding stress. Reducing injection speed, increasing mould temperature, and avoiding sharp corners reduces residual stress. Nordmould flags chemical-contact risks during DFM.
Can polystyrene be used for outdoor applications?
Standard PS is not UV stable and yellows and embrittles within months outdoors. UV-stabilised grades and ASA (an outdoor-grade PS alternative) are available. For long-term outdoor exposure, Nordmould would typically recommend ABS or ASA rather than PS, unless cost is the overriding constraint.
What tolerances can be held on polystyrene injection-moulded parts?
PS shrinks only 0.3–0.6 %, which is among the lowest of commodity plastics. This low shrinkage, combined with a rigid amorphous structure, enables relatively tight dimensional tolerances — PS parts are easier to hold to close dimensions than semi-crystalline plastics such as PP or PE. Exact tolerance figures are confirmed at quote stage.
How much does polystyrene tooling cost at Nordmould?
Polystyrene tooling starts from €3,000 for simple single-cavity bridge tools. PS is not abrasive, so aluminium tooling is fully viable for low-to-mid volumes, keeping costs down. Production orders from 100 pieces upward are available with lead times of 4–11 weeks depending on tool complexity.
Is HIPS suitable for vacuum forming as well as injection moulding?
HIPS is one of the most common vacuum-forming materials. However, this guide covers only injection moulding. Nordmould processes HIPS exclusively by injection moulding; if vacuum-formed HIPS parts are needed for the same project, that process is handled outside Nordmould's scope.