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PS

Poliestireno

PS·Polystyrenes·Amorphous

PS (Polystyrene, also called GPPS or "crystal") is the cheapest transparent thermoplastic on the market, and the founding father of an entire family of polymers including HIPS (impact-modified), ABS (acrylonitrile butadiene styrene), ASA, SBR, SBS and EPS (foam/styrofoam). With nearly 92% light transmission, easy to process at low temperature, no drying requirement in normal climates and at 30–50% the cost of PMMA, it is the material par excellence for CD/DVD cases, disposable transparent cups, electronics blisters, low-cost commercial displays and thousands of applications where crystal clarity at the lowest possible price is the goal.

But it has a major weakness: it is brittle to impact (much more than PMMA, no comparison with PC), tends to develop stress cracking from contact with oils/greases/UV, and there is ongoing debate about styrene monomer migration in food-contact applications with fats or heat. Here we have compiled the reference ranges from the PDS, plus the questions that come up over and over on the shop floor: GPPS vs HIPS vs ABS vs EPS, food-safe yes but with asterisks, code #6 and real recyclability, and when PS vs PMMA makes sense.

Share your experience in the comments — ranges vary by manufacturer and grade, and collective discussion is what gets us out of trouble on the floor.

The ranges shown in these data tables were compiled by the MVPS team from various parameter sheets and literature, integrating the lower and upper limits for each material type.

This information must be carefully reviewed when developing injection molding processes. Final ranges and processing tolerances are the responsibility of the engineer in charge.

These ranges are not recommended for developing specific process tolerances. MVPS always recommends requesting and consulting the supplier's data sheet.

General Properties

Chemical StructureAmorphous
Specific Gravity (Density)1.05:1
L/D Ratio17 – 24
Compression Ratio1.5 – 4
Tonnage Factor1.54 – 6.18kN/cm²
Thermal Diffusivity0.1505mm²/s
Max Shear Rate40,0001/s
Shrinkage0.1 – 0.7%
Regrind25%
Heat Deflection (HDT) @ 1.82 MPa71°C
Glass Transition (Tg) @ 10°C/min90°C
Vicat Softening @ 50N98°C

Drying

Drying Temperature74 – 79°C
Drying Time1 – 2h
Recommended Moisture0.05%
Recommended Dryer TypeAir
Dew Point-40°C

Temperatures

Melt166 – 229°C
Nozzle166 – 229°C
Front166 – 229°C
Middle154 – 216°C
Rear146 – 199°C
Demolding38 – 82°C
Mold (Cooling)21 – 71°C
Feed Throat35 – 79°C

Processing

Back Pressure3.4 – 5.2bar
Screw Speed60 – 150RPM
Injection SpeedHigh
Barrel Occupancy15 – 85%
Injection Pressure350 – 1,400Pbar
Holding Pressure88 – 1,120Pbar
Cushion6.4 – 12.7mm

Mold

Runner Diameter3.05 – 6.1mm
Gate Diameter0.76 – 1.52mm
Gate Area0.46 – 1.82mm²
Wall Thickness0.99 – 3.99mm

Venting

Depth (Vent Depth)0.0102 – 0.0203mm
Land (Vent Land)0.508 – 1.02mm
Width (Vent / Clearance)3.05 – 7.62mm
Relief (Relief Channel)0.127 – 0.254mm

Frequently asked questions

PS (Polystyrene) is an amorphous thermoplastic synthesized by polymerization of styrene monomer (vinylbenzene) — a liquid derived from benzene and ethylene. Its linear chain with pendant aromatic rings gives it optical transparency (88–92%), rigidity, inherent brittleness and very low cost. Density ~1.04–1.05 g/cm³. It exists in grades GPPS (General Purpose, crystal transparent), HIPS (High Impact, opaque with rubber), EPS/XPS (expanded, known as styrofoam) and SAN (with acrylonitrile, better chemical resistance).
GPPS (crystal PS): transparent, rigid, brittle, cheap. CD cases, disposable cups, displays. HIPS (High Impact): opaque with 5–10% dispersed polybutadiene that absorbs impacts. 5–10× tougher than GPPS, matte. Refrigerators, economical toys, yogurt cups. ABS: acrylonitrile + butadiene + styrene — better impact/rigidity/surface balance but 30–50% more expensive than HIPS. Premium housings, toys (LEGO). EPS/XPS: PS expanded with gas (white/blue styrofoam). Thermal insulation, fragile packaging. Rule of thumb: need clarity → GPPS. Need impact + opaque finish → HIPS. Need premium finish + impact → ABS. Need insulation or cushioning → EPS/XPS.
Yes, FDA and EFSA approve it for food contact in normal use. PS migrates micro-amounts of residual styrene monomer into food — the FDA set a limit of 0.1 mg/L in water and commercial products are 10,000× below that limit. But there are nuances: heating PS significantly increases migration (don't microwave PS cups not marked 'microwave safe'), and fatty/oily foods extract more monomer than aqueous ones. Current consensus: PS is safe for short cold disposable use, doubtful for repeated hot or fatty foods. For reusable tupperware/microwave, always PP (#5).
Its rigid aromatic ring and the absence of elastomeric chains make it inherently brittle — it breaks on impact like glass. It is the main reason for the existence of HIPS and ABS (which add rubber to fix this). In GPPS part design: (1) avoid sharp angles (radii ≥1 mm), (2) uniform walls (<15% variation), (3) avoid stress-concentrating corners in load zones, (4) adequate holding pressure for thin parts (cups), (5) consider HIPS if you will have any impact during use. Stress cracking appears especially with contact with oils, fats, alcohols and prolonged direct UV.
Technically yes, practically no in many countries. Recycling code #6. The problem: rigid PS can be processed but few municipal centers accept it (especially food-soiled cups); EPS (styrofoam) takes 90% volume vs 10% weight, making it uneconomical to transport to recycling centers. Many cities worldwide have banned EPS in food packaging (New York, San Francisco, several European countries). Clean rigid PS (CD cases, commercial blisters) is recycled, but less than 10% globally. The PDS marks 25% regrind without significant property loss.
PS wins on: cost (~30–50% cheaper), processing ease (lower temperature, flows better), no drying in normal climates, faster cycle. PMMA wins on: optical clarity (92% vs 88% for PS), UV resistance (PMMA doesn't yellow, PS yellows in 6–12 months), much better scratch resistance, better chemical resistance, biocompatible (PS is not). Rule of thumb: for disposable or low-cost interior transparent parts (cases, packaging) → PS/GPPS. For premium transparent, outdoor, optical or signage parts → PMMA. For parts that see UV or solvents → never PS.
The PDS marks 38–71°C. Hotter (50–65°C) = better gloss, less residual stress, better detail reproduction, but longer cycle. Cooler (30–45°C) = short cycle, ideal for high-volume production runs of disposable cups and blisters. For cosmetic or transparent optical parts: 50–60°C. For cheap disposable parts: 30–40°C maximizes throughput. Never below 30°C — risk of frozen stress that causes brittleness and crazing weeks later.
Normally no in temperate climates. PS absorbs very little moisture (<0.05% at equilibrium). The PDS marks 'Air' as the dryer type. In tropical/coastal climates (sustained humidity >70%), a light drying at 60–70°C for 1–2 h can help avoid silver streaks in optical transparent parts. If material came in sealed bag and you opened it just to use, you can skip it. If exposed to humid shop floor for more than 2 hours in tropical climate, preventive drying.
UV radiation attacks the aromatic rings of styrene, causing oxidative degradation that produces progressive yellowing, brittleness and gloss loss. A GPPS outdoor part visibly yellows in 6–12 months, much worse than PMMA. For outdoor use, do NOT use PS — switch to PMMA (native UV resistant) or ASA (acrylonitrile-styrene-acrylate, formulated for weathering). If you insist on styrene for cost, there are PS grades with UV stabilizers (HALS) that extend service life to 2–3 years, but they still don't compete with PMMA.
Despite environmental criticism, PS still dominates: CD/DVD cases (transparency + rigidity + cost), disposable transparent cups (clarity + cheap + light), blisters for electronics/cosmetics (heat-formed from sheet), refrigerator interiors (HIPS) — most fridge liners are thermoformed HIPS, yogurt cups (printed HIPS), hangers and cheap trays (HIPS), scale modeling (Tamiya, Revell) (molded GPPS + colors), short advertising panels (interior signage), and all the EPS/styrofoam packaging and insulation. When cost dominates and long durability isn't critical, PS wins.

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