SAN Injection Moulding: Clear Engineering Plastic Guide
SAN (styrene-acrylonitrile) is the rigid, transparent matrix resin inside every ABS part — the same polymer, minus the rubber phase. Take out the butadiene rubber and you get optical clarity and higher stiffness; add it back and you get ABS's impact resistance but lose the transparency. For applications requiring glass-like appearance at injection-moulding cost with no drop-impact requirement, SAN is the practical choice.
What are the mechanical and thermal properties of SAN?
SAN's rigidity and clarity come from the tight molecular packing of the amorphous copolymer. The acrylonitrile content (typically 20–30% by weight) governs chemical resistance, heat performance, and colour — higher acrylonitrile deepens the slight yellow tint but improves heat deflection and solvent resistance.
| Property | Typical Value | Test Standard |
|---|---|---|
| Tensile Strength | 65–80 MPa | ISO 527 |
| Flexural Modulus | 3,500–4,200 MPa | ISO 178 |
| Izod Impact (notched) | 15–25 J/m | ISO 180 |
| Rockwell Hardness | M75–85 | ASTM D785 |
| Heat Deflection Temp (0.45 MPa) | 95–110°C | ISO 75 |
| Vicat Softening Point | 100–115°C | ISO 306 |
| Density | 1.07–1.08 g/cm³ | ISO 1183 |
| Mould Shrinkage | 0.3–0.7% | ISO 294-4 |
| Light Transmission | 88–90% | ASTM D1003 |
| Water Absorption (24 h) | 0.15–0.25% | ISO 62 |
SAN's flexural modulus (3,500–4,200 MPa) is notably higher than ABS (2,000–2,700 MPa), making it stiffer under bending. The trade-off is much lower impact strength — SAN is brittle by engineering standards and is not suitable for applications with drop or impact risk.
Where is SAN injection moulding used?
SAN is specified where transparency and stiffness at moderate cost are the design drivers, with no requirement for impact or drop resistance.
Housewares and consumer goods: Mixing bowls, measuring cups, refrigerator crisper drawers and door shelves, salad spinners, and storage containers. SAN's combination of clarity, food-contact compliance in certified grades, and dishwasher stability (HDT 95–110°C) makes it the workhorse of the kitchen-goods segment — a role it fills more cheaply than PMMA.
Packaging and dispenser components: Transparent dispenser bodies, pump covers, cosmetic jars, and personal care packaging. Glass-like appearance at a price point PMMA cannot match.
Automotive interior lighting and instrument clusters: Instrument panel lenses, light diffusers, and small transparent covers where rigid clarity in a dimensionally stable material is the specification.
Medical and laboratory equipment: Specimen containers, pipette stands, centrifuge tube racks, and diagnostic instrument windows. Some grades are available with gamma-radiation stability for sterilisable applications.
Display cases and point-of-sale fixtures: Retail display stands, protective covers, and transparent showcases. SAN's stiffness keeps large transparent panels from flexing, which GPPS cannot always guarantee.
Electrical component covers: Transparent protective covers for indicators, switches, and LED arrays, where rigidity and moderate chemical resistance cover the typical enclosure environment without the cost of PC.
What are the moulding characteristics of SAN?
SAN is easy to process — similar to polystyrene but with better flow predictability and wider melt temperature latitude. Its low sensitivity to processing variation is a practical advantage for transparent parts, where cosmetic consistency across the run matters.
Melt temperature: 200–260°C. Standard grades process well at 230–250°C. Degradation above 270°C discolours the melt — visible in clear parts — so residence time in the barrel should be kept short.
Mould temperature: 40–75°C. For the clearest parts, 60–75°C combined with a polished cavity surface is standard practice.
Injection pressure: 60–130 MPa. SAN flows well and fills moderate-complexity cavities readily. Jetting causes visible flow lines in transparent parts; controlled, gradual injection is preferred.
Drying: 2–3 hours at 80°C in a dehumidifying dryer before processing. Surface haze from moisture is the primary aesthetic failure mode in SAN. Unlike some engineering resins, SAN re-absorbs moisture quickly — material should be processed promptly after drying.
Shrinkage: 0.3–0.7%. Amorphous SAN exhibits near-isotropic shrinkage, which simplifies tool compensation. Exact values depend on gate design and pack pressure.
Draft angles: 0.5–1° on polished surfaces. Textured surfaces require 2–3° per 0.025 mm of texture depth. For optical parts, draft and surface polish are co-designed to avoid diffraction artefacts.
Sink and warp: Warp is low due to the amorphous structure. Sink marks appear at walls above 4 mm or at rib-to-wall junctions above 60%. Gate location is critical for optical homogeneity — gate shadows and knit lines are fully visible in clear parts and must be designed out, not processed around.
Which SAN grades and variants should you consider?
| Grade / Variant | Key Feature | Typical Application |
|---|---|---|
| Standard SAN | Clarity + rigidity baseline | Housewares, packaging, display |
| High-acrylonitrile SAN | Higher HDT, improved chemical resistance | Automotive, appliance components |
| Food-contact SAN | EU 10/2011 / FDA 21 CFR compliant | Kitchen goods, food packaging |
| Medical-grade SAN | Low extractables, gamma-stable options | Lab equipment, diagnostic covers |
| UV-stabilised SAN | Hindered amine stabilisers | Outdoor display, lighting diffusers |
| SAN + GF (10–20%) | Glass fibre reinforced | Structural transparent brackets (note: GF reduces clarity) |
Note that glass-filled SAN is not optically clear — the glass fibres scatter light significantly. GF-SAN is used for structural brackets and components where dimensional stability matters more than transparency.
Advantages and limitations of SAN
Advantages:
- Optical clarity superior to PP; comparable to lower grades of PMMA at lower cost
- Higher stiffness and better chemical resistance than polystyrene
- Good food-contact compliance in certified grades
- Near-isotropic shrinkage simplifies tool compensation
- Processes easily on standard equipment with short cycle times
Limitations:
- Brittle: notched impact strength is low; unsuitable for applications with drop or impact risk
- Yellows under prolonged direct sunlight in standard grades
- Attacked by ketones, esters, and chlorinated solvents
- Gate marks, knit lines, and flow marks are fully visible in transparent parts — gate design is not optional
- Not suitable for living hinges or snap-fits requiring high flex fatigue resistance
When to choose SAN over alternatives
SAN vs polystyrene (GPPS): Use SAN when chemical resistance, stiffness, or HDT above 90°C is needed. GPPS is marginally cheaper but less chemical-resistant and slightly less stiff — not enough of a saving to justify it for most transparent applications.
SAN vs PMMA: Use SAN when impact tolerance or complex geometry at lower cost matters more than maximum optical perfection. PMMA is preferred when scratch resistance, the highest optical clarity, or UV stability in an optical component is the priority.
SAN vs PC: Use SAN for cost-sensitive transparent applications at modest stress levels. PC is preferred when high impact resistance, service above 110°C, or thin-wall structural integrity is required.
SAN vs ABS: The choice is simple — use SAN when transparency is required, ABS when impact resistance matters and opacity is acceptable.
Nordmould regularly guides customers through the SAN vs PMMA vs PC decision based on the actual optical, structural, and chemical demands of the application.
Is SAN recyclable?
SAN carries resin code 7 (Other) and is not collected in residential streams. Industrial mechanical recycling is practised in packaging and consumer goods sectors. Recycled SAN retains acceptable stiffness but loses clarity with each reprocessing cycle, limiting it to opaque or lightly coloured secondary applications. Food-contact certification does not extend to recycled grades without separate compliance testing.
Frequently asked questions
Is SAN optically clear?
Yes. SAN is an amorphous copolymer with good optical clarity, light transmission around 88–90%, and a slightly blue-white tint. It is clearer than polypropylene and stiffer than polystyrene. Nordmould uses SAN for transparent housings, lenses, and display covers where optical quality is required at a lower cost than PMMA or PC.
What is the shrinkage rate of SAN in injection moulding?
SAN shrinks 0.3–0.7%, comparable to ABS. The near-isotropic shrinkage of this amorphous material makes tool compensation straightforward. Nordmould factors the exact grade shrinkage into tool design.
Does SAN require drying before moulding?
Yes. SAN should be dried at 80°C for 2–3 hours before processing. Moisture causes surface haze, splay marks, and reduced clarity — a visible defect in transparent parts. SAN re-absorbs moisture quickly after drying; process promptly.
How chemically resistant is SAN?
SAN has better chemical resistance than polystyrene and resists dilute acids, alkalis, and aliphatic hydrocarbons. Ketones, esters, and chlorinated solvents attack it. SAN is appropriate for housewares and food packaging that contacts mild cleaning agents, not for solvent-exposure environments.
Can SAN be used for food-contact applications?
Food-contact compliant SAN grades certified to EU 10/2011 and FDA 21 CFR 177.1640 are widely available. SAN is used for dishwasher-safe cups, mixing bowls, blender carafes, and refrigerator fittings. Confirm the specific grade data sheet during DFM review.
How does SAN compare to PMMA for transparent parts?
SAN is tougher and easier to process than PMMA, with lower melt viscosity and less sensitivity to processing variation. PMMA offers superior scratch resistance and slightly higher optical transmission (around 92% vs 88–90%). Use SAN when geometry complexity or modest impact tolerance matters more than optical perfection.
Is SAN the same as the matrix resin in ABS?
Yes. The rigid SAN copolymer is the matrix phase of ABS; the polybutadiene rubber phase converts SAN into ABS. SAN without rubber retains the clarity and hardness that the rubber phase eliminates. For transparent applications requiring some impact tolerance, discuss whether SAN or a specialist transparent ABS variant is appropriate.
What is the minimum order quantity for SAN parts at Nordmould?
Nordmould's minimum order quantity is 100 pieces, applicable to SAN and all other materials. Tooling starts from €3,000, with aluminium bridge tooling available for prototype and bridge volumes.
Send your STEP file to Nordmould for a free DFM review and written quote — typically returned within one business day.