PET Injection Moulding: Properties, Grades & Design Guide
Injection-moulded PET (polyethylene terephthalate) is a semi-crystalline polyester specified for structural applications demanding high stiffness, chemical resistance, and dimensional stability. It is not the same product as beverage-bottle PET — injection-moulding grades are nucleated to crystallise rapidly at elevated mould temperatures, producing opaque, semi-crystalline parts with heat deflection temperatures above 210°C in glass-filled form. PET injection moulding is used for food-processing equipment, precision connector bodies, pump and filter housings, and automotive fuel system components where the combination of strength, chemical inertness, and compliance is hard to match with alternatives.
What are the mechanical and thermal properties of injection-moulded PET?
PET's semi-crystalline structure is the source of both its strengths and its processing demands. Full crystallinity requires controlled mould temperatures and nucleation additives. The properties below reflect properly crystallised, injection-moulding-grade PET.
| Property | Unfilled PET (nucleated) | 30% GF PET | Test Standard |
|---|---|---|---|
| Tensile Strength | 55–75 MPa | 140–165 MPa | ISO 527 |
| Flexural Modulus | 2,800–3,200 MPa | 9,000–12,000 MPa | ISO 178 |
| Izod Impact (notched) | 25–45 J/m | 60–90 J/m | ISO 180 |
| Heat Deflection Temp (1.80 MPa) | 70–80°C | 210–225°C | ISO 75 |
| Melting Point | 250–265°C | 250–265°C | DSC |
| Density | 1.33–1.38 g/cm³ | 1.55–1.65 g/cm³ | ISO 1183 |
| Mould Shrinkage | 1.5–2.2% | 0.3–0.8% | ISO 294-4 |
| Water Absorption (24 h) | 0.10–0.20% | 0.05–0.10% | ISO 62 |
| Dielectric Strength | 15–20 kV/mm | 15–20 kV/mm | IEC 60243 |
The 30% GF grade achieves a flexural modulus of 9,000–12,000 MPa — among the highest of the commodity engineering thermoplastics — which matters for thin-section structural parts where deflection under load is a design constraint.
Where is PET injection moulding used?
Injection-moulded PET occupies a specialist niche: applications requiring the stiffness and heat resistance of glass-filled polyester combined with food-contact compliance, chemical inertness, or both.
Food-processing and food-contact equipment: Conveyor chain links, valve components, pump impellers, nozzle bodies, and filter housings that contact food or beverages during processing. GF-PET grades meeting EU 10/2011 are used here because they combine food-contact compliance with heat and chemical resistance that HDPE or PP cannot match.
Medical device components and pharmaceutical packaging: Sterilisable trays, device enclosure panels, and packaging components for pharmaceutical manufacturing. GF-PET withstands ethylene oxide and gamma sterilisation at dose levels used in medical device manufacture.
Automotive fuel and fluid system components: Fuel pump housings, fuel-line connectors, and emission control system parts. PET's resistance to petrol, diesel, and aromatic hydrocarbons makes it suitable where polyolefins fail due to fuel permeation.
Precision electrical connector bodies: Where the combination of high glass content (for stiffness and dimensional stability), low moisture absorption, and SMT compatibility is required — particularly in high-pin-count connectors where even 0.1 mm of dimensional drift causes mis-mating.
Industrial pump and filter housings: Centrifugal pump bodies, filter bowls, and valve seats for chemical process equipment. PET resists a broad range of acids, alkalis, and organic solvents in mild to moderate chemical process environments.
Precision structural brackets and housings: Instrument components, measurement equipment frames, and brackets where the high flexural modulus of GF-PET minimises deflection in thin-section designs.
What are the moulding characteristics of PET?
PET is the most process-demanding of the standard polyester thermoplastics. It rewards disciplined process control with excellent mechanical properties; it punishes moisture, incorrect mould temperature, or extended residence time with severely degraded parts.
Melt temperature: 270–290°C. PET's melting point (250–265°C) is higher than PBT's (220–225°C). The barrel temperature must be set above the crystalline melt with minimal overshoot — thermal degradation and discolouration begin above 300°C.
Mould temperature: 100–140°C. This is the most critical and distinctive aspect of PET injection moulding. High mould temperatures are required to achieve full crystallisation — parts moulded at 40–60°C are amorphous, brittle, and dimensionally unstable. Hot-runner tooling is strongly preferred to maintain melt temperature without extended residence in cold runners.
Injection pressure: 80–160 MPa. GF-PET is highly viscous; adequate injection pressure is needed to fill complex cavities. Multiple gates are standard on larger parts to prevent short shots and minimise weld-line formation.
Drying: 4–6 hours at 160–175°C to achieve less than 50 ppm moisture. This is more demanding than any other commodity engineering thermoplastic. A calibrated moisture analyser check before production start is best practice.
Shrinkage: 1.5–2.2% unfilled; 0.3–0.8% at 30% GF. Flow vs transverse anisotropy is significant in unfilled grades. GF grades show lower anisotropy due to the restraining effect of glass fibres on matrix shrinkage.
Draft angles: 1–2° on polished surfaces; 2–3° on textured surfaces. GF-PET's surface fibre orientation can increase ejection resistance; generous draft and polished tooling faces are advisable.
Sink and warp: Unfilled PET warps significantly without proper mould temperature and gate design. GF-PET is substantially better but still benefits from balanced gating and symmetric part geometry. Parts with large flat surfaces and low-symmetry gate placement are most at risk.
Which PET grades and variants should you consider?
| Grade / Variant | Key Feature | Typical Application |
|---|---|---|
| Nucleated PET (unfilled) | Fast crystallisation, moderate stiffness | Housings, consumer goods, food trays |
| PET GF15 | Moderate stiffness increase, better aesthetics | Enclosures, precision housings |
| PET GF30 | Maximum stiffness, HDT 210–225°C | Connectors, pump bodies, SMT components |
| PET GF30 + FR (V-0) | Flame retardant, halogen-free options | Electrical connectors in GF-dominated designs |
| Food-contact PET (GF or unfilled) | EU 10/2011 / FDA 21 CFR 177.1630 | Food-processing equipment, pharma trays |
| Hydrolysis-stabilised PET | Improved hot-wet stability | Automotive under-bonnet, outdoor weathering |
| Mineral-filled PET | Reduced anisotropy vs GF alone | Precision housings requiring isotropic shrinkage |
Mineral-filled grades (typically calcium carbonate or talc) are used when the fibre-anisotropy of GF-PET is unacceptable for precision parts with tight positional tolerances in all axes.
What are the advantages and limitations of PET?
Advantages:
- Highest stiffness-to-weight ratio among the common polyester engineering thermoplastics in GF grades
- Excellent chemical resistance to fuels, oils, dilute acids, and most organic solvents
- Low moisture absorption maintaining dimensional stability in service
- Food-contact compliant grades widely available
- Good dielectric properties for electrical connector applications
- GF30 grades withstand SMT solder reflow (HDT 210–225°C)
Limitations:
- Most process-demanding of the common polyesters — high mould temperature (100–140°C) required
- Rigorous drying protocol (160–175°C, 4–6 h) is non-negotiable; moisture causes irreversible property loss
- Susceptible to stress cracking in contact with polar solvents and alkalis
- Brittle in unfilled form; notch sensitivity requires generous corner radii
- Higher tooling cost than PBT due to hot-runner requirement and elevated mould temperature demands
- Not suitable for prolonged steam or boiling-water exposure in standard grades
When should you choose PET over alternative materials?
PET vs PBT: Both are polyesters. Choose PBT for most connector and housing applications — it processes more easily at lower mould temperatures and is the established connector standard. Choose GF-PET when maximum flexural stiffness, food-contact compliance in a glass-filled grade, or slightly higher tensile strength is the deciding factor.
PET vs nylon (PA66): Choose PET when low moisture absorption, better dimensional stability in humid service, or chemical resistance to fuels and oils is required. Nylon is preferred for higher impact resistance, better tribological performance, and greater flexibility in gate and tool design.
PET vs PPS: PPS offers superior heat resistance above 230°C continuous and better chemical inertness. Choose PET where cost is a constraint and the application temperature stays below 200°C. PPS is the upgrade path for harsher chemical environments.
PET vs PEEK: PEEK is justified only when continuous service temperatures above 240°C, aggressive solvents, or medical implant biocompatibility are required. GF-PET at a fraction of PEEK's material cost suits the vast majority of structural polyester applications.
Nordmould recommends raising PET as a candidate during DFM review whenever a customer is specifying GF-PBT for food-contact applications or needs the maximum stiffness available in an injection-mouldable polyester.
Is PET recyclable?
PET carries resin identification code 1 (PETE) and is one of the most widely recycled thermoplastics globally, primarily driven by bottle recycling infrastructure. Injection-moulded engineering PET — especially GF grades — does not generally enter the bottle-recycling stream and is classified as industrial waste. Clean production scrap can be mechanically recycled into lower-performance applications. Hydrolysis during reprocessing is a concern: PET regrind must be dried to the same standard as virgin resin. Carbon footprint comparisons between virgin and recycled PET favour recycled content where quality permits.
Frequently asked questions
Is PET suitable for injection moulding?
Standard PET is difficult to injection mould because its crystallisation is slow and incomplete at typical mould temperatures, producing warped, brittle parts. Nucleated and glass-filled PET grades engineered for injection moulding are commercially available and used widely for precision connectors, food-contact housings, and high-barrier packaging components. Nordmould specifies only injection-moulding-grade PET.
How does injection-moulded PET differ from PET bottles?
Bottle-grade PET is processed by injection stretch blow moulding at low crystallinity to achieve transparent, biaxially oriented walls. Injection-moulding-grade PET is nucleated to crystallise rapidly at elevated mould temperatures (100–140°C), producing opaque, semi-crystalline parts with high heat resistance. They share the same base polymer but are different product forms.
What is the shrinkage rate of PET in injection moulding?
Unfilled, nucleated PET shrinks 1.5–2.2%; glass-filled PET (30% GF) shrinks 0.3–0.8%. Anisotropy is significant in unfilled grades. Nordmould designs tool compensation from the specific grade shrinkage data provided by the resin manufacturer.
Does PET require drying before injection moulding?
Yes — PET is highly sensitive to moisture. It must be dried at 160–175°C for 4–6 hours in a dehumidifying dryer, achieving a moisture level below 50 ppm before processing. Moisture causes hydrolytic chain scission at melt temperature, permanently degrading molecular weight and mechanical properties. Nordmould treats PET drying as a critical process parameter.
Is injection-moulded PET food-safe?
Food-contact compliant injection-moulding PET grades are available, certified to EU 10/2011 and FDA 21 CFR 177.1630. These are used for food processing components, medical device trays, and pharmaceutical packaging. Nordmould advises confirming the specific grade data sheet during the DFM review, as not all PET grades carry food-contact clearance.
How does injection-moulded PET compare to PBT?
PBT and PET are both polyesters with similar chemical backbones, but PBT crystallises faster and processes more easily on standard tooling. PET in glass-filled grades achieves marginally higher tensile strength and a similar HDT to GF-PBT, but demands higher mould temperatures and tighter process control. PBT is the preferred default for connectors; GF-PET is specified where maximum stiffness or food-contact compliance drives material selection.
What applications use injection-moulded PET over other materials?
Injection-moulded PET is used in food-processing equipment parts, pump housings, filter housings, automotive fuel system components, and precision electrical connector bodies. Its combination of high stiffness (in GF grades), chemical resistance, and food-contact compliance makes it difficult to substitute with cheaper alternatives in these niches.
What is the minimum order quantity for PET parts at Nordmould?
Nordmould's minimum order is 100 pieces, applicable to PET and all other engineering thermoplastics. Tooling starts from €3,000, with aluminium bridge tooling available for prototype and bridge-volume runs.
Send your STEP file to Nordmould for a free DFM review and written quote — typically returned within one business day.