PET

What Is PET Injection Molding?

Polyethylene terephthalate (PET) is a semi-crystalline polyester and one of the most widely used polymers in the world — best known for beverage bottles, but also an important injection-molding material for preforms, food packaging, technical parts and electrical/electronic components. PET combines high strength and stiffness, excellent clarity in the amorphous state, good gas-barrier properties and very good chemical resistance to most solvents and weak acids. It is fully recyclable, and food-contact grades are approved for direct food packaging under the major global regulations.

In injection molding, PET behaves differently from most commodity plastics because its final properties depend strongly on thermal history. If the mold is held hot (120 – 140 °C), PET crystallizes and yields a white, opaque, dimensionally stable part with better heat resistance. If the mold is cold (20 – 30 °C), the melt freezes into a glassy, transparent, amorphous part — the route used for bottle preforms, which are later reheated and stretch-blown. Controlling this crystallization window is the key to a successful PET molding program; get it wrong and the part comes out hazy, warped or brittle.

PET is also available in glass-fiber-reinforced grades that substantially raise stiffness and heat resistance for technical applications, competing with PBT and nylon in many E/E components. Because the base resin is inexpensive and widely recycled, PET is one of the most cost-effective engineering polyesters on the market.

Key Properties of PET

Property	Typical Value	Why It Matters
Density	≈ 1.33 – 1.38 g/cm³ (amorphous); ≈ 1.40 g/cm³ (crystalline)	Lightweight packaging material with a good strength-to-weight ratio.
Tensile strength	≈ 50 – 80 MPa depending on orientation	Strong enough for pressure-retaining and structural parts.
Optical clarity	High transparency in the amorphous state	Clear containers and display parts without additives.
Gas barrier	Good O₂ and CO₂ barrier	Preserves carbonation and protects packaged food from oxygen.
Water absorption	Low, ≈ 0.1 – 0.2% (24 h immersion)	Stable dimensions in humid environments.
Heat deflection temperature	≈ 70 °C (1.8 MPa, amorphous)	Suits cold-fill and short thermal exposure; not a hot-service polymer.
Chemical resistance	Good to acids, alcohols and oils; poor to strong alkalis and hot water	Defines the compatible service environments for the part.

Processing Parameters We Control

PET is strongly hygroscopic and hydrolytically degrades when molded wet: moisture levels above roughly 50 ppm (0.005%) cause a drop in intrinsic viscosity (IV) and yield brittle, weak parts. Drying is therefore mandatory and non-negotiable. The industry-standard approach is desiccant drying at 150 °C for 4 – 6 h to a dew point of −40 °C, with the dried resin kept sealed until it enters the barrel.

Parameter	Typical Range
Pre-drying	150 °C for 4 – 6 h, dew point ≤ −40 °C (moisture below 50 ppm)
Melt temperature	260 – 290 °C
Mold temperature	120 – 140 °C (crystalline parts) or 20 – 30 °C (amorphous, clear parts)
Mold shrinkage	≈ 0.2 – 0.6% (amorphous); ≈ 1.5 – 2.0% (crystalline)
Injection speed	Fast fill for preforms to prevent premature crystallization

Because PET degrades above roughly 300 °C, residence time must be kept short: the barrel should be sized so the polymer moves through quickly, and long idle time in the machine should be avoided. Regrind use is common but must be carefully controlled, kept dry and limited to a modest percentage of the shot.

Common Applications

• Bottle preforms: injection-molded amorphous preforms that are reheated and stretch-blown into bottles and jars for beverages and food.
• Food packaging: trays, containers, lids and blister packs using food-contact-approved grades.
• Technical and mechanical parts: gears, housings and structural components in glass-reinforced grades.
• Electrical and electronic components: connectors, bobbins and insulators benefiting from good electrical properties and flame-retardant grades.
• Fasteners and buckles: high-strength injection-molded closures for textiles, bags and strapping systems.

Design Guidelines for PET Parts

• Uniform walls: keep wall thickness consistent; thick sections crystallize unevenly and cause haze, sink marks or warpage.
• Crystalline vs. amorphous decision: specify mold temperature early in the program — hot molds give opaque, stable parts with better heat resistance; cold molds give clear, glossy parts with lower heat resistance.
• Ribs and bosses: use ribs at 0.5 – 0.6 × wall thickness to add stiffness without creating thick sections.
• Draft: provide 0.5 – 1.5° of draft to avoid ejection damage, especially on crystalline parts.
• Gates and runners: large, well-polished gates and runners reduce shear heating that can degrade the melt.
• Venting: adequate venting prevents gas burn and trapped-air defects in fast-fill preform tooling.

Common Defects and Prevention

• Hydrolytic degradation (brittle parts): wet resin drops IV and embrittles the part. Dry at 150 °C to below 50 ppm moisture and never expose dried resin to humid air.
• Haze and opacity: unintended crystallization from a hot mold or a slow fill; lower mold temperature or increase fill speed for clear parts.
• Warpage: uneven cooling or crystallization; balance cooling channels and keep walls uniform.
• Black specks and streaks: degraded polymer from excessive residence time or melt temperature; shorten residence and purge the barrel before shutdown.
• Gate blush: surface whitening at the gate caused by shear stress; enlarge the gate or reduce injection speed.

PET vs. PP vs. PC

PET competes with polypropylene (PP) and polycarbonate (PC) in packaging and transparent-part applications. Against PP, PET wins on clarity, stiffness and gas barrier — PP containers are cheaper but let oxygen and CO₂ through far more readily, which is why carbonated beverages are packaged in PET rather than PP. Against PC, PET is significantly cheaper and has better chemical resistance, but PC offers roughly double the heat deflection temperature and far higher impact strength.

The practical rule: for cold-fill packaging, food contact and clear technical housings, PET is often the most cost-effective choice; for hot-fill containers, impact-critical parts or anything near a heat source, PC or a copolyester is preferable. When dimensional stability at elevated temperature matters, glass-reinforced PET is a strong contender against PBT at a similar price point.

MOLDITQUICK Capabilities

MOLDITQUICK injection-molds PET for preforms, packaging components and technical parts. Our process control centers on the two factors that make or break PET: drying discipline and mold-temperature management. Dried to below 50 ppm moisture and processed inside the 260 – 290 °C melt window, PET produces strong, clear, dimensionally stable parts run after run.

We also mold glass-reinforced PET grades for structural and E/E components, and we coordinate secondary operations such as printing, assembly and packaging. Share your part file or drawings with us and we will return a DFM review with a grade recommendation, tooling plan and pricing — start with our quote request page or read more about our injection molding service.