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Resin Data
PET

Tereftalato de Polietileno

PET·Polyesters·Semi-crystalline

PET is the resin that rewrote modern packaging — and one of the most demanding to injection-mold. We're talking about polyethylene terephthalate (CAS 25038-59-9), the polymer behind virtually every clear beverage bottle on the planet, plus ovenable trays (CPET), polyester textile fibers, and technical films. Trade names like Indorama RamaPET, Far Eastern Cleartuf, Octal, and Alpek dominate the bottle-grade market, while Eastman and DAK Americas are references in specialty grades.

The key with PET is drying: if you process with moisture above 0.02% it hydrolyzes, molecular weight drops (IV falls), and parts come out brittle. Dry at 120-150°C for 4-6 hours in a desiccant dryer, never with ambient air. Melt temperatures run 260-280°C for natural grade and 275-290°C with glass fiber.

The other central topic is crystallinity: with a cold mold (15-30°C) you get clear amorphous parts —typically preforms that later go to SBM stretch blow molding—; with a hot mold (~140°C) you get crystalline CPET for ovenable trays. Are you running PET? Share your experience with drying, IV, and blowing in the comments.

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 StructureSemi-crystalline
Specific Gravity (Density)1.43:1
L/D Ratio20 – 30
Compression Ratio2 – 3
Tonnage Factor6.18 – 9.27kN/cm²
Thermal Diffusivity0.063mm²/s
Max Shear Rate50,0001/s
Shrinkage0.3 – 3%
Regrind50%
Heat Deflection (HDT) @ 1.82 MPa66°C
Glass Transition (Tg) @ 10°C/min71°C
Vicat Softening @ 50N205°C

Drying

Drying Temperature129 – 141°C
Drying Time3 – 6h
Recommended Moisture0.05%
Recommended Dryer TypeDesiccant
Dew Point-40°C

Temperatures

Melt282 – 299°C
Nozzle279 – 302°C
Front266 – 296°C
Middle260 – 296°C
Rear254 – 291°C
Demolding96 – 132°C
Mold (Cooling)79 – 121°C
Feed Throat35 – 79°C

Processing

Back Pressure3.4 – 6.9bar
Screw Speed30 – 70RPM
Injection SpeedHigh
Barrel Occupancy30 – 70%
Injection Pressure300 – 1,400Pbar
Holding Pressure75 – 1,120Pbar
Cushion6.4 – 12.7mm

Mold

Runner Diameter4.06 – 7.11mm
Gate Diameter1.02 – 1.52mm
Gate Area0.81 – 1.82mm²
Wall Thickness0.76 – 3.05mm

Venting

Depth (Vent Depth)0.0102 – 0.0305mm
Land (Vent Land)0.508 – 1.02mm
Width (Vent / Clearance)3.05 – 7.62mm
Relief (Relief Channel)0.127 – 0.254mm

Frequently asked questions

PET (polyethylene terephthalate) is a semi-crystalline thermoplastic polyester. In injection, its #1 use is making **bottle preforms** —small amorphous threaded tubes that are later stretch blow molded (SBM) into PET beverage bottles. It's also injected for ovenable CPET trays, cosmetic containers, and technical glass-fiber-reinforced parts.
PET is **strongly hygroscopic** and reacts with water at processing temperature (hydrolysis). With moisture above 0.02%, chains break, IV drops, and parts lose strength and clarity. Dry at 120-150°C for 4-6 hours in a desiccant dryer with dew point ≤ -40°C. Conventional hot air doesn't work.
**APET**: cold-mold quench (15-30°C), transparent, used in preforms and cold trays. **CPET**: hot-mold (~140°C) that promotes crystallization, opaque, withstands up to ~220°C in oven —these are the trays for ready meals that go straight from freezer to oven.
**PETG** is glycol-modified PET —it's amorphous, doesn't crystallize, doesn't need as critical drying, and has a wider processing window. Use it when you need thick clear parts, pharmaceutical blister, or 3D printing. **PET** when you need real mechanical properties, gas barrier (carbonated bottles), or thermal resistance.
**PBT** crystallizes faster and is easier to injection-mold structurally —used in electrical connectors and automotive parts. **PET** has higher stiffness and mechanical strength once crystallized, but demands longer cycles and higher mold temperatures. For mass-production technical parts, PBT usually wins; for barrier and clarity, PET has no rival.
**IV** measures the average molecular weight of PET. Typical grades: 0.60-0.65 dL/g for trays, 0.74-0.82 dL/g for bottle preforms, 0.85+ dL/g for wide-neck mineral water bottles. Every bad drying step or recycling cycle lowers IV. PET with IV < 0.70 isn't suitable for pressurized bottles.
**Yes, under strict conditions.** The FDA issues case-by-case "letters of non-objection" for mechanical recycling processes that demonstrate adequate decontamination. EFSA in Europe does the same. Food-grade rPET grades (e.g. Indorama Deja, Evergreen rPET) undergo SSP (solid-state polymerization) to raise IV and remove migrants to approved levels. Not all rPET is suitable for direct contact.
For amorphous PET (preforms): **0.2-0.5%**. For crystalline PET (CPET, technical parts): **1.2-2.0%** longitudinal, up to **2.5%** transverse. The high variability is because final crystallinity depends on the mold thermal profile —wall temperature differences translate into warpage. Designing the cooling system is critical.
The **preform** —that thick tube with the thread already formed— is injected amorphous, clear, with thick walls. Then it's reheated to **strain-induced crystallization** temperature (~100-120°C) and blown biaxially: material orients its chains in two directions, gains gas barrier (CO₂ for sodas) and mechanical strength. Making the bottle directly by injection wouldn't give that orientation.
**Hydrolysis from bad drying** is #1 by far. Symptoms: opaque parts with bubbles, sprue that comes out wet, low viscosity. The cure: desiccant dryer with -40°C dew point, 4-6h at 130°C, verify residual moisture with Karl Fischer. #2 is **acetaldehyde** (AA) in water bottles, which alters taste —controlled with low residence time and moderate melt temperature (no more than 280°C).

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