Why Do PET Thin-Wall Injection Molded Parts Become Brittle?

PET is a polyester material. At high processing temperature, residual moisture can trigger hydrolysis and cut polymer chains.

Once molecular weight drops, the part may show: - Lower impact strength - Brittle fracture - Reduced elongation - Lower melt strength - Unstable processing

Drying is therefore not optional for PET injection molding. It is a material protection step.

Thin-wall parts cool quickly and have limited cross-section to absorb stress. Small changes in molecular weight, crystallinity, or orientation can cause large changes in toughness.

Common failure locations include: - Corners - Snap-fits - Weld lines - Gate areas - Ribs and bosses

If the design contains sharp transitions, PET brittleness becomes much more visible.

PET is a semi-crystalline polymer. Its crystallization behavior strongly affects clarity, shrinkage, stiffness, and toughness.

Excessive or uneven crystallization can make thin-wall parts more brittle and dimensionally unstable. Fast cooling may freeze orientation, while local hot spots may allow higher crystallinity.

This imbalance can create local stress concentration and delayed cracking.

Thin-wall injection molding often uses high injection speed and high shear rate. This can stretch polymer chains along the flow direction.

If orientation is frozen before relaxation, the part becomes anisotropic. Strength may be higher in one direction and weaker in another.

This is why thin-wall PET parts may crack along predictable flow paths, weld lines, or sharp corners.

PET brittleness can be amplified by unstable processing conditions:

• Insufficient drying
• Excessive melt temperature
• Long residence time
• Overly high injection speed
• Low mold temperature
• Poor venting
• Excessive regrind ratio

The correct process should minimize degradation while allowing adequate filling and stress relaxation.

Practical improvement steps include:

• Dry PET thoroughly before molding
• Verify actual moisture content, not only drying time
• Reduce residence time and avoid overheating
• Improve corner radii and wall transition design
• Optimize gate location to reduce weld-line weakness
• Use impact modified PET when toughness is critical
• Validate parts after conditioning and aging

For high-risk applications, material grade selection is often more important than small parameter changes.

YicaiPlas provides material selection and compounding support for injection molded engineering plastics, including drying-sensitive materials, impact modified systems, and application-specific plastic compounds for thin-wall molded parts.