Factors affecting molding shrinkage of thermoplastics

The shrinkage of thermoplastic plastics is influenced by a variety of factors, which can significantly affect the final dimensions and properties of molded parts. Understanding these factors is crucial for optimizing the manufacturing process and ensuring product quality. Here are the main factors that affect the shrinkage of thermoplastic plastics:

▶ 1. Polymer Composition and Material Properties

• Crystallinity: Semi-crystalline polymers tend to have higher shrinkage rates than amorphous polymers due to their ordered molecular structures forming during cooling. The degree of crystallinity directly affects shrinkage; higher crystallinity generally leads to more shrinkage.
• Molecular Weight: Polymers with higher molecular weights typically exhibit lower shrinkage because they have better flow properties and form denser structures.

▶ 2. Processing Conditions

• Temperature: Both the melt temperature and mold temperature play significant roles. Higher melt temperatures can decrease shrinkage by promoting better polymer flow, while higher mold temperatures often increase shrinkage due to slower cooling rates.
• Pressure: Maintaining high pressure during molding reduces shrinkage by compacting the material more effectively. Longer pressure holding times also contribute to reduced shrinkage.
• Cooling Time: Extended cooling times in the mold can lead to reduced shrinkage as it allows more time for the polymer structure to stabilize.

▶ 3. Part Design and Geometry

• Wall Thickness: Thicker sections of a part tend to cool more slowly, resulting in higher crystallinity and greater shrinkage compared to thinner sections.
• Complexity and Inserts: Parts with complex geometries or inserts may experience less shrinkage due to constraints that limit material movement during cooling.

▶ 4. Additives and Fillers

• Fiber Reinforcement: The addition of fibers, such as glass fibers, can reduce shrinkage in the direction of fiber alignment but may increase it in other directions due to anisotropic effects.
• Inorganic Fillers: Fillers like talcum powder or calcium carbonate can reduce overall shrinkage by altering the polymer matrix’s structure and reducing crystallinity.

▶ 5. External Factors

• Environmental Conditions: Variations in ambient temperature and humidity can also impact the degree of shrinkage as they affect the cooling rate and residual stresses within the material

By carefully controlling these factors, manufacturers can minimize unwanted shrinkage, thereby improving the dimensional accuracy and mechanical properties of thermoplastic parts.