Shenzhen Alu Rapid Prototype Precision Co., Ltd.

Shrinkage in injection molding refers to the volumetric or linear contraction of a plastic part as it cools from the molten state to room temperature. This occurs due to the material's thermal expansion and crystallization properties, and it's a critical factor in designing molds to achieve accurate part dimensions. Without accounting for shrinkage, molded parts can end up undersized, leading to fit issues in assemblies.

Steps to Calculate:

1.Design or measure the mold cavity dimension at room temperature.

2.Inject and mold a test part under controlled conditions (e.g., consistent temperature, pressure, and cooling time).

3.Allow the part to cool fully (typically 24 hours) and measure its dimension using calipers, CMM (coordinate measuring machine), or similar tools.

4.Plug the values into the formula.

5.Iterate if needed, as actual shrinkage can vary based on processing parameters.

Factors Affecting Shrinkage

Shrinkage isn't constant and depends on several variables:

1.Material Type: Amorphous plastics (e.g., ABS, polystyrene) shrink 0.4–0.7%, while semi-crystalline ones (e.g., nylon, polypropylene) shrink more (0.7–2.5%) due to higher crystallinity.

2.Processing Conditions: Higher melt temperature, longer cooling time, or lower injection pressure can increase shrinkage. Mold temperature also plays a role—warmer molds reduce shrinkage by minimizing thermal gradients.

3.Part Geometry: Thicker sections shrink more than thin walls; flow direction can cause anisotropic (directional) shrinkage.

4.Additives: Fillers like glass fibers reduce shrinkage by 20–50%.