Meaning
Volumetric reduction that occurs as a molten polymer cools and solidifies inside a tool cavity is a predictable physical phenomenon. Calculating mold shrinkage is a primary step in the design of injection molds to ensure the final part meets its specifications. This value varies significantly between amorphous and semi-crystalline resins.
Cooling Influence
Parts that cool slowly generally exhibit more contraction than those that are cooled rapidly. Variations in mold shrinkage across a single part can lead to warping or internal stresses. Maintaining a consistent cycle time is necessary to keep the dimensions within the required tolerances.
Material Properties
Semi-crystalline polymers like polyethylene shrink more than amorphous materials like polystyrene due to the way the chains pack together. Molders must account for this mold shrinkage when switching between different resin types in the same tool. If the material changes, the part may no longer fit into its intended assembly.
Process Control
Injection pressure and melt temperature also play a role in how much the material contracts. Increasing the packing pressure can reduce mold shrinkage by forcing more material into the cavity to compensate for the cooling. Precise control of these variables is the method for producing high-precision components that meet tight tolerances.
