Today's KNOWLEDGE Share
Understanding Shrinkage in Injection Molding: The Role of the Packing Phase
In injection molding, shrinkage is fundamentally linked to thermal expansion.
However, this relationship can become complex, especially when we factor in the "Packing Phase."
During this phase, we apply significant pressure to the molten material, allowing us to inject more grams of material into a predefined mold volume, assuming we disregard mold deformation for now.
As a result, the final shrinkage can vary widely—ranging from high shrinkage, dictated by the room pressure PvT curve (in cases where no packing is applied), to even negative shrinkage in situations of overpacking.
While the basic principles of shrinkage are driven by Coefficient of Thermal Expansion (CTE), the reality is much more nuanced.
For instance, with glass-filled polymers, increased packing pressure can influence the anisotropy-driven warpage of the material; it may even suppress warpage without affecting the CTE anisotropy itself.
source: Vito leo
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