Thermoforming against Injection for Volumes below Fifty Thousand Units
Thermoforming delivers lower total landed cost below fifteen thousand units by saving upfront tooling capital, beyond which injection piece price efficiency dominates.
Mold Shrink
Meaning
Dimensional contraction occurs when a molten polymer cools and solidifies within a mold cavity. This physical change, known as mold shrink, is the percentage difference between the mold cavity dimensions and the final dimensions of the molded part after twenty-four hours of cooling. It governs the design of the tooling and the tolerances of the molded part.
The exact value depends on the polymer chemistry, part geometry, and processing conditions.
Contraction Measurement
Semi-crystalline polymers like polyethylene display high rates of mold shrink due to the tidy alignment of polymer chains during cooling. Amorphous polymers like polycarbonate shrink far less because their disordered structure does not pack as tightly. Molders measure this contraction to ensure the final parts meet the required dimensional drawings.
Processing Influence
Melt temperature and hold pressure affect the rate of contraction during the molding cycle. High hold pressure forces more material into the cavity, which compensates for volumetric contraction and reduces the mold shrink. Adjusting the cooling time also helps to stabilize the part dimensions before ejection.
Tool Design
Toolmakers must cut the mold cavity larger than the desired part dimensions to account for this contraction. If the resin exhibits variable behavior, the final part dimensions will fall outside of tolerances.