Meaning
Variation in temperature across a distance within a material or a system determines the rate and direction of heat transfer. This temperature gradient is a necessary variable in both the melting phase and the cooling phase of injection moulding. The difference in temperature between the polymer melt and the mould steel dictates how the plastic solidifies.
Thermal Distribution
Unbalanced heating or cooling rates generate non-uniform temperatures within the mould. A steep temperature gradient across the wall thickness of a part causes the outer layers to freeze quickly while the core remains molten. This uneven cooling rate creates internal stresses that can compromise the mechanical integrity of the component.
Moulding Consequence
Shrinkage variation arises when different areas of a part cool at different rates. The presence of a high temperature gradient leads to warpage, sink marks, and dimensional inaccuracy in the finished product. These defects are particularly severe in semi-crystalline resins where crystallinity is highly dependent on the local cooling rate.
Cooling Management
Optimizing the cooling channel design within the mould inserts reduces thermal variance. Controlling the temperature gradient ensures that the polymer cools uniformly, minimizing internal stress and reducing cycle time. Conformal cooling channels that follow the part geometry are often used to achieve this uniform thermal distribution in complex parts.
This uniform cooling is necessary for maintaining tight dimensional tolerances and ensuring that the finished parts do not distort after ejection.
