Meaning
Variation in temperature change per unit of time across the thickness of a plastic part dictates the local crystalline morphology during solidification. In injection molding, the cooling rate gradient describes the difference in cooling speed between the polymer layer touching the cold tool steel and the insulated core of the part. This difference is a major source of internal molded-in stress.
When the flow is unevenly cooled, differential shrinkage causes the part to warp or bow after ejection.
Thermal Stress
Molding processes are inherently uneven due to the low thermal conductivity of plastic resins. A steep cooling rate gradient leads to a highly crystalline core and an amorphous skin layer. This structural disparity creates unbalanced residual stresses within the part.
These stresses can release over time, leading to warpage.
Molding Control
Adjusting the coolant temperature and flow rate allows moulders to manage the heat extraction across the part profile. When the cooling rate gradient is minimized, the part cools more uniformly, which improves dimensional repeatability. It also reduces cycle times by allowing faster safe ejection of the part.
Proper runner design and gate placement further help to distribute heat evenly.
Defect Mitigation
Unbalanced thermal profiles are often resolved by altering the mold cavity surface temperature or using core pins. A severe cooling rate gradient can be avoided by optimizing the wall thickness of the part.
