Meaning
Non uniform temperature distribution exists within a cooling part when the outer skin loses heat faster than the inner core. Managing the transient thermal gradient is a primary factor in preventing warpage and internal stresses in thick walled parts. The gradient changes constantly from the moment the melt hits the cold tool until the part reaches room temperature.
Solidification Rate
Cooling starts at the surface and moves inward. A steep transient thermal gradient causes the outer layers to freeze into a rigid shell while the center remains molten.
Warpage Mechanism
Residual stress builds up as different sections of the part contract at different times. If the transient thermal gradient is not symmetrical, the part will bow or twist toward the side that cooled more slowly. Balanced cooling through well designed water channels is the primary method for controlling this effect.
Cycle Efficiency
Reduction of the time spent in the tool requires a fast but controlled heat removal process. Understanding the transient thermal gradient helps engineers determine when a part is structurally sound enough to be ejected. If the part is removed while the core is still too hot, the sudden change in environment can cause further deformation.
