How a part cools has a dramatic effect on part quality and dimensional accuracy. The ideal part is of uniform thickness cooled in a mold of uniform temperature. This assures that the part shrinks at the same rate in all directions. As we move away from the ideal conditions, we induce variable shrinkage in the part.
The areas that freeze first will be pulled on by the areas that shrink last. This induces molded in stress and warp in the part. The cooling plots show areas where this will occur. The cooling quality plot highlights the problem areas in the part. The Surface Temperature Variance and the Freeze Time Variance plots show the magnitude and areas of differential cooling.
The plots show you where heat tends to stay in a part due to its geometry (Surface Temperature Variance) and its thickness (Freeze Time Variance). Keep in mind that the Adviser results are ISO thermic. This means that the walls of the mold are kept at a constant temperature. This differs from actual conditions in which the tool is coldest near the water lines and warmer between them.
Surface Temperature Variance Results
The Surface Temperature Variance result highlights areas where the part’s geometry will cause local heat concentrations. The high surface temperature variance areas in the part are usually interior regions with deep cores. This is due to the fact that there is not sufficient thermal mass to remove the heat. Therefore these areas are natural “hot spots” that are difficult to cool. Bubblers and heat pins are often used to improve the cooling in these areas.
Note that the core pin that forms the inside of the part shown below is hotter than the outside. This is because it has the same heat load as the outside surface but has less thermal mass. Also note that it is hotter at the center of the pin. This is because the sink is at each end of the pin, forcing the hottest area to the center.
Freeze Time Variance Results
The freeze time variance result displays the time required for each element of the model to freeze completely. The Freeze Time Variance results indicate places on the part that might require a redesign, such as reducing the thickness of a wall, or places in the mold that will require additional cooling capacity.
Looking at the image below, there are three distinct freeze time zones. The fastest and first to cool is the thin rim of the part (-2.95). The second area is the thin area of the tube (0.63). The third is the thick section of the tube (4.22). To resolve these issues the flange was thickened and a flat was added to the thick section to thin that area. These changes were important to minimize warping and differential shrinkage in this tight tolerance part.
What problems can poor cooling quality cause?
- Excessive warping and/or sink in areas with large cooling variances.
- Short shots or poor weld line formation in colder areas.
- Increased molded in stresses.