Bulk temperature is used to indicate the weighted average temperature across the thickness.
The temperature of polymer melt changes not only with time and location, but also with thickness during the entire injection molding cycle. It is difficult to illustrate all these changes in a single display. For these reasons, Bulk temperature is used. Bulk temperature has more physical significance than a simple average temperature as the polymer melt flows. It also represents the energy that is transported through a particular location.
Bulk temperature is a velocity-weighted average temperature when the polymer is flowing and a simple average temperature when the flow stops. For each element, a plot of bulk temp versus time shows that the switch-over from bulk temperature to average temperature gives a smooth curve. Uniform bulk temperature distribution during filling is desirable for mold design.
Bulk temperature displays are an alternate way to examine the flow distribution. Areas with continuous flow (heat convection) typically have a higher bulk temperature. The bulk temperature drops quickly when the flow stops in that area.
Hot spots are indicated on the bulk-temperature contour or data-shaded plots during filling. Hot spots are due to excessive viscous heating during the filling stage.
If the maximum bulk temperature is close to the degradation temperature, consider redesigning the part geometry near the hot spot or changing the process conditions.
Differential temperature can also cause non-uniform shrinkage and warpage.