Cavity Balance
Pressure Drop
Process Window
Gate Seal
Cooling Study

 Procedure to estimate Cavity Balance

  • Set the holding pressure to zero.
  • Set the holding time to zero.
  • Set the screw recovery delay time to about a value close to an estimated holding time.
  • Set the cooling time to a value such that you know that the part will be cool enough to eject.
  • Set your injection speed to the value obtained from the Viscosity Curve study.
  • With the rest of the settings the same as you had in the viscosity study, start molding.
  • Only by adjusting the transfer position, mold parts that are just short. If there is a visible cavity imbalance, then the ‘biggest’ part should be just short.
  • Make three such shots and take the average weight of each cavity and plot a graph.

A typical graph from an eight cavity mold is shown below.

How to use this information: 
(The assumption here is that the gate and runner sizes are the same.)
Check the %variation between the maximum and the minimum fill cavities. In most cases, the % variation should not be greater than 5%. For tight tolerance parts, the variation should not be more than about 3%. If the tolerances are large, variation more than 5% is acceptable. More importantly, it is the final quality of the part that should be checked to see it there is need to tighten up the cavity balance. For example, the Cpk values of all the parts from all the cavities is a good measure. The following should be considered:

  • Amorphous materials can tolerate more imbalance than crystalline materials.
  • Tighter the tolerances, lesser should be the variation.
Venting is a very big contributor to plastic fill and can have a big impact on the cavity balance although the gate and runner sizes are the same. Make sure that the venting is the same for all cavities.
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