Plastics Design Principles: Constant Pressure Gradient

By Jay Shoemaker

The pressure gradient while the part is filling should be uniform through the part. Figure 4: Pressure Gradient shows a part that does not have a constant pressure gradient during filling. The XY graph is the pressure at the injection location. Just at the beginning of fill, there is a spike in pressure. However, the big problem is at the end of fill. The part is mostly filling by radial flow. As the flow front meets the center of the sidewalls the flow front starts contracting. This corresponds to a slight increase in the pressure gradient. The big spike occurs when the three corners fill and the remaining upper right corner is the only area that remains unfilled. All the material exiting the gate enters the upper right corner causing the pressure spike. The volumetric flow rate entering the part is constant. The pressure gradient is an indication of a balance problem, or it suggests an injection velocity profile should be used.

Figure 4: Pressure Gradient at Plastics Injection Location

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