HOME  > Plastic Molding Tutorial  > #034 Pressure Loss in the Runners
Plastic Molding Tutorial

#034 Pressure Loss in the Runners

Category : Technical Information
March19, 2010

In plastic injection molding, the molten plastic material flows through the runners, passes through the gates, and then reaches the cavity. The pressure gradually decreases in the process of this sequence of flow. Molten plastic is a viscous fluid having a certain amount of viscosity. In addition, it also has the feature that the viscosity changes depending on the temperature of the plastic, and when the plastic temperature falls below a certain range the plastic can no longer flow and starts to solidify.

Further, whenever a viscous fluid flows through a flow path, there is always a loss in pressure. This is the same as with the flow of water or oil. Now, under what situations is the pressure loss high? Pressure loss is known to occur under the following conditions.

1.In the vicinity of the inlet to the flow path

Whirls are generated in the vicinity of the inlet to the flow path into which the fluid enters thereby causing a pressure loss. It is possible to reduce the pressure loss by making the rounding diameter R large for the corners of the inlet.

2. Where the flow path bends

A pressure loss occurs in the parts where the flow path bends at an angle because there is a change in which the fluid first gets compressed and then expands.

3. Where flexures and bends are present

Whirls are generated in parts where there are flexures and bends thereby causing a loss in pressure.

4. Locations where the flow path expands or contracts

Whirls are generated in parts where the cross section of the flow path becomes wider or narrower thereby causing a loss in pressure. In particular, since a large pressure loss occurs with very sudden expansions or contractions of the flow path, it is necessary that they be strictly avoided.

Although it is possible to know the trends in pressure loss due to the predictions in pressure loss using CAE tools, in order to optimize the detailed flow conditions that become necessary in actual injection molding, it is very important to understand the reasoning according to theory and to utilize the know how obtained by the trial and error of fine adjustments.

MISUMI USA eCatalog