Introduction to Pressure Drop in Injection Molding - 5/1/2007

Plastic flow differs from the flow of conventional fluids such as water. (See related article on Polymer Rheology.) As the plastic flows through a flow channel, there are multiple phenomena affecting the flow. In relation to injection molding, consider plastic flow in a runner. As the molten plastic enters a runner it has a temperature T and a pressure P, the mold is colder as compared to the plastic, and is typically set around the crystallization temperatures (for crystalline materials). As the plastic flows through the runner, because of drag and frictional effects there is a loss of the applied pressure at the flow front of the plastic. The pressure P will now be P-ΔP. Additionally, as the plastic hits the walls of the mold, it begins to cool. The temperature is now T-ΔT. This increases the viscosity of the plastic and thereby requiring additional pressure to push the plastic. The skin of plastic that is formed at the walls decreases the cross sectional area of the channel. All these factors show that with increasing flow distance and time, the plastic needs increasing pressure to maintain the required flow rate. This results in the reduction of the pressure at the head of the flow front and is called pressure drop.

The molding machine has a limited maximum amount of pressure available to push the screw at the set injection speed. The required pressure to push the screw at the set injection speed should never be more than the maximum available pressure. If so, the process becomes pressure limited. The maximum pressure will be used but the required speed will never be achieved. For example, let us consider that the max available hydraulic pressure on the machine is 2200 psi. In order to move the screw at 5 in/sec, if we need 2500 psi, then we can never achieve the required speed of 5 in/sec. This is what is called pressure limited.