What is Back Pressure in Injection Molding Machine?

What is Back Pressure in Injection Molding Machine?

At the time when the injection molding machine takes charge, we apply pressure from the back side of the screw inside the barrel. This allows the screw to push the molten plastic material to move forward. This pressure exerted from the backside is called back pressure. If we hadn’t applied that pressure on the screw, the screw would move backward, allowing the material to remain distinct throughout the barrel itself without any change. Within the injection molding machine, the screw is circulated around the barrel at the back, with a hydraulic motor placed at the back. The back pressure prevents the linear motion of the plastic particles inside the barrel. The amount of back pressure applied should not be very high or low. It should be optimum enough for the material comes out of the nozzle at the time of charging. We can keep back pressure high only when we are mixing two materials, like at the time of the master match. After setting, if the back pressure keeps being given continuously, drooling might start. So, to avoid drooling, 4 mm or 10 mm back pressure is given depending on the type and amount of material to linearly make the screw move. This is called a suck-back situation for an injection molding machine. In the case of materials like PVC, PBT, ABS, HIPS etc., a little more amount of backward pressure is given. If back pressure is missed due to drooling, there might be problems in various components of the injection molding machine.


We can increase back pressure in the following circumstances:

  • In the case of the short molding in frequent cycles is required.

  • When bubbles are formed.

  • If air is trapped by the machine.

  • If there appear sink mark, flow mark, silver lines if/any/ etc. Back pressure is attached to the back side of the cylinder or barrel. The knob that sets the back pressure must be set in increasing order; it is rotated clockwise and vice versa.


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