Procedures
Viscosity
Cavity Balance
Pressure Drop
Process Window
Gate Seal
Cooling Study

Procedure to determine Pressure Drop

Consider the plastic flows through the following sections: The nozzle of the machine, the sprue, the primary runner, the secondary runner, the gate and the end of fill. The procedure to determine the pressure drop is as follows.

  • Set the machine to the maximum available pressure.
  • Build a shot on the molding machine and take an air shot. Note down the peak pressure required to do so. 
  • Next start molding but mold only the sprue and note the peak pressure.
  • Mold the primary runner only and note the peak pressure.
  • Mold the secondary runner only and note the peak pressure.
  • Mold a shot such that it just enters the gate and note the peak pressure.
  • Mold a shot such that it just reaches the end of fill and note the peak pressure.  
  • Generate a graph as shown and look for the sections that have a high pressure drop.   

Note: Set a short screw recovery delay time. In doing so you will prevent immediate back flow into the cavity. If the gate is not frozen, the back pressure will influence the cavity fill.  
 
 

How to use this information: 
The maximum pressure used in the process should never be equal to the maximum available pressure at the machine. For example, if the maximum available hydraulic pressure is 2200 psi, then the end of fill pressure should not be equal to 2200. If this is the case, it means that the screw needs more pressure to move at the set injection speed and cannot do so because of the limited pressure. Such a condition is called ‘Pressure Limited’. Typically, you should a maximum of about 90% of the maximum available pressure. Therefore in this case, where the maximum is 2200, the end of fill pressure should not be more than 1980 psi.

In the generated graph, if you are pressure limited or more than 90% of the maximum, look for steep increases in the pressure and try to reduce these. For example, If the secondary runner section shows a steep increase, then this means it takes a lot of force to move the plastic through this section. Increasing the diameter of the runner will help in reducing this pressure.  

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