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

 Procedure to determine Holding Pressure and Process Window 

  • Set the barrel temperatures to attain the lower value of the recommended melt temperature.
  • Set the injection speed to the value obtained from the viscosity curve experiment. 
  • Set all holding times and pressures to zero
  • Set the cooling time to a value more than what would be typically necessary. E.g. If the estimated cooling time is 10 seconds, set the cooling time to 20 seconds.
  • Start molding and adjust the transfer position to make a part 95 t o98% full.
  • Let the process and the melt stabilize by molding the parts for about 5 to 8 shots.
  • Now set the hold time to a value such that you are sure the gate is frozen. (In the next section we will learn how to optimize this time.) This would be based on some previous experience. For example, for a 30% glass filled PBT or Nylon with a gate size of 0.070” this time ends up between 6 and 8 seconds. In this case, for this part of the experiment, we would set the hold time to 10 to 12 seconds.  
  • Increase the holding pressure in small increments and note the pressure where an acceptable part is made (no shots, flash, etc. …).
  • Note down this pressure as the ‘Low Temperature - Low Pressure’ corner.
  • Increase the pressure further in similar increments and note down the pressure where there is an evidence of an unacceptable situation such as part sticking in the mold or flash, warpage, etc. ….. Note down this pressure as the ‘Low Temperature - High Pressure’ corner.
  • Repeat steps (9) and (10), but at the high end of the recommended melt temperature. This time the two extreme pressures would be the ‘High Temperature - Low Pressure’ and ‘High Temperature - High Pressure’ corners.
  • Joining these four corners would now generate the process window or the molding area diagram.
  • Set the process to the center of this window.
Note: In case of crystalline materials, the recommended melt temperature range is small. In such cases you can perform the window study at one temperature only and generate a high and low limit for the holding pressure.

The figure below shows a typical process window generated at the molding machine.
 

How to use this information: 
The process window is an indicator of how much can you vary your process and still make an acceptable part. An ideal situation is you have a wide process window. If the process window is very narrow, then there is always a danger of molding parts with defects. For example in the above graph, if the process window is very small, then one could get occasional short shots or occasional flash due to the natural variation in the process. A robust process is one that has a large process window and accommodates the natural variation. 

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