Cavity Balance
Pressure Drop
Process Window
Gate Seal
Cooling Study

Procedure to generate a Viscosity Curve

  • Set the melt temperatures to those recommended by the manufacturer. If there is a range, set the temperatures to the center of the range.
  • Set all the holding phase parameters to zero. This means that there will not be any holding phase and only injection.
  • Set the injection pressure to the maximum available.
  • Set the cooling time to a safe value such that the part will be cool and has reached the ejection temperature before mold opening.
  • Set the injection speed to ‘slow’ and make a part. The part should be short. If not adjust the transfer position to make the part such that it is filled only about 50%.
  • Increase the speed in steps and make sure that the parts are still short. Mold a part with close to the maximum injection speed and make sure that it is still short. If it is full, then adjust the transfer position, such that it is about 95 % full part. If it is less than 95 % full, then also adjust such that the part is 95% full. This means that at close to the maximum injection speed you have a 95% full part with no holding time or pressure.
  • Make another shot and record the fill time and the peak hydraulic pressure required to fill the part. Note: The peak hydraulic pressure will be the pressure required to move the screw at the set injection speed. This is taken from the available pressure from the machine. For example, the machine is set to 2200 psi but may require only 1850 psi to move the screw at the maximum speed of 5 in/sec.
  • Next, lower the speed by a small amount, for example from 5 in/sec to 4.5 in/sec or from 90% to 80%. Note the fill time and the peak injection pressure.
  • Repeat the above step all the way till you get to the lowest injection speed possible. Divide the available injection speed range into about 10 - 12 speeds so that you get as many data points.
  • Find the Intensification Ratio of the screw from the machine manufacturer. If this number is not available, pick it to be 10. It does not really matter since this is a constant used in the equation and will factor the viscosity proportionately.
  • To get the viscosity, use the following formula: Viscosity = Peak Injection Pressure X Fill Time X Screw Intensification RatioPlot the graph of viscosity vs injection speed.

The figure below shows a typical viscosity curve that as generated at the molding machine.

How to use this information:
Looking at the above curve, one can notice that the viscosity stays fairly constant after about 60% of the injection speed. Therefore, setting the injection speed to 70% would ensure that the filling stage of the process will stay consistent. Any small natural variations will not cause large changes in viscosities resulting in shot to shot variations.

Shot to shot variations should be reduced in order to achieve repeatable quality of parts. This is especially important in case of tight tolerance parts and multi cavity molds. Optimizing the injection speed through In-mold rheology is only the first step to achieving a robust process. Later, the holding phase and the cooling phase must also be optimized.
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