July 14, 2022

How to define the viscosity curve of injection molding?

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1) Set the melt temperature in middle of range a described by manufacture.
2) Set all the holding phase parameter to zero. This means only injection no holding.
3) Set the Injection pressure to maximum to available.
4) Set the cooling time sufficient to ejection.
5 Set injection speed slow, and make sure component should be short only 50% filled, if not adjust the material position.

6) Increase the injection speed in steps and sure that part should be still short. When injection velocity is full adjusting the material position that only 95% component is filled.
7) Now make another shot and note the Fill time and peak pressure.
8) Now lower the injection speed to 90%, 80% so on and note down peak pressure and fill time.
9) Find the screw intensification factor of screw (for electric machine it is 1:1, for hydraulics it is Area of ram/section area of screw)

10) To get viscosity use the formula.

Viscosity = Peak Injection pressure* fill time * Intensification factor

11) Plot the graph Injection speed vs Viscosity.

You get the region in curve where viscosity is constant w,r,t to Injection speed . This is you region of processing for injection speed, where the consistency in maintaining shot to shot. (Now shearing at nozzle and at gate point not changed the viscosity).

Sometime solution is depending on shape of the component. For example we face warpage problem in Car interior view mirror housing, As a result mirror is not assembled properly. But I did, tighten two component of housing with rubber band after produced from machine and store these as it is. These are opened on assy line when mirror is to be fitted problem is resolved.

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