Friday 17 April 2015

Injection molding filling simulation - My method for clear identification of regions with high shear rates

For showing you a method for a clear identification of high shear rate areas , I want to use again the perforated plate as a demonstration part.

We put lots of emphasis on parameters like filling pressure needed, gating position and temperature raise during filling.
But when you work with polymers which are sensitive to mechanical stresses like shear rates then it is worth to have a plan of action how to locate critical areas and solve them by using simulation or in a simple way with analytic methods.

In this post I show you a way of using CFD- based filling simulations, not only with contour plots but  also with shear rates tracers.

First, let's have a look at the video I made to have a better insight:

In the video you can see the perforated plate in the version of side gating and central gating. This applied method of shear rate tracer release is possible in the virtual molding package Sigmasoft.
For sure, the other suppliers like Moldflow or Moldex3d have similar options, which you can use in the following way.

General definitions:
A tracer is nothing else then a velocity vector colored when passing through a defined cell of your part. In case of the shear rate tracer, defining an area of cells is not possible. Furthermore, the definition of a critical shear rate is done.
You know at which level the shear rate can be dangerous regarding degradation of the polymer. This value you can use as a maximum border and a value underneath (25% less) presents as the releasing border of the tracer. So, you still have a safety pillow.  

My procedure:
This is my 4th step procedure I use in the post-processing after I have done a process simulation:

1) Watch the shear rate contour plot to get the "big picture"
2) Activate the shear rate tracer
3) Analyze the release places and where the sheared material will end up in the part (to predict if there will be a decrease in the mechanical properties of the part)
4) Make geometry changes or process changes (melt temperature; inlet velocity profile)

The final aim of every part and mold design should be that the critical areas are located at the gating. Here we use higher shear rates for decreasing viscosity and ensure a final mixing of the polymer, especially if it is colored). There should be no critical shear rate areas in the part itself.

The shear rate tracer method is helps you to locate the punctual critical areas. So far, those are the advantages of such kind of approach. Another aspect is that the allover simulation will take more time and more memory as well as more working space.