Strength of injection moulded composite

General Composites Discussions Engineering Talk
#1

Hi,
I’m working on a complex part of injection moulded polyester reinforced with glass fibres. The fibres are of various length and orientation. I need to understand the anisotropic behaviour of the strength of the part.

My idea is to test the tensile and flexural properties of the material and make an estimation of the anisotropic behaviour. Since I’m new to composites I have searched the Internet to get some input, but all I can find are michromechanical models that are difficult to understand. Is there any easy way to make the estimation that does not involve too much mathematics?

#2

Difficult question. Only remark I have is that the fiber will orient into the flow direction of the melt while injected. (the longer the fiber, the more it will be oriented.

#3

There are more macro-mechanical theories/estimates out there (using critical fiber lengths, ROM equations), but if you are going to test the composite material in tension and flex, I would recommend doing so in both the perceived fiber orientation (parallel to the fibers), and perpendicular to the fiber orientation. This would theoretically give you the maximum and minimum strengths and moduli, and you can use classical laminate theory (matrix based algebra) to estimate any off-fiber axis properties. Of course, that can be rather mathematically intensive but there are some programs out there that can help you.

#4

No. Every part will be different, and every processing parameter change will change your results. are you talking thermoplastic Polyester, or thermoset? Thermoplastic has issues of shell cooling on the mold surface that thermoset molding will not have. So right there, changes a lot.
You will have to understand fluid dynamics. Suresh Advani is a person to look for books from. He deals a lot in these matters.