For Processing injection molding for POLYPROPYLENE (PP) with long fibers, what care should be taken? like injection rates, shear rates in gates, gate type , gate sizes & gate location..etc

Hi Madhukeshwar,
using a high back pressure during plastication (screw-back) is known to lead to high breakage of the fibers. Assuming you want to preserve as much fiber length as possible, you should aim to use the lowest back-pressure which still allows adequate plastication.
Regarding gates, narrow pins gates have been known to lead to bridging (or blockage) of the fiber. So you are right that gate design needs to be considered.

Hi Madhukeshwar

Franco's comments are all things you need to consider. You need to be sure you have the proper tip and screw for the resin installed as well. CHeck with you machine maker and confirm what is is installed is correct in the machine. THen check with the resin suppliers recommendations. There are differences for these resins. ALso screw speed is importatnt to keep as slow as you possibly can. Good luck!

Yet another screaming example of how important a proper cooperation between the analyst, the technologist, and the machine operator is!

Since material contains long fibres, our aim should be process in such away that it can retain maximum fibres length in part. So we have to provide easy flow of material into injection mold impression.
We can get it by
1) Low Screw RPM,
2) Low back pressure,
3) High barrel temperature,
4) High mold temperature,
5) Low injection rate/speed,
6) Increasing nozzle & runner diameter,
7) Tab gate instead of Submarine or film gate (Direct sprue gate is mostly preferred if possible)
8) Gate location on thick portion of the part e.t.c.


If we follow first 5-steps in process we can get poor aesthetic part surface. So we have to also think about that.

I agree with you up to a point:
you generally do not need to have low screw RPM, but low tangential velocity which depend also on screw diameter.
Regarding points 3 and 4 they generally improve the aesthetical surface.
Correct holding pressure and uniform mold temperature are also very important for good aesthetical surface.

With according my experience in additional to process parameters very important remove sharp edge on the runner and if you use tunnel gate - need increase they thickness and improve fillets.
Result is so easy to check - gather samples and put to furnace.
Plastic burned, glass rest.
Very accurate remove from furnace and check what fiber length you have.

considering fiber breakage is the point because it will affect the fiber concentration. Of course, the stiffness will be different.