random short fibres reinforcement are used in the moulding by extrusions and moulding processes by pressing and injection. these manufacturing processes cause degradation of long fibres

Long fibres are usually better from the point of view of the resultant mechanical properties, but cannot be processed by many methods. Short fibres are much more versatile from the point of view of processability, and can be sprayed with thermodurcissable resins or be used in most extrusion techniques with thermoplastics, i.e. in much more methods compared to long fibres.

Alain

I would also add that, even if short fibres are not as good as long ones, composites correctly prepared always present higher properties than pure polymers, so it is always interesting to reinforce them, even with short fibres. And this is especially true when these fibres are cheap: not only the composite will be cheaper than the polymer alone (as polymer is generally the most expensive, except if high-performanecs carbon fibres are considered ...), but it will also present higher mechanical properties. Therefore, most plastics are almost never used alone but as composites.

Alain

CAE analysis shows good mechanical results but during part manufacturing like injection or compression molding there is huge chances of fibre breakage... so test result will be differ negatively from CAE..  Isotropic property is major advantage for short fiber  reinforcement. My question is really above two factor affect the long fiber..?

The properties of short fibers composites depend the stress on the rupture and length L > LC (critical see article Aveston ,couper and Kelly or wells and Beaumont) and the nature on the interface matrix fiber. The optimization of these parameters is essential

We have identical parts that are a composite base that components are mounted to with threaded fasteners into a cored hole in the base.  One is compression molded(longer fibers) and the other is injection molded(shorter fibers).  The screw forms its own threads in the cored hole.  For the same torque parameters, the screw strips material from the cored hole in the injection molded part more often than in the compression molded part.  The injection molded part is cheaper than the compression molded part.