How the recycled materials affect virgin plastic and its mechanical properties?

Adding recycled material simply reduces the effective molecular weight of the material you are processing. So, whatever product characteristics are affected by lower effective molecular weight will suffer. Even of the effects on the final product are minimal or acceptable, it will affect processing since it affects the viscosity of the melt -- the same as if you changed resins or resin formulation. In many extrusion blow moulding processes or any other process where it is an inherent part of the process, the key is to keep it consistent. Statistically designed experiments are efficient and effective at helping you optimize the settings for a given blend of virgin and recycled material. Statistical process control charting is your best friend for providing guidance to the effects of the recycled material. Control chart the injection or extrusion pressures, fill times (injection moulding), melt temperatures, product weight and other key quality characteristics like shrinkage. Use Xi/R charts and EWMA charts to best determine the need for adjusting the process to accommodate variation in the mix.

For high quality products and products that are designed to optimize the use of materials by careful attention to their design in shape, thickness etc., you may want to avoid the use of recycled materials. If you are making products for disposable applications or those with limited life, then the use of a proportion of recycled materials may be acceptable. For other products, it may be acceptable if the quality of the recycled material is uniform and you keep its use to a minimum. In any event, it will also depend on which polymers are involved. It may affect your processing and product surface finish and color as well as the mechanical properties. For specific applications you would need to conduct manufacturing tests and tests on the final product. The type of testing that you can do will depend on the product.

Regrind can also result in an overdosing of pigment (It's already colored so doesn't need anymore). This can have the added effect of increasing MFI (higher % of 50MFI master batch as an example). You can obviously fix this with a gravimetric blender adjusting for regrind %.

Another point is that the "fines" are smaller (obviously) so they become fully molten earlier. Easy test throw some fines and some graduals on a hot nozzle to see if they melt and which one does first. Add to this that the fines will sink to the bottom of your hopper faster those larger pieces giving a high concentration of fines. So this means that you will have a larger % of the screw with molten polymer on it which equates to higher torque required to rotate the screw, longer and varied heat history for regrind and especially fines.

Also for the x linking of the polymer after processing. I know that PP and LDPE are not known for their X-Linking but just ask your local Blown Film manufacturer about gels (yep you guessed it x-linked). The answer that comes out of the tests that you carry out on your machines using your materials.