The screw speed during plasticizing of the mixture is responsible for the homogene­ity of the NFCs. Higher screw speed leads to the high shear forces leading to heat generation resulting in the reduction in viscosity of the mixture. At the same time high screw speed leads to fiber attrition (fiber breakage) and in extreme cases causes the polymer properties to degrade (due to high heat generation). A proper balance between the screw speed and the backpressure should be maintained to allow mix­ing of the fibers and matrix during the entire cooling cycle.

8.3.1.1 INJECTION SPEED

Injection speed is an important process parameter during injection molding of ther­moplastics. Injection speed is dependent upon the melt viscosity of the polymer and the fiber load. The injection speed has to be increased with the rising viscosity of the melt as it becomes difficult for the viscous mixture to flow into thin cavities. High injection speed causes high shear rates while passing through the mold, which reduces the viscosity of the melt. The same has been reported in a study where, when the fiber content of natural fiber was increased above 30% the injection speed was increased in order to overcome the increased viscosity and fill the mold cavi­ties completely.6Injection speed should be faster in case of precision parts having thin cross-section or in case of multi cavity mold. Injection speed is kept compara­tively slower for thick parts. Injection molding machines now come equipped with programmable injection speed control, which can vary the injection speed during injection process.