Residual stresses are the internal stresses that exist inside the molded part in the ab­sence of the external load. The residual stresses in case of injection molding are flow induced and thermal induced stresses. The flow-induced stresses in polymer chains depend on orientation and packing pressure while thermal stresses are induced as a result of non uniform cooling of the molded part.29 In other words, residual stresses in case of injection molding are a result of temperature variations, high pressure generated and polymer chains orientation after cooling.30 The characteristic residual stress distribution in an injection molded part shows tensile stresses at the core and surface, and compressive stresses at intermediate region.2531 The major causes of residual stresses in case of fiber reinforced composites have been listed as high pressure gradient, orientation of the polymer chains, non uniform temperature pro­file and the difference in the thermal expansion coefficient of fibers and the matrix. These stresses are introduced during injection molding process stages of filling, packing, and cooling. Residual stresses in the molded composite cause an early fracture. The stress distribution in injection molded part depends upon the pressure history to which the melt mixture is subjected from start to filling up of the mold cavity.25 The residual stresses in case of injection molded parts may cause defects like stress cracking, warpage and long-term deformation.32 The residual stresses can be reduced by the gradual cooling of the molded part. The gradual cooling of the injection-molded part is achieved by setting higher mold temperature. Heat treat­ment of the injection-molded parts is another way of relieving the residual stresses developed due to non uniform heating. The residual stresses can also be reduced by carefully adjusting the process parameters like screw speed, injection pressure, melt and mold temperature as well as taking the rheological properties of the polymer matrix into consideration. The appropriate mold design helps in the reduction of residual stresses. The mold designing includes the injection gate size and location, cavity shape and vents for air to escape.

8.2 CONCLUSION

The driving forces behind the use of NFCs are environmental as well as economic considerations. The use of NFCs is rapidly increasing in aerospace, automobile, house hold, electric, sports goods and biomedical applications. This has made NFCs as a potential candidate for research efforts. The literature available on NFCs has shown that NFCs have the potential to replace the traditional PMCs in many ap­plications. As the demand is increasing; fast, easy and economical processing tech­niques are required for fabrication of NFCs. Injection molding of NFCs has been reported by many researchers but still much work has not been presented on the selection and optimization of the process parameters. In other words, there is no set of thumb rules available for the processing of NFCs. In this chapter different as­pects regarding fabrication of injection molded NFCs have been discussed. Injection molding process parameters like screw barrel temperature, screw speed, injection speed, injection time, injection pressure, mold temperature and back pressure have been discussed. The study of the effect of these parameters on the injection molding of NFCs is important to ensure a defect free injection molded composite. Distribu­tion and orientation of fibers, fiber attrition and residual stress generation are the main issues that have to be taken care of during injection molding of NFCs. These issues can be minimized by careful selection of process parameters and by optimal mold design. The rheological properties of the polymer matrix should also be taken into consideration while selecting the parameters and designing of the mold.

Finally, it can be concluded that injection-molding process has a huge poten­tial for processing of natural fiber reinforced composites. A judicious selection of process parameters and an optimal mold design can further enhance the application areas of injection molding process in context of the natural fiber reinforced com­posites.