Injection molding process is the most extensively used molding method in the in­dustry used for the production of polymer composites due to its simplicity and fast processing cycle. Injection molding machine mixes and injects a measured amount of matrix and fiber mixture into the mold resulting in the desired product. It consists of three major sections: the injection unit, mold, and ejection and clamping unit (see Fig. 8.4).

The injection unit consists of a heated screw barrel having a compression screw, which can rotate as well as reciprocate. The function of the heated barrel is to pro­vide heat to the polymer matrix to melt before injection. The function of the recip­rocating screw is to carry and compress the pellets from the hopper into the heated barrel, mix the polymer matrix and fiber, provide heat to the matrix by viscous shearing and inject the mixture into the closed mold by acting as a piston. In other words injection unit consists of an extruder with an added function of reciprocating screw that injects the mixture into the mold. The cavities in the mold are the replica of the desired geometry of the product. The molds consist of cooling and/or heat­ing coils to regulate the mold temperature. The mold temperature determines the cooling rate of the product. The clamping unit clamps the mold tightly against the injection pressure to prevent burr formation and the ejector unit actuates the ejectors in the mold to eject the part when the cycle completes.

A typical injection molding process cycle is shown in Fig. 8.5. Generally the injection molding cycle is assumed to start from mold close position. After the mold closes it is tightly clamped against the injection pressure by the clamping unit. In the mean time the screw is retracted to its back position and then it injects the molten mixture with the desired injection pressure and speed into the mold cavity. The in­jected mixture undergoes shrinkage during solidification and to compensate that the screw is kept forward by the desired holding pressure for some time. After this point the screw starts to retract and plasticize the mixture while the part is being cooled in the mold. The part is allowed to cool sufficiently to be able to bear the ejector force and meet the desired dimensions. In the mean time the screw is being pushed backward as it is rotating and accumulating the mixture in the front. The part is then ejected and the cycle repeats itself.

FIGURE 8.5 Injection molding cycle.