Injection molding is sometimes referred to as a “net shape” manufacturing process because the molded parts emerge from the molding process in their final form with no or minmal post-processing required to further shape the product. An operating injection molding machine is depicted in Fig.1.1. The Mold is inserted and clamped between a stationary and moving platen. The mold typically is connected to and moves with the machine platens, so that the molded parts are formed within a closed mold, after which the mold is opened so that the molded parts can be removed.
The mold cavity is the “heart” of the mold where the polymer is injected and solidified to produce the molded part(s) with each molding cycle. While molding processes can differ substantially in design and operation, most injection molding processes generally include platication, injection, packing, cooling, and ejection stages. During the plastication stage, a screw within the barrel rotates to convey plastic pellets and form a “shot” of polymer melt. The polymer melt is platicized from solid granules or pellets through the combined effect of heat conduction from the heated barrel as well as the internal viscous heating caused by molecular deformation as as the polymer is forced along the screw flights. Afterwards, during the filling stage, the platicated shot of polymer melt is forced from the barrel of the molding machine through the nozzle and into the mold. The molten resin travels down a feed system, through one or more gates, and throughout one or more mold cavities where it forms the molded product(s).
After the mold cavity is filled with the polymer melt, the packing stage provides additional material into the mold cavity as the molten plastic melt cools and contracts. The plastic’s volumetric shrinkage varies with the material properties and application requirements, but the molding machine typically forces 1 to 10% additional melt into the mold cavity during the packing stage. After the polymer melt ceases to flow, the cooling stage provides additional time for the resin in the cavity to solidify and become sufficiently rigid for ejection. Then, the molding machine actuates the moving platen and the attached moving side of the mold to provide access to the mold cavities. The mold typically contains and ejection system with moving slides and pins that are then actuated to remove the moldes part(s) prior to mold closure and the start of the next molding cycle.
Remark: this artical is from book <Injection Mold Design Engineering> 2nd Edition. The author: professor David Kazmer, Department of Plastics Engineering, 1 University Ave, Lowell MA 01833 USA.