Plastic injection molding is a common manufacturing process that uses hot molten plastic to create a physical representation of a modeled part. Injection molded parts can be found in everything from the keys on your keyboard to the dashboard of your car or even your X-Wing Lego set. It is used for manufacturing large numbers of identical plastic parts because it can produce them at a relatively low cost with a high level of quality.
The plastic injection molding process begins with pelletized resin that is heated to a melted state. It is then injected into a preformed mold that has been designed specifically for the final product’s shape. Once the plastic has been injected it will cool and solidify into its final form. The mold is then opened and the molded plastic part can be ejected automatically or manually. Injection molded plastic parts can be made from a variety of thermoplastics and can be manufactured to meet a wide range of performance requirements such as stiffness, strength, corrosion resistance, heat resistance, electrical conductivity, etc.
Injection molded parts can also be reinforced with glass mats, fiber meshes or preforms to enhance the material properties of the finished product. The reinforcement is added to the liquid polymer before injection and is incorporated into the final product as a structural component. The addition of reinforcements to a plastic part increases the overall strength, rigidity, and hardness of the finished product, while reducing weight. Reinforcements can be inserted directly into the mold or they can be added through the Structural Reaction Injection Molding (SRIM) process where the preforms, mats and fiber meshes are inserted into the mold prior to injecting the liquid polymer.
During the injection molding process, the molten plastic flows through channels in the mold called runners and gates. The sprue is the main passageway that distributes the molten plastic from the injection nozzle to the front half of the mold. The gate is a small opening in the mold that directs the flow of the molten plastic into each cavity. The sprue and gate have perpendicular features that narrow the flow of the molten plastic entering the gates to control shear and void formation in the resulting part.
Manufacturers rank design for manufacturing (DFM) as the most important factor in reducing overall unit costs for plastic injection molded parts. DFM ensures that the plastic injection molds and processes are optimized for the materials, dimensions, and features of the product being manufactured. DFM helps to create efficiencies that will reduce production costs without sacrificing the integrity of the finished part.
The injection molding machine consists of a hydraulically rotating screw and barrel that are attached to the injection nozzle. A hopper holds the raw pellets of plastic that is fed into the barrel and then the hydraulically rotating screw melts the plastic and transfers it to a runner system that runs around the outside of the injection mold. The injection pressure is controlled and the screw reaches a transfer position when the plastic is 95% full. This pressure is sufficient to complete the injection of molten plastic into the cavities and compensate for thermal shrinkage.