Injection molding is a manufacturing process for creating custom parts by injecting material into a mold. It can be achieved with a host of materials, including metals, glasses, elastomers, confections, as well as thermoplastic and thermosetting polymers. First, the material for the part is put into a heated barrel, mixed, and then output into a mold cavity where it cools and hardens to the configuration of the mold. Molds are made by a moldmaker from either steel or aluminum. Injection molding is commonly utilized for manufacturing a variety of parts, from the smallest components to entire body panels of cars. In the case of plastics being molded, pellets of raw material are put into a hopper into a heated barrel with a reciprocating screw. Once the raw material enters, the heat increases and the Van der Waals forces that resist relative flow of individual chains are weakened as a result of increased space between molecules at higher thermal energy states. This makes the plastic malleable and able to take on the shape of its mold.
The plastic injection molding usually includes the following steps: feeding, plasticization, injection, pressure holding, cooling and mold release.
- Feeding: Plastic pellets or powders are fed into the injection hopper.
- Plasticization: The plastic is heated in the barrel and turned from solid pellets into a molten flow, which possesses great plasticity. This process is referred to as plasticization.
- Injection: The plasticized molten flow will be pushed to the forepart of the barrel by the plunger or the screw, and then injected to fill the mold cavity through the injection nozzle and sprue & runner system of the mold. This step is called injection.
- Pressure holding: When the molten plastic is shrinking inside the mold due to cooling, the plunger or the screw will continuously force the molten material in the barrel into the mold for replenishment, to ensure that a complete structured and dense textured plastic product is produced. This step is known as pressure holding.
- Cooling: The in-mold cooling process of the plastic part usually refers to the entire process from the moment the molten plastic at the gate is fully solidified to the plastic part is ejected from the mold cavity; but actually the cooling step starts the minute the molten plastic flows into the cavity, covering the time period from completion of injection, pressure holding to the moment before mold release starts.
- Mold release: Mold release is allowed when the plastic part is cooled to a certain temperature, through which the plastic part is pushed out of the mold by the ejectors.