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What is Plastic Injection Molding?

What is Plastic Injection Molding?

After the plastic material is heated and plasticized in the injection barrel, the molten plastic flow will be injected into the mold cavity through the sprue and runner system, and finally, take shape after being cooled. This process is defined as plastic injection molding.

The Detail of the Process

During the molding process, the plastic pellets will be first delivered into the high-temperature injection barrel through the hopper, where they are heated, melted, and plasticized into a sticky molten flow, which will then be injected into a lower temperature closed mold through the injection nozzle at a high speed under the great pushing pressure exerted by the plunger or the screw.
Under great pressure, the molten plastic will fill the entire cavity and will also be compacted. After that, the plunger or the screw will return after a period of pressure holding. At this point, it is possible to flow back from the cavity into the sprue and runner system. When the mold opens after cooling and forming, the product will be released from the mold cavity.
The common process usually includes the following steps: feeding, plasticization, injection, pressure holding, cooling, and mold release.
Feeding: Pellets or powders are fed into the hopper.
Plasticization: The plastic is heated in the barrel and turned from solid pellets into a molten flow, which possesses great plasticity.
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 an injection.
Pressure holding: When the molten material 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 completely structured and densely textured product is produced. This step is known as pressure holding.
Cooling: The in-mold cooling process usually refers to the entire process from the moment the molten material 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 the injection, pressure holding to the moment before mold release starts.
Mold release/Ejection: Mold release is allowed when is cooled to a certain temperature, through which the plastic part is pushed out of the mold by the ejectors.