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What is plastic injection molding process?

Views:2     Author:Site Editor     Publish Time: 2017-04-27      Origin:Site

Moulten plastic is injected at high pressure into an injection mould tool. The plastic cools inside the tool and takes the inverse form of the cavity machined into the tool. 

The process takes several stages: 

1/ Closing - The tool is closed (the 2 halves are brought together) and clamped under high force. This force is maintained until the end of step 4.

2/ Injection - Plastic is injected into the tool at pressure (up to about 2000 bar thus high clamping force required in step 1) 

3/ Hold Pressure - Plastic shrinks as it solidifies. The pressure of moulten plastic is maintained for a period of a few seconds (depends on part) to keep filling the tool as this happens. 

4/ Cooling - The tool remains closed whilst the plastic cools and becomes solid enough to be ejected. 5/ Opening - The tool is opened. 6/ Ejection - The part is ejected from the tool. The process continues at step 1. The total time for these 6 steps is usually between about 8 seconds and 3 minutes depending on the thickness of the part being moulded. (Steps 3 & 4 take much longer with thick parts.) Times of 20 to 40 seconds are typical.


Plastic injection molding is a complex and intricate process. Simply put, the injection molding uses a plunger to force hot, liquid plastic into a mold cavity.The thermoplastic injection molding process at Proto Labs is a standard process involving an aluminum mold with no heating or cooling lines running through it, which means cycle times are a bit longer. It allows our molders to monitor fill pressure, cosmetic concerns, and the basic quality of the parts.

plastic injection mold

Resin pellets are loaded into a barrel where they will eventually be melted, compressed, and injected into the mold’s runner system. Hot resin is shot into the mold cavity through the gates and the part is molded. Ejector pins facilitate removal of the part from the mold where it falls into a loading bin. When the run is complete, parts (or the initial sample run) are boxed and shipped shortly thereafter.

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