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Injection Molding Cycle

May 23,2019 | Views: 2768

The plastic injection molding cycle consists of four phases:

1. Clamping is the closing of the mold.

2. Injection is the filling of the mold.

3. Cooling is the hardening of the plastic within the mold.

4. Ejection is the removal of the plastic part from the mold.



During the clamping portion of the injection molding cycle, the clamp should close quickly at first and then slowly until closed (without slamming). Once closed,the press builds high clamp pressure or tonnage before injecting plastic. The function of the clamping portion of the injection molding cycle is to quickly andsafely close the mold. Once closed it must build tonnage to resist the injection unit pushing material into the mold.



During the injection phase the hot molten plastic material is first pushed as fast as possible into a cool closed mold by the screw.Once the mold is full the injection unit packs and holds extra material into the mold to compensate for shrinkage. It does this until the gate freezes off.The function of "fill" is to push plastic into the mold cavity as fast as possible until about 95% full. The function of "pack" is to fill the last 5% of plastic into the mold and to put extra plastic into the cooled mold. This is desirable because as thermoplastics cool they shrink. The function of "hold" is to hold the extra plastic in the cooled mold until the gate freezes off. This prevents backflow of material through the gate.



The cooling portion of the injection molding cycle starts after the hold function ends. During the cooling phase, when the mold is in the closed position, the hot liquid plastic is cooled into a solid plastic part until it hardens enough for the part to be ejected. During this cooling phase, the screw is rotating to build a new shot for the next cycle. The function of the cooling portion of the injection molding cycle is to quickly solidify the part and to build the next shot. Building the next shot is commonly referred to as plastication.



The function of the ejection portion of the injection molding cycle is to quickly and safely open the mold and eject the part. During the ejection portion of the injection molding cycle, the clamp should open slowly at first and then quickly until it slows down to a stop without slamming back. Either during opening or after the mold is open, the ejectors will come forward to eject the part from the mold. Once the part is ejected, the ejectors retract. Then the press is ready to begin the next cycle.

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