The plastic injection molding cycle

The complete injection process is called a molding cycle. The period between the start of the injection of the melt into the mold cavity and the opening of the mold is called the clamp close time. The total injection cycle time consists of the clamp close time plus the time required to open the mold, eject the plastic part, and close the mold again, the injection molding machine transfer the resin into molded parts through a melt down, injection, pack, and cool down cycle. A plastic injection molding machine includes the following major components as below.

Injection system: feed the raw materials into cylinder, heat up and melt it down, push the melted materials into the cavity through the spure.
Hydraulic system: to provide the force of injection.
Mold system: to load and assemble the mold.
Clamping system: to provide packing force.
Control system: to control action, cooling system.

Clamping force is commonly used to identify the capacity of the plastic injection molding machine, other parameters include shot volume, injection rate, injection pressure, screw, layout of inject bar, mold size, and the distance between tie bars. Plastic injection molding machines can be divided into several categories, besides general-purpose machines for normal plastic parts without high precision or unusual design, there are tight-tolerance machines especially for high precision parts, and high-speed machines for thin-wall parts.

A whole injection molding process includes following six steps

1) The mold closes and the screw begins moving forward for injection.


2) Filling, eject melted raw materials into the cavity.


3) Pack, the cavity is packed as the screw continuously moves forward.


4) Cooling, the cavity cools down as the gate freezes off and closed, the screw begins to retract to plasticize material for the next cycle.


5) Mold open and part ejection, the mold opens and parts are injected out by ejection system.


6) Close, the mold closes and the next cycle begins.