The so-called injection molding is that the plastic materials in the barrel of the injection molding machine undergo external heating and the shear heat generated by the rotation of the screw to plasticize the resin materials into a melt, and then apply a certain pressure to inject the melt into the cavity with a certain shape. After cooling and shaping, the products produced are injection molding.

1. Injection molding process

The injection molding process of plastic parts mainly includes four stages: filling, pressure maintaining, cooling and demoulding. These four stages directly determine the molding quality of products, and these four stages are a complete and continuous process.

(1) Pre plastic stage

The screw starts to rotate, and then the plastic delivered from the hopper is delivered to the front end of the screw. The plastic is uniformly plasticized under the effect of high temperature and shear force and gradually gathers in the front end of the barrel. With the accumulation of molten plastic, the pressure becomes greater and greater. Finally, the screw is pushed back gradually by overcoming the screw back pressure. When the plastic in the front of the barrel reaches the required injection volume, the screw stops retreating and rotating, and the pre molding phase is ended.

Plastic Fan Mold

(2) Injection stage

The screw moves forward under the action of the injection cylinder, pushes the plastic stored in the front of the barrel forward at multiple speeds and pressures, and injects it into the closed mold cavity through the runner and gate.

(3) Pressure maintaining and feeding process

The injection pressure in the pressure maintaining stage is called the pressure maintaining pressure. Under this pressure, the melt in the mold cavity is fed due to cooling, and the products are compressed and densified. In this process, the holding pressure plays a leading role.

The characteristics of the pressure maintaining stage are: the melt flows slowly under high pressure, the screw has a small feeding displacement, and the products are gradually shaped with the increase of cooling and density of materials. In the pressure holding stage, the melt flow rate is very small and does not play a leading role, while the pressure is the main factor affecting the process. In the pressure holding stage, the die pressure and specific volume are constantly changing.

(4) Cooling and shaping stage

The plastic is maintained in the mold cavity to prevent the plastic from flowing back until the plastic solidifies, and the pressure in the cavity disappears. The cooling and setting time accounts for the largest proportion in a production cycle.