Principles of plastic injection moulding

Injection moulding is the process of injecting a plastic melt at high pressure into a closed mould cavity at high speed, after cooling and shaping to obtain a plastic product with the exact shape of the mould cavity. 

Injection moulding must meet two necessary conditions: firstly, the plastic must be injected into the mould cavity in a molten state; secondly, the injected plastic melt must have sufficient pressure and flow rate to completely fill the mould cavity. Therefore, injection moulding must have the basic functions of plasticisation, melt injection and holding pressure moulding. 
 

 (1) Plasticisation process In the plasticisation process of injection moulding, the solid plastic is continuously moved forward in the direction of the screw groove by the conveying action of the rotating screw, and is heated, compacted, sheared and mixed by the screw threads, etc. and heated up into a viscous flowing plastic fluid with uniform density, viscosity and components and uniform temperature distribution. The heat required for the plasticisation of solid plastics comes mainly from the heating of the plastic by the external barrel and the frictional shear heat of the injection screw on the plastic. In the plasticisation process, whether the temperature of the plastic melt meets the injection requirements and whether the temperature distribution is uniform are important parameters to measure the plasticisation function of the injection moulding machine, while the plasticisation function refers to the size of the amount of molten plastic that the injection moulding machine can provide in a unit of time. 
 

The solid plastic is plasticised into a melt and then pushed by the rotating screw to the head of the screw and stored in the storage area at the front of the barrel, where the molten plastic has a certain pressure, which acts on the screw and pushes it backwards against various resistances. The screw stops rotating after a certain distance and the volume of molten plastic in the storage area (called the injection volume) is determined and the plasticising process is completed.

(2) Injection process The plasticized plastic melt is stored in the storage area of the barrel, when injecting, the screw moves axially, under the action of the screw injection pressure, the plastic melt flows at a certain rate through the nozzle installed in the front of the barrel, mould pouring system, etc. and is injected into the mould cavity. 

 (3) Cooling and shaping process The molten plastic injected into the mould cavity overcomes various flow resistance and fills the cavity, the molten plastic filling the cavity is subjected to great pressure from the mould cavity, this pressure has driven the molten plastic flow back to the barrel drive; and, due to the cooling effect of the mould cavity so that the molten plastic produces cooling shrinkage, at this time the injection screw continues to provide pressure to keep the molten plastic filling the cavity without reflux, and The melt is replenished as appropriate to fill the shrinkage space in the cavity until it cools and cures into a product.