Injection moulding is mainly divided into the following 4 stages:
1. Filling stage
Melt conveying and injection initiation: The plastic raw material is added to the hopper in the form of granules or powder, and the plastic is conveyed forward, compacted and plasticised by the rotation of the screw of the injection moulding machine (screw-type injection moulding machine) or by the movement of the plunger (plunger-type injection moulding machine). When the plastic is heated to a molten state, the screw or plunger pushes the plastic melt through the nozzle into the gate system of the mould.
Cavity filling process: The melt enters the mould cavity under pressure through the gate, a process that requires the melt to fill the entire cavity uniformly. The flow behaviour of the melt is influenced by a number of factors, such as the viscosity of the melt, the injection speed, the position and size of the gate, etc. At the end of the filling phase, the cavity is basically filled with melt, but the product is not yet fully formed.
2. Pressure maintaining stage
Pressure maintenance and compensation: After the cavity filling is completed, the screw or plunger is not immediately returned, but continues to exert a certain pressure on the melt in the cavity, which is called holding pressure. Because the plastic melt shrinks in volume during the cooling process, the holding pressure stage is designed to replenish the plastic melt in the cavity that has been reduced due to shrinkage, in order to ensure the dimensional accuracy and density of the product. If the holding pressure is insufficient, the product may have shrinkage marks, especially in the parts of the product with large wall thickness; while excessive holding pressure may lead to excessive internal stress in the product, or even produce defects such as flying edges.
Characteristics of filling flow: During the holding pressure process, the melt flows in a different way from the filling stage. In the filling stage, the melt fills the cavity quickly at a high injection rate, while the melt flow in the holding pressure stage is a slow replenishment process. At this point, the melt flow is mainly controlled by the holding pressure and the internal resistance of the mould cavity, and the quality of the product can be optimised by adjusting the holding time and pressure. Too long holding time will prolong the moulding cycle and increase the production cost; too short holding time can not effectively compensate for the shrinkage, which affects the product quality.
3. Cooling stage
Heat transfer and solidification: after the end of the holding pressure, enter the cooling stage. In this stage, the plastic melt in the mould cavity exchanges heat with the outside world through the mould wall, and gradually cools and solidifies into a solid product. The mould is generally cooled by circulating cooling water to take away the heat, so that the plastic melt is cooled quickly. The cooling speed has a great influence on the quality of the product, and different plastic materials have different cooling requirements.
Preparation for demoulding: When the plastic melt has cooled to a temperature low enough to give it the strength and stiffness to hold the shape of the product, the demoulding operation can be carried out. The determination of this temperature needs to take into account factors such as the thermal properties of the plastic and the size and shape of the product. Generally speaking, the cooling time accounts for most of the time of the whole injection moulding cycle, shortening the cooling time can improve the production efficiency, but must ensure that the product can be smoothly demoulded and the quality is qualified.
4. Demoulding Stage
Product out of the mould: demoulding is the last stage of injection moulding. When the product cools down to a suitable temperature, the moving part and the fixed part of the mould are separated by the opening action of the injection moulding machine. The product is then ejected from the mould cavity using an ejector mechanism. The ejection process needs to ensure that the product comes out of the mould intact and undamaged. If the release force is too high or the release method is improper, it may result in damage to the product.
Mould cleaning and reset: After the product is released from the mould, the mould needs to be cleaned briefly to check whether there is any plastic residue or other foreign matter left. If there is, it needs to be cleaned up in time to ensure the quality of the next injection. After cleaning, the mould is closed and restored to its initial state, ready for the next injection cycle.
