Basic process of injection moulding machine
The pressure in the injection moulding machine process includes plasticizing pressure and injection pressure, and directly affects the plasticization of plastics and product quality.
1. Plasticizing pressure: (back pressure) When using a screw injection machine, the pressure on the top of the screw when the screw is turned back is called plasticizing pressure, also known as back pressure. The size of this pressure can be adjusted by the overflow valve in the hydraulic system.
In the injection, the size of the plasticizing pressure needs to be changed with the design of the screw, the requirements of the product quality and the types of plastics. If these conditions and the screw speed are the same, increasing the plasticizing pressure will strengthen the shear Function, that is, it will increase the temperature of the melt, but it will reduce the efficiency of plasticization, increase the reverse flow and leakage, and increase the driving power.
In addition, increasing the plasticizing pressure can often make the temperature of the melt uniform, the mixing of the pigments and the discharge of the gas in the melt. In general injection molding pressure curve operation, the plasticizing pressure should be as low as possible under the premise of ensuring the quality of the product. The specific value varies with the type of plastic used, but usually rarely exceeds 20 kg /Square centimeter.
2. Injection pressure: In the current production, the injection pressure of almost all injection machines is based on the pressure applied by the plunger or the top of the screw to the plastic (converted from the oil pressure) as the standard. The role of injection pressure in injection molding is to overcome the flow resistance of the plastic from the barrel to the cavity, to give the melt filling rate and to compact the melt.
Injection molding cycle
The time required to complete an injection molding process is called the molding cycle, also called the molding cycle. It actually includes the following parts: injection molding cycle, which directly affects labor productivity and equipment utilization. Therefore, in the production process, the relevant time in the molding cycle should be shortened as much as possible under the premise of ensuring quality.
In the entire molding cycle, the injection time and cooling time are the most important, they all have a decisive influence on the quality of the product. The filling time in the injection time is directly inversely proportional to the filling rate, and the filling time in production is generally about 3-5 seconds.
The holding pressure time in the injection time is the pressure time for the plastic in the cavity, and it accounts for a large proportion of the entire injection time, generally about 20-120 seconds (extra-thick parts can be as high as 5-10 minutes). Before the melted material at the gate is frozen, the holding time has an impact on the accuracy of the product size, and if it is later, it has no effect.
The holding time also has the most favorable value, which is known to depend on the material temperature, mold temperature and the size of the main runner and gate (also related to the wall thickness of the product). If the dimensions of the sprue and gate and the process conditions are normal, usually the pressure value that gives the smallest fluctuation range of the product shrinkage rate shall prevail. The cooling time is mainly determined by the thickness of the product, the thermal and crystalline properties of the plastic, and the mold temperature.
The end of the cooling time should be based on the principle of ensuring that no changes are caused when the product is demolded. The cooling time is generally between 30 and 120 seconds. Too long cooling time is unnecessary. It will not only reduce production efficiency, but also It is difficult to demould, and demoulding stress may even occur when demolding forcefully. The other time in the molding cycle is related to whether the production process is continuous and automated, and the degree of continuity and automation.