injection molding explain
Injection molding, also known as injection molding, is a method of injection and molding. The advantages of the injection molding method are fast production speed, high efficiency, automatic operation, many varieties of designs, simple to complex shapes, large to small sizes, accurate product sizes, easy to update products, and complex shapes. Injection molding is suitable for large-scale production and shape complex products and other molding processing fields.
At a certain temperature, the completely melted plastic material is stirred by a screw, injected into the mold cavity with high pressure, cooled and solidified, and then the molded product is obtained. This method is suitable for the mass production of complex shape parts and is one of the important processing methods.
The temperature of the nozzle is usually slightly lower than the maximum temperature of the barrel, which is to prevent the possible "salivation" of the molten material in the through nozzle. The temperature of the nozzle should not be too low, otherwise, the nozzle will be blocked due to the early setting of the melt, or the performance of the product will be affected due to the early setting material injected into the mold cavity.
Mold temperature has a great influence on the internal performance and apparent quality of products. The mold temperature depends on the crystallinity of the plastic, the size and structure of the product, performance requirements, and other process conditions (melt temperature, injection speed, injection pressure, molding cycle, etc.).
The pressure in the injection molding process includes plasticizing pressure and injection pressure, which directly affects the plasticizing of plastics and the quality of products.1.Plasticizing pressure:
(back pressure) when the screw injection machine is used, the pressure on the melt at the top of the screw when the screw rotates and retreats is called plasticizing pressure, also known as back pressure. This pressure can be adjusted by the relief valve in the hydraulic system.
In the injection, the plasticizing pressure needs to change with the design of screw, the requirements of product quality and the type of plastic. If these conditions and the rotating speed of screw remain the same, increasing the plasticizing pressure will strengthen the shear effect, that is, it will increase the temperature of melt, but it will reduce the plasticizing efficiency, increase the countercurrent and leakage, and increase the driving power.
In addition, increasing the plasticizing pressure can make the melt temperature uniform, the color mixture uniform and the gas in the melt discharged. commonly
2.Pressure curve in injection molding
During operation, the plasticizing pressure shall be determined as low as possible on the premise of ensuring the good quality of the products. The specific value varies with the type of plastic used, but it is rarely more than 20 kg / cm2.
In the current production, almost all the injection pressure of the injection machine is based on the top of the plunger or screw to the plastic
The applied pressure (converted from the oil circuit pressure) shall prevail. The function of injection pressure in injection molding is to overcome the flow resistance of plastic from the barrel to the cavity, to give the rate of filling mold and to compact the melt.
The time required to complete an injection molding process is called the molding cycle, also known as the molding cycle. It actually includes the following parts:
Molding cycle: molding cycle directly affects labor productivity and equipment utilization. Therefore, in the production process, we should try to shorten the relevant time in the molding cycle on the premise of ensuring the quality. In the whole molding cycle, injection time and cooling time are the most important, which have a decisive impact on the quality of products.
The filling time in the injection time is directly inversely proportional to the filling rate, and the filling time in the production is generally about 3-5 seconds. The pressure holding time in the injection time is the pressure time of the plastic in the mold cavity, which accounts for a large proportion in the whole injection time, generally about 20-120 seconds (the extra thick parts can be as high as 5-10 minutes).
The amount of holding time before the melt is frozen at the gate has an effect on the dimensional accuracy of the product, but in the future, 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 sprue and gate. If the dimensions of the sprue and gate and the process conditions are normal, the pressure value of the minimum fluctuation range of the shrinkage rate of the product is usually taken as the standard.
The cooling time mainly depends on the thickness of the product, the thermal and crystalline properties of the plastic, and the mold temperature. The end point of cooling time shall be based on the principle of ensuring no change in product demoulding. Generally, the cooling time is about 30-120 seconds. It is unnecessary to have too long cooling time, which will not only reduce the production efficiency, but also cause difficulty in demoulding complex parts, and even produce demoulding stress when forced demoulding. The other time in the forming cycle is related to the continuity and automation of the production process and the degree of continuity and automation.
The injection pressure is provided by the hydraulic system of the injection system. The pressure of the hydraulic cylinder is transmitted to the plastic melt through the screw of the injection molding machine. Driven by the pressure, the plastic melt enters the vertical channel (which is also the main channel for some molds), the main channel and the shunt channel of the mold through the nozzle of the injection molding machine, and enters the mold cavity through the gate. This process is called the injection molding process, or the filling process. The existence of pressure is to overcome the resistance in the melt flow process, or conversely, the resistance in the flow process needs to be offset by the pressure of the injection molding machine to ensure the smooth filling process.
In the process of injection molding, the pressure at the nozzle of injection molding machine is the highest to overcome the flow resistance in the whole process of melt. After that, the pressure decreases gradually along the flow length to the front of the melt. If the cavity is well vented, the last pressure at the front of the melt is atmospheric pressure.