Plastic injection molding, also known as injection molding, is an injection-molding method. It has the advantages of fast production speed, high efficiency, automation of operation, variety of designs, simple to complex shapes, large to small sizes, precise product dimensions, easy product upgrades, and complex shapes.
At a certain temperature, the completely melted plastic material is stirred by the screw, and injected into the cavity with high pressure, and the molded product is obtained after cooling and solidification. Injection molding is suitable for mass production of parts with complex shapes and is one of the important processing methods.
The temperatures that need to be controlled during the injection molding process include barrel temperature, nozzle temperature, and mold temperature. The first two temperatures mainly affect the plasticization and flow of plastics, while the latter temperature mainly affects the flow and cooling of plastics.
Each plastic has a different flow temperature. The source or grade of the same plastic has different flow temperatures and decomposition temperatures. This is because the average molecular weight and molecular weight distribution are different. The plasticization process of plastics in different types of injection machines is also different, so the choice of barrel temperature is also different.
The nozzle temperature is usually slightly lower than the maximum temperature of the barrel. This is to prevent the melt from “salting” when the nozzle is straight through. The temperature of the nozzle should not be too low, otherwise it will cause the early setting of the melt and block the nozzle, or affect the performance of the product.
The mold temperature has a great influence on the intrinsic properties and apparent quality of the product. The temperature of the mold depends on the crystallinity of the plastic, the size and structure of the product, performance requirements, and other process conditions.
The pressure in the injection molding process includes two kinds of plasticizing pressure and injection pressure, which directly affect the plasticization of plastics and product quality.
When a screw-type plastic injection molding machine is used, the pressure on the top of the screw when the screw rotates and retracts is called the plasticizing pressure, and it is also called the back pressure. This pressure can be adjusted by the relief valve in the hydraulic system.
In the plastic injection molding, the size of the plasticizing pressure varies with the design of the screw, the quality requirements of the product, and the type of plastic. If these conditions and the rotation speed of the screw are unchanged, increasing the plasticizing pressure will strengthen the shearing effect. That will increase the temperature of the melt, but it will reduce the efficiency of plasticization, increase countercurrent and leakage, and increase driving power.
In addition, increasing the plasticizing pressure can often make the temperature of the melt uniform, mix the color material uniformly, and discharge the gas in the melt. In general operation, the determination of plasticizing pressure should be as low as possible on the premise of ensuring good product quality, and its specific value varies with the type of plastic used.
In current production, the injection pressure of almost all injection machines is based on the pressure exerted by the plunger or screw on the plastic. 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.
Plastic injection molding
Molding cycle, the time required to complete an injection molding process is called the molding cycle, also known as the molding cycle. The molding cycle 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 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. The filling time in production is generally about 3-5 seconds. The holding time in the injection time is the pressure time on the plastic in the cavity, which accounts for a large proportion in the entire injection time, generally about 20-120 seconds.
Before the melt is frozen at the gate, the amount of holding time will affect the accuracy of the product size. The dwell time also has the best value. It is known that it depends on the material temperature, mold temperature, and the size of the main runner and gate. If the dimensions of the main flow gate and the gate and the process conditions are normal, the pressure value that results in the smallest fluctuation range of the shrinkage of the product is usually used.
The cooling time is mainly determined by the thickness of the product, the thermal and crystallization properties of the plastic, and the mold temperature. The end of the cooling time should be based on the principle of ensuring that the product does not change when it is demolded. The cooling time is generally between 30 and 120 seconds. Excessive cooling time will not only reduce production efficiency, but also make it difficult to demould complex parts, and even demolding stress will be generated when forced demolding.