Injection molding process
1. Temperature control 1. Barrel temperature: The temperatures to be controlled during injection molding include barrel temperature, nozzle temperature and mold temperature. The first two temperatures mainly affect the plasticization and flow of plastics, while the latter mainly affects the flow and cooling of plastics. The flowing temperature of each plastic is different, and the flowing temperature and decomposition temperature of the same plastic are different because of its different source or brand. This is because the average molecular weight and molecular weight distribution are different, and the plasticizing process of plastics in different types of injection molding machines is also different, so the temperature of the barrel is also different.
4. Nozzle temperature: The nozzle temperature is usually slightly lower than the highest temperature of the barrel, which is to prevent the "salivation phenomenon" that may occur in the straight-through nozzle. The nozzle temperature should not be too low, otherwise the molten material will solidify prematurely, blocking the nozzle, or the product performance will be affected because the solidified material is injected into the mold cavity.
3. Mold temperature: Mold temperature has a great influence on the internal performance and appearance quality of products. Mold temperature depends on the crystallinity of plastics, the size and structure of products, performance requirements and other technological conditions (melt temperature, injection speed and pressure, molding cycle, etc.). ).
Second, the pressure control in injection molding process includes plasticizing pressure and injection pressure, which directly affects the plasticization of plastics and product quality.
1. Plasticizing pressure: (Back pressure) When a screw injection molding machine is used, when the screw rotates reversely, the pressure on the melt at the top of the screw is called plasticizing pressure, also known as back pressure. This pressure can be adjusted by the safety valve in the hydraulic system. In injection, the plasticizing pressure varies with the design of screw, the requirement of product quality and the type of plastic. If these conditions and screw speed remain unchanged, increasing the plasticizing pressure will strengthen the shearing effect, that is, it will increase the melt temperature, but it will reduce the plasticizing efficiency, increase the countercurrent and leakage, and increase the driving power.
In addition, increasing the plasticizing pressure can often make the melt temperature uniform, the pigment mixed evenly, and the gas in the melt can be exhausted. In general operation, the plasticizing pressure should be as low as possible under the premise of ensuring good product quality. Its specific value varies with the type of plastic used, but rarely exceeds 20 kg/cm2.
2. Injection pressure: At present, the injection pressure of almost all injection molding machines is based on the pressure exerted by the plunger or the top of the screw on the plastic (converted from the oil pressure). The function of injection pressure in injection molding is to overcome the flow resistance of plastic from the barrel to the cavity, give the melt the speed of filling the mold and make the melt dense.
Third, the molding cycle 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, it is necessary to shorten the relevant time in the molding cycle as much as possible on the premise of ensuring quality. In the whole molding cycle, injection time and cooling time are the most important and have a decisive influence on product quality. The mold filling time in injection time is directly proportional to the mold filling rate, and the mold filling time in production is generally about 3-5 seconds. The holding time in injection time is the time to press the plastic in the cavity, which accounts for a large proportion in the whole injection time, generally about 20- 120 seconds (the ultra-thick part can be as high as 5- 10 minutes). Before the melt at the gate is frozen, the holding time has an influence on the dimensional accuracy of the product, and if it is later, it has no influence. There is also a maximum holding time, which depends on the material temperature, mold temperature and the size of the main runner and gate. If the dimensions and process conditions of the main runner and gate are normal, the pressure value with the smallest fluctuation range of product shrinkage is usually taken as the standard. The cooling time mainly depends on the thickness of the product, the thermal and crystallization properties of the plastic and the mold temperature. The end point of cooling time should be based on the principle of ensuring that the product will not change when demoulding. Generally, the cooling time is about 30~ 120 seconds, and the cooling time is not too long, which not only reduces the production efficiency, but also makes it difficult to demould complex parts, and even produces demoulding stress when forcibly demoulding. Another time in the molding cycle is related to whether the production process is continuous and automatic, and to what extent.
Injection molding process
1. Injection pressure
The injection pressure is provided by the hydraulic system of the injection system. The pressure of the hydraulic cylinder is transferred to the plastic melt through the screw of the injection molding machine. Under the pressure, the plastic melt enters the vertical runner (also the main runner of some molds), the main runner and the shunt runner of the mold through the nozzle of the injection molding machine, and enters the mold cavity through the gate. This process is the injection process, or filling process. The existence of pressure is to overcome the resistance in the process of melt flow, or conversely, the resistance in the process of flow needs to be offset by the pressure of 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 pressure to overcome the flow resistance of melt in the whole process. After that, the pressure gradually decreases along the flow length to the front end of the melt. If the cavity is well ventilated, the final pressure at the front end of the melt is atmospheric pressure.
There are many factors that affect the melt filling pressure, which can be summarized into three categories: (1) material factors, such as the type and viscosity of plastics; (2) Structural factors, such as the type, quantity and location of gating system, the shape of mold cavity and the thickness of product; ⑶ Process elements of molding.
2. Injection time
The injection time mentioned here refers to the time required for the plastic melt to fill the cavity, excluding the auxiliary time such as mold opening and closing. Although the injection time is short, it has little effect on the molding cycle, but the adjustment of injection time has great effect on the pressure control of gate, runner and cavity. Reasonable injection time is helpful to the ideal filling of melt, which is of great significance to improve the surface quality of products and reduce the dimensional tolerance.
The injection time is much lower than the cooling time, which is about110 ~115 of the cooling time. This rule can be used as a basis for predicting the total molding time of plastic parts. In mold flow analysis, only when the melt is completely driven by the rotation of the screw to fill the cavity, the injection time in the analysis result is equal to the injection time set in the process conditions. If the pressure holding switch of the screw occurs before the cavity is full, the analysis result will be greater than the setting of process conditions.
3. Injection temperature
Injection temperature is an important factor affecting injection pressure. The barrel of injection molding machine has 5 ~ 6 heating sections, and each raw material has its suitable processing temperature (please refer to the data provided by the material supplier for detailed processing temperature). Injection molding temperature must be controlled within a certain range. If the temperature is too low, the melt will be poorly plasticized, which will affect the quality of molded parts and increase the technological difficulty. If the temperature is too high, the raw materials will decompose easily. In the actual injection molding process, the injection temperature is often higher than the barrel temperature, and the higher value is related to the injection rate and material properties, up to 30℃. This is because shearing generates high heat when the molten material passes through the main channel. In model flow analysis, there are two ways to compensate for this difference. One is to try to measure the temperature of molten material in the process of empty injection molding, and the other is to include nozzles in the modeling.
4. Keep the pressure and time.
When the injection molding process is over, the screw stops rotating and just pushes forward, and the injection molding enters the pressure maintaining stage. In the process of holding pressure, the nozzle of injection molding machine keeps filling the cavity to fill the volume vacated by the shrinkage of the parts. If the cavity is full and the pressure is not maintained, the part will shrink by about 25%, especially at the rib, which will form shrinkage marks due to excessive shrinkage. Generally, the holding pressure is about 85% of the maximum filling pressure, but it should be determined according to the actual situation.
5. Back pressure
Back pressure refers to the pressure that needs to be overcome when the screw rotates backward to store materials. Adopting high back pressure is beneficial to the dispersion of pigments and the melting of plastics, but it also prolongs the screw retraction time, reduces the length of plastic fibers and increases the pressure of injection molding machine, so the back pressure is lower, generally not exceeding 20% of the injection pressure. When injecting foam, the back pressure should be higher than the pressure formed by gas, otherwise the screw will be pushed out of the cylinder. Some injection molding machines can program back pressure to compensate for the decrease of screw length during melting, which will reduce the input heat and temperature. However, because the result of this change is difficult to estimate, it is not easy to adjust the machine accordingly.
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