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Precautions in injection processing

2019-11-02

1. Barrel temperature: The temperature of the melt is very important. The temperature of the shot cylinder used is only a guide. Melt temperature can be measured at the nozzle or using air jet method. The temperature setting of the shot cylinder depends on the melt temperature, screw speed, back pressure, shot volume and injection cycle. If you have no experience processing a particular grade of plastic, start with the settings. In order to facilitate the control, the injection cylinder is divided into zones, but not all are set to the same temperature. If the operation time is long or operating at high temperature, please set the temperature of one zone to a lower value, which will prevent the plastic from melting and diverting prematurely. Before injection molding begins, make sure that the hydraulic oil, hopper seal, mold, and shot cylinder are at the correct temperature.

Shanghai injection processing

2. Melt temperature: The melt temperature plays a major role in the melt's flow performance. Because plastic does not have a specific melting point, the so-called melting point is a temperature section in the molten state. The structure and composition of plastic molecular chains are different, so it flows. The effects of properties are also different. Rigid molecular chains are significantly affected by temperature, such as PC, PPS, etc., while flexible molecular chains such as PA, PP, PE and other fluidity are not obvious by changing the temperature, so it should be adjusted according to different materials Calibrate reasonable injection part temperature.


3. Mold temperature: Some plastic materials require higher mold temperature due to high crystallization temperature and slow crystallization speed, and some require higher or lower temperature due to the size and deformation control or the need for demolding, such as PC. It is required to be above 60 degrees, and in order to achieve better appearance and improve fluidity, the mold temperature sometimes needs to be above 160 degrees. Therefore, the mold temperature has an inestimable effect on improving the appearance, deformation, size and rubber mold of the product.


Injection processing


4. Injection pressure: The melt overcomes the resistance needed to advance, which directly affects the size, weight and deformation of the product. Different plastic products require different injection pressures. For materials such as PA and PP, increasing the pressure will make it fluid. Significantly improved, the injection pressure determines the density of the product, that is, the appearance of gloss. It does not have a fixed value, and the more difficult the mold filling is, the higher the pressure of the injection molded part.


Injection molding is a processing method used in the mass production of certain complex shapes. Specifically, the heat-melted material is injected into the mold cavity under high pressure, and after cooling and solidification, a molded product is obtained.


I. Temperature Control


1. Barrel temperature: The temperature to be controlled in the injection molding process includes 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 same plastic has different flow and decomposition temperatures due to different sources or grades. This is due to the difference in average molecular weight and molecular weight distribution. Plastics are injected in different types. The plasticizing process in the machine is also different, so the barrel temperature is also different.


2. Nozzle temperature: The nozzle temperature is usually slightly lower than the barrel temperature. This is to prevent the "salting phenomenon" that may occur in the straight-through nozzle of the melt. 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 because the early setting material is injected into the mold cavity.


3. Mold temperature: The mold temperature has a great impact on the intrinsic performance 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 (melt temperature, injection speed and injection pressure, molding cycle, etc.).


Second, pressure control


压力 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.


1. Plasticizing pressure: (back pressure) When a screw injection machine is used, the pressure on the top of the screw when the screw rotates and retracts is called plasticizing pressure, also called back pressure. This pressure can be adjusted by the relief 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 quality of the product and the type of plastic. If these conditions and the speed of the screw are unchanged, increasing the plasticizing pressure will strengthen the shear The effect is to increase the temperature of the melt, but it will reduce the efficiency of plasticization, increase the countercurrent and leakage, and increase the driving power.


In addition, increasing the plasticizing pressure can often make the temperature of the melt uniform, the color mixture is uniform, and the gas in the melt is discharged. In general operation, the determination of plasticizing pressure should be as low as possible under the premise of ensuring good product quality. The specific value varies with the type of plastic used, but it rarely exceeds 20 kg / cm2.


2. Injection pressure: In the current production, the injection pressure of almost all injection machines is based on the pressure applied to the plastic by the top of the plunger or screw (converted from the oil pressure). 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.


III. Molding cycle


时间 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: 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.


注射 In the entire molding cycle, the injection time and cooling time are important, and 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.


的 The dwell time in the injection time is the pressure time on the plastic in the cavity, which accounts for a large proportion of the entire injection time, generally about 20-120 seconds (special thick parts can be up to 5-10 minutes). Before the melt is frozen at the gate, the amount of holding time will affect the accuracy of the product size. If it is later, it will have no effect. The dwell time also has a benefit, and 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 of the fluctuation range of the shrinkage rate of the product is usually used.


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