The entry technology of injection molding machine adjustment: injection molding is an engineering technology that involves transforming plastics into useful products that can maintain their original performance. The important process conditions of injection molding are the temperature, pressure and corresponding action time that affect the plasticizing flow and cooling.
1. Temperature control
1. Barrel temperature: The temperature that needs to be controlled during the injection molding process includes barrel temperature, nozzle temperature and mold temperature. The first two temperatures mainly affect the plasticization and flow of the plastic, while the latter temperature mainly affects the flow and cooling of the plastic. Each plastic has a different flow temperature. For the same plastic, due to different sources or grades, its flow temperature and decomposition temperature are different. This is due to the difference in average molecular weight and molecular weight distribution. Plastics in different types of injection The plasticizing process in the machine is also different, so the cylinder temperature is also different.
2. Nozzle temperature: The nozzle temperature is usually slightly lower than the maximum temperature of the barrel. This is to prevent the "salivation" that may occur in the straight-through nozzle. The temperature of the nozzle should not be too low, otherwise the nozzle will be blocked due to the early solidification of the melt, or the performance of the product will be affected due to the injection of the early solidification material into the cavity.
3. Mold temperature: Mold temperature has a great influence on the internal performance and apparent quality of the product. 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 and injection pressure, molding cycle, etc.).
2. Pressure control
The pressure in the injection molding 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 rotates 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 is constant with the rotation speed of the screw. When the plasticizing pressure is increased, the temperature of the melt will be increased, but the speed of the plasticizing will be reduced. 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 operation, the decision of 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/cm².
2Injection 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 by 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, give the molten material the rate of filling the mold and compact the molten material.
3.Three, 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:
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, injection time and cooling time are the most important, and 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 in the entire injection time, generally about 20-120 seconds (extra-thick parts can be as high as 5-10 minutes). Before the molten material at the gate is frozen, the holding time has an impact on the accuracy of the product size, 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. 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 the product does not cause changes when the product is demolded. The cooling time is generally between 30 and 120 seconds. It is not necessary for the cooling time to be too long. It will not only reduce production efficiency, but also affect complex parts. 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.
General injection molding machines can be adjusted according to the following procedures:
According to the temperature range provided by the material supplier, adjust the barrel temperature to the middle of the range and adjust the mold temperature.
Estimate the required injection volume, and adjust the injection molding machine to two-thirds of the estimated maximum injection volume. Adjust the inverted cable (pumping glue) stroke. Estimate and adjust the secondary injection time and adjust the secondary injection pressure to zero.
Preliminarily adjust the first-level injection pressure to half (50%) of the limit of the injection molding machine; adjust the injection speed to the highest. Estimate and adjust the required cooling time. Adjust the back pressure to 3.5 bar. Remove the degraded resin in the barrel. Use semi-automatic injection molding mode; start the injection molding process and observe the action of the screw.
If necessary, adjust the injection speed and pressure appropriately. To shorten the filling time, the injection pressure can be increased. As mentioned earlier, since there is a process before full mold filling, the final pressure of mold filling can be adjusted to 100% of the first-level injection pressure. The pressure must eventually be adjusted high enough so that the maximum speed that can be reached is not limited by the set pressure. If there is flash, the speed can be reduced.
After observing a cycle, adjust the injection volume and switching point. Set the program so that the filling of 95-98% based on the injection weight can be obtained in the first level injection.
When the injection volume, switching point, injection speed and pressure of the first-level injection are adjusted properly, the second-level holding pressure adjustment procedure can be carried out.
Adjust the holding pressure appropriately as needed, but do not overfill the cavity.
Adjust the screw speed to ensure that the melt is completed just before the cycle is completed, and the injection cycle is not restricted.
Reduce cycle time to increase productivity
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