What factors affect the under injection of metal powder injection molding
time：2020-06-09Views：356 Author：Best Seiko
What is backlog? Under-injection is also called short shot, insufficient filling, and dissatisfied parts. Commonly known as under-injection, it refers to partial incomplete phenomenon at the end of the stream or part of the filling in a multi-cavity mold, especially the thin-walled area or the end area of the flow path. This is manifested by the fact that the melt condenses without filling the cavity, and the melt does not fill completely after entering the cavity, resulting in a shortage of product. The following is the analysis of the causes of defects caused by under injection of metal powder injection molding (MIM) by Shenzhen Best: 1. Improper equipment selection: When using equipment selection, the large injection volume of the metal powder injection molding machine must be larger than the plastic parts and The total weight of the nozzle, and the total weight of the metal powder injection should not exceed 85% of the plasticizing amount of the metal powder injection molding machine. 2. Insufficient feeding: The current commonly used method for controlling feeding is the fixed volume feeding method. Whether the volume of the roll material and the raw material are uniform, and whether there is a "bridging" phenomenon at the bottom of the feeding port. If the temperature at the feed opening is too high, it will also cause the blanking. In this regard, the feed port should be unblocked and cooled. 3. Poor material flowability: When the raw material flowability is poor, the structural parameters of the mold are the main reasons affecting the under-injection. Therefore, the stagnation defects of the mold pouring system should be improved, such as rationally setting the position of the runner, expanding the size of the gate, runner and injection port, and using larger nozzles. At the same time, an appropriate amount of additives can be added to the raw material formula to improve the flow properties of the resin. In addition, you should also check whether the amount of recycled materials in the raw material is excessive, and reduce its dosage appropriately. 4. Excessive amount of lubricant: If the amount of lubricant in the raw material formula is too much, and the wear gap between the metal powder injection screw check ring and the barrel is large, the backflow of molten material in the barrel will seriously cause insufficient supply, resulting in underfill . In this regard, the amount of lubricant should be reduced and the gap between the barrel and metal powder injection screw and check ring should be adjusted to repair the equipment. 5. Cold material impurities blocking the material channel: when the impurities in the molten material block the nozzle or the cold material is blocked, the gate and the flow channel, the nozzle should be folded down to clean or expand the mold cold material cavity and the flow channel cross section. 6. Irrational design of the pouring system: When a mold has multiple cavities, the appearance defects of the plastic parts are often caused by the unreasonable design of the gate and runner balance. When designing the pouring system, pay attention to the balance of the gate. The weight of the plastic parts in each cavity should be proportional to the size of the gate, so that each metal powder injection molding cavity can be filled at the same time. The gate position should be selected at the thick wall. The design scheme of balanced layout of shunts is adopted. If the gate or runner is small, thin and long, the pressure of the molten material will be lost too much along the flow, the flow is blocked, and it is easy to cause poor filling. To this end, the runner section and gate area should be expanded, and if necessary, a multi-point feeding method can be used. 7. Poor exhaust of the mold: When a large amount of gas remaining in the mold due to poor exhaust is squeezed by the flow material and generates a high pressure greater than the pressure of the metal powder injection MIM, it will prevent the molten material from filling the metal powder injection molding cavity and cause underfill. In this regard, it should be checked whether there is a cold cavity or its position is correct. For a mold with a deep metal powder injection molding cavity, a vent groove or vent should be added to the under-injected part; on the mold clamping surface, Ventilation slots with a depth of 0.02~0.04mm and a width of 5~10mm can be opened. The exhaust holes should be set at the mold filling point of the metal powder injection molding cavity. When using raw materials with excessive moisture and volatile content, a large amount of gas will also be generated, resulting in poor exhaust of the mold. At this time, the raw materials should be dried and volatile materials removed. In addition, in the process operation of the mold system, auxiliary measures such as raising the mold temperature, reducing the metal powder injection MIM speed, reducing the flow system's flow assistance, and reducing the mold clamping force, and increasing the mold gap can be used to improve the poor exhaust. 8. The mold temperature is too low: After the melt enters the low-temperature mold cavity, it will not fill up all corners of the metal powder injection molding cavity because it cools too quickly. Therefore, the mold must be preheated to the temperature required by the process before starting up. When starting up, the cooling water flow in the mold should be appropriately controlled. If the mold temperature does not rise, check whether the design of the mold cooling system is reasonable. 9. The temperature of the melt is too low: usually, within the range suitable for metal powder injection molding, the temperature of the material and the length of the mold are close to a proportional relationship, and the flow performance of the low-temperature melt is reduced, which reduces the length of the mold. When the material temperature is lower than the temperature required by the process, you should check whether the barrel feeder is intact and try to increase the barrel temperature. The temperature of the barrel is always lower than the temperature indicated by the gauge of the barrel heater when it is just turned on. It should be noted that after the barrel is heated to the temperature of the instrument, it will take a while for the temperature to turn on. If low temperature metal powder injection MIM has to be taken in order to prevent melt decomposition, the metal powder injection MIM cycle time can be properly extended to overcome under-injection. For the screw-type metal powder injection molding machine, the temperature of the front section of the barrel can be appropriately increased. 10. The temperature of the nozzle is too low: during the metal powder injection MIM, the nozzle is in contact with the mold. Because the temperature of the mold is generally lower than the temperature of the nozzle and the temperature difference is large, the temperature of the nozzle will drop after frequent contact, resulting in melting. The material is frozen at the nozzle. If there is no cold cavity in the mold structure, the cold material immediately solidifies after entering the metal powder injection molding cavity, so that the hot melt material behind the auxiliary plug cannot fill the metal powder injection molding cavity. Therefore, when the mold is opened, the nozzle and the mold should be separated to reduce the influence of the mold temperature on the nozzle temperature, and the temperature at the nozzle should be kept within the range required by the process. If the nozzle temperature is very low and cannot rise, check whether the nozzle heater is damaged and try to increase the nozzle temperature, otherwise, the pressure loss of the flow material will be too large and it will cause under-injection. 11. Insufficient MIM pressure or holding pressure for metal powder injection: The pressure of metal powder injection technology is close to a proportional relationship with the filling length. The injection pressure of MIM technology is too small, the filling length is short, and the metal powder injection molding cavity is not filled. In this regard, the injection pressure of MIM technology can be increased by slowing down the advancement speed of MIM technology injection and appropriately extending the injection time of MIM technology. When the pressure of the metal powder injection technology cannot be further increased, it can be remedied by increasing the material temperature, reducing the viscosity of the melt, and improving the melt flow properties. It is worth noting that if the material temperature is too high, the melt will thermally decompose, affecting the performance of the plastic parts. In addition, if the holding time is too short, it will also lead to insufficient filling. Therefore, the dwell time should be controlled within an appropriate range, but it should be noted that too long the dwell time will also cause other failures. The molding should be adjusted according to the specific circumstances of the plastic parts. 12. The MIM speed of metal powder injection is too slow: the MIM speed of metal powder injection is directly related to the filling speed. If the metal powder injection MIM speed is too slow, the melt fills the mold slowly, and the low-speed flowing melt is easy to cool, which further reduces the flow performance and causes under-injection. In this regard, the MIM speed of metal powder injection should be appropriately increased. But it should be noted that if the metal powder injection MIM speed is too fast, it is easy to cause other metal powder injection molding failure. 13. The structural design of the plastic parts is unreasonable: when the thickness of the plastic parts is not proportional to the length, the shape is very complex and the molding area is large, the melt can easily be blocked at the entrance of the thin-walled parts of the plastic parts, making the metal powder injection molding cavity It's hard to fill. Therefore, when designing the shape and structure of plastic parts, it should be noted that the thickness of the plastic parts is related to the limit flow length when the melt is filled. In metal powder injection molding, the thickness of plastic parts is 1~3mm, large plastic parts is 3~6mm, and the generally recommended small thickness is; polyethylene 0.5mm, cellulose acetate and cellulose acetate butyrate plastic 0.7mm, ethyl cellulose plastic 0.9mm, polymethyl methacrylate 0.7mm, polyamide 0.7mm, polystyrene 0.75mm, polyvinyl chloride 2.3mm. In general, the thickness of plastic parts exceeding 8mm or less than 0.5mm is unfavorable for metal powder injection molding, and such thickness should be avoided when designing. In addition, in the process of metal powder injection molding of complex structural plastic parts, the necessary measures should also be adopted in the process, such as rationally determining the position of the gate, appropriately adjusting the layout of the runner, increasing the MIM speed of metal powder injection or using rapid MIM technology injection. Increase the mold temperature or choose a resin with better flow properties.