The causes of defects in metal powder injection molding (MIM) underinjection were analyzed

What is an undernote?

Underinjection, also known as short shot, underfilling, and parts dissatisfied, commonly known as underinjection, refers to the phenomenon of partial incompleteness at the end of the material flow or partial filling in the cavity of one module, especially at the end of the thin wall or flow path. It is shown that the melt condenses without filling the cavity, and the molten material is not fully filled after entering the cavity, resulting in a lack of materials.

The following is an analysis of the causes of the defects caused by underinjection of metal powder injection molding (MIM) by guangdong dahong new materials:

1、Improper equipment selection:When using the selection equipment, the maximum injection weight of the metal powder injection molding machine must be greater than the total weight of the plastic parts and the water inlet, while the total weight of the metal powder injection machine cannot exceed 85% of the plasticizing amount of the metal powder injection molding machine.

2、Underfeeding:At present, the commonly used method to control the feeding is the fixed-volume feeding method. Is the amount of roller equal to the fruit grain of the raw material? Is there any "bridge" phenomenon at the bottom of the feeding mouth? If the temperature at the feeding port is too high, it will also cause poor blanking. To this, should dredge and cool feed mouth.

3、Material mobility:In the case of poor fluidity of raw materials, the structural parameters of the mold are the main factors affecting underinjection. Therefore, it is necessary to improve the stagnation flow defects of the mold pouring system, such as reasonable setting of sprue position, expansion of sprue, runner and sprue size, and the use of larger nozzles. At the same time, an appropriate amount of additives can be added in the raw material formula to improve the flow performance of the resin. In addition, should also check the raw materials in the excess of the amount of re - raw materials, appropriately reduce their dosage.

4、Lubricant excess:If the lubrication dose of the raw material formula is too much, and the wear gap between the screw stop and the barrel of metal powder injection material is large, the backflow of molten material in the barrel will cause insufficient feed, resulting in insufficient injection. Therefore, the lubricant dosage should be reduced and the gap between the cylinder and the metal powder injection screw rod and the check and reverse ring should be adjusted to repair the equipment.

5、Cold material impurities block the feed channel:When the impurities in the molten material block the nozzle or the cold material block, the gate and the flow channel, the nozzle should be broken down clean or enlarge the mold cold material hole and flow channel section.

6、The pouring system design is not reasonable：In the case of multi-cavity mold, the improper design of gate and gate balance often leads to the appearance defects of the plastic parts. When designing the pouring system, attention should be paid to gate balance. The weight of plastic parts in each mold cavity should be proportional to gate size, so that each metal powder injection cavity can be filled at the same time. If the gate or runner is small, thin, long, and the pressure of molten material along the flow process is too large, flow obstruction, easy to produce filling poor. Therefore, the flow section and gate area should be enlarged, and the method of multi-feed can be adopted if necessary.

7、Poor mold exhaust：When a large amount of residual gas in the mold due to poor exhaust is squeezed by the flow material, resulting in a high pressure greater than the MIM pressure of metal powder injection, it will prevent the molten material from filling the metal powder injection cavity and cause underinjection. In this regard, it is necessary to check whether the cold sluice hole is set or whether its position is correct. For the mold with deep metal powder injection cavity, the vent groove or vent hole should be added in the part of underinjection. The vent groove with a depth of 0.02~0.04mm and a width of 5~10mm can be opened on the formwork surface, and the vent hole should be set at the final mold place of the metal powder injection mold cavity. When using raw materials with excessive moisture and volatile content, a large amount of gas will be produced, resulting in poor mold exhaust. At this point, raw materials should be dried and cleared of volatile matter.

In addition, in terms of the process operation of the mold system, the exhaust defects can be improved by improving the mold temperature, reducing the MIM speed of metal powder injection, reducing the flow assist of the pouring system, reducing the clamping force and increasing the die clearance.

8、Mold temperature is too low：Once the molten material enters the cryogenic mold cavity, it will cool too quickly to fill with metal powder and inject into the corners of the mold cavity. Therefore, the mold must be preheated to the temperature required by the process before starting up. If the mold temperature does not rise, check whether the mold cooling system design is reasonable.

9、The melting temperature is too low：Generally, in the range suitable for metal powder injection molding, the material temperature and mold length are close to a direct proportion, and the flow performance of cryogenic melt decreases, making the mold length shorter. When the temperature of the material is lower than the temperature required by the process, it is necessary to check whether the feeder of the material barrel is intact and try to raise the temperature of the material barrel. When starting up, the temperature of the cylinder is lower than the indicated temperature of the cylinder heater. If low temperature metal powder injection MIM is necessary to prevent the decomposition of molten material, the circulation time of MIM injection can be extended appropriately to overcome underinjection. For the screw type metal powder injection molding machine, the temperature of the front section can be appropriately raised.

10、Nozzle temperature is too low：In the MIM process of metal powder injection, the nozzle is in contact with the mold. Since the mold temperature is generally lower than the temperature of the nozzle and the temperature difference is large, frequent contact between the two will lower the temperature of the nozzle, resulting in the freezing of molten material at the nozzle. If there is no cold hole in the mold structure, the cold material will solidify immediately after being injected into the mold cavity, so that the hot melt material behind the plug cannot fill the mold cavity. Therefore, the nozzle should be separated from the mold during mold opening to reduce the effect of mold temperature on the nozzle temperature and keep the temperature at the nozzle within the range required by the process. If the temperature of the nozzle is very low and cannot be raised, check whether the nozzle heater is damaged and try to raise the temperature of the nozzle.

11、Metal powder injection with MIM pressure or insufficient pressure：The pressure of metal powder injection technology is close to a direct proportion to the length of metal powder injection mold. The MIM technology injection pressure is too small, the length of metal powder injection mold is short, and the metal powder injection mold cavity is filled with dissatisfaction. In this regard, the injection pressure of MIM technology can be increased by slowing down the advance speed of MIM technology injection and extending the injection time of MIM technology appropriately. If the pressure of metal powder injection technology cannot be further improved, it can be remedied by improving the feed temperature, reducing the melt viscosity and improving the melt flow performance. It is worth noting that if the material temperature is too high, it will cause the heat decomposition of molten material, which will affect the performance of plastic parts. In addition, if the holding time is too short, it will cause insufficient filling. Therefore, the pressure keeping time should be controlled within the appropriate range, but it should be noted that the pressure keeping time too long will also cause other failures, and the molding should be adjusted according to the specific circumstances of the plastic parts.

12、Metal powder injection MIM is too slow：Metal powder injection speed is directly related to mold filling speed. If the metal powder is injected with MIM at a slow speed, the filler filling is slow, and the low-speed flow melt is easy to cool down, further reducing its flow performance and resulting in underinjection. The speed of MIM injection should be increased. However, it should be noted that if metal powder is injected with MIM too fast, it will easily cause other metal powder injection molding failures.

13、The structural design of plastic parts is unreasonable：When the thickness and length of the plastic parts are out of proportion and the shape is very complex and the forming area is very large, the molten material can easily block the flow at the entrance of the thin wall of the plastic parts, making it difficult to fill the mold cavity with metal powder injection. Therefore, when designing the physical structure of the plastic part, it should be noted that the thickness of the plastic part is related to the ultimate flow length of the molten metal filling. In metal powder injection molding, the thickness of the plastic part is 1~3mm at most, and that of the large plastic part is 3~6mm. Polyethylene: 0.5mm, cellulose acetate and cellulose acetate: 0.7mm, ethylcellulose plastics: 0.9mm, polymethyl methacrylate: 0.7mm, polyamide: 0.7mm, polystyrene: 0.75mm, polyvinyl chloride: 2.3mm. Generally, the thickness of plastic parts exceeding 8mm or less than 0.5mm is unfavorable for metal powder injection molding. Such thickness should be avoided in the design.

In addition, necessary measures should be adopted in the process of metal powder injection molding structural plastic parts with complex forms, such as reasonably determining the location of the gate and properly adjusting the flow channel layout to improve the MIM speed of metal powder injection or adopting rapid MIM technology injection. Improve the mold temperature or select the best flow performance resin.

For more information about metal powder injection molding, please click on guangdong da-hong new metal powder injection MIM technology.