Analysis of Causes of Undershot Defects in Metal Powder Injection Molding (MIM)

What is arrears?
Undershot is also called short shot, insufficient filling, unsatisfactory part, commonly known as undershot, which refers to the partial incompleteness of the end of the material flow or the partial filling of a mold with multiple cavities, especially the thin-walled area or the end area of ​​the flow path. The manifestation is that the melt is condensed before the cavity is filled, and the melt is not completely filled after entering the cavity, resulting in a product shortage.

The reasons for the defects formed by insufficient injection of metal powder injection molding (MIM) are analyzed as follows:

1. Improper equipment selection: When selecting equipment, the maximum injection volume of the metal powder injection molding machine must be greater than the total weight of the plastic part and nozzle, and the total weight of the metal powder injection cannot exceed 85% of the plasticization volume of the metal powder injection molding machine. .

2. Insufficient feeding: The current commonly used method of controlling feeding is the fixed-volume feeding method, whether the amount of roll material is uniform with the fruit grain of the raw material, and whether there is "bridging" phenomenon at the bottom of the feeding port. If the temperature at the feeding port is too high, it will also cause poor blanking. In this regard, the feeding port should be dredged and cooled.

3. Poor material fluidity: When the material fluidity is poor, the structural parameters of the mold are the main reason for the lack of injection. Therefore, the stagnation defects of the mold gating system should be improved, such as setting the runner position reasonably, expanding the gate, runner and injection port size, and using larger nozzles. At the same time, an appropriate amount of additives can be added to the raw material formula to improve the fluidity of the resin. In addition, check whether the amount of recycled materials in the raw materials is excessive, and appropriately reduce the amount of recycled materials.

4. Excessive amount of lubricant: If the amount of lubricant in the raw material formula is too much, and the metal powder injection material screw non-return ring and the barrel wear gap is large, the serious reflux of the molten material in the barrel will cause insufficient material supply, resulting in insufficient injection . In this regard, the amount of lubricant should be reduced and the gap between the barrel and the metal powder injection screw and the non-return ring should be adjusted to repair the equipment.

5. Cold material impurities block the forehearth: When impurities in the melt block the nozzle or the cold material blocks, the gate and the runner, the nozzle should be folded down to clean or expand the mold cold slug and runner section.

6. Improper design of the pouring system: When a mold has multiple cavities, the appearance of the plastic part is often defective due to the unreasonable balance design of the gate and runner. When designing the gating 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 the metal powder injection molding cavities can be filled at the same time, and the gate position should be selected at the thick wall. It adopts a design scheme of balanced arrangement of split runners. If the gate or runner is small, thin, and long, the pressure of the melt will lose too much along the way during the flow process, the flow will be blocked, and poor filling will easily occur. In this regard, the runner cross-section and gate area should be enlarged, and multi-point feeding can be used if necessary.

7. Poor mold exhaust: When a large amount of residual gas in the mold is squeezed by the flow material due to poor exhaust, and produces a high pressure greater than the MIM pressure of the metal powder injection, it will prevent the melt from filling the metal powder injection molding cavity and cause underfill. In this regard, check whether there is a cold slug hole or whether its position is correct. For a mold with a deep metal powder injection molding cavity, an exhaust groove or vent hole should be added to the undershot position; on the clamping surface, An exhaust slot with a depth of 0.02~0.04mm and a width of 5~10mm can be opened. The exhaust hole should be set at the final filling position 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 mold exhaust. At this time, the raw materials should be dried and volatiles removed.

In addition, in terms of the process operation of the mold system, the poor exhaust can be improved by increasing the mold temperature, reducing the metal powder injection MIM speed, reducing the flow assistance of the gating system, reducing the clamping force and increasing the mold gap.

8. The mold temperature is too low: after the melt enters the low-temperature mold cavity, it will cool too fast to fill all corners of the metal powder injection molding cavity. Therefore, the mold must be preheated to the temperature required by the process before starting up. When starting up, the amount of cooling water 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: Generally, in the range suitable for metal powder injection molding, the temperature of the material and the filling length are close to a proportional relationship, and the flow performance of the low temperature melt decreases, which shortens the filling length. When the material temperature is lower than the temperature required by the process, check whether the barrel feeder is intact and try to increase the barrel temperature. When starting the machine, the temperature of the barrel is always lower than the temperature indicated by the indicator of the barrel heater. It should be noted that after the barrel is heated to the temperature of the indicator, it will take some time to start the machine. If low-temperature metal powder injection MIM is necessary to prevent molten material from decomposing, the cycle time of metal powder injection MIM can be appropriately extended to overcome the shortage of injection. For screw metal powder injection molding machines, the temperature of the front section of the barrel can be appropriately increased.

10. The nozzle temperature is too low: During the metal powder injection MIM process, the nozzle is in contact with the mold. Since the mold temperature is generally lower than the nozzle temperature and the temperature difference is large, frequent contact between the two will cause the nozzle temperature to drop, resulting in melting The material freezes at the nozzle. If there is no cold material cavity in the mold structure, the cold material will solidify immediately after entering the metal powder injection molding cavity, so that the hot melt that is plugged behind cannot fill the metal powder injection molding cavity. Therefore, the nozzle should be separated from the mold when opening the mold to reduce the influence of mold temperature on the nozzle temperature and keep the temperature at the nozzle within the range of process requirements. 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 insufficient injection.

11. Insufficient pressure or holding pressure of metal powder injection MIM: The pressure of metal powder injection technology is close to the 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 fully filled. In this regard, the injection pressure of MIM technology can be increased by slowing down the injection speed of MIM technology and appropriately extending the injection time of MIM technology. In the case that the pressure of metal powder injection technology cannot be further increased, it can be remedied by increasing the material temperature, reducing the melt viscosity, and improving the melt flow performance. It is worth noting that if the material temperature is too high, the melt will be thermally decomposed, which will affect the performance of the plastic parts. In addition, if the holding time is too short, it will cause insufficient filling. Therefore, the pressure holding time should be controlled within an appropriate range, but it should be noted that too long pressure holding time will also cause other failures. The molding should be adjusted according to the specific conditions of the plastic part.

12. The speed of metal powder injection MIM is too slow: the speed of metal powder injection MIM is directly related to the filling speed. If the metal powder injection MIM speed is too slow, the melt will be slow to fill the mold, and the low-speed flow of the melt will be easy to cool, causing its flow performance to further decrease and cause under-injection. In this regard, the MIM speed of metal powder injection should be appropriately increased. However, 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 failures.

13. The structure design of the plastic part is unreasonable: when the thickness of the plastic part is not proportional to the length, the shape is very complicated and the molding area is large, the flow of the melt is easily blocked at the entrance of the thin wall part of the plastic part, which makes the metal powder injection molding cavity It's hard to fill. Therefore, when designing the shape and structure of the plastic part, it should be noted that the thickness of the plastic part 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, and the thickness of large plastic parts is 3~6mm. Generally, the minimum thickness recommended is 0.5mm for polyethylene, 0.7mm for cellulose acetate and cellulose acetate butyrate plastics. mm, ethyl cellulose plastic 0.9mm, polymethyl methacrylate 0.7mm, polyamide 0.7mm, polystyrene 0.75mm, polyvinyl chloride 2.3mm. Generally, the thickness of the plastic part exceeding 8mm or less than 0.5mm is unfavorable for metal powder injection molding, and such thickness should be avoided in the design.

In addition, when metal powder injection molding complex structural plastic parts, necessary measures must be taken in the process, such as reasonably determining the position of the gate, appropriately adjusting the runner layout, increasing the metal powder injection MIM speed or adopting rapid MIM technology injection. Increase the mold temperature or choose a resin with better flow properties.