What to do if there is a short shot during precision plastic injection molding

What is a short shot? Short shot is also called short shot, insufficient filling, and product dissatisfaction, commonly known as short shot, which refers to the partial incomplete phenomenon at the end of the material flow or the partial filling of a part of the multi-cavity in a mold, especially the thin-walled area or the end area of ​​the flow path. It is manifested as the melt condensing before filling the cavity, and the melt is not completely filled after entering the cavity, resulting in a lack of material in the product. The main reason for short shots is that the flow resistance is too large, causing the melt to be unable to continue to flow. Factors affecting the melt flow length include: part wall thickness, mold temperature, injection pressure, melt temperature and material composition. If these factors are not handled properly, they will cause short shots. Short shot (Short Shot)-defect analysis Due to the generation of gas surface short shot Product wall thickness causes rib short shot High viscosity and slow flow rate cause short shot in the corner of the part
1 Improper equipment selection When selecting equipment, the maximum injection volume of the injection molding machine must be greater than the total weight of the plastic part and the sprue, and the total injection weight cannot exceed 85% of the plasticizing amount of the injection molding machine.
2 Insufficient feeding At present, the commonly used method for controlling feeding is the fixed volume feeding method. Check whether the particles of the raw materials are uniform and whether there is any blockage at the bottom of the feeding port. If the temperature at the feeding port is too high, it will also cause poor material drop. In this regard, the feeding port should be unblocked and cooled.
3 Poor material fluidity When the raw material fluidity is poor, the structural parameters of the mold are the main reason for underfilling. Therefore, the stagnation defects of the mold casting system should be improved, such as reasonably setting the runner position, expanding the gate, runner and injection port size, and using a larger nozzle. 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, it should be checked whether the recycled material in the raw material is excessive and its dosage should be appropriately reduced.
4 Excessive lubricant If the amount of lubricant in the raw material formula is too much, and the wear gap between the injection screw check ring and the barrel is large, the molten material will reflux severely in the barrel, which will cause insufficient feeding and lead to underfilling. In this regard, the amount of lubricant should be reduced, the gap between the barrel and the injection screw and the check ring should be adjusted, and the equipment should be repaired.
5 Cold material impurities block the material channel When impurities in the molten material block the nozzle or cold material blocks the gate and runner, the nozzle should be folded down to clean or the cold material hole and runner section of the mold should be expanded.
6 Unreasonable design of the pouring system When there are multiple cavities in one mold, the appearance defects of the plastic parts are often caused by unreasonable design of the gate and runner balance. When designing the pouring system, attention should be paid to the gate balance. The weight of the plastic parts in each cavity should be proportional to the size of the gate so that each cavity can be filled at the same time. The gate position should be selected at the thick wall. The design scheme of the split runner balance layout can also be adopted. If the gate or runner is small, thin, and long, the pressure of the molten material is lost too much along the flow process, the flow is blocked, and poor filling is easy to occur. For this, the runner section and gate area should be expanded, and the multi-point feeding method can be adopted if necessary. 7 Poor mold exhaust When a large amount of gas remaining in the mold due to poor exhaust is squeezed by the flow material, generating a high pressure greater than the injection pressure, it will prevent the molten material from filling the cavity and causing under-injection. In this regard, it should be checked whether there is a cold material hole or whether its position is correct. For molds with deeper cavities, exhaust grooves or exhaust holes should be added to the under-injected parts; on the mold surface, exhaust grooves with a depth of 0.02~0.04mm and a width of 5~10mm can be opened, and the exhaust holes should be set at the final filling of the 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 should be removed. In addition, in terms of the process operation of the mold system, poor exhaust can be improved by increasing the mold temperature, reducing the injection speed, reducing the flow assistance of the pouring system, reducing the clamping force, and increasing the mold gap. 8 The mold temperature is too low After the molten material enters the low-temperature mold cavity, it will not be able to fill every corner of the cavity due to too fast cooling. Therefore, the mold must be preheated to the temperature required by the process before starting the machine. When the machine is just started, the amount of cooling water passing through the mold should be properly controlled. If the mold temperature cannot be raised, check whether the design of the mold cooling system is reasonable. 9. The melt temperature is too low. Usually, within the range suitable for molding, the material temperature and the filling length are close to a positive proportional relationship. 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 the machine is just turned on, the barrel temperature is always lower than the temperature indicated by the barrel heater instrument. It should be noted that after the barrel is heated to the instrument temperature, it is necessary to keep it warm for a period of time before turning on the machine. If low-temperature injection is necessary to prevent the decomposition of the melt, the injection cycle time can be appropriately extended to overcome under-injection. For screw injection machines, the temperature of the front section of the barrel can be appropriately increased. 10. The nozzle temperature is too low. During the injection 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 the freezing of the melt at the nozzle. If there is no cold material hole in the mold structure, the cold material will solidify immediately after entering the cavity, so that the hot melt behind the plug cannot fill the cavity. Therefore, the nozzle should be separated from the mold when opening the mold to reduce the influence of the mold temperature on the nozzle temperature and keep the temperature at the nozzle within the range required by the process. If the nozzle temperature is very low and cannot be raised, check whether the nozzle heater is damaged and try to increase the nozzle temperature. Otherwise, too much pressure loss of the flow material will also cause under-injection.
11 Insufficient injection pressure or holding pressure The injection pressure is close to a positive proportional relationship with the filling length. If the injection pressure is too small, the filling length is short and the cavity is not fully filled. In this regard, the injection pressure can be increased by slowing down the injection forward speed and appropriately extending the injection time. If the injection pressure 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, affecting the performance of the plastic part. In addition, if the holding time is too short, it will also lead to insufficient filling. Therefore, the holding time should be controlled within an appropriate range, but it should be noted that too long holding time will also cause other faults. During molding, it should be adjusted according to the specific situation of the plastic part.
12 The injection speed is too slow The injection speed is directly related to the filling speed. If the injection speed is too slow, the melt fills the mold slowly, and the low-speed flowing melt is easy to cool, which further reduces its flow performance and produces under-injection. In this regard, the injection speed should be appropriately increased. However, it should be noted that if the injection speed is too fast, it is easy to cause other molding failures.

13 The structural design of the plastic part is unreasonable When the thickness of the plastic part is not proportional to the length, the shape is very complex and the molding area is large, the melt is easy to flow at the entrance of the thin-walled part of the plastic part, making it difficult to fill the cavity. Therefore, when designing the shape structure of the plastic part, it should be noted that the thickness of the plastic part is related to the limit flow length of the melt when filling the mold. In injection molding, the thickness of plastic parts is 1~3mm, and that of large plastic parts is 3~6mm. The generally recommended minimum thickness is: 0.5mm for polyethylene, 0.7mm for cellulose acetate and cellulose acetate butyrate plastics, 0.9mm for ethyl cellulose plastics, 0.7mm for polymethyl methacrylate, 0.7mm for polyamide, 0.7mm for polystyrene, and 2.3mm for polyvinyl chloride. Generally, a plastic part thickness exceeding 8mm or less than 0.5mm is not conducive to injection molding, and such thickness should be avoided during design. In addition, when molding structural plastic parts with complex shapes, necessary measures should be taken in terms of process, such as reasonably determining the location of the gate, appropriately adjusting the flow channel layout, increasing the injection speed or using fast injection. Increasing the mold temperature or selecting a resin with better flow properties, etc. Methods for eliminating short shot defects Short shot defect elimination checkpoints (1) Improper process control. Appropriate adjustments should be made. (2) The injection capacity of the injection molding machine is less than the weight of the plastic part. A larger injection molding machine should be used. (3) The cross-section of the runner and gate is too small. They should be increased appropriately. (4) The flow distance of the molten material in the mold cavity is too long or there is a thin-walled part. A cold material hole should be set. (5) The mold venting is poor, and the residual air in the mold cavity causes under-injection. The mold venting system should be improved. (6) The flowability of the raw material is too poor. Resin with better flowability should be used. (7) The barrel temperature is too low, the injection pressure is insufficient, or the injection time of the refill is too short, which can also cause under-injection. The control amount of the relevant process parameters should be increased accordingly.