The explanation with the short shot of the mold

Improper equipment selection

In order to select proper equipment, the request is that the maximum injection volume of the injection molding machine must be greater than the total weight of the plastic part and the gate. Besides, the total injection weight cannot exceed 85% of the plasticizing volume of the injection molding machine.

At present, the commonly used method to control feeding is the fixed volume feeding method. Check whether the roller material volume is uniform with the particle size of the raw material and whether there is a bridge formation at the bottom of the feeding inlet. If the temperature at the feeding inlet is too high, it will also cause poor blanking. In this case, it should unblock and cool the feeding inlet.

When the raw material fluidity is poor, the structural parameters of the mold are the main reason affecting short shot. Therefore, it should improve the viscous flow defects of the mold feed system, such as setting the runner position reasonably, expanding the gate, runner and runner gate 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 is also necessary to check whether the recycled material in the raw material is excessive and appropriately reduce its amount.

If the amount of lubricant is too much in the raw material formula, in the meanwhile, the wear gap between the check ring of injection screw and the barrel is large, the molten material will flow back severely in the barrel, resulting in insufficient feeding and short shot. In this case, it need to reduce the amount of lubricant and adjust the gap between the barrel and the injection screw and the check ring, which can repair the equipment.

When impurities in the molten material block the nozzle or the cold material blocks the gate and runner, it should make the nozzle remove and clean, besides, expand the cold slug well and runner section of the mold.

When a mold has multiple cavities, the appearance defects of the plastic parts are often caused by unreasonable design of the gate and runner balance. When designing the feed system, it should be paid attention 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. Besides, the gate position should be selected at the thick wall. It can also adopt a design scheme of runner balance layout. If the gate or runner is small, thin and long, the pressure of the molten material will be lost too much along the flow process. In this case, the flow will be blocked and be likely to occur the poor filling. In this situation, it had better to enlarge the runner cross-section and gate area, as well as adopt a multi-point feeding method if necessary.

When a large amount of gas remaining in the mold due to poor venting is squeezed by the material flow, in the meanwhile, generating a high pressure greater than the injection pressure, it will prevent the molten material from filling the cavity and cause short shot. Therefore, it should check whether the cold slug well is set or its position is correct. For the molds with deeper cavities, it should add exhaust slot or exhaust vent at the short shot part. On the mold surface, it can open the exhaust slot with a depth of 0.02~0.04 mm and a width of 5~10 mm. Besides, the exhaust vent should be set at the final filling position 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, decreasing the flow resistance of the feed system, reducing the clamping force, increasing the mold gap and so on.

After the molten material enters the low-temperature mold cavity, it will cool too quickly to fill every corner of the cavity. Therefore, the mold must be preheated to the temperature required by the process before starting the machine. When the machine is just started, it should control the flow of cooling water passing through the mold appropriately. If the mold temperature cannot be raised, it need to check whether the design of the mold cooling system is reasonable.

In general, within the range suitable for molding, the melt temperature and the filling length are close to a positive proportional relationship. Then the flowing property of low-temperature melt decreases, which shortens the filling length. When the melt temperature is lower than the temperature required by the process, it need to check whether the barrel feeder is intact and try to increase the barrel temperature.

When turning on the machine, the barrel temperature is always lower than the temperature indicated by the barrel heater instrument. It should be noted that the barrel is heated to the instrument temperature, and then keep constant temperature for a while to turn on the machine.

If low temperature injection is necessary to prevent decomposition of molten material, in this situation, it can also adopt to extend the injection cycle time appropriately to overcome short shot. For screw injection molding machines, it can increase the temperature of the front section of the barrel appropriately.

During the injection process, the nozzle is in contact with the mold. Since the mold temperature is generally lower than the nozzle temperature and their temperature difference is large, their frequent contact will cause the nozzle temperature to drop, resulting in the molten material freezing at the nozzle.

If there is no cold slug well in the mold structure, the cold slug well will solidify immediately after entering the cavity, making it impossible for the hot melt behind to fill the cavity. Therefore, the nozzle should be separated from the mold when opening the mold. It can reduce the impact of the mold temperature on the nozzle temperature and keep the nozzle temperature within the range required by the process.

If the nozzle temperature is very low and cannot be raised, it need to 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 short shot.

The injection pressure is close to a positive proportional relationship with the mold filling length. If the injection pressure is too small and the mold filling length is short, the cavity will be not fully filled. Therefore, 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 the melt will be thermally decomposed with too high material temperature, 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. However, it should be noted that a long holding time may also cause other faults. Therefore, when molding, it should be adjusted appropriately according to the specific situation of the plastic part.

The injection speed is directly related to the mold filling speed. If the injection speed is too slow, the melt fills the mold slowly. Therefore, the low-speed melt is easily cooled, which further reduces its flow properties and causes short shot.

In this case, the injection speed should be appropriately increased. However, it should be noted that it is easy to cause other molding failures with too fast injection speed.

When the thickness of a plastic part is not proportional to its length, in the meanwhile, the shape is very complex and the molding area is large, the melt can easily be blocked 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 a plastic part, it should be noted that the thickness of the plastic part is related to the limiting flow length of the melt when filling the mold.