When selecting equipment, maximum injection volume of injection molding machine must be greater than the total weight of plastic parts and nozzles, the total injection weight cannot exceed 85% of plasticization volume of injection molding machine.

 

At present, commonly used method to control feeding is fixed-volume feeding method, whether amount of roll material is uniform with particle size of raw material, and whether there is "bridging" phenomenon at the bottom of feeding port. If temperature at feeding port is too high, it will also cause poor blanking. In this regard, feeding port should be dredged and cooled.

When fluidity of raw material is poor, structural parameters of mold are main reason that affects short shot. Therefore, stagnation defects of mold gating system should be improved, such as reasonable setting of runner position, expansion of gate, runner and injection port size, and use of larger nozzles. At the same time, an appropriate amount of additives can be added to raw material formula to improve fluidity of resin. In addition, check whether amount of recycled materials in raw materials is excessive, and appropriately reduce amount of recycled materials.

If amount of lubricant in raw material formulation is too much, gap between non-return ring of injection screw and barrel is large, serious reflux of molten material in barrel will cause insufficient supply and lead to underfilling. In this regard, amount of lubricant should be reduced, gap between barrel, injection screw and non-return ring should be adjusted, and equipment should be repaired.

When impurities in melt block nozzle or cold material blocks gate and runner, nozzle should be removed to clean or expand cold material cavity and runner cross-section of mold.

When a mold has multiple cavities, appearance of plastic part is often defective due to unreasonable balance design of gate and runner. When designing gating system, pay attention to balance of gate. Weight of plastic parts in each cavity should be proportional to size of gate, so that each cavity can be filled at the same time. Gate location should be selected at thick wall, design scheme of balanced arrangement of runners can also be used. If gate or runner is small, thin, and long, pressure of melt will be lost too much along way along flow process, flow will be blocked, and it is easy to cause poor filling. In this regard, runner cross-section and gate area should be expanded, and multi-point feeding can be used if necessary.

 

When a large amount of residual gas in mold is squeezed by material flow due to poor exhaust, a high pressure greater than injection pressure is generated, which will prevent melt from filling cavity and cause under-injection. In this regard, check whether there is a cold slug hole or whether its position is correct. For mold with a deep cavity, an vent groove or vent hole should be added to undershot position; on clamping surface, a vent groove with a depth of 0.02~0.04 mm and a width of 5~10 mm can be opened, and vent hole should be set at final filling of 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, raw materials should be dried and volatiles removed.
In addition, in process operation of mold system, poor exhaust can be improved by increasing mold temperature, reducing injection speed, reducing flow resistance of gating system, reducing mold clamping force, and increasing mold gaps.

After melt enters low-temperature mold cavity, it will not be able to fill all corners of the cavity due to cooling too fast. Therefore, mold must be preheated to temperature required by process before starting up, and amount of cooling water in mold should be properly controlled when machine is just started. If mold temperature cannot rise, check whether design of mold cooling system is reasonable. If mold temperature does not rise, check whether design of mold cooling system is reasonable.

Generally, within range suitable for molding, temperature of material and filling length are close to a proportional relationship, flow performance of low-temperature melt decreases, which shortens filling length. When material temperature is lower than temperature required by process, check whether barrel feeder is intact and try to increase barrel temperature.
When starting up, temperature of barrel is always lower than temperature indicated by indicator of barrel heater. It should be noted that after barrel is heated to temperature of indicator, it will take a while before it can be turned on.
If low-temperature injection is necessary to prevent melt decomposition, injection cycle time can be appropriately extended to overcome shortage of injection. For screw injection molding machines, temperature of front section of barrel can be appropriately increased.

During injection process, nozzle is in contact with mold. Because mold temperature is generally lower than nozzle temperature, temperature difference is large, frequent contact between the two will cause nozzle temperature to drop, causing melt to freeze at nozzle.
If there is no cold material cavity in mold structure, cold material will solidify immediately after entering cavity, so that subsequent hot melt cannot fill cavity. Therefore, nozzle should be separated from mold when opening mold to reduce influence of mold temperature on nozzle temperature and keep temperature at nozzle within range of process requirements.
If nozzle temperature is very low and cannot rise, check whether nozzle heater is damaged, and try to increase nozzle temperature, otherwise, pressure loss of flow material will be too large and it will cause under-injection.

Injection pressure and filling length are close to proportional relationship. Injection pressure is too small, filling length is short, and cavity filling is not full. In this regard, injection pressure can be increased by slowing down injection advancement speed and appropriately extending injection time.
In the case that injection pressure cannot be further increased, it can be remedied by increasing material temperature, reducing melt viscosity, and improving melt flow performance. It is worth noting that if material temperature is too high, melt will be thermally decomposed, which will affect performance of plastic parts.
In addition, if holding time is too short, it will also cause insufficient filling. Therefore, holding time should be controlled within an appropriate range, but it should be noted that too long holding time will also cause other failures. Molding should be adjusted according to specific conditions of plastic part.

 

Injection speed is directly related to filling speed. If injection speed is too slow, melt fills mold slowly, and low-speed flow of melt is easy to cool, causing its flow performance to further decrease and cause under-injection.
In this regard, injection speed should be appropriately increased. But it should be noted that if injection speed is too fast, it is easy to cause other molding failures.

When thickness of plastic part is not proportional to length, shape is very complicated and molding area is large, flow of melt is easily blocked at entrance of thin-walled part of plastic part, making it difficult to fill cavity. Therefore, when designing shape and structure of plastic part, it should be noted that thickness of plastic part is related to limit flow length of melt when filling mold.