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Measures to deal with the reasons for the lack of glue in injection molding products

by:Vowin Rapid Prototyping     2023-02-07
When plastic parts are processed, the finished product will lack glue, which seriously affects the quality of the product. There are many reasons for the lack of glue in plastic parts. Specifically, it can be analyzed from the following points. The injection molding engineers of Wo Neng Precision will answer them one by one.

1. The selected equipment is incorrect. When selecting an injection molding machine, the maximum injection volume must be greater than the total weight of the plastic part and nozzle, and the total injection weight cannot exceed 85% of the plasticized volume of the injection molding machine.

2. Insufficient feeding. At present, the commonly used method of controlling feeding is the fixed volume feeding method. Whether the amount of roller material and the particle size of the raw material are uniform, and whether there is a '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.

Three, the fluidity of the material is poor. When the fluidity of the raw material is poor, the structural parameters of the mold are the main reason for the under-injection. Therefore, the stagnation defect of the mold gating system should be improved, such as setting the position of the runner reasonably, expanding the size of the gate, runner and injection port, 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 amount of recycled materials in the raw materials is excessive, and reduce the amount appropriately.

4. Excess lubricant. If there is too much lubricant in the raw material formula, and the wear gap between the injection screw check ring and the barrel is large, the serious backflow 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, the gap between the barrel, 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 the impurities in the molten material block the nozzle or the cold material blocks the gate and the runner, the nozzle should be removed to clean or expand the cold material cavity and runner section of the mold.

6. The gating system design is unreasonable. When a mold has multiple cavities, the appearance defects of the plastic part are often caused by the unreasonable balance design of the gate and the 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 each cavity can be filled at the same time. A balanced layout design. 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, the flow will be blocked, and poor filling will easily occur. In this regard, the cross section of the runner and the area of ​​the gate should be enlarged, and the method of multi-point feeding can be adopted if necessary.

7. Poor exhaust of the mold. When a large amount of gas remaining in the mold due to poor exhaust is squeezed by the material flow and generates a high pressure greater than the injection pressure, it will prevent the molten material from filling the cavity and cause under-injection. In this regard, it should be checked whether there is a cold slug hole or whether its position is correct. For the mold with a deep cavity, an exhaust groove or vent hole should be added at the part of the underfill; 0.02~0.04 mm, the width of the vent groove is 5~10 mm, and the vent hole 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 point, raw materials should be dried and volatiles removed.

In addition, in terms of process operation of the mold system, poor exhaust can be improved by increasing the mold temperature, reducing the injection speed, reducing the flow resistance of the gating system, reducing the mold 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 all corners of the cavity due to cooling too fast. Therefore, the mold must be preheated to the temperature required by the process before starting the machine. When the machine is first started, the cooling water flow in the mold should be properly controlled. If the mold temperature does not rise, check whether the design of the mold cooling system is reasonable.

9. The temperature of the melting material is too low. Generally, within the range suitable for molding, the relationship between the material temperature and the filling length is close to a direct ratio, and the flow performance of the low-temperature melting material 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 up, the temperature of the barrel is always lower than the temperature indicated by the meter of the barrel heater. It should be noted that after the barrel is heated to the temperature of the meter, it will take a while to start the machine.

If low-temperature injection has to be used to prevent melt decomposition, the injection cycle time can be appropriately extended to overcome under-injection. For screw injection molding machines, the temperature in the front section of the barrel can be appropriately increased.

10. Insufficient injection pressure or holding pressure. The injection pressure is close to a proportional relationship with the filling length. If the injection pressure is too small, the filling length is short and the cavity is not filled enough. In this regard, the injection pressure can be increased by slowing down the injection speed and prolonging the injection time appropriately.

In the case that the injection pressure cannot be further increased, it can be remedied by increasing the material temperature, reducing the viscosity of the melt, and improving the melt flow performance. It is worth noting that if the material temperature is too high, the molten material will be thermally decomposed, which will affect the performance of the plastic parts.
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