Analysis and Solution of Dimensional Instability Defects of Router Injection Molding Products

Dimensional instability refers to the change in the size of plastic parts between each batch of router injection molding products or between each cavity product produced by each mold under the same injection molding machine and molding process conditions. Fluctuations in product size are usually caused by abnormal equipment control, unreasonable injection molding conditions, product design defects and changes in material properties.

Analysis and elimination methods of dimensional instability defects

1. Inconsistent molding conditions or improper operation

– During the injection molding process, process parameters such as temperature, pressure and time must be strictly controlled in accordance with process requirements, especially the molding cycle of each plastic part must be kept consistent and cannot be changed at will. If the injection pressure is too low, the holding time is too short, the mold temperature is too low or uneven, the temperature at the barrel and nozzle is too high, and the plastic part is not cooled enough, the size of the plastic part may be unstable. Generally, using higher injection pressure and speed, appropriately extending the filling and holding time, and increasing the mold and material temperature can help overcome the problem of dimensional instability. If the size of the plastic part after molding is larger than the requirement, the injection pressure and melt temperature should be appropriately reduced, and the mold temperature should be increased, the filling time should be shortened, and the gate cross-sectional area should be reduced to increase the shrinkage rate of the plastic part; conversely, if the size after molding is smaller than the requirement, the opposite molding conditions should be adopted. In addition, changes in ambient temperature will also affect the molding size of the plastic part, and the process temperature of the equipment and mold should be adjusted in time according to changes in the external environment.

2. Improper selection of molding raw materials

– The shrinkage rate of the molding raw materials has a significant impact on the dimensional accuracy of the plastic parts. If the molding equipment and mold precision are high, but the shrinkage rate of the raw materials is large, it is difficult to ensure the dimensional accuracy of the plastic parts. Generally speaking, the greater the shrinkage rate of the molding raw materials, the more difficult it is to control the dimensional accuracy. Therefore, when selecting molding resins, the impact of the shrinkage rate of the raw materials after molding on the size of the plastic parts must be fully considered. The shrinkage rates of different resins vary greatly. The shrinkage rates of crystalline and semi-crystalline resins are usually higher than those of non-crystalline resins, and the shrinkage rate variation range is also large. In addition, factors such as uneven raw material particle size, poor drying, uneven mixing of recycled materials and new materials, and differences in the performance of each batch of raw materials can also cause fluctuations in the size of plastic parts.

3. Mold failure

– The structural design and manufacturing accuracy of the mold directly affect the dimensional accuracy of the plastic part. During the molding process, insufficient mold rigidity or excessive molding pressure in the mold cavity may cause the mold to deform, thereby affecting the dimensional stability of the plastic part. If the clearance between the guide pin and the guide sleeve of the mold exceeds the tolerance due to poor manufacturing accuracy or excessive wear, the molding dimensional accuracy of the plastic part will also be reduced. In addition, hard fillers or glass fiber reinforced materials may cause serious wear of the mold cavity. When using one mold with multiple cavities, the errors between the cavities and the errors of the gate, runner, etc. will also cause inconsistent filling and lead to dimensional fluctuations. Therefore, when designing the mold, sufficient strength and rigidity should be ensured, the processing accuracy should be strictly controlled, and wear-resistant materials should be used. Heat treatment and cold hardening treatment should be performed when necessary. For plastic parts with high dimensional requirements, it is recommended to avoid one mold with multiple cavities, or set auxiliary devices to ensure mold accuracy.

4. Equipment failure

– Insufficient plasticizing capacity of the molding equipment, unstable feeding of the feeding system, unstable screw speed, abnormal stop function, failure of the check valve of the hydraulic system, failure of the temperature control system, etc., may all lead to unstable molding dimensions of the plastic part. Once these faults are found, targeted measures should be taken to eliminate them.

5. Inconsistent test methods or conditions

– Different methods, times and temperatures for measuring the size of plastic parts will lead to large differences in the test results. The temperature conditions have a particularly significant impact on the test because the thermal expansion coefficient of plastics is about 10 times that of metals. Therefore, the structural dimensions of plastic parts must be measured using the methods and temperature conditions specified in the standard, and the plastic parts must be fully cooled and fixed before they are tested.