Reasons for Unstable Plastic Injection Molding Dimensions

Dimensional instability refers to the change in the dimensions of Overmolding for injection molded parts between each batch of molded products or between each molded product of each mold under the same injection molding machine and molding process conditions. The change in product size is caused by abnormal equipment control, unreasonable injection molding conditions, poor product design and changes in material properties.

1. Inconsistent molding conditions or improper operation

During injection molding, various 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 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 shape and size of the plastic part will be unstable. In general, using higher injection pressure and injection speed, appropriately extending the mold filling and holding time, and increasing mold temperature and material temperature are beneficial to overcome dimensional instability failures. If the external dimensions of the plastic part after molding are larger than the required dimensions, the injection pressure and melt temperature should be appropriately reduced, the mold temperature should be increased, the mold filling time should be shortened, and the gate cross-sectional area should be reduced, thereby increasing the shrinkage rate of the plastic part. If the size of the plastic part after molding is smaller than the required size, the opposite molding conditions should be adopted. It is worth noting that changes in ambient temperature also have a certain impact on the fluctuation of the molding dimensions of the plastic part. 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 great influence on the dimensional accuracy of the plastic parts. If the molding equipment and mold have high precision, but the shrinkage rate of the molding raw materials is large, it is difficult to ensure the dimensional accuracy of the plastic parts. In general, the greater the shrinkage rate of the molding raw materials, the more difficult it is to ensure the dimensional accuracy of the plastic parts. Therefore, when selecting molding resins, the influence of the shrinkage rate of the raw materials after molding on the dimensional accuracy of the plastic parts must be fully considered. For the selected raw materials, the range of change in their shrinkage rate cannot be greater than the requirements for the dimensional accuracy of the plastic parts. It should be noted that the shrinkage rates of various resins vary greatly, and the analysis should be conducted based on the degree of crystallization of the resin. Generally, the shrinkage rates of crystalline and semi-crystalline resins are greater than those of non-crystalline resins, and the range of shrinkage rate variation is also relatively large. The shrinkage rate fluctuations of the corresponding plastic parts after molding are also relatively large; for crystalline resins, the crystallinity is high, the molecular volume is reduced, and the shrinkage of the plastic parts is large. The size of the resin spherulites also affects the shrinkage rate. The smaller the spherulites, the smaller the gaps between molecules, the smaller the shrinkage of the plastic parts, and the higher the impact strength of the plastic parts. In addition, if the particle size of the molding raw materials is uneven, the drying is poor, the recycled materials and new materials are mixed unevenly, and the performance of each batch of raw materials is different, it will also cause fluctuations in the molding size of the plastic parts.

3. Mold failure

The structural design and manufacturing accuracy of the mold directly affect the dimensional accuracy of the plastic parts. During the molding process, if the rigidity of the mold is insufficient or the molding pressure in the mold cavity is too high, the mold will be deformed, which will cause the molding size of the plastic parts to be unstable. If the matching 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 size accuracy of the plastic parts will also decrease. If there are hard fillers or glass fiber reinforced materials in the molding raw materials, which cause serious wear of the mold cavity, or when one mold with multiple cavities is used, there are errors between the cavities, errors in gates, runners, and poor balance of the feed port, which cause inconsistent filling, and will also cause dimensional fluctuations. Therefore, when designing the mold, sufficient mold strength and rigidity should be designed, and the processing accuracy should be strictly controlled. The mold cavity material should use wear-resistant materials, and the cavity surface should preferably be heat treated and cold hardened.
When the dimensional accuracy of the plastic part is very high, it is best not to use a one-mold-multiple-cavity structure. Otherwise, in order to ensure the molding accuracy of the plastic part, a series of auxiliary devices to ensure the mold accuracy must be set on the mold, resulting in a high mold production cost. When the plastic part has a thickness error, it is often caused by mold failure. If the wall thickness of the plastic part has a thickness error under the condition of one mold and one cavity, it is generally due to the relative position offset between the mold cavity and the core due to the installation error and poor positioning of the mold. At this time, for those plastic parts with very precise wall thickness requirements, they cannot be positioned only by guide pins and guide sleeves, and other positioning devices must be added; if the thickness error is caused under the condition of one mold with multiple cavities, in general, the error is small at the beginning of molding, but the error gradually increases after continuous operation. This is mainly caused by the error between the mold cavity and the core, especially when hot runner mold molding is used. This phenomenon is most likely to occur. For this, a double cooling circuit with a small temperature difference can be set in the mold. If a thin-walled round container is molded, a floating core can be used, but the core and the mold cavity must be concentric. In addition, when making the mold, in order to facilitate mold repair, it is generally accustomed to making the cavity smaller than the required size and the core larger than the required size, leaving a certain mold repair margin. When the inner diameter of the molding hole of the plastic part is much smaller than the outer diameter, the core pin should be made larger. This is because the shrinkage of the plastic part at the molding hole is always greater than that of other parts, and shrinks toward the center of the hole. On the contrary, if the inner diameter of the molding hole of the plastic part is close to the outer diameter, the core pin can be made smaller.

4. Equipment failure

If the plasticizing capacity of the molding equipment is insufficient, the feeding system is unstable, the screw speed is unstable, the stop function is abnormal, the check valve of the hydraulic system fails, the thermocouple of the temperature control system is burned out, the heater is short-circuited, etc., the molding size of the plastic part will be unstable. Once these faults are found, targeted measures can be taken to eliminate them.

5. Inconsistent test methods or conditions

If the method, time, and temperature of measuring the size of the Overmolding for injection molded parts are different, the measured size will be very different. Among them, the temperature condition has the greatest impact on the test, because the thermal expansion coefficient of plastic is 10 times greater than that of metal. Therefore, the standard method and temperature conditions must be used to measure the structural dimensions of the plastic part, and the plastic part must be fully cooled and fixed before measurement. Generally, the size of the plastic part changes greatly within 10 hours of the demolding mode, and it is basically fixed after 24 hours.