Analysis of dimensional instability injection molding defects and elimination methods

Dimensional instability refers to the change in the size of plastic parts between each batch of molded products or between each cavity of molded products produced by each mold under the same injection molding machine and molding process conditions. The change of product size is caused by the anomaly of equipment control, unreasonable injection conditions, poor product design and change of material properties, etc.

1. Inconsistent molding conditions or improper operation

When injection molding, the temperature, pressure and time, and other process parameters must be strictly controlled by the process requirements, especially the molding cycle of each plastic part must be consistent, not arbitrary changes. 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 cooling of the plastic part is not enough, it will lead to the unstable size of the plastic part.

In general, the use of higher injection pressure and injection speed, appropriately extending the mold filling and holding time, improve the mold temperature and material temperature, is conducive to overcoming the size instability failure.

If the size of the molded part is larger than the required size, the injection pressure and melt temperature should be reduced, the mold temperature should be increased, the mold filling time should be shortened, and the gate cross-sectional area should be reduced to increase the shrinkage rate of the molded part. If the size of the molded part is smaller than the required size, the opposite molding conditions should be adopted.

It is worth noting that the change of ambient temperature also has a certain influence on the fluctuation of molding size of plastic parts, and the process temperature of equipment and mold should be adjusted timely according to the change of external environment.

2. Improper selection of molding materials

The shrinkage rate of molding raw materials has a great impact on the dimensional accuracy of plastic parts. If the molding equipment and mold precision are very high, but the shrinkage rate of molding raw materials is very large, it is difficult to ensure the dimensional accuracy of plastic parts. Generally speaking, the larger the shrinkage rate of molding materials, the more difficult it is to ensure the dimensional accuracy of plastic parts.

Therefore, the selection of molding resin must be fully considered after the shrinkage of raw materials molding on the impact of dimensional accuracy of plastic parts. For the choice of raw materials, the shrinkage rate of the range of change can not be greater than the requirements of the dimensional accuracy of the plastic parts.

It should be noted that the shrinkage rate of various resins varies greatly and is analyzed according to the degree of crystallization of the resin. Usually, the shrinkage rate of crystalline and semi-crystalline resins is larger than that of non-crystalline resins, and the range of shrinkage variation is also larger, and the shrinkage rate generated after molding of the corresponding plastic parts fluctuates more.

For crystalline resin, high crystallinity, molecular volume reduction, the shrinkage of plastic parts is large, the size of resin spherical crystal also has an impact on the shrinkage rate, spherical crystal is small, the gap between molecules is small, the shrinkage of plastic parts is smaller, and the impact strength of plastic parts is relatively high.

In addition, if the molding raw materials of uneven particle size, poor drying, uneven mixing of recycled materials and new materials, the performance of each batch of raw materials are different, will also cause fluctuations in the molding size of plastic parts.

3. Mould failure

The structural design and manufacturing accuracy of the mold directly affects the dimensional accuracy of the plastic parts. In the molding process, if the rigidity of the mold is not enough or the molding pressure in the mold cavity is too high, which makes the mold deformed, it will cause the unstable molding size of the plastic parts.

If the clearance between the guide pillar and guide bush of the mold is poor due to poor manufacturing accuracy or too much wear and tear, it will also make the molding dimensional accuracy of the plastic parts decrease.

If there is hard filler or glass fiber reinforced material in the molding raw material which leads to serious wear of the mold cavity, or when using a mold with multiple cavities, there are errors between the cavities and gates, runners and other errors and poor balance of the inlet and other reasons to produce inconsistent mold filling will also cause size fluctuations.

Therefore, in the design of the mold, should design sufficient mold strength and rigidity, strict control of processing accuracy, mold cavity materials should use wear-resistant materials, cavity surface heat treatment, and cold hardening treatment is best.

When the dimensional accuracy of the plastic part is very high, it is better not to adopt the structure form of one mold with multiple cavities, otherwise, to ensure the molding accuracy of the plastic part, a series of auxiliary devices must be set on the mold to ensure the accuracy of the mold, which leads to the high production cost of the mold.

When the plastic part has the error of partial thickness, it is often also caused by mold failure. If the wall thickness of the plastic part under the condition of one mold and one cavity produces deviated thickness error, it is generally due to the installation error of the mold and poor positioning which causes the relative position of the mold cavity and the core to be shifted. At this time, for those wall thickness size requirements are very accurate plastic parts, can not rely only on the guide pillar and guide bush to position, must add other positioning devices.

If the deviation thickness error is produced under the condition of multiple cavities in one mold, in general, the error is small at the beginning of molding, but the error gradually becomes larger after the continuous operation, which is mainly caused by the error between the mold cavity and the core, especially when using hot runner mold molding is most likely to produce this phenomenon.

In this regard, a double cooling circuit with a small temperature difference can be set in the mold. In the case of molding thin-walled round containers, a floating core can be used, but the core and cavity must be concentric.

In addition, in the production of the mold, to facilitate the repair of the mold, it is always customary to make the cavity smaller than the required size, the core to make a larger than the required size, leaving a certain amount of repair margin.

When the inner diameter of the molding hole is smaller than the outer diameter, the core pin should be made larger, which is due to the shrinkage of the molding hole being always larger than other parts, and shrinkage in the direction of the hole center. On the contrary, if the inner diameter of the molding hole 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 rotational speed of the screw is unstable, the stopping action is out of order, the check valve of the hydraulic system is out of order, the thermocouple of the temperature control system is burnt out, the heater is disconnected, etc., it will lead to the unstable molding size of the plastic parts. These faults can be eliminated by taking targeted measures as long as they are identified.

5. Inconsistent test methods or conditions

If the method, time, and temperature of measuring the size of plastic parts are different, the measured size will be very different. Among them, the temperature conditions have the greatest impact on the test, which is because the coefficient of thermal expansion of plastic is 10 times greater than that of the metal industry.

Therefore, it is necessary to use the standard method and temperature conditions to determine the structural dimensions of the plastic part, and the plastic part must be sufficiently cooled and set before measurement. Generally, the dimensional change of the plastic part is very large within 10 hours of demoulding, and it takes 24 hours for the part to be set.