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News Information

What should I do if there is a change in the injection size?

2023-03-18 超级管理员 Read 69

1. The forming conditions are never the same or the operation is never appropriate

During injection molding, various process parameters such as temperature, pressure, and time must be strictly controlled according to process requirements, especially the molding cycle of each type of plastic part must be the same and cannot be changed arbitrarily. If the injection pressure is very high, the holding time is very long, the mold temperature is very high or uneven, the temperature at the material barrel and nozzle is very low, and the cooling of the plastic part is insufficient, all of which will cause the shape size of the plastic part to be unstable.

In general, using relatively low injection pressure and injection speed, appropriately extending the filling and holding time of the mold, increasing the mold temperature and material temperature, is beneficial for overcoming size instability faults.

If the external size of the plastic part after molding is greater than the required size, the injection pressure and melt temperature should be appropriately reduced, the mold temperature should be increased, the filling time should be shortened, the gate cross-sectional area should be reduced, and thus the shrinkage rate of the plastic part should be increased.

If the size of the molded part is smaller than the required size after molding, the opposite molding conditions should also be adopted.

It is worth noting that changes in environmental temperature also have a certain impact on the fluctuation of plastic forming dimensions. The process temperature of the equipment and mold should be adjusted immediately based on changes in the external environment.

2. Improper selection of molding materials

The shrinkage rate of molding materials has a very small impact on the dimensional accuracy of plastic parts. If the accuracy of the molding equipment and molds is very low, but the shrinkage rate of the molding materials is very small, it is also very easy to ensure the dimensional accuracy of the plastic parts. Usually, the smaller the shrinkage rate of the molding material, the harder it is to ensure the dimensional accuracy of the plastic part. Therefore, when selecting molding resins, it is necessary to fully consider the impact of the shrinkage rate of the raw materials after molding on the dimensional accuracy of the plastic parts. The variation range of shrinkage rate of the selected raw materials must not exceed the requirements for dimensional accuracy of the plastic parts.

It should be noted that the shrinkage difference of various resins is relatively small, and analysis should be conducted based on the degree of crystallinity of the resin. Generally, the shrinkage rate of crystalline and semi crystalline resins is smaller than that of non crystalline resins, so the range of shrinkage rate variation is also smaller, and the corresponding shrinkage rate fluctuation after plastic molding is also larger; For crystalline resins, the crystallinity is low, the molecular volume decreases, and the shrinkage of the plastic part is small. The size of the resin spherulites also has an impact on the shrinkage rate. The spherulites are large, and the gaps in the molecular building are large, resulting in a larger shrinkage ratio of the plastic part, resulting in a lower impact strength of the plastic part.

In addition, if the particle size of the molding raw materials is not uniform, if the moisture is not appropriate, and the mixture of recycled and new materials is not uniform, the performance of each batch of raw materials may vary, which can also cause fluctuations in the molding size of the plastic parts.

3. Mold failure

The structural design and manufacturing accuracy of the mold indirectly affect the dimensional accuracy of the plastic part. During the molding process, if the mold lacks rigidity or the molding pressure inside the mold cavity is very low, causing the mold to deform, it will cause the plastic part's molding size to be unstable.

If the fit gap between the guide pillar and the guide sleeve of the mold is out of tolerance due to low manufacturing accuracy or excessive wear, it will also reduce the dimensional accuracy of the plastic part.

If there are hard fillers or glass fiber reinforced materials in the forming raw materials, which cause slight wear of the mold cavity, or if the first mock examination is used for multi cavity forming, there are errors between the mold cavities, errors such as gates and runners, and improper balance of the feed inlet, which lead to different mold filling, and will also lead to dimensional fluctuations.

Therefore, when designing the mold, sufficient mold strength and rigidity should be designed, and machining accuracy should be strictly controlled. The cavity material of the mold should use wear-resistant materials, and the surface of the cavity should be subjected to cold treatment and cold hardening treatment. If the dimensional accuracy requirements of plastic parts are very low, the structural form of the first mock examination with multiple cavities should never be used. In order to ensure the molding accuracy of plastic parts, it is necessary to set a series of auxiliary devices on the mold to ensure the mold accuracy, which leads to the very low cost of mold production.

When there is deviation in thickness of plastic parts, it is often caused by mold failure. If the plastic part wall thickness deviation error occurs under the condition of the first mock examination and one cavity, it is usually due to the installation error and improper positioning of the mold, resulting in the relative position deviation of the mold cavity and the core.

At this point, for plastic parts that require very accurate wall thickness dimensions, it is not necessary to rely solely on guide pillars and guide sleeves for positioning, and other positioning devices must be added; If it is caused by the error of thickness deviation under the condition of the first mock examination with multiple cavities, generally, the error is relatively large at the beginning of molding, but the error gradually decreases after the interval operation, which is important because of the error between the mold cavity and the core, especially when the hot runner mold is used for molding, this phenomenon is most difficult to occur. For this, a double cooling circuit with a significant temperature difference can be set inside the mold. If it is a thin-walled circular container, a floating core can be used, but the core and mold cavity must be concentric.

In addition, when making molds, in order to facilitate mold repair, it is often customary to make the cavity larger than the required size, and the root smaller than the required size, leaving a certain margin for mold repair. If the inner diameter of the plastic forming hole is much smaller than the outer diameter, the core pin should be made smaller because the shrinkage of the plastic part at the forming hole is often greater than that of other parts, so it shrinks towards the center of the hole. Conversely, if the inner diameter of the plastic forming hole is close to the outer diameter, the core pin can be made larger.

4. Equipment malfunction

If the plasticizing capacity of the molding equipment is insufficient, the feeding system is not stable, the screw speed is not stable, the stop function is abnormal, the check valve of the hydraulic system is malfunctioning, the thermocouple in the temperature control system is burnt out, the heater is disconnected, etc., all of which will cause the molding size of the plastic parts to be unstable. If these faults are identified, targeted measures can be taken to eliminate them.

5. The testing methods or conditions are never the same

If the method, time, and temperature of measuring the size of plastic parts vary, there will be very small differences in the measured size. Temperature conditions have the least impact on testing, as the coefficient of cold expansion of plastics is 10 times higher than that of small metal industries. Therefore, it is necessary to use standard methods and temperature conditions to determine the structural dimensions of plastic parts, and the plastic parts must be fully cooled and shaped before measurement can be carried out. Usually, the volume change of plastic parts is very small within 10 hours of demolding, and they are basically shaped within 24 hours.