Xinxiang Bolite Rubber Products Co., Ltd. Address: Daqu Town Industrial Park, Fengquan District, Xinxiang City Phone: 0373-5788884 Contact: Manager Han Mobile phone number: 13007663261 Mobile phone number: 13569839119 QQ: 1171150508 Fax: 0373-5788884 Email: firstname.lastname@example.org
What to do if the product size is unstable during injection processing
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When the equipment is working for a long time during the injection molding process , the product may sometimes be deformed, depending on the size of the product. In fact, there are many reasons for this, such as temperature setting, whether the water channel is not smooth, pressure setting problems, etc. This is the reason for the deformation of the product. The following editor will briefly explain our understanding of this aspect based on our many years of injection molding experience, and I hope it will be helpful to our friends.
1) Inconsistent molding conditions or improper operation During injection molding, various process parameters such as temperature, pressure and time must be controlled in strict accordance with the process requirements, especially the molding cycle of each plastic part must be consistent and cannot be arbitrarily changed. 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 the nozzle is too high, and the plastic part is not cooled enough, it will cause the size of the plastic part to be unstable.
In general, a higher injection pressure and injection speed are used to appropriately extend the mold filling and holding time, increase the mold temperature and the material temperature, and help overcome dimensional instability.
If the shape of the plastic part is larger 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, and the cross-sectional area of the gate should be reduced to increase the shrinkage of the plastic part.
If the size of the plastic part after the molding is smaller than the required size, the molding conditions opposite to that should be adopted.
It is worth noting that the change of the ambient temperature also has a certain effect on the fluctuation of the molding size of the plastic part, and the process temperature of the equipment and mold should be adjusted in time according to the change of the external environment.
2) The shrinkage of improper selection of molding raw materials has a great impact on the dimensional accuracy of plastic parts. If the accuracy of the molding equipment and mold is high, but the shrinkage of the molding raw material is large, it is difficult to ensure the dimensional accuracy of the plastic part. In general, the larger the shrinkage of the molding material, the more difficult it is to ensure the dimensional accuracy of the plastic part.
Therefore, when selecting a molding resin, it is necessary to fully consider the influence of the shrinkage rate after the molding of the raw material on the dimensional accuracy of the plastic part. For the selected raw materials, the variation range of the shrinkage rate cannot be greater than the requirement of the dimensional accuracy of the plastic parts.
It should be noted that the shrinkage of various resins varies greatly, and analysis is based on the degree of resin crystallinity. In general, the shrinkage of crystalline and semi-crystalline resins is larger than that of amorphous resins, and the range of shrinkage changes is relatively large. The corresponding shrinkage fluctuations after molding of corresponding plastic parts are also relatively large. For crystalline resins, High crystallinity, shrinking molecular volume, large shrinkage of plastic parts, the size of resin spherulites also affect the shrinkage rate, small spherulites, small gaps between molecules, small shrinkage of plastic parts, and high impact strength of plastic parts .
In addition, if the particle size of the molding raw materials is uneven, the drying is poor, and the recycled materials are not uniformly mixed with the new materials, the performance of each batch of raw materials is different, and it will also cause the fluctuation of 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 part. During the molding process, if the rigidity of the mold is insufficient or the molding pressure in the cavity is too high, the mold will be deformed, which will cause plastic molds. The forming dimensions of the pieces are unstable.
If the fitting gap between the guide post of the mold and the guide sleeve is too poor due to poor manufacturing accuracy or too much wear, the precision of the molding dimensions of the plastic part will also decrease.
If there are hard fillers or glass fiber reinforced materials in the molding material that cause severe wear of the cavity, or when using a mold with multiple cavities, there will be errors between the cavities, errors in the gates, runners, etc., and poor balance of the inlet. Inconsistent mold filling 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 cavity material of the mold should use abrasion-resistant materials. The cavity surface should preferably be heat treated and cold hardened. When the dimensional accuracy requirements of plastic parts are very high, it is better not to use the one-mold multi-cavity structure. Otherwise, in order to ensure the molding accuracy of the plastic parts, a series of auxiliary devices to ensure the precision of the mold must be set, which will lead to the production of the mold High cost.
When plastic parts have thicker errors, they are often caused by mold failure. If the thickness of the plastic part has a thicker thickness error under the condition of one mold and one cavity, the relative position of the mold cavity and the core is generally shifted due to mold installation errors and poor positioning.
At this time, for those plastic parts that require very accurate wall thickness dimensions, it is not possible to rely solely on the guide post and guide sleeve for positioning, and other positioning devices must be added; if it is an under-thickness error in a mold with multiple cavities, in general, The error at the beginning of molding is small, but the error gradually increases after continuous operation. This is mainly due to the error between the cavity and the core. This phenomenon is most likely to occur when using hot runner molding.
To this end, a dual cooling circuit with a small temperature difference can be installed in the mold. If it is a thin-walled round container, a floating core can be used, but the core and the cavity must be concentric.
In addition, when making a mold, in order to facilitate mold repair, it is generally used to make the cavity smaller than the required size, and the core is made larger than the required size, leaving a certain amount of 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 larger than other parts, and it 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 burns out, and the heater is disconnected. And so on, it will lead to unstable forming dimensions of plastic parts. As long as these faults are found, targeted measures can be taken to eliminate them.
5) Test methods or conditions are inconsistent. If the method, time and temperature of plastic parts are different, the measured size will be very different. Among them, the temperature condition has the greatest influence on the test, because the thermal expansion coefficient of plastic is 10 times that of the metal industry. Therefore, the method and temperature conditions specified in the standard must be used to determine the structural size of the plastic part, and the plastic part must be sufficiently cooled and shaped before measurement. Generally, the dimensional change of plastic parts within 10 hours of the release mode is very large, and the shape is basically set within 24 hours.