What Causes Dimensional Variations Of Injection Molding And How To Fix

1. Inconsistent molding conditions or improper operation

期间obb体育 ，根据工艺要求，必须严格控制诸如温度，压力和时间的过程参数，尤其是每个塑料部件的成型周期必须一致，无法默许变化。如果注射压力太低，则保持时间太短，模具温度过低或不均匀，桶和喷嘴的温度太高，塑料部件的冷却不足，形状和尺寸不足，形状和尺寸塑料部分将是不稳定的。

解决方案:

– In general, higher injection pressure and injection speed, longer filling and holding time, higher mold temperature and material temperature are beneficial to overcome dimensional instability.

– If the shape size of the molded part is larger than the required size, 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, so as to improve the shrinkage rate of the plastic part.

– If the size of the molded part is less than the required size, the opposite molding condition should be adopted.

– The change of environmental temperature also has a certain impact on the fluctuation of the molding size of plastic parts. The process temperature of equipment and mold should be adjusted in time according to the change of external environment.

2. Improper selection of molding materials

原材料有很大的influen的收缩欧宝官网首页ce on the dimensional accuracy of plastic parts. If the precision of molding equipment and mold is very high, but the shrinkage of raw materials is very large, it is difficult to ensure the dimensional accuracy of plastic parts. In general, the larger the shrinkage of raw materials, the more difficult it is to ensure the dimensional accuracy of plastic parts. Therefore, in the selection of molding resin, we must fully consider the influence of the shrinkage of raw materials on the dimensional accuracy of plastic parts. For the selected raw materials, the variation range of shrinkage should not be greater than the requirements of dimensional accuracy of plastic parts.

It should be noted that the shrinkage of various resins is quite different, and it should be analyzed according to the degree of crystallization of the resin. In general, the shrinkage of crystalline and semi crystalline resin is larger than that of non crystalline resin, and the range of shrinkage is also larger, and the corresponding shrinkage fluctuation of plastic parts is also larger; For crystalline resin, the crystallinity is high, the molecular volume is reduced, and the shrinkage of plastic parts is large. The size of resin spherulites also has an impact on the shrinkage. The spherulites are small, the intermolecular space is small, and the shrinkage of plastic parts is small, while the impact strength of plastic parts is relatively high.

In addition, if the particle size of molding raw materials is uneven, drying is poor, recycled materials and new materials are mixed unevenly, and the properties of each batch of raw materials are different, the molding size of plastic parts will also fluctuate.

3. Mold failure

模具的结构设计和制造精度直接影响塑料部件的尺寸精度。在模制过程中，如果模具不够刚性或模腔中的模制压力太高，则模具将变形，这将导致塑料部件的不稳定性。如果引导柱和模具的引导套管之间的拟合间隙由于制造精度差或磨损太多而超出公差，则塑料部件的成形尺寸精度也将减少。如果第一模拟检查磨损或硬芯充满刚性填充物或玻璃纤维增​​强材料，则模腔的误差和转轮的误差和进料口的余量将导致模具填充物。

解决方案

– In the design of the mold, we should design enough mold strength and rigidity, strictly control the machining accuracy, the mold cavity material should use wear-resistant materials, and the cavity surface should be heat treated and cold hardened. It is the first mock exam to ensure that the size of the plastic parts is very high. It is better not to use the structure of a multi cavity mold. Otherwise, in order to ensure the molding accuracy of the plastic parts, a series of auxiliary devices to ensure the accuracy of the mould must be set up, which will lead to the high cost of the mould manufacturing.

- 当塑料部件出现部分厚度误差时，它通常由模具故障引起。对于第一模拟检查，塑料壁的厚度误差通常由模具安装的误差和模具的位置引起，这导致模腔和芯的相对位移。此时，对于具有精确壁厚的塑料部件，必须添加其他定位装置，而不是仅依靠导向柱和引导套筒;如果部分厚度的误差是第一次模拟检查，则形成的成形误差首先较小，但连续操作后误差更大。这主要是由于模腔与芯之间的误差，特别是当使用热流道模具时。因此，可以在模具中设置具有小的温差小的双冷却回路。如果是薄壁圆形容器，则可以使用浮芯，但芯和腔必须同心。

- 另外，在制造模具时，为了容易地修理模具，总是用来使腔体比所需尺寸小，而芯大于所需尺寸，留下一定量的模具修复津贴。当成形孔的内径远小于塑料部件的外径时，应使芯销变大，因为在成形孔处的塑料部件的收缩总是大于其他部件的收缩，而且缩小到洞的中心。相反，如果成型孔的内径接近外径，则可以使芯销变小。

4. Equipment failure

如果模塑设备的塑化容量不足，馈电系统是不稳定的，螺杆速度不稳定，止动功能异常，液压系统的止回阀超出顺序，温度控制系统的热电偶燃烧了出去，加热器是开路的，塑料部件的成型尺寸将不稳定。只要发现这些故障，就可以采取目标措施来消除它们。

5. Inconsistent test methods or conditions

If the measurement method, time and temperature are different, the measurement size will be very different. The temperature condition has the greatest influence on the test, because the coefficient of thermal expansion of plastics is 10 times larger than that of metals. Therefore, the standard method and temperature conditions must be used to measure the structural dimensions of plastic parts, and the plastic parts must be fully cooled before measurement. In general, the size of plastic parts changes greatly within 10 hours of the stripping mode, and basically takes shape within 24 hours.