模具排气与毛边

  模具的排气效果对注塑过程的影响，进而对模制品品质的影响远比我们认为的要大。行业老师傅总结的关于排气的经验很多都不合理。在模具人的观念中，排气以不产生毛边为原则，而不是以充填顺畅为原则。而现场的工艺人也很多没有真正理解排气的重要性，纠结在充填压力和排气效果的矛盾之中。

  塑胶熔体流动遵循最小阻力原则（任何流体的流动都遵循这一原则）。因此决定流动分布的不仅有驱动力，还有阻力的影响。阻力取决于型腔的几何形状，模具温度，以及排气。黏度越高的材料对流动阻抗越大，因而对流动通道的阻力越敏感。比如PP料对型腔壁厚变化的敏感性就比PC料低。而排气在阻力中扮演着重要的角色。

  任何注塑缺陷或多或少都与排气效果有关，就如任何缺陷都与注塑压力有关一样。因为排气效果的一个直接影响就是注塑压力。型腔中的任何联通到模具外面的间隙都有排气作用，并不只有特意设计的排气槽才能排气。注意，关键词是联通到模具外面，没有联通到模具外面的间隙没有排气作用。

 模具排气槽出现毛边，极大地误导了模具人的判断。因为毛边是间隙过大的结果，而并不是排气槽本身的结果。间隙过大是多种因素的综合结果:封胶面吻合不密；注塑压力过大（模具刚度不够）导致封胶面变形涨开；注塑机模板平行度不够；等等。如果这些因素综合作用产生了毛边，而毛边位置恰好是排气槽，于是得出结论:排气槽太深。在这种错误结论引导下，模具排气槽做得越来越浅，因而排气效果不好导致许多缺陷发生，却不敢从改善排气方面着想。问题解决于是陷入一个死循环。

  行业盛行着许多关于排气效果的错误观念。很多人认为只在需要的位置做排气，也有很多人认为只要产品不出现烧焦，排气就是很充足。而将排气不充分导致的气纹，复制性差，缺胶，等缺陷归咎于参数不合理。一旦出现这些缺陷，不去检查模具排气效果，而是逼着工艺人员变着花样调整速度，只恨机器功能设定的段数不够，仿佛不用完所有段数就不足以证明手段高明似的。然而这样走钢丝般的手段无法获得稳定的注塑生产。

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The effect of mold ventilation to molding process, thus to the quality of molded part is far more than we thought. Experienced technicians had concluded many rules of thumb about venting, but many of it is not correct. In the mold maker's mind, the rule No.1 of venting channel design and machine is that must have no flash occurred, but not to make filling easy. And for the molder, many of them are lack of understanding about the relationship between flash and venting, so suffer for fight to find a compromise between reduction of fill pressure and flash.

The flow of plastic obeys the general rule which it intend to flow into the area with least resistance (every fluid follow the rule when it flows). So the flow of plastic melt during injection not only depended by driving force which provided by the screw of injection machine, but also be affected by the resistance that flow front will face. The resistance to flow is determined by geometry, temperature, and venting of cavity. The viscosity of melt higher, more resistance to flow, and this melt also more sensitive to resistance variation. For instance, PP is less sensitive to thickness variation than PC, because PP is much less viscously than PC. The ventilation of cavity plays a big role in resistance to melt flow.

Any defect of molded part, almost its root cause could be chased to venting effect; just like that injection pressure too high almost is the source of any defect. Because venting effect directed affect the injection pressure. Any gap inside cavity which connected outside of the mold can vent, it is not only the designed venting channel have the function to vent. But note that the key word is: connecting to outside of mold. The gap has no venting effect if it does not connect to outside.

If the flash happen to occurred on venting channel, could mislead to wrong judgement about depth of venting channels. Because of flash is the result of excessive gap, not only caused by too deep venting channel. Excessive gap is the result of many factors, or the combination of them, such as sealing surface touched not well; mold deflection caused by pressure (mold stiffness is not enough); machine plateau is not parallel or has permanent deflection, etc. Once the flash occurred, and happen to on venting area, then concluded a result: the depth of venting channel is too big. This is a wrong conclusion, and result in that venting channel become shallow and shallow, then many defect happened due to this, but very few people dare to face the risk of flash that maybe caused by venting improvement. The troubleshooting has fallen into a dead circle.

There are many wrong idea of venting which is popular in this industry. Many people think that venting channel only need to be located where it is necessary; venting is enough if there is no burning mark shown on molded part. But blame that parameter setting is not proper if some defect occurred which is caused by insufficient venting, such as air mark, bad surface quality, short shot, and others. Once these occurred, nobody go to check the venting, but push the processer to try to adjust injection velocity, and they wish that the machine has the function with limitless injection step to meet the requirement of velocity setting. In some people’s mind that a molder is not skilled if the machine he set still has some injection still be available to set. But this “best balance” way cannot make production stable and robust.

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