Why Do Injection Molds Need a Venting System?
Sources of Gas in Injection Molds
A. Air trapped in the pouring system and mold cavity.
B. Water content that has not been dried out of some raw materials, which vaporizes into water vapor under high temperatures.
C. Gas generated by the decomposition of certain unstable plastics due to excessively high injection temperatures.
D. Gas generated by the volatility or chemical reaction of certain additives in the plastic raw materials.
Hazards of Poor Venting
Poor venting in injection molds can bring a series of hazards to the quality of plastic parts and other aspects. The main manifestations are as follows:
1) During the injection molding process, the melt will replace the gas in the mold cavity. If the gas is not discharged promptly, it will cause difficulty in filling the melt, resulting in insufficient injection volume and inability to fill the mold cavity completely.
2) Gas that is not discharged smoothly will form high pressure in the mold cavity and penetrate into the plastic under a certain degree of compression, causing quality defects such as air holes, voids, and silver streaks.
3) Due to the highly compressed gas, the temperature in the mold cavity will rise sharply, causing the surrounding melt to decompose and burn, resulting in local carbonization and burning of the plastic part. This mainly occurs at the confluence of two melts and the gate flange.
4) Poor gas discharge will cause the melt entering each cavity to have different speeds, thus easily forming flow marks and fusion marks, and reducing the mechanical properties of the injection-molded parts.
5) The obstruction of gas in the mold cavity will reduce the mold filling speed, affect the molding cycle, and lower the production efficiency.
Bubble Distribution in Injection Molded Components
The sources of gas in the mold cavity are mainly divided into three categories: trapped air in the mold cavity, gas produced by decomposition of raw materials, and water vapor evaporated from residual water in the raw materials. The position of the bubbles generated due to different sources will also be different.
1) Bubbles produced by trapped air in the mold cavity are often distributed at locations opposite to the gate.
2) Bubbles generated by decomposition or chemical reactions in the plastic raw materials are distributed along the thickness of the plastic part.
3) Bubbles produced by the evaporation of residual water in the plastic raw materials are irregularly distributed throughout the plastic part.
Characteristics of Venting Channel Design in Injection Molds
1) Design of Venting Ports
Venting ports are slot-shaped outlets designed in molds to discharge original and incoming gases brought by the melt. When the melt is injected into the mold cavity, the original air in the cavity and the gas brought by the melt must be discharged through the venting ports at the end of the material flow to the outside of the mold. Otherwise, the plastic product will have air holes, insufficient filling, and even the accumulation of air compressed to high temperatures, which may burn the plastic product.
Generally, venting holes can be set at the end of the melt flow in the mold cavity or on the parting surface of the plastic mold. The latter involves creating a shallow groove with a depth of 0.03-0.2 mm and a width of 1.5-6 mm on the concave mold side.
During injection, there will not be much melt leaking out of the venting holes because the melt will cool and solidify, blocking the channel. The location of the venting ports should not be directed towards operators to prevent accidental melting material from spraying and injuring them. Additionally, the gap between the ejector pins and ejector holes, as well as the gap between the ejector blocks and the core, can also be used for venting.
2) Functions of Venting Channels
The main functions of venting channels are: first, to discharge the air in the mold cavity during the injection of molten material; second, to discharge various gases generated during the heating process of the material. For thin-walled plastic products, especially those parts far away from the gate, the creation of venting channels is particularly important.
For small injection-molded parts or precision components, attention should also be paid to the creation of venting channels because they can not only avoid surface burns and insufficient injection volume but also eliminate various defects in the product and reduce mold contamination.
How can we determine if the venting in the mold cavity is sufficient? Generally, if there are no scorch marks left on the product when the melt is injected at the highest injection rate, it can be considered that the venting in the mold cavity is sufficient.
3) Venting Methods
There are many methods for venting the mold cavity, but each method must ensure that while the venting channels discharge gas, their size design should prevent the material from spilling into the channels. Additionally, they must be prevented from becoming clogged. Therefore, when measured from the inner surface of the mold cavity to the outer edge, the part of the venting channel with a length of 6-12 mm or more should have a groove height that is enlarged by approximately 0.25-0.4 mm.
Too many venting channels are harmful. If the clamping pressure acting on the part of the mold cavity parting surface without venting channels is very high, it can easily cause cold flow or cracking of the mold cavity material, which is very dangerous.
Apart from venting the mold cavity on the parting surface, it can also be achieved by setting venting channels at the end of the material flow in the pouring system and leaving gaps around the ejector pins. However, if the depth, width, and location of the venting channels are not chosen properly, flashing may occur, affecting the appearance and accuracy of the product. Therefore, the size of the above-mentioned gaps should be limited to prevent flashing around the ejector pins.
4) Venting Design Methods
Based on JSJM Mold's years of experience in injection mold design and product testing, this article briefly introduces several venting channel designs. For molds with complex geometric shapes, the creation of venting channels is best determined after several trial runs. The biggest disadvantage of the overall structural form in mold structure design is poor venting.
For solid mold cavity cores, there are several venting methods:
A. Utilize slots or insert installation locations in the cavity
B. Utilize the joint seams of side inserts
C. Locally create a spiral shape
D. Install grooved strips or open process holes in longitudinal positions
E. Use splicing structures when venting is extremely difficult
F. If it is difficult to create venting channels in some dead corners of the mold, the mold should be appropriately modified into splicing processing without affecting the appearance and accuracy of the product. This not only facilitates the processing of venting channels but can also improve the original processing difficulty and facilitate maintenance.
After a comprehensive discussion on the design of the venting system in injection molding, we can clearly recognize the importance of the venting system in the injection molding process. The presence of gas in injection molding molds is inevitable, which may originate from plastic raw materials, the interior of the mold, or other links in the injection molding process. However, if these gases are not effectively expelled, they will significantly impact the quality of the product and the operating efficiency of the mold. Therefore, through rational design and setup of the venting system, JSJM mold can effectively solve the problems caused by poor venting, improve the quality of the product and the operating efficiency of the mold, reduce production costs, and bring greater economic benefits to the enterprise.
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