Gas Assisted Molding

Basing on the fundamental concept of gas flow, it develops ten part- design rules to facilitate the application of the gas assisted injection molding process.

Rule 1: Prioritize layout design of gas channel
Designing the layout of the gas channel at first according to the purpose of applying gas-assisted injection molding process, no matter it’s for coring out the central portion of the part, saving material, enhancing structural strength by gas channels, avoiding warpage, or merely using the pressured gas at some local area to avoid a sink mark there.

Rule 2: Clearly define the path of gas flow. Avoid branched gas flow.
Gas is sensitive. It prefers the least resistance so much that it flows towards that direction at first. It is hard to realize for a gas channel design to have gas split equally into two identical branches, as illustrated in Figure 4. The possibility of creating identical resistance conditions in reality at the two branches during the actual molding process for leading to identical gas flow and distribution within the two branches is quite remote. Minor condition differences between the two branches, such as tool dimension, melt temperature, melt front advancement, and mold temperature, cause a difference in gas flow resistances, resulting in the expected identical gas distribution in the gas channel non-identical. It leaves a gas-unfilled segment of the gas channel where a high risk of sink mark issue. A part designer shall clearly define the path of gas flow. The branched gas channel, which is ambiguous for gas to flow forwards, shall be avoided.

Rule 3: Design layout of the gas channel across the entire part and in a symmetrical manner.
Packing and holding are important process stages during which the injected plastic material is compressed, making the molded part’s density as high as possible and as uniform as possible. In the traditional injection molding process, it is the machine screw to exert the packing/holding pressure a long way from the machine nozzle through the sprue, runner, gate to the inside cavity through the melt injected. Instead, in the gas-assisted molding process, it’s the injected gas within the part already to exert packing/holding pressure by itself. For a flat part, it is important to design the layout of the gas channel across the entire part to provide the molding part with an overall nearby source of packing/holding pressure and its uniform effect along the gas channel. It is also important to design the layout of the gas channel in a symmetrical manner to provide the molding part with a uniform and balanced packing/holding pressure effect transverse to the gas channel (Figure 2). Additionally, the symmetrical layout of the gas channel can reduce the complexity of process conditions about gas control and delivery.

Rule 4: Thinning part overall and thickening part locally wherein designed the gas channel.
Compared with the traditional injection molding process, the overall nominal part thickness for gas-assisted injection molding can be thinner for saving material. Then the part strength can be enhanced by a gas channel, where it acts like a rib but with an unusually thicker base without getting sink problem if adequately designed (Figure 3). Additionally, before injecting the gas into the gas channel, the gas channel plays the role of a flow leader at first to help the melt fill across the thinning part overall. After the gas distributes within the gas channel, the gas channel plays the second role as a packing/holding pressure source. And finally, after the process, the gas channel plays its third role as a thickening rib to perform the part’s strength avoiding warpage with less complexity of mold structure and tooling process.

Go with part thickness to design the gas channel’s height and width. Comparatively, too large a section of a gas channel might bring about too strong a flow leader effect during the melt filling stage, leading to the melt in gas channel flows much faster than that of the adjacent area and resulting in an air trap problem (Figure 4).

Rule 6: Avoid the fingering effect caused by too small a gas channel section.
Go with part thickness to design the gas channel’s height and width. Comparatively, too small a section of a gas channel might not offer the least resistant direction for gas to flow in the intended gas channel, resulting in that gas penetrates the area adjacent to the gas channel during the gas filling stage and packing/holding stage, which is called fingering effect (Figure 5). Typically, designing the height of the gas channel, not including the part thickness, one and a half times the adjacent part thickness as a start. It is necessary to avoid the fingering effect lest it weakens the part’s surface structure at the place where it happens.

Rule 7: Avoid closed-loop gas channels.
The expectation that gas flows around and forms an entirely closed-loop gas channel hardly comes true (Figure 6). No matter how well-balanced is the gas flow in the closed-loop gas channel, anyway melt fronts in the gas channel from the two directions will meet sooner or later, forming a solid portion where the gas can’t flow further. It is essential to avoid designing a closed-loop gas channel because the residual solid portion mentioned causes a high risk of sink mark problem and a longer cooling time and cycle time.

Rule 8: Extend the gas channel to the area where melt fills the last.
Where there is a proceeding melt front, there is a path with the least resistance for gas to flow towards. Extend the gas channel to the area where melt fills the last also helps the gas channel across the part overall, as mentioned in Rule 3. Following this rule, the design of the gas channel must go with a melt filling pattern which is determined by melt gate location, melt gate number, part thickness, and gas channel size. Change in melt filling pattern caused by any changes of the mentioned determinants often means that an inevitable modification in gas channel layout design is also required.

In other words, the melt filling pattern must be designed by optimizing the mentioned determinants to have the gas flow in the intended gas channel and penetrate in it only without any air trap problem and fingering effect.

Rule 9: Gas injection point to be far away from the area where melt fills the last.
Assuming a design for a flat part has been done by following Rule 1 to 8, as shown in Figure 10, gas injection points shall be placed at point 1 and point 2. By such a design, it is expected for the gas injected from point <1> to flow in the right gas channel and that from point <2> in the left, pushing melt forwards to the ends of both gas channels, the area where melt fills the last. In case that gas injection points are placed at point <3> and point <4>, the injected gas will also directly flow downwards the ends of gas channels, leaving the segments of gas channels from point <1> to point <3> and point <2> to point <4> solid without being cored out by gas.

Rule 10: Fine-tune the melt filling pattern and gas penetration length by adjusting the size of the gas channel.
Usually, the primary melt filling pattern and gas distribution are decided by means of the designs in part thickness, melt gate location/number, gas injection position/number, and gas channel layout/size. If needed, a minor change in melt filling pattern and gas penetration length, especially at the end of the gas channel, could be done by adjusting and fine tuning the size of the gas channel nearby.

The behavior of gas in the melt is sensitive, dynamic, complex, and difficult to predict by experience. The consequence of producing a part with a solid gas channel is severe and expensive because it can hardly get resolved at the same mold. Part design for gas-assisted molding process must involve integrated and systematic considerations in, part thickness, melt gate location/number, gas injection position/number, and gas channel layout/size. So, doing it with the help of Computer-aided Engineering (CAE) is highly suggested, especially for melt and gas filling analysis. Applying the ten part-design rules with CAE could help reach a low-risk solution more systematically and efficiently.

Quoted “TEN PART-DESIGN RULES FOR GAS-ASSISTED INJECTION MOLDING PROCESS” By Hank Tsai., Effinno Technologies Co., Ltd.

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