By Hank Tsai., Effinno Technologies Co., Ltd.
Process introduction:
In general, it has four stages in a gas-assisted injection molding process (Figure 1).
Stage 1 – Filling by Molding Machine Injection:
Molten plastic material is injected into a mold by a molding machine to fill the part cavity. The amount of the material injected is about to fill 60~90% volume of the cavity, depending on the types of part in which how much the volume to be cored out by gas is designed.
Stage 2 – Filling by Gas Injection:
After the material filling stage has finished, gas (usually nitrogen) is injected into the center of the molten plastic material, pushing it forwards to the rest of the cavity space to fill the part cavity completely.
Stage 3 – Packing, Holding, and Cooling:
The injected gas exerts uniformly distributed pressure to the previously filled plastic material from the inside of the part outwards against the mold wall. Instead of the machine screw, here it’s the injected gas to act as the pressure source of packing and holding, while in the way that provides across the part with much more uniform pressure and a much closer and more effective pressure source. It results in much less pressure gradient and brings this process advantage over traditional injection molding, especially from viewpoints of part dimension control and avoiding part warpage. When the injected gas is exerting its packing and holding pressure, the part is under cooling simultaneously.
Stage 4 – Gas releasing:
After the part is cooled and solidified, the injected gas is released or recycled before mold opening for part ejection.
Two Types of Part in Application:
It is diverse to design a part in its geometry, shape, size, thickness, etc., for an injection molding process. Generally, molding parts can be categorized into two types as follows for applying gas-assisted injection molding process.
(1) Handle-like part:
So-called handle-like part refers to those such as various kinds of handles and chair arms. This type of part is so wholly thick that cycle time has to be very long for cooling if using the traditional injection molding process. It also probably brings in appearance defects such as sink marks caused by the insufficient packing effect to such a thick part. To fit in with the traditional injection molding process and eliminate the mentioned problems, this type of part is typically designed as two splitting halves, each with a regular (much thinner) nominal thickness of part surface and structure reinforced ribs beneath it. Ideally, it requires two molds and two machines to produce the two halves individually. After the two halves are injection molded, a secondary process is needed to join them, becoming a finished good.
When applying gas-assisted injection molding process at such a part, it is the gas injected to push melt forwards coring out the central portion of the part (Figure 2), then followed by the packing and holding actions pressure-exerted by the gas itself in a uniformly distributed manner from right beneath the residual thickness across the part. Compared to the original solid part design produced by the traditional injection molding process, it can result in a much shorter cycle time and meanwhile without sink mark problem. Additionally, compared to designing part with two-half components, the time and cost saved are significantly in the tooling and the finished-good manufacturing points of view. With the gas-assisted injection molding process, it requires only one and less complex mold instead of two, each more complex caused by more complex part design with structure-reinforced ribs, and it needs only one molding machine and molding production instead of two that eliminates the need for the subsequent secondary process to join the two-half components.
(2) Flat part:
So-called flat part refers to various kinds of table, panel, housing/cover parts in electronic devices, home appliances, automotive, etc., industries. This type of part is nominally thin in thickness in comparison with its overall length and width. The most challenging issue of producing such parts by traditional injection molding process is warpage. To overcome it, a part designer has to design structure-reinforced ribs beneath the cosmetic surface across the part to resist its trend to warp. It might require a multiple-gate design for the mold to provide a balanced melt filling and packing/holding effect. And hot runner system might be required to provides the part with a closer and more effective source of packing/holding pressure, etc.
When applying gas-assisted injection molding process at such a part, in lieu of the situation that central portion of the whole part is cored out as a handle-like part, gas is guided into a purposefully designed gas channel only (Figure 3).
Compared to the traditional injection molding process, the advantages it brings about include a closer and more uniform pressure source at packing and holding stages that right within the part, and fewer ribs are required to make the part equivalently strong. Both facilitate to avoid warpage problem with less cost of the mold.
Concerning a handle-like part, the part itself has acted as a gas channel already. Under proper melt gate location, gas insert location, and process conditions, the injected gas is restricted; it can flow towards one direction only within the part from one end to the other without a doubt. However not the case in a flat part; the injected gas must selectively flow along the designed route of the gas channel rather than core out the part everywhere. And if not properly design in the melt gate number/locations, gas insert number/locations, and layout of the gas channel, the gas will not necessarily flow into, along, and core out the entire length of the gas channel. In such a situation, the gas-unfilled portion of the gas channel is more like being processed under traditional injection molding process but with unusually much thicker part thickness at the base of the rib. It tends to result in serious sink mark defect at the part surface and generally is of no use in trying to solve it by adjusting parameters of process conditions. For a flat part, it is crucial that the gas channel layout is tailor-made to guide the injected gas accurately flow into/through the entire network of the gas channel; and the injected gas exists only within the gas channel without penetrating the adjacent area.
One Fundamental Concept:
There is one fundamental concept about the gas-assisted injection molding process, as follows, which indicates how gas will flow and towards which direction the injected gas will flow within a part.
“Gas flows towards the direction in which the resistance to its flow is the least.”
Simply put, there are two significant factors in the part cavity which influence resistance to gas flow: pressure and temperature. Generally, the direction with the least resistance to make gas flow towards refers to the direction in which lower melt pressure and higher melt temperature exist.
The pressure-related variables include:
1. Location of melt front.
2. Section size of gas channel.
3. Distance from melt gate.
4. Distance from other gas channels, etc.
And the temperature-related variables include:
1. Part thickness.
2. Shear-heating effect.
3. Mold temperature, etc.
Ten Part-design Rules:
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 viscoelastic melt injected. Instead, in the gas-assisted injection 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 5). 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 6). 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 7).
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 8). 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 9). Nomatter 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 shallbe 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 finetuning 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 injection 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.
Complement
Gas-assisted injection molding might have its niche in small or micro parts, on which the space for designing gas channel is not available or not necessary. Here small or micro part refers to those each has much less part volume when compared with the cold runner volume of its mold. It has two types of small or micro parts at which this technology could apply, and it appears to be much easier.
(1) For solid short-fat-like micro part without core-out structure, the gas injection point is placed at runner so that gas is controlled to low in runner only and stop before melt gate, the closer the better (Figure 11). In such a case, the part is filled with the melt pushed by the gas. It is packed and held by the gas, too, right from outside of the melt gate.
(2) For a thin-slender-like small part with a complex core-out structure designed with rules for traditional injection molding, the gas injection point is placed inside the cavity. In such a case, the part is filled by melt at first at melt filling stage; then, skipping gas filling stage, the process is directly followed by gas packing/holding stage so that gas penetrates in part along the path where the least resistance is, generally along the path of the intersection of part walls (Figure 12). The formed gas channel acts as a linear pressure source inside the part to provide an effective and uniform packing/holding effect, which helps improve quality in part flatness and avoid warpage. Although the intersection of part walls acts as a natural channel for gas to penetrate, still care should be taken not to have gas penetrate adjacent part walls resulting in the inferior mechanical performance of the part.
About the Author:
Hank Tsai is the owner and consultant of Effinno Technologies Co., Ltd. in Taiwan, an injection molding training and consulting service provider. He has more than 25 years of experience in the injection molding industry. He has expertise in injection molding technologies and practices, production efficiency management, part costing, troubleshooting, simulation, mold/process/machine performance evaluation, and process optimization by Taguchi DOE. Contact: hank.tsai@effinno.com.
