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Computer-aided engineering for gas-assisted injection molding.

With current CAE technology, both the filling and post-filling stages can be analyzed, allowing the behavior of polymer and gas to be predicted for the entire process.

Gas-assisted injection molding is a versatile process that provides great flexibility in the design and manufacture of plastic parts. This process has the potential to produce rigid parts incorporating both thick and thin sections with fewer residual stresses (and therefore less warpage) and better surface finish than conventional injection molding, while offering even more design freedom and cost savings from weight reduction and lower tooling and press capacity requirements. Typical applications include:

* tube- and rod-like parts--clothes hangers, hammer handles, chair armrests, etc.--primarily for saving material and reducing the cycle time by coring out of the part;

* large sheet-like structural parts--automotive panels, business machine housings, outdoor furniture, etc.--primarily for reducing warpage and press tonnage requirements while enhancing rigidity and surface quality; and

* complex parts combining both thin and heavy sections--such as automotive window guidance channels--primarily for decreasing manufacturing cost via parts consolidation.

Because the process involves dynamic interaction between two dramatically dissimilar materials flowing within complex cavities, the product, tool, and process designs for gas-assisted injection molding are quire complicated. Further, previous experience with conventional injection molding is no longer sufficient to deal with this process, especially in designing the gas-channel network and optimizing the processing window.

A viable computer-aided engineering (CAE) analysis can provide crucial insight to help overcome this lack of experience. In the meantime, expertise and design guidelines can be quickly established by numerically iterating and evaluating alternative product/tool designs, processing conditions, and material selections until the optimal result is achieved. Another important benefit of CAE technology is the replacement of sequential design by concurrent design by enabling all parties to interact during the design phase via computer simulation. As a result, quality products can be developed more quickly and at lower cost.

This article begins with a brief description of gas-assisted injection molding, followed by a discussion of recent developments in CAE technology, in particular, experimental verification of the numerical analysis; versatile functions for simulating different methods of gas-assisted injection molding; and post-filling simulation to account for the secondary gas penetration resulting from volumetric shrinkage of the polymer melt. General guidelines are then established for the design and manufacture of gas-assisted injection molded parts.

Process Characteristics

Basically, the process consists of a partial or nearly full injection of polymer melt, followed by an injection of compressed gas into the core of the melt to assist the filling and packing of the cavity. Depending on the method employed, the gas can be injected via the nozzle, the sprue, or the runner, or directly into the cavity, at regulated gas pressure or volume. The gas hollows out a network of built-in, thick-sectioned gas channels, effectively transmitting a fairly uniform pressure distribution as it penetrates to the extremities of the part.

A detailed description of the underlying process mechanisms is given in Reference 1. Figure 1 schematically illustrates how the gas pressure required to fill the mold cavity can be lower than the required entrance pressure for conventional injection molding because of the effective transmission of pressure by the inviscid gas. Further, the resulting pressure distribution is more unifrom in a gas-injected part, which induces fewer residual stresses as the polymer cools during the post-filling stage. The gas-assisted injection molded part can thus be produced with a lower gas-pressure requirement (at normally lower clamp tonnage) and has less tendency to warp.

Since the first literature devoted to the numerical simulation of the gas-assisted process was published in 1991[2], this CAE analysis has been applied in a number of actual applications for verification[1,3,4]. These trials provided useful guidelines for further development. Meanwhile, the numerical analysis was enhanced with several new features to simulate various gas-assisted molding processes. These features include options to analyze processes that employ either gas-pressure or gas-volume control, permitting the proper process to be chosen for a particular application. An automatic gas-pressure-profiling option is also available to determine an ideally profiled gas pressure based on the user-specified melt-flow rate. This feature provides the user with a range of reasonable gas pressures that deliver proper melt viscosity without hesitation or acceleration, and a reference in setting the processing parameters with the existing process.

Simulating Gas-Pressure Controlled Processes

These processes regulate the pressure profile (constant or step) of the injected gas during the gas-injection stage. For the analysis, the user specifies a variable gas-pressure profile that will be imposed at the gas-injection point(s), with an optional delay time for the onset of gas injection after the end of resin injection. Figure 2a illustrates the predicted gas penetration at the end of fill for a spiral part with the gas-pressure control option. The imposed stepwise gas pressure at the melt/gas entrance (Node 67, Fig. 2b), together with pressure traces at other nodal locations, is shown in Fig. 2c. In Fig. 2c, the entrance pressure is seen to increase monotonously with time during the resin injection stage (from 0 to 1.0 sec). Based on the current incompressible-fluid assumption for polymer melt in the filling stage, the entrance pressure drops to zero during the 0.2-sec delay time (from 1.0 to 1.2 sec). Upon gas injection, the pressure trace for Node 67 in Fig. 2c shows the specified stepwise gas-pressure profile. Because of the delay time and the lower initial gas pressure compared with the melt-entrance pressure at the end of resin injection, the predicted melt-front advancements in Fig. 3 exhibit some hesitation immediately after the melt/gas transition, as indicated by the density of the contour lines. As the gas advances toward the melt front with increasing gas pressure, the melt ahead of the gas tip starts to accelerate, as evidence by the large distance between contour lines toward the end of the spiral part.

Simulating Gas-Volume Controlled Processes

The gas-volume control option is designed to simulate processes that compress a fixed amount of gas within a compression cylinder, which pushes the gas to displace polymer melt in the cavity. Conceivably, as the gas is being compressed, the resulting gas pressure will increase with time. This is because the gas volume reduction in the compression cylinder is generally greater than the gas volume in the cavity generated by displacing the polymer melt. The required process-condition inputs for this option include the initial gas pressure and volume in the compression cylinder, gas-plunger compression speed profile, and an optional delay time. Figure 4a illustrates the predicted gas penetration at the end of fill for the same spiral part with the gas-volume option. By adjusting the user-specified process conditions, the resulting entrance gas pressure at Node 67 (Fig. 4b) was made comparable with the imposed gas pressure in the previous example. This leads to similar results in terms of predicted gas penetration for the two processes. The pressure traces during filling at other nodal locations (Fig. 4b) are also plotted in Fig. 4c. The predicted melt-front advancements (Fig. 5) exhibit the same initial hesitation immediately after the melt/gas transition and final acceleration toward the end of gas injection in filling.

Simulation of Secondary Gas Penetration

The simulation was extended to the post-filling stage to account for the secondary gas penetration resulting from polymer shrinkage. As examples, the gas penetrations for the previous two cases are plotted in Figs. 6a and b, respectively. Recall that in the filling stage, gas cannot penetrate the shorter blind portion to the right of the melt/gas inlet because that portion was filled early by polymer melt, which cannot be physically displaced by the incoming gas (Figs. 2a and 4a). However, as the polymer melt begins to shrink during post-filling, the gas begins to penetrate into the solid portions at both ends of the part, as shown in Fig. 6.

General Design Guidelines

These design guidelines are aimed at providing basic know-how in design and manufacturing of gas-assisted injection molded parts.

Guideline 1. Process feasibility for the application should be considered prior to adopting gas-assisted injection molding.

This process is different from either extrusion or injection blow molding, in which a much larger volume of air (than polymer) is used to inflate and expand the plastic parison to a finished shape inside the mold. In gas-assisted injection molding, gas is primarily used only to hollow out thick-sectioned gas channels and/or to compensate for the volumetric shrinkage of the polymer melt within the part. In practice, it is a challenge to hollow out more than 25% of the cavity volume without gas blow-through at the polymer melt front before the part is completely filled. Typically, the volume of the gas-channel network is less than 10% of the total part volume.

With the current technology, the process can be applied to virtually all thermoplastic materials (with or without fillers), and initial applications to thermosetting bulk-molding compounds also show encouraging results. Because product and tool designs are different from conventional injection molding, an initial learning period is required in the product development cycle. Finally, a licensing fee is generally required for using the process, which should be taken into account in the cost evaluation.

Guideline 2. For part design, first determine the thick sections to be cored out by the gas within the part, and then connect those gas passages to lay out a clearly defined gas-channel network.

In general, the gas-channel network should guide the gas penetration to the extremities of the cavity without introducing air traps and gas permeation. For example, a diagonal gas-channel-network configuration, which leads the gas to the four corners of a rectangular part, is generally preferred over a cross gas-channel network.

The dimension of the gas-channel network should be distinctly larger than that of the adjacent areas to confine the gas penetration. However, care should be taken to avoid air traps and gas permeation resulting from improper layout and sizing of the gas-channel network. More specifically, during the resin-injection stage, polymer melt will flow preferentially along the gas channels that serve as flow leaders, leading to a "racetrack" effect. The significance of the racetrack effect is that it could lead to entrapment of air, as shown by the melt-front advancements in Fig. 7 (left) for a quarter-scale automotive hood panel. In this case, ejector pins have to be used to vent the trapped air. The relatively balanced gas penetration for this part is shown on the right.

Guideline 3. The tool and part designs should deliver a balanced filling pattern to promote even gas penetration.

One of the common problems associated with gas-assisted injection molding is uneven gas penetration resulting from an unbalanced filling pattern[1]. Because gas cannot penetrate into regions that are filled by polymer melt, the product and tool design features, such as the gate location(s) and gas entrance(s), should offer a well-balanced filling pattern so that the subsequent gas penetration can be evenly distributed.

That the gas will generally take the least resistant path to catch up to the polymer melt front, where the pressure is lowest if proper venting is provided, must always be kept in mind. Conditions that provide a path of less resistance for the gas include: a short flow length from the gas tip to the polymer melt front; thick gas channels in which the polymer melt is still hot and less viscous; and a place to which the polymer displaced by the gas may flow. A balanced filling pattern should provide approximately equal resistance among all the gas channels.

Guideline 4. Mold-wall temperature, shot-size control, and part dimensions are more critical in gas-assisted than in conventional injection molding.

Gas penetration can change dramatically with a small variation in any of these parameters. Figure 8a shows uneven gas penetration in a window guidance channel during filling[5,6]. This part has two parallel, closely spaced gas channels with gas injected at the one end of the part. Any small variation in melt (or gas) advancement arising from variation in mold-wall temperature, shot volume, or part dimension will increase significantly because of the racetrack effect.

Recall that the pressure drop from the gas injection point to the gas tip is small because of the inviscid nature of gas. Given the same pressure drop from the gas tips to the melt fronts along the two gas channels, the polymer melt in the channel that has the shorter flow length (due to longer gas penetration) will move faster because of the higher pressure gradient, yielding its space to the incoming gas. Accordingly, the gas will penetrate more in that particular channel, which, in turn, produces an even higher pressure gradient in the melt domain. With CAE analysis, the racetrack effect can be clearly seen. Figure 8b shows the improved gas-penetration pattern after slight modification of the gas-channel dimension.

In addition to air traps, gas permeation into thin sections, and uneven gas penetration, there are several other important flow-related problems. These include gas blow-through because of insufficient polymer melt or poor tool design; "switch-over" or "hesitation" marks occurring along the suddenly decelerated melt front as a result of the delay time between gas and resin injection; material degradation arising from the gas-driven acceleration of the polymer melt; and short shots resulting from insufficient gas pressure or poor tool design. CAE analysis can take into account all these factors contributing to the success or failure of the application at an early stage of the design.


The current CAE analysis generates intermediate results for both the filling and post-filling stages, enabling the filling behaviors of polymer and gas to be "visualized" throughout the entire process. Based on these progressional results, molding problems associated with the process and their causes can be detected and resolved before the tool is cut. These comprehensive predictions also provide valuable insight from which crucial engineering know-how and design guidelines can be quickly acquired. All the results reported in this article were obtained from C-GAS FLOW, a three-dimensional CAE analysis program for gas-assisted injection molding.


[1.] L.S. Turng, SPE ANTEC Tech. Papers, 38, 452 (1992).

[2.] L.S. Turng and V.W. Wang, SPE ANTEC Tech. Papers, 37, 297 (1991).

[3.] L.S. Turng, in Advances in Polymer Technology, to be published.

[4.] H. Potente and M. Hansen, 8th Annual Meeting, Polymer Processing Soc. (March 1992).

[5.] S. Shah and D.Hlavaty, SPE ANTEC Tech. Papers, 37, 1479 (1991); PLASTICS ENGINEERING, October 1991, p. 21.

[6.] P. Medina, L.S. Turng, and V.W. Wang, 19th Annual Conf. Structural Plastics, SPI (April 1991).
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Title Annotation:Injection Molding
Author:Turng, Lih-Sheng
Publication:Plastics Engineering
Date:Sep 1, 1993
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