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Appendix B

Gas-Assisted Injection Molding

General Design Guidelines for Gas-Assisted Injection Molding


Due to the unique characteristics of gas-assisted injection molding processes, conventional design concepts are no longer sufficient to take full advantage of the benefits inherent in these processes. The design guidelines presented here are intended to communicate basic know-how in design and manufacturing of parts using gas-assisted injection molding.

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

Gas-assisted injection molding is different from blow molding (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 used primarily to hollow out only thick-sectioned gas channels and to compensate for the volumetric shrinkage of the polymer melt within the part.

With the current technology, gas-assisted molding can be applied to virtually all thermoplastic materials (with or without fillers), although difficulty has been reported with some high-shrinkage and temperature-sensitive materials. The initial application for thermosetting bulk molding compounds has also begun.

Most conventional injection molding machines can be adapted for gas-assisted injection molding. However, product and tool designs for gas-assisted injection molding are different than for conventional injection molding and might require an initial learning period in the product development cycle. Finally, a licensing fee is generally required for using any of the commercially available gas-assisted injection molding processes, 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, before connecting those gas passages to lay out the gas-channel network.

The areas to be cored out by gas would depend on the product application. For example, for tube- or rod-like parts, it is typically desirable to let gas penetrate the entire part length, which is well defined, for saving material and reducing cooling time.

For large, sheet-like, structural parts, gas channels can be built-in by incorporating ribs with enlarged bases or a thick passage for enhancing the polymer flow and increasing the part rigidity.

Complex parts consisting of both thin and thick sections should have gas channels designed to pass through (or pass by) the heavy sections to eliminate sink marks and reduce cycle time. Gas channels can also be employed at transition points of different wall thicknesses to avoid potential high stresses and warpage.

In general, gas channels should be continuous, but a closed-loop design should be avoided if complete coring is required, since a solid slug can occur when two gas fronts merge. Gas channels with sharp bends could introduce structural weakness due to the tendency of gas to travel close to the inner radius or even to jump across in thin sections.

Guideline 3: Gas channel dimensions should be defined clearly to enclose the gas penetration with a limit to minimize the racetrack effect.

The dimensions of the gas-channel network should be distinctly larger than those of the adjacent areas to give the gas a well-defined path in which to travel. These dimensions should not be too large, either. Large gas channels could cause undesirable racetrack behavior of polymer and gas. As a rule of thumb, the ratio of channel dimension to nominal wall thickness for structural parts is 2 to 3. This range normally generates a well-shaped, hollow cross-section. Irregular hollow sections with sharp angles could cause stress concentrations when the part is subjected to service loading.

Use extreme care to avoid air traps and gas permeation caused by improper layout and sizing of the gas-channel network. During the resin injection stage, polymer melt will flow preferentially along the gas channels, which serve as flow leaders, resulting in a racetrack effect. The significance of the racetrack effect depends on the material properties, processing conditions and cavity geometry. For example, if air traps result, you might need to incorporate ejector pins to vent the trapped air.

When designing gas channels, the recommended approach is to start with relatively small gas-channel dimensions to minimize the racetrack effect.

Guideline 4: Extend gas channels as close to the last-filled areas as possible to avoid gas permeation into those areas at the end of cavity filling.

The gas-channel network should guide the gas penetration to the extremities of the cavity and should be extended as close to the last-filled areas as possible to avoid gas permeation into thin sections.

The gas will take the path along which the polymer melt has least resistance and largest pressure gradient. Since the pressure drop from the gas tip to the (presumably vented) melt front is approximately a constant (pgas-patm), the flow path that has the least flow length (between the gas tip and melt front) will result in a high pressure gradient. Such a high pressure gradient will drive the polymer to flow toward the empty areas, leaving its place to be filled by the incoming gas. This why the gas sometimes permeates into thin sections.

Gas channels should also be oriented somewhat in the direction of melt flow. As a result, many gas-channel networks are either symmetrical, or they are unidirectional from a single gas injection point.

Guideline 5: Overflow wells can be incorporated at the ends of the gas channels to promote a better gas penetration pattern.

For some gas-assisted molding applications, an overflow well can be incorporated at the end of a gas channel to promote a desirable gas-penetration pattern. The passage to the overflow well is generally closed during the resin injection stage, while the resin is filling the rest of the cavity (refer to Figure B-1, top). At the end of resin injection, there is an optional delay time, which allows the polymer over the thin sections to solidify.

Immediately before gas injection is triggered, the passage to the overflow well is opened, creating an additional volume to accommodate the resin that is displaced by the incoming gas (refer to Figure B-1, bottom). After the part is ejected from the mold, the overflow can be trimmed off if it is undesirable.

The advantages of using overflow wells are to reduce the complexity in the filling dynamics of resin and gas, and at the same time, to offer an additional control to guide the gas penetration. The inherent disadvantages are the secondary trimming operation that might be required, and the additional material used to fill the overflow well.

Figure B-1. (top) Part with built-in overflow. (bottom) Gas displaces the resin in the gas channel, forcing it to flow into the overflow.

Guideline 6: The design of the tool and selection of polymer and gas entrances should deliver a balanced filling pattern to promote even gas penetration.

Flow balancing is one of the key factors in designing the part and tool and in selecting the polymer and gas entrances. One of the common problems associated with gas-assisted molding is uneven gas penetration due to an unbalanced filling pattern.

In the filling stage, primary gas penetration cannot occur in regions filled by polymer melt that cannot be displaced to other areas. Hence, the design of the part and tool, such as the gate location(s) and gas entrance(s), should offer a well-balanced filling pattern so that the subsequent gas penetration will be evenly distributed.

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. The conditions that provide less resistance for the gas include:

A balanced filling pattern should provide approximately equal resistance among all the gas channels to be hollowed out by the gas.

Guideline 7: The volume of unfilled areas prior to gas injection should not exceed 50 percent of the total volume of the gas channels.

Gas should be confined within the gas channels without gas blow-through or gas permeation into thin sections. Accordingly, polymer displaced by gas from the hot core of gas channels should be sufficient to fill the empty regions and pack out the entire mold. The optimal polymer volume to be injected into the cavity can be obtained by subtracting the volume that can be cored out by the primary gas penetration from the total cavity volume.

For example, to hollow out a simple circular tube with an average polymer skin thickness of half the radius, the part has to be pre-filled at least 75 percent by volume with polymer. In other words, 25 percent of the part volume will be cored out by gas to form a hollowed, circular tube with thickness equal to half the part radius. In this case, given the projected skin-thickness average and the part geometry, 25 percent of the part volume is the maximum amount of material that can be saved.

Guideline 8: Mold-wall temperature and shot-size control, as well as part dimensions, are more critical in gas-assisted injection molding than in conventional injection molding.

To ensure product repeatability, shot-size control within 0.5 percent is desirable. With only a small variation in mold-wall temperature, shot volume, or part dimension, gas penetration can change dramatically.

Suppose there is a small variation in melt (or gas) advancement due to variation in mold-wall temperature, shot volume, or part dimension. That difference will increase significantly due to the racetrack effect. More specifically, given the same pressure drop from the gas tips to the melt fronts along two gas channels, the polymer melt along the channel that has the shorter flow length (due to longer gas penetration) will move faster because of the higher pressure gradient, giving 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. However, by slightly modifying the dimension of the gas channel, the gas penetration pattern can be improved. This racetrack effect is the reason why control of a multi-cavity system is more difficult with gas-assisted injection molding: because gas races among different cavities.

Guideline 9: The effect of material properties and process variables on the gas penetration should be taken into account in determining the processing window.

Some material properties and process variables have profound effects on the molding outcomes (see Table B-1). Remember that the primary gas penetration is determined by the polymer volume fraction and is strongly coupled with flow dynamics, whereas the secondary gas penetration depends on amount of polymer shrinkage, occurs only along the thick sections, and extends in all directions.

Further, higher gas pressure and shorter delay time generally result in shorter primary gas penetration length with thinner polymer skin, and vice versa. On the other hand, volumetric fill time, which reflects the melt velocity, decreases with increasing gas pressure, higher melt temperature, and lower melt viscosity. The secondary gas penetration is more significant with semi-crystalline polymers and longer, solid gas-channel length ahead of the gas tip.

Finally, gas should be injected at the proper gas pressure to avoid excessive deceleration or acceleration of polymer melt, which can cause short shots, hesitation marks, material degradation, or discoloration.

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Table B-1. Qualitative effect of relevant material properties and process variables on molding outcomes
   
Extent of gas
penetration 
Polymer skin
thickness 
Volumetric
fill time 
Material
properties 
High thermal diffusivity  Low thermal diffusivity 
longer 
shorter 
thicker 
thinner 
longer 
shorter 
  High viscosity  Low viscosity 
longer 
shorter 
thicker 
thinner 
longer 
shorter 
Process variables  Higher gas pressure  Lower gas pressure 
shorter 
longer 
thinner 
thicker 
shorter 
longer 
  Higher melt temperature  Lower melt temperature 
shorter 
longer 
  Longer delay time  Shorter delay time 
longer 
shorter 
thicker 
thinner 
longer 
shorter 
  Longer gas injection time  Shorter gas injection time 
longer 
shorter 
** 
** 
** 
** 
  Higher polymer pre-fill  Lower polymer pre-fill 
shorter 
longer 
** 
** 
** 
** 
* Trend depends on other parameters

** Data not available or not applicable



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Copyright © 1997 Advanced CAE Technology, Inc. All rights reserved.