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.
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.
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.
High thermal diffusivity
Low thermal diffusivity
Higher gas pressure
Lower gas pressure
Higher melt temperature
Lower melt temperature
Longer delay time
Shorter delay time
Longer gas injection time
Shorter gas injection time
Higher polymer pre-fill
Lower polymer pre-fill
* Trend depends on other parameters