Expanding design horizons with gas-assist molding

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External-GasAssist-Mack-DiagramGas-assist molding, the process of using nitrogen gas pressure to fully form a part, increases design and manufacturing options for injection molded components.

Scott Rishell, Mack Molding

Traditional plastic processing brings design and manufacturing benefits through repeatable, high-volume and high-quality part production coupled with cost reductions, but its versatility can be expanded in certain scenarios by the introduction of gas-assist molding. There are two methods – external and internal – with both allowing for improved flatness and the packing out of atypically-thick geometry.  The external process can also reduce press tonnage, leading to cost savings, while internal gas-assist can form a lighter part that may benefit the end application.

Examining external gas-assist

In external gas-assist, a micro-thin layer of nitrogen gas is introduced during the packing phase on the non-aesthetic side of the part, after the part is filled with plastic but not yet fully packed out. This process packs the part evenly, replacing the more traditional holding phase. The gas is held at a high pressure, forcing the uncured resin onto the opposite side of the tool. The aesthetic side ends up with a clean, uniform appearance while the gassed side will have a wavy or “sinky” appearance.

External gas allows designers to modify traditional molded-part design parameters, such as rib-to-wall ratio. Generally held below 50% to 70% to avoid creating sink marks on the aesthetic side, external gas designers can increase this ratio up to and even above 100% depending on the part and resin requirements.

This process also allows for thicker ribs, offering a key part advantage by reducing differential shrink – a primary culprit in causing part distortion – between nominal wall and the ribs. In components with critical flatness specifications this can be a game-changer.

In typical molding the required pressure to fully pack a part out can be extremely high, due to pressure loss from gate to end of fill. Because external gas molding replaces some press work with evenly distributed nitrogen gas pressure, less clamp force is required – molding pressures of 1-2 tons/in2 can be achieved leading to lower part costs. Some considerations, however, include the fact that sealing gas from the external environment requires more intricate tool and part design leading to increased tool cost and the gassed side of the tool needs to be unseen in the final application if aesthetics are a factor.

Internal-GasAssist-Mack-DiagramInvestigating internal gas-assist 

Internal gas-assist, on the other hand, is used specifically to mold very thick sections in parts. In general the process is similar to the external method, except the gas is introduced inside the part geometry – creating a hollow channel through the thick region of the part and forcing the resin against the external walls to create a packed-out appearance in the thick section.

When the resin is evacuated from the center of the thick section, it leaves a cavity that can be 20% to 30% of the cross section. This can result in significant part-weight savings that could be critical to product performance. Additionally, getting this resin out of the part means the cooling time can be reduced in comparison to molding the thick geometry as a solid. Press cycle times are typically dependent on the thickest part’s geometry.

Internal gas-assist allows extremely thick sections to be molded with high aesthetics; however, as with external gas-assist, the more intricate tool design and increased tool cost must be considered when determining the best course of action for the application.

Scott Rishell is the technical lead on several programs at Mack Molding (Arlington, Vt.), developing part designs for production processes with a focus on diagnosing and implementing solutions for design for manufacturability (DFM) issues, as well as reduce manufacturing costs through part simplification and process improvements.

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