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Large, thin, plate‐shaped parts usually are strengthened with structural ribs. Ribs also serve as gas channels with gas‐assisted technology. The layout and geometry of these gas‐channel ribs are critical to the gas‐assisted injection molding (GAIM) process. In this study, the effects of rib geometry, including aspect ratio and fillet geometry, on the GAIM process are investigated. Experimental results indicate that increasing the rib width widens the allowable operation range and thus improves the moldability. Adding fillets to the rib corner significantly enhances the moldability. Adding fillets also reduces the loss of rigidity due to void formation in the rib. A curved fillet improves moldability and rigidity more than one that is straight.

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During gas-assisted filling, gas penetrations in symmetrical rib channels are found never symmetrical. The development of this unsymmetry is observed and its cause is analyzed. The system of gas penetrations in melt in symmetrical rib channels is found unstable. Symmetry is upset upon perturbation. Symmetrical gas-channel ribs in gas-assisted injection molded parts should be avoided.