Fight sinks with 'external' gas assist.

The ability of gas-assist injection molding to ward off sinks over bosses and ribs is well known. But what can you do when the part design precludes putting a gas channel underneath the rib or boss? Two new ways to address this problem turned up at the recent Molding '98 Conference in Cleveland. The meeting was sponsored by Executive Conference Management, Inc., Plymouth, Mich., and co-sponsored by PLASTICS TECHNOLOGY.

Rather than allowing the gas to enter the melt and hollow out channels within the part, both these gas-assist methods apply the gas to localized areas of the part surface. Here's a look at how these two "external" gas-assist methods work.

No sink plus faster cycles

The new Airmould Contour system from Battenfeld of America, West Warwick, R.I., applies gas pressure to a circumscribed external region of the part during the holding and cooling phases after the mold has filled completely with plastic [ILLUSTRATION FOR FIGURE 1 OMITTED]. Using standard Airmould gas-injection and control devices, the Contour system introduces the gas between the mold surface and the part, where it can compensate for the volumetric shrinkage that takes place as the part cools. Thomas Betts, Battenfeld product manager for advanced processes, explains, "The gas pressure is applied over a predetermined surface area and acts as a pressure cushion during the cooling of the plastic." The tendency to shrink on the visible side opposite the ribs is compensated for by the gas pressure pushing the plastic against the mold surface from the rib side of the part, Betts says. Just before the mold opens, the gas is vented out of the tool.

Betts also cites some secondary advantages from external gas pressure: Cooling times drop because of the improved contact between the melt and the mold surface. And the external gas pressure can enable reductions in holding pressure and holding time in parts with thick cross sections, which in turn can reduce overall cycle time, machine tonnage requirements, and part warpage.

Some of these benefits were revealed in an early Contour application, a ribbed cosmetics cover with a high-gloss exterior surface. According to Betts, the cover required a 90-sec cycle time and a clamp force of 30 tons per cavity when molded conventionally. The same part molded with Contour needed only 40 sec and a clamp force of 15 tons/cavity. Betts adds that only the gas-molded part came out sink-free.

External vs. internal gas

Gas Injection Ltd. of the U.K. recently established a U.S. presence to market its gas-assist technology here - through Advanced Injection Solutions, Clinton Township, Mich.. One of these is "External Gas Molding." As Terry Pearson, chairman of Gas Injection Ltd., explains, this process applies gas pressure through a hollow core pin directly to the internal base of a rib or boss [ILLUSTRATION FOR FIGURE 2 OMITTED]. Here, too, the pressure in this localized area works to eliminate sink on the opposing part surface.

Pearson notes that this external gas molding method can be susceptible to "fingering" or gas penetration into the wall section. For these cases, he recommends a design alternative: Enlarge the joint between the rib and wall section. This joint area can be hollowed out locally with gas using a similar type of core pin. As with the external method, these hollowed out rib or boss bases still exert pressure on the opposing part surface, according to Pearson [ILLUSTRATION FOR FIGURE 3 OMITTED].

Pearson points out that if the gas pressure stays on through the mold-opening stage, it can assist in part ejection and may even replace a conventional sleeve ejector.