Utilization of steam for keeping a mold at elevated temperatures during in-jection is gathering momentum in Japan. The technique reportedly can enhance a number of part attributes including surface finish. Competing technologies from auxiliaries manufacturer Sysko Corp., Soka, Japan, and processor Ono Sangyo Co., Tokyo, allow molders to maintain the surface temperature of injection molds above the glass-transition temperature (Tg) of resin during the injection phase before switching over to water for the cooling phase.
“The process is essentially a fast annealing process,” says Les Goff, application technology manager at GE Plastics, Pittsfield, ma. Parts can have one or more enhancements: higher-gloss finish, elimination of weld lines, increased knit-line strength, improved transfer of embossed patterns, elimination of residual stress, improved part tolerance, or minimal roughing of the surface in highly-filled parts. GE Plastics worked with both Japanese firms to commercialize the technology in Japan. It is now working with five Japanese processors, including Ono Sangyo, and says another 20 have expressed interest in the process. Four use equipment from Sysko.
While the concept of keeping the mold hot during injection is not new, its application has been rare. The first commercial parts appeared five years ago, but growing efforts by Japan’s electronics, business machine, and automotive industries to introduce paintless parts have accelerated its deployment.
Ono Sangyo, for example, employed its Rapid Heat Cycle Molding (rhcm) process for a glossy, black polycarbonate/abs housing of a Sony laptop computer. A boiler is used to generate pressurized steam of up to 150°C to heat the mold. A valve switches over to cooling water after injection.
rhcm was also employed, together with gas-assist molding, for the enclosure of a 50-in Hitachi plasma display that uses a pc/abs alloy incorporating a high loading of talc to reduce shrinkage and eliminate warping. The display is only 4 in thick. Kiyoshi Ikeda, manager of process licensing at Ono Sangyo, says, “Despite the high talc filling, rhcm realizes a resin-rich surface finish that does not require coating.”
Computer maker Fujitsu plans to adopt rhcm for a laptop due for release in 2004. Its housing will be molded from biodegradable polylactic acid-based resin. Keeping the mold temperature above the resin’s Tg will allow crystal growth to proceed further to realize a harder, scratch- and chemical-resistant surface, in addition to high gloss. Ikeda says up to five laptop makers may adopt rhcm for future products.
Ono Sangyo charges licensees – it has several in Japan – a one-time fee of ¥10 million ($84,000), in addition to equipment costs, such as for the steam generator. This includes guidance for the first mold design and tryout. Ikeda says licensing costs will be slightly higher offshore.
Sysko, meanwhile, sells a steam boiler, controller, and manifold package for a single injection press for ¥7.5 million ($63,000). Provided sufficient steam-generating capacity is available, a single boiler can be used for multiple injection machines. Maximum mold temperature achieved is 170°C.
Sysko president Toshio Shimoda says, “Steam is a more efficient medium for quick heating of the mold, although air must be used to purge the steam from the channels before cooling water can be introduced.” This increases the time of the molding cycle. Commercial examples that are using Sysko’s process include rear spoilers, bumper skirts, and lenses.
GE Plastics’ Goff says that cycle time could be lengthened significantly depending on the application, due to the extra time needed to cool the mold down from elevated temperatures, so a systems-cost approach is required. In certain uses, such as large parts with thick sections, the cycle time could double, so processors need to calculate whether cost benefits and productivity gains offset this.
Although both the Ono Sangyo and Sysko technologies are theoretically retrofittable on standard injection machines and tooling, with the heating and cooling media sharing the same channels, separate cooling channels can accelerate cooling for parts with thick sections.