Gas Assist '98: A Process at the Crossroads

Digital Process Imaging

Gas Channel Design

The Blowback Technique

Cleveland, OH was the site of Molding '98 (March 23-24), the eighth such conference and exhibition on emerging technologies and business trends in injection molding. As at the previous seven, the topic of gas-assist injection molding held center stage. There were no less than seven presentations directly dealing with gas assist, and a number of others throughout the conference indirectly referencing the topic. It's the best of times for gas assist. But it's also the worst of times.

Legal issues continue to hamstring the further growth of gas assist. Case histories of successful applications of gas assist's productivity-enhancing benefits mount in number. Technology improvements are being developed by suppliers and molders alike. Designers and prototype houses experienced in speeding new products to market are finding that their customers' projects can be accelerated even faster by stepping on the gas. Nevertheless, industry worldwide still is hobbled by claims of patent rights and cases of alleged patent infringements. Most of the speakers, exhibitors, and attendees that IMM spoke to at Molding '98 could only shake their heads in utter frustration after explaining how even the "possibility of costly legal battles scares many molders away from investing capital into a process technology that could be putting money into their pockets." The gas-assist story could be facing a sad ending before it's really even begun.

Here's an example: "The legal front is relatively quiet. The House of Lords has decided not to hear the final appeal on the use of overflow wells in gas-assist injection molded parts. Since the use of overflow wells had been established prior to the litigation, the input from Cinpres Ltd. is that the pre-established right to use them exists. Thus, overflow wells can be used without being in jeopardy of litigation."

This paragraph opened a segment entitled "Legal Issues" in the first presentation at Molding '98. In what he's called his "annual rite of spring," GE Plastics' Jack Avery was updating all on the state of the gas-assist union. Though the preceding paragraph appeared as is in the book of Molding '98 presentations, Avery had to modify this part of his report when speaking from the podium, explaining how Gain Technologies' Mike Ladney is "clarifying" this particular legal situation, and that the use of overflow wells still requires licensing. There was little anyone could do from that point on to inspire any sort of up-beat mood in Cleveland. It's gas-assist business as usual, a business that's either about to boom, or about to go to the dogs.

Will There Be a Second Wave?

The specter of litigation is the major reason why growth in gas assist during the past year has primarily been through multiple installations in existing licensees, rather than through expansions via new licensees. Automotive OEMs have driven what Avery calls the "first wave" of gas-assist molding. GM, Ford, Chrysler, Mercedes Benz, Toyota, and Mazda all use the technology. But the OEMs, the Tier One suppliers, and the other molders who have demonstrated capabilities of supporting gas-assist programs are returning to the same gas-assist vendors. "They do not want to risk delays and/or cost penalty by going to an unproven supplier," Avery explains.

The global economy also has kept any headlines about a rapid global proliferation of gas assist out of the news. "Areas that would use this technology for new applications are Asia and Europe. The economies of Japan, Korea, and Indonesia are precarious. Also, Germany and France have been in the doldrums. As a result, new programs are not being developed at the rate they would be expected." Avery estimates that there are about 1000 gas-assist licenses at present around the world. Including consultants and technology and equipment suppliers, he says there are more than 45 gas-assist suppliers. So, gas assist has become a mainstream process and the first wave of technological application is in place, according to Avery. Whether business trends will allow a second wave remains to be seen. Nevertheless, gas-assist technology continues to evolve.

External Gas Molding (EGM) is one of the newer technological innovations Avery noted in his report. EGM, developed by cross-licensees Gas Injection Ltd. and ICP Systems, is based on the application of gas pressure on one surface of the plastic in the mold in a localized, sealed area. This eliminates sink marks on the opposite part surface adjacent to ribs and bosses. Battenfeld offers a somewhat similar approach in its Airmould Contour process (see photo).

Both allow molds designed for conventional molding to benefit from gas-assist technology. DJ Inc.'s use of two cavity-pressure signals per cavity, rather than a timing signal to control gas injection, was another development Avery covered. He says this "flow-front control" approach has reduced standard pressure deviation for PP appliance handles from 100 psi to 39 psi. Epcon Gas Systems supplied the control unit to DJ; RJG Technologies supplied the cavity-pressure instrumentation.

Meanwhile, Germany's Institute for Plastics Processing has rolled out "gas-assisted fiber-braid injection molding" (Gafim) to produce continuous reinforced flexible hollow parts with high torsion and burst strength. Here's how it works: woven braided fibers (in glass, carbon fiber, polymer, or metal) are fed through the mold cavity and fixed at one or both ends. Melt expands, permeating the woven tube and holding it against the cavity wall. Then, a second component is injected, a barrier layer perhaps, followed by nitrogen gas to form the hollow core, producing a constant diameter part with a smooth surface.

Cooling is improved, and warpage is reduced. Production costs for Gafim are 30 percent lower than conventional extruded barrier tubing, though strength and flexibility are higher. High-pressure fuel and brake-fluid tubing in automobiles is a possible application that Avery says is currently being investigated by German processors.

Hardware, Software, Applications

Taking a hard look at the new gas-assist equipment debuting at NPE '97, Avery concludes developments were more evolutionary than revolutionary. For example, Battenfeld's latest Airmould system package is more compact, making it more economical and easier to move around the plant. Gain Technologies' Satellite 1 is a compact, portable, single-valve unit with a built-in nitrogen reservoir designed for a remote location from the main station. The LGX-1 control module from Alliance Gas Systems features a nitrogen pressure intensifier and twin in-mold gas injectors for power and control versatility. And Cinpres introduced its Full Shot conversion unit, engineered for molding thin-wall parts like TV housings. Refinements, rather than breakthroughs. On the softer side of things, newer gas-assist molding simulation software from C-Mold gives users full control over gas injection into the part at any time in the cycle.

Applications - Avery says that's where the action is in gas assist these days. Among the examples he offered was Chrysler's molded-body CCV, a sub-subcompact concept car designed for sale in the Third World for $6000. Gas assist is used to reduce cycle times and increase rigidity. C-Mold gas-assist simulation software was used in this project. Avery says Chrysler has two more molded-bodied "green" concept cars in the works. One, the Dodge Intrepid ESX2, which could represent about 150,000 units/year should it go into production, features a part consolidation of from 80 parts down to six panels, and a 50 percent weight reduction. Gas assist and sequential ejection may be used to speed production. Hettinga, Battenfeld, and Cinpres were among others discussing similarly interesting applications of their gas-assist systems.

Will gas-assist injection molding ever enjoy a second wave of application development among newcomers? "When will we see the second wave?" Avery asks. "I believe it has already begun. It may take longer to develop and will not be as dramatic, but will have just as significant an impact on the plastics industry." Education is what Avery says is needed to grow the process. Product designers, materials suppliers, consultants, and prototypers familiar with the process must be brought in early on to optimize gas-assist molded products from the beginning, rather than being brought in later to be "Mr. Fix-its." Customers need to be educated as to when it is appropriate to use gas assist. Still, from the looks of things today, a legal education in patent rights and royalty claims also wouldn't be a bad idea for those promoting the process. For copies of all of the Molding '98 presentations from Executive Conference Management, call (734) 420-0507. - Carl Kirkland

Digital Process Imaging

Epcon Gas Systems has uncovered a new use for thermal imaging technology in nondestructive quality testing of gas-assist molded parts. Originally developed in top-secret military research, the technology combines a radiometer and a thermal imaging system with software. The technology makes it possible to view real-time digital video or still imaging of gas bubble penetration into a part - you can look right through the mold. The factory-hardened infrared camera and the software also determine the temperature changes that may cause dimensional instability at more than 12,000 temperature data points in each image by measuring infrared radiation.

Bubble penetration can be more accurately verified. And the system provides an early alarm to the changing temperatures of the mold and the material if preset limits are exceeded. That means less scrap due to sinks, gas penetration into the wall, or warpage due to differential cooling. Actual results can be compared with computerized process simulations. Greg Crawford, Epcon's customer development manager, says his company continues to evaluate this infrared camera technology in hopes of providing better tools to help molders make better informed decisions.

Gas Channel Design

Terry C. Pearson, director of Gas Injection Ltd. of the U.K. (now represented Stateside by Advanced Injection Solutions, Clinton Township, MI), presented a number of intriguingly different approaches to gas channel design and to methods of applying gas pressure within mold cavities. Some involved patent-pending solutions to preventing sink marks on surfaces opposite screw bosses and pillars. In parts like TV cabinets, it's a common practice to direct gas channels to the base of screw pillars when injecting gas at positions remote from the pillars. Pearson says that it is not always convenient or practical to include gas channels in remote positions. Gas

Injection has traditionally offered its External Gas Molding (EGM) technique. EGM applies gas pressure through a hollow core pin to the internal base of the pillar. However, Pearson admits that with this approach, it's often difficult to prevent gas "fingering," rather, penetration of the wall section by gas.

Another solution Pearson proposes lies in channel designs that offer no resistance to gassed-up resin dynamics. Take a design that goes ahead and lets the gas penetrate into thicker web sections supporting the pillar, for instance. Strength is pumped into the area of the joint between the pillar and the general wall section. And the design can be modified to apply pressure to other pillars or webs, as shown in the accompanying graphics. Surface sinks can be eliminated, so paint-free TV cabinets can be produced.

Gas Injection's gas channel development efforts have yielded some startling results, not the least of which are extremely flat thin-wall parts, even in PP, thanks to channeling that achieves consistent uniform wall thickness by inducing lateral shrinkage. Pearson recommends that designers look beyond many of the traditional ways of channeling gas into parts. Rather, wherever possible, he says you should locate gas channels at a change in contour, at an inside corner, or at the end of wall sections to avoid witness marks. .

The Blowback Technique

Shot size repeatability is a major concern in a short shot process like gas assist. One solution is the use of spillover. A pin is pulled after the cavity is completely filled, so gas vacates the material in the channel into a spillover area. But spillover has spilled over into court battles over patents. Epcon Gas Systems has another idea (U.S. Patent # 5,204,050). It calls it the Blowback Technique. Blowback involves the manipulation of two independently controlled gas pins, one at each end of the channel. First, you shoot the resin short, then gas is injected into the channel, vacating resin into the last-to-fill area, just as in conventional gas assist. But, just as in conventional gas assist, there's still that small bit of material left at the end of the channel that can cause sinks as the part shrinks. What Epcon proposes is the use of a second pin.

As pressure is relieved in the first pin, pressure is ramped up from the second pin at the other end of the channel. That small bit of material at the end of the channel now moves back toward the first pin. Material is forced against the wall of the channel until the secondary bubble created breaks through into the primary channel. At this point a holding pressure is applied to both pins through the cooling phase that takes up volume shrinkage until the gas is vented back through both pins prior to mold opening.