Gassing Up

Would you like to add a third or more to effective press tonnage without changing machine internals? Most injection molders would, and an increasing number are adding the unique technology which makes this possible: Gas assist. Gas assist injection molding (GA) has matured into a stable, off-the-shelf technology which promises and delivers larger, lighter, more complex parts from less press tonnage than any conventional injection

molding process.

Gas assist works by injecting nitrogen gas at very high pressures into the melt stream, forcing resin against the mold walls and forming a hollow cavity in the part core. This assist creates the effective tonnage gain, while incidentally reducing sink marks. Full-shot and short-shot processes can be used, and gas can be controlled by pressure or volume. Currently, seven percent of U.S. molders report gas assist capability, a number surprisingly low for a successful technology with a 15 year commercial history. The primary reason for the market’s early reluctance to embrace GA had nothing to do with the cost-effectiveness or market potential of the process, but instead was based on a series of patent disputes which added enough uncertainty to deter potential licensees of GA technology.

This situation has now changed. Steve Johnson, vice-president of NitroJection Corporation, Chagrin Falls, Ohio explains the shift: “If you follow the patents, there are no more patent disputes in this technology than there are in resins. Instead of a manufacturer of gas assist threatening a user, you now see most of this is manufacturer versus manufacturer. That’s a profound difference.” (140, in Canada, Plastics Machinery Inc. Newmarket, Ont.)

While processors investigating the various GA technologies will discover that licensing fees may or may not be rolled into the price of the hardware, the licenses do buy the right to use the technology. The end of the “litigation chill” has brought rapid expansion in the installed user base for GA molding. The largest market growth segment is OEM molders that started with gas two or three years ago. This reflects increasing confidence with OEM engineers to design for the gas assist process combined with reliable moldmaking capabilities. Simulation software is also available. The newest release of Advanced CAE Technology’s C-Mold (141) displays gas penetration patterns and allows gas-no-gas comparisons of parts. The system can also account for the relationship between the gas bubble and fibre orientation in filled resins.

Manufacturers such as Battenfeld and Engel sell turnkey gas assist press systems, and retrofit equipment suppliers are growing in number. In England, Gas Injection Ltd. (142) has launched a range of presses with integral GA in sizes ranging from 20 to 600 tons. The firm claims significant operating cost reductions over ancillary installations. Suppliers are also broadening the range of services available to GA users. Bauer Compressors, Inc., Norfolk, VA (143), for example, has formed the Bauer Plastics Technology Group, which offers part and mold filling analysis, design assistance and training seminars. Will the gas assist process continue its steepening growth curve? Nitrojection’s Steve Johnson thinks so: ” I believe that in the next five years, five to 10 percent of the injection molding presses installed will have gas as an option. I don’t see another technology which can replace it”.

With one notable exception, high-volume GA processes operate with lowest cost where the nitrogen supply is generated on-site. Gas generators may use a membrane diffusion process to produce high-purity nitrogen which must then be boosted to operating pressure. In a single press installation, this feature may be incorporated into the gas controller.

How pure is pure? The answer to that question depends on the reactivity of the resin and on melt temperature. Three percent residual oxygen can produce significant oxidation and gas pin residue in many applications. An alternative technology which offers very high nitrogen purity is pressure swing adsorption (PSA). PSA uses alternating columns containing a carbon molecular sieve which effectively traps oxygen and other impurities.

An example of a generator design using PSA technology is domnick hunter’s (144) MAXIGAS series. The N2MAX generator is designed specifically for gas assist and can deliver nitrogen in purities as high as 10 parts per million of oxygen. ECS Sales, Toronto, produce turnkey gas assist systems using domnick hunter generators. Once generated, the gas pressure may be raised by an intensifier located at the controller, or within the generator unit. Both must boost nitrogen gas pressure to operating levels on the order of 5000 PSI and higher. Multiple press operations use “satellites” designed to receive pressure from a separate high-pressure generator. Costs are lower because the internal intensifier is not duplicated for each machine. Both industrial computers and PLCs are used, with differing opinions on overall durability and performance. An example is the NitroJection Satellite Unit (NSU). The NSU is available in one, two, or four outlets, and operates with an Allen-Bradley SLC 503 controller with up to 30 recipe files. The cost is in the $US35,000 range. A smaller unit for mounting on the mold or platen is available from Epcon Gas Systems, Troy, MI, (145), the MC2, which features two outlets and is priced under $US25,000.

The exception mentioned earlier is the Hettinga HELGA (146) process, which uses a proprietary liquid which vaporizes when injected into the melt stream. The process expands the gas at lower pressures than conventional gas assist, minimizing “fingering” and allowing tooling costs comparable to blow molding. The system allows injection at up to 16 points in the mold or runners. Another variation on the process is Battenfeld’s Airmold Contour (147) process, which uses gas on one side of the part, rather than within a hollow core, to produce large parts without sink marks.

Exactly where the gas is injected, at the nozzle, in the runners or in the mold, depends on part requirements and on which license is used, since the injection location formed the basis for much of the litigation surrounding gas assist. Current pin designs emphasize ease of maintenance and reduced clogging. New gas pin designs and readily available gas generation and control equipment will encourage more players to enter the market, especially firms with experience in hot runners or specialty tooling. INCOE Corporation, Troy, MI, (148), for example, manufactures and distributes equipment developed by UK-based Gas Injection Ltd.

Processors interested in expanding their business, especially OEM production, should study the gas assist process. As John Blundy, vice-president of business development for INCOE says:

“Because the technology of gas assist has become a viable process, processors now say ‘there are advantages, things I can do that I couldn’t do before’. Processors who want to remain competitive need to look at the gas assist process. Not only is it a weight and cycle savings, and in many cases a structural savings, there is also the cost issue. They’re getting a double benefit.” CPL

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