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March 1, 2004

6 Min Read
Water-assist molding bears fruit, homes in on automotive applications

Most of the early bugs, often made evident by pools of water under the machine, have been eliminated, and important potential users are keen to reap the benefits of large potential cost savings.

Six years on from the first public demonstrations of water-assist injection molding technology (WIT), developers are quietly confident that early promise can now be turned into profit. The initial suspicion from potential users of an untried technology are being overcome, assisted by pressure from part specifiers in the key automotive market, who are keen to see savings resulting from the remarkable cycle time reductions WIT offers.

WIT is being marketed as complementary to GIT (gas injection technology) for fat parts like door handles and components in fluid transport systems like car engine coolant pipes. It can slash as much as 50% off the cooling time of a part made using GIT. As pointed out by John Heasman, general manager of Cinpres Gas Injection Inc. (Ann Arbor, MI), that can easily yield sufficient savings to pay for the equipment investment in less than 1000 production hours. (Heasman was speaking at the Molding 2004 conference in New Orleans in February, where he gave details of CGI''s version of WIT, which it calls WAM—Water Assisted Molding.)

But as Heasman points out, the shock-cooling effect of the water also has a downside. In parts with thick wall sections, the fast freezing of the inner wall inhibits transmission of pressure to the core, and can result in incomplete filling and/or sink marks. Latest developments in WIT—including WAM—combine it with GIT to obtain (in theory at least) the best of both worlds.

A leading exponent of this hybridization is a Freiburg, Germany-based engineering bureau, TiK Technology in Kunststoff, set up last year by Marcel Op de Laak, who previously led WIT development efforts at Rhodia Engineering Plastics'' automotive applications center.

A process he calls TiK-WIT involves an initial injection of a small amount of gas, immediately followed by water, through the same nozzle. This gas bubble acts as a thermal cushion between the hot plastic and the cold water, and Op de Laak says it yields two major benefits.

First, the inner walls of the part are even smoother than is achievable with WIT—which itself produces smoother walls than can GIT—because turbulent water flow is avoided around the injection nozzle and at points where the cross-section of the part changes rapidly. Second, it stops water from penetrating the walls of the part, a phenomenon that can cause premature failure in components like cooling pipes that have to resist hot glycols for hours on end.

Op de Laak notes that several materials companies are developing grades of glass-reinforced nylon 66 specifically for WIT. TiK can carry out application-specific tests on parts made in these grades. In fact, it offers a complete suite of product development services, including concept, simulation, and prototype processing.

A pioneering WIT developer, Friedrich Westphal, claims to have earlier tried and subsequently dropped a procedure similar to TiK-WIT. He says parts still failed in dynamic stress tests. However, Op de Laak can show parts made with ''regular'' WIT and TiK-WIT to back up his case.

As with GIT, some gas does dissolve into the melt, but the problem of subsequent out-gassing, which is one reason why GIT parts have poorer inner surfaces than WIT parts, is not an issue with TiK-WIT, both because of the small amount of gas used and because of the cooling effect of the water; the gas remains dissolved within the plastic.

For a similar reason, Op de Laak also says the problem of void formation in the walls of parts is overcome with his process. This can occur in WIT and GIT when internal pressure is suddenly reduced while the core of the part wall is still mobile. But with TiK-WIT, the walls are too cool to move by the time the pressure is reduced.

A further advantage of having the gas bubble ahead of the water comes at the end of the molding cycle, when the part is drained. The gas, previously compressed by the water, forces it out of the part once the pressure has been removed.

Second, the gas used in TiK-WIT does not have to be nitrogen, as in GIT. Nitrogen, an inert gas, is used in GIT to prevent burn marks in the part caused by the high pressure. Op de Laak says the small amount of gas used with TiK-WIT means that air can be used instead.

TiK-WIT has yet to be used in commercial production. Op de Laak hopes a project may go live around mid-year, depending on approvals. The process can be used on any suitably adapted WIT equipment, although TiK is working most closely with German equipment maker Maximator (Sinsheim).

CGI''s WAM can also be operated in various ways, including gas/water/gas, like TiK-WIT. The company has its own water/gas control, as well as a Combi nozzle for gas and water. The console includes a hydraulic power pack for operating three different circuits within the nozzle. CGI is more restrained in its enthusiasm for WIT, partly because it has a variant of GIT that it maintains can handle some of the applications others are targeting with WIT (Gas Cool works by flowing gas through the part, and can also use gas cooled down to -25C).

Nevertheless, CGI Chairman Terry Pearson, based in Middlewich, U.K., says the technology "is making its mark." Controllers are in operation for customer trials in Middlewich, and Ann Arbor—"We have a queue of people waiting to do trials there"—as well as at CGI''s partner in Germany, SKS GmbH (Goslar).

But Friedrich Westphal, with his company PME Fluidtec in Sasbach, Germany, appears to be well ahead of the pack. He claims to have sold no fewer than 32 WIT power modules—already in their third generation—and in January said he had orders for a further eight. Around 80% of applications are automotive, but few details are available. He says he has six projects running for engine cooling pipes alone.

Westphal has developed a version of his equipment specifically for door handles. It is already in use on a two-cavity mold at German company Witte-Velbert GmbH, in Velbert, which is making parts for the new Opel Astra, launched this month. In January, a four-cavity mold was also being built.

Speaking at a Krauss-Maffei open house in Munich last year, Westphal said that a door handle normally produced in 58 seconds using gas-assist technology took 32 seconds using WIT. "I am sure we can get it down to 28 or 29 seconds," he predicts.

PME Fluidtec is working particularly closely with compound producer A. Schulman (Kerpen, Germany; Akron, OH), which has developed several compounds specifically for WIT. Westphal says interest from the U.S. market is substantial. Speaking in late January, Schulman''s Director for Technical Service, Thilo Stier, said the company was close to presenting new technology for water pipes that would yield further benefits for processors. He also said the company would be making WIT pipes on its stand at K 2004.

At Battenfeld GmbH, based in Meinerzhagen, Germany, another WIT technology developer, process engineering technology director Helmut Eckardt cites "very good progress with the Aquamould technology" in recent months. He says the system "works perfectly...we have developed new water injection nozzles."

Peter Mapleston [email protected]

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PME Fluidtec   


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