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May 1, 2002

9 Min Read
WEB EXCLUSIVE:Let's just call it AIM—Assisted Injection Molding

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Measuring 78 by 36 inches, this gas-assist molded Volvo truck bunkboard supports the sleeper compartment's mattress. Gas assist helped consolidate various functional design components, resulting in a 20 percent part cost reduction vs. its pressed wood counterpart. It was designed by Richard Langhoff of Volvo and produced by Mack Molding Co. (headquartered in Arlington, VT).

As part of his presentation at Molding 2002 (March 4-6, New Orleans, LA), Terry C. Pearson, chairman of the recently consolidated Cinpres Gas Injection Ltd. (CGI, Cheshire, U.K. and Ann Arbor, MI) called on the molding community to consolidate its abbreviations. Whether molders are using gas assist, water assist, supercritical fluid assist, or combinations of these and more, he says that in spite of their differences they share a common bond. They all involve add-on systems to assist in improving processing and part designs. So, he asks, why not let them share a common name, namely, AIM (assisted injection molding)?

Time will tell whether or not Pearson's suggestion is adopted. Actually, the main topic of his presentation was the latest technological development in gas assist from CGI called the Plastic Expulsion Process, which we'll discuss later. In fact, there was a lot of other AIM news at this annual conference.

Jack Avery, manager of operational assets at GE Plastics (Pittsfield, MA), led the talks with his annual overview of the latest AIM trends. Avery says gas assist is no longer news. It has matured to the point where the technology itself is routinely being applied in product designs for most major markets, including appliance, building and construction, IT, materials handling, medical, sports and recreation, and transportation. Growth in the use of gas assist also has matured.

Citing published statistics, Avery says 48 percent of the largest 100 U.S. molders (in terms of sales) use gas assist to the greatest extent. On the whole, use of gas assist has remained unchanged for the past two years—about 14 percent of survey respondents, says Avery. The most active users are still automotive molders.

Supply-side News
On the business front, Avery says the biggest thing to happen last year was in July when BI Group Plc, the parent company of Cinpres, purchased Gas Injection Ltd. (www.gasinjection.com). "This is the first consolidation in the industry," says Avery, "and is a departure from what has been more typical—legal battles over patent infringement."

He doesn't think it will be the last. "In the current economic environment and with a larger number of suppliers of gas-assist injection molding technology vying for a limited amount of business, I will not be surprised if additional consolidation/contraction occurs."

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At K 2001 Engel demonstrated its Watermelt water-assist system in molding this curved automotive coolant tube. There were three water injectors (WI): one at the mold gate (G) and two at the inlets of the two secondary overflow cavities (SC1 and SC2). Water assist reduced cycle time to 35 seconds.
OD: 25 mm
Wall thickness: 3-4 mm
Material: 30% glass-filled nylon 6/6



In fact, there was a change of hats at the conference. Factor Maschinen & Anlagentechnik GmbH (Hainburg, Germany), a European equipment supplier and sales rep, represented Gas Injection Ltd. at K 2001. Andreas Janisch, engineering and sales director for Factor, announced at Molding 2002 that it now represents Gain Technologies (Sterling Heights, MI, www.gaintechnologies.com).

Avery is upbeat about the M&A activities of AIM suppliers. He says, "The good news is that more energy and resources are being focused on technology, supporting customers, and developing new business, rather than on legal hassles."

Cool Technology
Rui Magalhaes, research fellow at the University of Warwick's Warwick Mfg. Group (WMG, Coventry, U.K.), discussed the results to date of a five-year research project. WMG wanted to determine whether cooling the gas prior to injecting it could further reduce cycle times vs. standard gas assist. It apparently does—by 40 to 60 percent, according to Magalhaes. WMG has trademarked its technology KoolGas.

KoolGas systems control the injection of cryogenically cooled nitrogen gas at temperatures from 0 to -196C. Electronic flow control of the super-cooled gas enhances heat transfer at the gas channel interface, resulting in faster cycles. The heart of the system is a proprietary cryogenic heat exchange unit installed inline with the gas injection system, downstream of the pressure regulation module, and as close as possible to the gas injection nozzle.

It does more than speed things up. Studies reportedly show that KoolGas allows the molding of parts with thinner walls. Tests also show that KoolGassed parts have gas channels with smoother bores throughout the gas channel than those molded with conventional gas assist. This can be a key point to consider when designing parts such as fluid conduits.

Magalhaes says the University has assigned its KoolGas intellectual property rights to a spin-off company, which will commercialize the process.

Control Over Overflow
As mentioned above, CGI's Pearson discussed his company's

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Plastic Expulsion Process (PEP) in New Orleans this year. PEP provides a way to accurately control the amount of plastic injected into each cavity through the use of shutoff valves.

In one version, PEP 1 (see drawing, below), following the initial pressurization of the melt, a shutoff valve in each runner that connects the part cavity to a secondary overflow cavity is opened. The amount of material flowing to the secondary cavity is controlled both by the fixed volume of this secondary cavity and by the timing of the shutoff valves. The initial timing of gas injection is inconsequential to the volume of material expelled into the secondary cavity. A multicavity mold will use as many shutoffs and secondary cavities as are necessary.

In CGI's PEP 2 process an injection nozzle shutoff valve is used. Rather than expelling material into a secondary cavity, it is forced back into the injection cylinder by pushing back the injection screw against a controllable backpressure.

Pearson says both methods offer a number of benefits over conventional overflow techniques, including the elimination of hesitation marks and the elimination of the recovery and regrind of expelled material. CGI can provide all the necessary valves and controllers. Licensing will involve a one-time fee, he says, and will be competitively priced.

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The amount of plastic expelled is controlled by the fixed volume of the secondary overflow cavity(ies) and by the timing of a shutoff valve in CGI's PEP 1 process. Part surface quality is improved, and since the initial timing of gas injection has no effect on the volume of plastic expelled, cycle times and part weight can be reduced through more gas core-out. Also, gate locations are less critical.

Water, Water Everywhere
The levee broke this year in New Orleans when it came to reports on water assist, adding to the flood of developmental activity that was so much in evidence for anyone wading through K 2001. GEP's Avery says that, since water is incompressible, when it is injected into the melt, a higher pressure can be generated than with gas. Water also has a greater heat exchange capacity than gas, and parts are cooled inside and out.

So larger, thick-walled parts—components with 1- to 3-inch-diameter channels and greater—can be molded in relatively fast cycles. Cycle time reductions of up to 75 percent have been achieved, according to Avery.

And, of equal importance in many applications, water improves the centricity and smooths the bore of the channel, reducing distortions, say, in an automotive intake manifold or cooling water pipes. Water also is easier to control than gas, while providing the part surface quality benefits of gas assist without the cost of nitrogen.

Avery cited a number of companies that have been very involved in developing water AIM, including Alliance Gas Systems (Chesterfield Twp., MI, www.gasassist.com), IKV (Aachen, Germany, www.rwth-aachen.de/ikv/), and Factor Gmbh. Battenfeld (Aquamold, www.battenfeld.com), Engel (Watermelt, www.engel.at), and Ferromatik Milacron Europe/North America (Aquapress, www.ferromatik.com) also are actively involved. Some even propose using water and gas injection combined. Most were at Molding 2002 and included mention of their water-assist systems in their presentations.

Ready to Take the Plunge?
Hermann Plank, managing director of Ferromatik Milacron Europe/North America, says that despite the commercialization of water-AIM systems, the technology is still in the red and has limited possibilities today that must be assessed on a case-by-case basis.

Plank, Factor's Janisch, GEP's Avery, and other speakers repeatedly stressed the fact that switching over from gas assist to water assist is anything but a drop-in process change. It's just too different. Specially designed injectors are required. Injectors designed for gas injection are not suitable for water. Injector sealoff is critical, as is sequence timing.

Then there's resin compatibility—will water assist work with resins like nylons, polycarbonates, and polyesters? Is there enough application-specific know-how around? And then there's the big question: What do you do with the water?

"Leakage is not a problem with gas-assist molding. With water, it's different," says Factor's Janisch. He says this is especially true when running water through a part in a water-cooled mold on an all-electric machine.

Other AIM News
Avery noted a few other intriguing developments in AIM.

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For example, researchers at the University of Bradford in the U.K. are fine-tuning 3-D FEA and moldfilling analysis for gas assist. He also cited four University papers presented at the SPE's Antec 2001 in Dallas, TX that covered a wide range of topics—everything from optimizing processing conditions to the effect of liquid cooling of gas channels.

He also noted that IKV researchers have been applying gas-assist technology to PIM. In Aachen, gas assist has been found to reduce debinding time in large PIM parts with thick wall sections.

Editor's note: The complete Molding 2002 proceedings can be purchased from the conference's organizer, Executive Conference Management Inc. of Plymouth, MI, (734) 737-0507, www.executive-conference.com.



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Water dissipates more heat than gas, so a high-viscosity membrane forms over the flow face, as in this Aquapress water-assist system from Ferromatik Milacron Europe/North America. The membrane has a greater displacement potential than gas, so more melt is moved in the direction of flow. Less is moved to the outer layer. Therefore, the remaining wall thickness is reduced and cavity centricity is improved.

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The plastic expelled back into the injection cylinder in CGI's PEP 2 process can immediately be remolded in succeeding shots, with no recovery and regrinding. No trimming of the expelled plastic is required, and the cost of expulsion shutoff outlet valves is eliminated.

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