Appliance makers plunge into gas-assisted parts

Several years ago, Mike Nelson, a senior tooling engineer at Maytag, and some of his colleagues decided they wanted to try something they'd never done before: mold a part using gas assist. They were looking around for a suitable part, something that would benefit best from everything gas assist has to offer. "Then it just kind of popped into place," Nelson says. "Why can't we use gas assist on this part?" "This part" is an agitator auger for a Maytag washing machine and proved particularly challenging to mold with gas. The results, though, are impressive.

Maytag Appliances, based in Newton, IA, operates a captive operation of 24 presses ranging from 125 to 2000 tons. The plant molds parts for clothes dryers and horizontal and vertical washers. The auger was to go in a new washing machine. Maytag's gas assist efforts on this project were encumbered by several sticky wickets: The company had never molded an appliance part with gas assist, it did not want to take a trial-and-error approach to tooling and molding, and the part itself consists of relatively thick and thin sections abutting each other - gas penetration would have to be carefully controlled.

The polypropylene auger is part of a larger agitator assembly. It's that tower-like part used in washing machines to gently agitate, turn, and roll clothes for cleaning. The auger itself consists of a tapered cylinder with a fin spiraling down its length. It's 9.5 inches long, 4.5 inches in diameter at the large end, and 2.5 inches in diameter at the smaller end. Although the photo shows the auger with the tapered end up, when assembled the auger sits vertically with the tapered end pointed down toward the bottom of the washer.

Nelson says the goal of using gas assist was to core out some of the thicker sections - the fin in general, and where it contacts the cylinder in particular. Nominal wall thickness of the part is about .12 inch; where the fin contacts the cylinder, thickness jumps to .34 inch, a three-fold increase. Maytag saw an opportunity here to eliminate shrinkage along the fin base, as well as reduce weight and cycle time by not having to cool the thick sections.

Maytag decided to go with a NitroJection gas-assist system early on and called on the company to help it design the part. To avoid the trial-and-error approach the tooling could require, Maytag turned to the C-Mold Gas-Assisted Injection Molding simulation package to help on the design. The real trick, Nelson knew, would be to keep the gas in the fin without it traveling straight down the length of the cylinder. "We knew that the fin had to be the path of least resistance," Nelson says.

Steve Johnson, vice president, sales and marketing at NitroJection (Chagrin Falls, OH), says gate placement and timing would be the keys to success. Although NitroJection projected a healthy weight savings potential of 12 to 17 percent, Johnson says his concern was about gas permeation from the fin into the cylinder. "In a perfect world, the gas would stay in the fin, but you always have some secondary penetration," he says.

Using the simulation software, Maytag, C-Mold, and NitroJection tested a variety of runner and pin placements, injection timing and gas timing and compared the gas-assist part to one molded without gas. They analyzed shrinkage patterns, gas fingering patterns, cycle time savings, weight savings, and other parameters. Before any steel was cut for the mold, Nelson says, he had developed data for injection timing, gas timing, and gate placement that showed they could mold the part and keep the gas in the fin.

Nelson says Maytag decided to build a two-cavity mold and settled on a strategy that uses one large gate (at the small end) and four subgates. Gas is introduced in the runner of the primary gate. Nelson says the part is molded using the short-shot process. About 50 percent of the gas is injected during the fill stage; the remaining is injected during postfill. "The gas fills it out during fill and postfill as well," Nelson says. Gas is kept to the fin as the cylinder stiffens appreciably early enough to force the gas down the fin. Although there is some gas permeation into the cylinder, Nelson says these regions are not critical to the performance of the part and the auger still meets structural requirements.

"The benefits offered by gas assist are now a reality for this application that seemed at first nearly impossible," says Nelson. Based on the C-Mold simulation data, the cycle would have been 70 seconds using straight injection molding. Under gas assist, it was reduced to 43 seconds. Part weight was also reduced by 12 percent, within the range predicted by Johnson. Nelson also reports that because volumetric shrinkage was reduced, part roundness improved by 50 to 75 percent - an important consideration in such washing machine applications.

As for the NitroJection dual-zone gas-assist unit Maytag bought, Nelson says, "We've gone through some growing pains with it." Most of the pains, however, he attributes to Maytag's inexperience with gas assist in general. Now that he's familiar with the unit and how it operates, Nelson says he's had no problems worth mentioning and that the device runs almost continuously. The mold for the auger was produced by Chicago-based Ostram; it's cut from 420 steel and polished to an A2 finish. It runs in a 500-ton press and uses a Kona hot runner system. The washing machine in which the auger is used was introduced in August 1996.

For fascia panels, appearance is everything

By Michelle Maniscalco

Whirlpool, arguably the world's largest appliance manufacturer, naturally pays a great deal of attention to product appearance. This focus has transformed the company's Findlay, OH dishwasher manufacturing plant into a successful gas-assist molding venture. And the changes are helping Whirlpool clean up on more than just surface finish.

Engineers at the Findlay facility, which produces dishwashers under the Kenmore, KitchenAid, and Whirlpool names, recently announced their intention to mold all exterior console panels for 1998 models using gas-assist technology. A major success story with one part gave processors the confidence to make this conversion, which will affect the total annual production.

After evaluating gas-nitrogen molding, according to senior process engineer Gus Vargo, Whirlpool installed a Battenfeld Airmould system in 1995 to produce the rigid PVC console, or fascia panel, for a deluxe-model Kitchen-Aid dishwasher. "With conventional molding, we consistently fought sink marks," Vargo tells IMM. "The back side of the panel contains four bosses for mounting to the door. To avoid sink on the opposite side, we increased pack and hold pressures and times. As a result, internal mold stress shot way up. There was always a certain amount of warp induced into the part, and scrap rates ran as high as 20 percent. Aesthetics are the key on dishwasher panels, so our standards are very high."

Another complication involved the decorating method. Whirlpool purchases a decorated PC overlay, which workers adhesively attach to the molded console. Even slight imperfections became glaring errors once the overlay was in place.

Sampling results at Battenfeld's facility were favorable, and according to Vargo, the system blended well with Findlay's operations. "Battenfeld gave us great technical support in setting up the system," he says, "and our material supplier, Geon, has an Airmould system at its tech center, so it's able to help us sample prototype molds and set up processing parameters."

As a result of the switch to gas-assist molding, sink marks on the console were eliminated, and straightness improved significantly. "We are now consistently molding parts within 1mm of being flat," Vargo adds. Also, aesthetic scrap has been reduced to roughly 2 percent, with a maximum of 5 percent. Pack and hold times are down to one quarter of a second. Cycle time has also been reduced significntly, which Vargo partially credits to both gas-assist and berylliium-copper mold cores.

Weight of the 1.75 lb part, already slightly reduced, could be even further reduced. But the part must meed UL impact requirements. Whirlpool began converting other models to gas-assist early this year, and currently runs the parts - 24 hours a day, seven days a week - on a maximum of five machines in the 500-ton range. Two Airmould pressure generators equipped with nitrogen bottles supply gas to each of the machines through individual pressure control modules and 5-mm fixed needles, which inject gas into the mold core.