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NitroJection Presentation

Excerpted from Injection Molding Magazine

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An early NitroJection success!

1997

IMM Magazine Article Archive

Appliance makers plunge into gas-assisted parts

Both Maytag and Whirlpool are taking advantage of gas assist.

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.

This auger was molded without colorant during Maytag's testing of the part. The gas fingering here shows the maximum gas penetration into the cylinder that can be achieved. The auger is gated at the narrow end, and the gas pin runs through the center of the cylinder and injects at the primary gate that meets the part at the terminus of the fin.

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.