Resin moisture

This used to happen to Teena Henson about twice a week: A part would start to show signs of splay or degradation. The molder would shut down the press and call Henson over, claiming that the resin was too wet. The molder would increase the drying time, without knowing for sure what the culprit was. Thus began a series of guesses as to what the cause of the splay might be, without data to back up assertions.

Henson is a material handler at Teledyne Water Pik in Loveland, CO. The facility there has 28 presses ranging from 30 to 500 tons running about 150 materials, many that like to hold water, such as nylon and Isoplast (polyurethane). With 150 to 200 mold changes a month, material and parts come and go quickly, leaving little margin for error. Two particular products, the Shower Massage and the Oral Irrigator, are both manufactured with molded Isoplast parts; if too wet they quickly become brittle and easily break.

Henson says she was frustrated by her inability to know what was happening in the dryer hopper - especially in the rarefied air at the foot of the Rocky Mountains, where material specs for drying time and temperature do not always apply. "If the molding people came to us and said the material was wet," Henson says, "we had no way to dispute it."

At a similar altitude and in a similar situation was Michael McPheron, process manufacturing engineer at Complex Tooling and Molding in Boulder, CO. Complex runs a lot of engineering materials on its 22 presses, ranging from 25 to 750 tons. Like Henson, he was doing a fair amount of guessing when it came to troubleshooting processing problems, ranging from yellowing nylon to splay to degradation.

Then last November McPheron decided to stop guessing and started trial testing resin moisture analyzers. He settled on the Mark 2, an analyzer from Omnimark Instrument Corp. (Tempe, AZ). The device heats a sample of resin and measures the moisture evolved, telling McPheron just how wet or dry his resin is. The immediate effect was simple: "Now, I know if it's wet or dry. It used to be just a guessing game."

Henson also has adopted uses of this analyzer, and agrees. "In truth, it's saved us a lot of money by not running wet material, not getting bad parts," she says.

But a moisture analyzer is not a device to hide in the broom closet to be whipped out just because your operator thinks your nylon is wet, or because parts are starting to show splay. McPheron and Henson both use the analyzer as preventive medicine, and that's where the long-term payoffs are.

The drying process has a lot of variables: dryer, material, air lines, ambient temperature, ambient humidity. To attenuate the problem, McPheron says every resin in his shop is tested for moisture before it runs on a press. And for his hydrosensitive materials - nylon and polycarbonate - the material gets checked once or twice during the run. Henson too checks resin for moisture before it runs, and at least once a shift after that.

Henson and McPheron both say that using the moisture analyzer helps make them less reliant on material drying specs that may or may not apply. By knowing the exact moisture content of their resin, they dry it only as much as needed, which is often less at a mile high.

The Mark 2, McPheron also points out, tells him if his resin is too dry. He says Complex had a series of nylon parts that frequently emerged from the press yellowed. With the moisture analyzer, he determined that the resin was being overdried in the hopper. "We'd wind up throwing away hundreds of pounds in bad parts," he says. "I'd have to say it's already paid for itself, just in scrap alone." By Henson's calculations, Teledyne reduced material downtime from 55 hr/week to 35 hr/week in one year, saving the company approximately $20,000.

Although the concept of the moisture analyzer is relatively simple, the device itself is actually more sophisticated. Gary Beebe, sales manager at Omnimark, says his company spends a lot of time testing, tracking, storing, and maintaining material data used by the analyzers. For every type and brand name of material, Omnimark develops ideal parameters for temperature, sample weight, minimum testing time, and end point setting. When a customer buys a Mark 2, Omnimark preprograms the analyzer with this resin data, specific to the materials the customer runs. So, when Henson is ready to test her nylon, she simply selects it from the menu and the analyzer prompts her for the rest.

Testing time of samples varies depending upon the material, but generally ranges from 3 to 10 minutes. Results are printed by the analyzer and stored in memory. Beebe says up to 255 consecutive tests per resin can be stored, allowing you to go back and pull data for almost any specific date, time, or material. For more sophisticated analysis you can attach a PC. Results of moisture content can be expressed in parts per million or as a percentage; Beebe says most customers use percentage.

If you have a new material that's not been preprogrammed into your Mark 2, just send it to Omnimark. If the company doesn't have data for it, Beebe says it'll test it for you and send you the specs, which you can enter into the analyzer yourself. Prices for the Mark 2 start at about $7500.