Molding Polyurethane Can Be A Sticky Business

BeverlyStout went into business for the same reason many people do: Shefelt she could do it better than anyone else. And, since her areaof expertise happens to be polyurethane, Stout set out to be thebest molder of that material in the country. It appears, withfive years under her belt as president of Stack Plastics, thatStout has achieved her goal. In fact, she has helped develop,with the aid of business partner Mark Rackley, a fast-growingcompany with a reputation for its technical aptitude in moldingpolyurethane and other engineering-grade materials for very demandingcustomers.

Stack primarily serves the medical industry in the SiliconValley, operating eight molding presses. Most of the presses areToshibas, from 30 to 150 tons. There is also a 60-ton Nissei verticalpress in which Stack does insert molding.

Many of Stack's customers present the company with difficultdesign and molding challenges. Finished products require complex,multistep processes resulting in components used in such criticalapplications as open-heart surgery. One component Stack manufacturesis a comolded part that overmolds a long-glass-fiber PUR witha medical-grade TPU.

So what is Stack's secret to success? First, it approachesmolding analytically. Second, it pays attention to the details.And in molding polyurethane, the details are critical and numerous.

Critical Factors

"We pay attention to screw design, mold design, moisturelevels, and various processing variables," Stout says, notingthat as shear-sensitive resins, urethanes require special screwdesign considerations. Having molding machines capable of tightcontrols also is required, as is top-notch mold design and quality.

Stack uses several high-end mold shops in the area; the choicedepends on the size, complexity, and whether or not the mold isa prototype or production tool. For complex production molds,for instance, Stack frequently uses United Mold Corp. in San Jose.

Another, if not the most, critical factor is moisture, notesStout. Moisture in the material will cause molecular weight reduction,resulting in a drastic loss of physical properties in the moldedpart, she explains. "When molding medical products, uponwhich peoples' lives depend, you can understand the importanceof this factor," she adds.

To combat moisture problems, before being molded urethanesmust be dried to moisture levels of less than .02 percent, andthe material must stay dry throughout the process. "Urethaneswill absorb surface moisture within 15 minutes that will renderthem too wet to process.

"They will, however, let you know if they're not dry,but people don't pay attention," Stout adds. "Wet urethaneswill appear foamy or have bubbles in the purge and can cause significantnozzle drool. On screw recovery, the purge should have the lookand consistency of honey." Still, Stack analyzes the moisturecontent of the material to ensure optimum molding conditions.The company uses moisture analyzers from Arizona Instrument (Phoenix,AZ).

Getting resin dry is critical, but keeping it dry is just asimportant. The dryness of the resin can be checked with an airshot. Stack mounts the dryer hopper directly on the infeed ofthe processing equipment. Stack has also eliminated as many deadspots as possible, even stripping off the bottom of its Matsuidryers to eliminate dead space below the cone.

Worth the Price

To complete its service package, Stack has developed severalsecondary operations. Many of the parts Stack molds require padprinting of logos or other markings, which can be tricky withurethanes, says Rackley. It's extremely difficult to get the inkto stick to a surface that is inherently chemically resistant,he explains. After having problems with outside vendors, Stackbrought its pad printing in-house to preserve the quality andfurther develop the technology. Now it considers the process almostproprietary. In addition to pad printing, Stack also does somesonic welding and light assembly.

Stout notes that focusing Stack's niche on high-end medicalmolding has paid off in more ways than one, including raisingits level of expertise across the board in all engineering thermoplasticsand elastomers.

Rarely is Stack the low bidder on work the company wins. However,the confidence that Stout and Rackley have built in their customersmakes it worth the price.

"Technical ability is only one component necessary forthe relationships we've developed," Stout explains. StackPlastics prides itself on being responsive, taking care of itscustomers' demanding needs, and manufacturing the highest-qualityparts possible by paying attention to details.
"Details will kill you," says Stout; Rackley agrees."We pay attention to details."