Prototypes become rapid parts

Fitness fiends couldn't run on this machine without its rubber footpads, so Star Trac needed spare parts fast.

Irvine, CA —From trendy gyms to personal trainers and celebrity workout videotapes, exercise is a big business.

So Star Trac Inc., a maker of indoor running machines, takes its job seriously. The company designs and manufactures cardiovascular fitness equipment, such as treadmills, stationary bikes, and stair climbers. It also makes an aerobic trainer called the Elliptical EDGE™, a person-powered running machine with a 24-inch rolling stride, so exercisers can get a low-impact workout.

But on the eve of bringing the Elliptical EDGE to market, the company nearly got sidelined when it found it had 400 of the new machines in its warehouse, all waiting for a supplier who was late in delivering three injection-molded rubber components.

The machines were useless without a missing rubber grommet and left and right footpads, since those pieces had been carefully designed for aesthetic appeal and for direct contact with the user. Star Trac couldn't just swap in a cheaply-made part, since the footpads had to have a specific texture, feel good to the customer, have tough wear characteristics, and appear consistent from one machine to another.

So the company called for help from Solid Concepts (Valencia, CA), a rapid prototyping (RP) firm that had done many complex designs in the past, but usually for prototypes, not final products, says Solid Concepts' Marketing VP and engineer Tom Vorgitch.

The company's choice: Use rapid prototyping to make 400 finished parts, not prototypes.

"The big difference was these would be used on a large volume of saleable items, where usually RP is used for small volumes," says Dave Cawley, Solid Concepts' director of cast urethane operations. "This job stepped over the line toward production quantities."

One of their biggest challenges was repeatability, to make all the parts look identical. "It's pretty easy when you're making ones and twos, but with quantities in the hundreds it's different," Cawley says.

This temporary SLA footpad (in green) forms a cavity in the silicone mold, so the urethane can fill the final part.

And geometrically, the challenges were different for each part. The L-shaped, flanged grommet was complex, but not visible. And the 13×6-inch footpads were geometrically simple, but highly visible, so aesthetics were crucial—especially when fitness buffs could choose between rows of machines lined up before enormous mirrors.

Solid Concepts had to match three main material properties:

hardness (durometer) had to be consistent, since the user's foot would contact the pads

durability had to be high, so exercisers could log many miles on the machines

aesthetics had to be good, since the parts would bear logos with the manufacturer's name on them

So Cawley chose a castable shore-A urethane he had used on previous prototype projects, including the shift boot for a Mack truck. The casting process occurs when he pours an A and B liquid into the mold, and they have an exothermic reaction to form a solid.

But first he had to create the mold. Cawley took an SLA (stereolithography) part, dropped it into a box, and poured silicon around it. When the material set, he removed the box and sliced the silicon in half, removed the SLA part, then poured urethane into the resulting mold cavity. Each of the molds was good for about 25 uses before it had to be replaced.

Since they had to make 400 copies of each of the three parts, Solid Concepts bought an injection machine to pour precise amounts of the urethane liquids into 20-30 molds at once.

"It worked out great—they saved us in a pinch," says Randy Bergstedt, Star Trac's director of product management. The first production run was so well received that the company is now making 300-500 machines per month.

One of the biggest surprises of the project was that Solid Concepts has continued to use their new injection machine for subsequent jobs, since increasing numbers of companies are choosing to use rapid-prototyped components in their finished products, Cawley says.

"Say you make only 50 units a year of a specialized ophthalmic medical product, and you need a housing. You're not going to invest in expensive tooling," he says. "We've seen that more and more as production cycles and product lifecycles get shorter."

For instance, short shelf-life products like trendy cell phones seem to change appearance every week. And other products have very small production runs, such as some specialized medical products which might sell only 20-30 units per year.

Aside from speed, the main variable was cost. Injection molded parts typically cost just pennies per item over the long run, but that efficiency is lost for small production runs since it costs several hundred thousand dollars to set up the tooling, Vorgitch says. In comparison, rapid-prototyped items might cost $20-$50 apiece, but the total cost remains low if your production run is small enough.

And as materials technology improves, RP can better match the part characteristics of more final products. Solid Concepts has made final parts for such sensitive components as the housing around lasers used in eye surgery and the EMI shielding inside an infrared camera.