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May/June Feature

Stack Plastics Has a Clamp
on the Medical Industry

California-based Stack Plastics Inc. specializes in engineering thermoplastics and elastomers primarily for the healthcare industry.

By Karen Koenig

Manufactured for Guidant, this disposable cardiovascular system blade stabilizer arm contains multiple plastic parts which were injection molded from thermoplastic materials purchased from The Dow Chemical Co.
The medical industry is a very healthy niche market for Stack Plastics Inc.. The company estimates that 85 percent of its injection molded products are manufactured exclusively for that field.

The Menlo Park, CA-based custom firm specializes in molding engineering resins such as polyurethane. “(Before starting Stack Plastics) I realized all the best polyurethane molders were captive and there was a huge need for a custom molding shop that could process polyurethanes. We also saw a great potential for a captive audience in medical and high-tech equipment right here in Silicon Valley,” says Beverly Sidders, co-owner of the company.

“The medical industry was also a very attractive market because it’s very stable and not as cyclical,” adds Mark Rackley, co-owner. “For these companies, too, it doesn’t always come down to price — service and quality are the two most important factors.”

Stressing Service and Quality

Service begins from the moment the order is placed. Stack Plastics offers recommendations on the material, gate locations and the moldability of the design. The company works with outside vendors to build the tools.

“The design and quality of the mold is key to a good part,“ Rackley says.

Once the mold is made, Stack Plastics guarantees a two-week turnaround time or less on most parts. “We try to make sure we have some shop capacity open so we can handle emergencies for our customers. Even if it means putting another machine on the shop floor, it’s worth it in the long run,” Sidders adds.

The majority of Stack Plastics’ 5,200-square-foot manufacturing area is set up in a Toshiba/Matsui platform. Currently, the company has seven Toshibas, ranging in size from 30 tons to 150 tons, a 60-ton Nissei vertical press for insert molding and an older Boy 50-ton press. Barrel sizes on the machines range from 1.2 ounce to 10.5 ounces.
The design and quality of the mold is key to manufacturing a good part. A Toshiba 150-ton press is used to mold blades for the Guidant cardiovascular rib spreader. A newly added robot helps manage the flow of parts from the machine.

The Toshiba ISG Series injection molding machines are equipped with real-time shot-to-shot statistical process control. “Matsui dehumidifying dryers allow us to dry hygroscopic resins with a consistent air dew point of -40F,” Sidders says.

Tight tolerances are essential when manufacturing medical devices, especially clamps and other surgical instruments. “In this business, variation is the biggest enemy,” says Rackley.

Some of the products molded by Stack Plastics include disposable components for open heart surgery manufactured for Guidant from various materials, including The Dow Chemical Co.’s Calibre and Isoplast (trademarked) materials. Stack Plastics also molds a surgical clamp for Novare Surgical. “We worked with Novare engineers to develop the best combination of materials to ensure their requirements for rigidity, flexibility and adhesion were met,” Rackley explains.

In addition, Stack Plastics has also manufactured products for many high-tech firms, such as Anritsu and Silicon Graphics, and other manufacturers such as Nike and ORAL-B.

“We offer custom molding, so we do all types of projects,” Rackley says. “We have no minimum run requirements — we base the job on its value, not on its run size.”

Among the medical equipment manufactured at Stack Plastics is this vascular shunt tag. As part of its specification, the suture on the shunt tag must withstand a 1-pound pull test. The product was manufactured on a Nissei vertical injection molding machine.

Managing the Moisture Content

No matter what the product is, the success of the molding process depends on the moisture content of the material. For best results, urethanes, for example, must be dried to a 0.02 percent moisture content or less. An Arizona Instrument MAX 2000 moisture analyzer is used every morning to test the day’s materials and ensure their moisture levels.

“You can have the best dryer in your shop, but unless you test the material, you can‘t know for sure whether your hitting the 0.02 percent moisture,” Sidders says. If too much moisture is present in the polymer, the material will not mold properly.

Keeping the resin dry prior to the molding stage is critical as urethanes can absorb surface moisture within a few minutes. In an innovative move, Stack Plastics has eliminated the bottom portion of its Matsui dryers and, when possible, mount the dryer directly onto the infeed of the barrel. “We try to eliminate as many dead spots as possible. It allows us to get the best molecular weight in a molded part,” she says.

Stack Plastics inventories approximately 10,000 to 15,000 pounds of resin, much of it supplied by The Dow Chemical Co. “Because we manufacture on a just-in-time basis, we need to make sure we have an inventory of material ready. The (resin) manufacturers can’t necessarily ship to us on a just-in-time basis,” Sidders explains.

Quality and Added Value

Because of the emphasis Stack Plastics puts on the quality of its products, as well as its value-added services, much of its business has been from repeat customers. “We get a lot of business through repeat people — people who have switched jobs and recommend us to their new companies,” Sidders says. “We also get a lot of business from word-of-mouth referrals,” she adds.

The company’s Web page,, functions as an online brochure for the company, describing the features and capabilities of the firm.

In addition to its injection molding expertise, Stack Plastics also offers insert molding, overmolding, ultrasonic welding of parts and pad printing.

Stack Plastics brought its printing in-house and invested in a Trans Tech Duo Seal 90 pad printer in order to maintain quality control over its printing process. According to Rackley, printing on urethanes can sometimes be difficult in that the ink has trouble sticking to a material that is essentially chemically resistant.

“We’re also always open to adding new technologies as well to our shop,” Sidders says. Any additions to its capabilities will most likely be driven by customer demand.

The main thing to remember for success is to know what you’re doing and do what you do best. Find your niche and take care of your customers,” Sidders adds.

ISO Registration

As of this writing, Stack Plastics Inc. is awaiting its certificate for ISO 9001:2000 registration. The company passed its audit in early April.

“ISO 9001:2000 registration is the international standards for excellence. The new 2000 standards require the monitoring, statistical control and documentation of processes as a way to improve the end product,” says Mark Rackley, co-owner of Stack Plastics.

Beverly Sidders and Mark Rackley opened Stack Plastics almost six years ago. They began the company after noticing a gap in the custom injection molding field for a company specializing in urethanes and other engineering thermoplastics.
Stack Plastics: a merger of names, experience and a desire for quality

The name Stack comes from a merger of the owners’ names: “St” from Stout, Beverly Sidders’ maiden name, and “ack” from Rackley. Stack Plastics evolved almost six years ago from a desire by Sidders and Mark Rackley to offer quality custom molding of engineering resins and elastomers, including polyurethanes.

Rackley relates, “I was involved in manufacturing plastic parts my whole career, working for both small and large companies. I met Bev (who was working for Dow Chemical at the time) when she came on-site at one of my former employers to provide technical service on a urethane project.”

“At the time,“ Sidders says, ”not a lot of molders were handling the material correctly. When I would visit these shops, I saw that there was a huge need for a custom molding shop that could process urethane. We also saw a great potential in the medical and high tech industries. We started making some cold calls and also got involved in research and development for these companies,” she continues.

Sidders and Rackley say the philosophy behind Stack Plastics helps set it apart from other injection molding companies. “We pride ourselves on being responsive, taking care of our customers’ demanding needs and manufacturing the highest-quality parts possible,” Rackley says.

For more information, or to contact Stack Plastics, please call (650) 361-8600, attn. Leigh Fancher, or visit

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