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The Right Material, the Right Design

Pressured Formed Plastics

How a molder, a die caster, and a thermoforming specialist are helping their customers achieve success

Mark Shortt
Editorial Director
Design-2-Part Magazine

For many companies, contract manufacturing is becoming less of a "build to print" environment as OEMs increasingly look to partners that they can trust to provide an ever-widening range of services. This is especially true in medical product manufacturing, where contract manufacturers are playing an increasingly greater role in pre-production areas like materials selection, tooling and part design, and prototyping. A company that combines materials expertise (and, in some cases, materials development) with the ability to provide design and engineering assistance, for example, can take on more of a full-service role that eliminates a variety of manufacturing problems before production even begins.

Optimizing the design of a part, which includes selecting the right material for the application, can pay big dividends. It can lower part costs by reducing tooling costs, the number of manufacturing steps required, or part count. And choosing the right material for a part has a clear and direct impact on the quality of the product, enabling it to function as intended and contributing to longer product life. Whether a company offers injection molding, die casting, thermoforming, or any other manufacturing service, it raises its standing among competitors to the extent that it helps its customers succeed. One of the best ways that contract manufacturers can help their customers succeed is by delivering a part that not only meets functional expectations, but enhances the performance of the device that it's designed for.

Surface Friction Factors Critical to Design of Pass-Through Seals for Trocars

Materials and coatings--particularly their ability to overcome friction--are among the most important factors to consider when designing pass-through seals for endoscopic trocars. Specifying the correct material and coating can reduce friction in these applications by as much as 90 percent, according to Minnesota Rubber and Plastics (MR&P), a provider of precision-manufactured rubber and plastic components and assemblies. The company, known for its science-based approach to manufacturing, offers services that include materials R&D, engineering, and design assistance.

"The design of these seals is developing into a science of its own, with friction playing a major role," says Ted Ahrenholtz, technical support manager for Minnesota Rubber and Plastics. "Friction in these pass-through seals is extraordinarily complex. The amount of force it takes to slide one surface past another is affected by many variables influencing the surgeon's skill and quality of the procedure."

In designing pass-through seals for the medical industry, MR&P considers a number of variables that have a profound impact on friction. Among these variables are the lubrication state, the material modulus, and the surface finish, temperature, and geometry of the part, as well as direction of the relative forces. Inserting and retracting a surgical instrument through a seal must feel smooth and easy; the more the material "grabs" the shaft of the instrument, the more force will be required to manipulate the instrument.

Quantification of seal friction is important in designing the highest quality trocar seal, according to Ahrenholtz. Seal friction is measured as frictional force (lbs), which is directly related to the coefficient of friction (COF) of the material. For the most part, MR&P uses COF (measured using ASTM D1894 test) for comparisons, because it is a sealing system measurement rather than a measurement of the property of the material.

When a surgical instrument or cannula is moved through a trocar seal, the inertia at the start of the movement is different from the inertia present when the instrument is already in motion. This difference between the static (starting) COF and the dynamic (moving) COF varies tremendously by material, application, and--most of all--by the lubrication state or coating, and is a central element of the "feel" of a trocar, according to Ahrenholtz.

"Contrary to most thinking, sliding a rod (cannula) through a seal is never smooth," he says. "While it may feel smooth, in reality, it is not. There is always a stick/slip force that is present where the seal flexes to accommodate the cannula's movement, which then rebounds back to a stable state. In general, the smaller the difference between the static and dynamic COF, the smoother the inertia feels. The best seal design and the most appropriate material formulation will minimize this stick/slip phenomenon. However, it is important to note that some degree of friction will always be present in all seals used for this purpose.

"Surface finishes and coatings on the trocar seal material can substantially reduce the coefficient of friction," continues Ahrenholtz. "The smoother the surface, the greater the coefficient of friction. A matte finish can greatly reduce the amount of friction on a very smooth surface. However, maximum friction reduction is achieved with the correct coatings applied to the material surface. Depending on the substrate and its application, PTFE, parylene, chlorination, and MR&P's proprietary processes can reduce friction by as much as 90 percent."

Minnesota Rubber and Plastics (www.mnrubber.com) molds and assembles medical devices requiring high tolerance components that facilitate the function of the device. Design and materials engineering are among the key strengths of MR&P, which operates U.S. manufacturing facilities in Litchfield, Minnesota; Mason City, Iowa; Watertown, South Dakota, and Los Angeles, California. In addition to operating under a corporate-wide Environmental Management System registered to the International Standards Organization series ISO-14001, all of the company's facilities are certified to ISO 13485, ISO-TS 16949, and ISO 9000.

Precision Die Castings Meet Functional and Esthetic Requirements

Precision Die CastingMany medical products today require electronics shielding, lightweight materials with good structural properties, and finished parts with a pleasing cosmetic appearance. Twin City Die Castings Company (www.tcdcinc.com), a full-service supplier of precision aluminum and magnesium die castings to the medical industry, is able to meet these functional and esthetic requirements with a variety of custom-manufactured housings, display panels, heat sinks, and chassis. The Minneapolis-based company, which also operates plants in Monticello, Minnesota, and Watertown, South Dakota, runs fully automated die casting machines that range from 350 to 1000 tons in clamp force and are capable of producing components ranging in weight from a few fractions of an ounce up to approximately 15 lbs.

"Twin City Die Castings Co. will meet or exceed tolerances as defined by NADCA (North American Die Casting Association) in their Product Specification Standards - Precision Tolerances guidelines," wrote Andrea Mudrey, the company's marketing coordinator, in an email to Design-2-Part Magazine. "Additionally, all of our manufacturing plants are TS-16949 certified, and we operate to the highest industry standards for quality and accuracy."

Twin City Die Castings (TCDC) complements its die casting specialty with secondary operations that include vertical and horizontal CNC machining, drilling and tapping, and finishing. Through outside sources, TCDC can offer a variety of coatings. At the front end, the company provides a number of key pre-production services, including part design and prototyping.

"During pre-production planning, we are able to identify the critical design elements, and provide a solution that incorporates them into the design," said Mudrey. "We won't hesitate to build or reconfigure equipment to support customer requirements."

Twin City's engineering services are evident from the initial design review, when its engineers help to ensure that the part design incorporates "all of the capabilities and advantages of a die casting." At the same time, the design review aims to decrease manufacturing costs, reduce time to market, and ensure that the right material for the part is used. In addition to CAD and product design assistance, Twin Cities offers complete simulation packages to identify metal flow, distortion, and thermal issues.

"Twin City Die Castings Co. is unique in the custom die castings business because we have a PhD metallurgist on staff whose role is to work directly with metal needs," said Ms. Mudrey. "Having this expertise, we offer advanced capabilities in helping our customers select the right alloy for their application."

Engineering continues through a complete project management process, in which a five-person project launch team--including engineers in die casting, machining, quality, and sub-contracting, as well as a project manager that tracks every step--integrates new work into the company's three plants. Being able to draw upon the company's more than 90 years of die casting experience is a big asset, according to Mudrey. "We understand what it takes to move a concept from prototype to production," she says. "Dedicated personnel work through the launch effort with the customer to develop the right solution, keeping within their required timeframe and budget."

During production, the company uses robotic cells to maximize efficiency, and automated machining and inspections (such as infrared and optical comparators, as well as coordinate measuring machines) to control accuracy and quality.

In one example of an engineering challenge that Twin City was able to solve for a customer,

Twin City Die Castings converted a part--previously designed for multiple processes--to die casting. Previously, the part had been manufactured using plastic injection molding and sheet metal stampings, and it required assembly. Twin City's solution allowed the customer to eliminate a number of components, and its assembly, with a single casting that also eliminated the need for shielding.

Twin City Die Castings Co. has received numerous awards in recent years from NADCA, IMA (International Magnesium Association), and AMC (American Metal Castings Consortium) for design, safety, and industry development (R&D). In 2009, TCDC won a total of five design awards from NADCA. The company also won an appreciation award in 2010 from AMC for its efforts in industry development and involvement.

Pressure Formed Enclosures Have Look of Injection Molding

Thermoforming

Another process used to manufacture housings for medical products is pressure forming, a thermoforming process that uses high-pressure air assist to achieve better part definition than vacuum forming. Freetech Plastics (www.freetechplastics.com), an ISO 9001:2008 certified thermoforming specialist in Fremont, California, has been providing precision pressure formed parts to the medical, scientific, telecommunications, and electronic enclosure markets for more than 25 years. In addition to pressure forming, the company employs vacuum forming, CNC machining, painting, and finishing to produce enclosures for surgical devices, laser systems for skin and dental procedures, and systems for vascular procedures.

"We manufacture custom plastic enclosures to enhance the look of the device," said Freetech Vice President of Business Development Jack Stritch in an email to Design-2-Part Magazine. "Most are assemblies, with large footprints in medium production volumes. Many have special considerations of look, finish, fit, and operating conditions."

According to Freetech, parts produced by pressure forming show sharp definition and a high-end appearance comparable to structural foam or injection molding, but with a much lower tooling cost. They can also incorporate high levels of design complexity--deep undercuts and contours, as well as fine detailing--as a result of advances in tooling and process technology.

"Our biggest strength is being able to successfully produce challenging projects or complex design concepts," continues Stritch. "Our motto is ‘We'll make it like you want it,' so we never ask the designers to compromise on their ideas. Rather, we utilize our many talents to bring those designs to life. We do so by offering mold design and engineering, part design assistance, and in some cases, even contract design and mechanical engineering service."

Mold design and engineering are included, free of charge, for any part that the company is asked to quote. Freetech also offers material selection assistance and finishing recommendations for paint, color, gloss, and texture.

"We have substantial knowledge and experience in the areas of plastics," says Stritch. "Our engineering support staff boasts more than 20 years each, on average, in a variety of manufacturing methods of everything from common commodity plastics to engineering thermoplastics. We've even worked with major materials suppliers to develop new materials for a variety of markets.

"The materials we work with include low-toxicity plastics, high-temperature materials, electronics-specific sheet, and plastics that offer hydrocarbon, ultra violet, and chemical resistance," he continues. "These options enable us to manufacture products for the aerospace, transportation, and medical device markets by providing low-cost solutions to the engineering challenges many of our customers face. Obviously, our experience in these areas provides a broad knowledge base from which our customers get materials recommendations."

One of Freetech's customers designed a vascular surgery device that needed to be produced in a single form, without bonding two "halves" together as other companies had suggested. According to Stritch, Freetech developed a molding technique to enable forming of a part that violated draw ratios which, until then, had been thought to be impossible.

Parts coming off the production line at Freetech are held to "the tightest tolerances available in the industry," according to Stritch. "We can typically hold tolerances that exceed the customer's expectation for any machined surfaces, and typically, the only place where tolerances fluctuate (closer to standard thermoforming tolerances) is due to the shrink and thermal characteristics of the polymer," he says. "Typically, +/- 0.005 inch to 0.010 inch are general machining tolerances for us."

Stritch says the company's CAD/CAM software allows the company to efficiently program its 3- and 5-axis CNC machines to obtain tight tolerances. "This allows us to recommend design considerations that engineers and designers use to make their products look and fit better," he explains. "Productivity is enhanced by periodically making changes to CNC programs to do the job faster and better."

Freetech's thermoforming achievements have been well recognized throughout the industrial design and manufacturing industry. The company has received multiple awards from industry organizations, including five "People's Choice" awards from the Society of Plastics Engineers (SPE), according to Stritch.

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