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Molding

ASK THE EXPERT

Molding

Fred Carr is vice president of Plastic & Metal Center Inc. (Laguna Hills, CA), a multiservice provider to the medical device manufacturing industry that he cofounded in 1993. The company provides a range of services, including CAD 3-D design and modeling, CNC machining, thermo-vacuum forming, injection molding, fabrication, and assembly and finishing. Carr has a background in plastics and metals processing.

Although molding and assembly applications represent discrete processes, how is the molder influenced by the assembler’s choice of adhesives and assembly operations?

Carr: It is up to the molder and the engineering team to have a good idea about how a device is going to be assembled. To save manufacturing time, molders should involve themselves at the design stage to help decrease subsequent assembly problems. Consequently, molders must consider the types of adhesives that will be employed during assembly. While adhesives have always been a great friend to the medical device industry, they also present challenges. For example, the use of new bonding technologies or materials creates a smaller window for the selection of plastic materials that molders can use, because molders must ensure that the adhesives that eventually will be selected will fuse the molded parts.

When materials are selected for a given product, engineers can choose from an array of adhesives, including epoxies, urethanes, silicones, cyanoacrylates, and solvent-based systems. However, adhesives must also meet strict biocompatibility standards. In addition to employing traditional adhesives, manufacturers can choose other bonding technologies, such as curing at ambient or elevated temperatures or the use of UV-cured solvents and adhesives.

More and more designs are moving toward the use of ultrasonic or radio-frequency welding. By employing these processes, engineers can calibrate, measure, and repeat the diffusion process consistently. These processes also open the door to other materials that may be able to meet medical manufacturers’ specifications. For example, many adhesion methods meet USP Class VI medical specifications protocols. The use of ultrasonic welding or RF systems enhances production efficiency while helping products to retain their structure when they are exposed to sterilization procedures such as radiation, EtO, autoclaving, or cold sterilants. However, many variables—including the types of fluids, stress, and environmental conditions that materials will be exposed to—must be addressed to ensure optimal performance. While different technologies are available to meet specific parts designs, it is incumbent on engineers—starting at the molding stage—to pick the right processes that are suitable for final assembly.

The introduction of new techniques, production processes, and materials is good for the medical device manufacturing industry, but implementing changes is challenging. That’s why all industry specialists—including molders and assemblers—should be educated in the use of new materials and strive to work together.

How has the rise of ceramic and metal molding influenced medical device manufacturing?

Carr: With increased demand for sophisticated designs, ceramic and metal injection molding applications are becoming more prevalent. Because many medical device engineers are hoping to use these processes to manufacture their products, the technologies are improving rapidly. While the cost of implementing these processes is still an issue, unit prices are falling as the production equipment becomes more efficient.

The use of ceramic materials has enhanced medical device manufacturing capability. For example, metal and ceramic molding processes are particularly beneficial for manufacturing machined parts with many complex contours. Composed of very tightly compressed aluminum oxide, ceramic materials are second to diamond in terms of substance hardness. They are used primarily for parts that require strength, hardness, and stiffness. Their only disadvantage is that because they are inflexible, they run the risk of breaking under stress.

Metal injection molding applications are being performed using standard plastic molding machines, with the addition of some postmolding steps. The technology can now produce medium-sized parts with fine details that are not geometrically possible using other machining or casting methods. In addition, the technology is efficient and cost-effective.

What are some challenges facing the introduction of new resins and other materials into the injection molding process?

Carr: The plastics industry is always working on resin cocktails that exceed the specifications set by medical device manufacturers, but it takes a lot of testing to introduce new materials to meet specific criteria. Since many companies use proven materials that have been available for a long time, they are either hesitant to use new materials that they are not familiar with or resistant to increasing their budgets to perform biotesting on new materials. Both resin suppliers and medical device manufacturers need to create an alliance and educate each other on material resources and on the needs of the medical industry to develop the next generation of materials.

A challenge facing the introduction of new devices has been encouraging resin manufacturers to come up with innovative mixtures such as polyarylsulfone and engineered thermoplastics made from glass-reinforced polycarbonate/acrylonitrile-butadiene-styrene (ABS) alloys, which are compounded and custom colored to meet aesthetic and functionality criteria. In this connection, biocompatibility standards are key: Resins must usually meet the ISO 10993-1 standard pertaining to the biological evaluation and testing of medical devices. While manufacturers consistently deliver resins to meet this standard, the biggest hurdle is to create materials with Pantone Matching System (PMS) colors. Precolored polycarbonates and low-density polyethylene compounds enable engineers to meet color-specification requirements.

How is the introduction of statistical process control (SPC) affecting molding equipment and the molding process?

Carr: A major improvement in molding machines is in the area of controls. New programmable logic controllers are more accurate than previous-generation systems and can deliver many data variables that are used in quality SPC to better control material flow and to deliver quality parts with the fastest cycle times possible. Control technology may move toward the implementation of refined fuzzy logic to help machines self-calibrate and control processes more efficiently, thereby consistently producing accurate parts.

The maintenance of process and quality control has always been an issue in the molding sector. In the old days, people used to think that once the machine is set and running, it will make the same part over and over again. Nowadays, industry engineers know that process control is an ongoing procedure that is required to mold any part. That’s why more and more machine manufacturers are refining their controllers to meet SPC requirements. In fact, process control actually does reduce costs because molding manufacturers end up producing very few rejects, thus transferring their cost savings to the customer.

Modern production efficiency requires the use of control technologies that can measure injection pressure, clamp force, screw speed, clamp-open and clamp-close speeds, and ejector pressure. Control systems now usually meet Six Sigma functionality, which employs a deviation band for monitoring machine functionality and provides alarms to keep all parameters within allowable tolerances. Meeting Six Sigma process control standards is key to deploying new equipment because it helps molders to provide point-to-point progressive parts control while increasing productivity, parts quality, and energy savings.

What breakthrough molding techniques hold future promise?

Carr: The medical device manufacturing industry would benefit from the expanded use of multishot molding, which is employed heavily in the automotive industry. This technology’s greatest advantage is that it can reduce production costs by eliminating the need for multiple components and assembly procedures. Multiaction machines would produce different materials and diffuse them together. However, the medical industry must first take an interest in the technology with an eye toward enhancing existing production processes. Sumitomo (Norcross, GA) offers all-electric double-shot machines that are precise and efficient. The design permitting, the deployment of double-shot machines would help maintain cost-efficiency and would also perhaps eliminate the need for secondary assembly.

Another potential breakthrough area is small-parts molding for components weighing 0.1 to 3.0 g. Small machines with a flat-clamp mechanism manufactured by Nissei America Inc. (Anaheim, CA) perform this technique. To date, this technology is still the exclusive preserve of micromolding.

Do you have a question for next month’s testing expert? Or do you have a question on another topic that one of our experts could answer in a future column? If so, e-mail MPMN managing editor Bob Michaels at [email protected].

Copyright ©2008 Medical Product Manufacturing News
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