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Plastic Injection Molding

Silicone Over-molding Achieves Design Goals for Impact-resistant Dental Tray/Tool Holder

WAYNE, Pa.—New processing technology and shortened time-to-market have helped make the over-molding of silicone onto engineered plastics an increasingly popular choice for the medical device market. An example of this trend is the manufacturing evolution of a recently introduced medical device, which also demonstrates how a turnkey contract manufacturer’s experience can enhance the benefits of manufacturing science and technology. 

A major manufacturer of restorative and esthetic dental devices and systems recently developed a combination presentation tray and tool holding system. The new product and its introduction presented numerous production challenges, including program timing, material selection, color matches, assembly, and silk screening. Besides requiring various tray/cover sizes and assemblies, the company also needed a tray/holding system that could withstand repeated point-of-use sterilizations.

The Experience

For this project, the polymer vendor supplied the science and ATP Engineered Rubber and Plastics Group, a global contract manufacturer with expertise in plastic, rubber, and silicone  injection molding, provided the technology and experience. A specialist in two-shot molding, over-molding, and insert-molding, ATP Engineered Rubber and Plastics Group also offers clean room and white room capability, assembly, and packaged product programs for the medical device and health care custom commercial markets.

The ATP Elkhorn facility, Elkhorn, Wisconsin, was selected as the lead manufacturing source for this project because of its capabilities for silicone and rubber over-molding and insert-molding, as well as its experience in the molding of engineered plastics. ATP routinely molds a variety of polymers, such as polyphenylsulfone (PPS), including RADEL®; polyisoprene; polyisobutylene (Butyl); nitrile rubber (NBR, Buna); ethylene propylene diene monomer rubber (EPDM); fluoroelastomer; and silicone.

The Science

Polyphenylsulfone was chosen as the engineering plastic for the tray and cover because of its clarity (for the cover), coloring characteristics (for the tray), minimum sink rates, ability to withstand repeated steam sterilizations, and its exceptional toughness and impact resistance. Another advantage of the polymer is its ability to tolerate the heat produced during the silicone over-molding process.  

The tray was designed to have a soft outer surface for esthetics and impact resistance. To achieve this soft texture, silicone is overmolded onto the plastic tray. Silicone was specified as the over-molding material because it readily accepts coloring agents and, more importantly, can maintain material integrity during sterilization on a commercial or point-of-use basis. Silicone also has excellent color stability, an important esthetic property.

Silicone polymers are playing an increasing role in today’s health care market because of their manufacturing versatility, unique chemical structure, and improved mechanical properties. Silicone maintains its functional characteristics in both low and high temperature ranges, is resistant to chemical and ozone attack, and is resistant to sterilization systems. Silicone is one of the most extensively tested materials for biocompatibility. Its inert properties make it suitable
for implantation in the human body for periods greater than 29-days.

The Technology

ATP designs and builds its own tooling systems, a capability that minimizes lead-times and reduces time-to-market. Mold design of both the silicone and plastic tooling is also important because silicone tends to flash, even with excellent shut-off systems and clamping forces. ATP offered the medical device customer the tri-capabilities of tool design, plastic molding, and silicone/rubber molding from a single source. This minimized lead-time and enhanced quality because the control remained with a single source.

An early decision made by ATP Elkhorn was the type of delivery system that it would employ to transfer the liquid silicone to the molding press. The method of accomplishing this transfer has significant implications on machine utilization and efficiency. One of the factors that entered into the decision was that the annual demand forecast for this product was considered less than optimal for a press that employs a two-shot molding system, which is geared for very high production rates. ATP engineers determined that a more efficient approach was to mold the plastic trays and then transfer them to the liquid silicone injection machine in a batch operation.

The next manufacturing step involved part transfer between presses. Although ATP Elkhorn has automated pick-and-place robotic arms available, the decision was made by the manufacturing engineer to manually transfer the trays from the plastic injection press into the liquid injection molding unit. This better suited the production process because different size trays were being molded. With an eye for efficiency, ATP met the customer’s budget and unit cost drivers by working within volume constraints.

Achieving a robust chemical bond between rubber and plastic, or rubber and metal inserts, requires a thorough knowledge of material compatibility and bonding technology. A self-bonding liquid silicone was selected for this application because it does not require the use of an adhesive.

ATP molds the clear plastic cover at its sister plant in Monticello, Iowa, because of the size of the cover and the facility’s in-house silk screening capabilities. Final assembly and packaging are also carried out at this location. Adding to the synergy, ATP Monticello also designed and produced the tooling.

For more on ATP Engineered Rubber & Plastics Group, visit www.medplastgroup.com.

RADEL is a registered trademark of Solvay Advanced Polymers.

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