Colin Macqueen/Director Technology/Trelleborg Sealing Solutions
Today’s medical devices require seals to contain fluids, exclude contaminants, protect electronics, and assist in metering doses of medication. It’s common for manufacturers to complete the design of a device, including the seals, and then seek several competing quotes to find the best possible price for the seals. But this might not be the best approach to using the seal supplier’s full capabilities when developing an optimal sealing solution.
If everything fitted perfectly in a static application, it is quite possible that no seals would be required. But things don’t always fit perfectly, which makes seals necessary. In addition, the right tolerances make the seal work. Early seal supplier involvement in the design process allows for a better understanding of the entire tolerance scheme and a more reliable system. For example, instead of requiring tight tolerances on the plastic case of an insulin infusion pump and sealing it with an O-Ring or thin gasket, a taller gasket design will be self-retaining in the housing and accept a greater out-of-flatness on the plastic parts, thus reducing total cost.
The miniaturization of devices leads to the adoption of smaller seals which are harder to consistently install. For example, a recent request was for an improved gasket design on a battery pack for a cordless surgical hand piece. By replacing the homogeneous elastomer parts with a metal form carrying an edge-bonded elastomer gasket, the customer easily handled the rigid gasket and assembled it more accurately every time.
Design for manufacture
Liquid silicone rubber (LSR) has become a leading option for medical device sealing, due to its cleanliness, inertness, and capability to form complex, detailed shapes. An early consultation with the seal supplier’s engineering team can avoid overly complex manufacturing challenges and use the full potential of LSR. Allowance for mold-parting lines, features to aid in robotic demolding, avoidance of deep undercuts and other such details are best considered at the beginning of the design process.
Seals often form part of a sub-assembly prior to final device assembly. A capable seal supplier, especially one that produces rubber and plastic parts, can produce the whole sub-assembly and in many cases reduce the number of parts required by producing a single, co-molded component. For example, two-shot manufacturing can produce a nylon syringe pump piston with an LSR seal in a single molding operation and a single tool. This reduces:
- Tolerance stack-up and hence variation in friction
- Additional handling steps
- Potential leak paths
- Potential failure modes due to misassembly
As patients use more medical devices, it is vital to make them as easy-to-use and as foolproof as possible. For example, we recently worked closely with a supplier of an infusion pump for treatment of chronic disease. To ensure simple and secure tubing connections, the conventional EPDM O-Ring is now supplied with a friction-reducing ISO 10993/USP 1031-compliant coating that reduces insertion force and eliminates the risk of incorrect or incomplete connections.
Seals are critical elements in many medical devices and working with a supply partner who goes beyond “make-to-print” can truly pay dividends in the long run.