The Challenge:
A silicone rubber seal design issue was perceived as poor workmanship, manifested as visible gaps in the part assemblies of a household consumer appliance, potentially harming the brand image. Directly competing design requirements were at play. A high seal compression force was required to maintain the required internal pressure and contain the liquid spillage. At the same time, the seal compression force needed to be low enough to be generated by a typical user and not cause excessive mechanical deformation on the molded plastic parts of the assembly.
The Solution:
Figure 1: Custom seal design and optimization
The device needed to form a good seal against several types of flexible multi-layer plastic materials at all operating conditions such as high-low temperatures and pressures. The project truly needed a multi-pronged design approach to account for all the design variables such as operating conditions, variations in the material properties, tolerance stack analysis, structural rigidity, frictional interaction between the seal and the mating parts, etc. SES supported the seal design effort primarily through computer modeling and simulation (CM&S) expertise or the “Design by Engineering/Analysis” approach. SES created a design envelope for the relevant design variables through material testing as well as parts and assemblies testing to establish the worst-case scenarios. The worst-case scenarios were evaluated using CM&S (finite element analysis or FEA) through which SES provided critical understanding of interaction among various design variables. The information was further used in discussions with the appliance manufacturer to formulate design improvement solutions. 
A resolution was obtained to reduce seal compression force without compromising the sealing performance by iterative FEA on seal redesigns. The redesigns focused on changes in the seal geometry, seal material properties, and seal interaction with the mating parts. Through analysis, SES optimized the seal geometry by removing redundant material and refining the critical sealing features for the seal performance. SES was able to show that the compression of redundant material and friction adhesion between the rubber seal and the steel mount were significant contributing factors to high seal compression force. The frictional forces were reduced by reducing the contact area between the seal and the mount. SES also recommended PEEK (poly-ether-ether-ketone) as an alternative material for the seal mount to further reduce the frictional forces. Alternative material recommendations were provided based on the analysis results and other design requirements, such as structural integrity and stability over the product’s life cycle.
Figure 2: Effect of seal optimization and friction on seal compression force
The reduction in seal compression force directly reduced the gaps in the part assemblies by 50% of the target reduction value. The next 25% of the gap reduction was achieved through improved structural rigidity of the plastic parts around the seal. The remaining 25% of the gap reduction was achieved by specifying tighter tolerances on the assembly parts.
Rubber Seals in Medical and Drug Delivery Devices
Custom Rubber Seals
Rubber Seal Adhesive
Rubber Seal on Machinery
SES Force-4 supports seal design and development not only for consumer appliances or products but also across multiple industries such as medical and pharmaceutical devices. Rubber seals have been an integral part of medical and drug delivery devices, especially in the form of sterile barrier systems to ensure the sterility of the devices, sensors, and/or drug products until the point of use. These seals come in various shapes, sizes, and types of rubber material depending on the application. Design and documentation of these seals becomes even more challenging due to the sterilization methods being used and the FDA approval processes involved. SES Force-4 has helped several clients in the medical and drug delivery device industry to achieve robust sterile barrier systems and looks forward to continuing to take on technical challenges in this space.
THE SUCCESS
SES Force-4 functioned as a natural extension of the client’s design team to achieve the seal design goals. The client was not only able to successfully launch the appliance on the market but also use variations of the same seal design on several other appliances from their product line-up.
Author
Mandar M. Kulkarni, PhD – Engineering Consultant
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