Manufacturing TPE Fluid Seals
Sensitive applications demand close attention to critical (and often small) features. An effective fluid seal between components can only be achieved when both the thermoplastic elastomer (TPE) fluid sealing interface and the bonding components are designed and manufactured with careful attention to key elements.
TPE Material Choice
There is no one TPE material that solves all problems for all applications. Manufacturers must work closely with their TPE material provider to select the right material for their application. Important factors to consider include:
- Combination with the material composition of the bonding component: Effective adhesion between the TPE fluid sealing interface and the bonding component require the proper, most compatible combination of component materials.
- Hardness (or “shore hardness”): The softer the TPE material, the more flexible and compliant it is to accommodate surrounding substrates when under pressure. This creates a more effective seal. However, softer materials could deform under internal or external forces, causing issues like occlusion, which negatively affect flow characteristics. While harder materials produce robust geometries, they may require excessive compression forces to form an effective seal. This could have negative effects when dealing with delicate substrates.
- Compression set: The extent to which the TPE seal reverts to its original dimensions after being pressurized at different temperatures varies among different types. If the TPE does not fully recover its original dimensions, the amount that is lost is referred to as its “compression set.” This is commonly expressed as a percentage (e.g., “0%” means the TPE will fully recover its original dimensions). The compression set of a specific TPE material, combined with its intended use, can provide an indication of the TPE sealing interface’s life expectancy. Design of the TPE fluid seal is therefore critical to optimize this potential limitation, if repeated usage or prolonged pressure with or without temperature variation is expected.
- Thermal durability/service temperature: See Temperature and Sealing.
- Chemical resistance/compatibility: This is determined mostly by the application and the device function. In some cases, due to characteristics of either the reagent and/or the TPE material, the TPE fluid seal could absorb some amount of fluid. Inversely, some matter could leach into the fluid from the TPE, negatively impacting the reagent. Sample material such as nucleic acids (e.g., DNA or RNA), protein, blood, and cells, and reagents including enzymes, solvents, buffers, fluorescent labels, or detergents, must neither stick to nor damage the TPE.
Complex shapes, small features, intense pneumatic pressures—manufacturing a stable fluid-sealing interface can be complicated. Download our TPE fluid-sealing guide to learn how Enplas Life Tech can help.
Some of the design considerations described below go beyond basic design support provided by any manufacturer, called “Design for Manufacturability” (DFM). Typically, basic DFM focuses on achieving a manufacturable assembly. At Enplas Life Tech, however, the DFM process goes above manufacturability to actually improve product performance. In that sense, DFM at Enplas might better be called “Design for Functionality.”
- TPE fluid seal design: Regardless of whether or not the TPE fluid seal chemically adheres to the bonding component, supportive mechanical features may be required to strengthen the bond. Design features in the bonding component such as interlocks or undercuts can provide a strong mechanical grip between the materials. For TPEs that can chemically bond to the bonding component, a process called overmolding is used. When a bonding component has multiple interface areas, it may be more effective to consolidate the TPE as one component rather than create different TPE fluid seals for each. Such modification requires significant engineering expertise since the shape will be more intricate with wider surface area to bond.
- Bonding component design: In some cases (e.g., when there is motion affecting the TPE fluid seal), there could be features added to the bonding surface to prevent possible delamination and further leakage.
- Tool/mold design for the TPE fluid seal and bonding component: Expert tool design for both the fluid seal and bonding component is critical to ensure the assembly functions as the design intended. This is because thermoplastics do not always process in a perfectly uniform manner. Proper tool design can control thermoplastic material flow and keep imperfections at a minimum. For example, a simple choice as to where to place the “gate” (where the thermoplastic material enters the mold) could significantly impact fluid-sealing performance.
It is critical to select the right manufacturing partner with the experience and foresight to handle all critical production processes, so that the end product achieves its full potential.
- Mold-making for overmolding: Two molds are required for the fabrication of overmolded products. The first mold is for the rigid plastic substrate. The second mold is the one that the rigid plastic is placed in to be overmolded with TPE. This second overmolding tool is complex and requires specialized expertise to fabricate and use, as fitting and aligning the rigid plastic substrate into the mold requires precision. This precision ensures the TPE is overmolded in the right position. Delicate control and intricate shut-off systems are required in this process since the TPE pushes against not just stable steel but also a plastic part. Knowledge (of controlling the flow of TPE materials) and manufacturing capability (to construct the precision tool) are both critical to making a reliable mold for quality overmolded components.
- Quality of injection molding: If the proper processes are not practiced and parameters followed during injection molding, the components produced will not reflect the intended design. Refer to TPE material manufacturer PolyOne’s guide TPE Injection Molding for the wide range of expertise and consideration required to mold TPEs appropriately.
- Quality control/metrology: For applications requiring precision sealing, any minor defect in the shape can be detrimental. An efficient quality assurance system is required to ensure high-volume production without failure and delivery delays. Proper process validation should be conducted so that quality is systematically ensured. As part of the process validation, an optimal inspection processes should be set up, including appropriate checks at certain frequencies with traceability. High precision metrology and advanced equipment applied to measure and inspect parts should be prioritized.
It is a challenging science to balance the characteristics of various materials to produce a high-performance TPE fluid seal. It requires engineering knowledge, precision manufacturing techniques, and experience to achieve a reliable solution with short development time for robust mass production. To learn more about how Enplas Life Tech can assist you through this process, contact us today.