TPE and silicone (especially liquid silicone rubber, or LSR) are both materials that are often considered for fluid-sealing interfaces due to their elastomeric/rubber characteristics. They both share a certain degree of reliability in sealing applications due to their resilience to fluid and material flexibility.
How should one choose TPE vs. silicone to create an optimal fluid-sealing interface?
Although silicones have characteristics that make their sealing functions more resilient when heat and constant pressure are present, the long-term benefits of TPEs (e.g., fast and cost-effective manufacturing) can be attractive especially in high volumes.
Therefore, for consumables where the interface is used only once or a few times but are consumed in large volumes (such as on the disposables / cartridge side), finding a TPE solution that meets just the right requirements may be a better option to optimize production costs. If the component is for longer-term use under extreme heat / pressure with less frequency to replace (such as on the instrument side), silicone may be the option to meet the harsh requirements. Despite such generalities, material choice should depend on specific conditions of each project, and we are always open to inquiries.
For projects eyeing eventual scale-up, identifying a TPE solution at the onset of your development stage could save efforts and costs, as opposed to starting with an all-encompassing silicone and switching to TPE at a later stage.
Below are some general comparisons between the two, with special attention to life science applications.
TPE (Thermoplastic Elastomer) Raw Material
LSR (Liquid Silicone Rubber) Raw Material
High Temperature Resistance
Medical grade TPEs can be heat resistant to a certain degree (e.g., TPEs are used in PCR applications where the temperature could temporarily reach above 90°C [194°F] or applications requiring autoclave sterilization at around 130°C [270°F]). Exposure to a higher temperature or for a longer time at a certain temperature could affect the sealing function.
Has a high heat resistance, not significantly changing its elasticity and sealing properties in temperatures kept constantly as high as around 200°C (400°F).
Low Temperature Resistance
TPEs can typically withstand cold environments such as cryogenic storage at around -30°C (-22°F). Morphologic change could begin around -60°C (-76°F) where TPEs could harden, which, depending on the structure of component and application, could either work better or less effective in terms of sealing. The hardness will be gradually restored to the original state as the temperature returns to room temperature.
Colder temperature does not generally affect its sealing performance down to around -50°C (-60°F).
Each variety of TPE has a certain compression set, meaning that exposure to pressure under certain temperatures for a long period could reduce its sealing function.
Silicone has a tendency to return to its original form even after long exposure to pressure, making it almost free from compression set.
Just like rigid plastics, solid TPEs can return to liquid form upon heating. Thus, TPEs can also be recycled in general.
Once solidified, LSR does not return to a liquid state, making it impossible to recycle.
Injection molding: pellets of raw material are heated to melt, instantly injected into a mold cavity, and then subsequently cooled down to form a desired shape. Shares the same generic equipment and process used for standard thermoplastic injection molding with additional know-how required for TPEs to be produced properly. Injection molding machines come in different tonnage (how much clamping pressure it can apply to a mold), but the same machines can be used for various hard plastics and TPE materials.
For large volume production, LSR can be injection molded. Temperatures work the opposite from TPE injection molding. Two materials are constantly mixed in a cooled temperature, and then heated in the mold to solidify in desired form. A customized injection molding machine is required for different materials.
“Cycle time” is the time it takes for the injection molding machine to complete one cycle of part production from melting raw materials to cooling them as a solid part. Actual cycle time will depend on size, material and complexity of each part, but in general the process is simpler than LSR injection molding and ranges within the lower tens of seconds per cycle. “Multi-cavity” molds varying from two to more than sixteen cavities can produce copies of parts simultaneously, allowing the same cycle time to provide more parts.
Injection molding LSR generally takes longer than molding TPE as there are additional steps to LSR injection molding due to the different chemical reactions that take place. For example, the first cooling process for LSR is to mix two materials for the catalyst to cure the material. This is a more complex chemical reaction than the first heating process for TPE, which is simply melting pellets. Due to this complexity, cycle time could reach minutes. LSRs can be processed with multi-cavity molds as well.
The raw materials of the TPE are stored at room temperature.
The raw material of the LSR requires refrigeration while in storage.
Assembly Process (Overmolding)
Certain TPEs are formulated to chemically bond to rigid plastic substrates, making it an ideal material for overmolding/two-shot molding. Having both chemical and mechanical bonding features could increase the seal strength.
There are limited silicone materials formulated to stick to plastic substrates. Overmolding with mechanical gripping features are still possible. However, since silicone molding process is done in high temperature, the other plastic component may be affected if it has low heat resistance.
In addition to the above-mentioned production method, cycle time, and material storage requirements, TPEs have lower density / specific gravity per cm3 than LSR, producing more parts per same amount of material compared to silicone. Such elements make TPE’s production costs generally lower than injection molding LSR.
Due to the requirement of investment in customized equipment, longer cycle time, refrigeration facility, higher density/specific gravity per cm3 than TPE, and in some cases additional assembly process instead of overmolding, the overall production cost is usually higher than injection molding the same figure by TPE.
Debating whether to use TPE vs. silicone? For over 20 years, Enplas Life Tech has worked with industry leading clients to solve challenging TPE manufacturing problems, resulting in robust prototypes and industrial-scale manufacturing of the final product. With dedication to meticulous design review and engineering, tool building, injection molding, and assembly, Enplas Life Tech will help you speed up product development and accurately manufacture your fluid-sealing interface.