Pressure Sensitive Thin Membrane – Material selection
Thin membranes have always been sought for pressure detection devices, such as a pressure sensitive diaphragm. The thinner the membrane is the more it will be able to react to the slightest changes in pressure. Not only is it crucial for the membrane to have the smallest thickness possible, but it also must be able to operate within a broad range of temperatures that a pressurized gas may experience in different applications. Finally, the membrane must have a long life span and be resistant to the constant fatigue it will undergo.
Some materials used for thin diaphragms are made out of natural rubber and TPEs (Thermoplastic elastomers). Although these materials provide the elasticity required for the application, the average operating range of natural rubbers is around 150 degrees Celsius. The low operating range limits the applications that the natural rubber membrane can be applied to. Additionally, both natural rubbers and TPEs become brittle with time. The aging of the material will cause improper function and premature failure such as unresponsiveness and even cracking. Another characteristic with natural rubber and TPE is mold flow, as both do not flow well to form thin walls. With inadequate material flow the membrane may either be inconsistent, create pinholes, or not fully form which would cause a critical error to operation.
Many of the material problems that hinder natural rubber and TPE do not exist in silicone. Liquid Silicone Rubber (LSR) is a preferred replacement in thin membranes because it addresses all three of the problems: operating temperature range, brittle lifespan, and cavity mold flow. Silicone has an operating temperature threshold well into 250 degrees Celsius and is still able to perform at sub 0 degrees Celsius temperatures. Over time, silicone will not become brittle like its counterparts, and it does not degrade with constant cycling. Finally, the silicone has a respectably lower viscosity, so it will flow better in the mold, creating a more consistent thickness. With a broad durometer range available, typical diaphragms are made with a 30-50 shore “A” grade LSR material. Another key factor leading to the selection of silicone is the ability to be molded into a thin membrane at great lengths consistently. For example, a silicone diaphragm with a 50mm outer diameter is able to be 0.1mm thick while holding a ±0.02mm tolerance.