Hello all (long time, no see!) I have a query for the massed ranks of engineer / material science / designers out there... I'm designing a product with a seal which is to be compressed when not in use. The production material will be SEBS, but for a prototype, we silicone molded one from a Shore 60 A PU resin. It works very well, but when left compressed for a long period of time (over night for example), we noticed that the seal is slow to return to its original shape. This means that when you need to use the seal again, the compression isn't as great and air can escape. The first time we can test the production material itself will be when we have injection moulded production samples - by which time if the performance is found to be substandard, it will be too late. Is there a way i can work out given the material properties and seal profile what the memory retention is likely to be? Is it as simple as specifying a high modulus of elasticity? Or is this a case for some fancy CAD simulations? Thanks in advance Pete
Hi, I used to work with polymers and thermoplastics and I found a great reference for these materials. It's Polymer Foams Handbook:Engineering and Biomechanics Applications and Design Guide by Nigel J. Mills. You REALLY should take a look at this reference. Half your problem is understanding stress relaxation and creep. Stress relaxation is when a material is exposed to a constant strain at a certain temperature (often relatively high, which for thermoplastics really isn't) and the stress decreases over time. Creep is when a material is exposed to a constant stress at a certain temperature and the strain increases over time. You have stress relaxation when you consider your seal to be tightened and squeezed. At first, you impose a certain strain which will stay constant. The seal will adapt to the strain and the stress will decrease over time. However, at some point the seal will tend to retain a constant amount of stress and will start to compress further, relaxing the stresses in the bolts, this will go on until an equilibrium has been reached. Now when you untighten your assembly, the seal will immediately restore a portion of the strain it had. This amount depends on the yield limit of the material and it's geometry, and if the yield limit was surpassed. You could use LVDT sensors to obtain your joint deflection during tightening and compare the strain to the yield strain of the material. The strain exceeding the yield strain will be partially restored over time but residual strain/stress will remain, increasing the yield limit of your material. This model can be compared to a spring-damper model. Force = Material stiffness (spring) X displacement + Material stiffness (damping) X rate of displacement or in other terms Stress = Young's modulus X strain + Damping constant (different) X strain rate To identify the constants you could use basic material properties and run a few compression, decompression tests to adapt your model to better approximate the seal's behavior. After this, all you have to do is identify on you model at what residual strain, or after how much time, it is alright to reuse the seal. Hope this helps, P.S. : PM me if you want further information on the reference.
Hi again Pete, About my reference, it centers more around foams and I don't know why I thought this was your case. However, the same mechanics I explained apply. If you want more information about material properties, polymers and strenght of materials, I could recommend a better reference.
Hello Pete, In using A-60 PU to prototype a gasket, you will need to make sure the material is fully post-cured after casting, and then some more for good measure, before trying to test it out. I would recommend 36 hours minimum at mfr's max post cure temp in the mold or fully supported. Then the material will not compession set so easily when tested. Also make sure to use top quality resins; as this always seems to make a huge difference. The material will be more sucseptable to compression set if subjuct to any heat, as all thermo-set resins have limited heat deflection temps. Hope this helps.
