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REQUIREMENT: The code requires that fusible devices operate at no hotter than 250$% Customers want the device to work at as high a temperature as possible to prevent accidental releases. DEVELOPMENT PROGRAM: To get the precise formulation of the plastic you need, you will almost certainly need to blend, or compound various plastics. To begin the process,you’ll want to make samples for testing using pure compounds that have desirable properties:LDPE and HDPE are good starts as the they are likely to have oprating samples for testing using pure starts compounds that have desirable properties:LDPE and HDPE are good starts as the operating temperatures below and above the 250F requirement and have very good chemical resistance properties. Given that polyethylenes are susceptible to damage from UV and the service is expected to be outside, the mix should include blacking to provide good U8 protection The maximum service temperature (no load) for LDPE is 180-212 and for HDPE it is 175-250, and we would expect the yield temperature to be higher than the maximum service temperature. Each should be tested: then the fist guess at the final compound will be determined as follows: For example: if the LDPE fusnut yields at 220 degrees and the HDPE fusnut yields at 290, then the percent of LDPE to try=100%*(250-220/(290-220)=43%. So a blend of 43% LDPE and 47% HDPE would be the next best formulation to try, To account for possible non-linearities, it would be best to try the calculated blend and one with 5% more and less LDPE: in the example given, we'd work with the base 43/57 blend and also with 38/62 and 48/52. Each would then be tested to ensure proper activation at the required 250 degree temperature. 9 certain degree of trial an error is needed in this process: you're working to make a compound that meets your specific needs and so tuning in to a specific formulation is a necessary step. Note: it is important that you do a temperature test periodically and also whenever you're considering changing the specific plastic blends (such as new colors or additives) or even changing any features on the metal insert. Small changes may greatly impact the yielding temperature and as this is not a well defined property in the polymer formulating world. In the Plastic Engineering Handbook I use, I can find maximum-use-temperature(no load) when the material is designed to be sufficiently solid to use and references to recommended temperatures for injection molding where the material is designed to be sufficiently fluid to mold, but a temperature where the material is going to yield under some load will fall in between those and will also depend on the particular size and loads in the application. [ Last edited by yogidan on 2009-2-23 at 15:28 ] |
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