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Thermoplastic Acrylics
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<blockquote data-quote="TECHARTISAN" data-source="post: 91018" data-attributes="member: 1289"><p>I cannot speak to nylon as I have already raised question of this....I wouldnt use a nylon material personally as it is EVEN MORE hydrophillic than the thermal cure acrylics most of us use, so while it might help with monomer sensitivity it is no other way an improvement IMHO.</p><p></p><p> Previous conversations with Dave in this regard have always focused on PMMA. In thermoplastic injection the material does not need to cure. Its already cured. This improves the materials dimensional stability and limits variation to the thermal coefficient of expansion of the acrylic, which should in an ideal world be matched closely to your investment and model material. The critical issues of fit happen during phase change, in the first 45 sec to 2 minutes, after this the material has already passed its glass transition, continued pressure is unnecessary and wasteful. </p><p></p><p>These materials are often quick cooled allowing injection and devest in about 5 minutes. HOWEVER, Quick cooling can reduce impact resistance and if cooled waaay to quickly warpage that will damage fit.</p><p></p><p> Manufacturers usually state in their directions that material and fit benefit from progressive cooling overnight. This prevents crystal formation within the amorphous polymer matrix. Crystalization creates weaknesses in the lattice that become initiation points in impact related fracture. I suspect, This slow cool is what Dave is using the polymerization unit for.....temperature controlled cooling.</p></blockquote><p></p>
[QUOTE="TECHARTISAN, post: 91018, member: 1289"] I cannot speak to nylon as I have already raised question of this....I wouldnt use a nylon material personally as it is EVEN MORE hydrophillic than the thermal cure acrylics most of us use, so while it might help with monomer sensitivity it is no other way an improvement IMHO. Previous conversations with Dave in this regard have always focused on PMMA. In thermoplastic injection the material does not need to cure. Its already cured. This improves the materials dimensional stability and limits variation to the thermal coefficient of expansion of the acrylic, which should in an ideal world be matched closely to your investment and model material. The critical issues of fit happen during phase change, in the first 45 sec to 2 minutes, after this the material has already passed its glass transition, continued pressure is unnecessary and wasteful. These materials are often quick cooled allowing injection and devest in about 5 minutes. HOWEVER, Quick cooling can reduce impact resistance and if cooled waaay to quickly warpage that will damage fit. Manufacturers usually state in their directions that material and fit benefit from progressive cooling overnight. This prevents crystal formation within the amorphous polymer matrix. Crystalization creates weaknesses in the lattice that become initiation points in impact related fracture. I suspect, This slow cool is what Dave is using the polymerization unit for.....temperature controlled cooling. [/QUOTE]
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