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PLA object shinking during use

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I printed a replacement coffee pot lid with PLA.  The threads are 2mm deep.  After 3 or 4 uses (cycles of warm/cool from making/pouring coffee) the threads act as if they have been stripped out.  To the point that the lid will no longer stay in.  I have measured the diameter of the threads and find them within .1mm of spec.  The lid is never tightened, not even "finger tight".  It's just turned about half a turn till it stops, very gently.  The water is heated to 200' F but is cooled (an unknown amount) as it passes through the coffee grounds before it reaches the lid.  The pot is not heated.  It's a thermos type.


Can PLA shrink (further) after an object is used with the application of heat/cool cycles?

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PLA is totally unsuitable for hot liquids, or for any other hot application. It starts to soften and deform around 50°C (~122°F). It will even deform in a car in mild sunny weather, in spring or autumn. We are not even talking about summer. Each time you go over the softening temperature, of course it may deform further, also depending on the loads applied to it, and on residual internal stresses.


For your application you really need a high-temp 3D-printing material, thus one that has a glass transition temperature (=point where it starts softening) of more than 100°C. But I don't have enough experience with high-temp materials to recommend a specific product, so I will leave that answer to others. It will probably go in the direction of PET or polyester-like products: these are still easy to print (contrary to ABS and PC which are more difficult to print).


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Well PLA gets quite soft (think of clay) around 55C so I would avoid that anywhere near boiling water or steam.  I don't think it shrunk necessarily - it may just be moving slowly towards a sphere in shape.  Or a saucer.


I don't know much about plastics and food.  ABS seems like it wouldn't be good near food (I could be wrong) but it softens at 99C.  Nylon probably wouldn't be good because the high humidity would likely turn it into a very floppy object.  Or maybe it would be fine because most of the time humidity would be low.


PET/CPE/nGen I think might be mostly the same thing?  I think they soften around 80C which is significantly better than PLA and almost as easy to print.  Things that soften at 100C or higher are going to be hard to print (you'll want to enclose the printer and make sure fan is at 0% to get good layer bonding).


Polycarbonate might work.  ABS will definitely work.  here is some info on materials.  Unfortunately the "softening temp" is wrong on many of these because, well, measuring this with a standard measure is going to give you the wrong answer.  But it wil give you some ideas of where to start - you want the second graph which shows temperature related stuff:


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I used nGen to build a GPS holder for my car and so far, since last summer, that has survived warm sunny days in the Summer - of course Texas may be different! Colorfabb also do another material, sort of next step up from nGen, Colorfab HT which again is done in collaboration with Eastman like nGen (no personal experience with HT); this has a Tg of over 100c. I found nGen as easy as PLA to print and capable of better surface finishes. My personal view is to print nGen at at least 245; go lower and you may start to hit layer adhesion weaknesses.

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