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shrinkage problem

Posted: Sat Aug 10, 2013 10:32 pm
by Ian Smith
Folks ,
A little help if you will.

I am trying to print a small wheel with a "v" shaped groove around it's circumference.
It's 11mm in diameter with a 5 mm hole for shaft, and 6.7 mm wide.

No matter what I try, i.e. slow, fast, normal ,0.15mm layer, 0.2 layer, fan ,no fan , half fan, and all possible combinations, I am always getting shrinkage in the top half.
I am only printing with maximum fill and laying flat, as that's the way I need the layers to run for maximum strength, and I am only using ABS - ( genuine UP White 1.75 mm, and using an UP! Plus with Version 2.0 software)

The top half always ends up being smaller by about 0.2 mm. in both it's external diameter and the diameter of the hole for the shaft. The bottom half is near perfect, but of course it's anchored by the raft.

I am certain that shrinkage is the problem, but how do I avoid/compensate for it ?

Thanks

Ian

Re: shrinkage problem

Posted: Wed Aug 14, 2013 6:52 am
by wackojacko
Hi Ian

Try printing two at once, it might be because the part is retaining heat.

Re: shrinkage problem

Posted: Wed Aug 14, 2013 8:12 am
by Ian Smith
Yep Wacko,
I thought that that might work, and it did, to a degree.
I actually printed 12 at once, with the fan on, and still there is a small difference. But I guess it does point to the cooling process being the culprit.
Thanks for your response

Ian

Re: shrinkage problem

Posted: Wed Aug 14, 2013 11:58 am
by roller
I would leave the fan closed/off and you will get even better results. Cooling is exactly the issue as ABS shrinks a lot when it cools and it's the whole reason we need such a hot platform. Cooling is handy with ABS if your print isn't cooling enough to harden before the next layer is put over the top, or to make sure support is hardened well so the next layer adheres loosely rather than firmly but otherwise you generally don't want the fan with ABS. (a slight over generalisation on fan use there)

If you don't have to use ABS you might find printing parts like these in PLA is more reliable. PLA shrinks less and as such warps less. With PLA you will want the fan open and maybe even the cooling of an extra fan or more parts to ensure your layers are firming up sufficiently as they print. PLA is harder/less flexible and a little more slippery. I prefer it for things like pulleys because it's harder wearing for such uses but if friction will be generating heat above 40C it will start to undergo plastic deformation.

Re: shrinkage problem

Posted: Wed Aug 14, 2013 12:12 pm
by Ian Smith
Roller,
Thank you for your help.
I have only used ABS thus far, and I haven't got any PLA on hand, but I will give a try some time soon.

I will try to print a batch with the fan off and let you know how it goes

Ian

Re: shrinkage problem

Posted: Wed Aug 14, 2013 10:09 pm
by JuliaDee
Or nylon, which is easier to print with on the Up...

Re: shrinkage problem

Posted: Thu Aug 15, 2013 12:43 am
by roller
JuliaDee wrote:Or nylon, which is easier to print with on the Up...
I haven't tested Nylon for dimensional stability and shrinkage yet - does it shrink much?

Re: shrinkage problem

Posted: Thu Aug 15, 2013 3:57 am
by JuliaDee
roller wrote:
JuliaDee wrote:Or nylon, which is easier to print with on the Up...
I haven't tested Nylon for dimensional stability and shrinkage yet - does it shrink much?
I don't think it shrinks much, but I think printing itself might not be quite as precise as with ABS; I may have been printing at too high a temp, but it tended to seem a tad "sloppy". My parts were for a fairly complex rotary snap-lock mechanism with small features, though, and they worked quite well, mating with a die-cast metal part. I certainly didn't measure them down to tenths of mm for dimensional accuracy however.

Re: shrinkage problem

Posted: Thu Aug 15, 2013 4:29 am
by wilsonj
Nylon can expand after printing though, due to drawing moisture from the air.

The local engineer actually factors this in when creating parts from nylon ( not printed).