Advice sought on SLS-printed Nylon prototype - what tolerances & how to best finish?

Hello

Background:
I am new to 3D printing. I need to create a prototype for a product that will ultimately be made from PE (Polyethylene/Polythene). I have just received my first 3D prints back from a 3D Printing Service (in London, UK). They use SLS to print in PA2200 Nylon. Fwiw, the are using an EOS Formiga P100 (or P110) 3D printing machine. I think the layer height is 0.1mm.


A) GAP SIZE
My model I have designed involves a 'Male' part that goes into a 'Female' part so as to form a sort of 'piston'. The diameter of the male part is 7mm.
In my first 3D print, I made the diameter of the Male part of my piston 0.2mm smaller than that of the 'Female' part that it goes into. However with a bit of sanding the Male part is now seems too loose in the slide.

For next time, how much gap (if any!) would you recommend, given that I will need to sand down the surfaces to create a smoothly sliding mate?


B) FINISH
As a result looseness as described above, I now need to add bulk to the male part! What would you recommend I use for this?
I have seen people dip a 3D print made from ABS in ABS dissolved in a solvent such as Acetone or a solvent cement (e.g. Tamiya Extra Thin/EMA Plastic Weld).

I suppose that I could apply a thin layer of some sort of robust paint (or lacquer?), over the whole of the exterior of both parts including the area of the male part where the sliding surfaces of the 'piston' mate. This might help create a nice shiny looking model and thereby mimic the plastic finish of the ultimate product I am designing. A smooth paint or lacquer finish would also help stop my parts from attracting fingerprints & dirt.

However in order to create a nice smooth slide, I will need to be able to sand the mating surface down without ruining the finish. And moreover I would like to have a prototype with a mating surface with a smooth sliding action that will not get damaged too quickly through use of the slide.

Any thoughts?

Many thanks

J

I don't think you'll have much luck trying to add material to your nylon part. Nylon is very difficult to adhere anything to. Sanding is an inherently imprecise process; it's difficult to get an accurate sliding fit once you've attacked your part with sandpaper. You might consider re-doing your part and machining the critical surfaces rather than sanding them. In that case, printing the positive part a bit oversize (and making the bore a little smaller) will give you some material to remove.

Andrew Werby
www.computersculpture.com

Quote:

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Originally Posted by awerby

I don't think you'll have much luck trying to add material to your nylon part. Nylon is very difficult to adhere anything to. Sanding is an inherently imprecise process; it's difficult to get an accurate sliding fit once you've attacked your part with sandpaper. You might consider re-doing your part and machining the critical surfaces rather than sanding them. In that case, printing the positive part a bit oversize (and making the bore a little smaller) will give you some material to remove. www.computersculpture.com

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Oh - so are you saying that if I don't sand a part at all, then (and only then) there would be a sporting chance that I could paint onto it?

I already have a couple of other parts already on order, so maybe I could try deliberating not sanding them. The trouble is that they definitely wont fit in without at least some sanding. OK but I could possibly add some material (paint / lacquer / dissolved ABS?) and then sand all of that down ??

Do you think I could bond ABS onto Nylon by painting it on in solution?

My initial feeling was that I should re-print the part over-sized and then just sand it down to push-fit dimensions. (One trick I sometime use for sanding is to put pencil black onto one surface and see where the other is touching - a bit like dentist's "articulating paper".)

OK I give up - I have no idea what I'm talking about.


So I guess I could try and "machine" the parts, but how exactly ?
a) Turn the parts on a lathe? I don't have ready access to a lathe - the parts are too complex to fit into a lathe chuck in any case.

OR keeping the part stationary?

b) CNC ?

In my home workshop the only machine that might be useful is a hand drill, optionally on a press mount.
I could possibly drill the female hole out with a drill... but I have no idea how to machine the male part without a lathe.

:^/


EDIT: These guys seem to think that there are various options for painting onto Nylon/ nylon(polyamide 12) "Fine Polyamide PA 2200"
http://www.shapeways.com/tutorials/painting3dprintedsls

Including:
- acrylic paint, water based
- acrylic paint, solvent based
- cellulose spray paint, solvent based
- nail varnish, acetate solvent based or acrylic
- oil paint

It seems that sintered nylon 'sucks' the paint it.

Sanding it may well cause problems though... :^/

For tolerance and fit I suggest you find a similar document in english :
http://www.langlet.fr/bdd/docs/4-tol...justements.pdf

Nylon seems not easy to paint but it can be dyed. Rich rap wrote womewhat about it on his blog you should look into it.

SLS Nylon parts can definitely be printed.

We have a customer that uses them for casting patterns. We print them along the split line, he attaches them to boards and then does a process of spray painting and sanding down to build up the paint and get a completely smooth finish. Not sure what paint he uses, but I'll try to remember to ask next time he's in.

I've been very impressed lately by the results we're getting when requesting SLS prints go through polishing in a tumbler. Of course only on non dimensional critical pieces but it makes them impressively smooth.

you're asking the wrong people :-)

You should ask the print service who are printing the part for you - they will know what tolearances their printer can produce.

But logically if 0.2 is too large then reduce it :-)

But ask the people printing it first :-)

I was thinking of solid nylon; evidently SLS-printed nylon is a lot more porous, so that even though paint won't actually stick to it, it will build up in the pores. But I still doubt that this built-up paint layer will give you the smooth sliding fit you're talking about.

You say it's too complex to fit in a lathe chuck, but that's a common problem in machining. You can use a fixturing compound to surround the complex end of your part and make it grippable by a chuck; these typically soften when placed in hot water and can be removed. Jettset is a plastic one, there's also Cerrabend which is a metal.

You didn't show us the part you're making, but there are other machining tools besides lathes. If it's complex, you can probably make it or refine it using a CNC mill with a rotary 4th axis.

Your sanding technique sounds fairly controlled, but you may have made your interior piece too small to start with; if you had more material to remove it might have worked better.

Andrew Werby
www.computersculpture.com

Quote:

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Originally Posted by curious aardvark

you're asking the wrong people :-)

You should ask the print service who are printing the part for you - they will know what tolearances their printer can produce.

But logically if 0.2 is too large then reduce it :-)

But ask the people printing it first :-)

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The people printing recommended 0.1mm gap in the part sizes. This makes a 0.2 difference in diameter (i.e. 0.1 x2) which, once one has done some light sanding does seems to be too much in practice. I next time I shall half the gap.

Either way I'm still not sure what to do about finishing with paint / lacquer etc. I have bought some automobile wheel paint and some lacquer to experiment with. My plan is to build it up with a layer of this stuff and then sand down the mating surface of the male part, in a controlled manner with fine sandpaper.

I have also bought some more sand papers so that I can cut it to size and wrap around the male part and sand in a slightly more controlled manner. Watch this space :)

Well nylon is famous for being non-stick. Don't think I've ever seen painted nylon parts.
Also bear in mind that nylon flexes - so most paint, even if it will stick will flex off after a short period of time. Particularly if it's a moving part.

Depending on the model you're making - you might be better off in this case getting it printed in pla or abs or pet or poly carbonate on an fff machine. At least that way you'd be able to paint it :-)

Aardvark: Hmm.... I'm not so sure - have you ever tried using SLS sintered nylon yourself? As far as I can, tell sintered nylon behaves very different from solid/injection moulded nylon in that it appears to "suck up" paint by capillary action.

The 3D printing service I have been using (3DPrintUK in London) seem to ONLY offer an SLS service using white nylon (Nylon PA2200), which runs at 0.1mm layer height. No other materials are on offer.

I must say that so far I am very impressed with both the accuracy of the printouts and the strength of the printed material. e.g. I printed a 7mm piston with a 0.1mm radius gap (i.e. 0.2 mm diam) and after a little light sanding to get rid of surplus dust etc, it seems that was too large a gap so next time I will need to half that gap I think.

What is also quite surprising is that in most instances for some reason, with the naked eye the 0.1mm layers are mostly fairly hard to even see! This feels light years different from the 0.2mm FDM model that I got 3D printed ion ABS whether the layers are pretty in your face.

* * *

Meanwhile I am now experimenting with painting the models with car paint. I must say that I had forgotten just what a messy process spray painting is! For one thing you are not supposed to spray in cold air - tricky in winter, but if you spray indoors it's incredibly messy (!) and even if you spray from a few feet off the ground the paint is still wet when it lands and the paint droplets immediately stick to where they land. Potential nightmare!

Next time I'll try with a paint-brush I think.