3d-printing in SLS Nylon - hints and tips.

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Timothy Huff
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3d-printing in SLS Nylon - hints and tips.

Postby Timothy Huff » April 29th, 2019, 11:35 pm

I thought I'd "put pen to paper" and set down all that I've learned about printing in SLS Nylon, as it may help some of you more used to working in other materials, but for whom 3d printing offers an attractive means of producing light-weight, fairly strong and dimensionally accurate parts. I'll limit it to the production of parts from commercial printers, as "investing" in a printer is still a fairly expensive route, and the results and failure rates of using non-commercial printers are relatively poor and high respectively.

Thoughts:

SLS Nylon is capable of excellent dimensional accuracy. When delivered as a "polished" part, expect a finish akin to an "extra strong mint". If I have two parts of dimensions "A" and "B", then I reduce the mating surfaces by 0.1mm if I want the finished combined width to remain as "A+B". If the parts need to move in relation to each other, eg a peg rotating in a hole, then allow 0.2mm minimum reduction in the peg diameter relative to the hole. A little forethought here is valuable, so if one draws (say) a 0.75 mm hole on the model, but you actually need a peg circa 2.381mm (comes out in Imperial as 3/32"), then you'd need a 2.5 or 2.6mm drill to gain the needed clearance if the two are to rotate freely in relation to each other.

Use of a 0.75 hole in the model-drawing used for a part will NOT resolve as a hole on the printed model. However, it WILL result in a dimple that is perfect for centring a drill. SLS Nylon will happily drill to clean holes, and will likewise make a good thread if tapped. What it won't do is file or sand, which merely produces plastic "fur". If you need to shave material, a scalpel is about all you can use in my experience.


The key point is to always create holes in the your printed model well under the desired size, and then drill them out using the printed hole as a centre for the drill. When drilling, always use a dead-slow speed, as heat can cause all manner of issues. I would NOT recommend a tool incapable of slow speeds, however a pin-vice and small drill-bits (sub 1.5mm with a pin-vice) is more than practical, and with practice one can judge depths pretty accurately. I often use a pin-vice where I want the control, more than the speed. To give you an example, I had to drill and tap some blind holes today. I didn't want the drill to go right through the material, so by placing a finger over where the hole would emerge if you drilled too far, it becomes possible to "feel" the material begin to bulge as the drill nears the edge of the material, at which point one ceases turning the Pin-vice (drilling) else you'll end up with a drill in your finger! The pin vice, with practice, gives the control required for this sort of work
Obviously, never ever use this strategy with a powered drill of any description.

Most of my turret parts are assembled with very fine screws, typically M1 or M1.6, and almost never involve glue. The reason for this is that glued parts cannot be dismantled without damage, and by far the hardest part of assembling parts from CAD drawings is the build-sequence and whether or not you can actually get a jewellers-screwdriver actually onto the required screw-head with all the gubbins in the way. It's a bit like building an aircraft where the smallest screwdriver you can employ is 3 feet long! The importance of the ability to back-out of an assembly cannot be overstated.


Thin flat sheets are problematic, as the corners tend to warp and/or shrink. As one can use other sheet materials instead, this isn't too much of an issue, however, if your 3d printed sheet goes under 2mm, you can start to see dimensional issues. I found that if I needed holes towards the corners of any sub 2mm thickness part, it was better to drill them after printing, preferably using a thicker different part as a key to where the holes were drilled. On one occasion I printed the sheet material over-size, printed a special "tool" which registered the desired holes when aligned with other features/edges, and ended up with perfectly accurate holes - and a little scalpel trimming to obtain the correct sheet material dimensions.


3d printed parts can be drawn in more less any CAD package - I use Autodesk Fusion 360. The finished 3d model is then exported as an .stl file, which is essentially a set of instructions for the printer to fabricate the desired part, using incremental deposits of material and fusing same as it goes. This leaves striations, so again a little planning here will yield better results. If we consider an Instrument panel, then printing it so that the instruments are "facing up" will give beautifully circular bevels around each instrument. If the panel is printed vertically, then each bevel will have a start and end point that differs on each and every pass, resulting in what can only be described as a "pixellated" finish. If you look at objects on my turrets that have compund curvatures - eg the ammunition feed chutes, then one can see this now unavoidable problem. Consideration of aspects, curvatures and printing orientation is a bit of a black-art, but it's worthwhile to try and and get au fait with.


Parts smaller than 125 cubic mm (5x5x5mm) are frequently lost in the production process. It's very possible to print smaller parts, but it's essential to sprue them and then you cut away the sprues later. By the same token, "nesting parts" (making an .stl file with smaller parts inside big ones, but with a miminum seperation of 1.6mm will help keep costs down. However as prices usually vary with the volume cubed, it can sometimes be cheaper to print 3 smaller nests than one containing all the parts in one larger volume .stl.

If you've read this far, well done! Vaseline is a sensible lubricant for moving parts, painting them is straightforward, priming is recommeneded, as the SLS Nylon is slightly permeable - priming can save expensive paint.

Although I've not tried it, I suspect that SLS Nylon could be turned on a watch-maker's lathe nicely, again at a slow rpm and feed-rate.

If anyone has questions, or wants help in preparing a drawing into a viable .stl file, I'd be happy to help.

Tim

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