Joining several parts properly


I was wondering what you, as the Formlabs community, use to join different parts together. I printing some cilinders that need to fit together (airtight), and I’m not happy with the way i’m joining them right now. Perhaps you all have some advice from past experiences which can help me find a proper way to join the printed parts.

Right now i’m trying to fit the cilinders together by adding two (or more) small rods on one end of the model and two slots in the other model. I dip both sides in resin and them cure them together. However, for an airtight fit (especially under pressure), I don’t know if my solutions is the very best.

Perhaps you guys have some advice!


For a round shape like that I would make some lovejoy shaped cuts in each so they fit together nice and strong. Here’s a picture of a typical love-joy. The black is just a rubber piece you don’t need since it is not a drive-line.

PS - If that is a silencer for a gun it will probably explode like a pipe bomb and blind you. :frowning: If not on the first shot, surely after a little fatigue. This stuff has lousy mechanical properties.


What color resin are you using and what is the wall thickness?

Basic epoxy works well to bond pieces together, but my favorite is using resin and a 405nm laser pointer. Once they are “stuck” I put them in a curing chamber. It works quite well on clear, but for parts of other colors, you will have to chamfer the edge to be bonded, and build it up layer by layer (sort of like a weld on thick plate that is done in many passes). We have also had good luck with CA glue (aka superglue).


Another good way to get an airtight seal would be to taper them relative to each other. This gives you a large surface area to bond together, resulting in a higher strength joint. Throw some epoxy in there and that sucker is never coming apart. Also, it keeps the seams to a minimum, meaning less finishing if you want to hide the joint afterwards.

Put a thread on each of the parts and screw them together

Thanks for the reply everyone!

@JoshK, very nice idea. I shall give it a try and see how it comes out! Thanks! For the record, it’s not a silencer for a gun, it’s the inside of tailpipe of a moped (though you were close with your guess, as this part does act partly as the silencer ;)). I’m going to design and print a full exhaust and see how well it’ll work compared to the original metal exhaust.
Yes, I am fully aware of heat issues and the spec sheet of the different resins. That still isn’t going to hold me back. Furthermore, I’m printing it in clear to see/show how the exhaust fumes act inside the exhaust itself. Of course it would be great if it would run and ride (at least once :P).

@Aaron_Silidker, as said to Josh, I’m will be printing in clear resin (V2). Epoxy is a definite contender for sealing. I’m trying to stick them together by dipping them in clear resin and curing them in a curing chamber. This definitely works, however I wonder how strong the bond is under pressure. Creating a joint which relieves part of the stess at that point is therefore essential (in my opinion). Eventually i’d like to mask the joints as much as I can so that it looks like it’s made out of 1 part. Of course there is a limit as i’m printing in clear resin.

@Steve_Thomas. Indeed another great idea. there might be an issue as the wall is only 2mm thick which doesn’t leave a lot of room for tapering, however, together with Josh’s idea, it’s a great contender. Thanks!

@Gary_Cairns. I did think of that, and I do have stl’s with all metric threads, however, as the wall is only 2mm thick, there is hardly any room to screw them together. Or I would have to design my own type of thread which uses only several microns as the screwing part. However, to ensure this will work, i’ll probably have to print it at .0025mm for the threads to work properly. As the parts are getting bigger and bigger (eventually taking up more than half of the build platform, and the full height of the build volume), this might cause easy print failures, as printing large parts at .0025mm is not a Form1+ favourite! ;).
I will definitely keep it in mind though, as it is absolutely a good way to join parts in a solid way. I would just need to find a way to do that on such a thin surface area.


The right choice of joining method depends on the strength of the joint. Assuming the resin cures completely and you have totally uniform coverage, the strength of the joint should be every bit as strong as the material adjacent to the joint. The joint is essentially just another layer between all the layers.

If the cured resin attachment method can’t guarantee complete cure and uniform coverage, butt-splicing is the worst possible method to use. Where adhesive strength is lower than it needs to be, increasing the bonding surface area improves the strength of the joint. Both the tapered and castellated methods shown above do this quite nicely.

The tapered method (especially a long taper as shown) will require really good dimensional accuracy in the print. If you designed the two pieces line-to-line, you’ll probably have immense trouble getting them to fit together all the way. You may have to iterate the print a few times, adding a little more “draft” each time until you’ve “tuned” the print for the degree of accuracy of your printer.

The castellated approach is probably your best bet. It should be more tolerant of the printer’s inherent printing inaccuracies and will be easier to work by hand if it needs tweaking after you’ve printed it.

When I need parts that are dimensionally accurate, I always print the key surface features slightly oversize and then machine them with my CNC. 3D printers (at least, the ones I can afford) are not particularly high-precision output devices.

Thanks for the advice @Randy_Cohen. I shall definitely take this into account! I don’t have a CNC machine to sand down the part, but manual labor can do the trick as well :wink:

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