Printing Threaded Connections

[Harv 0]

Hello all,

 

I searched, but did not find much information on designing 3D printed threads. I thought I would share a success for others to reference. I came across a recommendation of using trapezoidal threads, which was the approach I used. I have printed 4 sets of threads now, and all worked very well. I used Tough PLA, major diameters around 1.3 inches, dual start threads, and 5 revolutions per inch. Please see the pictures for thread fits (dimensions are in inches / degrees). Some of them were a little tight, but wear in to a good fit after a few twists. I also chamfered the male and female thread starts with a 45 cut revolved around the thread axis.

 

Harv

[Framar 38]

Hello Harv, 

 

What clearance did you use between male and female threads ?

[geert_2 558]

For dimensioning threads, I think you could reuse the guidelines for plastic injection moulding. These threads generally use shallower angles than typical metal threads (M20, etc...), wider teeth, rounded corners, to improve the molten plastic flow, increase strength, and reduce stress-concentrations in corners under load. Indeed very similar to what you have done.

 

If you use a standard thread, you can screw existing caps on it (e.g. recovered from PET bottles).

 

Search for "injection moulding guidlines pdf", or: "part design guidelines for injection moulding pdf", "lanxess part and mold design guide", "injection moulded threads" (and then go to "show images"), and similar, but without the quotation marks. Most plastic manufacturers do have excellent guides on injection moulding.

 

[Harv 0]

Framar,

 

The theoretical clearance is .006" at the thread root and crest, with .004" clearance on the thread flanks. They could be a little looser. I only had one set that came out easily free-running, where I guess the male and female shrank a similar amount at cool down. That set had the internal and external threads both on thin wall cylinders.

 

Geert,

 

Thanks for the good tip. I have quite a few of those design documents saved from previous projects. It didn't occur to me to reference those for a baseline. I'm far from an expert in the injection molding realm and never designed plastic threads.

 

I should have mentioned in my first post that these were made on an S5 with .1 mm layer heights.

 

regards,

Harv