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At POP we are often asked for recommendations about threading plastic 3D printed parts. Tapping is an option, but we rarely recommend it. Plastic threads, even if cut perfectly, are inherently fragile compared to their metal counterparts. In addition, cutting them is time consuming (and therefore expensive). 3D printing them directly isn’t a great shortcut because getting the fit just right is tough, and supporting/cleaning threads can be challenging. It can reduce thread strength as well, relative to cut threads. Where possible, the better approach is to use threaded inserts. The specifics vary slightly depending on the print technology, however the gist of it remains constant: insert a metal (usually brass) piece that contains the chosen thread into an otherwise finished 3D printed part. With this approach, you get the best of both worlds:
Getting started with inserts can feel daunting, but it’s really pretty simple. At POP we stock all major thread sizes and insertion tips, and have even custom fabricated all sorts of tips, fixtures, and tools to make installing inserts as efficient and precise as possible (ask us if you need inserts installed in your prints!) For most applications, though, simple tools and some design tips are all that’s needed. We’ll break this guide down into two sections: first addressing heat inserts for FDM parts, and then expanding inserts for PolyJet parts. Heat Inserts for FDM PartsThese metal inserts melt their way into plastic parts, and their barbed and knurled geometry ensures that they never come out or slip without a big fight (usually the part breaks or the metal threads strip first). We like the tapered variety best, as they are easier to install and offer extremely high pull-out resistance.  McMaster-Carr offers a wide variety of these inserts, in addition to general recommendations on how to use them. McMaster-Carr’s inserts are generally accepted as an industry standard. We’ll use those as a starting point, with a few extra tips that we have learned along the way. Let’s say, for example, that we are trying to use a ¼”-20 insert. Filtering for that size gives us two options in brass (generally our go-to material for inserts):  Which you choose will have everything to do with your application – generally if you have the space (length) and want the strength, pick the longer variety. But do keep in mind if inserting into a blind hole that the hole depth should be at least ~25% deeper than the insert is long – this allows space for the molten plastic to pool as the insert is pushed into position. Without this head room, you’ll likely make a mess of things. In our example, we have a blind hole that’s only 0.45” deep, so we need to use the “short” version: PN 93365A160. Clicking on the PN, we see more information provided by our friends at McMaster:  I’ve highlighted the two fields that we are asked about most:
To answer these:
 Once the part is printed, it’s time to get the insert in. We have a dedicated, temperature-controlled soldering iron set aside for inserts. For an ABS part, we will generally use an iron temp of ~140°C. Fight the urge to heat things up too much: going hotter will speed the process up slightly, but will often cause bubbling or even charring of the plastic that looks bad, smells bad, and increases the odds of a crooked insert. Slow and steady wins the race! Using the recommended brass insertion tips from McMaster is the tool of choice here, but we have also machined custom tools for hard-to-reach areas.  Similarly, using a handheld soldering iron works in most cases, but it can also be helpful to have a method of ensuring an orthogonal pressing direction; we have adapted a drill press for the purpose which comes in handy, especially for larger runs of bridge-production parts.   Once the insert is heated up and pushed into place, remove the iron and let it cool for a minute or so before handling. That’s it!      Inserts for PolyJet Printed Parts: Because PolyJet resins aren’t thermoplastics (they don’t melt, but instead react to heat more like a polyurethane would), it’s not proper etiquette to use heat set inserts with this material (or other SLA resins, for that matter). Yes, we’ve seen people get away with it… but that’s not how we handle your work here at POP. Instead of heated inserts, we use “screw to expand” inserts for PolyJet parts. These inserts have a grippy knurled exterior, and push into the plastic part with a (relatively) loose fit. When a screw is driven into the insert, however, it expands slightly, locking itself against the wall of the printed hole and holding tight forevermore. We prefer (and stock) the flanged variety, as the extra meat makes them easier to place and can help to provide some extra holding power. If there’s no room for the flange, though, a standard version can be used as well. PolyJet resins (depending on the type and application) can be brittle, so it’s important to treat them accordingly, and take care when pushing inserts into parts. It’s better to have a slightly loose fit (which can be remedied with a drop of CA glue before the insert) than to break the part. So, we again recommend taking the McMaster recommendation and adding 0.005” or so. For the purpose of this example, let’s say that we are aiming for a #4-40 thread in a hole that’s 3/16” deep. We will be using the “short” variety again as such (NB: expanding inserts don’t require as much of the extra head room in blind holes that heat inserts do, though we generally recommend some breathing room to allow space for reworking and glues if needed). Our hole will be modeled as 0.1613” diameter (5/32” + .005)   After printing, these are pretty easy to install. We recommend reaming the hole out to size (0.005” under is fine for this step if the oversize bit is not readily available), to remove any residual support and ensure that everything is to size.  We also recommend warming the printed part slightly, to around 25-40C before insertion. This helps to ensure some extra flexibility – a frozen part is much more likely to crack, especially if it’s made of a brittle resin.  Pushing these inserts in is easy once the prep work is done. If you are worried about slippage, use a small drop of CA (“super”) glue on the exterior of the insert as it goes in; this will chemically bond to the acrylate-based PJ resin and make things a lot stronger.   Finally, expand the insert by carefully driving a screw into it, and then backing the screw out.    Comments are closed.
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