Tongue & Groove Joint for a Mold

We made a 1mm tongue and groove joint for the top and bottom halves of a mold.  With 0.1mm clearance in each direction, they fit together quite snugly.  We just made sure to orient the parts so that no supports were attached to the joint surfaces.

Wow. Great!

Very cool!

I am excited to see more interesting joints like this

Very cool! I wonder what this is for?

I am very curious about fitting parts and would love to hear more about this!

When you say 0.1mm on both sides, I wonder if that would also apply to a “peg in hole” test. I am currently designing with only 0.1mm diameter difference as I am worried that 0.1 on both sides  (0.2mm total) would be too lose.  I have measured a few fitting pieces I have around the house (plastic models and battery cases) and 0.05mm on both sides or 0.1mm total difference seemed to be a reoccurring theme.

Have you tried any other fittings/shapes with different tolerances?

This mold is for casting a silicone model of the spinal cord of an infant with a congenital defect (spina bifida) to be used as part of a surgical training model.  You can see a video of the surgical simulation on the model at

We have not yet tested holes & pegs in our Form1, but will probably do so soon.  When we design parts for larger SLA machines, we use 0.1 mm radial (0.2mm total) clearance for holes and pegs as well.  As you have figured out from your measurements, that is indeed twice the clearance you would specify if you were machining those  parts.

It would probably be a good idea to make a few small test parts with slightly different clearances to determine exactly the clearance works best for your build settings, size & aspect ratio of the pegs, etc.

Crispin, this looks great. A piece of advice for silicone molding with the plastic. You will most likely not need a mold release, since silicone will not stick with the plastic.

Also, depending on your grade of silicone, you may find that it does not fully cure right against the surface of the plastic (Sylgard 184, for example, remains a little tacky). This is useful in many ways to aid release from the mold. If the silicone feels a little tacky after releasing from the plastic mold, put it in the oven overnight at 80C, and this will help to further cure the silicone. We have found some grades of silicone do not exhibit this surface tackiness.

Crispin, thanks for sharing your experience. A very interesting project as well! Since we are without printers for a little while longer, I am trying to get the design work done, unfortunately this means I can’t do any test work on the finer details just yet. If you do get to experiment with peg and hole geometry, I would absolutely love to hear about your experience! Thanks again for sharing! readyGo

Ready Go, we tried some peg & hole tests and the results are bit messy. The results depend on on geometry - diameter of pegs, aspect ratio of pegs - and on build angle, as well as on design clearance.  We tried making pegs with diameters of 1, 1.5, 2, and 5mm.  In some cases, we looked at different aspect ratios (height/diameter).  We tried radial clearances from 0.02 to 0.2mm.  For most parts we oriented them with pegs & holes facing up (from build platform)  in order to avoid problems with supports on the features. Here are a few preliminary findings:

-Not surprisingly, some 1mm pegs failed to build and  some holes were partially filled in with resin.

-1.5mm pegs are too small for consistent results.

-2mm and 5mm pegs can work.

-Aspect ratios of 1:1 and 2:1 seem to be best. Lower and higher aspect ratios gave less consistent results.  Higher aspect ratio pegs required greater clearance.  Many pegs seem to have bulges, especially at higher aspect ratios.

-Largest clearances are loose.  In many cases the smallest clearance pegs fit snugly at one angle, but not at all another.  This suggests that the cross section of the pegs and holes are either not circular or vary along their lengths (possibly the bulges seen in some pegs?).  This variation seemed greater at higher aspect ratios.

-One caveat: these tests are on single pegs & holes.  Any warping or alignment errors in building across larger distances might require slightly larger clearances when you are trying to fit together multiple pegs and holes.

-In at least one test, holes built in a horizontal orientation seem to be larger than holes of the same designed diameter built in a vertical orientation.  We will pull out the pin gauges to verify this when we have some time to look at it.

In summary, you probably want to start with pegs at least 2-3mm in diameter, aspect ratio around 1:1 to 2:1, and radial clearance around 0.05mm, but final choice may depend on geometry and how snug a fit you desire.

Hi Crispin,

Wow that’s a lot of great information!

I will examine it in great detail (when I am off work), but I wanted to write a quick note to say thank you very much for sharing your results!