3D Printed Interlocking Parts: Guide To Design & Production

If you want to print complicated parts, it can require a significant amount of technical know-how and experimentation. 

Interlocking parts, especially those that have to move after you put them together, can be extremely difficult to print right. However, anyone can 3D print interlocking parts, provided you’re willing to test and get everything right. 

In most cases, getting 3D printed interlocking parts right relies on carefully designing parts to maintain strength and clearance. Otherwise, your printer should have no issues making the parts as you design them – providing they fit on the bed. 

Techniques To Design Interlocking Parts

Designing interlocking parts is the most important part of 3D printing them. Here, you’ll have to create parts that fit together well but also have the strength to hold up to pressure or force. 

In addition, you’ll want to use the best interlocking design for your needs – which might mean doing your research and seeing how those parts work. 

Understanding the Science 

There are several factors to keep in mind when designing interlocking parts.

These will all impact how your part works and whether it interlocks at all. 

• Friction – This force holds the interlocking part together. The tighter the joint or interlocking part, the more friction, and the more difficult it is to remove the parts from each other. Friction is determined by tolerance or the space between the parts, which is quite a bit impacted by the material you choose. However, less space between the joints means more friction, or a better hold. 
• Tension – Tension is the force that pulls the joints apart. This is normally determined by the weight on each side of the interlocking part. You can minimize tension by using smaller or lighter parts or by strengthening the interlocking part. 
• Shear – Shear is a “sideways tearing force” and also relates to the weight of the model you’re printing. If your joints shear or rip, it means the model is too heavy. You can make lighter parts, figure out how to support them, or make a stronger interlocking part. 

Snap Fit Joints

Snap fit joints are the “standard” 3D printed joint because they offer strength, durability, and ease of printing. 

Most are printed flat, which means that most printers can pull them off. A snap belt buckle is a good example of this type of joint. 

• Cantilever – Cantilever joints use a two-part construction. The external connector has a recess that the second part slides and hooks into. The inner part can be squeezed together so that it slides past the recess and can hook in. This is very common in the 3D printing world and can be found in many snap-fit designs. 
• Annular – Annular snap joints are often used for circular and ball joints. However, they can also be used for nearly any other kind of shape. These joints feature an outer part with a groove at one part of the connector and an inner part with a tongue at one part of the connector. The joint can only be assembled by inserting the joint so that the tongue and groove connect, after which, the joint is rotated to move the two away from each other. In addition, if you later fill the groove, the joint becomes permanent. 
• Torsional – Torsional snap joints rely on flexible plastics and cannot be printed with resin. However, they allow you to create a joint with a hook that fits into a latch assembly, allowing it to hold the piece in place. Gutter joints are one example of torsional joints, although these also utilize a tongue and groove mechanism. 

Tongue and Groove Joints 

Tongue and groove joints are the simplest form of joint you can make. However, they enable a very large amount of construction, including hinges, strong corners, and much more. 

That makes these parts extremely useful for mostly non-moving joints – although, with a pin or dowel, you can make them moveable.  

• Box – Box joints use a simple box-shaped recess in each of the interlocking parts to fit the piece together. These can then be pushed into each other – and depending on design can be quite difficult to take apart. For example, using triangular dowels with very small tolerances creates very tight joints. 
• Dovetail – Dovetail joints use a wedge that widens as it reaches the end, which allows you to create a joint that has no possibility of coming apart in a single direction. These joints are often made as tight as possible to ensure that they don’t slide apart in other directions. However, loose dovetail joints are commonly used to create boxes that can be picked up from the top, but slid apart from the side. 
• Tongue and Groove – Tongue and groove joints are similar to box joints but with longer profiles, meaning they can be more easily slid and moved around. These are normally created with a lot of tolerance, allowing the part to move in a single direction. 

Captive Joints 

Captive joints, or those that you print together in the printer, are one of the most complicated things you can print. 

In addition, these parts typically rely on being able to create parts with clearance between the internal components, using supports that are as small as possible, so you can manually break the supports by moving the ball joint around. 

SLA printing with resin can be extremely useful for this kind of printing. However, it may also be too brittle, meaning that you break the part when twisting it. 

In addition, SLA can have problems with UV curing the inner part of the joint. These joints do not ever come apart, which can be good for creating moveable pieces. 

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How To 3D Print Interlocking Parts

3D printing interlocking parts is normally the same as printing any other material.

However, you will have to double-check the settings and get everything right. 

Materials

Choosing the right materials will greatly impact the strength and durability of your interlocking parts. 

In addition, different materials have different clearance tolerances, which means there might be positives to choosing a brittle material for a joint, if it means you can get away with less clearance. 

Material	Durability	Flexibility	Tolerance (mm)
PLA	High	Low	0.5
ABS	High	Medium	0.5
Resin	Low	Low	0.2
High Strength Resin	Medium	Low	0.2

In addition, if you’re using an SLS printer, you can normally get other materials down to about 0.2 mm as well. 

For this reason, SLS is the most-recommended printing technology for interlocking parts (unless you can go with material jetting, which is unlikely).  

As discussed above, tolerance affects the amount of space between the joints. 

With PLA and ABS, it’s unlikely that you’ll get below a 0.5 mm gap between parts. With resin, you can expect as little as 0.2 mm. And, with SLS, you can also get to 0.2 mm. 

Test Your Prints 

The most important step in printing interlocking parts is to print and test your joints first. 

You can avoid wasting time by printing the joints as a standalone thing – which allows you to test how the joints fit together, tolerances, and the overall design. However, it won’t tell you how the joints hold up to stress and tension. 

It will point out when your joints are simply unworkable, for example, if you’ve made mistakes in how the joints fit together, if they break when you snap them together, or if the angles don’t work together. 

You can then correct those issues before printing the full model. 

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Best Practices For Printing Interlocking Joints

There are many best practices to use for 3D printing joints. 

Most of those involve good design and getting the settings right in your slicer. 

Get Clearance Right 

Clearance is the most important part of any joint. 

In most cases, you’ll need 0.2 to 0.5 mm of clearance at a minimum. However, the larger your joint, the more clearance you can get away with. 

If you have friction on the joint, you likely want clearance to be as low as possible. 

However, you can’t just lower the clearance as far as your printer is able to manage. Most materials and printers have different levels of tolerance – or the range at which they are accurate – and that will impact the clearance you’re able to achieve. 

One good way to figure this out is to use a tolerance test model, like this one from Thingiverse. That will tell you what tolerance you’re able to cleanly print with your machine, and you can design based on that data. 

Eliminate Stress Points 

Sharp or steep angles can create extreme stress points in connectors. It’s best practice to eliminate them. 

Here, a chamfer is a good tactic. Chamfering reduces angles to 45 degrees or less, whenever possible. However, you’ll have to chamfer both sides of your joint, and then test to ensure that they still fit together. 

You should also keep that in mind when designing thin parts, because they will always be under a higher amount of stress. The closer your connection point is to a base, the more stress it will be under when connected. 

Use 100% Infill for Connectors 

Connectors typically require as much strength as possible. For this reason, you should normally use 100% infill for them. 

If you’re printing large pieces that interlock, you can also add pins or dowels to increase the total stability. That will allow you to print without as much infill, while spreading the tension and shear load across more stress points. 

However, on average, increasing infill is more than good enough to increase the strength of the interconnecting part for your model. 

If you think 100% is too much, feel free to experiment and reduce it until you reach a balance of material, strength, and print time. 

Increase Width for Stability 

Increasing layer height and layer width increases the stability of the printed part. 

While it’s understandable that you might not want to print very large parts, you should try maximizing the Z distance width so that you have a more stable part. 

However, again, it may be overkill. You can always experiment and find a balance to minimize print time. 

Print Parallel to the Joint 

It’s always a good idea to print parallel to the joint, to reduce the shearing force against the print layers. However, that will mean going into your slicer and aligning those settings, for each joint. 

Here, a diagonal approach is often best, because it optimizes for weakness against both shear and tension. 

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Frequently Asked Questions

If you still have questions about 3D printing interlocking parts, these answers may help. 

Can you melt 3D printed parts together?

3D print welding is a normal thing when using PLA and other low-heat thermoplastics. However, if you do, it’s important to note that welding 3D printed parts together can cause unsightly joints. 

In addition, you’re unlikely to create a clean weld all the way through the part, which means the joint may be weak. 

As a result, many people prefer to use Cyanoacrylate glue rather than welding if a permanent joint is preferred. 

What is the 45-degree rule in 3D printing?

If you use filament to print overhangs at 45 degrees or less, you can print without supports. If you print at a steeper angle, you’ll need supports. 

However, this rule does not apply to resin printing, which almost always needs supports. 

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Final Thoughts

3D printing interlocking parts can be a challenge because there are a lot of factors to consider. However, most often, those considerations come into play when you’re designing your own parts. If you’re using premade files, you can likely just try a print and see how it goes. It’s always a good idea to check settings, make sure you’re printing at the right width, and ensure you’re using a high enough infill, otherwise, you should be good to go.