Could you imagine a product made all in one piece? We can, because SLS 3D printing technology makes it possible to print very complicated structures in just one take. But in real life, we are surrounded by objects made from a lot of parts that require to be connected somehow. Designer, even knowing the possibilities of SLS, sometimes needs to create models that are a part of something bigger. From this article, you will learn how to design connections for 3D printed, SLS models.
Sometimes a model that you create would be bigger than the maximum size that your 3D printer can produce. In such case, it is necessary to split the model into pieces that are able to be later joined. So how to design it in a way that makes all the parts capable of being easily attached together. And how to make them detachable or create a movable interlocking connection?
Let me show you the types of connections that are perfect to be used with SLS technology and that can be easily printed with Sinterit Lisa and Lisa PRO. Thanks to the possibilities of this technology, we can do amazing things, and all that without any support structures.
Types of connections
We can distinguish two types of connections: permanent and separable.
It seems obvious, but to stay specific: we use permanent connections if there is no need to separate elements in the future. They are perfect if the only purpose is to connect parts of the model that is bigger than the maximum printing size of your device.
The elements can be of course joined using glue, rivets or screws, and it also requires designing dedicated surfaces or elements to make it possible. But in this article I will focus on connection types that don’t require additional resources and are simple to apply thanks to incredible possibilities of SLS 3D printing.
One of the possible solutions is a permanent connection using SLS 3D printed elements such as pins, prgs, hooks etc. If once attached, it is not possible to remove them without damaging the model. Between all these kinds of joints, we can distinguish also visible and non-visible connections. The former are used if they need to be visible (for safety or easy access) and the latter are perfect if the priority is the model’s aesthetics and surface.
Permanent connection: as a rock
This solution lets to connect elements if there is not much space for additional elements. It works well for closed housings because there are no protruding elements, so there is a low risk of accidentally detaching the parts.
2. Sleeve + channel + hook
It is a perfect solution to apply for elements with identical profile. The connection itself remains covered and if the clearances are designed right, it is very firm and stable.
3. A narrowing sleeve which expands after slipping in
A popular connection which is also a very simple solution. The pin consists of a few segments and is primarily bent towards the hole’s outside. It is narrows while being slipped in and then expands and locks in the gap with a larger diameter.
4. Additional pin
An additional pin is a modified version of the previous solution. The difference is that the expanding pin is a separate part. The advantage is a possibility to remove it after joining the elements, for example by drilling, and detach the parts.
A great inspiration for designing connections is the injection molding technology. With SLS it is possible to get similar parameters of thickness and precision, although the shape might be even more complicated as there are no limitations implicated by the mold’s shape.
Separable connection: a useful point of view
Separable connections are applied to elements which are used as a housing, plug or a cover and are used in cases requiring access to the covered parts.
1. Separable snap-fit
It is similar to one of the non-separable connections, but it gives access to the fastening elements which lets it to be easily detached.
2. A thread with a screw
Using SLS you don’t have to get a separate screw – simply print it. It’s a widely known solution which we can improve a bit. With this freedom of design, why not make the screw a movable part integrated with a model’s part? It can be tailored for any tool such as a Philips or a flat screwdriver, torx, hex key or anything else.
3. A pin with clearance
A pin with designed clearance of about 0.05 – 0.1 mm at each side lets the elements to be connected by friction forces between them. To reduce it a little bit, you can decrease the contact area by adding some cutouts.
4. A sleeve with a groove
It is very useful and simple at the same time. You can think of the pin as a some kind of a key. Properly designed fences and canals enables it to be pulled out.
After turning it by 90⁰ the fence blocks in the channel holding the whole connection. If you design the clearances properly, the whole structure can be printed in one piece. Isn’t that impressive?
Good old friction. If you combine the right wall thickness and clearances between elements, the elements can be simply joined like jigsaw puzzles. Simple, isn’t it?
I like to move it: Movable connections
If the model requires a movable connection, SLS is a great choice to make it. Why? Because there’s no need to assemble them – you can 3D print them at once!
If the model’s position in the printing bed would have a negative impact on the quality of the movable part (eg. it is impossible for the holes axis to be in direction with Z axis), you can always prints some parts separately and join them together after they’re printed..
2. A ball joint
A connection that gives the elements much freedom and is perfect for any joints that require to be movable and rotatable.
3. A detachable hinge
It is almost identical with a standard hinge, but it gives the possibility to detach it and attach it again.
Feel the force: Tensions to take into account while designing a connection
A crucial factor to take into account while choosing the right connection type and its size are all the forces that would press on the model. If the only purpose is to keep the elements together, you can use tiny connectors that would mainly position the parts. But if the model is designed to be heavily loaded, you would need thicker walls, deeper grooves, stronger sleeves etc.
Why SLS is the best technology to print these connections?
The SLS 3D printing technology is perfectly suitable everywhere where high precision and perfect fitting is required or the elements and walls are a few millimeters thin. And especially if the model consists of closed elements with tiny clearances of about tenths of a millimeter.
Remember that depending on the desired precision you need to choose an appropriate layer height and don’t forget to set the holes axes in direction with “Z” axis to assure the highest possible quality and fit.
If you design any elements desired do expand after putting it in, remember to make it previously bent to prevent it from detaching.
All these examples apply to hard materials (PA12 Smooth and PA11 Onyx). If you want to join a soft and hard materials with elastic or just two elastic parts – we’ll address this in future articles.
Product designer at Sinterit