Splitting and Assembling Large 3D Printed Parts

Split v4 copy
Mechanical 3D printed component that was split, printed and is about to be reassembled

Splitting and Assembling Large 3D Printed Parts

Written By: Ron Luther, Manufacturing Specialist - March 13, 2024

One of the major constraints encountered when designing for 3D printing is build size limitations. So what happens when you’ve designed a part that is too big? The best solution is to use one of our Large Format 3D Printers. Sometimes, however, material selection or other factors can limit manufacturing to small or standard build envelopes. In these cases, the part needs to be split and printed in sections, which then must be assembled. While this is a great workaround, it comes with some added complications that can affect cost, quality, timeline and dimensional accuracy.

Where to make splits

When deciding where to split your parts, optimizing for fewer seams is a good starting point, but there are many other factors that can influence seam placement.

Aesthetic

If an uninterrupted surface finish is important, then it may make sense to place seams in less visible areas of the part, or in areas that are easier to fill, sand, and finish.

Mechanical Properties

Seams can affect the mechanical properties of a part, creating weaknesses along the split not present in a solid part. Consider the physical requirements for your part and place seams in areas that experience less force or impact during use. Alternatively, you can add reinforcement across the seam to account for this, but this will increase the print cost.

Print Constraints

If you have a good knowledge of the additive technology you are using to produce your parts, you can make strategic decisions about seam placement based on printability. This could mean reducing the amount of support required for each piece, reducing print time, choosing splits that favor good part orientation, separating large parts liable to warp, opening trapped volumes, optimizing to reduce stepping or to increase strength, as well as many other potential print parameters. 

Using Multiple Technologies

Sometimes it may be necessary or desirable to use multiple technologies or materials for a single part. For example, a part may have some isolated high detail areas surrounded by much simpler geometry. This could be a good use case for printing with a mix of FDM and SLA. The SLA would be used for its ultra high resolution surface finish, and the FDM would be a good candidate for large forms that don’t require high detail but need to be strong. 

Assembly

It is crucial to make sure that seams are placed in such a way that assembly is possible, and ideally, easy. If fasteners are being used to connect pieces together from the inside, ensure that tool access is possible. Always carefully check the CAD to make sure that parts are able to actually move into position without interference.

A part too large for a standard industrial 3d printer build volume
The part must be split and printed as pieces

Parts too large for a standard printer must be segmented and printed in pieces, then reassembled.

Accuracy and Tolerances

Splitting parts comes with some accuracy penalties! For every seam, there is a chance that the mating parts don’t seat quite perfectly and dimensions across the entire part may be less accurate than a single solid piece. This is why testing is absolutely crucial. Rather than printing out an entire set of parts only to find they don’t quite fit, isolate a small section of each side of a join and print a test part. Dry fit, measure, and adjust the CAD as needed until you are confident the entire assembly will fit to spec. You can refer to our Design Guidelines for recommended clearance offsets and process tolerances for each technology.

offset tests done in SLA

A set of tests to determine the best offset for the keys on a particular feature.

Attachment features

The simplest possible seam involves gluing the unmodified split faces back together. This can work for parts that have split faces with high surface area and that are easy to align, but since most 3D printed parts are fairly complex hollowed shapes, the thin walls across a split are not likely to align well or glue strongly this way. There are two major strategies for designing features that aid assembly: joinery and fasteners.

wood joint types

Various wood joint types

image credit: dimensions.com

Joinery

Similar to wood joinery, this method consists of modifying the geometry around the split to create features that fit and align together in a specific position, ideally without adding more material. There are many different join types, but some common ones are lap joint, mortise and tenon, tongue and groove, dovetail, and doweled. 3D printing also allows for a lot more design freedom than wood, so it is easy to come up with highly tailored custom joint shapes.

It is important to factor in the process tolerances when designing joinery features so that they fit as intended. This involves adding a clearance or interference offset, depending on whether it is a press-fit or a glued joint. It’s also good practice to bevel the front edges of a key so that it can easily enter the slot.  

For technologies with more pronounced stepping (like FDM), it is beneficial to print matched joint features at the same height so that layer height rounding is the same for both halves. Otherwise, a large offset is required. It is usually better to stick to simple rectangular forms to avoid stepping artifacts generated by curved shapes.

fasteners are extremely useful for assembling parts made with additive manufacturing

There is a whole universe of fasteners available for assembling your parts.

Fasteners

Sometimes designers opt for fasteners instead of joinery features. This can be beneficial for providing adequate seam pressure on parts that are difficult or impossible to clamp properly, and it also allows for disassembly (given that the part isn’t being filled and painted).

Accommodating fasteners requires the addition of features inside the part on either side of the split that allow for nuts and bolts or other hardware (such as heat set inserts, magnets etc.) to be mounted or passed through. This will add material and considerable CAD design time. As mentioned above, always make sure tools can fit into the relevant areas through the entire assembly process. 

Designing jigs for accurate reassembly

If accuracy is a priority, it may be necessary to produce a jig that can hold the mating pieces in the correct position during assembly or gluing. Jigs can be 3D printed or machined, and usually take advantage of existing holes or other features that can be used to attach the parts to the jig. If you are using glue there is a risk that the part will stick to the jig, so it is a good idea to design no-contact areas where the glue might leak out. Jigs are an excellent way to ensure consistent dimensions during repeat production. To learn more about designing jigs for 3D printing, read our online guide.

jigs and fixtures are used to assist in assembly of multi-section parts

Use jigs to maintain part alignment during assembly

Finishing

Unless a part is strictly functional, it is usually desirable to hide seams and make the part look like a single piece. This involves filling, sanding, priming, and painting, and involves quite a bit of skill, equipment and labor to pull off well. We are able to provide high quality paint finishes, but if you are interested in learning more, take a look at our guide to painting 3D printed parts.

professional paint finishing of a large automotive component

Finishing a 3D printed automotive part.

Print it Big

All in all, the added time, labor, and material required to design, test, glue, assemble, clean up and finish a part can be significant and is usually not worth it unless multiple materials are being used or components need to be embedded inside of the part.

If you are considering a large format project, we recommend you take the time to estimate the total project cost and lead time and see how it compares to printing it as a single part by using our instant quote tool. Feel free to get in touch with us and we’d be happy to guide you through the process!