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If you’re making a 3D printed part with moving elements or you need to connect different pieces in the same part, using a 3D printed joint is the standard method.
In traditional manufacturing, the objective of joinery—the process of designing and making joints—is often to avoid using nails, screws, or similar. But this can result in you having to design complicated shapes.
With 3D printed joinery, making a complicated joint is likely to be no more expensive than printing the simplest element of your 3D printed part. You can also design the joint to be an integral part of your design.
But, because of all the potential options, it’s vital that you choose the right approach in the design stage.
To do so, you and your designers also need to be aware of the geometric possibilities offered by your 3D printer.
This article introduces you to some of the most effective ways in which you can design joints for 3D printing and their benefits.
The benefits of inserting joints directly in your 3D design
You save time
If you integrate 3D printed joints directly in your design, you can reduce the assembly time of your 3D printed parts, whether you’re prototyping or mass-producing.
Your 3D printed parts can be strong but flexible
The plastics you use with 3D printing, such as HP 3D Printing Materials for example, are ideally suited to create joints like 3D printed snap-fits that need to be strong and flexible. You can also experiment with arriving at minimum wall thickness more easily.
Extra precision
When you’ve designed complex joints, they need to be printed accurately so they can be assembled perfectly. 3D printing processes like HP Multi Jet Fusion (MJF) technology can deliver fine detail and extremely precise results.
Easier iteration
With 3D printing, if a joint doesn’t fit, you don’t have to make a mold every time you iterate. All you do is edit your 3D printed part and print again.
Designing different types of 3D printed joints
Threaded unions
Screws and threaded parts are the most widely used kind of joints—for example, in customized fasteners and caps or to join tubes. They make for durable, strong joints that can be taken apart.
3D printing enables you to print external and internal threads inside the part, removing the need for threads to be formed mechanically.
For 3D printing processes such as HP MJF it’s recommended to print external and internal threads in sizes larger than 6 mm, M6, or ¼ inch per the Imperial system, to achieve favorable results in all printing orientations. If a thread less than 6 mm is needed, it’s best to use self-tapping screws, threaded inserts, or to machine the thread for the small tolerances required.
Remember: Tolerances depend on the material, print mode, and post-processing selected. You should validate the design with different offsets before printing multiple parts.
Self-tapping screws
HP MJF technology allows you to print small features such as external and internal threads inside the part. But when a small thread, up to 6 mm, is needed, you should use self-tapping screws that tap their own threads as they are driven into the part.
Certain types of self-tapping screws require a pre-formed hole. Your screw supplier will recommend dimensions.
Machined threads
If you’re working with a thread of up to 6 mm, you can always machine the part after it’s printed to get the accuracy you need.
A standard machining process can achieve dimensional tolerances up to ±0.05 mm. The tools recommended for machining HP MJF parts are the same as for other technical plastics.
Internal threads
To machine an internal thread, it’s necessary to start from a pre-formed hole and then machine the thread using the required tap.
When you design the pre-hole on the printed part, you can refer to standard drill size recommendations for plastic and metal.
External threads
When you machine an external thread, you start from a solid printed cylinder and then machine the thread using the required die.
The diameter of the cylinder to be machined must be slightly smaller than the die's largest diameter. Typical cylinder diameter recommendations for plastic and metal apply.
Standard 3D printed threads
There are a few recommendations when designing threads larger than 6 mm under international standards – for example, DIN 13-1, ISO 965-2, ANSI/ASME B1.1. These usually specify tolerances relative to diameter and pitch of a thread.
When designing internal threads, less restrictive maximum tolerance values should be used, and when designing external threads, use more restrictive minimum tolerance values.
The HP MJF Handbook includes details about drill size recommendations, recommended thread tolerances and customized thread design tips, and post-processing information.
Snap-fits
3D printing allows for the designing and printing of parts with specific design features, such as snap-fits, to connect them integrated into the part itself.
A snap-fit joins plastic parts by using a protruding feature on one part, such as a hook that deflects during assembly and is inserted into a groove or a slot in the second part. After it’s assembled, the protruding feature goes back to its initial position.
The way a snap-fit is designed determines whether it can be disassembled and re-assembled several times and the force required to do so. This assembly method is suited to thermoplastic materials because of their flexibility, high elongation, and ability to be printed into complex shapes.
Snap-fits are a simple and economical way to assemble plastic parts by drastically reducing assembly time.
Types of snap-fit
These are some of the more popular types of snap-fit.
Cantilever snap-fit
The most commonly used type of snap-fit is the cantilever snap. It consists of a cantilever beam with an overhang at the end.
L-shaped snap-fit
An L-shaped snap-fit can be an alternative when it’s not possible to design a cantilever snap-fit without compromising robustness of assembly and strength due to material or geometrical constraints.
Adding a groove to the base of the snap-fit increases its flexibility while reducing the strain on the beam.
U-shaped snap-fit
When it’s necessary to increase snap-fit flexibility within a confined space, a U-shaped snap-fit is another alternative to the cantilever snap-fit.
U-shaped snap-fits are extremely flexible, which makes them easier to remove. They are usually used when parts need to be pulled apart repeatedly or when two parts don’t require a lot of force to stay in position. For example, in a battery compartment lid.
The HP MJF Handbook lists many more types of snap-fits along with design tips.
3D printed joints design – adhesive bonding
In certain 3D printing processes, it’s sometimes necessary to divide a part into different pieces and then re-join them.
There are two main reasons why: to split big parts and increase packing density.
Splitting big parts
Some 3D printed parts, such as car bumpers, are too big to fit inside the 3D printer build chamber. In this case, they can be split into several pieces and then re-assembled after printing.
Parts that need bonding to achieve a strong joint to function properly, like jigs and fixtures, can also be split.
Increasing packing density
With powder bed technologies such as MJF or Selective Laser Sintering (SLS) , the packing density of your build is one of your most important considerations.
Packing density refers to the percentage of the build volume taken up by printed parts. You achieve maximum efficiency when you print by increasing packing density.
Splitting parts is a possible option when packing limitations—for example, caused by geometry—make it otherwise impossible to achieve maximum printing efficiency and value.
So, you could optimize a 3D printed part’s packing density by adding hinges, so it folds. You could block these after printing using an adhesive or mechanical lock.
Figure: Example of packing density optimization. Box original design and number of parts that fit in the print bed.
Data courtesy2
Figure: Example of split part to optimized packing density: With the split, the packing density increased by 67%.
Data courtesy2
Bonding robustness
The union design of a bonded 3D printed part and the way it has been split or cut into different pieces is vital for your success.
Union design
The amount of time you spend on effective union design depends on the purpose of the 3D printed part you’re making. A visual prototype that doesn’t need to withstand any loads will require a simple union design. But an automotive part must be designed to optimize its performance.
Your design options depend on the thickness of the bonded parts and on the possibility of modifying the final geometry.
Thickness < 1.7 mm with no geometry modification allowed.
One of the objectives in the design of the union is to increase the bond area as much as possible. Including features that reference one piece to the other during bonding will help achieve the proper position between the parts and will optimize the final result.
The most recommended design option in this instance is a dove or jigsaw feature, as shown below:
Figure: Dove/Jigsaw design recommendations
This type of union increases the bonding area and positions and holds both pieces that will be assembled.
For 3D printed parts that need to be produced more quickly, you have other design options that deliver good results:
Figure: Square tongue design recommendations
Figure: Tooth design recommendations
Figure: Butt design recommendations
If union designs such as dove, square tongues, or a tooth are not possible because of geometrical constraints, you could try a butt union. But you need to bear in mind that a straight line in the bonding area with a smaller bonding area is the weakest design option.
All your union design options can be easily applied to a part using 3D software such as Materialise Magics, or they can be directly designed using CAD software.
The HP MJF Handbook offers more design options for different thickness specifications.
International Tolerance (IT) grades
Designing a part often involves the use of International Tolerance Grades. Tolerances should be considered at an early stage of the product development process.
You must also take into account the permissible range of variation in dimensions to make sure a part fits properly and works as intended at the part or product design for manufacturing and assembly design stage.
Depending on how the parts must interact to create a final product or achieve the 3D print assembly’s functional needs, the required tolerances will be tighter or wider.
The most common manufacturing processes have an associated IT Grade that specifies their capability to provide accurate parts. Full details about IT Tolerance Grades are in the HP MJF Handbook.
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Footnotes and disclaimers
- Data courtesy of Campetella Robotic Center
- Data courtesy of Henkel AG & Co.