Markforged Mechanical Features

Most 3D printing plastics are not strong enough to print threads. Their yield strength is significantly lower than that of metals. Even fibre reinforcements do not protect against high wear. Precision and high bonding strength can only be guaranteed by metal threads.

We have already explained before how this problem can be avoided by using heat and thread inserts. The metal insert melts and bonds with the plastic. This can often be a good solution, but there are design limitations. The insert must be placed on the front of a part and its pull-out resistance is limited by the material properties of the plastic around it.

A threaded insert is worked into a 3D printed part using heat.

You can work around the operation. You may know this technique as “overprinting”, “co-processing” or “embedded printing”. The process is similar to overmolding in injection molding, where parts are placed in the mold and the material is molded around them. For example, wheels of scooters are produced in this way. The rubber tyres are moulded around the metal hubs.

Wheels of a Razor scooter. The polyurethane tyre is moulded around the plastic hub of the wheel. (source: Razor)


We can also use this technique in 3D printing. We stop printing and embed external components. This enables us to produce assemblies that were previously considered impossible to manufacture. The insert is completely embedded in the print. If it is held in the workpiece by fibre-reinforced layers of material, the most durable screw connections are possible. All you need to do is design a recess of the appropriate size, stop the pressure just before adding the top layer, insert the component, and continue the printing.

Design guidelines:

  1. Tolerances: When embedding components in 3D printed parts, the printer’s tolerance is the most critical factor. On the Mark Two, a gap of 0.05 to 0.08 mm on each side ensures a nice fit. To be on the safe side, make sure to check this on your printer. If the gap is too large, the print material will not bond sufficiently with the insert, and if there is not enough space, the insert will not fit in.
  2. The surface: The surface of the part you are embedding is also important. If it is smooth, you will probably continue printing directly over it. You can apply some glue to the part. If the surface is not flat, you must not touch it during the rest of the printing process. In any case, the top of the embedded part must be under the print head, otherwise it will run in. During the design process, pay attention to what the surface looks like and when the printing must be stopped and the part inserted.
  3. Supporting material: Ideally, you don’t need supports where you want to embed parts, because otherwise they would collide. If it is unavoidable, you must remove them before inserting the part. You also need to make sure that there is no printing in the air or on the insert.
  4. Selecting the nut: Square nuts are actually the most suitable in this application because even if they are twisted a lot, they will have difficulty moving out of the shape of the recess. But since hexagon nuts are more common, I will work with them in the following. If you want to work more with this method, square nuts would be a useful investment.

Summary of the procedure:

  1. With your CAD program you insert a hexagonal cavity into the component to be printed at the desired position.
  2. Load the part into Eiger and activate the pause function before the top layer of the cavity.
  3. Start printing and insert the nut during the pause.
  4. Then simply click on “Resume” and observe that the nozzle does not move against the nut.

Embed nuts in the XY layer

1. Designing the recess: Once you have designed the hole, measure the nut to be embedded and insert a recess of an appropriate size. It is best to design an auxiliary layer for this purpose.

The layer on which I draw the sketch for the recess.

I use an M5 hexagon nut (width 7.85 mm, height 3.85 mm). The data in the data sheet are inaccurate (8 x 4 mm), so I measured manually. Now the tolerances must be added (0.05 mm on each side). So I have a width of 7.95 mm and a height of 3.95 mm. But I want to make sure that the nut fits and round to 8 x 4 mm.

Sketch the nut profile including tolerances on the plane.

Fill out the sketch as previously calculated. Do not round or chamfer the edges, otherwise you will not be able to insert the nut into the part later.

Extrude the sketch to create the cavity.

A cross-section of the nut holes for the bracket.

2. Add an interruption: In Eiger you can add breaks according to defined layers. First make sure that you have turned off support (unless absolutely necessary). This is done under “Advanced settings”.

The support is turned off in the advanced settings.

Find the first layer of the recess and scroll to the layer BEFORE. A click on “Pause after layer” inserts the break.

Insert an interruption with the “Pause after layer” function.

3. Add fibers: To increase the holding force of the nut, you can add fibers above or below your part. In the picture below, I have added fibers on both sides of the nut for better grip. They can also be placed on the six side walls, which prevents the nut from twisting.

Fibers above and below the nut prevent it from being pulled out.

4. Printing the part: Now it’s time to print. In the shift details in Eiger you can see when printing is interrupted, so you do not have to wait. During the break, push the component into the recess and continue printing. To ensure that the nylon or onyx holds securely on the component, you can put a little of our glue on it before continuing (Attention! On the printed material itself, the glue can lead to layer separation!) To embed nuts in places that are difficult to reach, you can remove the base plate from the printer without worrying and insert it again later. The kinematic couplings on the bottom snap back into their original position with 10 micrometer accuracy.

Simply remove the build platform, insert the nuts and continue the printing.

The embedded nuts of the 3D printed part.

5. Dealing with support material and more complicated geometries (if necessary): If you have to use support material, you can pull it out with flat-nose pliers during the interruption. However, this only works if your cavity has a flat ceiling. If you are embedding parts with more complex surfaces, you cannot use support material. You must either use curved or angled overhangs to keep the cavity clean, or print a secondary part with a flat surface to remove the material. This process is described below.

Printing of secondary parts for embedding nuts on other layers

The addition of embedded nuts on other layers involves design effort. A secondary part is required. As an example, I want to embed a hex nut into my part as shown in the cross section below with its axis parallel to the building board. (A square nut would be the simplest solution here, as it would provide a flat surface for printing). If I leave the cavity as shown, support material to be removed is necessary.

With a flat overhang the sides of the ceiling are not supported.

I could build an angled overhang into the cavity. But now I still can’t use any supporting material and furthermore the nut is not tight, but would twist when tightening a screw in the cavity.

With an angled overhang, the nut rotates in the cavity.

Instead, I can add a secondary part to the print, which secures the nut and gives the printer a flat top for printing. To do this, I create a recess with a flat top for the nut:

With a little more space in the recess we can integrate a secondary part to secure the nut.

Design a small part that fills the remaining space in the cavity. Take the tolerances into account here as well.

The nut can now be fixed inside the cavity with a secondary part.

This can be printed next to the main component, so I can insert it during the interruption with the nut, and continue printing on the flat top of the secondary component, as with the angled square nut below.

You can also use the same method to embed nuts at other angles, you just need enough space to place them. The small rectangular piece in the cross-sectional view below secures a square nut crosswise in the printed part:

This cross-section shows a diagonally embedded square nut, which is secured with a triangular gap filler.

Secondary inserts allow screws at all angles.

With this technique, nuts can be installed in 3D-printed parts at any angle and on any layer. But you do not have to limit yourself to nuts. Try out how you can best use nuts or other components and let us know your results.