Additive manufacturing has many advantages, including the possibility of printing sets of parts in a single print or complex geometries such as a thread. However, in the latter case, the thread design is not made so that there is the possibility of a regular assembly since, being made of plastic, they can deform and lose their dimensions or even fracture. 

When you want to manufacture a certain type of part in the 3D printing world, there are numerous occasions when it is necessary to use strong joints to make removable parts or to ensure the permanent bonding of large volume elements.

Mechanical components manufactured using traditional technologies become a support for additive manufacturing parts. And in the specific case of joining parts, inserts are particularly noteworthy.

What are inserts?

An alternative to incorporating nuts in the middle of a print is to use metal inserts. Metal inserts are threaded fasteners that are inserted into a hole to provide the part with a possible screw-thread connection.

Image 1: Inserts. Source: Filament2print.

There are two key factors to consider when selecting the best type of insert: pull-out resistance and twist resistance. 

• The pull-out resistance is the resistance of the insert to pull out of its housing when the screw is tightened. This is a value that can be increased by increasing the length of the insert.

• The resistance to rotation is the opposition to the insert turning on itself when the screw is tightened. In this case, the larger the diameter of the insert, the greater the torque capacity, as there will be more surface area clamping on the part.

Generically, factors such as the diameter and size of the insert will be taken into account (for which it is necessary to refer to the manufacturer's technical data sheet). And materials, both of the insert and of the plastic where the installation is to be carried out. There is a wide variety of inserts to be installed in any type of material, whether for metal, wood, plastic or similar. The most common material in the manufacture of inserts is brass, although other materials such as stainless steel (corrosion resistant) or aluminium (lighter weight) are also appearing.

For FDM (when manufacturing with thermoplastic materials) almost any insert would be suitable, but when manufacturing by SLA (which uses thermosetting materials and therefore once printed cannot be modified by heat or pressure) only self-tapping or helicoil inserts could be installed.

It is very important to emphasise that despite the fact that in FDM an insert made of almost any type of material can be used, the procedure for placing this insert is always going to be more problematic than in SLA parts. This is due to the non-isotropy of the part, which means that when faced with a cold-placed insert, the pressure is such that the part delaminates.

There are many types of inserts, and the common classification is according to their form of insertion.

Types of inserts

Heat insertion: Heat inserts are used with thermoplastic materials, which melt at low temperatures. The method of use is very simple and only requires a soldering iron or other heat source (Modifi3D Pro), which causes the insert to heat up by contact through heat transmission. To place this type of insert, there must be a hole in the part whose diameter is slightly smaller than the outer diameter of the insert. Using an insertion tool, and controlling the pressure (because it is possible to melt the part around the hole), press the insert over the edge of the hole to expand the plastic. The insert will be trapped between the fibres of the plastic when it cools down. The insert will be in the right position when both edges, part and insert, are at the same level. With the correct positioning of this type of insert, a tensile strength of 15 kg and a torsional strength of 2.5 kg can be achieved. In addition, heat tapped inserts have both holes open so they can be considered through holes, meaning that the length of the insert will not be a limitation even if the screw to be tapped has to be inserted deeper than the length of the insert itself.

Image 2: Heat insert. Source: Filament2print.

Pressure: Pressure inserts, or rivets, require a hole with the same diameter as the rivet. Their installation is very simple as, like any rivet, they simply need to be pressed into place with a compression tool. As in the previous case, the insert will be in the right position when the two edges are at the same level. It is very important to note that during installation, the inserts must be properly pressed as the parts can be crushed near the holes.

Self-threading: For certain materials used in SLS and resin 3D printing that melt at very high temperatures or do not exhibit stable behaviour when heated, self-threading inserts are ideal. Characterised by an internal and external thread for easy installation in the part, these inserts are placed using an ensat-type threading tool. The part must be manufactured with a hole that has the same nominal diameter as the insert, as these inserts create a thread on the plastic as they are installed, resulting in an assembly that can be subjected to much greater stresses compared to heat inserts.  In addition, these inserts are made of stainless steel, a material that is highly resistant to wear and aggressive environments.

Image 3: Self-tapping insert and tool. Source: Filament2print.

Helicoil: Like the self-tapping inserts, helicoil inserts also have 2 threads and function in the same way. The hole in the workpiece must be dimensioned according to the standard of the selected helicoil. To fit the insert, the thread must first be created in the hole using a tap and then installed using a tool. Finally, the small flange at the bottom of the insert, which acts as a stop for the installation tool, is broken off. While it is true that these inserts are not inexpensive, the appearance and practicality are remarkable.

Recommendations

With regard to the diameter of the holes, it must be taken into account that when printing the part there will be dimensional variations between the 3D design and the part, therefore when designing the holes it will be necessary to apply a tolerance to them. In addition, the hole must be long enough for the installation of the insert, also taking into consideration the extra length of the screw protrusion. Another thing to take into account related to the design is the wall thickness; try to apply values high enough so that when machining the hole, it does not enter the infill of the part.

Image 4: Insertion of the insert by heat. Source: Markforged.

In terms of installation, it is very important to ensure certain aspects such as positioning and speed of operation. Always ensure that the insert is, as far as possible, centred and perpendicular to the hole. This applies to heat, pressure or threaded inserts, as the insertion of the insert can lead to cross-threading and damage to the thread or even result in subsequent incorrect insertion of the connecting screw. Keeping in mind that the parts are made of plastic and that the insert is a metal element will affect the speed of installation, as the hardness of the metal can damage the printed parts. This is why the speed will have to be controlled so as not to damage the walls of the hole.

As we know, 3D printing is in full evolution and solutions like this are the ones that make the difference when it comes to obtaining parts with greater complexity and that respond more accurately to the needs of the industry.