Press Fits and Inserts

Press Fits and Inserts

Best Practice

p/n 33-D210 Rev A

Metal inserts are used in both traditional machined, injection molded, and 3D printed plastic parts. Metal inserts allow the use of standard fasteners such as machine screws. Further, threaded metal inserts allow for many cycles of assembly and disassembly.

Drill/Tap, Press, & Screw Diagnostic (Black – Projet 5600, Clear Projet 2500 M2R-CL)

From left to right the following features are shown: 1) Drilled and taped hole with hardware, 2) Press fit precision dowel

pins, 3) Screws added to 3D printed injection molded bosses, 4) Press-fit brass inserts, and 5) machined precision holes.

The most common type of insert for 3D printed parts is a press-fit insert. The knurled body holds these inserts in place in plastic parts. There is no need to tap the hole. These inserts are tapered, so they align themselves as they’re pressed into the hole. A hammer or arbor press is used. It is sometimes advantageous to thread a correct size bolt (say, about 1 inch long or 2-3 cm) into the insert first and then apply the insertion force (hammer or arbor press) to the end of the bolt.

Metal Dowel Pins inserted with hammer, arbor press, and with screw for extension

This method protects the surface quality of the part and can allow insertion into tight areas of a complex part.  The hole size for an insert is provided by the manufacturer and can be printed or drilled.  It is always

recommended to first try the insert on a piece of spare material before attempting to install the insert to the actual part.

Screw-to-Expand Inserts

For screw-to-expand type of insert, press or hammer the insert into a printed or drilled hole. Install a screw to expand the insert and drive the knurls into the surrounding plastic. Pull-out resistant versions have fins that cut into the surrounding material for a more secure hold than knurled inserts. Through-hole versions are installed in the underside of the material. The flange keeps the insert from being pulled through the wall of the part.

Flanged Screw-to-expand Inserts

On flanged inserts, the flange provides a load bearing surface for use in weaker plastics or for higher temperature applications. Press or hammer the insert into a printed or drilled hole. Install a screw to expand the insert and drive the knurls into the surrounding plastic.

Push-to-Expand Inserts

For push-to-expand inserts, an installation tool is used to push down on the inside of the insert so it expands the knurled material into the surrounding part before the screw is inserted. 

Fourth column from left are brass screw-to-expand inserts. 

These were inserted with either an arbor press and/or a hammer.

Tapping Inserts

Tapping inserts cut their own threads so you don't have to tap the hole. Install them into a printed or drilled hole with an installation tool and a ratchet wrench. These cut threads into plastics and have excellent pull-out resistance. Flutes in the insert channel loose chips up and out to keep threads clean during installation.  Installation tools can also be placed in the chuck on a drill press and turned by hand. 

Helical Inserts

While normally used in metal parts, helical inserts can be used with 3D printed threads and/or traditional drilled and tapped holes. To install, drill and tap a hole or 3D print a threaded hole. Use the suggested drill and threads size for the desired insert. Screw the insert onto the installation tool. The prong must be properly engaged in the tool’s driving contour. Install the insert a quarter to half turn below the surface of the hole. Remove the prong to allow full passage of your screw into the insert.

Installation of Helical Inserts into 3D printed plastic parts

General Considerations

If an insert is used with a thin-walled injection molded screw boss or if the insert seems very tight or likely to break the part, it may be advantageous to make the wall thicker or even solid for prototype purposes. A solid wall will tend to create a compressive stress instead of tensile hoop stress and will be more robust, especially for a brittle material. A tap with threads equal to that of the insert can be used to clean out the insert after installation. This will allow the machine screw hardware to thread in and out easier and with less torque. This is especially helpful for expansion type of inserts, which use the machine screw hardware to expand the insert. MJP acrylate parts will not melt when heat is applied like with traditional thermoplastics. However, the material will soften when heated and can assist in installing the insert. One can use pliers to hold the insert, or to hold a screw that is threaded into the insert. Use the tip of a soldering iron and apply downward pressure. The heat transfers to the insert and, after a moment, should begin to sink into the softening plastic. Continue with downward pressure until the insert is at the desired depth, then remove the heat and hold the insert in place long enough for the plastic to stiffen. When using a soldering iron, be cautions that the tip does not snag the insert and pull it back out. A knife or screwdriver can be used to hold the insert in place when retracting the soldering iron tip.

Slightly heating the part or the insert will soften the material and facilitate installation

For MJP parts, a soldering iron should be used on the lowest setting. The goal is to heat the plastic just enough that it softens. When using heat for installation, it is best to carefully position the insert in the final position. The plastic will form around the insert after cooling and it will be difficult to move. Repeated heating is not recommended. Heating the part with a blow dryer or heat gun can greatly improve the ability of the MJP material to accept an insert without burning the material or breaking the part. Finally, the engineering materials (M2G-CL and M2G-DUR) are less stiff compared to the rigid family of materials and might require a slightly smaller hole compared to the manufacturer recommendation in order to create a solid holding force. Using heat to soften these materials can assist in insertion. One of the simplest inserts is to use a standard nut as the metal insert. One can add a hexagonal pocket to the part in CAD or press the nut into a hole using force/temperature as described above. The screw can be used on either the front or back side of the nut. 

Standard nut used as a robust metal insert

When possible, a simple method to join parts is to use a threaded screw that passes through the entire assembly and it attached with a washer and nut. 

Standard nut and bolt penetrating through the part with nut and washer

This type of joint puts the part into compression and should create a robust assembly for all materials. Even if this is not the final design intent for the assembly, this method will provide a robust joint for early functional prototyping or jigs and fixtures, etc.