Hot Glue gets a Bad Rap

In this post, I’ll detail how I use hot glue and make instant setting joints that are stronger than I can break apart by hand.

I use hot glue for almost every 3d print that I need to glue. This video is a good watch for background info on glues. In summary, hot glue is fast, easy, and a great gap filler. It will let you down on smooth and cold surfaces. Let’s fix that.

For this technique, you will need a temperature controlled hot air gun and a can of freeze spray! Taking advantage of different melting temperatures, I can heat up the plastic to better fuse with the hot glue without melting the plastic at all. Then instantly cool the glue and move on.

1. Preheat PLA plastic with 140C hot air
2. Apply Hot Glue
3. More hot air to ensure glue and plastic are evenly heated
4. Push plastic joint together
5. Cool down with Freeze Spray

Surface Prep

Hot glue doesn’t penetrate the plastic, so all of it’s hold strength comes from gripping the surface area. First we want to increase the surface area as much as possible.

Most 3D printed parts already have a ridged surface from the print lines on the top and sides. The bottom of the 3d printed part is typically smooth. To make the bottom glueable, orient the part’s gluing surface away from the bed, or use a textured print bed.

Hot Air

The goal of using hot air is to get the hot glue and the plastic to an same temperature. This gives the hot glue a stronger bond, as it can not cooling during application.

I use 140C hot air to melt the Hot Glue but not my PLA plastic. About 10 seconds of hot air at 140C is enough to evenly heat the hot glue and plastic surface but not soften / deform the PLA plastic.
https://all3dp.com/2/pla-petg-glass-transition-temperature-3d-printing/

Freeze It

Instead of waiting around for the hot glue to cool down, I spray it with freeze spray. Which is just an upside down air can.

When I spray the canned air’s liquid on the hot glue, it instantly cools and frosts over. A 1-2 second blast and my joint is cool enough to handle and trim.

Warnings:
I have been burned so many times. Be careful!

• The same way that hot glue is very hot and can burn you, freeze spray is very cold and can burn you.
• Canned air is often packed with foul-smelling bitterant to prevent abuse. Be careful not to breath.
• Follow the directions and warnings on the air can. Keep away from flames, etc…

Cleanup

It is easy to use too much hot glue on a joint, causing it to squish out the sides. After freezing the hot glue, I can trim the excess back with a hobby knife. Then hit with more hot air and freeze spray. Causing the remaining glue to get pulled back into the joint.

After cooling, you can roll your finger over the remaining glue debris to remove. More hot glue to blast away and even out the remainders.

Removal

Separating two parts that have been fused with hot glue is tricky. Here are some tips:

• Cut apart, then remove the remaining glue with a pliers.
• Heat up again with hot air, then gently pull apart.
• Soak in high percentage isopropyl alcohol / rubbing alcohol. This breaks the bonds of the hot glue to the plastic.

Optional: Secondary Joint

The hot glue will do its job to hold your parts together. Now to keep the parts held together on hot days. You should include a friction fit joint or a extra clip to fall back on, incase the part is exposed to sunlight or is forgotten in a hot car. This will prevent your project from simply falling apart.

A friction joint can be a peg or snap designed into the 3d printed part, so the parts click together and require some force to separate. Make the hole diameter slightly larger than the peg diameter, and you got a peg joint.

0.04mm larger diameter = friction fit, snaps together
0.40mm larger diameter = sliding fit, no snap

For smaller parts, I use metal clips / inserts that get melted into the plastic.