When is a hot glue stick not a hot glue stick? When it’s PLA, of course! A glue gun that dispenses molten PLA instead of hot glue turned out to be a handy tool for joining 3D-printed objects together, once I had figured out how to print my own “glue” sticks out of PLA. The result is a bit like a plus-sized 3D-printing pen, but much simpler and capable of much heavier extrusion. But it wasn’t quite as simple as shoving scrap PLA into a hot glue gun and mashing the trigger; a few glitches needed to be ironed out.

Why Use a Glue Gun for PLA?

Some solutions come from no more than looking at two dissimilar things while in the right mindset, and realizing they can be mashed together. In this case I had recently segmented a large, hollow, 3D model into smaller 3D-printer-sized pieces and printed them all out, but found myself with a problem. I now had a large number of curved, thin-walled pieces that needed to be connected flush with one another. These were essentially butt joints on all sides — the weakest kind of joint — offering very little surface for gluing. On top of it all, the curved surfaces meant clamping was impractical, and any movement of the pieces while gluing would result in other pieces not lining up.

An advantage was that only the outside of my hollow model was a presentation surface; the inside could be ugly. A hot glue gun is worth considering for a job like this. The idea would be to hold two pieces with the presentation sides lined up properly with each other, then anchor the seams together by applying melted glue on the inside (non-presentation) side of the joint. Let the hot glue cool and harden, and repeat. It’s a workable process, but I felt that hot glue just wasn’t the right thing to use in this case. Hot glue can be slow to cool completely, and will always have a bit of flexibility to it. I wanted to work fast, and I wanted the joints to be hard and stiff. What I really wanted was melted PLA instead of glue, but I had no way to do it. Friction welding the 3D-printed pieces was a possibility but I doubted how maneuverable my rotary tool would be in awkward orientations. I was considering ordering a 3D-printing pen to use as a small PLA spot welder when I laid eyes on my cheap desktop glue gun.

A glue gun had everything I needed: good ergonomics, good tip visibility and tactical tactile feedback, and simple mechanical operation. If it could be made to extrude melted PLA instead of hot glue, it would be the ideal tool for the job. After some initial tests and a discussion with colleagues, it was clear that trying to make this happen was worth possibly wrecking a cheap glue gun.

Will a Glue Gun Melt PLA?

According to the RepRap Wiki entry for PLA, it softens around 60 °C to 65 °C and melts around 180 °C to 220 °C. Will a glue gun do the job? To find the answer to this question, I manually pushed a bundle of scrap PLA filament through a small desktop glue gun that I didn’t mind ruining. The glue gun was a hobby unit made for lower-temperature glue sticks. The small hobby unit eventually melted the PLA but only barely; the PLA came out more like softened putty. Based on this teardown of a cheap hobby glue gun the operating temperature is expected to be around 150 °C, which isn’t enough to really melt PLA properly.

Another thing that became clear during this process was that the glue gun had special needs for extrusion and feeding. To feed properly, the trigger mechanism needed to be able to grip and push on a solid cylinder, not a bundle of filaments. In addition, proper extrusion required a solid shape that filled the opening of the melting chamber completely to prevent backflow. Otherwise, molten plastic prefers to spill out the back instead of being forced through the nozzle. In other words, I needed:

A higher-temperature glue gun, and

A “glue stick” of the correct size and shape made from PLA instead of hot glue

After a bit of research, I purchased an economical high-temperature glue gun that claimed 80 W and an operating temperature of up to 208 °C.

Testing PLA “Glue Sticks”

To feed my new glue gun I needed a cylinder 11 mm in diameter and at least 5 in long. Happily, 3D printers exist for the sole purpose of turning 1.75 mm filament into other shapes and sizes. It felt a little strange to use a 3D printer simply to turn 1.75 mm diameter plastic into 11 mm diameter plastic, but in about an hour I had printed a high (75%) infill 11 mm x 150 mm cylinder of PLA for testing.

That first stick of PLA was enough to show that the 80 W glue gun was able to melt and extrude PLA acceptably; the only hitch was a ten to fifteen minute warmup in my cool workshop, compared to only five minutes or less for hot glue.

However, a problem was revealed. The feed mechanism for the glue gun has a small levered arm that bites into a glue stick and pushes it forward when the trigger is pulled. However, the PLA “glue stick” was smooth and hard, and the feed system could not properly bite into it. In fact, the ridged surface of the PLA cylinder quickly wore away the teeth in the little arm as it tried in vain to find something to grab onto.

Feeding Problem Solved by Adding Notches to the PLA Sticks

The solution was a small change to the 3D model for the glue stick. Adding a series of angled notches to the cylinder model allowed the raised arm to latch and push perfectly.

Testing The Bonds

With an 80 W glue gun doing the melting, and the notches in the cylinder of PLA allowing the feed system do its job when the trigger was pulled, molten PLA flowed easily and with excellent control. I performed some simple tests:

Using PLA like glue by depositing a molten blob, then mashing a part down on top of it. Results were good, but the PLA blob adds some bulk because it doesn’t smear out fully before it starts to cool and harden. Joining seams: the molten PLA melts only slightly to the surface being joined, but still ends up surprisingly strong. I was able to pull the seam apart but in the process always broke one or the other of the surfaces, or broke the seam itself in two (leaving the remains stuck to the parts I had joined).

Is There Really Any Welding Happening?

A true weld results when the material of the joint and both pieces meld together to become one. That clearly isn’t the case. There is some melting together happening, but it’s superficial at best. Nevertheless, the result easily passes a “tug test”. The cut-away test joint shows the red PLA has flowed into and filled every nook and cranny, which probably accounts for most of the strength.

Lessons Learned

A small hobby glue gun reliably softened but did not melt PLA. A glue gun made for higher-temperature hot glue melted PLA acceptably.

The test glue gun was an economical dual-heater 80 W unit that did the job, but takes a good ten minutes or more to warm up and melt the plastic filling the chamber (compared to five minutes or less for hot glue.)

Aside from heat, the biggest issue keeping PLA from being used in a glue gun is that glue gun feed mechanisms are not made to grip and push on smooth, hard PLA. They expect a much softer hot glue stick. The test gun functioned well with PLA sticks that had notches in the side, but this will vary depending on the design of the specific glue gun being used.

The STL file for my simple notched glue stick is available on GitHub. It may or may not be compatible with other glue gun feed systems.

It’s not much of a weld since it’s superficial only, but it still takes more than casual effort to pull pieces apart.

Experimenting is fun.

Does The PLA Glue Gun Do the Job?

My large multi-piece 3D print was a big hollow object with thin pieces fitting together like a jigsaw puzzle, and using the PLA glue gun to quickly and efficiently stick those 3D-printed pieces together with no need for clamping was better than expected.

For situations where a PLA glue gun may not do the job, check out this method to ease printing and assembly of multi-part models with a bit of help from OpenSCAD. But for my big hollow model, manually aligning the surfaces and bonding the inside seams was quick, easy, sturdy, and absolutely worth repurposing a $30 glue gun and giving it a place in the tool drawer.