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I made a print that involves joining two halves together to create the full model. I decided to use hot glue to bond the halves and on one model, the parts came together slightly skewed. Is there any way to ‘unbond’ the hot glue without ruining the model so I can realign the pieces? Touching the glue gun tip to the hot glue wouldn’t work, as the glue is inside the model.

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  • $\begingroup$ Welcome to 3dPrinting.SE! I think you have two options: 1) physically cut it in half using some type of blade or saw (destructive, most likely). 2) Reprint. Anything which would bring the two pieces apart (either chemically or physically) will most likely destroy the print in the process. $\endgroup$ Apr 23 '19 at 23:56
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What is on the market as Hot Glue - or rather "Hot Melt Adhedive" (HMA) is usually a slightly ductile, thermoplastic that sticks to most surfaces because it can easily mold into slight deformations.

HMA basics

One of the most common base thermoplastics is Ethylene-vinyl acetate, aka EVA, mixed with terpene-phenol resin (TPR) to make it tackier and wax, like paraffin, as a filler to make it more economical. These HMA's are usually rated for 30-50 °C ambient temperature and start to soften enough for stress creep at 60-80 °C. Using this low level of heat might allow forcing the parts to move slowly without getting into the more dangerous zone of 120-200 °C, where the HMA actually melts and PLA follows suit.

CAUTION!

Yet we got a problem: many PLA mixtures I have encountered do soften enough at temperatures around 60 °C (like sitting in a hot car) so the print can be deformed permanently. This finding for non-annealed PLA also matches with what Stefan of CNC Kitchen found in a temperature resistance test. He found in the conclusion that 55-60 °C starts to affect non-annealed PLA while tempered PLA can withstand 160 °C to 180 °C, depending on the blend.

Cutting

However, most HMA formulas result in a material that can be carved and cut with a sharp knife. As long as your halves don't have interlocking lugs that block access to the plug of HMA, you could use a long, thin blade to slowly cut the halves apart.

Localized heat

Another solution might be coming in the shape of a soldering iron: by using it to apply the heat more localized than with a hot air gun, the HMA plug might get softened and then either removed in pieces or realigned. This needs considerable control over the heating and a careful hand. As an alternative, if the halves are not too much skewed from their intended positions, a hot soldering iron might be used to smooth over the edge, but it would destroy surface details.

What to do better next time?

There are basically two things that could help to prevent misalignment before the HMA application step:

  • Alignment lugs/notches would make sure the pieces snap into position.
  • If none are present, holding the piece into position and spot-welding in some none visible places - for example using friction welding as explained by Polymaker - can help to keep the pieces in position before the addition of the HMA.
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Depending on the composition of your model, the heat you've applied by using the glue gun has distorted the surfaces, or they haven't. One would hope that the model is composed of material that is not unduly damaged by the heat of the gun.

I suggest that if you can test the material of the model independent of the misaligned item, you can determine if my answer will work.

Apply hot glue to a test section, let it cure. Secure another portion to the test, but apply it in such a manner as to be displaced by gravity if it is not cooled. The first part should be "locked" to a surface, while the second should be free to move if not glued/cured.

Place the section in a small heated chamber such as a toaster oven. Observe the parts as you slowly increase the heat. When you see the unsecured part begin to slide/move, you've determined the temperature at which the hot glue releases.

According to a quick search, hot glue melts between 120°C and 200°C. The latter temperature is for specific high temperature sticks, while the lower number is for low temp sticks. PLA printing temperatures are often above 200°C, which gives you some leeway.

Unfortunately, PLA will soften before it melts and that can happen well below the melting point.

Testing will give you more definitive results.

If your testing appears promising, you may be able to place the model in the oven at the predetermined temperature, let it reach softening temperature, then slide the parts into position.

EDIT: A second option...

I've had a new attack pop into my alleged mind. I recall reading that alcohol will help clear hot glue and a quick 'net search confirms that both isopropyl alcohol and denatured alcohol will work on cleaning up hot glue.

I can attest that denatured alcohol does no harm to PLA or to ABS and we use it at the maker space to create a faster release from the PEI bed. Of course, if the model has tight fit, it's going to be much more time consuming to get sufficient alcohol into the gap to free it up.

I'd consider an ultrasonic cleaner with a bin full of denatured alcohol as the first option, even over the pre-heat above.

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