Molding and Casting Hot Melt Glue

Hot Glue Casting

The "Problem"
We didn't actually have a problem, here so it was more of a hypothetical problem.  The premise is:

What do you do if you realize you need a flexible casting today, and you don't have access to traditional casting supplies and you don't have time to wait for them to be delivered?

This project really came about because I had some time on my hands and was looking for something to do.  It wasn't because I needed it, rather I needed to see if I could do it.  I have cast hot melt glue in silicone molds before, but I wondered if you could mold in hot glue and then cast hot glue into it.  My colleagues, Ainsley Furgason, Mason Wynder and I purposely did not look to the web or anywhere else to see if this had been done before.  This project was intended to be a Tabula Rasa kind of project.

I know some people in the industry who hate hot melt glue.  They think it is a tool that is beneath the dignity of the craftsperson.  My thought is that hot glue is just another tool.  You wouldn't say to a hammer, "Because you are not a screwdriver, you are not a tool."  No, you would keep it in your toolbox and bring it out when you needed a hammer.  I feel the same way about hot glue.

Project Goals
*Create a mold and a cast from non-traditional casting materials one would normally find in a scene shop.
*If the cast was flexible, that would be a bonus.
*Succeed or fail, blog about it.

The Process
Obviously, the first step is to find an item to mold and cast.  Naturally, I found a plastic Halloween costume belt buckle in the shape of a skull and crossbones.  Naturally.

Plastic belt buckle

The "hardware" has to go

The buckle part of this plastic bit needed to be taken off, so we used a Dremel Tool to grind it off.

Removing the unwanted plastic clasp

Mission accomplished

The next step was to build the box for the mold.  Remember, the point of this exercise was to use non-traditional things you might find in a scene shop.

The box was made from a square of MDF that was larger than the skull.  We attached the skull to the MDF with Super 77 spray adhesive.  The sides of the box were made from paint stir sticks we had just lying around.  We cut them to length on the chop saw and taped them in place with painter's tape.

Scrap of MDF

Super 77 spray adhesive

Skull attached to MDF

Stir sticks

Eyeball measuring with a screw for a scribe.  Really going old school on this one

Green painter's tape.  Blue probably would  work

Ready Freddy

Tape wrapped around the backside

We made scribe marks on the mold box walls to give ourselves a high glue mark.  To do this we set a Sharpie Pen on top of the skull and scribed a measurement on a craft stick.  Then we transferred this mark to the walls of the mold box.  We figured the thickness of the Sharpie Pen, resting on the skull would give us the thickness of glue we needed at the bottom of the mold to have an intact casting.

Measuring on a stick

Transferring the measurement to the mold box walls 

Done

A mold needs a mold release, and so in keeping with the theme of materials found in the shop, we used WD-40 and gave it a pretty thick coat.

WD-40

Sufficiently slimed

Apparently WD-40 is toxic in California.  Who knew?  Good thing I live in Idaho

So now we have a problem.  We want to mold in hot glue and cast hot glue in a hot glue mold.  Seems like hot glue might stick to hot glue.  All problems have a solution though.  Part of our planning for this project was that we would make the mold of High Temp hot glue and do the cast in Low Temp hot glue.

As we began filling the box with hot glue, it became apparent that our box was too big and it would take forever to fill it with melted glue.  Around the edges, we filled the space with un-melted glue sticks to take up room and facilitate filling.

Hi temp hot glue

Un-melted glue sticks

on four sides

Then this happened.

The waiting

is the hardest

part

The glue solidified but was still hot to the touch.  That much melted glue holds the heat for a long time, so we broke the walls of the mold and sped up the cooling with a secret weapon.

Peeling the walls back

The secret cooling weapon

We de-molded the skull using sophisticated pocket knife technology.

Knife work.  Might be illegal in some countries

Moment of truth

Popped right out

The paint stuck to the mold

As you can see, a pretty faithful replication

Now the tricky part.  How do you cast hot glue into hot glue.  The moment of truth was upon us.  Low temp glue drizzled into a high temp mold.  The first thing, obviously was to coat the mold with a generous coating of WD-40, again.  Then using low temp glue we dribbled the glue into the mold, making sure to hit the areas with the most detail and relief first and then back-filling behind.

More Idaho safe WD-40 not approved for California sale

Low temp hot melt glue gun

Filling the details first, then back-filling

We noticed that the lower temp hot glue didn't level itself as well as the high temp.  That was unexpected but not surprising.  So we used a utility knife to shave the raised parts off so it would be level with the mold.

Then the next moment of truth.  Would we be able to de-mold the skull?  Or would the cooler hot glue stick irrevocably to the higher temp hot glue?  Did we waste our time and blow an afternoon? or was this going to work...?

I need a shave

It appears to be working!

It worked!

This image doesn't show the amount of detail that was achieved .  From left to right:  Original buck for molding, the hot melt glue mold, the finished casting.

The image above doesn't show the amount of detail we were able to achieve with hot glue.  The indents between the teeth are visible.  Most of the detail transferred.  It isn't a perfect cast, but it's really quite good considering the non-traditional way in which we did it.

So what is the point of this?

If you have access to Smooth-On or other products, this would be a colossal waste of time.  But if you were getting ready for tech week and needed a casting today and didn't have access to the Smooth-On or had to order it in, this could make sense.

Because it is made of hot glue, the casting is flexible and could be applied to a curved surface.  I know there are many traditional products that produce a flexible casting as well.  As I said, this is a project that assumes you don't have immediate access to those materials.

So, if I had to do it over, I would create a smaller box for the mold.  This would be less wasteful of both time and product.

There is paint transfer from original to mold to cast.  I don't know what to do to mitigate that or if I even care to do so.

Eventually, when I am able to go back to work after the shelter at home order is lifted, I will paint the casting and post the picture as an addendum to this blog post.

The results of this experiment were encouraging and given the right circumstances I would do it again.