A Simple Way to Mold a Fluorescent Glue Sticks or Crayon Using Fusion 360

OK, the question you might ask is, what does fluorescence have to do with hot glue sticks and crayons?

There was an instructable entered into a recent contest that used hot glue and the ink from some special markers to create fluorescent art. It looked visually exciting and even better; I had a pack of the fluorescent markers they used. (Not to mention the four of five pounds of leftover hot glue sticks from when the kids were in primary school.)

So, the quest began; this should take a couple of hours, right? That was three weeks ago. (Latest update. I passed four months a while back.)

I will share the good with the bad if you are still reading. I am going to spell out each mistake I made as well as some good things I found. I will point you to the places that helped me find a path to an answer.

I had a couple of goals when I set out to do this project.

Create a glue stick that the feed mechanism on a glue gun can use.
Provide a nice, bright, and even color when exposed to UV light.
Be safe.

So what is the connection between glue sticks and Crayons? The answer is the mold. Working with hot glue is dangerous; working with hot wax is a little less dangerous. Hot wax was an easy-to-melt, nonsmelling analog for hot glue. It also turns out that a glue stick analog makes a crayon if you add a coloring agent and a suitable wax. So most of the molds (after a few false starts) were prototyped using wax.

Obligatory warning. This project uses hot materials that can burn you and are sticky. You are mixing coloring agents into a hot material that may produce fumes. Use the appropriate safety equipment, including but not limited to a respirator, eye protection, feet protection, and gloves. Undertake this project at your own risk.

Supplies

Fusion 360

Access to a 3D printer

Silicon mold-making kit.
Wax (carnauba)
Clear glue sticks
An old pan that will never be used for cooking again
Hot plates or other heat sources
Color pigments (fluorescent)

A respirator, gloves( welding and examination ), and eye protection.

Step 1: The Recipe for a Crayon

The setup.

A clean workspace covered in something that soaks up wax, like old newspapers or a silicon craft mat that wax will not stick to.
A hotplate.
Several containers for mixing the wax with the coloring agent. It can be disposable or silicon.
Coloring agent
The mold you created.

The Recipe.

Put a quantity of wax into a melting pot. We start with a large melting pot just used to melt wax over a hot plate. This is a link to the one we used in the end.
Melt the wax to a liquid state.
In a separate pouring container, add the coloring agent. ( See the glue stick recipe for all the warnings about handling fluorescent pigments. )
Pour the wax into the container with the coloring agent and mix with a disposable stick. (popsicle sticks work great.) You want to be quick because the wax has to stay liquid.
Pour the mixture into a cavity on the mold(s). You will probably not have enough to fill the last cavity. That is ok. Pour the next color on top and make a multi-colored crayon.
Fill all the cavities on the molds and put the mold aside for a couple of hours or a day.

Step 2: The Recipe for Fluorescent Hot Glue Sticks

The recipe.

15 small glue sticks (4.2g x 15 = 63g) or five large 4-inch glue sticks (10g x 5 = 50g)
candle wax TBDg. The amount of candle wax to be added is calculated by taking 0.1 of the weight of glue sticks.
Coloring agent.

A quick note on adding coloring agents to the glue. We used a fluorescent pigment in a powdered form we purchased online. Using a powdered pigment worked better than mixing paint or the fluid from cut-open markers. We could not obtain an MDS ( materials data sheet ) for the paints or pigments. We are unsure about heating these color elements to the temperature required to liquefy hot glue. Wear a respirator when measuring, mixing, and heating the mixture of pigment and glue. Heat the mixture in a well-ventilated area. The pigment is a super fine powder. Don't expose the pigment to any air currents when handling it. Always wear a respirator. Getting the powder airborne or spilling it will create a mess that will be difficult to clean up, and under UV light, the futility of trying to capture it all will be apparent for years to come. Did I mention you should wear a respirator?

Place the glue sticks in a container that makes pouring easy. It helps to speed up the melting if the glue stick is cut into smaller pieces. A utility knife can cut halfway through the stick. Then the two pieces can be twisted apart.
After the glue is in a liquid state, add the wax while stirring the mixture vigorously with a wooden stick or silicon spatula. The silicon spatula has the advantage because any mixture that sticks to the blade can be hardened and remelted in the mixture.
While stirring, slowly add the coloring agent. With the pigments, a little goes a long way. When the coloring agent is wholly integrated, raise the heat as hot as possible without burning the mixture. (it will start to smoke a little just before you get here. You are still wearing the respirator, so you should not be able to smell it, but it does stink. )
The goal is to make the mixture as fluid as possible so you can pore it into the mold.
Pour the mixture into the mold. The mold should be placed on a sheet of something disposable, like heavy cardboard or scrap wood. As you pour, be careful not to overfill the mold chamber. It is good practice to pour in stages and tap the side of the mold between pours to release any air bubbles that may be formed.
Put the mold aside for a few hours to let the contents firm up.
If you have material left and is not enough left to make more sticks, put it aside. You can remelt it later after the mold is free again.

You must be appropriately dressed while you mix and pour the gluesticks. That means good coverage of the skin. No sandals or open-toed shoes, heavy Levi's, heavy shirt, eye protection, and a respirator at a minimum. Gloves. The work should be done in a well-ventilated place. Have a fire extinguisher handy. If you have a spill, step back and don't touch it. It is hot glue and will stick. After it cools a bit, it is easier to manipulate.

When the sticks are cooled, they can be removed from the silicon mold by pushing on the bottom of the stick. You will need to cut any excess of the top of the stick.

Step 3: Dont Repeat This Mistake

One of the things that intrigued me when I saw the original project was the chance to use hot glue as an art medium. There are a number of hot glue guns around the house—the ones I use for woodworking and electronics projects. I would not consider putting a homemade glue stick in the expensive ones. They all use large-diameter glue sticks.

There are also the ones the kids use to use. ( I found seven.) They use small-diameter glue sticks that come in four-inch lengths. Most of them were under ten dollars. The children used them through primary and middle school. ( One class in drafting in high school, too, I think. ) Our home used to be filled with houses, forts, doll furniture, model airplanes, model ships, and the occasional trebuchet made from hot glue and popsicle sticks. While looking for glue guns, I found a bulk box of small glue sticks still left. ( 5lb) So I believed I had the perfect sacrificial platform if something went wrong.

Also, it turned out, wholly the wrong choice for this project. The critical thing I did not understand when approaching this project was how viscous hot glue is. It will not flow like liquid wax. It will remain slightly sticky even when molten and cool quickly when touched by a surface. Hot glue does not like to flow into small spaces.

The children and I have used hot glue in molds before. We have used cardboard covered in painter's tape to make cude, simple molds.

So the original molds I created were lined with painter's tape. Painters tape is a "rough" surface to molten glue. The hot glue sticks loosely to it and works well in large open molds. A significant problem was the glue liked to cover the opening to the mold cavity, which stopped the flow down into the cavity. This was because of the small size of the cavity and the opening to it. The mold failed utterly.

The key takeaways.

I picked the wrong hot glue gun from the start.
I started working with hot glue immediately.
Small-diameter cavities are wrong for this project.
Using tape to line the mold did not work in this instance.
The mold needs to have a closed bottom; tape is not enough.
Mold halves have to be joined with the flattest surfaces possible.

Step 4: A Less Stickey Solution

So, with failure everywhere, I went looking for a different solution.

First, I switched to the larger diameter glue sticks as a model. Next, I worked on the material for making the mold. I tried playing with aluminum tape to cover the mold cavity, making a fully metal two-part mold and semi-rigid silicon paper molds. Hot glue does not stick to silicon and can easily handle the temperatures of molten hot glue.

I was explaining all this to a friend while trying to borrow time on his CNC machine to cut an aluminum mold. He looked at me very confused and said, "Why don't you just make a silicon mold?" I tend to focus on a single solution and am an idiot.

The switch to a molded silicon mold was on.

Step 5: Desigining the Mold for the Mold

Having decided that a cast silicon mold was the way to go, I designed a 3D-printed mold to create the silicon mold. The mold needed to have several key features.

Be reusable. I was going to want to create several different copies of the molds.
Be capable of supporting an interchangeable core. The core is the inner part of the mold that will be the shape of the negative space in the silicon mold. Prior experience showed me that I would most likely be iterating this design part.
I wanted a scalable design, so the model used a parametric design.
The initial design was based on a glue stick, which I later discovered has parameters very close to a crayon.
I wanted an integrated funnel (sprue) off the top of each silicon mold.

The mold's core was the first thing I designed in Fusion 360. I modeled the sticks first and then added the sprues. I finished with a flat surface so there would be minimal trimming on the silicon mold after it was cast.

The other addition was the threaded holes for 1/4-20 eye bolts. The eye bolts server two functions, they make a great handle and provide a way to clamp the core to the top of the can. This allows a soft bottom on the silicon mold. A soft, flexible bottom is essential when pushing the sticks out of the silicon mold. The handle is excellent for extracting the core and silicon mold from the can and later extracting the core from the silicon mold.

Small Can and Large stick core: Printables Thingiverse

Large Can and Large stick core: Printables Thingiverse

Step 6: Designing the Can

The interesting side note about the can is it is designed around a can of tomato sauce. While trying to visualize this project, I took several gluesticks and a used (but clean) can of tomato sauce from the recycling bin to visualize the mold's arrangement. The can dimensions are stuck in the design.

The can was designed to provide a minimum wall thickness in the silicon mold of 16mm. The goal was to make the wall thick enough for the mold to be free-standing but flexible enough to push the sticks out. In Fusion 360, it was designed as two pieces and then cut in half for the final operation. The bottom of the can was designed first, and the top was done as a separate sketch. This allowed for the placement of holes in the top to act as a sprue to pour the silicon liquid into, vents so the air could escape, and a track to adjust the eyebolts holding the core.

Step 7: Making the Silicon Mold

Making the silicon mold will take about an hour of actual work and six hours of cure time. I usually let the mold cure overnight, with a few quick notes before starting.

The cleaner the 3d printed parts are, the easier the silicon mold will detach from the forms.
The silicon mold will capture and reproduce any artifacts from the printing process. So smaller layer heights and attention to print direction are recommended.
I have not experimented with smoothing the 3d prints, sanding, painting, or a mold release agent for silicon, but they will help to produce a better mold.

Ok, the setup. A clean, well-ventilated workspace that is covered in some protective material. We used cardboard, and it was fine.

Set up the 3D-printed mold first.

Put wingnuts and washers on the eyebolts and screw them into the mold core.
Place four screws into the threaded holes on half of the can mold.
Position the eyebolt from the core in the track on the side of the can mold with screws. Loosely tighten the wingnut holding the core.
Position the other side of the can mold over the remaining eyebolt and four screw posts. Check the mating surfaces for any obstructions. The fit must be tight in the next step.
Attach four washers and wingnuts to the screw post and tighten. Visual inspect the matted surfaces; if you can see any gaping, it will leak.
Looking through the holes on the top of the can mold, position the core in the center of the mold.
Tighten the wingnuts on the eyebolts.
Attach painter's tape to the outside seams on the mold.
Place the mold in position for the pour.

Mix the silicon mold solution according to the manufactures direction. I used a two-part, one-to-one mixture. It works well, but be sure to mix it well. (in the later stages of this project, I built a robot to do the measuring and mixing, but that is another instructable.) We used disposable 16oz plastic cups to do the combining and mixing. The mixture I used had a thirty-minute setup time, allowing five minutes of mixing with plenty of time to pour.

Pour the liquid silicon into the mold until it becomes visible through the vent holes. Tap the mold a couple of times on the work surface to release trapped bubbles. Finish filling the mold and tap again. Let the mold sit for the manufactures recommended cure time or longer. (Overnight)

You might also have a couple of different-size forms ready to use for any leftover silicon mix. Many designs are available on Printables and Thingiverse; search for "silicon mold."

Step 8: Decant the Mold

We have now arrived at the Zen part of the project. Decanting the mold is an exercise in patience. If you do not want to break the mold, take a deep breath and release slowly. Calm yourself as you collect up all the old screwdrivers you have dreamed fit to only use as chisels, any wood scraps 2mm to 10mm, smooth steel rods, etc. You can use anything as spacers and wedges while you pry the mold apart. The thin, flexible wood sticks you find in a craft store are helpful too.

This step will take about an hour and is done in two stages, removing the silicon mold from the can mold and the core from the silicon mold.

Removing the silicon mold from the can mold.

Remove all the painter's tape.
Remove all the wingnuts and washers from the four retaining bolts.
Loosen the two wingnuts on the eyebolts.
Clean up any loose silicon puddles from the outside of the mold.
Examine the mold to figure out where to split it. The corners of the outside rib are usually the easiest.
We use a small screwdriver or a spudger to wedge open the mold at one of the outside corners. The process we will follow has us wedging a small space, bracing it with something, moving further down, and wedging the next section open. This will slowly open the 3D-printed mold without breaking it.
Repeat until you have one of the ribs opened. Place a screwdriver or rod down the open side between the bolt post and the silicon mold. (see pictures)
When inserting a craft stick or tongue depressor into the top and bottom of the 3d printed mold, push the silicon mold away from the top and bottom of the half of the 3D-printed mold.
Let it rest. The silicon will slowly detach itself from the 3D-printed mold.
Repeat the process on the other side.
After half of the 3d printed mold releases, use your fingers to rock the silicon mold out of half of the mold.

Removing the core from the silicon mold.

Using the eyebolts as a handle, push on the bottom of each stick
wiggle the core up
repeat.

The core will slowly work itself free.

The bigger core (see below) may require a different technique. It is stiff enough that it doesn't wiggle. You may need to place a small hole at the tip of each stick to break the vacuum holding the core in place. You can cover the bottom of the silicon mold with painter's tape when you use it.

Step 9: A Word About Up Scaling the Molds

Scaling the molds is a mixed bag. If you are making crayons for a classroom, it can be a blessing; it can also be frustrating.

The actual construction and manipulation of the mold is more challenging as the silicon mold becomes larger. The first time I tried it, I had to cut the core apart to remove the silicon mold's sticks. The redesigned core, with the small holes, is much easier but still a struggle.

It may be easier to construct multiple small molds rather than a single large one.

The glue sticks or crayons are easier to remove from the smaller mold. The bottom of the large mold can be a bit stiff.

Step 10: Enjoying the Crayons and Using the Glue Sticks

Using Fusion 360, you can create a lot of other cores to create new and different silicon molds. I have used the same can to create several other designs, including a silicon "cup" to melt scrap wax in. I also have. I have been working on creating candle molds.

If there is any interest, I will do an instructable on the robot I created to mix the silicon for the molds/

Introduction: A Simple Way to Mold a Fluorescent Glue Sticks or Crayon Using Fusion 360