The objective of this assignment was to learn how to make molds and casts. I made a form to replicate a little panda figure, which I used to make a candle in the form of the figure. Also, I designed a mold to be milled by our CNC mill. The replicating mold is made out of silicone, the same material that we reviewed in our group assignment.

I used the following materials and tools to make my molds:

• My cute panda figure
• Two-component silicone
• A laser-cut box of plexiglass
• Tape to close leaks in the box
• Some dough to fill half of the box
• Wax, to create the replica of my figure

Making a mold to replicate an object

The first technique creates a mold from an existing object. First, I used the laser cutter to make a box out of plexiglass. The SVG file for the box was created by MakerCase, which is a very handy online tool that automatically generates vector graphics for use with a laser cutter, handling varying sizes, joints, and laser cutter parameters. Then I put the figure in the box and filled one half with dough; the other half is filled with two-component silicone. It's important to make sure that the box is free of leaks, otherwise the silicone will create a huge mess. Additionally, it is helpful to indent the dough a little in a couple of places, to ensure a proper fit between both parts of the mold. After I sealed the sides of my box with some tape, I mixed some silicone and poured it in the free half of the box. The silicone takes about three hours to solidify, so I went home and called it a day.

The laser cut box that will hold the figure & the mold.

Make sure no silicone will leak out.

Half of the box filled with dough.

Add two inlets: one for silicone, one for air.

Adding a couple of ditches should help both sides hold together.

Both components are mixed one to one.

Pouring in a thin stream reduces air bubbles.

Fill until the object is fully covered.

Let sit for three hours.

<

>

The next day, I opened the box and removed the half filled with dough. Then I mixed silicone for the other half of the mold. Before pouring it into the empty half of the box and waiting for another three hours, I sprayed the inside of the box with silicone spray to avoid the two halves forming a single block of silicone. But apparently, I did not use enough of the spray, because the halves did end up as one single block.

Fully solid!

Remove the box...

...and the dough.

Back into the box. Again, make sure it has no leaks.

Use silicone spray to avoid both halves of the mold sticking together and to aid removal of your object.

Fill the second half and wait for it to solidify.

Almost there!

I did not use enough silicone spray, as both halves would not separate cleanly.

<

>

I had to open the mold with a knife (carefully, I didn't want to destroy the figure). The I could insert a wick into the mold and heat some wax to make a candle. Pouring the wax into the mold was a little difficult because its air inlet was right next to the hole for the wax and it clogged up a little. Then the wax had to cool for some time. The candle turned out pretty nice, but there are lots of bubbles enclosed in the bottom and there is a little part of the ears missing, probably because I did not pour enough wax.

First, insert the wick for the candle.

Then, heat the wax and pour it onto the box.

I should have be more carefull while pouring the wax in order to not clog the air inlet.

The finished candle. Clearly looks like a panda!

<

>

Milling a mold to create a new object

When one wants to replicate an existing object, the above method of creating a mold is fine. However, often a mold for a totally new object is needed, so there is obviously no way to make it this way. In this case, one can design the mold with the help of CAD tools and mill the mold with a CNC mill. Or, alternatively, one might mill a prototype of the object out of a material that's easily machinable, and use this to create the mold with the above process.

I designed a small vibration damping element in FreeCad that was intended for my final project. In fact I did mill the cast for the damper in this assignment, but in the end, I didn't use it in my final project. I created the damper design by rotating a sketch in FreeCAD. Then I created a cube on its left and right side each and subtracted the object from the cubes, resulting in two shapes that form a mold for the object. Then I added pins and pockets that hold both sides together.

To mill the mold, I used our labs new Roland CM-20 CNC mill with a 3mm tool. There are three milling steps involved: first, flattening the material surface, then rough cutting of the form, and finally fine cutting to give the surface a nice smooth finish. The mold is made out of machinable wax.

The damping element in FreeCAD.

I milled the mold out of machinable wax.

I prepared the underside with double sided tape so it doesn't move while being milled.

Placing the material in the mill.

Adjusting the tool to the material surface.

Milling the mold produces lots of small pieces of wax, which need to be removed regularly.

<

>

After removing the molds from the wax block, I mixed silicone (the same as I used for my other mold and the same that we used in the group assignment) to cast the vibration damper. The result looked nice enough, but I didn't have the time to integrate it with my final project.

The mold after the finishing cut.

The resulting cast.

<

>

All in all, this approach is a nice way to make complex molds for complex objects. In particular, it's easy to make both solid and flexible molds. For solid molds, just mill them out of a solid, machinable material; for flexible molds, mill the negative of the mold, and cast the actual mold with a flexible material (like silicone) with the help of this "mold of a mold".