I wanted to make some custom chocolate santas as christmas gifts for quite some time now. This year i finally had the time to do it.
After some initial quite loose sketches i settled on a design that (besides the creative considerations) would be easy to mill and would only require a very simple two part mold.
This design was than drawn using 3d CAD software to be able to mill the santa on my CNC-mill. It would also have been possible to shape the form by hand, but since i have a mill at hand and the part didn´t take long to model, I took the digital course...
The part was than milled out of PU-foam (Ureol/Uriol/Renshape...).
This material is very easy to work with - similar to wood, but without the fibers. The "bridges" for instance, that held the part in place during milling were removed using an air grinder.
Sanding & Finishing
Milled parts initially have a quite rough surface finish unless you let the mill work for hours and hours with a very fine endmill. Its way faster to sand down the excess material by hand.
To better see what i was doing I gave my freshly milled santa a blue spray paint coat. This way i could see, where I had to sand - blue means there is still a slight indentation with excess material around it.
Finally a layer of primer and glossy spray paint was applied to make a smooth surface finish.
Casting the Mold
The final two-part mold with which the chocolate santas were produced consists of silicone rubber.
To cast the first half, I embedded the santa half ways in ordinary clay and built a box around it. I also did cut some groves (keys) which help to align the two parts later on.
To measure the needed volume of silicone I used rice.
The silicone for this santa costs about 50 euro so its a good idea not to waste it and mixing to less also is tedious.
The silicone I used asks for 2% hardener so I used a very precise laboratory scale. Some rubbers come in more convenient 50/50 mixing ratios but since i wanted a food-save material I was a bit more restricted.
The second half was than cast directly onto the first one. Vaseline was used as a release agent between the two silicone parts.
Molding the Santas
To make hollow chocolate santas, like the ones you can buy in the store I used a rotomolder, that I built earlyer that year.
This device is able to rotate the mold around two axis simultaneously. Molten chocolate is poured into one half of the mold, than the second half closes the mold and the whole thing is attached to the rotomolder. While the device is spinning, gravity forces the cooling chocolate to stick to the walls of the mold.
Here is a video, showing the movement:
It took some experiments to get the right amount of chocolate and cooling time and chocolate temperature. I ended up pre-heating the mold with warm water while washing off the bits of chocolate from the previous cast. I than quickly dried the mold using compressed air.
The chocolate must not exceed a temperature of 30deg Celsius, otherwise it starts to flocculate, resulting in nasty white or grayish stains that often occur several days later.
To prevent this I heated it in a bowl that sat on a pot with only very little water in it. This way only the steam would warm the bowl - just enough to let the chocolate melt. The mold then spent 11 minutes on the rotomolder and another 11 outside in the snow (about -5 degree Celsius)
thanks for reading and merry christmas :)