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How to create a silicone chocolate form

Creating a Chocolate mold of the Pilcrow character in silicone

This application story (called "Fabbie and the Chocolate Factory" :-) is about the creation of some chocolate characters, in a FabLab.
A Fab Lab (Fabrication Laboratory) is a workshop where any individual can drop in to create a product that he/she needs. For that aim a FabLab offers a number of computer controlled tools, like (for this project) CAD software, CAM software and a CNC milling machine.

FabLabs were invented in the USA, at the Center for Bits and Atoms (CBA) of the Massachusetts Institute of Technology (MIT) in Cambridge near Boston. Their idea proved to be successful, and has been spread around the world. Currently many FabLabs can be found worldwide.

The Pilcrow character
Sketch of the mold, on paper
The pilcrow sign, and a sketch for the mold.

This project has been done in the CabFabLab in the Netherlands, a FabLab named after the Caballero factory building in which it is located. And the man who created this product is Fabio Bartali, a graphic designer located in Rotterdam.

For the anniversary of his design studio Bartali he wanted a give a present to all relations of the studio, and decided to produce chocolate versions of the Pilcrow character. This is also called the Capitulum and the Paragraph mark: see the illustration above. For Fabio this sign is special as it is both old and new: it was used in ancient handwriting and manual printing techniques, and nowadays it is used in wordprocessing software (to indicate a Hard Return).

Screenshot 1 of SolidWorks
Screenshot 2 of SolidWorks
Renderings of the pilcrow, made in SolidWorks.

Chocolate products are produced by pouring molten chocolate into a mold, so Fabio had to create a mold: see the design sketch in the illustrations above. The mold in turn also has been produced by molding (here it gets complicated, sorry about that).

Here is how it works: the original design (made in SolidWorks, see the illustrations above) was the geometry of the desired product (a positive). That geometry also has been machined (see below): the first mold. That mold has been inverted (conversion male to female) by pouring silicone rubber. The result (a negative) is a second mold, made of silicone rubber, featuring a cavity for the actual product. Finally that actual product (again a positive) will be made by pouring molten chocolate in the rubber mold (the second conversion: female-to-male).

The design was made with an existing chocolate character as an example: copying weight, dimensions and draft angle to guarantee a producible design. Dimensions of the design are 75 x 110 x 18 mm, due to shrink the resulting product in chocolate will be a bit smaller.

Screenshot of DeskProto with roughing toolpaths
Machine doing the roughing operation
The roughing toolpaths: in DeskProto and on the machine.

For CNC machining the original FabLab used the Roland MDX-20 machine and the Modela Player CAM software that comes with this machine. As the capabilities of both this machine and the software are limited, many Fablabs have grown to use a more powerful machine and more powerful CAM software: DeskProto. DeskProto combines well with the FabLab philosophy as it is easy to understand for novice users.

CabFabLab decided to buy a High-Z 1000T machine, made by Heiz CNC-Technik in Germany. This is a lowcost machine, still large enough and with sufficient power for the type of projects as done in a FabLab. It comes with the WinPC-NC control software. As CAM software DeskProto is used, and for this mold a number of operations has been defined and performed. The pictures above shows the Roughing operation: quickly removing material with a thick cutter, leaving a skin of material around the part.

Screenshot of DeskProto with finishing toolpaths
Machine doing the finishing operation
The finishing toolpaths, in WinPC-NC and on the machine

The next two pictures are for the finishing operation: making the model exactly on size, with a smooth outer surface (BTW the skin needs not be as thick as shown here). Again left a DeskProto screenshot with the finishing toolpaths, and right the machine in action.
The mold is machined in Necuron tooling board (type 651), which is well suited as a base for later molding silicone rubber.

Detail of the machined mold
Picture of the machined mold
The machined mold: top-lining detail and complete.

After roughing and finishing also a detailing operation has been done, for all small details. It would of course also have been possible to use a very small cutter for finishing, however then finishing would have taken a lot of time ! Now detailing toolpaths are defined only where needed.

An interesting detail are the lines on top of the part. These improve the quality of the resulting product, and are also needed to make it possible to remove the chocolate character from the silicone mold, later in the process. While already present in the SolidWorks rendering shown above, it was quicker to use an STL model without these lines and then later "add" the lines with a thin cutter in an extra milling operation.

All stuff needed to work with silicones
Jar with freshly mixed silicone
Detergent, used as release agent
Preparations for casting silicone into the machined mold.

Silicone rubber comes as two components, in different jars. Mix the components well, and after that pour it in the mold and let your negative rubber mold cure.
For creating edible stuff a suitable type of silicone is of course needed: Fabio used a rubber called SmoothSil 940, by Smooth On. He tells that when reading all warnings on the website the process seems complicated, however that in practice it proves to be surprisingly simple.

In order to make removing the cured silicone from the machine mold possible, a release agent is needed (to be applied on the mold before pouring the rubber). Many release agents can be bought - Fabio used a simple detergent for this aim. He warns that testing is needed as detergent may contain a component that will react with the silicone.

Pouring the silicone into the machined mold
A filled mold, with the silicone curing
The resulting silicone mold
Casting silicone, curing, and the resulting mold.

Pouring and curing the silicone has been done at normal room conditions (pressure and temperature). It may be needed to use a vacuum chamber in order to remove any included air bubbles. Fabio minimized this problem by pouring directly around the part, thus pushing air bubbles towards the outside of the mold. Curing the mold took 16 to 24 hours at room temperature.

An alternative for pouring fluid silicone rubber is thermoforming sheet material over the machined mold. Here as well materials are available suited for edible stuff (PolyCarbonate), and this technique is also very well suited for small series production. For large series the PolyCarbonate molds will be produced by pressure injection molding.

All stuff needed to mold white chocolate
Silicone mold filled with white chocolate
Casting chocolate: before and after.

Molding the chocolate has been done in the workshop of specialist Chocoholic in Rotterdam. As chocolate is very sensitive to small variations in temperature it is difficult to do this at home.
After melting the chocolate it is poured in the mold and put in the fridge for 10 minutes. after that the chocolate character can be removed from the rubber mold in order to create a next one.
The chocolate used has been made by manufacturer Callebaut from Belgium: 100 gr per product.

Two Pilcrow characters in white chocolate
Series of packaged pilcrow characters
The final product in white chocolate, without and with bag.

The resulting products look great: makes you go to the story to immediately buy some chocolate...
Finally: after packaging the products are ready to be distributed.