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How to create a vacuum forming mold for chocolate forms

Creating a Chocolate mold via thermoforming

Chocolate molds are an ideal product to be created using CNC milling: by definition no undercuts are permitted as the chocolates have to easily come out of the mold. The chocolate designs will vary from intricate small bonbon designs up to large, simple shaped bars (possibly with some detail engravings).
In this case the letter S is created in chocolate, according to the ancient Dutch custom of giving a chocolate "S" for Santaclaus as a present at December 5.

Rhino design
The design of the letter S in Rhino. Note the differences in slope on the 'vertical' surfaces.

This chocolate letter mold was created for chocolate manufacturer Rijkenberg in the Netherlands. The geometry was not as simple as it would seem, as the slope of the side-walls had to be adjusted on certain locations. Still the design proved to be easy using the Rhino 3D CAD software: see the illustration above. From Rhino the geometry then was exported as an STL file for further processing.

DeskProto toolpaths
DeskProto window, showing the STL file geometry and the CNC toolpaths.

The geometry of the letter S has a simple form, without small details and without small inner radii. It is thus possible to use a rather large ballnose cutter, resulting in a very smooth surface.
The toolpaths are for a positive model (not for a mold), as the model that is machined is in fact 'a mold for a mold': the machined positive geometry is used as a thermoforming mold for plastic sheet, after which the resulting negative geometry in plastic is used to actually mold the chocolate.

On the milling machine
The actual machining of the thermoforming model in Metapor (tm).

First a prototype has been machined in tooling board for the customer's approval. Then a thermoforming model has been machined in Metapor, which a a micro-porous, 'air permeable' type of aluminum, specially suited for thermoforming as no small air ducts have to be drilled.
The picture shows the finishing operation, removing the small 'skin' of material left over during roughing. This two-step procedure results in a better surface quality. Note that the ballnose tool has to travel a bit below the bottom of the model in order to correctly machine any (almost) vertical walls.

Resulting model, mold and chocolate
The results, from left to right: the Metapor model, the thermoformed plastic mold, the packaged chocolate letter.