The biggest amount of work when producing fibre reinforced parts is the mould manufacturing. There are many ways to design and manufacture moulds, which all differ in material, cost, time and the process eco-friendliness.
The common way when producing moulds is the use of polyurethane raw blocks with a density between 450 and 700 kg/m³. The material’s disadvantages are the weight and the price. For our solar car, the mould dimension of the main body alone is 5000 x 2000 x 1200 mm, which would result in an estimated weight of over two tons.
Therefore, our chosen mould material is a combination of extruded polystyrene foam (XPS) and the epoxy tooling paste SC 175. The main part of every mould is a raw block out of single XPS-plates. This material is much lighter and cheaper compared to a similar amount of tooling blocks. The tooling paste was applied on the machined XPS raw block on every mould and then machined for a second time. In doing so, we could reduce the amount necessary tooling material to 20% compared to conventional mould manufacturing.
The first principle in mould manufacturing says, that the mould surface defines the desired part surface. Therefore, our moulds needed to be designed as negatives. Starting with our solar car as positive form, the negative moulds for the different parts were designed as CAD-models. While modelling, it was important to focus on the later demoulding process and therefore to avoid undercuts inside of a mould.
The XPS-raw blocks were machined with an offset of nine millimetres on every outer surface. After the first machine cut, the epoxy tooling paste was applied with a thickness of 15 millimetres on every mould part with a Nodopox 50 mixing machine. This machine is especially designed for mixing high viscosity, two-component materials. One big advantage of the epoxy tooling paste is it’s fast curing time. The applied material is ready to be machined again after only 48 more hours.
After the tooling past was completely cured, every mould part was machined a second time to get its final shape resulting in a mould made of 95% XPS and 5% tooling paste.