In this article we will discuss the selection of mould types based on the design of the rotationally moulded part(s). There are a total of three types of mould construction that can be used for the creation of rotational moulded parts, knowing:
- Sheet Metal
- Cast aluminum
- CNC aluminum milling
For each mould we will discuss the tooling option and how the part design is affected by the particular choice.
Sheet Metal Moulds
There are multiple reasons for choosing sheet metal moulds. This mould is the most cost effective of the three types and well suited for large parts. For instance Pentas used this type of mould for the creation of rainwatertanks. But the moulds are not only useable for very large parts also for some very complex shapes. However for complex parts there are limitations when using sheet metal moulds. Shapes that could be created are shapes that also could be attained with paper or cardboard. So it can be cut, bent, rolled and bunched, but not rolled into shapes that have curves in two axes. Rolled surfaces are limited to simple cylindrical shapes. In the product design phase the choice of using Sheet metal moulds is restricted to geometric forms based on cylinders and planes. High-end products with structures on the surfaces cannot be created with the sheet Metal Moulds.
Cast Aluminum Moulds
Reasons for choosing Cast aluminum are threefold. It has cost effectiveness, design freedom and quality advantages. Any shape can bereplicated with a cast alumni moulds. The mould is cast from a pattern, as opposed to fabricated products from sheet material. Complex organic forms to defined shapes can be created. The limitation is that cast moulds cannot be chemically etched. An additional disadvantage is that design changes to the mold are difficult, as the heat distribution / absobering of a molded cast can be unpredictable. Examples of products created by Pentas by the use of cast aluminum moulds are: flowerpots.
CNC aluminum milling
Machined aluminum moulds are becoming more popular as they are more accurate, the part quality is typically better and it has the possibility to high standard surface finishing in opposite to the previous discussed moulds. Machined aluminum moulds are machined from a billet. This means that both sides of the slab must be machined to create a uniform 7 to 9 mm wall. It is best suited for shallow parts, as costs are competitive with cast aluminum. Pentas has 8 CNC milling machines only for making aluminum moulds, 85% of the moulds that are used at Pentas are made with this technique. For example to create Fuel/adblue tanks, airducks, luggage trays, fenders etc.