Moulds have many jobs to do, but their primary function is to define the shape of the moulded part.
The design of a mould is, therefore, dictated by the design of the plastic part. A moulded part can be no better than the mould that produced it.
Designers of plastic parts should, therefore, consider the mould construction requirements when designing a plastic part to be made by the rotational moulding process.
In addition to giving the plastic part its final shape, moulds must simultaneously provide for many other important functions. For example, the mould must allow for the transfer of heat into and out of the cavity.
Provisions must also be made for;
- attaching the mould to the rotational moulding machine;
- opening and closing the mould;
- removing the moulded part;
- keeping the two or more parts of the mould in alignment;
- holding the various parts of the mould together during the moulding process; and
- venting the cavity, etc
The majority of moulds for rotational moulding are currently made from metal, usually sheet steel or they can made from cast aluminium. The sheet metal moulds are relatively thin, shell like structures which are designed to allow the heat source to quickly transfer heat through the metal into the powder.
Although most fabricated sheet-metal mould shapes are simple, such as tanks or piping junctions and joints, more complex shapes can be manufactured. For highly detailed parts where a high quality finish is necessary, and multiple moulds are required, cast aluminium can be used.
The rotational moulding process doesn't use pressure to form the part, so the mould itself needs only to have enough strength to support itself. As the material cools, it naturally shrinks away from the surface of the mould; and mould design must take this into account to create an accurately size final part.
Most moulds are typically manufactured in two pieces, however for complex designs more separate pieces can be used. The area where the pieces of the mould connect to each other is known as the parting line and it can include a complex curvature.
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FABRICATED MOULDS
Fabricated moulds or cavities are the most common type of mould used for rotational moulding. They find their greatest use in large cavities of relatively simple shapes.
It is difficult to be definitive about the relative costs of a fabricated cavity versus a cast aluminium cavity. For example, a cavity for a 200litre drum could be made by casting or fabrication. If more than one cavity was required casting could be the most economical approach. If only one cavity was required sheet metal fabrication would be faster and lower in cost.
On the other hand, a complicated dashboard cavity could be easily made economically cast in aluminium. A cavity for a dashboard could also be fabricated, but the labor would be high and the cost would probably be prohibitive.
Sheet steel moulds are fabricated using conventional metal forming methods and welding. While conventional arc welding is usually satisfactory for most low-volume applications, MIG or inert gas welding is recommended where porosity and blowholes might be problems.
Although most sheet-metal mould shapes are simple, such as tanks or piping junctions and joints, more complex shapes can be manufactured using more advanced metal forming techniques such as pressure rolling and hydroforming. Low carbon steel is usually considered satisfactory for most low-volume applications.
Fabricated cavities are made by bending, stretching, cutting and welding sheet metal plates. The most common material is mild steel, although you can also use aluminum and stainless steel. This process requires great skill. Surprisingly complex shapes can be made. The thickness of fabricated cavities can be whatever is required and one advantage of this approach is the ability to provide different thicknesses or different materials in different locations.
Although aluminium has better thermal conductivity than steel (approximately three times higher) aluminium moulds need to be thicker to compensate for its lower modulus. Mild steel and stainless steel cavities are typically 2mm, while cast aluminium cavities range in thickness from 10-15mm in thickness.
Large sheet metal cavities sometimes show marks where they have been welded together. Complete removal of all of the weld marks would greatly increase the mould cost. Improperly finished welds can result in porosity.
The weld on a stainless steel cavity is very hard and it would be a lot of work to buff the cavity walls completely smooth. The appearance marks left by welding do not affect the functional quality of the part. If a large fabricated sheet metal cavity with a completely smooth surface is needed for appearance, the best material to use would be sheet aluminum. The aluminum is much softer than steel, and the weld marks can be removed and polished smooth with less effort. If needed, the cavity could be polished to a mirror finish or simply smoothed and then etched.
The same tool could be made in mild steel or stainless steel. The stainless steel is tougher and the surface will resist rust, but it is higher in cost
It is difficult to hold close dimensions in fabricated moulds or to reproduce the same dimensions on additional cavities. An aluminum fabricated mould can incorporate castings where difficult contours are required. Sharp corners can be welded in and radii can be worked in without too much difficulty.
