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Moulding with Aluminium Moulds
One of the most crucial aspects in favour of the use of aluminium moulds with respect to steel moulds is surely the possibility of increasing the productivity of presses which permanently use light alloy moulds.
The main reason for the increase in press' rhythm is aluminium's thermal conductivity value, which is up to 7/8 greater than that of steel; some types of high chrome content steel range from "relatively" to "definitely" insulating.
Approximately 70/75% of the press cycle time is dedicated to the cooling phase (consolidation) of the moulded part or in any case to the lowering of the temperature of the mould (see figure).
The advantages obtained can be summed up as follows:
- Up to a 50% increase in press moulding rhythm.
- Considerable reduction of mould preparation time.
- Easier to design cooling channels: larger holes and further away from the form to be cooled.
The following diagram shows the difference in thermal conductivity values compared to the mould materials more commonly used, even with respect to Beryllium Copper used frequently in critical moulding areas and difficult to reach with cooling channels.
So far, moulding process simulation with aluminium moulds is still not possible as the main types of software do not feature thermal conductivity parameterisation, being that they are set mainly for steel moulds. However, field tests lead us to consider a 30% minimum percentage saving in cycle time, and in some cases even up to 60%, when the thickness of the thermoplastic material to be moulded reaches a thickness greater than 2/2.5 mm.
Another important factor when choosing the mould material depending on the press cycle is, as mentioned above, that of the thickness of the part to be moulded. It is a well-known fact that when it is necessary to reproduce considerable thicknesses, there can be some inconveniences regarding the quality of the product obtained: in fact, the latter may show excessive shrinking due to the ineffectiveness of the cooling system or, in any case, to the intrinsic difficulty of cooling some mould materials.
The aluminium solution allows a considerable reduction of deformation risks on the final product, a reduction in investments set aside for the mould cooling system (less machine hours for making holes), and more rapid cycle time.
 
The photos above represent the articular and mould of a Polypropylene office chair with a 30% fibreglass charge. At first, the mould was entirely made of steel and only after useless attempts to mould it, it was decided to make the form out of aluminium. In this way many cooling problems were solved, especially in the central area of the mould, where the part reached 3-4 mm in thickness.
 
The part featured in the photo above is an element mounted on a piece of gym equipment.
The mould material is Certal SPC, while the moulded part is made of Copolymer Polypropylene.
The photo on the right illustrates a section of the part. Three different sections can be seen with 20, 8.5 and 5.25 mm walls, respectively. According to the die-caster, this part was practically impossible to make with a steel mould.
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From Old Tooling 