Technology Aluminium butts heads with steel in injection tooling

Editor: Eric Culp

As materials technology continues to develop, aluminium is playing an even larger role in injection moulding. Alcoa, one of the world’s largest manufacturers of the lightweight metal, compares its performance with that of tool steel.

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Blowmoulding tools are only one use for aluminium. The material is being increasingly favoured for injection tooling.
Blowmoulding tools are only one use for aluminium. The material is being increasingly favoured for injection tooling.
(Source: Alcoa)

Aluminium as a mould material has proven itself in low temperature and low pressure applications, such as in the blow moulding and injection moulding of simple thermoplastics like polypropylene and polyethylene.

More Al moulds coming

Aluminium mould alloys are no longer limited to 5083 and 6061. There are now a myriad of high strength aluminium products in the market, including ones with improved through-thickness and property consistency. In addition to increased strength and wear resistance, aluminium alloys such as Alcoa’s Alumec series display a more consistent through-thickness strength and hardness, which further improves its machinability.

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Moulders have started to capitalise on the benefits of aluminium, and it is now being used in full volume injection moulding practices for more complex thermoplastics and elastomers. As shown in the following table, development and prototyping is taking place with increasingly aggressive resins.

It is also possible to use aluminium for rubber moulding, and this is where it is important to discuss the application with the aluminium supplier, so that the right aluminium product can be selected for its purpose.

No panacea for toolmakers

Aluminium is not suitable for all applications, however. Whilst some aluminium alloys can be used at moulding temperatures up to 180C, non-aluminium alternatives are recommended for moulding at higher temperatures and high pressures. Additionally, certain resins and thermosetting plastics are not suited for use with aluminium moulds, due to processing requirements.

Benefits for those using steel

Aluminium’s properties give it a number of distinct advantages as a mould material. The thermal conductivity of an aluminium mould alloy (~160 W/mK) is about four times higher than that of mould steel (~40 W/mK), which leads to three principal benefits:

  • Simpler mould design. A less complicated cooling system is required, and issues associated with hotspots in mould design are reduced.
  • Reduced part distortion. Parts made using high strength aluminium and P20 steel moulds were compared. The level of internal residual stress in steel mould-produced parts was higher.
  • Reduced cycle time. Within studies, a high strength aluminium mould extracted more than 1.6 times the thermal energy of a P20 steel mould. This leads to quicker melt solidification, which in turn reduces the moulding cycle time.

Aluminium alloys generally have lower rigidity and lower mechanical strength compared to steels at moulding temperatures. Even though mould wall thicknesses need to be increased by up to 40% to take this into account, an aluminium mould is still less than half the weight of a steel one for the same application. This weight decrease reduces load on process machinery.

Welcome to the machining

Process modelling software can compare the machinability of steel (P20) with high-strength mould grade aluminium. Aluminium can be machined nine times faster than if there is no limit to the machine capability. When power consumption is a limiting factor, research shows that the same machine can machine aluminium four times faster than steel.

Wearing down the tool

In a study by the Warwick Manufacturing Group, a department of the UK’s University of Warwick, the level of tool wear was compared when flat end milling was carried out on high strength aluminium and steel injection mould alloys. For the aluminium, no significant tool wear was observed after 418 minutes of machining. For the Stravax ESR, however, 45.41μm wear was observed after 29 minutes of machining. Furthermore, chip form could be maintained throughout the machining cycle for the high strength mould-grade aluminium, which is said to be an indication of good machinability.

Good results with EDM, too

Within an EDM milling study, the benefits of aluminium’s use were faster milling, lower current and savings on EDM wires. It produced data showing a 58-60% reduction in mill time for the high strength aluminium alloy.