Back both horses (plastic and metal) to win

A Norse Precision Castings product story

Edited by the Engineeringtalk editorial team Oct 22, 2007

By working with both aluminium and rigid polyurethane, Norse Precision Castings offers impartial advice on materials.

The ease with which plastics form complex shapes, their low weight and economy of production have always made them attractive engineering materials.

Metals however have the ultimate strength and hardness and as cast can offer equally high fluidity, as well as retaining a high degree of perceived quality.

Few companies offer unbiased advice and the designer's choice is not easy, but Norse Precision Castings offers both and reckons it is well positioned to give impartial advice.

Recognising some years ago the parallels of processing rigid polyurethane and sand casting aluminium, Norse Precision has developed the facility to work both materials, crossfertilising many processes and optimising designs to the client's advantage.

This development continues particularly on such pivotal aspects as modelling, where the company has begun sourcing three dimensional models built up from powder deposition, structured from electronically generated models.

When strengthened with an epoxy coating these can be used in either sand casting or rigid polyurethane moulding with the real advantage of high accuracy prototypes and very short time to market.

Melting at about 660C, the excellent fluidity of aluminium enables intricate three dimensional shapes to be formed easily to near net shape, with minimum draft angles.

Cavities and strengthening ribs, even shallow logos can also be incorporated into a casting.

Techniques at Norse Precision Castings can achieve tolerances of +0.4mm and wall thickness down to 3mm.

Of the various alloys available to the caster, LM6 and LM25 are most useful.

Alloy LM25, with slightly higher copper and magnesium content offers marginally better mechanical properties and would be the natural choice for a high integrity part needing later machining.

Alloy LM6 is most ductile and especially useful for leak tight fittings like "on-deck" marine castings, water cooled manifolds, pump parts, paint and food industry components and domestic goods.

Alloy LM25 however has superior mechanical characteristics, is easier to machine, and is available in four conditions of heat treatment, each with significant benefits.

It is, in practice the all purpose alloy for high strength, high integrity castings and as such has found application in aircraft and nuclear parts, road vehicle components requiring high reliability, such as wheels, cylinder blocks, heads, other engine and body castings.

Although the choice of plastic materials seems beyond limit with additives and fillers to enhance physical properties, rigid polyurethane compares favourably with most, offering a high quality engineering material with some exceptional benefits.

Rigid polyurethane is cured by chemical reaction at room temperature.

This allows other materials to be cast into a moulding without stress or thermal damage.

The integrity of other materials and components set into a moulding can therefore be assured.

Electrical resistivity is high at 1.3 x 10e14ohm/cm3.

Density at 1.2g/cm3 is on the lower end of the plastics range of 1.2 to 1.7g/cm3, and well under even light metals such as magnesium, at 1.75g/cm3.

Typically minimum wall thickness in sand cast aluminium is 3mm and although normally 4 to 6mm in rigid polyurethane, Norse Precision Castings can go down to 1mm.

Tolerances on moulded polyurethane at +0.3mm also compare favourably with aluminium castings +0.4mm.

In terms of physical properties, pattern shrinkage on aluminium of 0.006mm/mm is lower than most common metals: bronze is 0.018, iron is 0.010 and zinc ranges from 0.006 to 0.013.

The tensile and yield strength of aluminium are comparable with magnesium, bronze and cast iron, and the Brinell hardness is better than magnesium and bronze, a little down on zinc.

Physical data for compact rigid polyurethane include: density of 1.2g/cm3; Shore D 72; tensile Strength of 35N/mm2; elongation 15%; flexural strength 65N/mm2; flexural deflection 17%; flexural modulus 1800N/mm2; impact strength 21kJ/m2; heat distortion temperature 70C; dielectric strength 22kV/mm (test plaque 4.5mm thick); and volume resistivity 1.3 x 10e14ohm/cm3.