Rigid Polyurethane as an engineering material

A Norse Precision Castings product story

Edited by the Engineeringtalk editorial team Jul 21, 2000

Whereas today's designers have a wide choice of engineering plastics at their disposal, compact rigid polyurethane offers unique, often hidden benefits.

Whereas today's designers have a wide choice of engineering plastics at their disposal, some of which compare favourably with metals, compact rigid polyurethane offers unique, often hidden benefits.

The low density of most plastics generally equips them well to compete with metals on weight/strength ratio, and rigid polyurethane at 1.05g/cm3 is well down the 'plastics' range of 1.1 to 1.7, and all well under the lightest of metals, magnesium at 1.75.

Further the ability of rigid polyurethane to form variable wall thicknesses, and strengthening ribs in recognised areas of stress, allows the designer the luxury of putting material only where it is needed.

Material thickness can range anywhere from 1mm to 30mm.

In terms of rigidity and toughness therefore rigid polyurethane compares favourably with other plastics and although it may seem to fall short of the common metals, in practice a component can be designed which adequately fits the mechanical needs and then excels in low weight.

Further its primary benefits emerge when we consider its ability to be moulded cold.

The material fluidity at moulding means an ability to form complex three dimensional patterns to near net shape, with minimum draft angles, often zero.

Whereas moulded tolerances can be as low as + 0.3mm, excellent machinability enables the finished part to be offered with very low tolerances.

The polyol resin is mixed with an isocyanate hardener and fed to accurate but inexpensive moulds at room temperature.

The process uses techniques very similar to sand cast metals enabling Norse Precision Castings, a company which has built a high reputation on aluminium castings from a firm base of high quality patternmaking, to cross fertilise skills.

Mould material like casting patterns is basically wood.

Mould design is of course critical, but material flow characteristics are very similar to gravity cast metals but with few of the problems found at the higher temperatures necessary for metals.

Casting at room temperature, the process lends itself to accommodating inserts, not only to minimise the shortfalls of tensile strength for example by insetting threads or bearing surfaces but also more delicate items such as electronic packages, connectors, co-axial components etc.

and without risk of damage.

High electrical resistivity at 1.3 x 1014 Ohms/cu cm, comes into play here and with good mould design, excellent material fluidity.

The versatility of finish offers the designer both a decorative external appearance and inner surfaces capable of incorporating EMC/RFI screening or mechanical strengthening components.

External finish can also be textured to give a pleasant feel.

Turning finally to commercial aspects, quickly produced, low cost tooling, capable of producing up to 2000 off moulds and material costs favourably below metals suggests anything from pre-production prototypes to low even medium volume production runs.

Norse Precision Castings run the process in parallel with their aluminium sand casting operation offering the advantages of castings experience, a modern CAD design facility, mould production and precision finishing machines.

Experience and production capability in both materials thus gives them the ability to compare the benefits of each without favour, advise the designer and offer production of either or in some cases an assembled combination of both.

Physical Properties of Compact Rigid Polyurethane - Density 1.05 g/cu.cm; Shore D 72; Tensile Strength 35 N/mm2; Elongation 15%; Flexural Strength 65 N/mm2; Flexural Deflection 17%; Flexural Modulus 1800 N/mm2; Impact Strength 21 kJ/m2; Heat Distortion Temperature 70 degrees C; Dielectric Strength 22kV/mm (test plaque 4.5mm thick); Volume Resistivity 1.3 x 1014 Ohm/cu.cm.