Xencast PX60 Medium Flexible Polyurethane Resin

Yes we have a range of specific PU pigments which are compatible with the PX range of resins https://www.easycomposites.co.uk/polyurethane-solid-colour-pigment 

Our tinting pigments are also compatible with PU resins https://www.easycomposites.co.uk/translucent-tinting-pigment

You can certainly use this resin without degassing however we do recommend degassing to ensure maximum performance of the finished product by removing all traces of air. Bubbles can form weak points in the casting which promote tearing and ultimately early failure.

Due to the short pot life, if you do choose to degas the resin then we would recommend a degassing system that uses a high capacity pump so that there is enough time to degas within the resins pot-life. Our Professional Vacuum Degassing System - EC20-1 Vacuum Pump is the ideal one stop solution for degassing highly reactive resins with short pot-lives.

Assuming your skateboard wheels are perfect cylinders then the volume is quite easy to calculate using the formula Πr²h which in your case r = 25mm and h = 30mm giving a volume of 58.9 cm³. For Example, the density of PX90 is fractionally over 1g/cm³ which means you need 58.9g of resin per wheel or 235.62g for a set of 4. However check the density of the specific resin you are using in all cases so you multiply by the right figure to get an accurate weight.

However, most skateboard wheels have cut outs or hollows so the reality is you will use less resin. Calculating the volume in that case is a little more complicated as you need to calculate the volume of the cut outs then take it from the figures above to give the actual volume.

...the shore should be around 70 ish. Also, how well does it bond to metal, and can I treat the metal with a product to increase the adhesion?

Most commonly motor mounts (engine bushes) are made using something more like an 80 or 90 shore resin. In fact our PX90 has been used by a number of customers to create engine mounts.

If you're confident that you do need a 70 shore for your application then our closest shore hardness would be our PX60 but my opinion is that this would be too soft for most typical engine mount applications.

As for the bond to metal, we don't really have, and you shouldn't really need, an adhesion promoter for PU to aluminium - they should bond well. As usual, just make sure the surface is clean, completely grease-free (using acetone) and lightly keyed (using abrasive paper).

Glass Bubbles can be added to most resins to reduce the density of the cured material. However, bear in mind that they will reduce the clarity of the GlassCast resin, making it more of a translucent 'milky' appearance rather than transparent.

Polyurethane based resins will bond to a keyed and degreased epoxy surface so the resin will bond and act as a rubber-like surface on an epoxy part however the suitability for your application may be affected by environmental conditions such as weather and sunlight exposure and the appearance you're trying to achieve. For example, the resin would not be suitable as a 'clear' coating to a carbon fibre part because the PX60 is amber in colour and also will discolour with exposure to UV light. 

No, there are currently no additives designed to help improve the rubber's UV performance however, adding bold/dark coloured pigments (such as black) can help significantly by reducing the exposure of the resin to the UV.

Yes the PX60 will cast well and release well from the CS25 Condensation Cure Silicone as found in our XenCast Resin Casting Starter Kit

No. Our PX series of polyurethane resins are not designed to be UV stable and as such will yellow under prolonged UV exposure, especially in strong sunlight.

...Could this be used to create rubber intensifiers for moulding carbon prepreg parts in an autoclave cure? If release coated I assume it will not stick. But would it inhibit curing of the epoxy at all? It would need to be removed (not re used) after cure.

Polyurethane is known to have quite a strange inhibiting effect on the cure of epoxy when both are at elevated temperature (this effect is seen clearly if you've ever tried using PU tooling board to make a prepreg carbon fibre part) and therefore I would be at least wary of this combination. I cannot say for sure it would inhibit the cure of the epoxy but it does have all ingredients of a potential problem (uncured epoxy in contact with polyurethane and elevated temperature).

A more reliable alternative would be our AS40 Addition Cure Silicone Rubber. Silicone is a much more common material to use as an intensifier in prepreg laminating and our AS40 product would probably be about right in terms of shore hardness. Silicone also has the great advantage (in this application) of having a very high CTE which makes it expand at elevated temperatures, thereby increasing the intensifying effect. Epoxy will also naturally release from silicone.