Unlike injection moulded plastic kits which require a high volume of sales to justify the investment in expensive machined dies,  resin kit technology is suited to short runs from moulds which are typically made from a synthetic rubber.  Whilst castings detailed on all sides are possible with resin technology it is a specialist process which requires pressure or vacuum filling of moulds to do the job properly.

In most cases the resin castings supplied in Nine Mill Scale Products are made in open backed moulds so that one surface (e.g. the inside face of a coach or van side) is flat and undetailed. The resin used in kits listed on this site at present is a filled polyester which is stronger than some of the more brittle resins but not as resilient as polystyrene or ABS mouldings.  Starting in 2004 we have been working on a transition to polyurethane resin instead of the filled polyester.  Polyurethane has considerable strength advantages over polyester and is less prone to heat distortion prior to assembly.    As and when kits are changed over to polyurethane they will be noted as such in the product listings.  There will inevitably be a price differential - polyurethane is considerably more expensive than polyester and, in some cases,   customers may be given a choice of resin until old stock runs out.  Nine Mill Scale Products resin kits are so called 'flat' kits.  That is to say the sides and ends of coaches or wagons are flat castings which require to be joined to form the 'box' shape of the vehicle being modelled.

Resins castings require more work than the average styrene or ABS moulded kit.  Resin kits are in more of a 'craft' tradition and can be likened to scratch building with a lot of the basic shaping already done. 

Kit builders can expect to have to clean up some flash from castings and generally fashion the parts to ensure a close fit.

Whilst generally referred to as 'resin kits',  Nine Mill Scale Products kits also require skills in assembling white metal and brass details and some basic wood working skills (e.g. fitting MDF floors and/or ceilings).

An excellent article dealing with American resin kits was published in the August 1999 issue of Model Railroader.   I recommend that article as background reading for any modeller who has not previously built a resin kit.