Paul Woods <paul@...>
Without pretending to be an expert toolmaker, but speaking as a mechanical engineer with more than a passing acquaintance with manufacturing methods, I believe there can be considerable differences in the properties of the material used to make injection moulds. Harder materials can be had, which will last practically forever but require expensive manufacturing techniques such as spark-erosion and grinding, because a milling cutter will barely scratch them. Even a softer material can be made more durable by surface hardening or hard-plating. On the other side, there are softer materials which can be machined on a milling machine and polished by hand in someone's garage, but obviously won't last as long. This would explain why some kits never seem to degrade no matter how many have been made.
Eliminating flash by facing off the mating surfaces of a mould seems like an act of desperation to me - admittedly one which I might be tempted to do if short on funds and the tooling had not yet earned a reasonable return, but I would have my limits. I have worked on jobs where badly worn fine-tolerance parts such as shafts that run in plain bearings (NOT 'friction' bearings, that is not an engineering term!) were restored to original size by metal spraying (a bit like welding spatter but finer) and refinishing. Lesser wear can be restored by metal plating and polishing, but both these methods are more costly than simply skimming the mating faces in a surface grinder.