On Mon, Nov 11, 2019 at 01:02 PM, Paul Woods wrote:
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.I was going to state that, but Paul beat me to it. Over the years a LOT of model railroad tooling has been "soft" tooling, because it's cheap. The choices are, in order of increasing hardness/durability:
Kirksite (a cast zinc alloy)
Mold steel (a tool steel such as P-20 used in a semi hard state, soft enough to cut with conventional milling, with no further heat treatment.)
Tool steel (which is fully hardened and then worked by Electro Discharge Machining and grinding)
Kirksite could be cast over metal patterns, same as rubber molds are made. If damaged it is not repairable.
Brass was a favorite of people who came into toolmaking from engraving. A lot of Grandt Line tooling is brass. It is difficult to repair, due to its softness and low melting temperature.
Aluminium is a favorite for CNC machining of cavities because it cuts easy and doesn't break small cutters. The old Front Range line was completely aluminum tooling. It is slightly harder than brass and more easily repaired by welding.
P-20 was a favorite of the old model car manufacturers. It is easily damaged, but weldable.
Tool steel is the gold standard, and welded repairs should be undetectable.
While there are surface treatments that can be applied to give a more wear resistant surface to most of these mold materials, the problem is, in our world, most damage is denting from closing on stuck parts rather than abrasive wear, and surface treatments are ineffective. However, for items produced with quality tooling, the millionth part should look no different from the first.