Does anyone reading this have any technical data on how long the
flask core should be at maximum temperature? I am not referring the
holding time for the oven since the core temperature obviously lags
behing the oven temperature.
I doubt you can come up with any real hard and fixed times here. It
will vary according to how much carbon needs to be oxidized in the
investment (how much wax was in the flask, and how much of it soaked
into the investment instead of melting and flowing out). And it will
then vary according to both the size of the flask and thickness of
investment through which oxygen must travel to do that job, and
finally, and perhaps even harder to quantify, the amount of oxygen
available to the investment, which will depend on the amount of
ventillation in the oven, as well as air flow patterns. Even the way
the flasks are placed and supported will affect this. It's important
since if you're burning out without oxygen, the carbon won't burn
away at all. One reason some casters find differences in performance
between electric and gas fired burn out ovens is precisely this, that
the gas fired furnaces have greater rates of ventillation, even
though much of the oxygen in the furnace is being burned by the fuel
gas. And it's also one of the reasons why some small furnaces,
enamelling and PMC types especially, may not be suited to casting
burnout, if they lack sufficient ventillation to allow an adequate
inflow of oxygen and gas circulation to carry that oxygen to the
With all that said, of course, one certainly can come up with times
required for typical burnouts in most situations. For small flasks,
usually a hold time of around an hour is sufficient. Given that this
is with the usual slow burnout, it probably is is close to the time
the flask core is at these temps too. Larger flasks with more waxes,
need more. Steam dewaxing reduces the times needed.
Trial and error may be the best way. Easy enough for a caster to
keep notes and find the best times for their own burnout method.
Great little book you've got there, by the way... Well done indeed.