Much can be said about typemetal temperatures for casting of slugs and type. Fry's "Printing Metals" book covers this in reasonable detail, and is worth a read if you want more information.
Linecasting machines have a number of things they're trying to achieve:
The Nebitype attempts to do all these things with two heating mechanisms, the Pot heaters and the mouthpiece heater.
The Pot heater has two elements, one on either side of the metal pot at the top rear of the machine. This heater runs at a slightly higher
temperature (530-560 degrees F, or 277-293 degrees C).
The Mouthpiece heater runs slightly cooler (520-525F) than the pot, as typemetal needs to remain fluid while passing through the mouthpiece but
will need to quickly solidify at the casting point. With a cycle time of around 9 seconds (depending on the machine), slugs cannot simply rest
in the machine to cool.
The job of the mould is to form the tang of the slug whilst also delivering molten typemetal to the matrices in the stickholder. The mould is water cooled so the tang will solidify quite quickly - as will the molten typemetal when it gets to the matrices.
The challenge at casting is to deliver thei right amount of typemetal to both the matrices and the mould to solidify - whilst also ensuring that the metal doesn't solidify or chill part way through the process, OR be delivered too hot. To do all of this the designers must take into account the various heater temperatures, the mould cooling effect, the pump and coolant delivery system and even the speed and size of the metal plunger
If this weren't enough, the machine has to be able to account for wear, ambient temperature, and changes in coolant flow.
Because of this, there are a few adjustments which can be made to the Nebitype:
What may seem to be counterintuitive is that the manual states that the mouthpiece temperature should be raised when casting 6pt blank slugs.
I assume this is because the mould cools the narrower slug more quickly than it would a full 12pt slug, so the additional heat
ensures the slug is fully cast.
It's also worth noting that the casting of blank slugs requires the Plunger Auxilliary Spring Control rod (#2289, Plate 7) to be pushed in,
which prevents the Plunger spring assembly from adding additional force to the plunger, which will limit the amount of pressure exerted and, most
likely, the velocity, "dwell" and amount of typemetal delivered when the plunger is activated.
Verifying the temperature of the typemetal is an important tool in investigating poor casting. In recent years, infrared temperature sensors
have replaced many older style sensors, however these sensors can have issues, particularly with reflective surfaces, like the shiny typemetal
in a well-skimmed pot. Ideally the temperature should be measured inside the typemetal, and not at the surface, and old-school typemetal
thermometer (which looks a bit like an oversized eyedropper is probably best for this task.
The Nebitype uses different methods to heat the pot and the mouthpiece. The POT heating is an ON/OFF system controlled by a thermocouple
in the pot. When the temperature gets below the low setpoint the elements will be switched on, and when it gets above the high setpoint
the elements will be switched off. The mouthpiece heater does not use a thermocouple but appears to enagage heating in an on/off duty
cycle controlled by the mouthpiece temperature setting knob.
All this said, the control mechanisms for the Nebitype heaters changed a few times over the various models and became increasingly complicated
using Wheatstone Bridges in later models).
In the event that the pot temperature control system became unreliable it could be replaced with a modern PID controller in combination with
a 25 Amp solid state relay and a K type (0-400 degrees C) thermocouple. This modification had already been made to my Nebitype prior to my
receiving it.
Replacing the mouthpiece temperature control system is a little more complicated as the mouthpiece temperature would need to be detected, which
would require the installation of a thermocouple in contact with the mouthpiece but without affecting the lockup of the mould, etc or
interfering with the pot-plunger pathway. It might also involve a bit of fine tuning of the setpoint ranges to ensure that the mouthpiece
temperature is reasonably constant, probably with a smaller acceptable temperature range than the pot has).