............Is 400 degrees the general target for most moulds before casting is started, or does this tend to vary?
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For me, it "tends to vary." Not by a lot, but it's not a hard and fast XXF.
I know it doesn't sound like much and may just be part of my personal superstition, but MOST of my moulds seem to hit their happy place just under 400 - like 370 to 390.
I have one 4C NOE 360-180 which seems to do its best with the alloy hotter than usual (800F) and the mould cooler (350F) than usual, but the important thing to remember is that..........
Any of these discussions is based on RELATIVE terms among members/members' sensors. Not everyone's thermocouples (TCs), analog thermometers, non-contact thermometers, etc. are calibrated to "agree," and even among the several sensors I personally employ, I can't guarantee that they are all within 10% of one another. It's all relative.
I know you all know that, but I wanted to throw that in for anyone "new" or just lurking so they had a more realistic perspective.
Now that someone's brought this up again, I still think my cheap hot plate is the one (embarrassingly) best/biggest help in casting for me. Not that I couldn't without it, but it sure makes life easier. If I have to stop and load clothes from the washer to the dryer, take the dog out, fix lunch, take a leak, etc. I can turn on the hot plate, set the mould on it and walk away (not too far, mind you) and come back and roll right back into the original pace. It's also nice to let the mould slowly come up to temp while the alloy melts or you dink with adding some magical component.