Ofc there are better things for ~250C, like using matched pairs of hermetically sealed thermistors, and there are other sensors like RTC (Pt thin film), thermocouples, etc. But we reprapers are fatally attracted to cheapest things possible, and that seems incurable. However dont get me wrong, thermistors are ok for use in reprap, evidently just need to note that operating range is bigger than interchangeability range, and overall its not a bad solution. One just needs to understand the implications and how to deal with them, other than that thermistors are perfectly fine.
One of the implications is that when we exchange the extruder thermistor tables we assume our thermistors are "interchangeable" at 250C. They are interchangeable for bed temperatures, with a fair margin up to 150C maybe, but higher than that they stop being interchangeable, so our tables shouldnt be either. However absolute values dont really matter in the absolute sense, its all relative, even celsius scale itsels is relative. And just dont think if some1 on forum says "extrude this as xxx temp" and if you set on your printer "xxx temp" you will get that same temp, coz you wont. Beside that your temps should be consistent, meaning your xxxx temp today will be exactly same tomorrow, and thats what is important.
About print surface, i use some pcb blank (copper cald or what the name is), double sided. Its 1,5mm and very easy to cut into shape. I put kapton on one side, and i clamp them on top of the bed with paper clamps. After print i take it off to cool down and flex a little and the parts easily snap off. Tried metal plates like 1mm thick, but arent so good coz when it accidentally bends, it remains bent. The pcb is ideal coz it can flex alot and always recovers to the proper shape.
The glass/mirror is only good for pla so you can print pla directly on it, without kapton tape, that is sort of economy.
One of the implications is that when we exchange the extruder thermistor tables we assume our thermistors are "interchangeable" at 250C. They are interchangeable for bed temperatures, with a fair margin up to 150C maybe, but higher than that they stop being interchangeable, so our tables shouldnt be either. However absolute values dont really matter in the absolute sense, its all relative, even celsius scale itsels is relative. And just dont think if some1 on forum says "extrude this as xxx temp" and if you set on your printer "xxx temp" you will get that same temp, coz you wont. Beside that your temps should be consistent, meaning your xxxx temp today will be exactly same tomorrow, and thats what is important.
About print surface, i use some pcb blank (copper cald or what the name is), double sided. Its 1,5mm and very easy to cut into shape. I put kapton on one side, and i clamp them on top of the bed with paper clamps. After print i take it off to cool down and flex a little and the parts easily snap off. Tried metal plates like 1mm thick, but arent so good coz when it accidentally bends, it remains bent. The pcb is ideal coz it can flex alot and always recovers to the proper shape.
The glass/mirror is only good for pla so you can print pla directly on it, without kapton tape, that is sort of economy.