Hot End and Extruder Building Questions

Hi,

I'm quite new at this 3D printers world but I'm starting a project where I'll try to develop my won 3D printer arduino based.

I've been doing lots of research, reading articles and watching lots of videos, but most of the information refers to already-built printers, kits or upgrades for commercial printers so I actually miss some details abot how it works. Anyway I'm now specially centered on hot ends and extrusion and there are some base questions to which I can't find an answer, let's see:

As far as I know a hot end is the device that heats the filament until it's melted. So, the filament is pushed by a stepper motor, it melts down in the hot end and it goes out through a nozzle. My questions are:

- I can see everywhere "hot end temperatures" (like 240 C for ABS), can I assume that is the temperature on the inside surface of the hot end? or even further, if I were using nichrome wire around a barrel, could I assume for the calculations that 240 is the temperature I should get on the nichrome wire?
And related to this, where should I set the temperature sensor? Does it go inside the hot end or does it measure the barrel temperature?

- About extrusion: the way to extrude the plastic is simply melting it and letting it fall through the nozzle hole? I mean, I'm wondering if the hole through which the filament goes inside of the hot end was so tight that the movement of the filament generated pressure inside and that madethe melted plastic go out. Maybe this guess is just a fool idea but as I said I couldn't find this info anywhere

- A bit to do with the previous one, how do you prevent the reflux of the melted plastic, i mean, how do you get the melted plastic not going up through the conduction where the filament comes in?


Just to make it clear, I'm not trying to make you work for me, I think these are basic questions and probably some of you won't answer because of that, but I couldn't find an aswer anywhere and I think you guys know a lot about this. I'm so excited with this 3D printing world and its enormous potential.

Thank you all in advance

Toalla

Hot end temps are usually measured by a thermistor on or in the block
of aluminum / copper being heated by a resistor or cartridge heater.
The thermistor feeds data to firmware / software to control the hot end heater temperature.
A cartridge heater is nichrome wire in a thermal conductive matrix which
is transferred to hot end heated block.
Hot ends usually have a steel insert with a precision taper from 1.75mm (or 3mm)
to the nozzle diameter (0.2 to 0.5mm). Some use High temperature PTFE plastic
to provide very smooth patch for filament.

Hot end block temperature PLA 180 - 195 deg C, ABS 200-225 deg C Nylon- Poly 230 - 260deg C

The solid cold filament acts like a piston to force polymer thru the melt zone and out the nozzle
Filament retraction prevents ooze when not laying down polymer

Start with a known extruder / hot end (SeeMeCNC extruder & E3D hot end)
After using it for a couple of years you can work on your hot end pHD thesis.

First of all thank you for your answer,

With some help from here, some readings on the internet and info from the wiki I think I'll try and build my own system, even though I'll probably end up buying a commercial system that will be much more precise and calibrated.

This are just those basic doubts you have when you approach any problem in a field that is quite new for you, that don't let you go on.

Thanks again