Heatbreak hole's

Why do (almost) all heatbreaks have larger holes on the top side (the long thread inside the cooling grill)?
I must put a teflon tube inside or else my filement gets stuck when i put new filament into the extruder.
I want to print some high temperature filament and flexible filaments in the future and i was thinking that the teflon tube melts when printing at such high temerature.
If the top of the heatbreak had the same hole-dimention as the bottom side (heatblock-side) with a champfer, then the teflon tube is not needed anymore so the filament would be guided into the heatbreak by the champfer.

Is my thinking very strange or do i have a point?

There are many hot ends designed so that the PTFE tube stops near the bottom of the heatsink and never gets near the heater block. Look for "all metal" hot-ends. The E3D v6 has been a standard for years, but there are many others.
If you have PTFE going into the heater block you will be severely limited in the materials you can print because the PTFE swells and even breaks down at temperatures used to print ABS and PETG.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

Yes i know, i have also a E3D v6 heatbreak but my point is "Why need a teflon tube atall?".
Why isn't the heatbreak designed so you don't need the tube?
When the hole is 1.75 mm from top to bottom, with a champher on top then the filement wouldn't get stuck and you could print very high temeratures.
Question: What is the maximum temperature you could print without damage the teflon tube on a E3D v6 heatbreak?
Does someone know of these heatbreaks? Please send me a link.
Thank you

Edited 3 time(s). Last edit at 11/29/2020 05:58PM by VenHeelun.

AFAIK, filament sliding against PTFE has lower friction than filament sliding against metal, especially as the filament warms and expands. The PTFE probably provides some thermal insulation to keep the filament cool right to the neck of the heatbreak, too.
The top end of the heatbreak - i.e. the heatsink- should never get hot enough to have much effect on the PTFE. If it does, the heatsink is poorly designed or the cooling fan isn't doing its job.
The E3D V6 has several problems, but PTFE doesn't seem to be one of them. For example, there is no way to secure the heatbreak part inside the heatsink. It just threads in and you have to hope it will stay tight even though the thin walls of the neck limit how much torque you can apply before it bends/twists/breaks. They frequently loosen after a few heating cycles, wrecking the print when they do. If you put thread locker on it, you won't be able to take it out of the heatsink if you ever need to because you'll break the neck when you try (and chew up the threads from the end that goes into the heaterblock- there's no way to grab it with a tool, except on the threads or the fragile neck.

I've used a cheapo hotend from China (XCR3D, $15 IRIC) on two printers for the last few years with no problems. The unthreaded heatbreak tube is secured with set screws and it never comes loose. Likewise, the fan is on a metal bracket that is secured with screws to the heatsink and it never spins around the heatsink and never gets near the heater block and melts like the E3D fan bracket. The neck of the heatbreak has thicker walls than the E3D part, so if you have a head crash it doesn't bend like the E3D part does. The fan that comes with it is trash (as is the one that comes with the E3D V6), so plan to spend a few more $ on a decent fan to replace it. It comes with a 50W heater so it heats up quickly and can go to very high temperatures is so desired. I've never gone above 270C but it will happily do so.

[drmrehorst.blogspot.com]

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

I also use that XCR-BP6 hotend. I got one with the setscrews screwed into the heatbreak so tight that I stripped a wrench just trying to remove them (as shipped, without having heated it) so I think quality control isn't perfect. I replace those setscrews with long enough M3 FSC screws to stick out just past the fins. That way, they are easier to get at and have a larger socket, but they still don't get in the way of anything. I prep the screws with anti-seize compound as well. They didn't previously ship with a sock, but they now include a silicone sock and my hotend temperatures have been more stable with the silicone sock than they used to be with multiple layers of kapton tape wrapped around it as a makeshift sock. I've printed PETG at 285° at 120mm/sec with no problem. The plastic doesn't get that hot when you push it that fast, it's not as crazy as it sounds; it's measuring the temperature of the block not of the plastic.

I made an adapter and put a 40mm noctua fan on in place of the 30mm that it shipped with. I do so little part cooling that I haven't finished adding a duct from the part cooling fan to the nozzle, but what I have is here: [gitlab.com]

I did have it jam from heat creep when printing PLA slowly with the all-metal design. I don't print much PLA any more so I haven't cared enough to experiment with speeds. Probably I should print at least PLA with volumetric extrusion and it would probably just solve that problem, but I'm lazy, or something like that. They sell both throats separately, so you can order some of each if you want both high temperature and PLA printing available and don't mind recalibrating Z. ☺