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Saturday, May 25, 2013

No slipping, no grinding: not always a good thing!

Let your filament slip, your bowden tube pop up... or break something !

If the filament cannot slip nor be ground a bit,
excessive pressure has to "break" something anyway.
Now for sure, and I can prove it: my driving system is very efficient and my bowden stays well clamped...

When I tighten the idler completely to the maximum, the driving bolt grips the filament so well that the weakest point in the feeder mechanism becomes the PTFE bowden tube resistance itself.

As you can see on the pictures, my PTFE bowden tube got stretched and stripped in the pneumatic push-fit connector!

This seriously clogs the nozzle and it becomes a real drawback in the end: check my analysis below.

My home made driving bolt can grip the filament so hard that it is the bowden tube that gets stripped in its place! :(
Note: the push-fit connector is reversed on the pictures, sorry for that! The tube clamps on the blue side!

Why did it happen?

It happened here after a dozen minutes, while I was reducing the temperature to the lowest in order to print very small and detailed objects. The filament finally was too cold to get properly extruded, at the expected rate. Pressure accumulated in the bowden tube, and as it started to slip on the feeding bolt, I tightened the bolt to the maximum, so the pressure accumulated elsewhere. Well, it had to break somewhere at some time...

Which consequences?

First, the bowden tube was stripped out of the push-fit connector metal fins (it would not slip on the other side with my setup). And my print was ruined. But much more annoying is that these little strips of my PTFE/Teflon tube got through the push-fit connector, and they were dragged by the filament towards the nozzle. Since PTFE does not melt, it clogs the nozzle very efficienty...

How to fix.

Obviously I had to disassemble the head completely to remove the PLA/Teflon mix, until the head was perfectly clean. I failed the first time as I probably left a tiny bit of PTFE on a sidewall, which clogged the nozzle again shorty after. I sticked pieces of PLA quickly forth and back to remove most of the molten plastic and used the thin wire trick (see my post about cleaning a hot end).

I also knew I had reached the lowest extrusion temperature, so I had the choice to reduce the speed or to increase the temperature back a bit.

What I learnt and my conclusion.

Tightening the bolt completely is not a smart solution to force the filament into the nozzle when it cannot, such as when overfeeding, or with too low an extrusion temperature.

This also illustrates my opinion on the weird race for the "best ever gripping driving bolt"... My experience tells me I just need to give the filament a chance to slip on the driving bolt unless I add a sensor that can detect an excessive pressure and reduce the feeding rate.

This is mostly where and why a spring is very useful on the feeder idler: do not thighten it completely, unless your bolt is not very efficient and that it lets the filament slip anyway before havoc.

As a rule of thumb, better let a system "break" on an expected place than see it break something unexpected, that may be much more annoying to fix... For this very reason, I probably do not want to use the same efficient clamp I used on the other side of my bowden, as the printer would then probably destroy a support structure, like stripping the push-fit connector off of its wooden threads.

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