MK8 Plastic Version Extruder Not Working

[laudeza]

Hi all there!,

I've bought a Prusa I3 Pro B kit from an EBay seller and I have spent last week and part of the previous one assembling it and trying to get rid of it.

Assembly instructions were very clear and quite easy to follow. The kit was with extra units on some small parts like nuts, bolts, washers,... The quality of the laser cut acrylic parts was nice and all the movement mechanics and electronics worked perfectly at the first try.

I am writting this because it may be helpfull for others with the same issue.

The problem was the extruder the MK8 plastic versión direct drive extruder. It has a problem very common here, extrusions were lower than needed or no extrusion at all.

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I've been reading this forum and a lot others, and tried every posted trick (increasing power from drivers, lubricating the wire, putting washers in the spring, ...). Some of them worked partially but no good results at all. I only was able to print something with one manufacturer's PLA, tried many PLAs and HIPS.

It was very strange because when I pressed the arm and pushed manually the wire, it flowed easily with almost no effort. But apparently the motor had no enough power to push the wire.

I dissasembled it and the free Wheel and the motor moved easily by hand. The holes for the wire were perfectly aligned and there was no clearance for the wire to bend...

Yesterday I dissasebled it one more time, but this time deeper. I've never disasembled the freewheel from its arm, never noticed a malfunction. The axle was broken!!

There are friction signs inside the bigger part. When the spring was pressing, the force to spin the wheel was huge.

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I've made a 1.6mm guide hole in the middle and after that, a partial 2.8mm hole. This second doesnt pass from side to side to avoig the axle to pass through.
Sorry, no picture of that.

I reassembled the Wheel and pushed the axle get into the partial hole. In the next pic, the axle is pushed down.

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There is only de guide hole pasing the other side.

IMG_20170320_175716.jpg (98.55 KiB) Viewed 5171 times

After this, the extruder is working properly. I've made the changes instead of ordering another one (warranty) because I understand this is a weak point and in a new one is going to break sooner or later.
And I also don't wanted to wait for another one

Best regards and printings for all,

Laude.

[alang]

I had similar issue with plastic pressure arm - placed packing under spring which worked for a while, then had all sorts of problems with poor extrusion. The L shaped pressure arm turned out to be cracked, so replaced it with the aluminium replacement and has been working perfectly since.
The only issue is that the aluminium kit is slightly wider than the plastic version, so the lower mounting screws were not long enough to reach through the fan, extruder kit and mount to the stepper. Problem was solved by counter boring the holes in the fan so that the screws could reach. Perhaps the aluminium kit could come with some sligthly longer screws included?

Best US$8 I spent - saved a lot of frustration.