Assembly and startup report (one more)

This was my first step in the 3D printer community and I selected the K8400 for several resaons:

  • robust housing
  • small nozzle size
  • high axial resolution
  • twin extruder

The assembly itself is well described, nevertheless, I needed to remount the back plate of the housing and the sidewalls because I flipped these parts in the first run and didn’t mention the details which are important for the proper orientation.

There were no major problems during assembly, and after several hours I was able to start the printer. I installed the firmware version 1.4. It worked properly and I could start the first test printing. (a small chip) using the slic3r software (I’m running linux as standard).

I was completely displeased with the print results.

I experienced layer shifts in the object.
According to the feedback in the forum this is mainly due to mechanical blockings during printing.
To avoid these mechanical problems I initially cleaned all rods and coated them with silicone oil during the assembly. I put some oil on the rods and could print three test chips, then the layer shift reappeared. The silicone layer was not stable enough for a longer printing session.
I switched to silicone grease, which makes the printhed movement harder, but this film remained stable the whole time. neverteless, it is blocking high accelerations, which are necessary for the print dynamics (filament flow). Finally I switched to PTFE coating, which is stable since several days now.
I would remove the recommendation for silicone since silicone lubrificant is diffusion very fast, so your surrounding is silicone tainted in short time (also the print bed!).
An improvement to think about is the replacement of the brass tubes in the print head with PTFE tubes. When using the silivone grease I noticed some friction coming from the movement.
My conclusion: after cleaning of the rods they shall be coated with PTFE.

Mechanical barriers
I faced an effect of mechanical blocking, which was not mentioned in the forum yet: the printhead movement was hindered at three positions along the full axis length. I was wondering about the reason and found out, that the blocking was related to a dedicated angle of one outer rod. When moving the print head (here: in y-direction) the front and backside rods are rotated by the belts. The blocking appeared at a dedicated angle for each rotation. Finally I found the reason on the front rod: the rod is fixed in the sidwalls where beareings shall ensure the easy rotation of the rod. On the outer side the rod is fixed between two plastic end caps, which are screwed on the sidewalls. By loosing or thightening of these caps I could remove or increase the blocking of the rotation of the rod.
My conclusion: take care of manufacturing tolerances (as well as asembly tolerances too)!

Too much material was deposited.
After some trials with modification of filament extrusion factor I checked the traction of the filament. Within the firmware settings I found a value of 200 steps/mm for the filament movement. with this setting I commanded an extrusion of 50mm and got a pushing of 64mm. While I found all other settings (x, y, z) correct, this is a major error which made my first prints very poor. Actually I corrected to 156 steps/mm, the resulting extrusion factors are around 1 now. A drawback of this modification is the filament load/unload procedure. Obviously there is only the number of steps programmed, niot the filament length. With the new option the load/unlead length for the filament became shorter.
My strong Recommendation: Correction and update firmware settings (new revision)!!!

Temperature sensor
The location of the temperature sensor is quite critical in my mind. Since I was facing a lot of over-extrusion, the print head was covered with excess filament several times, which also was smeared into the hole of the temperature sensor. by several mechanical cleaning steps the isolation of the connection becomes damaged. Here I would recommend a temperature sensor housing (e.g. ball), which is only accesed from the top side.
My recommendation: review of the temperature sensor.

Since I would like to print other materials than PLA and ABS, I will need some spare parts. I didn’t manage to print an ABS part yet, so I’m very keen in testing some carbon filled materials. These are promising mechanical hardness compared to ABS and printing behaviour like PLA. Unfortunately they claim an abrasion of the printer nozzle, so I will need some spare parts, optionally in another material (e.g steel). Actually I couldn’t find any provider, which is shipping to Germany.
My request: please provide an support link for spare parts, especially for the type specific parts like nozzle, temperature sensor.

In the meantime the print results are improved, but I’m still not please. Actually I have some trouble with the slicer software.

I hope this will help somebody, finally if an updated firmware and a support link for spare part can be provided.

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