extruder does not extrude

I have bought a extruder kit on ebay and I can´t extrude. The stepmotor pulls the filament through the hotend chamber, then it starts to stop extruding by itself. I set hotend temperature at 220C. I tried 220C - 240C range tempetrature with no success. I have also tried to lower the extruder speed down to 5mm/s, then 1mm/s and 0.5mm/s. I assume I have no clogged nozzle because I can extrude the filament through 0.3mm nozzle by hand. It may be a product design problem? Does anybody have this product?

When it 'stops extruding', what exactly happens? Does the extruder motor stop turning? If it keeps turning, try increasing pressure of the filament against the extruder gear.

What kind of hot end do you have? All metal? Or with a PTFE liner? I found that all-metal ones tend to clog... PTFE-lined ones seem to be OK (so long as the PTFE is OK). Do you have a fan on the heatsink, continuously on?

What kind of filament are you trying to extrude... PLA should be about 190C, ABS about 210C

I try to extrude 1.68mm ABS filament. When it stops extruding, the extruder gear theeth begins to slide over the filament surface. The gear keeps spinning and it starts digging the ABS filament.

the extruder has a PTFE liner in the space right after the gear until 5mm above the heat cartridge. Hotend inside is metal. I have a fan continuously on. It has a teflon washer inside the nozzle.

If the gear is spinning and the filament not moving, increase the pressure of the filament against the gear. If it grinds away the filament you have clogging.

Try turning down/off retraction. Retracting hot, soft filament into a cool part of the hotend can cause clogs.

Try turning temperature down to 210... too hot causes heat to creep back up into the cool part of the hotend.

If your nozzle is too low, you can get clogging.

If you're over-extruding it can cause clogging.... 1.68mm filament is unusual. 1.75mm is normal. Are you sure it is 1.68? Try telling your slicer it is 1.75. Also check the calibration of your extruder. If you tell it to extrude 100mm, how much does it actually extrude?

Spool says ABS 1.75mm, but actually measures 1.68mm (micrometer ). As you suggested, I tested the gear extruding 100mm sample (without hotend and nozzle hooked up). This turned into 103.38mm.

I am testing the pressure against the filament. I have noticed I can easily pull the filament and the gear start spinning.