Extruder

A 3D printer consists of 2 main parts: an extruder which produces a very small flow of molten plastic, and a platform which is accurately moved in 3 directions: left-right (x), forward-backward (y) and up-down (z). In future blog entries I will try to address some specifics of each of these parts of the 3D printer, to start with the extruder in this post.

The extruder is the makerbot extruder MK4. This extruder comes as a kit with a motor pulling the filament in and pushing it into metal tube. Around the tube is a heating element which has a temperature sensor. The kit does not include the electronics. The included assembly is made of laser cut acrylic.
The extruder in general is excellent, it has been running for dozens of hours and it has never gotten stuck or given any serious problems. There are 2 things which are worth noticing though:

• The motor is a dc motor with a fixed gear box. This works well when running at maximum speed. Since the motor is loaded very heavily it does not work very well at lower speeds. Sometimes there are situations in which I would like to slow down the flow of plastic, but I can not slow down the motor enough without risking it does not start properly. To prevent molten plastic to continue to flow though the extruder also after the motor has stopped, the software actually reverses the direction from the motor for a while. This works, but with a gearbox with a big ratio there will always be considerable play and reversing does not work very well. Newer generation of extruders in the reprap project are all designed to have a stepper motor to drive the filament, giving finer control of the flow of plastic which results in significantly better quality prints. I do plan to upgrade the extruder in the future with a stepper motor.

• Filament is pulled in between a gear on the motor and an acrylic idler wheel. The idler wheel has to push really hard to make sure that the filament does not slip resulting in failure of a printing job. The idler wheel in the makerbot extruder has an M8 bolt and nut to prevent it from moving, but this is a weak part of the design. The bolt needs to be tightened really hard to make sure it does not slip, but since it is clamping acrylic sheets there is a serious risk that it is clamped too hard and the whole extruder is ruined. Tightening it too little causes the filament to slip so some solution had to be found. Since the 3D repstrap printer is built partly of aluminium profiles (10x10 mm square mostly), it was easy to add 2 bolts to the vertical profiles which prevent the M8 bolt from moving sideways. See picture, click to enlarge. An M3 bolt is going through a tapped hole in the vertical aluminium profile on the right; a second M3 bolt is present at the back of the extruder. Since fitting these additional bolts there has been no slippage and the extruder has worked fine.
  

I have read several reports of people ruining their extruders because of a software or control issues which caused the Z-axis to go too low. A strong motor in the Z-axis can ruin the extruder when it crashes into the base. In this printer the X-carriage is held down only by gravity. This has the advantage that in case the Z-axis is lowered too much, the extruder will be lifted of the rails and has a better chance of surviving. More in a future post about the x-axis.

Another example of an object printed recently on this printer is the dodecahedron as found on thingiverse: