Setting up the Extruder
From Wiki
Copied from Tony's blogpost at http://rapmanv3.blogspot.com/2009/05/extruder.html and edited a little bit.
There are not too many tips for building a successful extruder, if you build it to the instructions then you should not have too many problems. However it is worth a few notes and observations on the subject.
The hot end is fundamental to the whole machine. The main factors are getting the heat into the plastic as close to the nozzle as possible and keeping the hot zone as small as possible. The design ensures these criteria are met by using a compact nozzle and PEEK/PTFE combo insulator. The heater element can be wound at 4 to 8 Ohms, anything in this region will work. Lower than 4 Ohms and it wont work, 5 Ohms is a save value.
Take care with the heater and thermistor wires to ensure they do not touch and that they remain insulated from the support structure. Confirm the resistance of both devices when the extruder is fitted to the machine for the first time.
The Heater/PTFE interface
(This whole section needs editing for the new extruder)
The back end of the heater barrel has to be cut off and set on the PTFE just clear of the main heater body, this is required to introduce a thermal break. This prevents the full heat being transmitted to a long section of the PTFE. The result of not doing this would be very difficult starting, the plastic goes soft a long way up the PTFE. This will work the first time but after it has cooled the extended melt zone takes a long time to soften again. If you run the extruder like this it will most likely cause a mechanical failure.
Even with the backup ring, the plastic melts in the end of the PTFE, how far it extends depends on the temperature you are running at but the zone certainly extends to the end of the section inserted in the heater and on for a few mm above the heater. The plastic swells a little as it approaches melting point but as we are putting a high load on the material, it also deforms to the point it fills the bore of the PTFE. As soon as the filament rubs on the side of the PTFE the extrusion force goes up. A long transition zone will need a high extrusion force to keep it moving. Drilling of the PTFE enables the end of the filament to expand to a larger diameter, this exposes a lot more surface area of plastic to the top of the heater barrel improving heat transfer to the filament at the critical point. I also run a 5mm drill into the end of the heater barrel, just the conical tip, this gives even more area for heat transfer and helps funnel the material into the heater.
In all of the above I assume that your control system maintains the temperature and speed correctly. Again, stating the obvious, you are wasting your time if you try to run the motor at low temperature, 10 or 20 deg under temp the plastic is so viscous the extruder will slip or break.
The Version 3 machine takes things a “step” further. The new design uses a stepper motor drive, this gives a very stable speed and has plenty of power for the job. The speed range is has been greatly improved also the stability of the speed has given us another significant quality improvement. The motor is certainly powerful enough to cause damage if things are not set correctly. Ian is just finishing off some changes to the hot end that should enable the full power of the motor to be utilised. This will help pulling material from the drum and increase the safe temperature range for the hot end.
I would encourage anybody who has the standard PTFE (All of you at the moment!) to build the original design to start with. If you do this and take care with your settings it will last indefinitely. I have used the version that ships with the kit for many hours of printing and I not had a failure of the PTFE or any other part of the hot end. See the attached picture, the only mod is use of a 10mm olive as my backup ring, the PTFE has been in there for weeks.
If you are having problems with the PTFE the following may give a few clues of what to tweak. The main modes of failure for the PTFE as seen during destruction testing.
Extrusion to hot The problem here is the PTFE looses its mechanical properties around 240Deg. The insulator either balloons out above the heater or deforms and gets pushed out of the heater barrel.
Extrusion too cold. Here the plastic viscosity goes through the roof and the pressures required to extrude it are very high, this combined with the temperature, either cause the PTFE to get blown out of the heater or more likely, the heater assembly is damaged in some way.
Extrusion too fast. The higher the flow through the nozzle the more pressure is required to force it through, failure mode could be either of the above.
To avoid damage, get your temperatures right before you start the motor.
Assessment of speed and temperature settings
Its worth mentioning at this point that any temperatures you find stated on the forum or in the wiki will always be a ball park figure. You can find somewhere that ABS extrudes at 235DegC, you may try this and damage the extruder.
The problem here is twofold, the calibration of your machine and the material. Hopefully calibration errors will be small as we will both be using the same design heater with the same thermistor in the same position, even so + or – 1 or 2 DegC is possible. The material is a greater issue. There is ABS that will extrude comfortably at a setting of 227 to 230DegC. There is also ABS that needs to run at 245to 247DegC. Run the Higher temp stuff at 230 and you would almost certainly break the extruder. If you run the lower temp material at 247DegC I get a very messy print.
You will need to establish the correct setting for the material you are going to use, take any settings you see on the web and run your own test just to be sure. This applies to new coils of the same material, different colours or different source for the same product.
You will find a tool set up option on the main menu, this should be used for initial assessment of any new material you use in the machine.
- Slacken off the filament tension clamp all the way. If the motor starts its not going to cause a problem. The last thing you want is to drive the material through if the temp is low.
- Load the material.
- Set the target temperature:
The screen will be showing the heater status, the set temperature (0), in the middle, the ambient temperature reported by the thermistor, and on the right, the extruder rpm set at 1rpm. Note: the ambient reading of the thermistor will be unstable, this temperature is almost off the bottom of the scale for the device. Readings stabilise when >100DegC. Max for the thermistor is 300DegC If you have a ball park figure that others are using you can set that temperature and let it warm up.
- Push the material through by hand:
If you can get material flowing from the nozzle refit the filament clamp and tighten the screws. If its impossible to get a flow by hand feeding, increase your temperature in 5 degC steps until you can.
- Adjust temperature speed and spring tension to achieve a reliable extruded filament at the print settings you intend to use.
For hand feeding the extruder, I use a pair of small nose pliers to grip and push the filament as it enters the top of the extruder.
If you do not have any idea of the setting, take the filament clamp off and feed the material through by hand, increase the temperature in steps until you can push the filament through. Refit the clamp and tighten up the clamp springs half way. Ramp up the motor to 10 or 15rpm. See if it extrudes. Continue to adjust the settings to how you intend to print. Turn the machine off and let it cool down.
Turn on again and immediately enter the print settings. Observe the extruder come up to temperature and start the motor. Does it work? If it starts again reliably then you are ready to try a print. Continue to Printing
