There was an immense amount of work that went into getting to this point, and I can only stake claim to a tiny portion of it. It was really cool to see our Makerbot throwing down the plastic… on the 20-30th try.
There were two main wrinkles. One we fixed, the other we lived with.
- Getting the plastic to stick to the foam-core was hit or miss. Covering the foam-core with double-sided tape created a much better surface. We printed out 4 sets of tweezers without replacing the tape!
- The issue we lived with was the extruder. It’s not quite gripping the welding rod correctly yet, so we had to apply downward pressure manually. not that big of a deal for a 9 minute print, but it needs to be fixed before we print 4 shot-glasses on the same raft. Man, if only most of us were members of a kickass hackerspace with access to machine tools. Oh wait… we are. Cool!
The other area just begging for improvement is the temperature control. Because the makerbot is using simple on-off control, we’re getting temperature swings of +/- 10 °C. The plan is to put in some data logging to see how bad the problem is, then implement progressively more complicated control strategies until we’re happy with it.
Tags: Makerbot
Brett,
I am a project manager in the automation industry and have a fair bit of experience with various methods of heating/melting of plastics. I do not know what what heater-nozzle on your Makerbot looks like, however my experience (some good . . . some not so good) has taught me that many systems designers try to control heat by increasing the complexity of the controllers, using multiple thermocouples placed very near the working surface and implementing complex multiplex PID software . . . usually without success.
FWIW: Compared to electricity or hydraulic and pneumatic pressure; heat has a relatively slow moving influence within a system.
That is, heat takes time to soak in; when the heating element comes on, it usually does not reach temperature instantaneously and therefore the “working surface of heating system†. . . I believe a nozzle in this case needs some time to come up to temperature.
Conversely, heat takes time to drain off or cool. You may have noticed this when the system “overshoots†your target temperature, it can take quite a while to cool down . . . especially if you stop extruding and wait until you reach the target temperature again before restarting the system. Unfortunately if your system is too hot and you wait for it to cool down, then the long residency time of the plastic in the barrel that is at heat, will usually degrade the material and cause a loss of desired physical characteristics even if it does not burn or discolour (Canadian spelling).
Now this slow to heat, slow to cool characteristic can sometimes be used to your advantage, by increasing (where possible) the mass of the heated tool . . . a nozzle in this case. I understand that your Makerbot system will have physical limitations as to the amount of mass it can move or the physical size that can be accommodated . . . however in this case, mass is your friend.
All things being equal, a heater tool/nozzle with a larger mass will take longer to bring up to working temperature and it will take longer to cool down when you shut the system off. However, it will also not be drawn down (cooled) to the same degree by running your plastic rod through it. You will see fewer temperature fluctuations because the larger mass holds more heat and is less affected by the influence of heating the plastic rod, air drafts or parasitic loss to its mounting hardware.
A final word to the wise, if possible locate your thermocouple midpoint between your driven heating element and the working surface of your tool/nozzle. In this way, you will have a better indicator of the actual working temperature with less influence of surface temperature draw down.
Please contact me if you would like some more information.
Best Regards,
Christopher (Captain Balsa) Singleton
csingleton@meikleautomation.com
WOW Christopher! thank you for all that input. I’m a Controls Engineer by trade, but I don’t have a lot of experience (any really) with plastics. We’re definitely in agreement when it comes to controls complexity. Often it hurts as much as it helps.
As far as our makerbot, it looks like I won’t need to change anything for the time-being. After machining a new idler pulley for the extruder, everything pretty much came together. The on-off (30%-0%) control strategy appears to be sufficient.
For the record, the order of increasing complexity I was thinking of was:
1: Shrink the on-off window from 30% to ~10%, and bias with a linear feed-forward based on feed speed. So more plastic being fed, more heat added, with the on-off trimming.
2: same as above, but using the PID Library instead of on-off.
3: praying to the controls gods. (very complex. lots of dancing, sacrificing of goats, yelling. who wants to go through all that?)
Thanks Again, and I’ll be sure to post any strategy changes we make in the future!
Brett