Lessons Learned: HotBox Project
Fayette County Additive was born out of the various successes and failures, of the Ender-5 HotBox Project.
Based on the fundamental idea that there is nothing innately difficult about using the super-hi-temp industrial and engineering polymers commonly processed on large multi-thousand dollar machines, especially when using a common build p[ate size like 235×235. Very few mechanical limitiations stand in the way which can be easily tackled on a point-by-point basis.
This is a living document. Check back later
Hot End Choices-Part One
Hobby Standard
The inital modification needed to be an all-metal hotend so the machine could handle temperatures required to make parts for itself that can sustain in a hot enviroment out of ABS plastic, since the initial Mk8 Hotend on the Ender Series machines cannot safely heat past 240 degrees on the nozzle. The initial, and seemingly obvious, choice was to use a MicroSwiss All Metal Mk8 Hotend however this was poorly suited for true high temperature printing for a variety of reasons, including the inability to make use of the sprawling and open-source V6 ecosystem. This hotend was able to effectively print ABS parts following enclosing of the machine, but from time to time struggled with PLA heat creep. I attribute this to the poor performance of Creality Stock fans and not with a flaw in the Micro Swiss hardware.
Once the limitations of the Mk8 Hotend design were made clear, including lack of water-cooling options, poor selection of heatblock materials, need too use bead-type thermistors as opposed to cartridge-type thermistors, and lack of the access to the V6 ecosytem, a V6-type hotend was outfitted to the machine. All hotend related issues were completely resolved and a standard-configuration V6 proved to be an extremely reliable hotend up until the 285 degree benchmark using stock Creality Fans, which is limited by the thermistor and heatblock. (Note: There is conflicting information as to the maximum safe temperature for the components of the V6 Hotend, even from E3D themselves. I recommend avoiding temperatures above 285c.
It is important to note, both the V6 and Mk8 were configured in a bowden setup, as to avoid the need to cool another moving object (Direct-Drive stepper).
Enclosure Considerations
Ambient drafts shock hot plastic. All printers should be enclosed, if only for the benefit of a consistently controlled enviroment improving reliability.
An enclosure is also imperative for keeping heat in, which is a must for the true high temperature materials were going for here. Issue arises, when a quality enclosure can be the most expensive change you make to your printer, one of the guiding principles of the concept was being low-cost.
The options which were considered are, in order of cost…….
A.) "Tent" style commercial enclosure
B.) Acrylic-Panel Enclosure
C.) Lexan-Panel Enclosure
The tent enclosure offers little true insulative capability but is certaily the most cost effective with both the Acrylic and the Lexan costing almost as much as the printer.
This is unfortunate because Lexan is the clear winner here with very high heat resistance of well over 100c, which would make it cooperate well with a chamber heater, easily winning out over Acrylic which is only temperature stable to 70c. This temperature rating ties with the Sainsmart Tent enclosure rating.
While the insulative capabilites of this solution are lacking, using a second "enclosure" around the printed part in the form of a draft shield allowed for warp-free and dimensionally accurate in all materials with the air inside of the "second" enclosure maintaining appreciably higher temperatures than the air only inside of the tent.
Coming to Terms with Glass
Replacing the heatbed is the first instance in which one modifaction canceled out or interfered with another.
The stock 220w Creality PCB Bed heater will only effectively heat to about 115c before you have serious issues with diminshing returns, with the aformentioned 115c. This should be replaced with a much more effective silicone pad heater, which gives you better heating per watt. Note that most silicone mat heater need to be secured with 3M Brand 468MP Adhesive which has a longterm use temperature of 149c. Using a product continuously at its failure point is a surefire way to shorten its service life, so a reasonable soft cap of 140c is recommended. A custom metal plate is placed below the hotbed insulation and above the springs to act as a failsafe against adhesive failure.
Pushing the bed to 150c is fairly straightforward, and possible on a direct current bed, however it comes with the inabilty to use magnetic beds due to delamination and adhesive failure. For printing on extremely hot printbeds, glass is the most effective option. Even the highest temperature rated magnetic mats avaliable tend to lose their magnetiscm at temperatures approaching 130c. It is possible to affix a ULTEM sheet to the bed of the priner mechanically with clips, if that is the route you would prefer.
I feel it would be a disservice to talk about the bed heater without mentioning the impact proper insulation can have on your heat-up times. It was impressive. Using the stock PCB heater, insulating the underside of the heatbed with approximately 3mm of cork decreased heat-up time by more than 50% (11:57 vs 5:35) on a PID tuned bed. I recommend Silicone as an insulator instead of cork since, while slightly more expensive, it does not burn and is therefore safer in the event of an electrical fault.
New Firmware, Same Board
In order to advance past Creality's stock maximum of 260 degrees on the hotend, Marlin firmware needed to be modified. Once you obtain and install the necessary components to safely allow your printer to reach higher temperatures than a "normal" all metal hotend (more on that later) you can safely increase your firmwre max temperature to 500 on the hotend and 160 on the bed. After you have installed the necessary components. Not before.
Honestly, the first time flashing firmware was the hardest part of the entire project thus far and I don't wish it on my worst enemy. That's why the V6-I hotend comes with firmware for Ender-3 series 3D printers.
High-temperature thermistors were elected to be used instead of thermocouples, so it is possible to run on a stock motherboard to minimize cost.
High-temperature thermistors are not like normal thermistors in that they struggle with room temperature and are horribly inaccurate until it gets to approximately 100c. The firmware must be modified to not interpret these expected incorrect readings as a thermal runaway. It is imperative that you -DO NOT DISABLE THERMAL RUNAWAY PROTECTION- and instead make the changes required by your specific thermistor. Contact the supplier for the firmware changes that you need to make when compiling since not all are created equal and some require inputting completely new thermistor tables.
Hot End Choices: Part Two
Really Turning up the Heat
At the end of the capability of the stock configuration the aluminum heatblock and the thermistor must be replaced. Intuitively, choosing a robust high-temp thermistor can read up to 500c, covering all printable thermoplastics. In changing the heatblock, your options include either copper or brass. The best choice is brass. Brass remains much stronger than copper at elevated temperatures, not considering how more beautiful machined brass looks. A common objection to this is that copper is more thermally conductive than brass, however this is not appreciably useful since the heating element is very very close to the filament path and is located closer to the filament than the thermistor is.
Concern about added weight is misplaced since, while less mass on the printhead is better, this is a bowden setup and is still much lighter than a direct drive set-up and did not experience issues with flexibles during testing.
Did it Work?
Yep
Following the changes made in the record above, the machine ws able to accurately print appropriate parts for its build volume in every material that we threw at it.
The biggest takeaways, in my opinion, was the shift in belief as to how important a 3rd heat source in the enclosure is. While an appropriate chamber heater is no doubt important for large parts, the need for additional active heating was not readily observed for reasonably sized parts, and the resistance of a standard V6 hotend to heatcreep even in a hot build enviroment is impressive as the machine presented with electronics-related heat issues (XYZ shifts due to overheating of the stepper drivers on the board) well before cooling of the cold-side of the hotend failed.