I’m half trough with modifications on my rapman’s hot end so I decided to share some of the experience with the rest of you.
First – why?
Shorter the transition zone from the solid filament to the molten plastic the better. Better means
- Higher precision (you can better control the flow)
- Less stringing (less ooze + better control)
- Smoother run / consistent flow
Now, the “smoother run” is the major problem you have to solve as it quickly go from “running not-smooth” to “not running at all”. What actually happens if the transition zone is too long is that between liquid top at the one side of the transition zone and hard part on the other side your filament is “soft”. By being soft when you push it in the heat barrel the filament compresses in length but increases in diameter pushing against the wall of the tube it is traveling trough. The “smoothness of the flow” depends solely on
- How smooth / slippery the tube walls are
- How much pressure the filament is putting on the wall itself
The first issue we usually solve by using the slipperiest material that can withstand temperatures up to 260C – PTFE (a.k.a TEFLON ). The tube must be properly machined so that inside walls are smooth. Drilling it with a standard drill reduces the slipperiness a great deal so try to get your tube properly machined from day 1. Big problem with PTFE is that at temperatures we run (up to 250C) it gets “soft” and cannot fully hold shape (this really depends on the actual mixture but the harder PTFE you have the less slippery it is) so some reinforcement is required. The smart people in BFB solved this by encasing the PTFE tube inside a PEEK tube, another material that can withstand temperatures we work on but without loosing structural integrity. This combination has proven to work very good and now I see other hot end manufacturers started to use it too. This is actually where we come to our first problem with BFB hot end. The PTFE tube does not look as it is made from quality machined PTFE but looks more like a “next best thing” – extruded silicone tube. It can also withstand high temperatures and is also slippery, only it is not as slippery as PTFE. Replacing this tube by properly machined hard PTFE tube is the first step in improving your hot end. At the moment when I write this post, the only supplier of machined PTFE tube replacements that I know of is László KREKÁCS. His website/blog/webshop are not working yet so just send him email ( order[AT]arcol.hu ) and check details directly, he is very nice person to talk to I can recommend him without hesitations. You can also find a local cnc shop and make your PTFE tube replacement machined locally, and you might want to check with the BFB if they start to sell upgrades to the existing hot end (or new improved version of one) but attm bfb only offer stock hot end.
Very important to the slipperiness problem is that you have no “gaps” in the flow of filament. This is often assumed to be so, but in reality the PTFE/silicone tube touches very hot (250+C) aluminium and it can deform in the joining point because of the heat. If a gap is formed between aluminum and tube the filament will expand into that gap and the force needed to extrude it would become more then 100 times greater then if there was no gap. There are also some other deformations in this point that can have nasty effects, breaking the seal can lead to all sort of nasty problems, like the last one Forrest had.
Since going “more slippery” then PTFE is hard, the next thing to do is to shorten the transition zone as much as we can. The only thing to do it (other than using other filament) is to
- prevent heating of the filament except on the very end where we want it to melt
- dispose of the heat propagated trough filament itself
The first point sounds simple – we don’t heat it anyway, well, it is not that simple, all our PTFE, PEEK etc are linked to the very hot aluminium heated barrel and they are pressed to it strongly so there is a lot of heat transfered from the aluminium nozzle to the PTFE and PEEK tubes. This tubes are also heated by the filament itself (second point) so in order to solve both problems BFB shortened the PEEK tube and continued PTFE reinforcement with aluminium tube. This tube now serves as a heat sink to cool the PTFE tube and by that to also get rid of the heat propagated up the hot end from hot barrel trough PEEK/PTFE tubes and trough filament itself.
Doing some measurements without filament in the chamber stock bfb hot end measures under 40C on both PEEK and aluminium tubes while tip is heated to 240C, but with the filament inside, the aluminium tube reaches up to 70C leading to conclusion that filament in that region is way over 90C so if you are using PLA the filament is already starting to “swell” at the top of hot end and your transition zone is way longer then it should be (the transition zone should never be higher then end of PEEK tube). Adding a fan to blow at the hot end improves cooling of the aluminium tube so helps a lot, but using fan provided by bfb I was only able to drop this temperature by another 10C. Now, this is all while we had “stationary” piece of filament in the hot end, when you start extruding, you are bringing cold new filament in and the heat propagation is slowed down by the speed new filament comes in. This is why, with default / stock bfb hot end when extruder drive rpm is over 30RPM you are able to print properly as with the fan cooling the aluminium tube and the “fresh/cold” filament coming fast you keep the transition zone short. Problem start to exponentially increase as you slow down the exuruder RPM so at 10RPM it is almost impossible to push filament as it is soft all trough the hot end and with 5rpm you can even see filament swelling outside the hot end which completely blocks it (as it gets stuck at entrance to the hot end). Since I need low RPM extrusion to print high precision objects (0.15mm layer height) printing more then 20 minutes is almost impossible with low melting point filaments like PLA. This is why I started to modify one of my hot ends (have bunch of them, different nozzle sizes …).
Let’s start from the tip – first mod I made to my nozzle is to get a blank from bfb and drill with 0.25mm drill. This does not at all sounds as simple as it sounds. I spoke with 5 different cnc shops and with over 20 lathe owners and every single one of them told me that “they cannot help me as drilling 1mm of aluminium with 0.25mm drill is incredibly hard”. I finally did it myself by hands! Moving up, you can see the big “bulk” on my hot end, this is an insulation ring made out of silicone and some baking paper, it allows even poorly powered heater to retain heat at the tip. Above it you can see some copper wire, this wire actually goes around the whole nozzle connecting the steel triangle with aluminium heat barren with 3 hexagonal rods. This wire is then connected to the earthing wire that earths the nozzle preventing buildup of large amounts of static electricity by plastic flowing trough aluminium nozzle. This reduced incidents with electronics to only a firmware problems (no more resets from static electricity). The latest modification is the added heat sink to the aluminium tube. The tube itself has a very low surface so it is not very effective heat sink. As I have some left over thermal adhesive that I purchased to glue heat sinks to my allegro stepper drivers (think pololu) I bended the aluminium heat sink around 10mm steal pipe to form it properly and then I glued it to the aluminum tube. I’d add a same thing to the peek but unfortunately the thermal glue I have don’t adhere to PEEK :(. There is one modification in this region that is not visible on the picture and that is thermal grease I applied between PTFE tube and the aluminium/peek tubes to increase thermal transfer from PTFE tube to get the heat faster out. The next modification I’m thinking about is to make the aluminium tube longer by 1cm and to shorten the PEEK tube by 1cm. On the picture you see original MDF insulation plate and then original plexy insulation plate. I future I want both of these to be cut in aluminium because the original idea of the MDF ring imo is that bfb guys wanted to prevent heat to move up and damage the acrylic mount plate and acrylic x-carriage. Since I already replaced original acrylic mount plate and acrylic x-carriage with aluminium parts having this insulation is actually contra productive as the huge chunks of aluminium (mount plate + x-carriage) are actually a huge heat sinks and puny heater could not keep them hot even if it was heating only them, so changing these insulation pads to aluminium (and coating them with thin film of thermal paste) will additionally improve heat disipation from the top of the hot end further improving the flow by further reducing the transition zone.
Now, as I work with limited tools that I have at my disposal there is a guy who invested a lot of time (and money) into designing a new hot end from scratch. I noticed he already incorporated many of the things I mentioned in this article and after talking with him I found out that some more improvements (not related to heat but to the flow, should increase quality a lot) that are not that obvious are implemented. I’m going to visit him in few weeks to get some of the prototypes (and of course a final 3.0) and test them myself. But before I gave his new hot end a try, I will continue to improve on the one I already have.