Scratch Build of PEEK and PEI (Ultem) Capable 3D printer
When moving across the country to Colorado for Grad School I knew I would be moving away from the CNSI labs I had helped manage for the past four years. This meant leaving behind my projects, the printers, the mill, the lathe I helped install, and most importantly my beloved HAAS CNC mill. Knowing I couldn't immediately buy a mill and lathe of my own (now acquired! stay tuned) I settled on the next best prototyping tool: a 3D printer. But I couldn't just build a 3D printer of course, it had to have some sort of angle to it. Speed printers, tool changers and filament swapping systems all crossed my radar, until I was looking through some Stratasys patent's and re-discovered their Fortus line of printers. Capable of printing high temp "super polymers" materials with nozzle temperatures in excess of 500 degrees? Count me in. I was going to have Stratasys at home: and infringe every single patent of theirs I could in the mean time. Let's get started.
The Nozzle(s)
|
The main hotend seen on right is a water-cooled beast of a hotend with a PT-1000 thermistor and 100 W cartridge heater. It's fully equipped with an E3D Nozzle X as we are getting dangerously close to the temperature at which high carbon steel loses its temper. The main reason I chose water-cooling for this hotend was process reliability. When the heat block is at 500 C, the chamber is at 120 C, and you're trying to troubleshoot a jam while a half finished part warps off the build plate and the hair burns off your arm you'll thank me for the water cooling. Due to the design of this printer the stepper motors driving the XY gantry as well as the main extruder are also inside the heated chamber and all require active cooling below printer chamber temp. Some NASA group and a couple high end printers such as the original HYLO achieve this active cooling by circulating chilled compressed air around the motors, but as I don't want an air compressor running 24/7 and don't have house air in my house, water-cooling was the move. On a side note, the newer Hylo uses a sick heat pipe system to move heat from the hotend into a cooler isolated section above the printhead which is a super sick idea. They should pull a Stratasys and patent it so I can infringe upon it in my next build.
|
|
The second hotend is... pretty boring by comparison. Except for the fact that it's mounted to a linear rail giving it 6mm of vertical travel so it can raise itself above the main print head while inactive, and lower itself below the main print head when printing. The main use of this hotend is to extrude soluble support material which can be dissolved away later to create complex internal geometry with perfect overhangs. Currently the main two soluble materials I print with are high impact polystyrene (HIPS) for use with ABS and AquaSys180 when printing with PEEK and PEI.
|
The Extruders
Personally I don't see much special about extruders. Bondtech has some large drive wheel extruders that work well, Prusa and the single drive Nextruder is pretty interesting, and of course I love the idea of a rotary extruder patented by FuseLabs especially because I could then infringe upon that as well. However after looking into cooling a through shaft BLDC motor and encoder I settled upon a standard pancake stepper driven setup using a Sherpa Micro for the main extruder, and a VORON M4 extruder for the support extruder connected using a Bowden tube. As I am not worried about tuning parameters such as pressure advance to get *beautiful* support material and only care about printing stiff support materials like HIPS and Aquasys180 a Bowden setup for the support material made sense and reduced complexity and weight on the printhead. Since I seem to be in the mood for shouting out cool designed, the Zortrax M300 dual has a sick design which uses a single stepper and a dual gear train on a lever arm to drive both extruders one at a time without the need for additional steppers or servos. If I was doing a scratch build I would definitely copy that design.
On a side note, I ended up having to tear apart a water block and found some very interesting galvanic corrosion occurring. Being a cheap bastard I purchased all aluminum water-cooling blocks and fittings but used stainless steel compression barb fittings because that's what I had on hand. As this loop doesn't have any thin fins or jet plates like a standard heat exchanger I wasn't too worried about clogging up my loop. The aluminum was also anodized and raw aluminum forms a rather robust oxide layer which helps prevent galvanic corrosion, however I believe some oxide within the threaded section likely got abraded when I tightened the fittings which likely helped accelerate the galvanic corrosion. All water blocks are also exposed to 120C ambient temperature which also accelerates the corrosion. Very cool to see this in real life, but I'm not concerned about this on a personal project printer which gets maintenance whenever I get bored and turn my kitchen into a printer surgery room.
The Printhead
|
Heating and Cooling (why waste energy doing just one?)
As I enjoy heating my house during the 100 degree (Fahrenheit) Colorado summer I threw a 1000W heating element on the top of this printer to heat the chamber to a nice and warm 120 degrees (Celsius). Two massive blower fans I stole from a server push air through the heating element and disperse it through the rest of the machine. These are high end Delta ball bearing fans but I don't expect them to last longer than a year or so in this temperature environment. The original design had these fans within the chamber but exposing the back of the stator to ambient air seemed like a better idea. This chamber temperature is incredibly important to the operation of this printer with high temp polymers such as PEI and PEEK. PEEK can be printed in an amorphous form at these temperatures without warping (if you nail the process parameters) and later annealed to form the semi-crystalline part. I prefer to print PEEK like this for two main reasons. First, the main difficulty of any super polymer is preventing it from warping of the build plate as the top layers of plastic cool and contract, tending to pull the edges of the print right off the build platform or even deforming the build plate material. With semi-crystalline materials you get the double whammy of crystallization occurring which induces shrinking as the uniform crystal structure is more dense than the amorphous polymer which can increase the chances of warping. The second reason is PEEK annealing temp is like 200 degrees C which is a bit warm for electronics and basically anything thats not Invar.
|
|
|
Cooling comes into play in only two areas in this printer: cooling the motors and hotends, and cooling the recently extruded plastic. As mentioned earlier cooling of the motors and hotend is done using water-cooling and 4 aluminum water blocks. The radiator is a 240 mm with a medium fin pitch as in reality we are not dissipating too much heat with the water cooling loop. Water temp never gets above 32 degrees C. The only change I've really made in the loop is switching out the cheap soft PVC tubing for high quality silicone tubing to reduce the permeability of the tubes within the chamber. With the printer running constantly I would lose about 3 millimeters of water per day. As a side note, I reversed two of the IEC connectors I'm using for pump and fan power, and was pulling 2.5 amps of 12 V for the pump off of a fan header on the controller board, and more impressively powering 12V fans with 24V for about a month. Very impressive Noctua....
|
|
Cooling the recently extruded plastic as it turns out is a much more difficult issue than cooling the motors and hotend, and yes I did think of water cooing it (just kidding). The fan duct seen here is printed out of FormLabs High Temp resin and is using some 5 mm aluminum standoffs to help support the arms. I tried printing this carbon fiber nylon, cf polycarbonate, and cf PPS but they all melted and warped when printing PEEK and PEI. The high temp resin seems to be holding up fairly well over time by comparison although the new hotend sock is definitely helping as well. 500 C is way too hot for silicone hotend socks so I made my own using raw fiberglass I have for a vacuum fiberglass bagging project. The woven fiberglass is held together with Kapton tape and wrapped around the hotend. Some nichrome wire is then wound around to tighten the hotend sock around the heat block. I used SolidWorks flow simulation and the kitchen sink (literally) to fine tune the duct because my ANSYS license is expired and the main addition I made was adding a vain down the center of each arm of the duct to prevent most of the air/water from sticking primarily to the outer edge of the arms. I'm using a 2 amp 40 mm fan that's running primarily on hopes and dreams because its exposed constantly to the 120 C chamber temperature.
|
|
A part cooling fan is not typically used for high temperature polymers but I have found it useful as long as you have a high enough chamber temperature. The goal is to cool the polymer from extrusion temp to just below the glass transition temperature as quick as possible, and a fan significantly speeds up this process as long as your chamber temp is somewhat close to Tg. I'm well aware that the Tg of PEI is 217 C, but for PEEK, PPS, Nylon, and the rest of the semi-crystalline materials significantly better overhangs can be achieved by using a part cooling fan.