Since my last post, there's been huge advancements in building a reliable extruder. After reading up on articles regarding it, I decided to scrap the current design and start anew for several reasons:
1. Long Heat Zone: Dimensions of the welding tip forces at least an inch-long heat zone (likely longer because the tip is connected to an aluminum rod). This will probably cause blockage.
2. Tapping Issues: As it turns out, you need to drill an inch in order to tap 1/4".
3. Cooling Issues: Although there are some successful PTFE-less extruders out there, relying on passively-cooled aluminum fins sounds unreliable, unless they were force cooled (which we want to avoid to keep the system simple).
Here are some design decisions chosen by comparing what's out there:
1. Power Resistor as Heating Element
What's Out There: Insulated nichrome wire (Makerbot Extruder MK4), power resistor(Makerbot Extruder MK5, Nophead's extruder)
Why Power Resistor?:
- Easier to Obtain: We've looked everywhere. There are plenty of uninsulated nichrome out there, but it's pretty much impossible to find one that's insulated (at least here in Canada).
- More Reliable: When I was assembling the Makerbot extruder hot end MK4, I spent some time trying to cut the wire to the right length as multimeters tend to act finicky when measuring resistance of a thin wire. On the other hand, all you need to do with resistors is just pick one up with the right value.
- Better Maintainability: Wrapping the nichrome wire was tedious enough. The whole "unwrap the old wire, cut a new one to the right resistance, and wrap it" sounds even more tedious, especially compared to "pull the old out and put the new one in."
MK5 heats up slowly, as its thermal footprint is larger, and power resistors can't wrap around the heater barrel like nichrome wires can. However, MK5 heater block is made out of stainless steel, while we're planning to build ours with aluminum. Aluminum has a thermal conductivity that's 15 times greater than stainless steel, meaning it'll heat up fast. Yes, it also means it'll lose heat fast too, but we're hoping to nullify that issue with thorough insulation.
2. Thermocouple as Temperature Sensor
What's Out There: Thermistor(Most of DIY 3DPs out there), thermocouple (RepRap Themocouple Sensor 1.0)
Less (or none at all?) Calibration Required: Sure, thermistor's easier to get, has a faster response rate, and doesn't require an interface IC, but it needs to be calibrated before it can properly measure temperature. To calibrate, you need... a thermocouple.
We don't know how a slow response time, and gradual drift of the measurement would affect reliability, but for now, calibration-free aspect of using thermocouple sounds too attractive to ignore (Provided we can obtain thermocouple wire, that is.).
3. Bowden Extruder
What's out there: Vanilla, everything-in-one-place extruder (Most of DIY 3DPs out there), Bowden Extruder (Erik's Bowden Extruder)
Why Bowden Extruder?:
- Larger Build Envelope: Our XY Stage carries the extruder, not the build stage (to maximize the build volume). Therefore, volume of the extruder is a major factor in determining the build volume. Incorporating the Bowden extruder drastically reduces the volume of the extruder (attached to the XY stage, that is).
- Less Intertia: Letting the XY stage carry less mass should bring up the speed at which we can print without stppers skipping steps.
- Relaxed Size Constraint for the Extruder Driver: Before this design revision, we were planning to use the worm gear driver for its footprint; however, as nophead pointed out in his blog post, it's not very efficient. Now that the extruder driver is out of the XY carriage, we can consider using other driving mechanisms.
However, use of Bowden Extruder introduces some feed delay between the hot end and the feed driver; consequently, NEMA 17 (Spur) geared extruder is used for more precise control.
Other design decisions include:
1. Aluminum / Stainless Steel Hot End Assembly: Since stainless steel has a lower thermal conductivity, it's used for the heater barrel. We bought a 3/8 threaded rod from Home Depot. Heater block and nozzle are still made from aluminum to ensure high thermal conductivity.
2. Custom Built Nozzle: For our new design, using a welding tip just doesn't cut it. Since we've found a good source for 0.5mm drills in Toronto, we decided to make a new nozzle. If there's one thing we're not short of, it's the machine shop access.
3. Bowden Tube Right to the Heat Zone: Since we're ordering PTFE tube for the Bowden extruder, we've decided to double its use as an insulation by putting the tube right above the heater core. This will help shortening the heat zone.
4. PTFE / Kapton Insulation of the Heater Block: Since we have a large amount of leftover PTFE sheet after making the XY stage slider, we've decided to use them to insulate the heater block. It also shields the nozzle from extruded ABS (We've had our share of problems with that).
Yep, our extruder is going to look a lot different (and hopefully more reliable). I'll share the new CAD files of the extruder next time (for real this time!).