The extruder is the heart and soul of your 3D printer and as such, close attention should be paid to its design, serviceability and ease of cleaning. Other important features to consider is what drive system does it use? Does it use a stepper motor or a dc motor to feed the filament?
Finally you need to take into consideration what type of hot end it uses as some are more prone to failure than others. I would only consider a printer that used a stepper motor to drive the filament and a hot end that offered easily changeable nozzles so you can fine tune your layer height and thickness for better prints. Below I have listed five of the most popular designs and what I like and dislike about them. While these specific designs may not be available on the kit, or preassembled printer you are looking at, chances are that its extruder is based off of one of these designs.
Makerbot StepStruder - The current extruder offered from Makerbot is the StepStruder MK7 and is their second stepper motor extruder design. Instead of a plastic based thermal transition barrier, the StepStruder MK7 utilizes a built-in heat sink and fan to keep the filament cool until the heat chamber. The MK7 has a small flaw in earlier models, though. The plunger design that forces the filament into the feed gear wears quickly and will lose tension causing the filament to strip, and the operator will need to disassemble the extruder to fix the issue. The MK7 does feature interchangeable nozzles. A unique feature to the Makerbot extruder is that it uses the stepper motor to directly drive the filament into the hot end. This is suitable for 1.75mm filament, but I feel that it does not provide enough torque for larger 3mm filament and could result in missed steps if a hard section of plastic were to pass through.
Makergear Stepper Plastruder - The Plastruder design offered by Makergear is unique to this list because it features a gear reduced stepper motor that provides additional torque to feed filament into the hot end. While I still consider this style "Direct Drive", one could agree that the gear box actually makes it an indirect drive. Unlike the Makerbot StepStruder MK7, the Makergear Plastruder is a little bulky and will require a larger frame to move around in. Also unlike the MK7, this extruder design utilizes a plastic insulator between the hot end and the filament feed to prevent filament preheating that causes jams. The Makergear Plastruder features the Makergear GrooveMount HotEnd, which is similar to another hot end we will mention later. While it has a removable nozzle, I feel that its nichrome wire heating method is antiquated, messy and very unreliable. Replacing the nichrome when it burns out is not only a time consuming and complicated task, but it is quite difficult for a non-tech minded person to accomplish.
For the next three items on our list, I have chosen to only list the hot ends as each one uses a version of the Wades extruder which has been modified by users to fit their specific needs. This extruder carriage features a nema 17 stepper motor and 3D printed gears. I personally am a fan of the Greg's Wade Reloaded design that features a filament tensioning system that utilizes springs instead of rubber O-rings like the Makerbot MK7. This makes changing filament, and clearing out clogs much easier. My gears of choice are the Parametric Herringbone Gears by Scribble J.
Modified Wades Extruder with Arcol Hot End - Pictured above is one of the more popular hot ends that has become one of the go-to hot ends for those wanting to print exotic plastics such as nylon. The Arcol V3's machined aluminum cooling fins make it a good candidate for plastics requiring extreme temperatures because of their cooling ability, which prevents heat from sinking up to the filament feed. Heat is provided to the Arcol hot end via a power resistor. This method is far superior to the nichrome wire method used on the Makergear Plastruder and is easy and quick to change out in the event of a failure. The only drawback to using the Arcol hotend is that its nozzle is built into the heater block and is not interchangeable.
Reifsnyder Precision Works J Head - The J Head hot end was one of the very first hot ends that I fell in love with. Its machined design is able to dissipate excess heat while still being able to heat up enough to print high tem plastics. Unlike some similar designed hotends, the J Head is machined from a solid piece of metal, making it highly reliable, and long lasting. Like the Arcol, its only downside is that its nozzle diameter is fixed and cannot easily be swapped out. I personally know the guy behind the JHead and have discussed, at length, the decision to make the nozzle a fixed unit. The reasoning behind it was to create the lightest and strongest design possible. This allows for faster head travel and a longer lifespan.
Lulzbot Budaschnozzle 1.3 - This is by far the best hot end ever made in my opinion. It combines all of the awesome features on the Arcol Hot End that it was based off of, with the removable nozzle design of the Makerbot MK7. It is heated by a power resistor, and comes prewired for ease of installation. This is the same nozzle that I use on my personal 3D printer and I have not experienced a failure in almost 10 pounds of filament. The Budaschnozzle 1.3 is designed to print so-called harder to print plastics such as nylon and wood infused plastics. When combined with a Greg's Wade Reloaded and a set of herringbone gears you get an extruder that is as robust as a tank and is capable of fine prints down to the sub 0.25mm range.
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