The majority of extruder hot-ends on the market at the moment come as kits of varying degrees. There's a certain amount of assembly you have to do yourself, but this is changing rapidly, and not too soon I think.
(J-Head IV-b shown above.)
It comes with a resistor and thermistor but they are not fitted. Fitting them is 'fun' the first few times, but after a while becomes a bit tedious. I can see the expanding market moving to some level of pre-assembly, as I can see the broader user population more willing to pay a premium for pre-assembled units and less interested in this doing this it themselves. Even for the enthusiast the process of gathering the heat resistant wiring and appropriate connectors is a bit of a chore and added cost. Another reason I'd favour pre assembly is the 'quality assurance' and consistency that would come from pre-assembled hot-end, which might include some level of thermal bedding-in and testing of the resistor/thermistor. These steps would certainly be worth the little extra, as well as the time saved.
[ J-Head heater block and integrated nozzle shown in picture. The advantage of this design is it's an all-in-one unit and can't leak. You have no worries about tightening on nozzles or loose fitting heater blocks needing lock nuts. the disadvantage is you can't just easily swap out the nozzle to change to a different orifice size. ]
There are many instructional sequences showing the assembly of the hot-end but I thought the process and sequence I've adopted might be worth sharing. I always like to insulate the heater block and have some photos showing that process also below.
Tip: I remove the pin from a small pop-rivet, snip it's head off, and use it as a joiner. I don't solder this joint as solder may melt. I use a PTFE lined, braided wire sleeve on the resistor. I salvaged the wire from an old iron. It's designed for high temperatures!
The resistor that comes with the J-head is 5.6 ohms. With a 12v supply, I've found this gives a very nice heating time and seems well able to keep the block hot in comparison to the 6.8 ohm resistor, also popular in the market. The 5.6 ohm resistor also has a more cylindrical shape and is a snug fit to the brass block. The other resistors (6.8 ohm, Green in colour) I've bough were slightly lozenge in shape and not a good fit.
This photos shows wires attached either side of the resistor. The choice of colours has no significance.
The tiny little 100k glass thermistor needs it's wires kept apart. I've tried the ptfe sleeve idea on one of the wires, but I've found this bulky. The extension wires are soldered and the joints head-shrinked. Be sure to scrape the wires bare first. They are slightly coated.
What I've found as a better way to keep the thermistor wires apart is to sandwich them between some high-temperature tape (above photo).
The thermistor is inserted into the hole in the brass block and the wires wrapped around the corner of the block, curved around more so than bent at a sharp angle. The tape holds it somewhat, but the addition of the other insulation jacket will hold it in place very well. I don't bother with any fire cement or silicone. It's a personal preference. Also note all joints are heat-shrink wrapped at this stage. The resistor gets removed one last time to fit the insulation jacket.
I use a very light sheet of ptfe to warp the brass heater-block. The material is rated to 280Deg C, and is commonly used as baking tray or oven liner. Wrapping the heater block in this material is more an art form than a science. You may be able to make out the folds and cuts I make in the sheet from the photos.
Once I'm happy that everything is fitting nicely I tape everything down with kapton tape.
And there it is, snug as a bug!
This jacket keeps the heat in allowing faster heat times, and means less work for the resistor to maintain a given temperature. It also protects the heat-block from the negative effects of stray fan air currents. Finally it reduces radiating heat from the heat-block to the newly printed job below it.
One final step is to cut out the holes for the resistor after the jacket is fitted. Mind your fingers!
With the hole cut out, I used a bar through the hole, and small wrench on the insulator notch to tighten home the head into the insulator. It's hard to describe how tight this should be. It's a 'good hand tight'. It came pre-assembled, so I actually marked the joint with a dab of Tippex before taking it apart. I reassembled until the two white marks realigned!
I've more extruder related photos to post but out of time for now. Thanks for viewing!