Template:RepRapPro hot end assembly

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You will need the following tools:

  • Allen key
  • Small screwdriver
  • Pliers
  • Adjustable spanner
  • Heat sources (small blowtorch plus hairdryer/soldering-iron)
  • Bench vice


Hardware Quantity Reprappro-huxley-hotend-components.jpg
The heatshrink is easy to confuse with the PTFE
for the extruder Bowden tube. The heatshrink
is the shorter one made from thinner material.
Fan and heatsink 1
Aluminium heater block 1
M3 x 25mm screws 2
M3 x 16mm screws 2
Cable ties 2
Stainless steel one-piece nozzle 1
Brass union 1
100K thermistor 1
Heater resistor 1
Brass tapered nut 1
PTFE spacer or 2 X double coil spring washers 1
Aluminium cooling block 1
Connecting wire 8-way ribbon cable 1.2m
crimps 4
PTFE (Clear) Heatshrink sleeve 1
Polyolefin (Black) Heatshrink sleeve 1

If you have a brass nozzle and separate stainless barrel like the picture below, you will need to refer to the archived instructions [here], page 78.


The Huxley heating resistor is a 6R8, whereas the Mendel one is 2R7.

There are several stages in this construction where you have to trim pieces of PTFE. It is essential to clear any swarf created away and not to let it get into the extruder. PTFE swarf will travel to the nozzle and block it if it is allowed to contaminate the device.

Please note, in newer machines double coil spring washers shown on the left below have replaced the PTFE spacer for extra height adjustment when using multiple heads. These often lock together during transport so please double check your spring washers are separated


Step 1: The hot part of the hot end

Check that the heater resistor fits in the larger hole in the heater block. The resistors can be a little variable in their diameter. The thermistor should fit in its hole - they are manufactured to a tighter tolerance.


If the resistor is too big, clamp the heater block firmly in a vice and run a 5mm drill down it. Be careful to run it square on. Running the drill up and down will shave a little off the sides of the hole. Repeat this until the resistor fits. The ideal fit is if it is snug, but not tight. You should only need light force to put it in its hole. If you have to push it hard, this will damage its insulation.

If the resistor is too small in the hole, wrap enough Aluminium foil around the body of the resistor to ensure a tight fit in the hole. Ensure that the resistor leads do not contact the Aluminium foil.


Next you will fit the thermistor. For this step, you will need the axial thermistor, and length of PTFE (clear) heatshrink.


Cut the PTFE heatshrink such that when slid over the thermistor, approximately 5mm of the leads are bare at each end. With the PTFE heatshrink in position, use a heat source to shrink the PTFE around the thermistor bead and leads. PTFE heatshrink requires temperatures in excess of 300C to shrink. A naked flame will do for this. Keep the flame moving whilst shrinking the PTFE.


Take the sheathed thermistor, and slide one lead into the small hole in the heater block. With the thermistor bead up to the side of the heater block, the PTFE heatshrink should protrude form the other side of the block. Grasp the PTFE heatshrink with some pliers, and pull the thermistor through until the bead is roughly in the centre of the heater block.

Reprappro-hotend-thermistor-half-in.jpg Reprappro-hotend-thermistor-fitted.jpg

Next you will terminate the resistor and thermistor leads. Crimp terminals onto the ends of the thermistor leads, then sheath those terminals using black Polyolefin heatshrink.

Reprappro-hotend-thermistor-crimped.jpg Reprappro-hotend-thermistor-sheathed.jpg

Finally, sheath the resistor leads using PTFE heatshrink


Set the heater block aside for use in a minute.

We will need a short length (about 10mm) of 3mm diameter PTFE tube to line the cold end of the one-piece nozzle.


Using a sharp blade, cut one end of the PTFE liner. Try to make the cut as square to the axis of the tube as possible. Push this into the counter-bore at the cold end of the one-piece nozzle. Again using a sharp blade, cut the PTFE liner flush with the cold end of the one-piece nozzle.


Take a 5mm drill and gently twist it against the end of the PTFE that you have just created to dish it slightly. Make sure you clear all swarf away. Now set the one-piece nozzle to one side.

Step 2: The cooling system

Now you will assemble the cooling system.

Take the free end of the extruder drive's PTFE tube. Use a pencil sharpener to make a small cone on about 2mm of the free end of the 4mm tube. Take care not to cut too far - PTFE is very soft.

Using a fine permanent marker pen, draw a line across the tube, 10mm from the end. Screw the brass union onto the free end of the extruder drive's PTFE tube until the brass union almost reaches the line you have drawn. By looking down the other end of the brass you will be able to see that the PTFE has reached the end of the thread (a magnifying glass is useful here).

Screwing the tube in will have reduced its internal diameter slightly. Gently twist a 2mm drill by hand in the end of the brass to thin the tube where it is inside the screw thread. If you have a small hand-chuck this is made easier:


Push a length of 1.75mm build filament down the tube from the other end to clear out any PTFE swarf (see the warning above about leaving any behind). Make sure the filament runs freely down the tube and comes out of the far end without impediment.

Some heatsinks come with a sticky backing which you will need to peel off. This can be quite tough - you may need to pull with pliers. Take care not to put stress on the delicate plastic fan. The easiest way once you have a corner off is to hold that with long-nosed pliers and to roll them over the back face of the heatsink like peeling the lid off a tin of sardines.


You can put a little heatsink grease on the aluminium cooling block if you like. Attach it to the fan with the two longer screws. Feed the screws in through the fan and into the cooling block.


In some fans the mounting holes are stepped partway down: the screw head should sink down into the recess and sit on the step. Depending upon the type of fan you received with your kit you may find that the screw heads are too big to fit into the mounting holes and thus do not reach the cooling block. If this happens, put the screw in the chuck of a drill and grind the head against a piece of coarse aluminum oxide sandpaper or a grinding wheel. Only take off enough to allow the head to sink down to the step. If you take off too much it won't catch securely on the step.

Now screw the brass bowden end piece (with the PTFE bowden tube screwed into it), into the Aluminium heatsink block (the long thin one with five holes in it).


Once fully screwed in, screw the free end of the barrel into the M5 hole in the Aluminium heatsink block until it meets the brass piece. Now unscrew the brass piece by 1/4 turn, screw the barrel in to meet it, and finally tighten the brass piece with some pliers. This will result in the barrel and bowden end pieces being locked together inside the heatsink block.


Step 3: Hotend final assembly

The final step is to assemble the hot part of the hotend with the cooling system.


Cut a small piece of Kapton tape and stick it to the bottom of the fan heatsink.


Now screw the heater block onto the one-piece nozzle, and follow it with the brass tapered nut. Tighten the brass tapered nut and heater block together tightly with spanners (more than finger tight!); this will ensure the threads make good contact with the nozzle, and heat transfers well. If you have problems with extrusion despite the temperature being reported correctly, it's worth checking that the brass tapered nut hasn't worked lose.


The last component to fit is the PTFE heat spacer. Attach it as shown below using two M3x16mm cap head screws.

Reprappro-huxley-hotend-heat-spacer.jpg Reprappro-huxley-hotend-heat-spacer-fitted.jpg

If your kit comes with with double coil spring washers, these are a direct replacement for the PTFE heat spacer and are used with two M3x25mm cap head screws.

You can put an M3 washer above and below each double coil spring washer, if you wish. Using double coil spring washers allows the height of the nozzle to be adjusted individually, so that multiple nozzles can be leveled.


Step 4: Hotend wires

The hotend is designed to be easily removed for maintenance, so a wire connector should be used to connect the hotend to the ribbon cable.

Using four spare lengths of motor wire, each approximately 4 inches long,


Crimp a terminal onto the ends of the heater resistor wires, then sheath it using black Polyolefin heatshrink. For the thermistor wires, solder a single header pin to each, and sheath using the black Polyolefin heatshrink.

Reprappro-huxley-hotend-wires-unsheathed.jpg Reprappro-huxley-hotend-wires-sheathed.jpg

Solder the four wires to six pins of the male header, in the following order:

  1. Heater resistor Wire 1
  2. Thermistor
  3. Fan + volts
  4. Fan Ground
  5. Thermistor
  6. Heater resistor Wire 2

One of the wires from the fan is not used.


Reprappro-huxley-hotend-wiring-unsheathed.jpg Reprappro-huxley-hotend-wiring-sheathed.jpg

Use two adjacent wires from the ribbon cable for each end of the heater resistor - four in all. This is to increase the current capacity.


The wires across the ribbon cable in order go like this:

  1. Heater resistor Wire 1
  2. Heater resistor Wire 1
  3. Thermistor
  4. Fan + volts
  5. Fan Ground
  6. Thermistor
  7. Heater resistor Wire 2
  8. Heater resistor Wire 2

Resistor wires 1 and 2 are arbitrary - the resistor has no polarity.

This order is designed to minimise the risk of cross wiring the different circuits to the hotend. Since the fan polarity is important, this wire arrangement makes it easy to flip the +ve and ground wires to the fan.

Crimp terminals onto the heater resistor wire pairs, and onto the thermistor and fan wires.


This step is easiest with the correct tool, such as this one http://www.rapidonline.com/Tools-Equipment/Ratchet-Action-Crimp-Tool-Ht225d-85-0262 If you do not have such a tool, this advice is worth following http://www.youtube.com/watch?v=KEm2PuHBt4Y

Push the terminals into the female housings and fit these to the male header strip.


Connecting up

Bend the wires up the side of the heat sink. Do not pull them tight - they need a little slack to accommodate movement and expansion. Attach them at the top of the heatsink with two cable ties chained together, one of them running through the top slot in the heatsink.

Trim the excess off the cable ties.

Use a meter to check that the resistance between the wires and the aluminium block is infinite and that nothing is shorting.

Also check the resistance of the heater resistor and the thermistor by measuring from the far ends of the wires. The Mendel heater should be just under 3 ohms or the Huxley just under 7 ohms. The thermistor should be about 100K.