Extruder improvements and alternatives

Under construction.

Direct drive.

This is a replacement for the flexible drive. It is simpler, and less likely to wear out. Its only disadvantage is that it does not allow the working material a straight run down the extruder - it has to bend. For most working polymers introducing a shallow bend as they enter the extruder is not a problem.

You will need a different motor coupling to the standard one. The design for this is in the motor-coupling.aoi file, called motor-coupling-direct-drive. Export this as an STL and print it.

The M6 screw drive is also simpler to make, though it is longer. The reduced diameter bearing lands are in exactly the same arrangement as for the standard extruder, but there is no need to drill an end hole nor to make a separate short section to hold the drive nuts. The drive screw needs to be 96 mm long.

Solder two M6 nuts on the end, taking care to align their flats. Clean all grease and oxide from the screw threads and nuts, apply a thin film of solder flux, screw the nuts in place, then heat gently with a blow torch. Use ordinary electronic solder.

As an alternative to soldering the two nuts on the end, you can try locking them against each other very tight. Get some long spanners and turn really hard to get the flats to line up. Don't strip the threads, but don't be frightened of damaging them slightly: they are never going to come undone again.

High-temperature heater design

This idea comes from reprapper Ian Adkins of Bits from Bytes.

JB Weld is fine up to about 190 ^oC. But if you want to go hotter, you need something tougher.

Fire cement fills the bill. It is available from hardware shops and plumbers' suppliers. It is used to fill cracks in chimneys and to seal round flues. The stuff I got goes up to 1250 ^oC, but don't regard that as a target - you'll degrade the PTFE to some nasty poison gases. The high two-hundreds are a sensible limit.

The picture on the right shows the parts you will need: fire cement, the standard PTFE cylinder, the standard brass M6 threaded tube, an 8 ohm length of nichrome heater wire, and an 18 mm length of 15mm diameter copper tube.

You will also need a K-type thermocouple. The extruder heater will get too hot to use thermistors. Thermocouples will be fine, though; unlike thermistors, they also give a linear output with temperature, which makes things simpler.

File two notches in the copper tube at the ends as shown. These will be used to lead the electrical connections through.

The nichrome heater wire doesn't need to be covered in fiberglass insulation, which will make it cheaper and easier to get (maybe from old electric heater?). You definitely can't solder connections to it as it will get too hot and melt the solder. Crimp on two small connectors as in the picture; alternatively use screw connectors.

Start by applying a layer of fire cement to the brass tube about 1mm thick and 18 mm long. You will probably find this easier to do with your fingers than with tools. It is also easier to apply too much, then to trim it away with a blade, and then to roll the result between your fingers to smooth it.

Partly set the fire cement with a hair dryer. You don't need to set it completely. Just get it to the point where it is firm and has a dry skin on it.

Use Blu-Tack to hold one end of the wire against the PTFE as shown. Wind the rest of the wire evenly over the fire cement, and secure the other end with Blu-Tack.

Check that the wire's resistance is still 8 ohms (that is, check that it is not shorted against itself). Also check that the resistance between it and the brass tube is infinite - again, no shorts.

Apply a second coat of fire cement. Don't completely cover the wire - you will need to unwind it and lead it back to the top as shown. Again dry the cement with a hair dryer.

Check the resistances again.

Push the copper tube over and get it roughly central. It may be simpler to have it slightly off-centre to accommodate the connections. This doesn't matter.

Push the thermocouple into the cement at the bottom.

Completely fill all gaps with fire cement. Push it in and tamp it down with a small screwdriver, but take care not to disturb the wire coil.

Again check the resistances, this time also making sure that the copper tube is isolated too, and that so is the thermocouple.

Clean the thread by running an M6 nut down the brass.

Attach the thermocouple to the RepRap thermocouple circuit and attach that to an Arduino as described on that page. Load the thermocouple program at the foot of that page into the Arduino and run it. The temperature should read room-temperature.

Attach a variable-voltage power supply to the heater coil and place the extruder heater vertically. It's going to get hot, so if (unlike me) you value your bench you may care to stand it on a small block of wood.

Turn on the power supply. Start with about 3v and check that the temperature rises slowly.

Over the space of a few hours come back and increase the voltage by about 0.5 v every so often. The idea is to ramp the temperature up very slowly to dry everything out without subjecting it to thermal stress. Stop when you get to about 250 ^oC.

Nozzle valve

This device allows the flow of polymer to be controlled with much more precision than simply turning on and off the extruder motor. It consists of a latching solenoid that pushes a piece of stiff piano wire across the exit to the nozzle, blocking it. This doesn't completely stop the flow, but it does eliminate a lot of leakage. It also allows tricks like starting up the extrude drive motor with the valve closed, building up a little pressure, then opening the valve to ensure a clean sharp start to polymer deposition. Controls for all that sort of thing are available in the RepRap Java software and in the Arduino (and PIC) controllers.

The design is not perfect - it does slowly leak a little polymer at the side, as you can see from the picture.

The solenoid I used was from RS, catalogue number 250-0827. This is held by the green RP parts shown. The designs for them are in the RepRap Subversion Repository here. The assembly attaches under the extruder using two of the four bolts that hold the extruder together, again as shown. You may need to add spacers to get the alignment right (the stacks of brass M3 washers you can see in the picture). The line of action of the solenoid needs to align with the movement of the piano wire in the valve, described below.

Here is a drawing of the two brass parts of the valve. This is in the repository here.

The nozzle is an 0.5mm vertical hole exiting from the bottom of the bottom half of the device. An 0.5mm diameter length of piano wire slides in an 0.5mm hole at 45^o to that along the axes marked A-A, which coincide when the device is assembled. The big problem that this design overcomes, and the reason that it is a little complicated, is that of getting those two holes to cross exactly in alignment in the middle of the device. It's quite easy to get them to cross at the exit (one just drills the 45^o hole using the nozzle flow hole as a centre), but that leaks - the crossing point has to be in the middle.

Making these parts has to be done in the right sequence. Start with the top one. The only tricky part here is drilling the 45^o hole, and that is done as just described: drill the vertical 0.5mm hole first, then clamp the nozzle in a drill's vice with that hole pointing at 45^o upwards, then drill the 45^o hole using the original hole to centre the drill.

Next make the bottom part. Drill and tap all the large vertical holes, and turn down the nozzle, but don't make the 3mm horizontal hole nor the 45^o hole, nor the 0.5 mm nozzle hole yet.

To make the flat-bottomed hole you will need a 4mm slot drill (conventional drills have a cone at the end, of course).

The important thing is to get the device so that, when you screw the two halves together, the two faces marked F-F are pushed hard against each other. You can check this by putting a little engineer's blue on the top face. Screwing the device together should transfer some of this by friction to the flat-bottomed hole.

On the top half, mark the side of the 8mm section to line up with where the 45^o hole emerges. Then screw the two halves together and continue the mark onto the bottom half. Take the halves apart and drill the horizontal 3mm hole to line up with your mark.

Now screw the halves back together again good and tight. (With a bit of luck, they'll never come apart again...) You should be able to see the top of the 45^o hole in the 3mm hole you just drilled, and you should be able to get the 0.5mm drill down it from the outside. Very gently drill this hole about another 1mm deeper to make the blind hole where the piano wire will locate when the valve is closed. Then, using the vertical 0.5mm hole in the top half to get the alignment right, continue the vertical nozzle hole right down through the valve. Clean the holes up as best you can without widening them further. A length of the piano wire is good for this, as is a compressed air line.

I took the one I made apart after I finished it - not a good idea, as this can misalign the holes. However, it allowed me to take this picture, and mirabile dictu the thing did go back into alignment when I re-assembled it.

The nozzle is rather longer than the standard no-valve one, and so it can get a little cold at the tip as all the heat has to be conducted down to it from the heater coil round the main barrel of the extruder. For the extruder using this new valve, I made one of the fire-cement heaters above with a 6 ohm, not 8 ohm, heater coil. Then I took an extra 2-ohm length of fibreglass-insulated nichrome heater wire, crimped a couple of connectors on the end, cut a short length of brass tube, and set the lot in more fire cement round the top of the valve nozzle.

Wiring the two coils in series gives 8 ohms, and 25% of the heat from that is generated in this valve to keep the tip hot.

The picture of the whole valve and solenoid above shows the device without insulation so you can see how it goes together. In operation I run mine with a wrapping of glassfibre roof insulation held round it by a couple of twists of wire.

Closed loop.

Locked clutch

Spacers at the top, very strong springs at the bottom

e.g.1.5mm x 9mm x 25mm.

Sharpened thread

That is the polymer drive screw thread - the sharper it is, the better it will drive hard polymers like polylactic acid and ABS. Sharpen it by running an M6 die down it several times, increasing the screw pressures on the die from the tightening screws in the die wrench. This will result in a slightly undersized thread, but the die will give much sharper peaks than you get on standard studding (which is forge-rolled, not cut).

Pipe clip on the PTFE

-- AdrianBowyer - 12 Jul 2008

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