I love ABS

I took delivery of some ABS just before the weekend and I have been running the repstrap for about 6 hrs a day since- what super stuff!

Having worked with HDPE for a long time the major problem was warping of any print. ABS nearly cures this although it smells a lot worse. I am now extruding onto a glass chopping board (heat resistant to 240 degrees ) and evostick.

The first major break through I got was reducing the extrusion rate. This had been puzzling me for a long time as every extrusion seemed to have an excess of plastic. I kept recalculating the and measuring the rate it was laid down at and the size but nothing seemed to work. Then after watching for a long time I noticed that when making small movements the machine never got to full speed- should be 720mm/min but on the on screen display I only got 2-300 mm/min- hence too much plastic! I now need to maximise the acceleration on all my steppers using the EMC set up programme.


I have also programmed the arduino so that when it switches off it goes backwards fast briefly and when it switches on it goes forward 1.5 times the reverse quickly to stop ooze and this seems to wok really well (although it stretched my limited programming capability)

The first thing I printed was Adrian's pinch wheel extruder. (I would show pictures but there are no batteries in my camera I will add them tomorrow). This came out really well and appears to be quiet strong- not quiet at Nopheads quality but good enough for now.

I then decided to print the wine glass as we were going out to a dinner party. Unfortunately the machine does not have enough height so I managed the stem- it looked great and only took 40 minutes. When I showed it to friends they did not understand the rep rap excitement. In fact Alex told her boyfriend Steve "Its a good job he already has a wife or he would stand no chance!"

I am now in the middle of printing a large assembly- it has been printing for over 9 hours now and is looking good despite the various breakdowns.

Next steps
1. Tidy all electronics/wires up.
2. Make sure the feed to the pinch wheel is constant using smaller hooks this appears to cause the pinch wheel jams
3. Fasten the extruder more securely to the machine
4. Fix the acceleration issue
5.Move the whole thing into the garage to reduce noise and smell
6. Increase the z axis movement

Questions to anyone

1. What is the biggest thing that anyone has made?
2. Using a pinch wheel extruder and the resistor/brass bar/welding tip/stainless set up could we just wack in solder and extrude it- what is the melting point?
3.Is acceleration an issue on Darwin?


Andy

Pinch whell stepper extruder

I have battled for a long time to get an extruder drive that works reliably, delivering a steady speed especially when extruding HDPE. The problems were that the DC motor would slow under load and was often variable in speed as the load varied. Also the screw drive did not positivley grip the plastic rod

I have come up with a solution based on all the other pinchwheel extruders but made from easily accesible parts (Thanks Adrian Zak and everyone else).

From the photographs you should be able to see a 90 degree angle bracket (from B and Q) which I have fastened a stepper motor to. I have machined a pulley and given it a rough surface using the technique described in another blog (sorry I can't rember who) where you put a tap into the lathe and the work piece into a bracket so it rotates effectively cutting a bevel gear. This is better than knurling as 1. I don't have a knurler and 2. it gives a groove for the plastic to sit in so keeps it in the correct place.

Initially the stepper did not have enough torque to drive the plasitc consitantly but I rewired the stepper so the coils were bipolar parallel and it worked fine.

I have also made it so that the stainless tube comes as close to the pinch wheels as possible to prevent any buckling.

I just now need to work out how programme the arduino so that when it gets a stop signal the stepper reverses for a few steps then stops to stop oozing.

90 minute nozzle

I have been experimenting with NOPHEAD's nozzle design for a couple of weeks. The second picture shows one made with brass and a Peek tube. I had problems with the thermistor coming out so I held it in with twisted wire and fire cement. This worked OK but I melted the Peek when the thermistor fell out also when the nozzle hit the work piece, it bent.

I then put in place mark 2 using a stainless steel tube. This works very well using virtually the same design as nophead but without the cooling fin. It does have a narrowed wall just above the heater block anmd it is tapered at the end. The cooling comes from the plate that the nozzle is mounted into. The tube is made from 8mm stainless studding and the brass is 3/4" bar. It uses the ceramic resistor as a heater and this is sealed at either end with fire cement.

The welding tip is drilled out but is not drilled in as far as it would go as I suggested earlier on and it seems to work.

The great thing about it all is that from deciding to make it to having it heating on my machine using a pillar drill, lathe and taps it only took 90 minutes. Much faster than the old nozzle. Well done Nophead, genius design.

New nozzle

I have spent a long time messing about with the extruder design trying to get over these problems.

1. The PTFE and nozzle seperate easily under pressure

2. The heater is awkward to make from nichrome wire

3. Nozzles are difficult to make especially the 0.5mm holes which break expensive drills.

In the process I have made a lot of junk.

My new design is based on NOPHEADS. It uses 22mm brass bar with 3 holes. One small hole to hold the thermistor held in with fire cement.

The next hole is 6.5mm and holds the ceramic resistor to heat the nozzle.

The third has a 5mm long 8mm tapped hole for the PEEK insulator and then is tapped all the way at 5mm and has the bottom drilled to 5mmm to insert a 0.5mm drilled out welding tip. The reason for the 5mm drilling out at the bottom is to get the welding tip as far in as possible to maximise the heat transfer.

I have tried this out and have found that I do not need the support that Nophead uses as the Peek/brass threaded section holds well under high temperature (300 degrees) and with it extruding as fast as possible. I have also tried to pull it out and failed. Only a long run at extrusion will proove if it works.

The drive design in the picture above does not yet work reliably as the threaded rod/ drive pulley has too much play and slips. I will stiffen this up with another support pulley when I get time.

Buiding an extruder

Following Adrian's efforts to build an extruder while I was waiting for a print I decided to have a go- not very succesfully but I think some parts have merit. I

Had some 23 mm ID aluminium tubing and an auger that was 23mm diameter. I put a goove in the top of the auger and then tapped a hole to stop the auger moving up and down.

The pipe is to supply granules to the auger .

The set up moved polymorph down the tube (And up if you reveresed it!). I then added a heated barrell and a 3mm hole and set it going and it made a small lenght of 3mm ish rod but it was very uneven and when I came to restart it from cold a plug had formed meaning I had to take it all to bits again.

Fixing Extruder issues

I have been experiencing the same problems as everyone else with the teflon barrell coming out of the bracket. This seems to happne most when the nozzle hits the work piece and it is extrremelyt annoying especially half way through a build.

I finally decided to fix it once and for all. My solution was to make a stepped piece of teflon

with a larger diameter at the top 22mm, in the middle 16mm and a jubilee clip bit at the bottom 14mm.

This is then placed into a piece of MDF with a 22mm hole at the top with a 16 mm

hole at the bottom to stop the teflon escaping. The idea of the 14mm bottom bit is to get more grip on the threaded rod so tha that part does not come out.

Black heart

After several hours of checking and changing parameters I have set up to make these hearts.

I found the width that I was extruding at by playing with speeds using a simple g code programme until I found that 150 mm/min onto gaffer tape was the best.(MDF and perspex did not stick and I was not using a raft)

I then extuded several lengths and measured the diameter with a vernier and found that it was 1 mm (the same size as the hole on my nozzle). I put these into skeinforge and then made the extrusion height over diameter 0.9 and these hearts came out.

As they are only 1 layer thick at the bottom there are small holes. To improve this I will make the walls thicker which will force Skeinforge to put 2 layers on.