ABSolution

I have been testing Acrylonitrile butadiene styrene (ABS) in the RepRap extruder and I have to say it works rather well. It is surprising how different each plastic I try is. For example, if I fold a short piece of filament double and let it go this is what happens :-



PCL is rubbery and springs back almost straight, HDPE is a lot less springy. ABS bruises when bent sharply and is a more opaque cream colour rather than white. PLA is transparent like glass and breaks when bent through a small radius. The progression from top to bottom is from rubbery to brittle. I think the reason for this is that ABS and PLA are below their glass transition (Tg) at room temperature, whereas with PCL and HDPE their Tg is well below 0°C.

I have a bit of a routine now for getting my machine working with new plastics. First I look at the extruder performance at different flow rates and temperatures. Then I have to experiment to find a bed material it will stick to. After that I make test blocks to fine tune the temperatures and find the best speed and filament diameter to build with. Finally I look at the warping with different infill densities.

So here is the flow rate versus motor duty cycle extruding at 190°C measured at the nozzle (the other plastics are at different temperatures):-


As you can see ABS works the motor harder. Part of this is due to the fact that it has to run faster (for the same flow rate) as the ABS filament I have is only 2.75mm rather than 3mm. However, I think that it is due mainly to the increased force and friction required to cut the thread in the pump. At first I had a lot of problems with the GM3 motor's clutch slipping. I got it working reliably by loosening the top springs and just tightening the bottoms ones. I also have the filament running through a felt washer soaked in oil, which I had to add when doing PCL.

Here is how the filament diameter varies with flow rate :-



ABS has much lower die swell than HDPE and is quite a bit better than PCL. That makes it good for extruding fine filaments at high speed.



I think the graph above shows that the viscosity increases a lot as the temperature drops but the motor duty cycle remains pretty constant showing that most of the torque is used overcoming friction in the pump.


I tried using PP and MDF as bed materials but ABS does not want to stick to them. Unlike PCL which stays molten for a very long time, ABS filament sets soon after leaving the nozzle. PCL and HDPE turn transparent when they are molten but ABS does not. I think its specific heat capacity is quite low compared to HDPE so the workpiece cools quite quickly and I can get away without a fan.

The best material I have found for a bed is plastic laminated board. My wife bought me a big sheet of it for 10p when I was experimenting with HDPE. It was cheap because it was scrap advertising material. She was disappointed when HDPE did not stick to it, but is made up now that I have found a use for it. I think possibly it works because the thin plastic lamination is actually polystyrene.



I can extrude ABS on to it and it sticks well enough without needing a raft. To remove it I cut the lamination around it with a penknife and pull the surface off. I can then peel the lamination off the base of the object leaving a clean finish. It works well but I don't know where to get any more and it is single use although you can use both sides.



It is good not to need a raft because while I can remove an HDPE raft with scissors or a sharp knife, ABS 1mm thick is too hard to cut off easily.

My theory about welding temperatures is that you need to extrude at least twice the melting point (105°C) minus ambient. That works out at 190°C. In practice I needed to go a bit hotter to get a satisfactory bond between the layers at high speed. I settled on 220°C for the first layer and 200°C for the rest of the object.

Layer bonding can be quite variable with ABS. It is easy to make objects which can be peeled apart again. I think this is because even at 220°C ABS is quite paste like and less fluid than the other plastics are at their critical weld temperature. That makes the filament contact points very tangential and so smaller. Plastics that are more fluid slump and get a bigger contact area, hence a stronger weld. Also, the time the plastic is in contact and above the melting point determines the amount of fusion. I think if I spend some time on this (and with the other plastics) I will be able to use plastics as their own support material for making overhangs. The layer height will be crucial to making this work so it will have to wait until I have replaced the PTFE insulator with stainless steel.

Here is my standard warp test shape: 40 x 10 x 20mm block made with 0.5mm filament at 16mm/s, layer hight 0.4mm, filament pitch 0.6mm: -



The warping figure I got after leaving it a few days was 0.38mm compared to 0.53mm for 100% HDPE and 0.21mm for 100% PCL. 50% filled ABS gives a figure of 0.15mm which is the lowest I have measured yet and ABS is still very strong at densities less than that, whereas PCL is not. Also, this was without a raft which gives worse figures for PCL and ridiculous warping for HDPE.

The filament is very soft and compliant when it is molten, with no spring in it, so it goes where the head leads it and produces good definition. Here is a top view showing how accurate the corners and infill are: -



Here is a 50% infill pattern: -



This is very accurate compared to the same pattern in HDPE shown here. 25% fill is also very good and the object remains strong: -



For some reason my 9M pixel camera doesn't like taking close ups of white things.

When I was extruding thick rafts I noticed some bubbling of the surface. I think this is due to absorbed moisture turning to steam because ABS has ten times more water absorption than HDPE. Oddly, it does not happen when extruding the object so is not a problem, at least with the current weather conditions.

ABS smells a little when it is hot but not enough to be objectionable.

My acorn nut nozzle, with the very shallow exit hole, is very incontinent with ABS as it was with HDPE but not with PCL. This means that even though I wipe it clean with the toothbrush it has extruded another few millimeters by the time it gets to where it has to start the object. The problem with that is that it sets so fast it is solid when it meets the table so will not stick and stops the following filament sticking. I am hoping the latest nozzle design will fix that.

On to the same test with PLA before I alter the extruder.