Mystery material

The bed material I am using for ABS works very well but the problem is I don't know what it is. My wife bought it for 10p when I was trying lots of things for HDPE and was disappointed when it did not work. She is now delighted I found a use for it as she loves to get a bargain.

It originally looked like this :-



It is about 3mm thick. Most of that is the plastic backing. On the front is printed paper, stuck on with a double sided sticky film and covered with single sided sticky film. This can be peeled off to just leave the back material so both sides of that can be used.

Here is a bit I destroyed developing rafts: -



It will bend a little but then it snaps, as you can see, when I pulled a raft off that was stuck too well.

I normally use this flowchart to identify plastic but it fails with this material because I think it is a polymer with a fibrous filler, possibly paper.

• It melts with a low temp soldering iron so is not a thermoset.
• It floats very well but it is not PE or PP, so that is probably due to a filler.
• It does not burn well and is self extinguishing.
• It gives black smoke and if anything has a yellow flame.
• It does not drip.
• The smoke has some odour but I don't recognise it. I don't know what phenol smells like though.

So perhaps it is PPO and a fiber. If anybody knows where I can buy it from I will be very grateful. It is quite reusable but I expect it will need replacing sometime and other people want to use it.

Stepping up production

As HydraRaptor seems to be working so well with ABS I decided to put my high temperature extruder design on hold and go for making a set of Darwin parts in ABS. This is how far I got before my extruder wore out again: -



The flexible drive cable disintegrated and most of the JB-Weld has fallen off.

Using Enrique's Skeinforge slicer I can make very sparse objects that are still strong when made in ABS. I set the infill to 25% but I am not sure exactly how Skeinforge interprets it. The infill lines are not parallel so they get further apart the longer they are. Large voids are very sparse indeed and smaller voids look like 25% fill.



The outer wall is always two filaments thick, one is the perimeter and the other is the ends of all the infill zigzags that meet each other. With 0.5mm filament and a layer height of 0.4mm the filament threads are 0.6mm wide so the side walls are 1.2mm thick. I set the number of solid layers to 3 so the top and bottom are also 1.2mm thick. Skeinforge is clever enough to make layers with some areas 100% fill (where they are less than three layers from the top or bottom or internal surface) and other areas sparse. Very clever stuff, which really speeds up the build process but still gives remarkably rigid and strong objects.

I made four of Darwin's eight corner blocks (taking about 2.5 hours each) but I was unhappy with the amount of warping I got when not using a raft. I decided to develop peelable rafts and reusable bed material, like commercial machines have, before making any more parts. That took a lot of experiments to get right but I now have a workable system for ABS.



The bed material is the advertising board I used for ABS before, but this time I am using the back. Unfortunately I don't know what it is. It is very buoyant in water and self extinguishing if I burn it. ABS bonds to it very well. If I extrude the object directly onto it then it is impossible to remove. If I put down a sparse raft first at a low temperature I can remove the raft with a penknife. It blisters the surface but that does not seem to matter because the raft presents a smooth surface to the object. It just gets a bit harder to remove the raft each time as the surface gets more blistered.

The board is not strong enough to resist the warping on its own so I stuck it to the back of some floor laminate with Evostick contact glue. Even that could not hold the edges down, hence the metal strip.

The first raft layer I put down is a 1mm filament zigzag with a 50% spacing, extruded at 4mm/s @ 200°C with a nozzle height of 0.7mm. Because the layer is so thick and extruded quite flat, it absorbs any surface irregularities and makes the initial head height less critical. Spacing it 50% allows it to spread sideways, if the head is too low, and also allows it to be removed. 100% fill is impossible to remove and the head height becomes critical. If it is a little too low, the filament is wider but there is nowhere for it to go, so it builds up on the nozzle and blobs.

The first layer is far too course to build upon so I put two layers of fine zigzag the other way on top. These are 0.5mm filament extruded at 16mm/s with a layer height of 0.4mm and spaced just wide enough to not bond with itself laterally. That makes it easier to remove from the base of the object. The temperature is raised to 230°C to give a strong weld to the layers below.

Two layers are needed because the first layer has a rippled surface as it spans the wide gaps in the layer below. I put them down on top of each other rather than alternating the direction of the zigzag. That makes them weaker laterally therefore easier to remove from the object with a penknife.

The raft uses horizontal and vertical zigzags so there is no correspondence with the object infill which is at 45°. Again that makes it easier to separate without risk of pulling a thread out of the bottom of the object.

To ensure the raft does not bond too well to the object it is cooled for a minute with the fan. The first layer of the object is then extruded at 8mm/s @ 215°C and subsequent layers at 16mm/s @ 230°C. The temperatures are critical, so depending on thermistor site and calibration, they will vary a bit from machine to machine.

This is what the bottom of the raft looks like: -



And this is the top: -



It does slow the build and waste plastic but it reduces warping and makes the bed reusable over and over again. I expect it won't last forever but you can certainly use it many times.

The base of the object is also pretty neat and tidy: -



Here are the stats for the objects I have processed so far: -

	Seconds	Filament @ 16 mm/s	Moves @ 32 mm/s	Build time	Plastic volume	Quantity required	Total build time	Total plastic
Corner bracket @ 25%	8866	122009 mm	34926 mm	02:27:46	24.0 cc	8	19:42:08	191.7 cc
Opto bracket @ 50%	1200	15902 mm	4661 mm	00:20:00	3.1 cc	3	01:00:00	9.4 cc
Diagonal tie bracket @ 25%	2178	31236 mm	3716 mm	00:34:28	6.1 cc	20	11:29:28	122.7 cc

I will update this table as I progress to make the Darwin parts.

Swiss cheese

HydraRaptor made a Darwin corner bracket in 50% filled ABS this evening: -



It took 2 hours 35 minutes. It feels pretty sturdy but there is some delamination through the thin section of the corner facing the camera. A bit of a weak spot in the design I think. Also the base is a bit warped as I didn't use a raft. I don't know if these matter as I haven't worked out what all the holes are for yet. I need to make seven more for a Darwin. I will probably do a 100% version for comparison.

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.