Thursday, 9 October 2008

Pot black

Having tried green ABS and found it a bit disappointing, I had a go with black ABS and was even more disappointed. I got it to try and make objects a bit more aesthetically pleasing but it was even harder to get working than green and has aesthetic problems as well.

The temperatures seem to be the same as those for green, i.e. higher than I have used for plain, but it is harder to get it to stick to the raft and it de-laminates more readily.

To make it stick I had to make the first layer a bit lower than normal. With plain ABS I can have it at the normal layer height above the raft (filament diameter * 0.8) but with HDPE and green ABS I had dropped it 0.1mm. With black I had to drop it 0.15mm as 0.1mm does not stick enough and 0.2mm sticks too much. 0.05mm makes all the difference and has about the same effect as changing the temperature by 10°C.

Another problem is that the extruded filament is not smooth. Close examination reveals that it has small craters in the surface where it has out gassed. It is particularly noticeable when laying the thick filament for the first layer of the raft.

It could be the effect of water absorption but, as all my ABS is stored in the same room, I think it is more likely a volatile component of the black dye. Another effect is that if the filament is stretched while molten, so that it is drawn into a fine thread, then it looks like a string of beads. I think that is an indication the black dye does not mix well with the ABS.

Fortunately, for some reason I don't understand the crater effect is less noticeable when making objects.

These are Darwin y-belt-clamps, a nice small part good for a quick test. The small one is made with 0.3mm filament and shrunk by 3/5. As you can see it is far less shiny. All I can think is that is related to the fact the filament is being stretched more.

Another downside of coloured ABS is that ABS turns white if it is stressed. This shows up far more when it is coloured and particularly with black. Because the base of the object is weakly welded to the raft then it gets bruised when it is peeled off.

This is the bottom of another screw topped pot.

As you can see it has white highlights where the welds to the raft have been broken. One way of fixing it is to wave a hot air gun over it to relax the stress points. That also flattened the base, which was a little convex due to warping.

I expect rubbing it with a little solvent like MEK would also solve the problem.

I now have a little family of pots!

I would not recommend coloured ABS as the dye introduces more variables and generally seems detrimental to strength and aesthetics.


  1. Hi nophead,

    Besides the fact that it doesn't work as well as the virgin plastic, it still looks pretty nice as far as I can tell from the pictures.

    Couldn't it be that, due to the dye, the adhesion to the extrusion head is different? Perhaps if you clean the orifice, sand down the head and polish it again it might become better?

    I'm building a new extruder, because the old one kept failing and because I had no spare FTFE rod. I just received the new bitsfrombytes extruder and started building it. I'd like to compare the results with yours. Is it possible that you ship me some small pieces of coloured plastic for that? I'll pay for the shipping and material charges.

    I'd like to compare results and photograph some of them under a USB microscope. I've made these pictures of the stratsys demo model to give you an example. Perhaps even one of you nice parts to demo at the Wikimedia conference I'll be speaking at on 1st of November? I'll talk mostly about RepRap and its relationship to open source. It would be nice to show something that was actually made with an open source extruder.

    Image 0100
    More images...

    Much higher resolutions are possible with this USB cam, but right now I'm sticking with the beta Linux driver for the webcam (I don't want to switch to windows just to take a few pictures). I'm already talking to the supplier about improving the Linux driver :)

    When I get milling working I'm going to design a nanotec microstepping PCB that will allow us to microstep with RepRap's allowing for more positioning accuracy. The PCB has to be able to be made on RepRaps fitted with e.g. a dremel and the reaction bars that tofletcher used. I want the controlling PCBs to be part of the genotype/phenotype that people will evolve.

  2. "I want the controlling PCBs to be part of the genotype/phenotype that people will evolve."

    Working on that right now with Tommelise 2.0. I think that with Darwin, Adrian's desire to be printing circuits directly with something like Field's Metal has been getting in the way of doing something more immediately practical like milling PCB's

  3. Hi Erik,
    Yes the objects seem to come out without extra blemishes even though the filament extruded into air has bumps on it. I don't understand why.

    I don't see what difference the adhesion to the nozzle would make.

    Yes I can send you some parts, send me an email detailing what you want.

    I have a sample salt pot from a Stratasys and it is very impressive, much better than the Strat at Bath produces, it must be a newer model I think.

    I planned to mill PCBs but I was disappointed to learn from Adrian that fibre glass will right off a milling bit making one board. You need to use SRBP. It is so cheap to get one off PCBs made these days I might not bother. Especially as I intend to move SMT and you really need a good solder mask for that.

  4. Nop: Apparently Adrian was cutting too deep into his PCB.

  5. Hi nophead,

    Could the outgassing be caused by air bubbles trapped in the filament?

    In injection molding, the pigment is often added to virgin resin right at the injection press, and although I do not know the exact ratio, I am always surprised to see how little pigment is required. Also worth noting is that the pigment comes in solid, quite large grain form (in the order of a mm) and has to be crushed and thoroughly worked into the resin by the injection screw.

    If the same pigments are used to extrude 3mm filament, it would certainly require a relatively large and somewhat complex apparatus to ensure that the proper amount of properly crushed pigment would be used... far from sure that a homegrown contraption could reliably achieve this.

    Perhaps, before discarding colored filament altogether, it would be wise to discuss with your supplier to find out how they do add the color, and perhaps work with them to resolve the issues (or try different suppliers).

  6. Hello Bert,
    Yes perhaps it could be trapped air; I don't know anything about commercial extruders.

    Oddly it is very apparent in the first layer of the raft, less apparent on filament extruded into the air, and seems to disappear altogether when making an object. The main difference in each case is how much the filament is stretched after leaving the nozzle.

    I am left with black working fairly well but the objects are perhaps not quite as strong.

  7. Hello nophead,

    Have you ever tried drying the abs before you extrude it.
    Use you oven set at aprox 120F. For afew hours.


  8. Hi Geirhart,
    No I haven't tried an oven. The room with my machines in is very dry and IIRC this black plastic improved over time.

    I am not sure a gas oven will dry very well as it produces steam as a combustion product.