Monday, 19 May 2008

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.


  1. Congratulations! This is really exciting progress.

    Have you documented your custom hardware/firmware? Whatever system you're using, you've certainly got it working effectively.

  2. One other question: how much post-processing, if any, did you do to those objects? I don't see any hair on the internal voids, which has been a problem in the past.

  3. Wow, those are some well printed parts. I did my first CAPA print yesterday but I only got a partial layer done before the extruder drive snapped.

    Is your extruder drive shaft glued with JB weld? I tried to solder mine but the cable was repelling the solder so I had to resort to JB weld, it only lasted a couple of hours, I've glued it again using more JB weld hoping it will last a little longer this time.

  4. This is brilliant work, Chris. LOL! Hell, you're going to have cracked all the printing problems with ABS by the time I get Tommelise 2.0 up and running. It's been so great to have you in the project team. :-)

  5. kyle,
    No I haven't documented it all in once place, only in various articles in the blog. I don't intend anyone to build exactly the same machine, just use ideas from it. When I make the Darwin I will make a new set of electronics that are easier/cheaper to copy and fully document that.

    All the parts had a lot of string in the internal voids, you can see in the picture of the parts still on the bed. I run a drill through each hole and scrape the teardrops with a penknife. Very time consuming on the corner bracket! I looked a Adrian's anti-ooze design but I think it needs support material.

  6. greenarrow,
    My last drive was soldered but the first one was JB-Welded successfully as follows:

    I cut a cross in the end of my screw thread.
    I roughed up the inside surface with a small spherical grinder.
    I stuck it with JB Weld and held it in alignment in my lathe.
    After letting it cure at room temperature for 15 hours I then baked it in the oven for two hours at 200C.

    The JB-Weld outlasted the cable!

    I never had a problem soldering the cable. It was just the first drive screw I made was stainless steel.

  7. Nophead,

    I'm trying to find a source for your mystery "marketing board" extrusion surface board.

    I had a friend of mine, who works at a marketing company, for some pointers and I ended up here:

    How close to what you have do you think this might be?

  8. I am familiar with that sort of foam board and that is not the stuff.

    The advert board is about 3mm thick. Most of that from the back is a tough plastic, not very flexible. I wouldn't describe it as foam, it is too dense but not as dense as solid plastic. It then has a layer of double sided clear sticky stuff, a layer of paper with the print on and then a layer of sticky back transparent plastic on the front.

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  10. OK, how about this:

    Poly-Coated Poster Board : "Laminated with extruded polyethylene on both sides"

    It's used for political signs... I'll have to see if I can snag one. :)

    Some links:

  11. No not even close. PE sticks to nothing and this is indoor material not outdoor. I will post an article with the burn tests in a few minutes.