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.
