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.

Cat's cradle

I hit another milestone today: HydraRaptor made the first part that I designed myself, using the ArtOfIllusion application. It is the first time I have done any 3D modeling and it is much harder than I thought it would be.

Adrian Bowyer has written a set of hints and tips here and I needed to use every single one of them. I don't know how anybody can use ArtOfIllusion without his guide.

The reason it is difficult is that you have to build up complex 3D shapes by composing primitive shapes like blocks and cylinders with boolean operations like union, intersection and subtract. That is fine but you are not allowed to do boolean ops between objects that have coincident or tangential faces. If you do, then you create non manifold objects which cannot be converted to STL files. However, you generally do want join things with a common faces. Here is the object I designed :-


It is a cradle to support the heatsink of my high temperature extruder design. If you take one of the upright legs as an example you see it's a cylinder that meets a rectangular lug with a common face at the bottom and tangential joints at the sides. It also meets the cone on the top with a common face. All of these are not allowed: I had to make the cylinder slightly too long and slightly bigger in diameter before unioning it with the cone and the block. That left it protruding slightly at the bottom, which is solved by subtracting a large flat rectangle from the base.

Another problem is that if you have long strings of boolean operations the application becomes very slow doing anything. That is solved by converting the results of boolean operations into triangle meshes. It solves the speed issue but then for some reason boolean operations on the resulting triangle mesh only offer intersection and subtraction. To restore the possibility of union you have to optimise the triangle mesh in the solid editor. Not hard, but not intuitive and very time consuming.

I tried to make the object in HDPE with my lash up stainless steel extruder but it was not reliable enough. This was the first attempt which stopped short due the filament slipping in the pump: -



I also realised at this point that two of the columns were too close to the heatsink. Other attempts resulted in either the filament slipping, or the GM3 clutch breaking free. I had stuck it with super glue, but that does not hold very well, so in the end I welded it with my soldering iron.

It takes an enormous amount of force to extrude with the stainless steel barrel and I am beginning to think the idea may be fatally flawed. I think that because there is a slow temperature gradient down the barrel you have a point where the filament is only just molten so it is very viscous, so is hard to push past that point. With the PTFE barrel the temperature will fall away quicker and the walls are also much more slippery.

I will try again with a much shorter barrel, but to get the object made, I put my old extruder back together and made it in ABS: -



As you can see lots of stringing due to extruder overrun, but easily cleaned up with a penknife and drill. It is much easier to remove strings from ABS and HDPE objects than it is from PCL.



I think the dark lines on the posts are grease from the extruder bearings.

All in all I think it worked very well: this is my first ABS object, other than test blocks, and it is also the largest and most complex object I have made so far. It is a bit warped underneath because I didn't use a raft and it is 100% filled. As it happens the underside does not matter at all for this part. It took just over 2 hours so I went for a walk and left it to it.

I designed the shape for HDPE, the objectives are for it to hold the heatsink rigidly and not restrict the airflow too much. Had I designed it for ABS I would have made it a bit less chunky.

Here it is with the heatsink installed: -



Next I need to make a new extruder support bracket / clamp to mate with this part to continue my attempt to make the high temperature extruder.