Golden wonder

My first attempt at extruding ABS onto hot Kapton had "all the stops pulled out" to make it stick, i.e. 120°C bed, nozzle height 0.1mm too low, very slow outline and infill on the first layer (4mm/s). The adhesion was very good so I decided to back off a bit. It is not a good idea to change more than one thing at a time but I did anyway. I got rid of the -0.1mm Z offset and sped up the first layer infill to 32mm/s, leaving the outline at 4mm/s. I also dropped the bed temperature to 80°C. That was too low, the corners lifted about 1mm during the build, but I think the part will still be usable.



The base is still glossy but you can see and feel some valleys between the extrusion "lanes". The next test was a binary chop with the bed at 100°C.



This is perfectly flat, even when off the bed for a day, but the extrusion lanes are still noticeable. The next test was at 110°C.



The extrusion lanes are gone in most places but a few are just visible. The first one that I did at 120°C has no extrusion lanes on it all, just some very slight graining from the Kapton tape that you can also see on the picture above. The tape lines and grain go from bottom right to top left. The extrusion infill slopes bottom left to top right and is only visible on the right hand side of the object. I think perhaps Z has to be a bit lower to get rid of them completely, but it is only important if you want to make something aesthetic, like an instrument panel, for example.

Of course there are the tape join marks. I used unbranded polyimide as it seems to be about half the price of branded Kapton. I got it from here, which is very cheap and free shipping if you don't mind waiting a while. You can get polyimide tape up to 250mm wide, but it is always on a 33m roll, so it gets very expensive. I have ordered a 150mm roll to cover the working area of HydraRaptor's build table. It was £53.71 from here, so very expensive, but a small price to pay for perfection! I don't know when it will arrive as post is a nightmare at the moment. I am still waiting for things from the 17th of December. Parcels are not being delivered because of the snow, so you have to go and collect them, but several letters and packets seem to have disappeared.

Here are all the tests side by side, notice the colour change with temperature, it is a bit exaggerated on the photo : -



I now have a full set of Mendel vertexes including two that I made in PLA that warped slightly (on a cold bed). I moved onto something more ambitious on the warping front: the Mendel x-carriage-lower_1off part. I don't think this is printable in ABS without a heated platform, or air stream, unless you use the apron method developed by Forrest Higgs. For this test I started the bed at 120°C and dropped it to 100°C after the first layer. The logic being that 100°C seems to be enough to prevent warping, but 120°C is needed to get a perfectly smooth finish. It takes a few layers before the temperature has dropped to 100°C as I don't wait for the plate to cool down.



Unfortunately the ancient version of Skeinforge that I use gets one layer wrong on this part. The layer has the central hole missing. The filament didn't span the void very well as it is a very big void, I have no fan running and there is a lot of heat rising from the bed. That caused some filament to stick up and collide with the head. It spun round 90° unscrewing it 1/4 of a turn. Amazingly it did not leak but the nozzle hole must be slightly off centre with respect to the barrel thread, so I got an offset in X and Y above the layer that went wrong. Still, the objective was to test warping and it came out totally flat.



The corners have a dimple that looks like an air bubble, but must be something to do with them trying to lift I think. Apart from these the base is as flat as glass and had it not been for the Skeinforge bug it would have been usable straight off the bed. I cut the membrane out with a knife and drilled through the blinded holes before taking these pictures.



I tried the x-carriage-upper_1off starting the bed at 120°C for the first layer and dropping to 90°C. Again Skeinforge got it wrong, not surprising as the topology is very similar. This time I also dropped the filament temperature to 220°C, so it spanned better and the head did not get spun. A longer snout on the nozzle might be a good idea to avoid collisions with build defects.



Again here it is with the membrane removed.



The corners lifted very slightly but the rest of the base is completely flat. It doesn't rock on a flat surface like an object made on a cold bed would. In fact, the raised corners made it easier to remove from the bed.



So it looks like 100°C bed temperature is the minimum to prevent warping when using Kapton. 120°C for the first layer gives a better aesthetic finish, perhaps with a small negative z-offset. Having the object kept warm seems to allow a lower filament temperature without losing strength. I used to build at 240°C and use 0.5mm for stronger objects. I can now use 220°C and 0.4mm with no sign of de-lamination so far. The lower temperature is good because the ABS out-gasses less and so smells less.

I can't recommend Kapton on heated aluminium highly enough. It has transformed my experience building with ABS completely. I no longer need a raft, which saves a lot of plastic, time and labour to remove it. My objects can be completely flat, smooth and glossy. Together with using a geared stepper extruder drive to completely eliminate ooze it means I just print an object, remove it from the bed and it is ready to use. There is a slight meniscus of plastic around the base, which you might want to remove with a file or a knife.

It has several advantages over acrylic: -

• Acrylic is a good insulator, so even 3mm reduces the surface temperature by about 15°C, making it take longer to warm up and harder to control.
• It tends to warp as it has a similar glass transition temperature to ABS.
• It can be hard to remove the object as it can be permanently welded if you deposit the ABS hot enough.

The way ABS sticks to hot Kapton is different. The Kapton does not melt at all so you don't get a weld no matter how hot the ABS is. I don't know what sort of bond it makes, but it is always peel-able.

While I have been writing this article a friend came up with a brilliant suggestion. Why not use non-adhesive Kapton film, clamp it on the table, possibly with a vacuum? When the build is finished just release it so it can be peeled off the object with ease. I realised that would enable a conveyor belt table to be made. People have suggested this would allow a machine to churn out parts unattended. E.g., stretch a band of Kapton over a heated plate and rotate it when the object is finished and has cooled. The object will then drop off the end.

I still have a couple of problems to solve with the heated bed. The heat spreads downwards and warms my X-Y table. It is not much, I haven't measured it but I would guess to mid 40's C. That is enough to expand the aluminium that the table is made from and open up a gap in the ways so that it has some play and starts rattling. I removed the foam-board to leave an air gap (the logic being that the movement of the table would generate some cooling airflow) and covered the top of the bed with aluminium foil to reflect the heat back. That helped, but not enough. I think I will need to blow cold air over the top of the table with a sheet of something like PTFE to cover the bottom of the heated bed.

The other issue is that having heat around the object rather than cold air blowing on it means that void spanning and overhangs don't work as well as they did. I think I need a jet of warmed air directed at the end of the nozzle to cool filament to freeze it quickly.

Hot metal and serendipity

I couldn't get to work today because we had seven inches of snow during the night and a couple more today, so I had an extra day of RepRapping.

So my extruder is back working after re-fixing the thermistor with some RTV silicone. I get a degree or two more temperature swing with silicone compared to Cerastil, so not ideal, but it is workable. I think the plastic has such a high specific heat capacity and thermal resistance that it probably averages out the temperature swings anyway.

I switched to ABS to make a change from PLA as I am now able to use my 5kg spool of oval ABS that has always been two wide for my previous extruders. The bore of this one is 3.6mm, which is actually a bit on the big side for 3mm filament. I think about 3.3mm would be the best compromise.

My first experiment was to see if I could extrude directly onto my heated aluminium bed. My initial attempts failed to stick, even at 110°C, but I found that I could lay down a raft. I always cool the raft before applying the first layer of the object (I also drop the temperature of the first layer to 190°C), otherwise it welds too strongly to remove. When I cooled the raft it detached from the bed, presumably because it shrinks.

I reasoned if I could get the raft to stick then I should be able to get the object to stick. The difference is I do the first layer of the raft at 4mm/s and have the head lower than I would normally, so that the filament is squashed more. I tried making the first layer of the object at 4mm/s and a little lower than it should be. It almost worked so I upped the temperature to 120°C and tried again. This time I was able to make one of Zaggo's whistles.



When it came to making the pea it got too hot and started moving around.



Normally I would use a fan on ABS to get small items to hold their shape, but obviously blowing cold air onto a hot base is going to waste a lot of power. The fix I have in mind is to blow a very small jet of air at the same temperature as the base and aim it just below nozzle. Hopefully by keeping the jet small I can avoid the sort of power that hair dryers use. Adding the heated bed has increased the power consumption of my machine by about 50W, which has more than doubled it.

When I cooled the finished object and the bed to 40°C, by running the fan, the object simply lifted off. At 120°C the ABS is like a soft rubber or gel. It clings to the aluminium, but will peel off with very little force. When it cools it becomes completely detached.



The bottom of the object is smooth and shiny and perfectly flat. I can actually see part of one of the swirls that are on my bed if I catch it right in the light. That means the plastic takes the texture of the base, so you could pattern and texture it in the same way as injection moulds.

The next thing I tried was a Mendel vertex bracket as these are big enough to warp. It managed the outline, but when it started doing the outlines of the holes the filament failed to stick so I aborted that build.

The obvious way to get more grip is to use a sheet of acrylic as many people report that works well. I have a couple of problem with that though. Acrylic is a good insulator so the temperature control becomes more difficult. It tends to warp unless it is held down at the edges. I don't have any bolts long enough to mount it on my bed with the frame on top. I ordered some 2BA studding last year, but all the post from just before Christmas has gone missing.

I looked around for a piece of metal with some texture and found some aluminium with a satin finish painted with metal primer, from a very old experiment. It looked promising to start with: -



But it soon snagged and started ripping it up again: -



However, as you can see, I held the plate down with Kapton tape and by accident part of the object was extruded onto the tape. It stuck well to the Kapton but was peel-able. This looked extremely promising. Kapton on top of aluminium could be the perfect bed material for ABS. It looks like it will be reusable many times, as masking tape is for PLA.



The bracket stayed perfectly flat during the build. I cooled it with the fan to 40°C. It was quite difficult to remove. In the end I put a penknife under one edge and tapped it with a hammer. It came off cleanly and with a perfectly flat base with a glassy appearance.



The only blemishes are the gaps in the tape, what looks like an air bubble in the tape, and the dent from my penknife.



The base is a slightly golden colour and that extends up for the first few layers so I think the bed was a bit too hot. I had it at 120°C and the first layer at 4mm/s, so I will have to back track a bit and see if I can get away with a lower temperature and faster first layer, but this is looking very good. No warping, no raft, a cheap reusable bed material and a mirror finish.

Extruder broke already

Well my best attempt at making a reliable extruder again resulted in one that only lasted a few weeks! The brass worm pulley that was pushed onto a splined shaft worked loose while extruding PMMA.



PMMA is quite hard work to extrude, but probably no worse than HDPE. On reflection splines into brass are not going to hold the massive force that occurs at 2mm radius. A better idea would be to have a boss on the side of the pulley and use a set screw onto a flat on the shaft. I would also add smaller diameter bosses at each side to meet the centre rim of the bearings. That would automatically position the pulley dead centre.

But to do that I would have to make a new pulley cutting jig and redesign the motor bracket to be a bit wider. I would need a working extruder to make the new bracket of course, so I decided to bodge the existing design.

I drilled out the centre of the pulley to 6mm and then reamed it to 6.4mm. I then turned a steel hub from a piece of hex pillar. I made it about a tenth of a millimetre oversized, added a chamfer to the hole in the pulley and forced it in with a vice, creating a very tight fit.



I didn't trust that to hold on its own so I left a hex flange on the other side and soldered it to the brass: -



Certainly not my best soldering, but bodging is bodging. The hub is twice as wide as the wheel and steel is harder than brass, so it should have a much better grip on the splines. I don't know if it will last or not. The constant back and forward motion of the anti-ooze fix means that if anything is weak it gets worked loose.

With the repaired extruder I made a third lamp shade clip leaving 1mm of the acrylic rod left above the pulley, how lucky is that?



Then I pushed my luck too far. When I bought the 3mm PMMA rod I also got a 2mm rod to compare results. Stiffness of a rod is a fourth power on diameter I think, so 2mm filament is five times more flexible than 3mm.



This would certainly be feasible to use in coils as it has a similar minimum bend radius to 3mm PLA, we just need to find somebody to supply it in that form at a reasonable price. 2mm rods are even more expensive than 3mm rods, £1.24 on eBay as opposed to £1.49, but are only 44% of the volume!

I decided to give it a try in my newly repaired extruder by printing a whistle. I had to scale it down because with 0.4mm filament it would use more than 1m of 2mm filament, so I printed the same g-code using 0.3mm filament and scaled the dimensions accordingly.

It managed to print a couple of layers and then the extruder jammed. I think the problem is that with a 3.6mm bore and 2mm filament there is too much of a gap, so molten plastic can flow upwards and freeze in the cold part of the tube above the taper. I think it would work fine with an extruder designed for 2mm filament. The drive mechanism just about works because although it does not have as much grip, it only needs 44% of the force that 3mm filament needs. The barrel and heater block would need a smaller bore though and could be made smaller. Similarly the smaller motor I used before would have plenty of torque, in fact a high torque NEMA14 should work.

So there are a lot of advantages to using 2mm feedstock like commercial machines do, BUT stiffness falls as a forth power, but force required only falls as a square law, so I expect soft plastics like HPDE, PP and PCL may buckle when being fed. Certainly the gap between the pinch wheel and the barrel entrance would need to be very small.

I fixed the jam by putting a drill down the hot barrel and hitting it with a hammer. That fixed it and I hand fed some ABS before reassembling the extruder. After assembly it would not work at all. The thermistor had shorted out to the metal work!

Nothing much to see from the outside, just a weird furry slimy deposit on the back of the AL tube and a green stain on the thermistor lead that was shorted.



I cannot get to the thermistor or heater without removing the PTFE cover, but that can't be removed without unscrewing the barrel, another slight design flaw. If I had tapped the stainless steel pipe all the way up I could just unscrew it from the AL tube that surrounds it, but it is really hard work tapping stainless steel.

I unscrewed the barrel while the extruder was hot to reveal this mess: -



The plastic that leaked when I first built the extruder has been stewing for weeks and has boiled down to something resembling bitumen. I expect the more volatile products condensed on the cold AL tube above it forming the Vaseline like deposit.

I couldn't tell why the thermistor was shorted because it came away with the PTFE cover. The Cerastil that I glued it in with seems to have decomposed in the chemical soup around it. My last few attempts at sticking thermistors with Cerastil have not been very successful. I am not sure if I mixed it to the wrong consistency, or if it is now too old to cure properly. It doesn't look any different, but instead of rock hard cement I seem to get something crumbly.

I cleaned it all up and stuck the thermistor back in with RTV silicone. I am sure it is not as conductive as Cerastil, but over such a short distance (between the thermistor and the wall of the hole it is in) I am hoping it will not have much effect.

I made the hole for it a bit deeper and opened out the top so it was big enough to accommodate the PTFE sleeving as well. That should keep it from touching the metal. It is surprisingly difficult to glue something into a small hole with a viscous glue. It is hard to get the glue to go down the hole without leaving an air pocket. A better idea might be to drill out a small screw, all the way through, fill it with glue from both sides. Then when it has set simply screw it into a tapped hole in the heater block.

I am waiting 24 hours for the silicone to cure now, so back to work tomorrow and less blog posts.