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.

Experimental extruder

I want to see how much of the Darwin design I can make out of HDPE as that is the plastic I have the most of and is the easiest to get hold of. It should also be the cheapest but I think I got a very bad deal with mine.

To extrude HDPE quickly, without losing accuracy, requires a fan blowing on the work piece while extruding at around 240°C. The PTFE insulator in the extruder starts to lose its strength under these conditions and it also extends about 0.5mm due to thermal expansion. The JB-Weld heater insulation also degrades rapidly. To address these problems I am working on a design using stainless steel as the insulator, which I first blogged here. Here is a second lash up I made to progress the idea :-



At the bottom is a brass nozzle made by the man himself, Adrian Bowyer, and is described here. It has already been superseded with the anti-ooze design shown here.

Above that is a brass barrel that came from BitsFromBytes, with my experimental Cerastil heater on it. I attached a thermistor to the barrel with JB-Weld.

The brass barrel is screwed into the end of a 1/4" stainless steel tube. The other end has been tapped with a 1/4" UNF thread and screwed into a small north bridge heatsink from a PC motherboard (40 x 40 x 15mm). I drilled through the centre and tapped it. To lock it in place and give a good thermal connection I made a square nut from a piece of 10mm aluminium bar. I spread heatsink compound on the threads.

The top of the stainless steel tube is screwed into an old PTFE barrel to join it to the pump. The barrel had swollen so that it wouldn't hold an M6 thread anymore, but fortuitously it seems to have swollen just enough to match 1/4" UNF.

This is by no means the final design, it is far too long and flimsy, it's just to test the concept using existing parts.

I also wanted to try insulating the barrel and nozzle with PTFE. I made an end cap that fits over the nozzle by plunging an 8mm end mill into a 12mm PTFE rod :-



The idea of this is to keep the fan wind off the nozzle and also give it a non-stick surface so that when filament curls upwards and will not stick to it. I also insulated the stainless steel tube with a piece of 12mm PTFE rod with a 7mm hole drilled through it. Here is the completed assembly :-



The gap in the PTFE where the heater and thermistor are and where the wires emerge is covered with fiber class wool. I hate the stuff, I only have to think about it for it to make me itch all over. It is a much better insulator than PTFE though, but I wanted something smooth and slender to not disrupt the airflow from the fan too much.

The wires are sleeved with PTFE insulation and then plugged into a floppy drive connector. So everything at the hot end is good for about 300°C.

How well does it work? Well it took me a long time to be able to get it to extrude HDPE semi reliably. Thermally it works well. With the fan off and the barrel at 250°C the heatsink only gets to about 45°C, easily cool enough to mate with HDPE, ABS and probably PLA and PCL as well. With the fan blowing it cools down to room temperature. The heater power goes from about 60% to 80% so the insulation works well enough. A better idea might be to lag the pipe with a thin layer of fiberglass wool and then wrap it with PTFE baking parchment to give it a smooth outer surface. Or maybe an outer metal pipe with fiberglass in between.

Mechanically it is not that great. It seems to a need lot of force to extrude. I had to open up the hole in the nozzle from Adrian's 0.4mm to my standard 0.5mm. I also had to up the temperature to 250°C. I think this is mainly due to where I am measuring it and how I calibrated the thermistor. Previously I measured the nozzle temperature and calibrated it with a thermocouple inserted into a hole in the nozzle. With this version the thermistor is in a notch on the surface of the heater barrel and I calibrated it with a thermocouple inside the empty barrel. Looking at the value of beta that I got I think that it is considerably hotter inside the barrel than the thermistor is outside. I am not sure how this is. With the heater on the outside of the barrel I can't see how the inside could be hotter. Perhaps the thermal connection of the thermistor to the barrel, via JB-Weld is not as good as it it could be. When sited in the acorn nut nozzle it was half buried in a hole.

Even with the nozzle removed it is quite hard to extrude 3mm filament by hand. Part of this has to do with how long the total barrel is and the fact that it has three joints. The inside of the stainless steel barrel is not as slippery as the PTFE. It might also be the case that the molten section extends further up the barrel causing more viscous friction. I plan to shorten the whole thing considerably: I will combine the clamp with the right angle bracket and take the tube right up to the base of the pump. I will support the heatsink with a cradle structure resembling an upside down table. More importantly, I will shorten the heater barrel by combining it with the nozzle and screwing the tube into it. Making it from aluminium, which is two and a half times a better conductor than brass and easier to machine, should make it easier to get a consistent temperature measurement.

As there is a continuous temperature gradient down the stainless steel, the point at which the plastic melts will be about halfway up so I think the heated nozzle can be quite short indeed. The limiting factor is how long it takes the heat to get to the centre of the filament with the very poor thermal conductivity and high specific heat capacity of the plastic.

Here is an HDPE version of the opto bracket with my best PCL version behind :-



I have no idea why it is so grey. It is not as neat as the PCL one but most of the errors are due to blobs forming when the extruder moves between extruding. These cause the nozzle to be displaced sideways when it gets close because it is so flimsy. Shortening it and supporting it properly will improve matters for sure. I also need to incorporate Adrian's anti-ooze valve somehow.

Fun with Python and G code

The current RepRap host software is a monolithic Java program that imports STL files, lets you place the objects to be made on the table, slices and dices them and controls the machine.

In my opinion the slice and dice code should be a separate program from the machine controller. Its inputs should be the 3D model in STL format plus the filament dimensions and the output should be an XML file with extruder paths grouped into layers, outlines and infills. The machine controller then reads the XML and controls the speed, temperature, fan, nozzle wiping, cooling delays, etc, according to the selected material and the machine characteristics. A third layer of software should be the communication protocol to the slave device, e.g. SNAP or G code over serial, USB, Ethernet, etc.

I have moved a little way towards that model by making my machine accept G code from the RepRap host or Enrique's Skeinforge script. I throw away most of the G codes, looking at just enough to build be an internal representation of the extruder path. This is simply a list of layers, which are lists of threads, which are lists of points. From that information I can control my machine, make animated GIFs or preview the paths in a GUI. All of this is trivial in Python.

Here is my first cut at the preview GUI: -



The preview shown is from G code generated by the RepRap host, and here is the object it made: -



Behind is the same object made from G code generated by Skeinforge.bsh. The RepRap one has sharper corners and the infill is a bit better but the Skienforge one is faster to produce because it has sparse infill.

Here is a video of it being made: -


Here is my first attempt to make Vik Olliver's shot glass: -



When it got to the stem the fan could not get the heat away faster than it was arriving and the whole thing became a molten mass. I fixed that by slowing down the extruder to 8mm/s when the layer gets small: -



It took five hours to process with Skeinforge and an hour and a half to build. I couldn't get the RepRap host to process it.

Python & Beans make object

Having got bored of making rectangular blocks for months I decided it was time to hook up my machine to the RepRap host software so that I could make arbitrary 3D objects from STL files. My original plan was to hack the host code to replace the serial comms with Ethernet and cope with the differences of my machine from the RepRap Darwin. Zach Smith added a G code back end so I decided to just add a G code parser into my Python to save me having to modify the host.

In the meantime Enrique Perez published a plug-in script called Skeinforge.bsh for ArtOfIllusion that also converts 3D objects to G code extruder paths. It is written in the Beanshell script language, which is Java like. I decided to try both approaches, as in theory a G code parser would allow me to use either.

Enrique posted some new scripts that process G code and drive the RepRap hardware using a Python SNAP protocol driver written by greenarrow, so I didn't even need to think about writing a G code parser, I just cut and pasted a few lines from Enrique's.

Before letting it drive my machine I thought it would be a good idea to look at the paths on screen. I knocked up a little script which used my HydraRaptor simulator to draw them. The script is just a few lines of Python that use TkInter.

It was soon apparent that Enrique's code had a bug that left off some of the outline, but apart from that it looked very promising because it has the ability to do sparse infill. That speeds up building objects, saves plastic and reduces warping so it is very worth while. Not only that, it had a novel infill pattern. Instead of parallel lines like this: -



He moves the ends together so that the outer wall is stronger: -



This looks like a good idea because it makes the outer wall effectively two layers thick but probably gives a bit less warping than a second continuous layer would give.

In order to communicate the results to the forums I came up with the idea of making an animated GIF showing all the layers in sequence. This turned out to very easy using Google and Python. The Python Image Library (PIL) can make GIF files and I found a script called gifmaker.py which takes a list of images and uses PIL to calculate the deltas and write out an animated GIF.

Enrique fixed the bug very quickly, here is the sliced extruder pump body: -



The red lines are moves without filament flowing (ideally) and the each new section of filament is a different colour.

And here is the same object sliced by the reprap host :-



A side effect of Enrique's algorithm is that the corners get rounded, however I don't think that matters too much because the filament has a minimum bend radius anyway. The main downside is that beanshell script is very slow, so it takes longer to slice than it does to extrude at the moment. A faster PC will probably sort that.

The first object I tried to make was this opto mounting bracket from the RepRap Darwin: -

I choose it because it is small, so does not take too long, but reasonably complex with a horizontal hole. Here is the sliced path from Enrique's script: -



And here is my first attempt at making it: -



This is PCL extruded onto MDF, 0.625mm filament extruded at 10mm/s with the fan on, no interlayer pauses.

A bit hairy due to the extruder not being able to stop the filament flow quickly, but I was quite pleased with it for a first attempt. It is too tall due to a bug in my code and its not the latest version, which has teardrop shaped holes to make the overhangs less than 45°.

Here it is cleaned up a bit: -



It is 50% filled which is probably not appropriate for this size object in PLA but that part is fully functional I think.

Enrique was pleased to see it as he doesn't have a machine to test his code with. A perfect partnership, he writes all the hard bits in beanshell script and I write the easy stuff in Python!

Basket case

I was using two old component spools to hold my feedstock, see all-wound-up, but I don't have any more so now that I have four polymers I decided to give Vik Olliver's design a try. It has the advantage that you don't have to spool all the filament on, you can just drop in an open reel if that is how your filament comes.

This is my take on it: -



The uprights are 15x15mm aluminium angle. The beam across the top is a piece of 20x10mm channel. The bearing is a standard ball bearing and I reduced its internal diameter with a couple of bushes I had lying around. I then used a bolt with holes through the head as an axle. I found it in the road while I was on a walk wondering what to use. That is the third piece of HydraRaptor that I have picked up in the street.



The baskets are £4.21 in B&Q and have a plate in the bottom with a central hole just right for feeding the filament through. As the machine pulls the filament from the centre of the reel, the basket rotates to prevent it becoming twisted.

It works very well and has the advantage I can buy as many baskets as I have plastics and just take them on an off as needed.

It also allows the filament to rotate in the extruder but ironically, since I tweaked my extruder, PLA no longer feels the need to rotate. Presumably it rotates if the friction between the screw and the plastic is higher than between the plastic and the filament guide. I think that gives a clue to which of my tweaks made all the difference in reducing the extruder torque needed. I think it was adding the washers to space the top of the pump apart so that the screw bites in progressively and sharpening the screw thread.

More PLAying

The tweaks I made to my extruder dramatically improved its ability to extrude PLA. I am not sure which one made all the difference or whether they are all needed. I can now extrude at my target 0.5mm filament 16mm/s with about 75% motor PWM duty cycle. That is less than I needed for ABS before the tweaks.

I can also extrude at lower temperatures. I think 180°C is a bit on the hot side as the plastic is very runny at the temperature. It will flow out of the extruder under gravity and has negative die swell. At 140°C it behaves more like the other plastics and swells to 0.6mm, which is very low.

In a previous article I stated that I could not make sparse filled objects because the filament slumped too much. With the reduction in temperature and increase is speed I now can. Here is a 50% filled block: -



It is still very strong. I expected the warping to be less but I have switched from MDF to balsa and I think that increased it. The balsa I have is only 2mm thick and was only stuck down with masking tape. I might try gluing two pieces back to back with the grains at right angles to get it stiffer.



So now with a slightly tweaked extruder I can do PCL, PLA and ABS at 0.5mm @ 16mm/s. I had to slow down for HDPE to prevent thermal damage to the extruder.

To get good definition at high speed I extrude with a fan running. The fan cools the nozzle which causes more heater power so the barrel temperature rises to the point where PTFE goes soft and the JB Weld turns to dust. PCL and PLA are no problem because the temperature is less. ABS does not seem to need the fan.

I plan to make a PTFE cover for the nozzle which will probably insulate it well enough and hopefully stop filament sticking to it and burning.

Fantastic PLAstic

I have managed to get the GM3 to extrude PLA reliably with the following tweaks: -

• I locked the clutch as per Solarbotics instructions.
• I lubricated the GM3 with silicon grease as per Adrian Bowers suggestion.
• I sharpened the thread a bit with a half round file following Vik Olliver's instructions.
• I spaced the top half of the pump slightly further apart with some thicker washers. That gives the thread a gentle lead in.
• Plenty of oil on the filament.
• I throttled back the flow rate to 3/4 of the rate I normally use (π mm³/s).

The motor runs warm, but not alarmingly so. For some reason the filament is no longer rotating in the extruder.

My first attempt curled away from the MDF bed so I used 2mm balsa wood as Adrian has been using for PCL. That worked well so it's good that we can use it for both. I have yet to try it with HDPE and ABS.

I made my standard test block with 0.5mm filament extruded at 180°C (at the nozzle), layer height 0.4mm, pitch 0.6mm, fan on constantly. The results are excellent: very good filament compliance, i.e. sharp corners and flat sides, excellent layer bonding.

It is slightly more warped than my first test. That is probably because balsa is softer than MDF.



No warts on this one!

Locking the Solarbotics GM3 clutch

All the adverts for the Solarbotics GM3 gearmotor say the clutch can easily be locked but don't say how. I emailed them today and got a quick reply from Dan: -

Locking the clutch is actually very easy... we should make an effort to get the instructions on-line. All you have to do is glue the little plastic clutching mechanism in its cavity, but to ensure a good contact you should first wipe it out with rubbing alcohol to get the grease out, then score the surface up a bit with the tip of an Exacto knife and then you can use either super glue, model cement, or epoxy to lock it.

So that is what I shall try next to see if i can get it to extrude PLA without warts.

A day with PLA

Adrian Bower has kindly given me a small sample of polylactic acid (PLA) filament to evaluate and some parts to make the new geared extruder are on their way. Being a bit too impatient I decided to try extruding it with my current extruder.

Each polymer I have tried so far (HDPE, PCL and ABS) has had very different characteristics and PLA is very different again. At room temperature it is very hard and brittle and is completely transparent. At somewhere between 50-80°C it has a glass transition temperature above which it becomes a rubbery jelly. If you put it in boiling water and then pick it out with tongs you can bend it as much as you want and when it cools it will set in that shape. These are the two bits of 3mm filament I showed earlier when I had bent a 150mm piece double and it snapped :-



After dipping them in boiling water I could tie knots in them but I had to be quick because it hardens in seconds. If I return them to boiling water they untie themselves and return to being a straight rod.

PLA melts at about 175°C, the highest of all the polymers I have tried so far, where it transitions from jelly to a liquid with the consistency of a thin syrup. If you extrude it quickly into mid air it sets almost instantly and forms a filament, but if you extrude it slowly it forms drops that drip from the nozzle like water from a tap, but are solid when they hit the deck. Very different from HDPE and ABS which extrude more like a paste. Like PCL, PLA is sticky when molten so it sticks to MDF quite well, unlike HPDE and ABS.

Getting it to extrude from the original extruder is next to impossible, I really should have waited for the gears. The problem is that the extruder pumps the filament by cutting a thread into it. PLA is so hard that it needs an enormous force to press the thread into its surface. That is no problem with the springs I have, but it also seems to have a high coefficient of friction, so the torque required to turn the thread is then too much for the GM3 gearmotor and its clutch slips.

Recently I had an idea to cut the torque requirement by shortening the thread. The reasoning goes like this: -

A substantial part or perhaps most of the force required to push the polymer is not the extrusion pressure but the lateral friction of pushing the filament through the filament guide and the tangential friction of cutting the thread. Both of these are equal to the respective coefficients of friction multiplied by the force exerted by the springs. But the force required to achieve enough pressure to push the thread into the plastic must be proportional to the length of contact. So once you have enough thread to push the filament without shearing off, any more is counter productive. It requires more spring force, which creates more friction, which makes the filament harder to push, a vicious circle.

Ian Adkins put the theory to the test and reported he could still extrude PCL with only 7mm of thread. The motor current dropped from 240mA to 190mA. The no load current was 62mA so that indicates about a 50% reduction in torque required. Not being as brave as Ian, I reduced mine a bit less radically to start with. I roughly halved it: -



I used a couple of washers at the top set of screws to space the pump halves apart, to keep them parallel, and just tightened the bottom pair of springs. That did reduce the torque required but even with plenty of oil the GM3 clutch was still slipping. It is a good modification because you save two springs, hence two less adjustments, and the extruder is quicker to strip down and rebuild. Plus future versions of the pump can be made much shorter.

All the adverts for the GM3 say the clutch can easily be locked but don't say how. I opened the gearbox and found the clutch is inside the last gear wheel. I tried putting some bits of thick wire into it to stop the springy bits from compressing.



That worked for a while until one fell out. I thought they would be trapped by the lid but seemingly not. My next attempt was to stick them in with super glue. That worked but the clutch still slips somehow.

Another issue with PLA is that the filament likes to rotate in the pump. Other polymers do that as well, but if you hold them so they can't rotate they still extrude. With PLA, if you don't let it rotate it slows down the motor. Presumably by letting it turn it removes the tangential friction leaving only the sliding friction. The problem with letting it rotate is that I think it means the forward motion is less so the extrusion rate is not what it should be.

When the clutch slips it jumps one notch. Because my shaft encoder is on the output shaft the firmware makes up the difference. If it does not happen too often I still get the right volume extruded. It does however cause a shock wave which makes a blob in the extruded filament. I decided to try and make an object anyway with it slipping occasionally.

Because I only had 8m of filament I decided to try a sparse filled object first. That does not work with PLA because the unsupported filaments sag: -



However, this messy object has a perfectly flat base so shows some promise.

Next I tried a 100% filled block, extruded at 180°C (at the nozzle) 0.75mm filament at 8mm/s, layer height 0.6mm, pitch 0.9mm, fan on continuously. A bit slow and course compared to my other tests but I always start slow and move up in speed.



During this build the clutch started slipping once or twice per revolution causing the warty surface. Near the end it started slipping continuously so this is only about 18.5mm high rather than my standard 20mm test. Never the less, it only has 0.19mm warping after more than 24 hours making it the least warping yet for a solid object. Added to that it is probably the hardest material of the four.

PLA's main downside is the low temperature at which it goes soft, not much better than PCL, even though it has a much higher melting point. In this respect it is very like PVC which also has a high melting point and a glass transition around 80°C.

Here is my warped league: -



I think I can explain why plastics are good and bad for warping. It is simply how much they contract between the point they go solid and room temperature.

HDPE has a high freezing point and high thermal coefficient.
ABS has a slightly lower freezing point and a low thermal coefficient.
PCL has a very low freezing point.
PLA has a glass transition point not much higher than room temperature.

If the object can be stuck to a rigid base while it is cooling then the warping is reduced.

My theory of needing to extrude at twice the melting point minus ambient certainly does not hold for PLA, otherwise I would need to extrude it at 330°C. I get very good bonding at 180°C. It may be that if I extrude very fast it would need to be hotter but I expect it would decompose at 330°C. It may be that the glass transition makes the theory invalid or maybe it is plain wrong. It does seem to be true for polymers which are paste like and not sticky, i.e. ABS and HDPE. PCL and PLA are both sticky when molten. I.e. they will stick to things like glue does whereas HDPE will only weld to things like itself. It has no adhesive quality.

I haven't entirely given up with making the non geared extruder work with PLA. The problem with the geared version is that it can't keep up with my extrusion speeds. I can try reducing the thread even more. I can try sharpening it as Vik Olliver has done. I can find a solution to locking the clutch. The motor does get quite hot so it probably won't last long. I do have three more to burn through before I find a better motor. They only seem to last a couple of weeks in my machine anyway.

Wear and tear

Half way thorough my evaluation of PCL the extruder's flexible drive coupling started to break up again. When I moved to ABS that was the final straw: -



The first one I made was only 2.5mm cable. This was a 3mm one from BitsFromBytes. I replaced it with some 3.2mm cable from B&Q. I drilled the hole out to 3.3mm so it is a snug fit. I also soldered it while it was held in alignment by my lathe so it is very straight.

I think the force required to bend a cable goes it with the fourth power of its diameter so this one is considerably stiffer. Possibly some of the motor torque is wasted in flexing it.

The good thing about the shaft I got from BitsFromBytes is that it solderable, so it makes it easy to replace. My original shaft was stainless steel so I had to glue the cable in with JB Weld, making it harder to replace.

My next extruder will be direct drive!

I also wore out the brushes on a second GM3 gearmotor. I replaced it with a 12V version which has to be ordered by phone from Solarbotics in Canada. It looks the same except that it has a black end cap instead of a white one. It runs a bit quieter but I don't know if it will last any longer. As you would expect the coil resistance is higher so the current through the brushes will be lower.

Stuck fast

In the previous article I said I could use both sides of the advertising board as bed material. Well the back of the board is a thick layer of a leathery sort of plastic, I think it may be PVC. I can certainly deposit ABS onto it, but it sticks so well and is so tough I found it completely impossible to remove.

The front of the board has a very thin layer of plastic with paper behind. That may also be PVC, but being so much thinner is easy to peel away.



Interesting that ABS appears to bond so well to PVC, if that is what it is.

ABSolution

I have been testing Acrylonitrile butadiene styrene (ABS) in the RepRap extruder and I have to say it works rather well. It is surprising how different each plastic I try is. For example, if I fold a short piece of filament double and let it go this is what happens :-



PCL is rubbery and springs back almost straight, HDPE is a lot less springy. ABS bruises when bent sharply and is a more opaque cream colour rather than white. PLA is transparent like glass and breaks when bent through a small radius. The progression from top to bottom is from rubbery to brittle. I think the reason for this is that ABS and PLA are below their glass transition (Tg) at room temperature, whereas with PCL and HDPE their Tg is well below 0°C.

I have a bit of a routine now for getting my machine working with new plastics. First I look at the extruder performance at different flow rates and temperatures. Then I have to experiment to find a bed material it will stick to. After that I make test blocks to fine tune the temperatures and find the best speed and filament diameter to build with. Finally I look at the warping with different infill densities.

So here is the flow rate versus motor duty cycle extruding at 190°C measured at the nozzle (the other plastics are at different temperatures):-


As you can see ABS works the motor harder. Part of this is due to the fact that it has to run faster (for the same flow rate) as the ABS filament I have is only 2.75mm rather than 3mm. However, I think that it is due mainly to the increased force and friction required to cut the thread in the pump. At first I had a lot of problems with the GM3 motor's clutch slipping. I got it working reliably by loosening the top springs and just tightening the bottoms ones. I also have the filament running through a felt washer soaked in oil, which I had to add when doing PCL.

Here is how the filament diameter varies with flow rate :-



ABS has much lower die swell than HDPE and is quite a bit better than PCL. That makes it good for extruding fine filaments at high speed.



I think the graph above shows that the viscosity increases a lot as the temperature drops but the motor duty cycle remains pretty constant showing that most of the torque is used overcoming friction in the pump.


I tried using PP and MDF as bed materials but ABS does not want to stick to them. Unlike PCL which stays molten for a very long time, ABS filament sets soon after leaving the nozzle. PCL and HDPE turn transparent when they are molten but ABS does not. I think its specific heat capacity is quite low compared to HDPE so the workpiece cools quite quickly and I can get away without a fan.

The best material I have found for a bed is plastic laminated board. My wife bought me a big sheet of it for 10p when I was experimenting with HDPE. It was cheap because it was scrap advertising material. She was disappointed when HDPE did not stick to it, but is made up now that I have found a use for it. I think possibly it works because the thin plastic lamination is actually polystyrene.



I can extrude ABS on to it and it sticks well enough without needing a raft. To remove it I cut the lamination around it with a penknife and pull the surface off. I can then peel the lamination off the base of the object leaving a clean finish. It works well but I don't know where to get any more and it is single use although you can use both sides.



It is good not to need a raft because while I can remove an HDPE raft with scissors or a sharp knife, ABS 1mm thick is too hard to cut off easily.

My theory about welding temperatures is that you need to extrude at least twice the melting point (105°C) minus ambient. That works out at 190°C. In practice I needed to go a bit hotter to get a satisfactory bond between the layers at high speed. I settled on 220°C for the first layer and 200°C for the rest of the object.

Layer bonding can be quite variable with ABS. It is easy to make objects which can be peeled apart again. I think this is because even at 220°C ABS is quite paste like and less fluid than the other plastics are at their critical weld temperature. That makes the filament contact points very tangential and so smaller. Plastics that are more fluid slump and get a bigger contact area, hence a stronger weld. Also, the time the plastic is in contact and above the melting point determines the amount of fusion. I think if I spend some time on this (and with the other plastics) I will be able to use plastics as their own support material for making overhangs. The layer height will be crucial to making this work so it will have to wait until I have replaced the PTFE insulator with stainless steel.

Here is my standard warp test shape: 40 x 10 x 20mm block made with 0.5mm filament at 16mm/s, layer hight 0.4mm, filament pitch 0.6mm: -



The warping figure I got after leaving it a few days was 0.38mm compared to 0.53mm for 100% HDPE and 0.21mm for 100% PCL. 50% filled ABS gives a figure of 0.15mm which is the lowest I have measured yet and ABS is still very strong at densities less than that, whereas PCL is not. Also, this was without a raft which gives worse figures for PCL and ridiculous warping for HDPE.

The filament is very soft and compliant when it is molten, with no spring in it, so it goes where the head leads it and produces good definition. Here is a top view showing how accurate the corners and infill are: -



Here is a 50% infill pattern: -



This is very accurate compared to the same pattern in HDPE shown here. 25% fill is also very good and the object remains strong: -



For some reason my 9M pixel camera doesn't like taking close ups of white things.

When I was extruding thick rafts I noticed some bubbling of the surface. I think this is due to absorbed moisture turning to steam because ABS has ten times more water absorption than HDPE. Oddly, it does not happen when extruding the object so is not a problem, at least with the current weather conditions.

ABS smells a little when it is hot but not enough to be objectionable.

My acorn nut nozzle, with the very shallow exit hole, is very incontinent with ABS as it was with HDPE but not with PCL. This means that even though I wipe it clean with the toothbrush it has extruded another few millimeters by the time it gets to where it has to start the object. The problem with that is that it sets so fast it is solid when it meets the table so will not stick and stops the following filament sticking. I am hoping the latest nozzle design will fix that.

On to the same test with PLA before I alter the extruder.

Chalk and cheese

I was curious to see how polycaprolactone (PCL, trade name CAPA) compares to HDPE. I bought some from BitsFromBytes a while ago but have not had chance to try it yet. It is the plastic RepRap was designed for and there is plenty of evidence on the web that it does not warp like HDPE does.

The first test I did was to run the extruder at various flow rates and look at the filament diameter and the amount of motor power required. Although I think I only need to extrude at twice the melting point minus ambient (~100°C) to get it to stick to the next layer, the extruder seemed to struggle a bit so I did the tests at 140°C (measured at the nozzle).

This is how the motor duty cycle varied with demanded flow rate: -

The first surprise: although the torque required for PCL through a 0.5mm hole starts off lower than HDPE through 0.5mm, it actually rises faster with flow rate and ends up needing more torque than HDPE through a 0.3mm hole. This became a problem when I started to try to make objects because the clutch in the GM3 gearmotor kept slipping. It never slipped when I was extruding HDPE. I tried loosening the pump springs to the point where the filament started to slip and I tried backing off the flow rate but to no avail. I even replaced the GM3 in case the clutch was worn. I solved it by lubricating the filament with oil, a tip I got from Vik Olliver who found it necessary for PLA, the other RepRap plastic. I did that by passing it through a felt pad with a hole in the middle, with a few drops of 3 in 1 oil applied.



I found the felt disc in the road, I have no idea what it is, but it I thought it might come in handy someday. If anybody recognises what it is please let me know.

The oil is very effective, a few drops lasts for many hours. Previously I was using PTFE spray to lubricate the pump for HDPE but that required opening the extruder occasionally.

The amount of spring pressure required for PCL is much less that HDPE, presumably because it is much softer so less force is required to make the screw bite.

Next I measured the die swell: -



PCL has far less die swell than HDPE, such that PCL filament from a 0.5mm hole is actually smaller than HDPE from a 0.3mm hole. Reducing the hole size to get smaller filament gives diminishing returns because the die swell as a percentage goes up with a small hole.

I also looked at how motor torque and die swell are affected by temperature. Once I had fixed the clutch slipping problem by lubricating the filament I had no problem extruding at low temperatures.


Quite a big variation in die swell indicating the viscosity changes a lot over this temperature range. This is also obvious looking at the filament. In fact some of the reason why it gets thinner at high temperature is that it is so runny that gravity probably stretches it. That may account for the inflection in the graph, or it may just be measurement error.

The next graph was another surprise: motor duty cycle plotted against temperature: -



This is essentially flat, the slight rise is probably due to the motor windings getting warm, increasing their resistance and thus lowering it's torque. The 160°C reading was taken after the motor had had time to cool down again. This is a good illustration of why a shaft encoder is necessary to control the feed rate.

So if the viscosity is changing, but it has no effect on the motor duty cycle, I have to conclude that most of the torque is required to overcome the friction in the filament guide. That also explains why more torque is required to extrude PCL than is required for HDPE, despite it being less viscous and requiring less spring force. If I rub my fingers over PCL it is obviously a lot less slippery than HDPE.

Having got the filament to extrude properly the next task was to get it to stick to the bed. I found that PCL does not stick to the PP board that I used for HDPE. I expect that is because it is too low a temperature to form a weld with PP.

The RepRap Darwin machine use MDF so I decided to try that.



I assumed that I could dispense with the raft that I lay down for HDPE, but that was not the case. I found that PCL objects still curled away from the base, so I went back to using the raft. That holds the object flat but is a pain to trim off. For some reason it is easier to cut HDPE with scissors even though it is stronger. One downside of using MDF is that some of it comes off with the object so it is not completely reusable and it leaves wood fibers embedded in the base of the the raft. This is nowhere near as good as a polypropylene bed is with HDPE. That peels away undamaged and leaves no trace on the object. I think I need to do some more experiments to find a similar solution for PCL.

The first test shape I made came out very grey. It must have picked up some contamination in the extruder but the only thing that should have been in it was left over HDPE. Perhaps for some reason white HDPE plus white PCL makes a grey plastic.



It was very flat to start with but over a few days it has warped slightly. The corners are lifted about 0.21mm compared to 0.53mm for a 100% filled HDPE block. That is also better than my polyurethane filled 25% HDPE block which had 0.25mm warping.

I think the reason PCL shrinkage is so much less is that although it melts at 60°C, it doesn't harden again until it is around 40°C. That means after setting it only cools a further 20°C back to room temperature. In contrast HDPE probably goes hard around 120°C so it cools a further 100° after that. Even if they had the same thermal expansion coefficient, PCL would shrink five time less.

I did the first test at 8 mm/s because that is as fast as I could go with HDPE with my current nozzle. However, I found that I can go at 16mm/s again with PCL. I have a fan running continuously to cool the object because otherwise PCL takes for ever to set.

I made a second block and that came out white: -



It was extruded at 100°C, 0.5mm filament at 16mm/s, 0.4mm layer height, 0.6mm pitch. The reason for having the height so much less than the pitch with HDPE was that the object shrinks in height while it is being built, otherwise the nozzle ends up extruding into fresh air. Perhaps with PCL I can get away with a smaller filament aspect ratio.

Here is a longer test piece with a 25% fill HDPE equivalent underneath for comparison: -



The PCL shrinks far less but at 100% fill is not as strong as the HDPE at 25% fill. I can also make 25% filled PCL objects but they are very flexible. Presumably PU injection would work with PCL as well and get the strength back.

The brush that I use to wipe the nozzle does not work as well with PCL. With HDPE any bits left stuck to the brush get knocked off on the next wipe cycle. With PCL they get picked up again by the nozzle on the next pass. I need to go back to using a knife I think, as shown here.

My acorn nut nozzle didn't work very well with HDPE compared to the one piece nozzle I used before, but it works much better with PCL. I get less extruder overrun and I can extrude quickly without the filament snapping.

PCL filament is much more compliant so the minimum corner radius is less and definition is generally much better. Some of this may be due to being able to run my fan again. I found that it improved HDPE definition but it pushes the heater temperature up above the point where the PTFE insulator goes soft.

So to summarise:

HDPE:

• Rigid.
• Cheap.
• Readily available.
• Handles high temperatures.
• Shrinks a lot leading to warping.
• High die swell.
  
• Doesn't stick to anything.

PCL:

• Springy.
• Expensive.
• Hard to get hold of in filament form.
• Doesn't handle high temperatures.
• Shrinks less leading to less warping.
• More compliant leading to better corner definition.
• Low die swell.
  
• Sticks to far more things.
• Has green credentials.

PCL seems better in all aspects that affect making accurate objects. HDPE is better in all other respects.

HDPE seems to push the extruder temperature wise and PCL seems to push it torque wise. I think a stainless steel barreled extruder with a PTFE lined filament guide will solve these problems.

The PCL results look easily accurate enough to make the Darwin parts so I need to hook up my machine with the host software and start churning them out. The HDPE results are probably good enough for some parts and probably beneficial for motor couplings and mountings which get hotter than 60°C.

Before that I will have a go with ABS and then do some more work on my high temperature extruder design.