Back to black

I have used up all of the 5lbs of ABS that I bought from the RRRF with just a couple of plastic parts left to make to complete my Darwin. I bought 2Kg of ABS from Tempatron but it is too oval to fit my current extruder, so it will have to wait until I build the new extruder for my Darwin. In order to complete that extruder I have had to go back to using HDPE. I always intended to make the filament guide for it out of HDPE anyway, because it is a lot more slippery than ABS, PCL and PLA.

It is quite a while since I did any work with HDPE and I didn't print many modeled objects with it before I switched to ABS, just lots of test blocks. In the meantime I have rebuilt and tweaked my extruder quite a lot and changed the way I do rafts. It took me a few attempts to dial in the parameters to get reasonable print quality.





In the end the result is not bad. The object on the far right is an ABS version, the one next to it is the best HDPE version.

The test piece is an alternative solution to the problem Vik mentions here: new-x-carriage-hot-off-reprap.



It replaces the y-belt clamp that holds a piece of filament (which carries the cables to the extruder) and also provides the bearing surface missing from the "no support" version of the Darwin x-carriage. You can see the gap above the bar created by the teardrop shaped hole below: -



Here is the new HDPE piece installed: -



It also fixes the fact that the screw hole pitch of the belt clamp does not match the holes in the carriage. HDPE is very good for making bearings so I intend to remake all the bearing inserts in it when the ABS ones wear out.

The first thing to do is get the raft temperature correct to make it stick, but still be peelable from the chopping board. I start with the temperature at a value I know will be too low and go up in steps of 10°C until it sticks enough, but not too much.

The next thing I do is get the first layer temperature right so that the object can be separated from the raft. I found that quite hard to control with ABS and much more so with HDPE. I increased the temperature until the outline started sticking properly and found a small test object was still peelable. When I made larger objects they were stuck fast. I normally use a small penknife to separate stubborn objects but after stabbing my fingers three times I resorted to a chisel! I think some of the variability is down to changes in ambient temperature and bed temperature. The XY table of HydraRaptor is made of several Kg of aluminium, which acts as a heatsink for the motors. They run a lot cooler than my Darwin's motors despite having more power through them. The table slowly warms up to about 30°C, about 10°C above ambient. 10°C is enough to make all the difference between sticking and not. I probably need to measure raft surface temperature and adjust the first layer accordingly. Another thing to try would be to do the first layer outline hotter than the infill.

I dropped the build speed from 16mm/s to 8mm/s for three reasons: -

• HDPE puts maximum strain on the extruder because it is the most viscous at extrusion temperature so it is both the hottest and the highest pressure.
  
• Heat builds up in the object limiting the minimum size that can be made without inter-layer pauses, something I have not implemented yet.
• HDPE likes to cut corners and not go where it should and I think going slower helps the accuracy.

I also had to set the infill overlay parameter to 0.5 in Enrique's software to make the infill join to the outlines correctly.

A few things that are different about HDPE:

• Extruder ooze creates blobs rather than strings with my current nozzle. They are harder to remove because they are a lot thicker. Faster head movement on my Darwin should drastically reduce this effect.
  
• The brush I use to wipe the nozzle works perfectly with HDPE but not very well with other plastics. This is because HDPE extrudes in a straight line and is not very sticky. In contrast ABS tends to curl upwards and sticks well to the nozzle.
• HDPE spans gaps a lot tighter than ABS does because when it is stretched it remains under tension whereas ABS doesn't.

• Holes always come out smaller than they should be, but with HDPE this effect is worse. Dr Bowyer published a correction formula based on there being too much material on the inside of circles but I get contractions an order of magnitude greater. I think it is related to how much I stretch the filament but I need to do some more work on it.

I then made the extruder clamp in HDPE. It is a bit warped of course but it doesn't really matter. The raft was strained upwards at the bottom left corner, not surprising as this is one of the longest pieces I have made.



Here it is cleaned up :-



The large hole for the PTFE barrel is undersized and my PTFE stock is oversized so I think I will have to turn it down to get it to fit. I drilled out the other holes.

Similarly the filament guide is warped on the underside but it is only the top side that needs to be flat.







As predicted it is slippery and my oval ABS glides through it very well.

I just have to make the metal extruder parts now and that completes the mechanical build of my Darwin.

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.

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.

HDPE + PU

Because my test objects are less warped while they are still attached to the polypropylene bed, I had the idea of filling them with something that sets hard to freeze them in that shape. That would also allow me to use a sparse fill pattern, which speeds up the FDM build time, but still get a strong object.

I needed something that was not too viscous so that it would flow in between the mesh of the fill pattern and would set hard.



Polyurethane was recommended to me because it has the consistency of milk before it sets and is strong enough to cast parts for Darwin. I bought some Smooth-Cast 300 which has a pot life of 3 minutes after it has been mixed, and cures in 15 minutes. I choose a fairly fast setting one because it gets hot while curing and I hoped it would soften the HDPE to relieve the stress. It only seems to get to about 50°C though so I don't think that it has much effect in that way.

This is the equipment I used :-



I know the internal volume of my objects pretty accurately so I measure out the required amount of plastic using separate labeled syringes for the two components. I mix it in a small pot before filling a third syringe to inject it. The syringes and pot are made out of polypropylene, which polyurethane does not stick to, so they can all be reused. I haven't found a way of unblocking the needles though.

I made a 50% filled object and drilled a hole the diameter of the needle in the middle that allowed the needle to go to the bottom. I also drilled a small riser hole at each end to let air out. Obviously, with cleverer software these holes could be made during the FDM phase.

The first attempt was a complete failure because the needle blocked when the object was only about 50% filled. Here is a cross section :-



For my second attempt I used a thicker needle, 1mm OD rather than 0.8mm :-



The object filled OK, but just as it became full the plastic in the needle set suddenly but I carried on pushing. The needle popped off the end of the syringe and PU sprayed all over the place. It was a good job I was wearing goggles and gloves but I should also have been wearing long sleeves, a mask and a hat! Fortunately PU does not stick to much, only untreated wood, skin and hair! Where it gets on your skin it burns slightly. Because it is transparent before it sets it is very hard to see where it has gone but when it sets it turns opaque white so it becomes obvious.



It actually sprayed around one quarter of the room. I even got some on my lips which I didn't notice until I tried eating.

What seems to happen is that if you subject the liquid plastic to pressure it accelerates the curing, which increases the temperature and pressure creating a positive feedback effect which makes it set suddenly in the needle. I only had two needles and they were now blocked so I did the remainder of my experiments using just the nozzle of the syringe into a bigger hole in the object.



The ideal solution is probably a very big needle that locks onto the syringe. It doesn't need to be sharp but the 45° slant at the end is handy because it stops the end being blocked if you press it against the bottom of the object.

Results
I left the objects on the bed overnight to make sure the PU was fully cured even though it sets in 15 minutes. The first object I made had a 50% fill and warped 0.36mm compared with 0.47mm without the PU injection.



Thinking that 50% fill leaves the PU fairly weak, I did another test at 25% fill. That gave 0.24mm warp, the lowest figure I have achieved yet for this shape.



I also tried a 100 x 10 x 20 mm test with 20% fill ratio. That gave about 50% less warping compared to the version without PU.



Conclusions
A useful technique for reducing warping and reducing the build time of FDM objects. The main disadvantage is that FDM is one of the cleanest and safest fabrication techniques whereas injecting PU is messy and somewhat dangerous unless you wear protective gear.

I was disappointed not to get rid of the warping completely. Instead of alternating the horizontal and vertical fill patterns, several layers of one followed by several layers of the other might make the PU lattice stronger. Raising the PU to 50°C for a few hours is supposed to harden it further, so I could try removing the bed and putting it in a very low oven for a while. I have a Peltier effect 12V beer fridge which can be reversed and used as an oven, so that would be ideal.

Using a harder plastic like epoxy might work better but it may be too viscous to inject. I believe heating it reduces viscosity.

Reducing HDPE warping feels much like banging ones head against the wall so I will try PCL and ABS next for some light relief.

Infill and warping

Now that I can create blocks with different infill densities I decided to experiment to see what effect it has on HDPE warping.

I have been using a standard test shape and a jig made of three nails to make comparative measurements.



I measure from the middle nail to the base with a pair of digital calipers and subtract the distance to a rule placed across the nails. The figure I get is an average of the amount each end warps upwards. Not very precise because the base is warped the other way as well.

The block is 40 x 10 x 20mm because you need about 40mm length before the warping becomes big enough to measure and 20mm height is about where things start to straighten out. Bigger shapes warp more but obviously take a lot longer to make. Each one of these takes about an hour including making the raft, extruding the block, separating it from the base and measuring it.

The block is held flat while it is stuck to the bed of the machine by the raft. It warps when I remove it. I have only recently noticed that it warps even more when left overnight, so some of my previous tests are not that accurate. For example I was quite pleased when I first produced this extruder sized block :-



But here it is again photographed some days later :-



Not easy to compare because of the angle but the uplift at each end probably increased from about 0.5mm to 1mm. It implies to me that HDPE creeps when under prolonged strain, not a very good engineering property. That is the main reason PTFE fails in the extruder.

I made the test blocks with different infill densities and left them overnight before measuring them :-



Here are the results: -

Density	Warp
20%	0.44 mm
25%	0.79 mm
33%	0.47 mm
50%	0.47 mm
100%	0.53 mm

The 33% value looks totally anomalous but that is because I tried a thicker base. Its base is 3mm of 100% fill including the raft, whereas all the other tests begin the sparse fill on the first layer above the raft.

I also tried 1mm filament 50% fill which gave 0.42mm warp showing that not stretching the filament does not give any improvement.

Conclusions: well sparser fill reduces the warping slightly. A thicker base, rather than resisting warping, actually contributes to it. I must point out that once you get less than 50% fill the object is considerably weaker than a solid block.

Finally here is a longer example, which illustrates how warping gets worse the larger the object is. This is 100 x 10 x 20mm with 20% fill. The first time I made it it lifted the raft away from the base. I got round that by increasing the raft temperature by 10°C to get a stronger weld. It was then quite hard work removing it and it caused some damage to the PP bed.



The 40mm section in the middle is only warped by 0.19mm but the ends are well over 1mm. That shows that you cannot compensate for the warping with a crowned bed because it is not a constant curvature. One could probably scan the shape of the base and lay down support material with the inverse curve. I expect it would then pull itself flat.

In my next experiment I will try filling the sparse blocks with polyurethane two part thermoset plastic.

To raft or not to raft?

When extruding HDPE onto foam board a raft needs to be laid down first to increase the anchorage at the corners to reduce curling. It becomes part of the object and has to be trimmed back to its outline with scissors or a knife. Now that I am extruding onto polypropylene cutting board I wondered if it was still necessary.

The temperature at which I lay down HDPE onto the cutting board is important. At 180°C it does not stick. At 200°C it sticks well but can be peeled off with the help of a penknife. Higher temperatures make it harder to remove and do more damage to the board.

Here are a couple of 15mm test cubes made directly onto the PP board without a raft :-



The one on the left had the first layer extruded at 200°C and subsequent layers at 240°C. As you can see it curled badly, particularly at one corner. The one on the right had its first layer extruded at 220°C. It looked promising but when I tried my standard warp test block the result was not good!



So it looks like the raft is here to stay. Here is an example :-



I lay down the raft at 4mm/s with a notional filament diameter of 1.1mm with the extruder head 1.3mm above the board. This is to get the filament as round as possible so that it doesn't form a solid weld. In actual fact, gravity causes it to slump to about 0.9mm high and spread to 1.3mm wide. The oval area calculation would give 1.34mm and a pitch of 1.3mm is sufficient to get adjacent filaments to stick together. My rationale for making the raft as thick as possible in one layer was to make it strong without taking too much time. It probably does not need to be as strong now that it binds to the PP.

I put the raft down at 200°C, then I do the first layer of the object at 240°C with the fan off to ensure it welds to the raft and then subsequent layers at 240°C with the fan on.

I calculate the amount the raft overlaps the object with this completely arbitrary function :-

def overlap(x):
  return x + 10 + 10.0 * (x - 20) / 80

I halved the overlap when I went from foam board to polypropylene.

Cutting corners

When making solid blocks with 0.5mm HDPE filament I noticed that the corners are not very accurate. The right hand edge of the 20mm cube below shows this effect at its worst :-



The problem is that, although the machine makes a perfect right angle, the filament appears to have a minimum bend radius and so cuts the corner. The amount it cuts the corner seems to vary from layer to layer giving rise to the rough edge.

I think the variation is due to the fact that my extruder spindle is a bit off centre. This causes the torque to go up and down as it rotates, which causes the flexible drive cable to wind up and run down again. This causes speed variations despite the fact that the motor speed is well regulated. At some point I will get rid of the flexible drive.

I expect the fact that I am stretching the filament doesn't help with the corner cutting. I improved it a lot by slowing down the drawing of the outline to 4mm/s and leaving the infill at 16mm/s. Here is the result :-



Still not perfect, another thing to try would be to recognise that there is a minimum corner radius and make the nozzle follow an arc of that radius around the corner. At least that way it might be more uniform.

Here is a close up of the top face taken with a scanner:-



As it goes round the corner the filament has an external radius of about 1.5mm and an internal radius of 0.9mm. As it is 0.6mm wide that is probably not bad. You can also see that the zigzag infill sometimes ends a bit short of the edge, probably also due to corner cutting.

To get sharper corners I expect I need to use a nozzle with a smaller hole, so that the filament can be fine without having to be stretched, but that has the disadvantage of slowing down the extrusion rate for a given pressure.

Warped

Having got an idea of the HDPE warping for thin walled open boxes, I decided to start investigating solid shapes. I made a solid block 40 x 10 x 20mm to compare with the open boxes of the same dimensions.



Obviously there are many ways to fill the interior so I started with the simplest, just alternate layers of horizontal and vertical zigzags. HydraRaptor seems quite happy extruding 0.5mm diameter filament at 16 mm / second. If extruded into free air it would actually be 1mm at 4mm/s, but that is too course, so I move the head at 16mm/s which stretches it.

From trial and error I have found that a good layer height to use is 0.8 times the notional filament diameter. If it is more, then as the lower layers shrink, the nozzle rises faster than the object and a gap develops. Once that happens the filament squirms about and does not follow the path of the nozzle accurately.

So the extruded filament is constrained to 0.4mm high. Measurements show the width to be about 0.6mm. Incidentally, if it squashed to a perfect ellipse with a height of 0.4mm then it would be 0.625mm wide to have the same area as a 0.5mm circle. I extrude the zigzag with a pitch of 0.6mm so that adjacent filaments touch, but it means the object is not actually completely solid. The space occupied by each filament is a rectangular channel 0.4 x 0.6 = 0.24mm² but the cross sectional area of the plastic is π x 0.25² = 0.20mm², so about 18% is air. I confirmed this by weighing the block. It weighs 6.5g but if it was solid HDPE then 8ml would weight about 8g. It takes about 45 minutes to make the object including laying a raft.

Before I tried it, I always imagined the amount of plastic deposited would have to exactly match the volume of the extruded object otherwise it would sag or bulge. I could never understand how FDM worked reliably. Now I know that the volume can be a bit less and the difference is made up by air. That means the amount of plastic deposited is actually not that critical, which is why RepRap can get away with an open loop extruder.

I measured the warping with the three nail jig that I showed in the last post. The thin walled box is warped 0.44mm and the solid box has warped 0.87mm so that answers the question whether solid objects warp more or less. Note that the thin walled box is made with 1mm filament because 0.5mm filament is too thin to be self supporting.

I expect I can make a less warped block by extruding a thick base and then a less dense infill above that. Something else to try.

It is amazing how strong 10mm thick HDPE is. You don't often get to see plastics in that form. Most end products have optimised strength against cost by having thin walls and ribs etc.

Chopping up chopping boards

Up until now I have been extruding HDPE onto foam board because it was the only thing that it sticks to well enough. However, it has a couple of failings: It is not strong enough to completely resist the warping caused by the HDPE and it is not reusable because the surface gets ripped off.

I have tried many other surfaces including various woods and metals (with and without primer), melamine and several other types of foam board but nothing worked. Obviously HDPE sticks to HDPE so I decided to investigate that further.

My first idea was to use a thin sheet of HDPE cut from a milk bottle. This makes a nice surface to extrude onto but the problem is holding it down. I first stuck it down with double sided tape but the heat melts the glue. Sticking it to a sheet of aluminium to take the heat away improved matters and I was able to get slightly less warping than with foam board.



To compare the warping on different base materials I made a test shape that is a 40mm x 10mm x 20mm open box with 1mm walls and measured how much the corners lift using a simple jig.



With foam board I was getting 0.83mm lift between corners and the middle. With HDPE stuck to aluminium I got 0.76mm. Not much better because the glue of the sticky tape stretches under the curling force.

I needed a thick HDPE base and I had heard that plastic kitchen chopping boards are made from HDPE. I bought a new one from ASDA which looks like this :-



It is 5mm thick, opaque and quite rigid. I realised it was very different from the other chopping boards we have which I think came from IKEA.



These are 10mm thick and made from a softer, more translucent plastic. To find out which was HDPE I used the flow chart on this website www.texloc.com/ztextonly/clplasticid.htm. I concluded the thin hard one from ASDA is HDPE and the thicker softer one from IKEA is PP. HDPE seems to stick equally well to both of them but the HDPE one warped a bit when it was only held down with masking tape, so I decided to go with the PP one. I cut it up and bolted it down to my XY-table. It was a bit curved due to years of dishwasher use but bolting it down pulled it flat.



Surprisingly, if I lay down a raft at 200°C it sticks well but can be easily prized off again with a penknife. The board is marked slightly but it can be reused over and over again.



I extrude the object at 240°C so that it welds to the raft and itself, and I turn the fan on after the first layer so that the object cools to room temp as fast as possible.

The board is strong enough to hold the object completely flat while it is attached but when it is removed it does still curl a bit. I measured 0.44mm on my jig so that is about half the curling I was getting with foam board. Other than extruding onto a convex surface, I think that is the best that can be achieved for that shape with HDPE at room temperature. Here are the three tests side by side :-



Next I will look at different solid shapes to see if they warp more or less.