Quick release bed

I am in the process of making a heated vacuum table to hopefully allow automatic ejection of finished objects. In a conversation with Laszlo he mentioned he was planning to use a heated steel bed and use magnets placed around the object to hold down a sheet of Kapton. I turned the idea upside down. Why not stick Kapton tape to a sheet of steel and clamp it to a heated aluminium bed using magnets underneath?

I found a thin sheet of bright springy steel that was part of an electric toaster. My best guess is that it is one of the grades of stainless steel that is magnetic. It is only 0.3mm thick so it is relatively flexible, but it always springs flat. It came from a Kenwood toaster that gave good service until our cleaner suggested to my wife that she should turn it upside down to get rid of some persistent crumbs. The next time it was used it burst into flames because a crumb got wedged between the element and the steel plate and burnt through the nichrome.

I made a tiny heated table from an off-cut of 6mm aluminium. It is only 105mm x 73mm, which is smaller than a MakerBot CupCake bed but I think it is just big enough to make all the Mendel parts.



I have run out of AL clad resistors so I made my own from vitreous enamel ones embedded in aluminium blocks with tin foil. I used two 6.8Ω resistors in series driven from ~ 26V AC. That gives about 50W and a similar warm up time to my larger bed driven with 200W.

I milled flat bottomed holes to within about 1mm of the surface and embedded five neodymium magnets which are held in with Kapton tape.

I used M3 threaded nylon stand-offs as insulated table legs and mounted it onto my XY-table using a sheet of 4mm aluminium / plastic laminate called Dibond. It is very nice material to work with.



The steel plate covered in Kapton tape then sticks to the top of the table. I heated it to 100°C and tried making some ABS objects.



This worked well and the objects were easy to remove by bending the plate and peeling them.



The magnets are strong enough to hold down even big objects. The only problem I had was that the nozzle snagged on the first layer of this object and managed to slide the steel plate, causing the first layer to be offset.



Contrary to popular belief, FFF does require significant force and benefits from a stiff extruder mounting.

A couple of pins in the corners to act as dowels would solve the sliding problem.

Here is a video showing how easy it is to remove the objects: -



It is still a manual process though, so I will pursue the vacuum table idea to attempt to make a bed that can eject the object itself.

Will it stick?

ABS sticks very well to hot Kapton, so I wondered what else would stick to it. The first thing to try was PLA. This sticks pretty well to cold masking tape and doesn't warp much, but large objects do have some warping. I figured heating the bed to around 50°C would fix that. Rather than changing from Kapton to masking tape I decided to see if I could stick PLA to Kapton and get a shiny surface as well.



The first bracket was made on cold masking tape so the base has a matt finish.

The second one is on Kapton at 50°C for the first layer, dropping to 40°C after that. My logic was to have the bed just above the glass transition to make it stick and just below afterwards to stop it warping. As you can see one of the hole outlines did not stick properly. The PLA was extruded at 200°C for the first layer and 180°C for the rest.

For the third one the bed was at 55°C falling to 45°C. The outline stuck properly and the base is nice and shiny. The surface imperfections you can see are from gouges in the aluminium bed caused by a slight accident with a decimal point. It caused the nozzle to be rammed into the bed and then the X-Y movement ploughed furrows. These show up through the Kapton tape.

The last one is my first ABS test for comparison.

It was looking good, so I tried something bigger, a Mendel belt splitter jig: -



The left hand corner lifted and the object ended up more warped than it would have been made on cold masking tape.

I tried again with the bed at 55°C all the way through the build. My extruder started jamming so I increased the PLA temperature to 210°C for the first layer and 190°C for the rest, the values I had been previously using on cold tape.

This time it was successful and stayed stuck down: -



The base came out perfectly flat and more transparent: -



The extrusion lines of the three solid base layers are less visible and you can see through to the sparse infill. This is only 25% but the object feels incredibly strong. I get the feeling the hot bed makes things stronger.

There is a bit of a meniscus around the edge. This is mainly because I had a bodge of a -0.1mm offset in the first layer outline to get PLA outlines to stick to tape reliably. I removed the bodge and made this object: -



The base layers are very transparent here, even more so to the naked eye than the camera shows. There is something a little odd with some of the extrusion lanes above the bottom left hole. I think those discontinuities must be the plastic squirming a bit while extruded, which is usually a sign of not being stretched enough.

The top of the object has a small defect: -



There is a small hole above and right a bit of the centre. I think this is because the plastic doesn't span gaps as well without a fan, so it fails to bridge the sparse infill properly. I wasn't watching so I didn't see exactly what went wrong.

The next plastic I tried was HDPE. Not surprisingly it doesn't stick very well to hot Kapton. With the bed at 130°C it stays molten but is quite rubber like. With the bed at 110°C it sets and turns white (because it crystallises I believe). I tried various combinations of these two temperatures but could not get it to stick reliably. I could lay down the first layer of a raft but then subsequent layers would rip it up as the adhesion is very low.



I think the way to do HDPE without a raft is to extrude it onto a thin sheet of HDPE, or maybe polythene, held down by a vacuum and heated to prevent warping. That will have to wait until I build a little vacuum table, hopefully this weekend.

Last on the list was PCL. That sticks very well to Kapton heated to 40°C but it never sets and makes a soggy object.



Before the heated bed I used to build with a fan, and at only 40°C the bed has no trouble holding temperature, so I tried with the fan next.



That worked OK and built a complete object: -



The infill did not stick very well to the outlines of the holes, especially on the downwind side. It probably needs a denser infill, and perhaps some overlap. 25% fill is not really appropriate for PCL as it very soft and flexible.



The bottom is smooth and shiny as expected and it took some effort to peel it off, so I expect large objects could be made. I couldn't experiment further though because the filament started buckling in my extruder.



I can't explain why it worked for a while and then stopped but I tried higher temperature and slower extrusion but could not get it reliable again. The pipe could probably be a few mm closer to the pulley but not much more because it would hit the pinch wheel.

I don't have a lot of use for PCL, other than using it up. Dropping it from the requirements for the extruder would allow me to use a smaller pulley. If you look at the table at the end of this article, you can see that it is only PCL that struggles for grip with a worm pulley. I think I could drop to half the diameter, which would just about bring the gear ratio into the range of a single pair of spur gears. I have a 4" Meccano gear that gives 7:1, so I might try that in my next extruder.

So hot Kapton works well for everything I have tried so far apart from HDPE.

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.

Hot bed

Making a heated bed to combat warping has been on my "to do" list for a long time. In fact I ordered the materials more than a year ago. My plan was to use an aluminium plate with many small power resistors screwed on the back.

The plate is 8" square to match my table and 6mm thick. A friend with a CNC machine shop kindly machined it for me. It saved me a lot of hard work with a hacksaw and file and looks a lot better as well.



I estimated that it would need about 50W to raise the temperature to 100°C, so I aimed for 100W to give a reasonable margin for control. I used 9 10W 12Ω resistors wired in parallel. Driven from 12V this would take 9A giving a power of 108W.

The holes in the resistors are only big enough for M2 screws. I drilled blind holes and tapped them with a plug tap, actually a broken tapered tap that I ground to a flat end.



Tapping small holes in aluminium is tricky, that was how the tap came to be broken in the first place. The correct size hole for M2 is 1.6mm but I drilled it 1.7mm to make it easier to tap. In fact aluminium is so ductile that the peaks of the thread are still the correct diameter. I.e. the 1.7mm drill would not fit the hole after tapping and the thread was a good tight fit on the bolts. I used paraffin for lubrication.

Soldering the resistors was fun.



I used stout wire to handle 9A and high temperature solder because I fancied using it as a hot plate for soldering. My 50W iron did not have enough power to melt the solder when the resistors were mounted with two thick copper wires leading from them. To get round that I placed it on a silicone matt and powered it up to raise the temperature to 100°C and then soldered it while hot and live, not something I would recommend. As the iron bit is grounded I had to solder all the 0V connections first and then swap the polarity.

The original plan was to power it from a 12V PC power supply and switch it with a big MOSFET. Initial tests with a bench power supply showed it took about 15 minutes to warm up to 80°C. When calculating the power I had forgotten take into account the specific heat capacity of the thick sheet of aluminium. I didn't want to add 15 minutes to the build time, so I decided to double the power. I have abused these resistors before and got away with it. I changed the wiring slightly to make a series parallel combination with a total resistance of 12Ω and fed it from 48V AC giving 192W.

I used a big 350W transformer and controlled the mains to it with a solid state relay. Since the temperature is controlled there is no real point in using a regulated DC supply. It is much more efficient to use AC and avoid the losses associated with rectification and smoothing. It also allows me to use the same control hardware and firmware that I used for the SMT oven.

I made some PEEK insulating stand-offs to mount it on my XY table with a gap of about 6mm below the resistors: -



I wrapped the feed points around two of these to make the transition to a lower temperature with PTFE sleeving before using normal flex to handle the movement of the table.



I also added some foam board to insulate the top of my X-Y table.



This just fills some of the air gap under the plate to prevent air circulating and convecting heat downwards.

I made some PTFE washers to go under the nuts that hold it down by slicing up a failed extruder insulator: -



These deformed considerably when I heated the table to 230°C, highlighting why PTFE insulators fail when used in an extruder.

Here is the final result mounted on the machine: -



I added Kapton tape around the edge as I thought it would stop hot air escaping from underneath, but it didn't seem to make a lot of difference.

Here is the open loop response at full power: -



Although it can reach the required temperature, it is much too slow for SMT soldering. It needs to be able to rise at about 1°C / second for that. So I will stick to using the oven for soldering for now. I was hoping to be able to paste boards, place components and then solder with the board still on the table, but it obviously needs a lot more power.

Here is the response using bang-bang control from the host at one second intervals.



Some analysis: the initial rise rate is about 20°C in 75 seconds. The specific heat capacity of aluminium is 0.9 J /gK and the total weight of the bed plus resistors is 700g. So with 192W the time taken to rise 20°C should be 0.9 × 700 × 20 / 192 = 66 seconds, reasonable agreement as we ignored any heat loss.

The initial fall rate is 5°C in 85 seconds while at a temperature of ~80°C above ambient. So the rate of heat loss is 0.9 * 700 * 5 / 85 = 37W. Looking at the steady state the power is on for about 1 in 6, which would be 32W, so again reasonable agreement.

The plate is ~ 200mm square so its area is 0.04m² so it looks like we need about 1kw / m² to reach the sort of temperatures needed for HDPE and probably twice that to have reasonable warm-up time and control. Mendel's build area is also 200mm square, so would require a similar power.

You might have noticed the thermocouple is covered with a piece of ceramic cloth in the photo above. This is what happens if it is just stuck down with Kapton tape:-



You can see that as the temperature rises you get increasing thermal noise. Even with the ceramic cover in place you can see similar noise on the open loop test when the temperature was much higher. I think the reason for this is the convective air currents causing chaotic air turbulence. If you think about it you have hot air rising but, away from the edges, the only way cold air can replace it is by falling through the rising air.

A better place to put the thermocouple would be under the bed to avoid the convection currents, but I wanted to try controlling the surface temperature when it was covered by a bed material. Here is what happens with the thermocouple on top of a 3mm thick sheet of smoked acrylic: -



The set point is 95°C in this case. Clearly a case where bang-bang does not work too well, with 5°C overshoot and 3°C undershoot.

The acrylic loses about 15°C between the bottom and the top surface. That makes it curl upwards, so it would need a frame around the edge to hold it down. Fortunately I have one made from HDPE laminated with aluminium so it should stand the heat. It also adds a significant time lag.

Another problem is that acrylic has a glass transition at about 114°C. When the control was of the top surface temperature, the bottom surface exceeded that during the overshoots and went soft.

So I will need to implement PID for top surface control, but I had a suspicion that a transformer was not going to like PWM into its primary much. Anyhow I put the thermistor back onto the plate and moved the bang-bang control from the PC to the firmware in preparation for building something. Bang-bang was an apt name for what happened next. When the temperature crossed the set point it started dithering the mains on and off. The transformer sounded like it wanted to jump off the desk and then blew its 3A anti-surge mains fuse.

The solid state relay turns the power on at the zero point crossing of the mains, and off when the current is zero. Current builds up slowly through an inductor so what could possibly be wrong? I had noticed big transformers thump when you connect them to the mains, but I had always assumed it was because the secondary usually has a big smoothing capacitor to charge up. However, this was a purely resistive load, and even with no load attached the transformer thumps on start-up, so some reading up on transformer theory was required!

It turns out that transformers take a big surge current and turning on at the zero crossing point is actually the worst point to turn them on. The reason is that when a transformer is running, being an inductor, the current lags behind the voltage by 90°. So normally when the voltage is crossing zero, the current is at its maximum reverse polarity and over the next half cycle of voltage it goes though zero and then to its maximum positive value: -

If the current starts at zero then over the first half cycle it will rise to twice its normal value: -

That would not be too bad except for the fact that transformers usually run with their core close to magnetic saturation for efficiency reasons. That means the core saturates during start-up. The inductance disappears and then the only thing limiting the current is the DC resistance of the primary, about 3.3Ω in my case, so the current can be enormous. Counter intuitively, the best time to turn a transformer on is when the mains is at its peak voltage.

So I learned something I didn't know about transformers. The fix was simple, I added a solid state relay to the secondary circuit and plugged the transformer into the mains. Bang-bang control then was able to pulse it very quickly due to the dithering caused by noise, which ends up with some proportional control, that reduces the temperature swings to a fraction of a degree.



So I can add PID control firmware and it can be shared with my oven control but I have an extra solid state relay and a big transformer. A better solution would be to pick the resistor values to give the correct wattage when wired in series across the mains. Of course mains on a moving table is not the safest design. I would use a heavy three core flex with an earth lead to the plate and a second independent earth strap for safety.

It turns out the first use for my heated bed was not to combat warping, but actually something more essential, details coming soon ...