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Thoughts on printing at ambient temperaturesSunday, November 11. 2007
I hit a technical problem back in September and you haven't heard
much from me since then. I've been able to print in HDPE and am well on
my way towards sorting out the problems with using brushed gearmotors
to run both the extruder and the Cartesian positioning system of
Tommelise. I'm publishing in the main blog rather than the builder's
blog this time in that the issues that I am addressing impact on both
Darwin and the various repstrap machines that are abuilding.
As you are all aware by now, I am sure, HDPE and to a lesser extent CAPA tend to curl when extruded at ambient temperatures.  This curling tends to happen at corners when the aspect ratio of object being printed approaches 1. When you have long objects you tend to get curling in the plane of the longest dimension. Nophead published some really compelling photos of this effect over in the builders' blog last month. I got very discouraged about the warpage issue and resolved not to share my misery with the rest of you if and until I came up with a viable solution to the problem. Some months ago, I printed an updated version of the polymer pump for Tommelise. I was able to defeat, to an extent, the tendency for the pump to warp during printing by using a lattice infill rather than a solid one.  It wasn't until I separated the pump from its HDPE printing raft and cleaned it up a bit that I noticed that the back side of the pump had warped all the same.  You can see that the curling is in the plane of the longest dimension in this case. You can also easily see a delamination occurring between layers roughly a quarter of the way from the bottom of the pump to the top. I will talk about this in more detail a bit later Also notice that the top surface of the pump, the critical one, is perfectly flat.  Here's what I think has happened.
Now whether or not the pump is trying to bow down into the foamboard, the extruder still tries to extrude in a flat plane. When the extruder head reaches the left and right extremes of the pump it is not able to put down as much plastic as it is in the centre simply because the distance between the extruder head and the last layer is smaller at the extremes and bigger in the middle. If I were trying to print a solid infill this situation would have probably caused more distortions in the print than it did. The top and bottom ends of the pump are square with the top simply because the cartesian positioning system keeps returning to the same perimeter with each layer regardless of the fact that the previous layer tries to shrink inwards a bit as it cools. The delamination mentioned earlier happened at exactly the layer where I stopped printing one evening and picked up again the next morning. I had a bit of difficulty repositioning the extruder to start up again. This likely weakened the weld between the new layers and the old ones. Proposed Solution It would appear that we might well be able to cancel out the downward bowing of the print by printing on a solid piece of HDPE instead of foamboard. Solid HDPE has a high thermal resistance which should preclude extruded plastic from cooling off too quickly and not welding properly. Keep in mind that this delamination developed over several days and was not apparent when I originally separated the pump from the foamboard and cleaned it. Such HDPE planks are easily acquired in the form of cutting boards at any hardware store. They are usually at least 3/8 - 1/2 inch thick and should have more than enough resistance to bending, especially if I use a loose infil in my print. Of course, getting the printed object off the HDPE plank will be considerably more difficult than getting it off of foamboard. All the same, using a heated wire to separate the printed object or, failing that, a hand jigsaw should do the trick. As well, printing the piece on top of a cross-hatched rick strong enough to resist downward thrust, but less solid than solid HDPE should make separation a bit easier. The HDPE baseboard should be relatively easy to resurface with a small belt sander. I checked and those are quite inexpensive. Keep in mind that constraining the printed object in HDPE in this manner does not mean that it will not warp after it is separated from the HDPE baseboard. I'm hoping that it won't to any great extent, however. Trackbacks
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Hi Forrest,
... as far as i know, the comercial FDM-printer from Dimension printing uses a start-sheet from another plastic then the body-material - more brittle and easy to remove (when i'm home, i'll post some photos of examples)
As Nophead mentioned, it's not only the 'pressing' in the foam-board, but an additive thermal stress in the sheets - when they are dispensed, the plastic is hot ... when it cools down, it contracts a bit and adds mechanical stress, so that the top layer curls upwards and 'tries' to rip up the underlying sheets from the surface.
As the plastic is merely elastic, this stress didn't adds cumulative over all slices, but it's there in the structure ...
So i think, it should be both problems together!
Viktor
Wow! That's news to me. I understood that Adrian's machine there at Bath printed onto some sort of foamed plastic substrate.
Yup, it does. I'm pretty sure its like this:
* foam substrate
* support material 'raft'
* ABS plastic.
... here photos of two Dimension-ABS-FDM-parts on a basis-sheet:
http://forums.reprap.org/file.php?0,file=332
... and here turned around :
http://forums.reprap.org/file.php?0,file=333
Viktor
I don't know much about the technicalities of this, but was just wondering: do you coat the HDPE board with any release agent before laying down the plastic?
While not directly comparable, I believe that injection moulding processes involve coating the mould with a release agent before injecting the material. This helps to prevent the hardened material from 'bonding' to the mould. Maybe the same principle could be applied here?
The problem we've been having is not to get the printed object to come off of the substrate but rather to get it to stick to the substrate. :-(
This is certainly a really annoying and exceedingly serious problem.
* The idea of using an HDPE baseboard doesn't seem workable to me - what happens in the future when you are using filler materials to build overhangs? In that case, it's very likely that most of the new part is only in contact with the filler material. That means that your filler material must have the same anti-warpage properties as the baseboard.
* This may be nuts - but if we had working filler material nozzle - would it be possible to calculate the curvature in the final block and simply build the inverse of that into the shape we make? Design it so that when it warps, it warps into exactly the shape we actually wanted?
* Is the nature of the warpage sufficiently consistant that we coulld simply build the inverse of that onto a curved base-plate?
* What do those hideously expensive commercial machines do to address this issue?
Stratasys machines, the only FDM system I'm acquainted with, keep everything in a heated enclosure set to a few degrees below the melting point of the plastic being printed. When the print is done the completed piece is then allowed to cool to ambient temperatures.
This approach simply uses too much energy to be practical for a cheap, consumer printer, imo.
forrest,
i was thinking about this the other day, and i came up with a potential idea.
remember when you were talking about extruding onto copper? the problem was that you couldnt extrude onto it until it was hot right?
the problem with HDPE we're seeing is that it cools at different times, so it contracts differently, right?
well, what if you combined the two things? here's what i'm thinking: you take a hotplate + a cookie sheet or some other metal baking pan and wrap the pan in aluminum foil. put that whole assembly onto a hot plate which then goes onto your stage. keep the hotplate on whatever its lowest setting is... it will probably still be hot enough to keep the HDPE warm/hot/soft. when you're done... simply let the whole piece cool, then take the aluminum foil + printed part off the sheet. you may have bits of aluminum foil stuck to the bottom of your object, but thats okay.
i'm not entirely sure if this would work, but perhaps it would work. is also fairly easy to try out. you just need a hotplate and some other common materials.
let me know if you try it. i'm about to start printing myself, so if you dont get to it first i will. good luck!
I've thought about taking that approach, however, I haven't been very enthusiastic about putting a hotplate on my xy positioning stage. I'll be trying the HDPE plate approach for now. If you get to trying the hotplate I will be following your efforts very closely. We've got to beat this warpage problem somehow. :-s
i agree... we absolutely have to beat this warpage issue. who knows, it may come down to a software thing. if we can calculate the shrinkage then perhaps we can apply some sort of math or matrix to all the points before they are printed.
thankfully, we arent out of ideas yet... not even close! =)
Yeah, I've felt that we're ultimately going to have to solve this in software modeling of the thermal/mechanical behaviour of the part as it is being built up. I did thermal modeling for my doctorate, mostly radiation transfer modeling, and later took a crack at the 3D heat transfer problem while I was lecturing at the Witwatersrand. Ordinary 3D heat transfer modeling assumes that the object that you are modeling already exists in toto. That doesn't work for what we're trying to do since our object is in the process of being created as we're trying to model it thermally. What that does is make it necessary to use a VERY fine 3D matrix of nodes to model. The resolution seems to be so fine because you are trying to lay a new layer onto an old one that doesn't quite fit but only by a tiny bit. As well, if you model corner curling and then lay plastic down as a flat layer you may be having to put down one layer of nodes at the edges of the print and several in the middle. My doctoral project was modeling long wave and short wave radiation transfer in a building which had some surfaces partially transparent to the short wave radiation (windows). That was a multi-year nightmare for me. From were I sit, however, it looks in retrospect like a walk in the park compared to modeling an object being printed in a reprap. :-s
Hi Forrest,
Good to see posting again. I think the main reason the upper layers are flat is because by the time they are extruded the bottom layers are strong enough to hold them down. That implies extruding on to chopping board will work provided the extruded object is thick enough to resist its own bending moment after it is separated.
I am going to revisit MDF if I ever get my machine working again.
Hi Nop! Your machine is broken? How did that happen? I'd wondered why you'd been so quiet. I assumed that you'd got discouraged with the HDPE curling, too. :-(
Small accident with a piece of wire http://hydraraptor.blogspot.com/2007/10/they-dont-like-it-up-em.html
I think everyone is missing the point. The problem with stuff like hotplates and clever base materials is that this isn't a fix in the case when we're building a complex object where much of the material under the HDPE is our filler material. Forget building on top of copper or HDPE or a hotplate - think about building on top of polyfilla or cake frosting or whatever we're ultimately going to use. That's a problem that has to be solved. You can argue "well, let's fix this problem first and then worry about that case" - but if you can solve it for HDPE laid onto filler - then you can simply start every build by laying down a solid layer of filler material.
So the problem that needs to be solved first is how to avoid curling when replicating on top of filler.
"So the problem that needs to be solved first is how to avoid curling when replicating on top of filler."
The problem with that approach for me is that I don't have a pollyfilla extruder or a support material extruder of any sort for that matter. It would be nice if I did, but right this minute I don't.
My main goal for now is to be able to print parts that I can use in better printers and other kinds of machinery that I'm interested in designing... like maybe a support material extruder. :-) It would be wonderful if I could go directly to the end goal, which is a 3D printer that costs virtually nothing that can print darned near anything while using tiny amounts of energy. I don't know how to do that, though, so the next thing I'm going to try is to see if printing on thick HDPE lets me get past the warpage problem I've been hammering at for the past several months. :-s
Ooh, this is a thinker. How's about setting up a couple of cheapy CCD cameras above and to the sides and comparing their image profiles to the ideal then applying some motion feedback to the extruder head upon each layer being built.
Another idea about base surfaces: has anybody tried double sided sticky tape? I've seen it at craft stores. It should stick down great, hold the new part well and release pretty easy, it just peels up.
I tried something of the sort back in May when I got annoyed with foamboard. I applied duct tape directly onto glass and printed onto that. The HDPE bonded to the duct tape beautifully. Unfortunately, the adhesive on the bottom side of the duct tape wasn't up to resisting the curling force of the HDPE layer. Here you can see what happened on the first layer.
http://3dreplicators.com/cgi-bin/cblog/index.php?/archives/264-Duct-tape-fails.html
I suspect that the heat generated by the extruder putting down the HDPE caused the adhesive to fail.
Hi Forrest,
... yes, if you heat the duct tape, the glue went fluid and didn't support distracting forces anymore ...
Another idea - it's maybe worth a try to 'flash' every top layer with an IR-heater (maybe with a moving IR-tube from 'newstyle toasters', where you heat your toast with glowing glass-sticks).
When the top-layer is re-melted, it should lost every mechanical stress and went much smoother too ;)
Viktor
Interesting! Okay, so we melt the layer again to get rid of the stresses. Don't we get them right back again when it cools again? Bear with me on this. I'm trying to understand what you're meaning here.
Hi Forrest,
... i hope, that remelting of the topmost layer would release a bit the 'memory-effect', which is in every tray of the HDPE, as it's sticked/pressed on the lower sheet and when cools down, then 'memorize' a mechanical stress, as known in Bimetall-sheets ...
When the layer melts, inherent stresses releases complete, so there's a real chance, that when cooling down, the stress in the next underlying layer is minimized too, so you only have to deal with the topmost surface - and this would be (hopefully) released with melting the next overlaying sheet ...
Viktor
Victor, I don't think memory effect is the main factor here. It's simply the fact that the HDPE shrinks 2% between the point that it solidifies and room temperature.
Hi Nophead,
... it isn't really the 'memory-effect' as in SMA's, but it's nearly the same basics with the mechanical stress, when cooling down and contracting (as in Bi-metal-sheets) ...
I think with melting the topmost layer the underlaying layer should release most of his stress too, so you can slicewise reduce the complete stress in the sheets ...
When you deal only with one-sheet-stress in the actual layer, the accumulating of rip-up-forces should be heavily reduced ...
Look on the images in my first reply - these parts are from ABS, which contracts a bit too, but i didn't notice any curling.
I have some other parts too, where some overhang is visible and i can see the bending of the ABS-filaments in the free space - the trays aren't pressed on the surface, but layer free down without pressingg force!
Viktor
The reason you don't see curling with commercial machines is that ABS only contracts by one fifth the amound HDPE does when going from liquid to room temp and it all cools down together because the object is kept just below melting point until it is finished.
"Stratasys machines ... keep everything in a heated enclosure set to a few degrees below the melting point of the plastic"
Ok, so how hard would it be to
1 - build/aquire a wooden/plastic/metal box of arbitrary dimensions larger than your printer
2 - aquire a hairdryer, or for bigger boxes, a heat-gun from a hardware store
3 - drill large hole into top of box, smaller vernt holes in bottom of box, and poke the hairdryer through the big hole?
4 - temperature probe/circuit of your choice to maintain internal temperature as described. I personally like the max6675 chip, and type K thermocouple/s.
I vaguely recall that at one time the design required a fan beside the deposition head to deliberately cool the workpiece so that new layers would stick. Is that gone now? Do we have a contradictory need?
Dissipation of heat is the enemy... You want the part to cool as evenly as possible... the whole part... Lattice infill is part of the answer, but you've got to balance that with thick walls that can retain some heat... start with some walls around the work surface to keep out breezes... a top would be nice but just the walls themselves will make a big difference... you might want to also outline the part with a wall that you build as you build the part, essentially a nearby barrier, that's also putting off heat, and can form an insulative barrier between it and the part... this would waste material at first, but you'll eventually be able to recycle that... and then if you want to add some heat to the environment... don't add too much... but you can start by harnessing the heat coming off the heat sinks on the stepper controllers... you also want to print the part quickly... in a single run.....
(I still don't have an extruder myself yet... but I've got a fair amount of experience making candles... and cooling wax shrinks and causes similar problems...)
I am using a fan to cool HDPE down as fast as possible. Otherwise you can't extude small things because the molten layers below move about as you try to build on top of them.
For the same reason I can't see how any of the support material ideas will work because the filament has to extruded onto something that it will bind to and not be dragged around.
I also extrude at 240C to guarentee a weld on top of a layer that has cooled to room temperature, otherwise you can't make a large object.
Seems like you either have to keep the whole object just below melting, and extude at just above tm, or keep it all at room temp and extrude at 2 * tm - tr. Any other combination will only work for certain sizes of object.
What kind of Tr are you looking at? I'm extruding at about 200 degrees C and only have problems with the smallest details. Mind, I took Vik's advice and pause between layers to allow the plastic to cool.
I assume room temp Tr is 20C, Tm is 130 so I extrude at 240.
If you are extruding at 200C that will account for your delamination on the layer where you let the object cool down to room temp. I think you rely on the layer below being at least 60C to get a weld when you put the next layer on.
I can make my 40mm hollow test cube at 200C but larger things will delaminate.
I haven't tried pausing between layers because of the extruder overrun problem. Don't you end up with lots of strings?
My strategy is to cool with the fan so that I am always extruding onto something at Tr but because of that I need to extrude at the higher tempeature to get a weld, and that needs more cooling from the fan so vicious circle.
One possible solution is to have a small aperture IR thermometer looking just ahead of the extruder and control the temperature to be the minimum to form a weld.
Hi Nophead,
... i visualize an optimized extruder-head, wher a diodelaser (from a salvaged DWD-writer or DPSS-laserpointer) remelts the very area of the underlaying layer, where the new material would contact - so the new material meets a melted phase and can perfectly combine ...
A second point could be the lowering of mechanical stress in the remelted layer.
Viktor
Hi Victor,
I can't see how remelting reduces stress. If we bring it from room temp to melting point it expands 2%, it then sets again and tries to contract 2%, putting a squeeze on the layer below. Unless you can melt the whole object, which would make it collapse, I can't see how it differs from laying it down originally. You have a solid layer with a liquid layer on top. The liquid layer solidifies and then shrinks 2%.
Hi Nophead,
... i think, with remelting the underlying slice spotwise i can homogenise side-stresses and insert new stress-dynamics, which comes from the melted spot.
The laser-melting is very small localized, so it should change the pulling-vectors radial - maybe in the right mode?
But it's anyway a perfect method to connect the actual tray with the cold underlying material, so the extruding-temp could go down to some grades over melting-temp ...
Viktor
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