Tuesday, March 27, 2007

 

Untimely Lapse Movie

I've had a go at printing one of Ed's Darwin corner brackets. On about the 5th layer, the output detached itself from the stage and got pushed around a bit. While this wasn't exactly a huge success, it does look very much like we're on the right track so I've been persuaded to post the video. As per the last one, there are approximately 2.5 seconds between each frame.



Right at the end you can see the deposited part start to detach and go springy before it gets pushed around. You can also see the second head appearing in the right of the frame in the latter half of the movie. The camera moves around a little as the USB lead was dragging on things, but I fixed that with some tape about half way through; not a problem as it actually gave a better camera angle!

Adhesion to the stage (or lack thereof) is my big bugbear, and I'm going to try a light coat of varnish as previous tests have shown molten CAPA sticks to it like glue. If that doesn't work, I'm onto foam bases like the rest of 'em.

Vik :v)

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Comments:
I see what you mean about the adhesion to the main surface being critical. It's too bad we can't print at something near that speed. That would be moving :)
 
Yeah, that's speeded up about 60x. I'm hoping Darwin will prove to be at least 5x faster than Zaphod.

Vik :v)
 
Craft stores sell heat resistant mats that are used by quilters. The tops are resin impregnated fabric to prevent fabric from slipping but at the same time serve as a ironing and cutting surface. They come with grids printed on them that might help with double checking positioning and part length.
 
oooh... good suggestion ghostwriter.
 
There was a suggestion I made a long while ago that might maybe be of value - if the surface had a couple of small holes drilled into it then the reprap could deposit a couple of squirts of plastic into each hole at the beginning and then replicate on top of that. The resulting object would then have two plastic spikes underneath that would prevent it from moving as it's fabricated - they could presumably be snipped off fairly easily when the object is complete.
 
Do you mean an pegboard between the surface and the printed object?
 
The current problem is that the corners lift up, so any pegs would have to go pretty deep. The lifting reults in object distortion. Hmm, angled holes filled manually maybe.

The quilter's mat sounds interesting - not too many appropriate stores around for that in my neck of the woods though.

Might have to settle for a more bloke-friendly spray can of varnish on the weekend...

Vik :v)
 
But if the workpiece is warping - don't we have much more serious and fundamental issues than it just coming unstuck from the baseplate? Why is it warping? Maybe the edges are cooling off more rapidly than the center or something? Maybe the initial outlining of the piece is putting a tension on the outer edges that the infill can't resist?
 
My guess would be it warps because it shrinks on hardening. In the video, unless the overhang is deliberate, you can see that the bottom layer is MUCH smaller than the top one - smaller by more than the depth of the object.

So anything that prints one layer, then lets it harden and shrink, then covers that with a molten layer, will cause warping of the first layer as the second layer hardens and shrinks, pulling the edges of the lower layer up.

If the material can't be made not to shrink during the printing, perhaps another idea might be to printing onto a slightly domed work-surface, to counteract this effect an end up with a flat result.

But the dome would need to be variable, because the more layers you were planning to print, the more warping you'd get... and for non-simple designs, not enough. There's also the consideration that with a lot of layers you're going to have a lot of internal stresses on the object from the differing shrinkage times.

Another solution would be something that remained liquid during printing, then had a curing phase. But then of course you can't rely on it to support itself, so you'd need to print within a walled chamber and have two substances, one for your object, one for a support gel that can be removed after curing.

Another option is to make something to hold down the edges - whether this is pegs of "ink" sticking down into receiving holes in the work surface, side-clamps with a very slight overhang that, if printed close enough to, the "ink" will flow under the overhang, or pins rising up into the work from the work surface, that need to be cut off after production.

If this is going to run unattended (and at current speeds, it MUST), then it will also need something to detect "bad stuff happened" and cancel the print. Sort of the 3D equivalent of a paper jam detector.
 
Here is another idea (in a bucket full of ideas):

Step 1: Create a 'net' of material by extruding a two layer cross hatch larger than the base of your part, and let it cool.

Step 2: Add clamps, screws, or other device to keep the "net" firmly attached to the working surface.

Step 3: Create the part on top of the net.

Step 4: Scrap/cut the net off part in areas it does not belong (Could possibly also leave holes in the net if you can guarantee the net does not move when clamping it.)
 
I've already added "feet" to some parts. This helps with adhesion and may well solve the problem. They only need to be one layer thick to work. Not tried it ont he bracket though - probably won't get the chance either as I've unexpectedly had to travel for a funeral this weekend.

Vik :v)
 
An idea I've had bouncing around for a bit is whether or not it would be possible to make a cutter on the side of the machine, perhaps just a tensioned, heated wire with a shelf behind it, and make the surface that's being extruded onto out of polymorph/HDPE/one of the materials the system uses. That way, the part is held firmly by virtue of being a single piece with the build platform until the time comes for it to be removed. Might even get a good enough attachment to do dremel-scale machining on the plastic with that sort of a rig...

It would also seem a way to have a refinishing cycle on the machine, that deposits a layer of polymorph onto the surface and uses the cutter to produce a reasonably nice finish on it if it gets messed up.
 
Wow, not only can it print 3D objects, it also does performance art. :^)
 
http://www.bloomingquilts.co.nz/ProductDetails.aspx?id=224&parent=75

NZ$40 plus shipping if you want to give it a try.

Ghostwriter/Ohiomike/Mike
 
Are those green mats heatproof?

Vik :v)
 
They are used as ironing boards for ironing the cotton fabric, so they should handle the temps we are discussing. My wife uses a rowenta steam iron on max without thinking twice about it.

Mike
 
You could make the base/net out of support material so it simply washes off the piece and main surface.
How strong is that stuff anyway?
 
You could make the layers seperatly and fuse them together after they harden, but that seems like it would make the whole process take even longer.
 
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