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A few thoughts on warpingWednesday, April 9. 2008Recently, Chris (nophead) has been doing yoemanry duty in sorting out the problems associated with printing the various polymers that we hope to use while the rest of us worry about other issues. He recently published this little graph...  The chart displays the amount of upward curling that could be expected from various polymers printed at several fill percentages at each end of a 40 mm bar with, iirc, a 10x10 cross-section. Chris, if I've understood correctly, has been doing such tests with the notion of identifying polymers and fill rates which give the smallest amount of warping after they've been pulled off of a printable surface. I've been toying with the idea of trying to correct for warping by printing on a convex surface. The brute force approach to this would be to develop a mathematical model for warping and then design a support rick for a particular piece that we wanted accordingly. This morning, however, I am wondering if we could simply center any print on a standard convex support rick, the curvature of which would be determined by the polymer being printed. It would seem that the first idea to test would be to try to print Chris' standard bars on top of a loose-fill support rick that corrected for the expected warping. If that worked a few other issues would have to be looked at. The first would be whether the aspect ratio in the xy plane made much of a difference to the curvature of the rick. If it didn't it would seem that we should double the width of Chris' standard bar a few times to get the shape of the convex curve of the rick. The second would be to see if the depth (z dimension) of the printed object affected the convex surface curvature as well. It would really be nice if we could get by with a standard convex support rick for printing parts of a particular polymer. It would certainly make life a lot simpler. Trackbacks
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Forrest,
I am mainly working with injection molding where the process you mentioned already exists. What we do is calculate the final warpage of the part and get the software to calculate the negative shape which we use for the mold. The process however is very dangerous and it is impossibile to fully control and everybody tends to avoid.
This is why I would like to eliminate (or reduce) shrinkage on my RepRap with the following way: create an envelope which can be heated (like commercial FDM machines). Thus I can stay above the Tg of the deposited material which gives me a solid but less warped state. When I finish printing I can get the final piece out of the "oven" and it does not shrink too much further (obviously if you have chosen the right material which is better to be amorphous with low Tg.)
Another solution I am working on is the extrusion speed/filament diameter coefficient. If you make the speed higher while making the diameter lower the molecular orientation gets better thus final shrinkage is more controlled. I am not saying less as orientated molecules work like springs, however in small dimensions everything works differently and as necessary cooling time is very low the molecules may freeze before they can start the "spring" behaviour. So more controlled...
But first I need to sort out my extruder problem as it keeps killing my PICs :-))
Hope this helps by the way...
Adam
"The process however is very dangerous and it is impossibile to fully control and everybody tends to avoid."
Could you tell me more about this, especially the "dangerous" part?
I personally use Moldex3D CAE software to estimate the material flow and final warpage in the cavity. After the calculation there is an option to get the software to calculate the "negative" warpage. This results a model which we have used a couple of times to create the mold. When we tried the mold on the injection molding machine the part deformed in a totally different way instead warping back to the original shape.
What is dangerous about it is the money. For us...
For a RepRap user it is not that dangerous :-)
BTW, the warpage is dependent on many other things, like geometry, flow lines (filament direction), unbalanced thermal conditions, molecular orientation, cooling and so on... I am sure the problem will be that there will be parts of your printed object that will warp more than others... And thus this will become impossible to control. Or once you figure out how to control it for a certain geometry and you move onto another geometry the same system will give different warping results.
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