Reducing Warping/Shrinkage in Large 3D Prints

I've been doing a bit more 3D printing lately and having problems with large 3D prints warping due to uneven cooling. Most of the prints I work on are fairly large, roughly 10x8x4 cm or larger, so this can be a big problem, particularly if the dimensions change enough to mess up the fit of parts. Basically the problem is that as layers cool, they contract and pull off the build-surface. Generally I'm printing PLA on painter's tape, so this will just pull up the tape. An example is shown in the photo below:

You can see how the tape is pulled up at the near edge; this is about 2mm of shrinkage which is not too bad. But this is also a relatively small part, perhaps 5x5x4 cm, and the problem gets worse as the parts get bigger.  A partial fix is to use a heated bed, but the Gen6 electronics for my printer do not support controlling a bed.

However a more practical solution was suggested by another member of the Vancouver Hack Space.  He's working on a very large printer for very large prints; I highly suggest visiting his site ottersoft.ca if you're interested.  Anyway, his suggestion is to space the part up from the bed using some dummy geometry and then print with support material.  The support material forms less connected layers than the object, which gives it more...'give' when printing. Here's an example:

 

The little block is some dummy geometry added to lower the bottom of the geometry about 1.5 cm. When printed with support, the extra flex of the support lets the object stay attached to the bed without the terrible shrinking.

Part of this seems to be the extra flex, but I suspect that another part is less direct contact with the large metal build-plate, which acts like a heat sink, so the whole part cools more uniformly.

After pulling it off the build plate, you can see that the support material on both sides is in contact with the table, indicating little to no warping.  And this is a big part, roughly 10x8x3 cm.

The downside is that you now have to deal with the support material. If this is the same as the print material, it can be difficult to clean off without leaving a nasty surface finish or changing the part accuracy. For example, filing the support out of the horizontal holes without accidentally changing the hole diameter.  But once done carefully, the part is remarkably accurate: the height as measured at all four corners only differs by about 0.15 mm (with a layer height of 0.3 mm), and the holes are actually round and not elliptical.

I wish that I could take credit for this trick, but that belongs to Loial (ottersoft.ca) of VHS (hackspace.ca)