This project is about 3D printing thin and large sheet-like forms that are larger than the printer's build volume. This is done by printing the model in a 'pre-rolled' form, then detaching it from its supports and unrolling after curing. The printer used is the Formlabs Form 2, which is an SLA (stereolithography) printer with a build volume of 145 x 145 x 160 mm.
Step 1: Background
The idea of compacting 3D prints to fit inside smaller print volumes isn't uncommon in the SLS (selective laser sintering) print process. A great example is Nervous System's Kinematic Dress, pictured above. It consists of a series of moving triangulated joints that can be printed in a connected and folded configuration.
The SLA print process, on the other hand, relies on a precision laser to cure layers of photopolymer resin. Unlike SLS, SLA involves curing liquid resin, which requires supports and doesn't let you print assemblies with moving parts in a single build.
However, with the right combination of resin type, part orientation and 3D modeling, it is possible to print a compacted model which is larger than the build volume with SLA.
We will be printing with the Formlabs Durable resin, which is more ductile and has higher elongation than general purpose resins. This helps create thin, pliable prints that can be used for anything - from architectural models with complex surfaces to lampshades - all at a scale that is larger than the given build volume!
Step 2: Gather What You'll Need
Step 3: Model and Tile Your Surface
Create a curved surface in a surface modeler of your choice (make sure it isn't a mesh). This surface will be used as an input to create a tiled surface.
The attached script (spiral_roll.gh, spiral.3dm) allows you to apply triangulated tiling on any input surface using the following workflow:
- Divide BREP surface
- Deconstruct BREP
- Find face centroid
- Scale each face – relative to the area centroid
- Convert face to mesh
- Thicken mesh face (Weaverbird)
- Thicken underlying mesh (Weaverbird)
- Join/weld scaled face meshes with underlying surface mesh (Weaverbird)
- Export STL file
See Step 4 for other tiling options and recommended thickness.
Step 4: Design Considerations
Although it is possible to print a thin sheet without any surface undulations, it is usually better to tile the surface with thicker regions so that it behaves more like a triangulated tessellation (think scored origami tessellations).
These thicker triangulated regions are easier to print and reduce the amount of extra-thin print layers, increasing the potential success rate of the print. Above are some tiling options and tests.
Attached are two other potential tiling options (uniform_tile.gh, voronoi.gh) that can be explored.
Formlabs recommends a minimum wall thickness of 0.4mm and 0.6mm for supported and unsupported walls respectively.
Step 5: Orient and Support Parts
- Import your STL file into the PreForm software. Because of the unique geometry, you must manually orient and add supports instead of applying PreForm’s automatic settings.
- Apply Durable material settings (0.1mm layer height is better as it often leads to lower chances of failure).
- Go to ‘Add supports’, turn to the bottom camera view and begin adding touch points to the bottom of the roll.
- Slice the model to make sure that there are no unsupported areas.
Step 6: Don'ts
- The peel mechanism on the Form 2 slides the tank to shear the connection between the part and the PDMS after every layer. The sliding action along with the viscous resin create a force that begins bending/arching thin, unsupported walls in one direction.
- Try not to print a free standing straight wall, because it will curve in a certain direction and future layers will stop adhering.
- Due to this, it is preferable to print with supports on the side, or print curved surfaces that are convex towards the slide direction, so that it flows around the part.
- In the example above, the overhangs from this catenary wouldn't meet at the apex of the arch if it weren't for the supports on the sides.
Step 7: Finish Part
Remove part from build platform, keep supports on and immerse in IPA (isopropyl alcohol) for 15 minutes.
Do not leave the parts in IPA for longer than 15 minutes, because super thin walls tend to disintegrate if left too long.
Step 8: Cure Part
- Remove part from supports with clippers. The sheet should unroll into a strip, which you can cure naturally in the sun or in a UV cure box (nail salon box will do).
- Cure for anywhere between 30-60 minutes.
- The unrolled surface will harden in the orientation and form that it has been kept in. It will remain pliable, just to a lesser degree.
- You can also consider fastening the print in the desired orientation (using clips or 3D printing a base mold to fasten to) and then cure, so that it hardens in the new orientation.
(images via Formlabs)
Step 9: Additional Ideas
- A dome-like surface is more immune to the peel process mentioned in the previous step. This will have increased hoop forces that prevent the viscous resin from pushing the model in a certain direction during the peel process.
- Alternatively, a curved surface that is convex towards the movement of the resin tank will allow the resin to flow smoothly around the surface and preserve this fragile print.
- Since the Form 2 prints upside down, parts are subject to gravitational forces. Due to this, printing a catenary shape would provide stability at small thicknesses and still be pliable. Although I was able to get thinner wall thicknesses, the structure was rather strong around the top of the catenary dome and hence was less pliable. It would be interesting to gradually reduce the thickness as one moves towards the apex of the dome.
- The process document here essentially provides a small peek of the possibilities provided by SLA 3D printing. This exploration is still in progress and I would like to begin printing a combination of joint techniques, combine 'folding' and 'rolling', as well as printer larger objects that occupy the entire build volume. Watch this space for updates!