PourStrap

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Subspecies

The two major subspecies of PourStrap are "monolithic PourStrap" and "lots-of-parts PourStrap".

A PourStrap forms one rigid base part out of some kind of cement -- perhaps with rods, fixture points, bolt holes, embedded bolts, etc. firmly embedded in the cement at precisely defined angles. By making it one large solid object, it becomes more rigid and rugged than a bunch of parts bolted together. By making it out of a liquid, the top surface is "self leveling" and automatically forms an optically flat base -- something that is difficult to achieve with other materials.

A distantly related cousin is "shelling" parts, extruding only the (hopefully water-tight) outer shell of a part, then pouring some kind of cement into the interior. Marcus Wolschon designed a shelled Mini-Mendel that greatly speeds up generation time.

By "some kind of cement", we mean "something that starts out relatively liquid and pourable, and later hardens up", such as but not limited to:

Several researchers have proposed that some or many of the parts of a RepRap could be made by pouring some kind of cement into a mold:

These proposals generally fall into 3 categories:

  • initial bootstrap RepStrap processes: it would be better (faster, cheaper, etc.) to print this part on a RepRap, but we make this part using this alternate process as part of the initial bootstrap process in order to get the first plastic-extruding RepRap. Once we have a RepRap, we don't really need to do it the hard way any more.
  • economy-of-scale: we could print this part on a plastic-extruding RepRap, but even after we have a RepRap it's faster and cheaper to make dozens of copies of this part using a mold than to print them with a RepRap. This may be an excellent way to provide many people with a RepStrap, but it seems to run counter to the rapid-improvement and self-replication goals of the RepRap project.
  • indirect self-replication: a machine makes molds, the molds are used to make new parts, the new parts are assembled into the next-generation machine. This process seems to make the "self-replication" people happy, and (for economy-of-scale reasons) seems to have lower cost than the direct replication system used in most RepRaps.

Monolithic

I think a monolithic or lots-of-parts Eiffel is the best way to begin with PourStraps.

Lots-of-Parts

Right now the best design (Mendel) is lots-of-parts and lots-of-metal-rod. I think that is a little silly, but it is the best working design we have, and ships well via the various postal and shipping services.

Monocoque

Also, if you are talking about making a Monocoque PourStrap, I would use paper-based foamboard or pegboard to make a cube, and then fiberglass or carbon fiber the outside of the cube.

Or burlap-reinforced plaster, on the outside of a LeCorb covered with vaseline and saranwrap as a Release Agent.

Everything is more fun with lots of vaseline and food-type plasticwrap. That's the one thing I learned at university.

Interior Shell

Try building a box of pegboard in the center with greased rods sticking out to align stuff. I plan on doing that, but building each edge of the cube from two strips of pegboard, and applying the fiber-stuff (burlap, fiberglass, carbon fiber, etc.) directly to the box. 24 edge-strips may be easier to disassemble and remove from the interior than 6 sheets, if they need to be removed. Although if I do that, I may not bother with the fiberglass shell.

Materials

Polymer Concrete Cement

Vitamins

Cement truck or cement mixer.

Cut and Paste

Also, if you are talking about making a Monocoque PourStrap, I would use paper-based foamboard or pegboard to make a cube, and then fiberglass or carbon fiber the outside of the cube.

Or burlap-reinforced plaster, on the outside of a LeCorb covered with vaseline and saranwrap.


Mold Release Agents

Everything is more fun with lots of vaseline and food-type plastic wrap. That's the one thing I learned at university.



Hi Sebastien,

... my idea for a monolithic frame is to construct a papercraft-model, then print/cut the contours in paper/cartonage or Dibond, fold them in 3D and start to harden the shell with glassfiber and 2K-epoxy until it can withstand the pressure of the infill ...

A multipart-frame will consist of mostly flat parts and some fixature-insets - this could be made from fabric for the hull and casted/milled aluminium- or moulded plastic-parts for the inserts ...

Try building a box of pegboard in the center with greased rods sticking out to align stuff. I plan on doing that, but building each edge of the cube from two strips of pegboard, and applying the fiber-stuff (burlap, fiberglass, carbon fiber, etc.) directly to the box. 24 edge-strips may be easier to disassemble and remove from the interior than 6 sheets, if they need to be removed. Although if I do that, I may not bother with the fiberglass shell.

How do you plan to build in geometry and keep your rods, fixture points, bolt holes, embedded bolts, etc at precisely defined angles?

Hydrostatic pressure

Epoxy and concrete has a high hydrostatic pressure and will burst through where you have sewn if you have tall column of concrete. This may be true unless you are using generic firehose-canvas used by firefighters.

Mold Sealant

Modeling Clay

The seams are not as much of a problem as the holes where your rods and static parts (fixtures, through holes, embeded nuts) are. Those holes in the fabric will be difficult to seal properly, and material will ooze out. You can seal them from the inside with modeling clay. ... You can frantically patch leaks from the outside with modeling clay.

Contact Paper

Self-adhesive paper or self-adhesive plastic sheet is useful and necessary if you are using RBS/Grid as your mold wall material.

Leaks

Your second mold will have many less leaks than your first mold. If you are lucky, you will not end up with a 50 kg PourStrap epoxied to your workbench or floor as a permanent desk ornament.

Water tight

Molds need to be watertight. If there is a pinhole in your mold, you will have 50kg of epoxy coating your workbench and floor.

Epoxy and most other Themosetting resins are heavier than water.

Geometry and mechanical degrees of freedom

If you are making a monolithic PourStrap, you have a number of fixtures and rods, and you need to align them with 1-2-3 blocks, lego bricks, masonite or steel pegboard, etc. (In which case you might make the mold from pegboard and food-type plastic wrap rather than fabric.)

If you are making a Lots-of-Parts PourStrap, like a Mendel clone, Darwin clone, Eiffel Clone, Delta-clone, you need each poured subunit to have 2 geometric ... "constraints"? 2 through-holes, 2 flats, 2 fixtures, 2 embedded nuts, 1 embedded rail + 1 throughhole, etc. Call it 2 geometric "features".

Because if we describe the Lots-of-Parts PourStrap as a 3D mesh or graph located in space, even if it is made from toothpicks and marshmellows, each marshmellow has 2 toothpicks and is connected to 2 marshmellows. (Ignoring 2 end marshmellows).

Locating and controlling the orientation of those 2 features requires RBS/Grid, 1-2-3 blocks, or RepRapped thermoplastic molds (as in Molding Mendel Parts) or fixtures (rod holders) (as in Moldmaking Tutorial).

You get 1 free flat area at the top of your cast, where gravity makes the top of the epoxy "perfectly" flat. But you need jigs and fixtures to locate and hold a rod at a specific angle and location with resepet to to this flat area.

Measurement

One thing you need for a pourstrap is a plastic drafting triangle, T-square, and lots of shim stock. Maybe a straight edge. Or a full machinist toolchest, if you have 100-500 dollars of measurement technology on hand.

Feedback

An octopus or hermit crab uses optical control and feedback (aka looking and correcting movement) to specify where the toolpoint is moving, and doesn't need 2 geometric "features".

Since most RepStraps are dumber than hermit crabs, I think it is necessary to build in precise geometry. (RepRapServo 1 0 has some ideas on how to use such crab-like optical feedback to make the machine slightly less dumb).

I apologize for not really understanding the real technical issues or mechanical engineering to describe the geometry requirements better.

Workbench

It may help to first build an optically flat workbench. Take a normal workbench or table, attach sidewalls, and pour several liters of epoxy into the volume.

Leak Test

You now have an optically flat basement or workroom floor. Try to figure out where the leaks are, seal the leaks.

Try Again

Having tested for leaks, and sealed leaks, pour several liters of epoxy into the volume.

You now have an optically flat workbench.

Embedded Nuts

Much like a chocolate bar, your workbench or CNC router table may taste better with Embedded Nuts. This is also a good Optical Table.