Goal

This week we're going to produce or assemble a 3D mold, either positive or negative and produce a composite fiber part from it using one of the available resins and fibers out there.

Last Week MIT lecture review

I am aware that I am starting to fall behind. Even I start the assignments and documentation I do not fully finish them on time. I need a plan to catch up. The first thing you need to do when you fall behind in something is to put yourself back on track. That is, not falling behind again. And then, schedule tasks to recover lost time. My plan for this week is to finish the current week assignment this afternoon (friday) and use the weekend, monday and tuesday to catch up week 10 and week 7. And then next week catch up week 9. That is was quite a realistic plan, but I faced further problems with week 10 assignment that I cannot solve without the use of an oscilloscope. And I have to learn how to use it. So I will be happy if I finish week 10 and 11 by Wednesday afternoon.

This week we are making composite parts. This is new this year in Fab Academy. I think it was mixed with molding and casting last year. They do it in MIT How to make (almost) anything. Composites along with 3D molding is a great technique to make shells, any kind of shell. Also, since you can reuse the mold it allows you to make repetitive things fast.

Making the mold

At the beginning my idea was making a carbon fiber composite part. But Neil took me off this idea when he explained that extra care (respirators not only for you, but for everybody in the room) should be taken. So here in Barcelona we are going to experiment with natural fiber composites: cotton, linen and sackcloth. I would like to make a lamp shade for a wall light.

Design

I drew half a solid paraboloid in Rhino measuring 25x10x5 cm which I compound with a box measuring the same as my foam and exported to stl. This week I didn't pull my hair out as you can see.

Preparing the foam

This step is optional but I just wanted to have the same amount of foam as my design. Actually you only need to worry about your material being at least big enough to contain your design. Also I did not want to have walls in my design, that is, carving inside a block.

Milling the foam

I used a piece of software named Cut3D which is actually the same as Partworks 3D that comes shipped with the Shopbot to produce the toolpaths. There are 3 types of toolpaths you can produce: Roughing that will remove most of the material quickly, finishing that will produce the final accurate surface and finally cutout if needed. For sackcloth you don't need superb detail, here you are the settings that I used. It is nice that Cut3D gives you an estimate of time that it will take to mill:

And the final mold result. There is a broken teeth near the origin in X axis in our Shopbot that causes a small imperfection, barely noticeable.

Cutting the textiles

There is a function in Rhino named Squish that flattens a non-developable (curved in two directions) 3D mesh or nurbs into a 2D flat  surface in order to be cut. It is quite a powerful tool with some settings you can adjust like preserving or not boundary length, type of material (solid or floppy), type of deformation (compress only, strech only, etc.) and some more. Squish can also tell you the percentage of deformation in a dot pattern matrix if you set it up. You can find more info on this command here.

I have just recalled my mother-in-law. She is a great stitcher and pays special attention when cutting fabric. She looks for the warp and weft threads and cuts the fabric at 45 degrees, this is a bias cut. She does this because a bias cut cloth wraps better a curved body. I would never have thought that I would put that theory into practice by cutting two biased pieces of sackcloth and seeing how they wrap in a paraboloid. When I tell her, she won't believe me.

Since my finishing toolpath settings were a bit rough I sanded a bit the surface. It's probably not necessary because the fabric I am using is quite thick.

I cut a couple of pieces of sackcloth about 35x35 cm

I checked that it was covering the whole paraboloid.

And I tested three different laser cut settings in the Epilog 36 EXT. Power set to 30% and speed varying 70% on the left, 65% in the center cut and 60% on the right cut. None of them made a full nice cut. Some thick fibers still needed to be cut manually with a sharp blade.

Finally I kept speed to 70% and raised power to 47%. And it cut it almost all the fibers. I just needed some manual cutting.

Here you are both pieces of fabric cut.

And laying in the paraboloid.

Preparing the composite

I surprised myself on how quick was this step. Just prepared a mix of two parts of resin and one part of hardener. It does not really need to be ultra precise, just eyeball it. I mixed it well ( for about 45 seconds) because it is important: resin does nothing, hardener does nothing, mixed they form the epoxy. Try to pour as little as you will use, this can be hard to guess but as a rule of thumb if you fold your fabric and try to make a cube, this is the volume you need of resin+hardener. My layers were set up as following: foam mold - food film - impregnated fabric - impregnated fabric - food film - felt breather. The whole process took less tan 10 minutes. I finally put the whole thing inside the vacuum bag.

While I was watching the vacuum process I noticed the vacuum pump was emitting a white smoke which smelled like burnt plastic. We stopped the machine and told our instructors. Luciano told us that someone put too much oil inside the pump by looking at a round indicator that was on the side of the pump. We removed some oil and the machine did not emit smoke anymore. After 5 hours of vacuum, this is the final result:

Final product review

I don't usually review the products I make but this time I got some unexpected results that I think are worth sharing. The first one is strength, the shell is incredibly stiff with just two layers of sackcloth. The second result is that bias cut proved to fulfill its function, there is not even a single wrinkle or crease on the surface, just a perfect wrapping. The third result is that the finish is commercial grade except on the boundaries. I didn't like the food wrapping film but it's giving the shell a smooth surface and it is supposedly also going to make it impervious if I wanted to put it outside. This nice finish does not include the boundaries, where the excess of resin has been accumulated. I think I can workaround it now by cutting and sanding it. With another mold design, that lets the excess of resin drain away from the fabric, this imperfection could be also avoided.

What I learned

Right after the lecture I thought that this assignment was a huge work with so many steps so I kept postponing it. The day I decided to face my laziness I finished it in just two to three hours. Sometimes I complain because I think I have too much work and so little time and a lot of excuses. Most of the time the real reason is either fear or laziness. When you are about to make something just like when you are about to swim in cold water, do not think in how long it will take, how little experience you have and so on. Just do it.

Download files

You can download all the files related to this week here.