Building Up to Maker Faire: Shawn Thorsson Brings ED-209 to Life, Part 2 — Casting

Craft & Design Robotics Workshop
Building Up to Maker Faire: Shawn Thorsson Brings ED-209 to Life, Part 2 — Casting
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Big, bad, beefy robots are the coolest thing in the world. If there’s one big robot that stands out as badder than beefier than all the others, it’s the original ED-209. In my ongoing quest to find new ways to wow the crowd at the Bay Area Maker Faire, I’m in a mad scramble to build a lifesize replica of the prop robot used in the original film. Done right, it’ll be seven and a half feet tall and look like this:

Hi there!

Now that you know what I’m up to, it’s time for the latest installment.

Thorsson’s Insane Project #209, Part 2: Prototyping and Molding the Fiberglass Parts

I had a few pics of the lower leg prototype in the last post, but it’s a good example of how these things are being made, so let’s walk through the process for that piece.  I started by making the basic shape out of MDF.

These shapes are the easy part.

Each side has a raised contour part needed to be built up. I planned to use auto body filler, but didn’t want to use an entire gallon just making this shape. To fill up space, I added blocks of scrap.

ED Lower Leg Progress 07
Small scrap blocks use up space and save money on filler.

Once I’d decided on the exact profile of the raised portion, I cut out a quick “contour gauge” from a scrap of MDF. This would help me ensure a uniform cross-section.

Nothing to see here.
A quick contour gauge ensures a uniform profile.

Then it was time to goop on some filler:

First filler pass completed.

After several iterations of fill, sand, repeat, it was smooth enough to make me happy. That’s when I moved on to some of the odd shapes around the bottom end.

ED Lower Leg Progress 10
Toe skirts are made of 1/8″ MDF and Geometry.

Once those bits were glued on, I set to work on the curved recess on the top of the shin.  In this case, I had to make a very tight curve. Since I didn’t have anything on hand that could bend into a small enough radius, I had to lay it up in a series of thin strips.

ED Lower Leg Progress 12
Can’t bend material into a tight enough curve? Just use smaller pieces.

Once I’d covered the round part, it was time to close up the rest of the opening.

Lower Leg Detailing03

The raised area above the front toe has some curve to it, but it’s gradual enough that I was able to bend a thin piece of plywood to cover it. So at the end of the day I had the first piece mostly roughed out.

He's got legs...
He’s got leg…
ED Lower Leg Progress 16
Bain is not impressed.

The next day I spent a few hours filling and covering the seams and then sanding everything smooth.

Sanding down the first layer of filler.

After a couple of rounds of filling and sanding, I decided to give the whole thing a coat of primer to see how I was doing.

The first coat of primer fills in the porous wood surface and gives you an idea of how horrible the smoothing is going.

At this point I realized I’d made a bit of an oversight and decided to add a bit more thickness to the bottom edge to facilitate moldmaking. In this case, I just glued on some strips of foamed PVC sheet.

Toe Skirt Flange
Adding to the bottom edge.

Then I used more filler to fair in the edges.

Bottom Edge Flange
Bottom edge complete.

Here it is after a bit more sanding and filling.

27 February Progress 14
Closeup shot shows some of the detailing at the top edge. We were inverted.

I must have gone through half a dozen iterations of sanding and filling and priming and sanding and filling and priming, but at this point I was confident it was finally smooth.

27 February Progress 17
Sand, fill, sand, fill, sand, fill, sand, prime, repeat…

I was wrong.

28 February Progress 02
Blast! It’s like no filling was done at all!

Since the remaining flaws were small and shallow, I was able to fill them with some glazing and spot putty.

28 February Progress 03
Spot putty to the rescue!

In the end there were a lot of them.

28 February Progress 06
So many spots.

From here, I did even more sanding with progressively finer grit sandpaper until I got to 320-grit and called it good enough.

28 February Progress 13
So much smoothing.

In order to make it easier to get the pieces out of the molds, I need them to be as shiny as possible. This means applying a coat of my choice prototype color. Bear in mind that at this stage, it doesn’t matter what color it is, only what shape it is.

28 February Progress 14
The color doesn’t matter.  So I make it as wrong as I can.

Here it is with few other pieces in all their glossy glory.

It's a new color.  Lightish red.
With a few coats of paint, you can’t tell that they’re just made out of cardboard and garbage.

After applying a 2nd coat and giving it plenty of time to dry, it’s still not shiny and smooth enough. So the next step is wet-sanding it with progressively finer and finer grades of sandpaper. Along the way you get to such a fine grit that it’s hard to believe it’s sandpaper at all.

04 March Progress 06
I use rougher paper to wipe my nose.

When you’ve graduated to sandpaper so fine that you might as well be rubbing it with your bare wet hand, you’re ready to start waxing.  Using a mold release paste wax, the process is identical to waxing a car. This fills in any scratches that you might have missed in the fine sanding process and gives everything a perfectly slick sheen. With that done, it’s time for…


Here’s a few terms you’re going to hear me use for the rest of the article.

Prototype Also called a “model” or a “plug” this is the original part to be molded (i.e. the thing I just made above).
Mold The device used to make a duplicate of the prototype.
Part The piece that comes out of the mold.  Ideally, this is the exact same shape and size as the prototype.
Release agent The stuff that keeps the mold from sticking to the prototype, and later the part.

Materials needed:

  • Masking tape, I use the blue painter’s kind available at your local hardware store
  • PVA Mold release
  • Mold release wax
  • FIberglass resin
  • Gelcoat
  • Fiberglass mat
  • Oil-based clay
  • Chip brushes (just buy a box, you’ll ruin a lot of them in the process
  • Time

One of the first things you learn about making molds and casting things is this:  If you want a rigid part, you need a flexible mold and if you want a flexible part, you need a rigid mold. If you depart from this simple rule, it requires very careful planning at all stages.

In this case, I’m going to need a rigid part.  While that would usually suggest I need a flexible mold in order to make it easier to remove the part, I really don’t have the budget for the hundreds of pounds of silicone or even urethane rubber that would go into molding pieces of this size.  Instead, I’ll be making rigid molds much like industrial manufacturers use to make bathtubs or small boats.

DISCLAIMER: At this point, I should mention that I’ve never built a bathtub or a small boat. While I have done a lot of work with fiberglass, most of the things that I’ve built have had no purpose other than to look good and be easy to move.  So although the tutorial you’re about to read will include all manner of useful information, if you finish reading it and your friend in the lifeboat repair business asks, “Can you handle some vital structural fiberglass lamination work for me without supervision?” the only answer you’ll be honestly qualified to give him is “Haha, no.”

Since some of the parts in question have fairly complex shapes, many of the molds will have to be built in separate pieces in order to be able to get the parts out.When designing a mold like this, the most important thing to do is avoiding any “undercuts.” These are areas that will lock onto the part and hold it in the mold.

The next consideration is how hard it’s going to be to slip the part out of the mold. Imagine pulling a sword from a sheath. Now imagine if the sheath was fitted to the sword so that it’s absolutely airtight with a thin film of semi-adhesive material applied to the inside right before inserting the sword. The only way to get it out is to break the vacuum between them. Clearly, the shorter the sword, the easier it will be to remove from the sheath.  Likewise, you really want to avoid molds that are excessively deep if at all possible.

With these two considerations in mind, I’ll be making the mold for the lower leg in four parts: front, back, left, and right. Since the back of the prototype is nice and flat and I can set it down to work on it, I’ll be starting with the front section of the mold. The first thing to do is determine where I want to have the “parting lines.” These will be the seams in the mold where it will come apart once the parts are done curing.

In order to avoid having the mold lock down on the little recessed details on the top end, I’ll split the mold right along their corners. To mark it, I lay down strips of blue painter’s tape.

04 March Progress 16
Putting the parting line on an edge will make it easier to smooth out the resulting mold line on the resulting part later on.

Using an oil-based clay, the next thing I will do is build up the parting wall. The key thing is to make it as smooth as possible.

04 March Progress 10 (2)
Clay parting wall in place.  Sexy.

Once the parting wall is smooth and watertight, I apply a coating of PVA mold release. This is a water-soluble material that will prevent the mold materials from sticking to the prototype.

04 March Progress 04
Applying mold release.

Once the mold release is dry, it’s time to start working with horribly toxic fumes. Don your respirator, open the doors and windows, and set up a fan or two. If it’s cold outside, toughen up, bundle up, or better yet, set the whole project aside for another day. Fiberglass work doesn’t agree with cold weather anyway.

To build up a nice, hard surface that will be shiny and smooth on the inside, start with a thick layer of gelcoat. This is the colored UV-protective coating you see on the presentable side of your fiberglass boat or bathtub. It can be pigmented to your choice of colors. I choose something bright that will make it easier to see if there are problems when I’m laying up the part later. I also like red.

04 March Progress 14
Bonus: gets extra attention from crime scene investigators .

When the gelcoat is applied evenly, you need to let it set until it cures nice and firm.  Do not give in to the temptation to touch the gelcoat too early.

04 March Progress 03 (2)
The shiny, red, candy-like gelcoat…
Don’t touch it.

Once the gelcoat is firm to the touch (I know I said don’t touch it.  You can do test touches with a mixing stick on the residue left in your mixing cup) you’re ready to lay up some fiberglass.  Huzzah!

Lots of different types of fiberglass are available. For the most part they can be broken down into two types: mat and cloth.  Mat is essentially a sheet of glass fibers laid out randomly and pressed together. Fiberglass cloth is made up of longer fibers woven together just like any other kind of cloth. Cloth is ideal for flat or nearly flat shapes while mat is ideal for compound curves and complex shapes.  In this case, I’ll be making the bulk of the molds with mat. It’s easy to tear off patches and fill in thin spots and it fits well with my generally impatient nature.

Once the parts are prepped for molding, I unroll my fiberglass and tear off enough patches to cover the entire area to be laid up twice. Then I mix up about 8-12 fluid ounces of polyester resin in accordance with the manufacturer’s instructions and get to work.

The trick is to get the resin soaked into the glass mat just enough to completely saturate it. You need to work the bubbles out along the way or they’ll end up being weak spots in the composite structure. There are specialized grooved rollers made for this exact purpose, but I usually just make do with a cheap bristle brush and a bit of elbow grease.  When your first layer is completely laid up, it should look like this:


Once you’ve layered over it a few times and built up some thickness (read: “strength”), it’s time to move on to the next section. I start by removing the clay from the flange where the edge of the first section was built up:

06 March Progress 05
Cleaning off the clay along the parting wall.

Peeling off the blue painter’s tape removes all of the residual clay from the prototype and makes me look like I really know what I’m doing. Then I build a new parting wall where the next section will separate:

06 March Progress 06
Set up for molding the next section.

After the new parting wall is built, the part gets a coat of mold release and it’s time to repeat the process for this side just like the first side:

06 March Progress 10
Gelcoat built up on the 2nd side.

The process for the second side is the same as the first side, ending with layers of fiberglass.

07 March Progress 02
Fiberglass laid up on the 2nd side of the prototype.

The same process is repeated for the remaining two sides.

07 March Progress 03
Parting walls prepared. Again.
07 March Progress 05
Side 3 gelcoated.
08 March Progress 11
Prepping the fourth and final side once the glass on the third side had cured.
08 March Progress 14
Gelcoat on the final side.

Once the fiberglass on the fourth and final side is cured, the next thing I do is clean up the rough edges with a flapwheel grinder. This is terribly messy work, so I do it outside in an area where I can contain the fallout.

09 March Progress 05
Fiberglass dust is icky.  Itchy too.

Now that the sharp pointy parts have been cleaned off of the edges of the mold, I drill a series of holes along these flanges so I can bolt the sections of the mold back together later:

Holes drilled for bolts.
Holes drilled for bolts.

The next step is to gently force a screwdriver or putty knife between the edges and pry them apart. Patience pays off here. The last thing you want to do is break the mold in the process. With slow and careful prying and a bit of gentle tapping, the mold will pop off of the prototype.

09 March Progress 07
First section of mold released cleanly.

For especially tricky shapes, it’s a bit harder to break the vacuum between the mold and the prototype. In this case, you can drill a small hole and force a blast of compressed air in. This will help pop the part out of the mold.

09 March Progress 09
You can see a tiny relief hole in the middle of this piece if you look very closely.
09 March Progress 10
As the last piece of the mold slowly releases it’s firm grip on the prototype, a deep and satisfying sigh of relief escaped from his lips.

Once you’ve got the mold apart, as long as everything’s in good order you no longer need the prototype. You can crush it or burn it or use it for target practice.  With a bit of imagination I suppose it could be turned into a cat tree. In any case, it has done it’s duty and is no longer needed.  Now it’s time to move on to polishing the insides of the mold.

Again, at this point you’re looking to make the surface as slick and shiny as possible to prevent the gelcoat and fiberglass of the part your making from bonding to the mold.  The first step is to apply at least three coats of mold release wax.  This is a great use for an otherwise bored teenager.

10 March Progress 12
Pictured: bored teenager.

Once your bored teenager finishes polishing the insides of the mold to a high sheen, I coat the inside of the mold with the same PVA mold release that I used to keep the mold from sticking to the prototype and apply a thick layer of gelcoat.

10 March Progress 06
The gelcoat in this picture is gray. It contrasts nicely with the red mold gelcoat which makes it easy to see any missed spots.

Once the gelcoat is nice and firm, it’s time to lay up fiberglass just like we did while making the mold. At this point, you need to keep away from the edges in order to avoid getting resin onto the mold flanges that might glue them shut later.

11 March Progress 10
Fiberglass resin: still stinky.

Once I’ve laid up a thick layer of glass inside of each of the mold pieces, I bolt pairs of them together and apply more glass and resin to the seams.

11 March Progress 12
Seam before being covered in glass to bond the part together.

Once I’ve covered this first pair of seams, I bolt the rest of the mold together and cover the last pair of seams. Unfortunately, my hands were full at this point and I didn’t snap any pictures of the mold bolted together while I stuck my respirator-clad head inside and laid up resin-soaked fiberglass over the remaining seams.

Once I was satisfied that the glass was laid up nice and strong, I set the whole mold assembly out in the sun to warm it up and accelerate the curing time.

A sunny afternoon in California does wonders for curing polyester resin.
A sunny afternoon in California does wonders for curing polyester resin.

A couple of hours later, it’s time to start pulling it apart again.

11 March Progress 14
Starting to feel like success.
11 March Progress 15
Even more optimistic.
11 March Progress 17
Cleanly separated mold parts.

As the final piece of the mold slowly releases it’s firm grip on the part, a deep and satisfying sigh of relief escaped from my lips. There’s always some lingering little bit of doubt in my mind every time I go through this process. Getting that first part out of the mold is pretty wonderful.

Fig. 832: Pretty wonderful.
Fig. 832: Pretty wonderful.

Since we know the mold works, the next step is to repeat the process to make the other leg. I’ll clean up the seam lines and mold flashing later.

Seam lines will disappear with the application of a little Bondo and some sanding.
Seam lines will disappear with the application of a little Bondo and some sanding.

So that’s fiberglass moldmaking in a nutshell. I’m repeating the same process for the toes, the arms, the main body, the pelvis, and more parts of the legs. Here’s a few pics of those parts in progress.

02 March Progress 12
Beginning the second half of the hip cover mold.
03 March Progress 03
Heel block with release agent brushed on.
08 March Progress 15
Gun pod prototype in primer.
08 March Progress 16
Leg rail cap prototype in progress.
08 March Progress 20
First copy of the heel block.

With all of the big pieces coming together, the next thing that’s going to be filling up my every waking hour will be the building of all of the little details including about 160 little socket head screws that this guy is riddled with. Since many of these details are duplicated more than once on the body, I’ll be describing the process involved in making silicone rubber molds to replicate small, intricately detailed parts. I’ll continue posting progress photos in the MAKE Flickr pool on a daily basis to keep you up to speed until the next  installment of this series.

Stay tuned…

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At a very young age, Shawn learned that the surest way to get the coolest toys was to make them yourselves. Now he makes costumes and props for films, promotional uses, and the occasional private commission. You can read more about his projects and scattered other adventures in his blog.

View more articles by Shawn Thorsson
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