- Lesson details
In this video lesson artist and mold maker Matt Lewis will take you from start to finish of a box mold. You will learn how to make a traditional silicone box mold, one of the most reliable and straightforward types of molds in order to mold a complex organic object. You will learn the materials and techniques needed to master the entire process of building this mold from start-to-finish
- 3/4″ x 18″ x 18″ Melamine Board
- Lazy Susan
- Foam Core Board
- Metal Ruler
- Sharpie Marker
- Utility Knife
- Plastic Dowel
- Hot Glue Gun
- Silicone Rubber
- Latex or Rubber Gloves
- 5 Quart Bucket
- Digital Scale
- Paint Stirrer
- Flat Palette Knife or Putty Knife
- Scalpel with Hooked Blade
- Needle Nose Pliers
- Wire End Sculpting Tool
- Gorilla Duct Tape
- Talcum Powder
- Odorless Mineral Spirits
- Clear Plastic Cups
- Urethane Resin
- So-Strong Color Tints for Urethane and Epoxy – Black and Green
- Popsicle Stick
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In this video lesson, artist and mold maker Matt Lewis will take you from start
to finish of a box mold. You will learn how to make a traditional silicone box
mold, one of the most reliable and straightforward types of molds, in
order to mold a complex, organic object. You will learn materials and
techniques needed to master the entire process of building this mold from
start to finish.
Hi, I'm Matt Lewis. I'm going to be walking you through some of the steps for making
a simple box mold. I'll be using foamcore for the mold, and silicone. It's a tin-based
silicone, which is perfect for pouring up polyurethane castings or even polyester
castings. It's a fairly straightforward mold; essentially you're building a box around
your original part and back filling with the rubber. A few things you want to
watch for when you're making a box mold are placement of cut lines and things that
are going to trap air. Essentially your piece is going to be inverted when
you're pouring the resin into it. So you need to look for things like the hair on
this piece: you're going to need to vent it. LIttle things like that. And then, also,
tracking your seams. You don't want to be zig-zagging through the piece too
much, so you want a fairly straight line when you're cutting it. You can either
use a key knife or just make jeweler's cuts, which is a zig-zag cut that helps
the rubber fit back together. This is a fairly simple and straightforward process,
so I think we can get started. OK, we're ready to start building our
box. First thing you need to do is measure the distance around the piece for putting
the box, and also the height. You want to cut your foamcore high enough so
it gives you some room for the rubber to have at least a good inch over the top of
the figure. The other thing: when you're measuring the lines around the bottom
edge, you don't want it too wide – you don't want the figure touching the
foamcore, but you don't want it too wide, either, or you'd just be wasting materials.
So the first thing I'm going to do is look down straight from the top and find the
best spot to cut around her. It seems like she's kind of tapered this way, so
I'm going to make the first line here. I've just got a Sharpie. And it's best,
when you're measuring around the edge of the bottom, to bring your
straight edge up and make sure you're not going to clip it anywhere.
I think I can bring it in a little more here. You don't want to be too close
to the bottom edge of the base of the piece either. All right, I'll come around
to this side. Now, her head is overhanging quite a bit here, so you are going to end
up with a little bit of dead space on this side. It's going to take a little bit of extra
rubber, but that usually happens with these box molds – you'll have one spot
that takes a little more rubber than another. That's a little close there.
That looks pretty good. OK, so now that I have that plotted, I'm going to
go ahead and measure this height here. It looks like ten inches should be fine;
that gives us a good inch over the top. So I'm going to cut a ten-inch strip off
of the foamcore board.
You can use an X-Acto blade for making these cuts.
All right, there's our first cut. Another thing: if you're using dense foamcore,
you can also use a box cutter. The blades are fairly rigid and you can get a nice,
clean cut with it. All right. Now we've got our height cut and you can just
double-check. We have our lines plotted out on here, but it doesn't hurt to just
rest it against here and double-check. That does look pretty good, so that
first one is true. So I measure that line there. Now, for this first cut, you're
not going to cut all the way through the board; you just want to score it. And mark
there. And I'm going to score this top edge of the board. You're just lightly
cutting in. And that bends pretty much any angle that you need it to. So there's
our first cut. Now we can check this angle here again. That looks pretty good as well.
So we're going to measure from the next cut down the board. And again, just
scoring the top – don't cut all the way through your board.
It's best, each time that you make a cut, to come back and double-check on the
lines that have been plotted out, and make sure she's still going to fit in there
nicely. All right, that one looks good also.
Each time, measure from the previous cut.
All right, that should be all of our sides. Now, on this last one, after you make
sure she's fit in there without touching any of the sides, mark just past this
first panel, so instead of meeting right on the edge, you have a little bit of an
extra lip sticking out. I just went about a quarter-inch past. So we'll measure
that from our last cut and you can go ahead and cut that piece off.
So when it's closed on there, you'll have a little bit of an overhang on this side
right here. That also gives you a little bit of adjustment back and forth if you
need to make that.
All right, that looks pretty good. Now your box is ready to be glued on. But
before you do that, the first thing you want to do is check over the piece
for anywhere that's going to trap air. Now this piece right here is going to
catch air when it's filled with the resin, so you want to make a vent through
here. I've got this regular plastic dowel. You can find these at any hobby store.
It doesn't have to be yellow. So what I want to do is make a connection from
here into the body. And it's just making a dead-end vent; it's not venting it all
the way back to the top. We're just connecting it right back into the body
here. So most of these plastic dowels you can cut with a regular knife, just
roll it across the edge of your carpenter knife, and it breaks easily like that.
Depending on what kind you get, some of them bend nicer than the others, but this
one bends pretty well, and it should hold the bend. And you want to make that
connection in there. If you're making your mold off of a rigid structure, you can use
a C.A. glue, or hot glue. C.A. glue is catalyzed superglue. But this is a clay
mold, so I can kind of press this piece in and tie it in there nicely and it should
just stay put on there. All right, we're ready to put our box on. Now, for
assembling the box, I'm just using regular hot glue. What you can do is always have
an extra handful of glue sticks set aside so you don't have to stop in the middle.
Fit your box around. And before you start to glue, make sure you look down into the
mold and make sure she's not touching anywhere on the sides. Just take an
extra second to look it over. That looks pretty good. I'm going to tack this at the
corners to hold it in place.
It just takes a second for the hot glue to cool on there and it should hold it fine.
There's really no such thing as an extra hand when you're doing this, so it's best
to not get ahead of yourself; just let the hot glue tack up, and we'll tack this top
corner as well. And you can let it drip down the seam.
We're using a tin-based silicone, which is fairly versatile. You can make a mold
off of sulfur-based clays with the tin silicone, and with the tin silicone, you
don't need any kind of mold to release; it should just peel right back off.
I'm going to start running a bead of hot glue all along these edges. You want to
make sure it makes good contact with the foamcore and also the board underneath it.
I'm going to let that cool before I finish this last line on here.
OK, that feels pretty good. I'm going to come around and make this last tack
here on the bottom and hold that together until it cools.
All right, that feels good. Now we can finish off the base and run a bead all
the way down this seam here.
You just need to be patient and let the hot glue cool before you let go of it.
OK, that looks pretty good. The last thing I'll do before I pour my rubber in is look
down in the top, decide where this will be cut, and leave a registration mark on
my box. We're not going to cut through the box, but we need to know where this
piece will be cut through in the rubber. I'm going to take a Sharpie marker and
follow the line going up the back side of the figure. Mark it on the board. And
right up the box. And even leave a little mark at the top.
I'm not going to cut her in half; I'm just going to cut from one side so I don't
need to mark the other side. I just know now that this is where that back arm is
and I can make that key knife cut, or jeweler's cut, right up this back side.
All right, she's ready for the rubber. We're ready to start mixing the rubber
for this box mold. We're going to be using a Shore 30 tin-based silicone. It's a 10-to-1
mixture, so we have a lined bucket. But this particular silicone is easier mixed by
weight, so it's on a scale here. I'm going to set it to grams and determine how
much you need to pour for your particular piece. It's going to take 10 parts of the
rubber to one part of the catalyst. So I'm going to pour this up. All right. I poured
up about 1,500 grams of silicone, so I'll need 150 grams of the catalyst.
If you have a paint mixer, it's so much easier to mix this quantity.
All right, so that should do it, and we're ready to mix. If you have a paint
mixer, go ahead and use that. If not, a stir stick should do the trick. And if you
also have access to a vacuum chamber, it's the easiest way to de-air after you've
entrained the air from mixing it. If not, you can pour over the edge of the bucket into
the mold and it should pop most of the air as it's going in. Now I'm going to mix it up.
All right, now that we've got the silicone good and mixed up, it's a nice,
even consistency and the air's all popped out of it, she's ready to pour. The best way
to start pouring is to find a straight shot directly to the bottom of the mold. You
don't want to pour this across the figure if you can avoid it. I see a nice little gap
up here in this front corner, so I'm going to get a slow stream pouring over that
edge. If you don't have access to a vacuum, you can just pour really slowly over this
edge and that sort of waterfall action will pop the bubbles as it comes up over the
edge. You can almost see them stretching out and popping as it's pouring down. You
just want to pour really slow, let it come up your part. If there are any spots that it's
going to trap air in there, let it creep slowly around and the rubber should
force the air out ahead of itself.
You may need to do this in a few different batches.
I've got a little space left here at the top, so I'm just going to mix one more
small batch and then top it off. I want to make sure I've got at least a half-inch
to an inch of rubber over the top of the mold, of the part.
All right, I'm going to mix one more batch. OK, I'm going to top this off with this
second batch here. Again, I want to pour away from the figure. Pour right into the
rubber that's already in there. Go slow; let it come up around the figure.
I want to make sure that I get at least an inch over the top. You don't have to fill
it to the top of the box, but if you have enough rubber, that's fine. It just needs
to be a half-inch to an inch over the top of the figure. And you need to let this
sit about 24 hours before you cut it open. And that's it.
All right, I've given this about 24 hours to cure, so it's ready to be cut out of the
mold. The first thing you want to do is release it from the base and cut it free
from the hot glue. So I get a regular paint scraper and kind of work it
loose all the way around. Poke through and loosen up the hot glue first, and it
should come right loose. There we go. Try not to tear your foamcore if you can avoid
it. The next thing is to cut the hot glue off of the piece. I'll cut it off the bottom
first. This is a regular drywall knife. I'll cut the hot glue from the bottom edge.
Now that that's free, I'm going to cut this whole strip off with the hot glue on
it that closed the mold.
This is a matte foamcore board, so we may get a little sticking on there. If you can find
glossy foamcore, it tends to release away from the silicone a little nicer. But now,
we've got an opening here you can stick the scraper in and try and loosen it up.
Go down the edge and loosen the foamcore from the silicone. Take your time going
down the edge. Release it along the top as well.
Yeah, we've got a little sticking right here on the edge. I can come around
All right, now release the top from this side – work your way around using the
scraper and just wedging it between the foamcore and the silicone. Slowly pry it
loose and release that surface tension.
I'm kind of twisting the paint scraper as I remove the foamcore. Last side here.
All right, we want to save the foamcore because that's going to act as the
mother mold when we go to put this back together. But we can set it aside for now.
Now, when you're getting ready to cut the silicone open, we've got registration
marks where we mark the board, where we know that this is the line we want
to cut. So about right here, on the bottom edge, you want to make your
initial cut. I've got a scalpel with a hooked blade on it. They make several different
blades for this, but this hook blade seems to work pretty nice and doesn't give you
too many double cuts. I'm just going to make a short cut to begin with.
And just slowly work my way in until I find that foot that I'm looking for. I'm
making very short cuts. Moving the scalpel from side to side creates a key; it's called
a jeweler's cut. Once you get right up on the piece, you want to make your cut as
clean and straight as possible. So once you see the sculpture inside of here, then
you can start making a straight line. That way, when the seam closes against the
piece and you fill it with resin or wax or whatever you're casting in your piece,
you get a clean, straight flashing – if any at all. It's much easier to clean up
a straight seam than it is a curved or jagged seam. Now, I've found the part
I'm looking for. I'm going to move in slowly. Just take your time.
OK. Sometimes, if you're having trouble finding where your line is that you want
to cut up, you can remove the first part of clay that's in here. This is essentially
the base of the sculpture. I'll pull this out and it'll give me a clear line of sight
as to where I need to aim the cut. You can see right in here, this is where the
foot comes down. So I'm moving up on it right now. I'll get a little bit closer here.
All right, I'm just working my way up this leg. You want to slowly work your way up
the seam and work down towards the piece. You don't want to cut too much because if
you end up going the wrong way and you've got to cut back, you end up cutting a piece
of the rubber off that's important inside of the mold.
You can see the jeweler's cuts I'm making. All of that will act as a key. When you close
this back together, those lock back in place together and close that up tight.
I'm going to keep working my way down until I get free and clear, and then I can
start working my way towards the other leg.
All right, you can see, I'm working my way up the figure here and making the short,
tiny jeweler's cuts all the way up. I had to go partway up the back leg and then
cut across to the leg on the other side and essentially pull the pieces out as
I go. I got some needle-nose pliers and removed the armature and scooped all
the clay out. I used a regular loop tool and scooped all the clay out that I
could see and worked my way up. You may need to even look back at photographs or
images you have of the piece just to remind yourself of the line that something takes, or
maybe the hand comes down a little further over, just so your seam isn't tracking too
far all over the place. Just because of the shape of the sculpture inside, you may have
to zig-zag your armature around a little bit. Another thing I use is these clamps.
If you put them in in reverse and release them, the handles will give you almost
a third hand to help you pry it open. This isn't cut completely out yet, but I want
to show you the route we have to take sometimes to get a figure out. Work a
little bit here, a little bit there, back and forth until you're all the way down and
you get your figure cut completely out.
All right, now that we've got the mold cut completely apart, I just want to show
you a few things inside. Essentially, I worked my way up the back leg until
I found the other leg, then we cut across into the other leg. You have to sort of find
the path as you're working your way up. The next thing I was looking for once I
got up the legs was the vent that I applied to the hair on the other side. Once I came
to the vent, I needed to open up this little window here. That's the key to the whole
thing: working your way up one spot and moving to the other side and the other
spot. There's a lot of tension when you're pulling on one side and you can't really
get in there to see, so then you move to the other side. Once you have this side
open, you can really see the spot and the line you need to take from the other side.
The key is to go slow and work your way up both sides. The other thing, if you can see
from there, is making your jeweler's cuts all the way in, just to the edge of the
figure. You want that nice, clean, straight cut right up against the figure. That way,
when you're chasing the resin or wax casting, you don't have a zig-zag line
you have to clean off. You get a clean, straight line you can scrape right off. So
this is the mold cut all the way to the top. The other thing that helps is
cleaning out the clay as you're working your way up. If you've got it cut to right
here, take your loop tool and scoop down in there and dig all that out. Then you can
see the next path you need to take to the top. The other key is to cut even. If you
cut down this far on that side, make sure you cut down the same distance on the
other side so when your mold opens up, it's not pulling up here. If you only cut to
here on this side, you get a twist like that; it'll pull on that side all the time
and it could rip the rubber. So make sure you have that nice, even placement on
your cuts. So there's the mold with the figure out of it. I'll close it up and put
the housing back on. You have to shift it around until you find the right spot.
Make sure all your keys are lined up inside. When you close it up - this
particular kind, I just use tape; you can use a rubber band if you want to.
If your rubber band is too tight, it might compress. If you see, when I'm compressing
this against our cut right there, sometimes it wants to push in like this, or if it's
not compressed enough, it could open up like that. So have good compression on
there, but not too tight. I'm going to put a couple of pieces of tape on here. If
you have packing tape, that works as well. Make sure you're good and lined up on
either side. Have even pressure on the top and bottom. There's really no need to tape
all the way around; you can just tape the seam. Again, double-check before pouring
anything in there and make sure that your seams are lined up. And that's a box mold.
All right, now that we have this de-molded, I got it cleaned up, and we're going to get
ready to do a urethane casting in it. The first thing to do is go through and make
sure you've got all the clay bits out of your mold. To prep it - this is silicone,
so you really don't need mold release with the urethane, but what I like to do is just
sprinkle a little bit of talc on the surface. You don't have to get the seams, you just
want to do the surface of the actual casted part. Just sprinkle a little bit
in there. And what that does is it releases the surface tensions of the air bubbles.
When you pour in the urethane, even if you use an anti-foaming agent, you're
going to get a little bit of turbulence when the resin's pouring through. So
if you're pouring over a sharp edge like this, the resin's going to want to go over
and down. The talc kind of helps keep it to the surface and pops those bubbles
as it goes around. So I just sprinkle a little bit of talc in there and - it's kind
of messy to shake it around in your shop, so close up your mold, take it outside, and
bang it around, shake it in all directions. Make sure you get it good and coated, then
dump it all back out. I'm going to do that right now and we'll get it ready.
Now that's good and coated. You can see in there, I just got a nice, even coat over
the whole thing. Pry the edges around. If you have any dead spots, like in this arm
down here, pry it open, look down in there, and make sure you don't have any talc
trapped in there. You really don't want any dry talc. If it's too much talc, the resin
won't saturate it and you'll have a dry spot in your casting. So make sure it's
just a thin, even coat before you start pouring. The last thing you want to do
is grease these seams. I have a mix of vaseline and odorless mineral spirits.
And I like to put a thin coat just on the cut seam. You don't want any of this
on your part, so just stay a good quarter- inch away from the part. We have our
jeweler's cuts all the way down, and we have our straight cut right up against the
part, so that can act as your guide. Just don't go past the jeweler's cuts. You
don't have to do both sides of the rubber; one side will do. Just be really careful not
to get it into the cast part. I'm kind of laying it on there in a very thin coat.
There are a couple of spots up here that I'm not able to get with the wide brush,
so I put a little on my finger and rub it around. When you close that up, it helps
all these little cuts in the jeweler's cut find their home and rock back together.
That really does lock it in there. That looks good. I'm going to set this up and
put our mother mold back on.
I originally put some tape on there to close that up. I cut through the tape to
open it back up, so now I know the best place to close it. I can put this tape over
the top of where that other tape was; that way you're not peeling the paper layer off
the top of your foamcore. Just keep going back over the same spot whenever you tape
them. I avoid rubber bands because you can't control the tension of a rubber band.
It can get too tight or too loose. With this tape, you can get it nice and snug
and you can check to make sure you're not over-tightening it on the other side.
Come down to the bottom here and check to make sure all your seams are
good and closed, nice and tight. And that's ready for casting. For casting, I'm using a
urethane resin. This resin sets up in 2 minutes, so you have about 2 minutes
of working time. It's about 20 minutes before de-mold. So it's a 1-to-1, A and B.
I'll try to get a full cup. I'll use these clear cups so you can eyeball. You can also
weigh this out; it's by weight or by volume. So if you have a scale, you can
use that. Just a little more in here. But it's nice, if you're doing irregular sizes
or something like that, to use clear cups. If you have lined cups or Dixie cups,
things with lines on them, those help out nicely too. The more material you pour,
the quicker it'll kick off. This will get pretty hot if you pour a large volume,
so you want to work briskly if you're pouring a lot of material.
When this resin sets up, it'll cure to a white. There are several different
pigments you can add to it. If you don't want white, you can pretty much get it
any color you want, but just remember that it's curing to white, so whatever pigment you
use is going to cure towards the white color. I'm going to use a few drops of green here,
maybe a drop of black. When you're adding the color, always add it to part B. Part B, in
this case, is the resin with the yellow cast to it. I'm going to stir that in.
Once you've got the pigment good and mixed in, you can add it to part A.
You want to stir this thoroughly, but at the same time, you don't want to entrain
a lot of air, so I just mix slowly, scrape sides as you go, make sure you're getting
the bottom mixed.
It doesn't take a lot of stirring; you just need to be conscious that you're getting
it all mixed together. If you wait too long, you're going to feel the sides of your
cup getting hot. That's the material curing. Make sure you get it good and mixed and
get it into your mold. I'm going to pour it into my mold. I've got two openings here –
I've got both the connections of the feet. One of them is a little closer to the edge
than the other, so I'm going to start out with that one and I'm going to start
pouring a gentle stream and rotate it as I go. So you're getting the entire thing inside.
Nice coat on it. You don't want to trap a lot of air. Keep that nice, clean stream.
You may want to stop for a second and rotate the piece around, make sure you're coating
everything in there. I'm going to switch to the other foot here.
You just slowly top it off.
That looks pretty good. If you still have time, if it's not gelled up too much, you
can tap the sides and try to release some of the air if there are any spots where it's
trapped. And as it cures, you'll see it go opaque at the top. Once it starts to cure
off and get opaque, just leave it. Don't move it around anymore because as it's
curing, it's gelling all along the surface of your mold. If you move that, it'll pull
away, it'll stretch away from the surface of your mold and you'll get a stretch in that
spot of your sculpture. All right, that's cured off, so we're going to give that
about 20 minutes and we should be able to pop it out of there, take a look at it and
see what we've got.
All right, this looks good and cured up, so I'm ready for de-mold. I'm going to
cut this open.
You want to be careful - start at the top and work your way down. You don't want
to tear any of your rubber off or anything like that in any of the delicate spots. Just
slowly work down the seam, kind of prying it loose as you go. That looks good. See,
our vent filled right there and acted exactly as we wanted it to. We had this hair – this
hair would've trapped there; it would've been a dead spot, so that's why we vented
this up to the hip, and that filled nicely, so our vent worked on that spot there.
I'll come around to this side and start working it down the same way.
This is still a little warm; it may have a little bit of flex to it, so you want to be
careful not to bend the piece as you're pulling it out. Most urethanes have a little
bit of a memory to them, so if they do bend a little bit as you're pulling them out, just
hang it from a coat hanger or something like that, or a wire – just something so it's
not supporting its own weight, and by the time it cools, it should go back to where it's
supposed to be. All right, so there's our casting right there. It looks really nice -
everything came out nicely. Now we did get a little bit of a flashing on here, so I just take
a scalpel, or if you have a curved X-Acto blade, you can run it down. Use the edge
of it and shave that off. It's so minor that it just disappears by scraping it off. You
might have to tool off – we'll cut off the vent here. If you have a Dremel or
something like that, you can tool that back anywhere that you have flashing
or gates that you want to cut back off. The other thing is the bottom. It's got
a little bit of crowning on it, so if you have a belt sander or something like
that, you can just kind of hold it down for a few seconds. Don't hold it too long,
because you may get it off-balance or take a little too much off one side or the other.
Just kind of touch it a little bit here and there. But other than that, she came
out really nice. And that's a casting from a box mold.