1. A how to guide
By
Christopher Poole
BA TPA 2

2. Casting has long been a passion of mine. It has aided
me in my work as a model maker and miniature
hobbyist for many years and am sure for many more.
Casting is the process of creating duplicates of a single
master in your chosen medium and creating an
identical copy in a fraction of the time taken to
manufacture the original or produce many copies.
I write this report more as a how to in the hope others
who read it can pick up on the techniques, tricks and
nuances I have learned and am still learning.

3. The first step in creating a cast is selecting or
producing the item or items you wish to duplicate.
Depending what your original (or master) is made
from and its general shape will determine the best
casting method used also you will have to decide
what the duplicates will be cast from, will they
need to be flexible or solid, hard wearing or short
lived.
During this report I will be using a few different
things for examples.

4. Knowing what you can produce a mold from and its
inherent properties can be the key to a successful cast. If
you make a one-piece mold out of plaster and the
master has an undercut then it’s a failure, where as if
you make the same mold from the same material but
use a two-part mold then it could be a success.

5. Molds can be made from a wide range of
materials, the most commonly used is silicone
due to its ability to capture fine detail, bend and
stretch to allow undercuts to be released, the
life of the mold and the range of thicknesses
(Shore). It is not the cheapest medium out there
but if you plan on making many high-end
copies it’s the one to go for.

6. Plaster is perhaps the cheapest to get your hands on. Its
easy to use and can catch some good detail, the molds
will last for some time if looked after properly i.e. not
dropped. The drawbacks to using plaster are that it can
take some time to dry if a particularly wet batch has
been made, it is rigid when set so planning your mold
will be essential, it is also harder to remove trapped air
from in the poring stage unlike silicone.

7. Resin can be a very good way to make a mold from if
you are familiar with it. Gellcoat is a thicker type of
resin is lightly brushed on to the master being careful
not to leave air bubbles trapped. Once it starts to
solidify (or cure) it is given a reinforcement of fiberglass
matting. This method is good to produce a mold for
many castings, as it is robust and easy to repair if
something should happen to it. It takes very little
looking after in terms of storage and will outlast most
molds. The down side is it is a messy process and
special safety gear is required throughout most of the
process, it can be very time consuming and is only
cheap if you buy everything in bulk or compare it with
silicone prices.

8. Latex is a nice cheap easy way to start of a mold. It
comes as a liquid but has the option of thickening and
does not require excessive safety gear, but does tend to
smell extremely bad. Set latex is very strong and can
bend and stretch better than silicone but it has the
unpleasant habit of shrinking. Unfortunately latex can’t
be pored like plaster or silicone and left to set but has to
be applied in layers and allowed to dry which adds on a
lot of time over all.

9. A cast can be made from a great many different materials, from
extremely solid concrete and metal to soft expanding foam. Each
material has its own particular quirks that have to be understood
and in some cases overcome to produce a good cast.
We’ll start with resin as it is the most widely used.
Resin is an exothermic material that requires a kicker to set it of
allowing it to set (methol ethol ketone peroxide). Exothermic is the
reaction where heat builds due to a chemical reaction, this is both a
help and a hindrance as heat is required to allow the resin to
solidify but too much heat and it could crack and fracture. During
the setting as the resin cools it can shrink and distort, some cheaper
resins do this more than the expensive ones. Even if all you can
afford is the cheap stuff help is at hand, in the form of filler
powders. Powders allow the excess heat to be redirected and
dissipated preventing cracking, it also bulks out the batch saving
money on resin and hinders distortion. Filler powders come in
many different types metal, marble, chalk and even glass bubbles.

10. Making a cast in foam can be an interesting choice.
Some foam solidifies hard when set and others end
up very soft and pliable. Some of the slightly more
expensive ones are self-skinning; meaning as the
foam expands (being made of bubbles) it will press
up against the mold walls and create a smooth
surface. Foam that is not self skinning will need a
thin brush of latex over the inside of the mold that
is left to dry then foam pored in.
Metal is a totally different substance to work with
so we shall go into further detail about that later.

11. During the coming examples I will be telling you to mix or add certain
materials, so you don’t get confused we will cover them here first.
Casting materials come in many different forms and from many different
places but will do the same job as their counterparts, with this in mind I
cant tell you how to mix everything precisely. I will press the point of
always read the bottle instructions first. Don’t assume the mix is the same
as the last one you may have used. Always double check labels.
As a general rule resin when mixed is a 100% to a 3% hardener. This is a
simple way of working it out if you don’t have a calculator to hand.
(Example. A cup is holding 600 grams of resin; we need 3%, which is 18
grams. 600 / 100 = 6 x 3 = 18)
Gel-coat takes around 5% and silicone about 5% also. Some resins take a
50/50 mix, that’s why you should always read the label. Other materials
such as thixotropic agents (for thickening latex, silicone etc.) are added to
eye, just add small amounts until you reach the desired consistency (one a
certain amount is added to will not thicken any more so don’t go nuts)

12. There are a few way to make a mold, we will be
covering single part and two part molds. As
stated earlier choosing the correct type of mold
can spell success or disaster for you’re casting.
The first obstacle we shall look at is an
undercut. These are parts of the master that will
prevent the new cast from coming out the mold
due to it being too large to exit the poring hole
or hooking the mold stopping a smooth pull.
Below are two molds with different shapes set
inside. Each is a plaster mold and a resin cast.

13. Example 1 shows how the pyramid shape with
its large poring base has no undercuts and so
can be pulled without any trouble.
Example 2 shows how a cast can become stuck
if not planed correctly.

14. There are three simple ways to amend the
trapped cast problem. Firstly is to make the
mold from a block of silicone, allowing the solid
resin shape to be manipulated out. Secondly
would be to make it a two part mold allowing it
to be parted releasing the cast, this method
mean’s we could still use plaster for the mold
(example A). Thirdly we could change the
position of the master if no detail will be lost
(example B)
Example A Example B

15. In this section we will go step by step on
constructing a two-part mold from silicon.
There are two ways to make one, depending on
the size and complexity of the master will
determine which would be more economical
and feasible to use.
For this example we will be using a piece of a
resin kit. I chose this piece because of its
particularly troublesome shape. For this piece
boxing up the master is the best option.

16. Step 1.
Fabricate a container that is taller than the piece
you are casting. Half fill it with oil-based clay,
do not use air-drying, as it will dry out while
the silicon sets, potentially trapping the master.
I used plasticine because it was cheaper to get
my hands on (clay is easier to work with). Now
remove enough clay so that the master will sit
snugly half way in. Make sure the mold surface
is smooth and a good connection is made with
the walls and the master.

17. Step 2
Now we need to add the keys. Keys are locating points used then
joining the mold back together to insure a good fit with no
stepping. Keys can be made from almost anything that makes an
imprint, the end of brushes is a common one to use, and as long it
creates a hole that can be filled it should do. Once this is done we
need to spray some mold release over everything the silicone will
come in contact will, If you don’t have any mold release spray,
petroleum jelly (Vaseline) will work just fine, only problem with it
is its almost impossible to apply it totally smooth and silicon picks
up ALL the detail. With our mold prepped were ready to pore the
base.

18. Step 3
Anyway once the silicone is mixed ideally you should
put it in a de-gassing chamber unfortunately not
everyone has one. So simply bounce the cup lightly of a
table top repeatedly and watch the trapped air rise to
the surface. Don’t do this for to long as the silicone
could start to set. Pore in from a single point allowing
all air to be pushed out of the way by the wave of
silicone. One the master has been covered de-gas it as
we did previously when it was in the cup just to make
sure there was no air in the mold. Wait for the silicone
to fully solidify before turning it over.

19. Remove the remaining clay being careful not to disturb the seals. If
there has not been any poring points added to the master before
this stage they need to be fabricated now. Anything that will act as
a channel to allow the resin to be pored through will suffice. Also if
there are any areas where air can’t escape a smaller channel can be
placed.
Once this is done apply more mold release and repeat step 3 to create the top.
Having applied the mold release the two silicone part’s will not have
bonded together and with a little teasing should split nicely.

20. This next method is used when dealing with
larger items where it is not practical or possible
to box up your master.
We will be using a person (life casting) for our
example in this one, as it is the main way to cast
a head. For simplicity sake we will imagine
were casting a cast of a person (a resin head and
shoulders). So we don’t have to emphasize on
the health of our model, also things are done a
little differently but we want to focus on the
casting method for inanimate objects for now.

21. Step1.
Again we will need oil based clay. Roll it out into long thin strips
about 5-10 mm thick. Look at the head and imagine or draw a line
from the base of the shoulder past the ear over the top of the head
and down to the other shoulder making sure it is at the highest
point. This will prevent any undercuts. This line is you’re guide
line, starting with the back build a wall (flange) round the head
making sure the clay is now covering the line (it will be on the face
side). Smooth it out and make sure there are no gaps between the
clay and the head, once this is done make keys all around.

22. Step2.
Lye the head down so when the silicon is
applied gravity wont take it to badly. Give a
coat of mold release and make up a batch of
silicone with some thixotropic agent through it
to make it less runny. Paint on a thin layer to
make sure it gets into all the creases and detail.
Once this is done layer on the remaining
silicone evenly.

23. Step3.
Once the silicone has solidified it will need to be
reinforced by a back layer of fiberglass or
plaster-bandage it really doesn’t make to much
difference except the rigidity in the long run.
Make sure that the fiberglass or bandage creates
a lip, this will be important later. Flip the head
when it’s hard and remove all the clay. Repeat
steps 2 making sure the reinforcement layer
meets up with the first creating a seal.

24. Step4
Drill securing holes around the flange big
enough for bolts to go though. These will hold
the two molds together whilst casting.
Tease the mold apart and remove the master,
now were ready to cast into it with whatever
you wish.