LATEX MOULD MAKING
Latex liquid rubber is an emulsion which will air dry on the
surface of a master pattern to form a thin peelable mould.
Mould made of latex have comparatively long life, are easily
made and will give good definition and accuracy of reproduction.
NOTE up to 10% shrinkage may occur using this material
MASTERS
These can be in a variety of materials.
PLASTER OF PARIS MASTERS. Probably the most suitable
material as the porosity of the plaster draws moisture from the latex causing
it to thicken.
CLAY MASTERS are suitable but should be allowed to dry out
or be once fired before dipping in latex.
WOOD MASTERS Experimental work has shown that moulds can be
made on wood masters, which must have a surface and be of a porous nature. The
dipping and curing is the same as for plaster and clay.
NON-POROUS
MASTERS. For these type of masters, the ‘paint on’ method (see below)
will be necessary. Some metals react with latex resulting in weak moulds.
PLASTICENE MASTERS can be used with latex but may result in
a reduced mould life.
MOULD MAKING.
Two mould-making techniques are used; the ‘dipping’ method
or the ‘paint on’ method.
DIPPING. This is best carried out with porous
masters, as the porosity draws moisture from the latex, thus causing it to
thicken on the surface. However, as the moisture enters, it replaces the air
already there, thus forcing air bubbles to form in the latex. To overcome this
re-dip for a few seconds, remove form the latex and with a brush, stick or
palette knife, burst the bubbles as they form and spread the latex over the
surface.
As this is being done, the latex will quickly turn to a
paste, which will prevent the release of further air. Re-dip in the latex and
leave for 15 – 20 minutes, remove and allow any surplus to drip off, a mould
thick enough to be used should have formed.
The latex should be touch-dry in 10 minutes at normal room
temperature
(20 °C),
turning from white to a semi-transparent creamy yellow colour, and ready to
peel from the master in 2-3 hours. Drying can be speeded up by using gentle
heat up to about 70° - 75°C.
Before attempting to peel the mould from the master, apply
talc or washing-up liquid over the surface to prevent it sticking to itself
when it is peeled. If the master is dipped for a longer period a thicker
coating will be obtained, needing a longer drying time.
In the event of an over thin mould being made with the first
dip, thickness can be increased by re-dipping as soon as possible but within
about 12 hours to ensure the latex bonding to itself. Best results are obtained
by trying to get the correct thickness of latex during the initial dip.
PAINT ON.
Paint a number of layers of latex with a soft brush allowing
a part cure at normal room temperature (20°C) before applying the next layer.
Since the master is non-porous the latex will tend to ‘run’ so only very thin
layers can be applied. Keep
repeating the process until a skin of sufficient thickness has been built up;
for small pieces 7 or 8 layers may be necessary, high spots can be given extra
layers. If the master shape is warmed in an oven before the initial layer is
applied a thicker skin will form and subsequent painting will give a better
finished mould.
LATEX THICKENER.
One of the difficulties of using latex for mould making is
it’s low viscosity and it’s inability to form easily on non-porous surfaces.
This can be overcome using a coagulant referred to later, but this can also
bring it’s own problems. Another difficulty is the amount of air which can be
trapped on the surface both when painting or dipping.
However, by the addition of a very small amount of latex
thickener well stirred in, the viscosity of the latex can be adjusted to
whatever method is used.
By adding enough thickener to make the latex similar in
consistency to emulsion paint, it can be painted on any surface, porous or not,
without running.
Care must be taken to brush the thickened latex on in thin
layers, although after the initial coat has been painted on it is possible to
dip, either in the thickened or the original latex, in the normal way.
While the addition of thickener at the correct rate will not
reduce the strength of the cured latex or have any adverse effect on the
finished castings, excessive use will produce a very rigid mould which will
also tends to be brittle.
The thickened latex can be used to take moulds form
non-porous vertical surfaces e.g. wood and stone carvings on walls etc., later
supported by a plaster of Paris or glassfibre case mould, before removing from
the original.
As a guide, about 10 drops of thickener will thicken 25-50g
of latex so that it can be painted on easily. This amount should be varied to
give best results on any particular job.
USE OF A COAGULANT
When using a non-porous master shape, it is possible to
build up a suitable thickness by making the latex coagulate when in contact
with the mould face.
The coagulant used is a mixture of 25 parts by weight of
calcium nitrate to 75 parts by weight of methylated spirits. Mix and shake
vigorously to dissolve the crystals. Gently warm the master and dip into the
coagulate for a few seconds, remove and allow to dry.
Slowly dip the master into the latex (do not try to brush
the latex on as it will ‘ball’ on the brush) and leave suspended for 2 – 10
minutes depending on the size of the master and thickness required; after
removal, allow to dry out, talc, remove from the master and prepare for use as
described earlier.
The surface finish of the casting using this method tends to
be shinier. A possible disadvantage in using this method, is the discoloured
and tacky nature of the resulting castings although the effect will disappear
after several casts have removed the residue of calcium nitrate. To help remove
this chemical, soak the mould in 0.2% washing up liquid or Teepol in water (10g
to 5 litres) for 24 hours. Brushes used with latex should be washed out
straight away with paraffin oil.
Follow these links for Youtube selections:
Face Latex Cast
Face Mould Making
Hand Mould Making
