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There are times when it is essential to use a
pre-thickened type of
rubber using a brush-on application to make a mould of an object. An
example of this would be to reproduce architectural ornamentation
attached to a wall that could not be removed. Another would be the
duplication of large pieces of sculpture. It would be difficult to
apply the regular type of silicone or polyurethane used to make moulds
by pouring, to do this type of vertical application without them
sagging, so a pre-thickened type of silicone such as Gelsil Silicone
is used.
1. Making Good the
Model
Surface: Before commencing
the mould making, make good the surface of the original by repairing
any cracks or damaged portions and allow the repairs to dry.
2. Some Models Must
Be
Sealed: The next step is to
seal the surface of all porous articles, such as plaster, concrete,
wood or stone. One or two coats of fast drying shellac may be sprayed
on using the aerosol form of shellac available at most art or hardware
stores, as this does not interfere with the surface detail. Spraying
the sealer has the advantage that the coverage of rough detail and
undercuts is easy. If sealing is not done the silicone will penetrate
into the interstices of the porous surface and would be difficult or
impossible to remove.
Sculptors please note, shellac is also suitable for sealing modelling
clays that contain sulphur such as Roma Plastelina and clays containing
moisture. It is not necessary to seal objects made of glass, glazed
ceramic, sulphur free clays and hard plastics such as acrylic and PVC.
In all cases the sealing agent must be allowed to dry thoroughly before
continuing on to the next step.
3. Application of
the
Release Agent: The release
agent should now be applied. The aerosol Stoner 234 is recommended for
silicones and should be lightly sprayed across all surfaces that will
be in contact with the rubber and allowed to dry. Another release agent
that can be used is a 10% solution of Vaseline or petroleum jelly
dissolved in naptha or white spirit. When using most silicones this
step is often not essential but until you have tested the surface it is
better to be sure than sorry. For all polyurethane rubber, followed by
a light spray, as polyurethane sticks to anything.
Before starting any major project, the effectiveness of the
sealer/release combination should be tested using similar surfaces, on
a small scale before starting the main work.
4. Measuring and
Mixing:
This will require two plastic containers as they must be
weighed. Gelsil requires 10 parts by weight of the catalyst
to 100 parts of the silicone. The pot life is 20 minutes and the model
can be de-moulded in 6 to 12 hours. We usually leave overnight. It is
Thixotropic by nature and is ideal for brush on application. It has a
shore hardness of 25 and is capable of picking up very small detail. It
has a low linear shrinkage of 0.40% and high tear strength. It is very
suitable for reproducing models with pronounced undercuts.
Mixing tools should be of metal, glass or plastic. The Gelsil catalyst
is coloured to ensure complete mixing. After weighing out the required
quantities of silicone and catalyst pour the catalyst into the silicone
container and mix thoroughly for 3 minutes making sure you scrape the
sides and bottom of the mixing container several times. Use a flat
paddle action rather than a brisk beating action to reduce the amount
of air you beat into the mixture. You will know when you have achieved
a good mix when you have a good even colour.
5. Rubber
Application:
The silicone must be applied in layers. The number of layers of rubber
required is between 4 to 6 dictated by the size and shape of the model.
Use one of the cheap Chinese white pigs bristle brushes sold in most
art shops for brushing on the silicone. The fist coat is important as
this will pick up all the fine detail of the model. Apply this "detail
coat" in a thin layer taking particular care to cover the entire
surface. Use a dabbing action rather than a painting action
especially around undercuts to reduce the amount of entrapped air and
to pick up all detail. Let this first coat dry till it becomes tacky
before adding the next coat.
Repeat adding layers of silicone in thin layers especially for the
first 3 or 4 coats till the necessary thickness is achieved. Later
layers can be applied thicker providing the silicone is not applied so
thickly it drips off the model. To prevent the possibility of
de-lamination do not allow any of the layers to fully cure but apply
each new coat when the previous coat has become "tacky". When working
in difficult or dark conditions, a mould maker’s trick is to
add colour to each layer to ensure complete coverage has been achieved,
but this is not essential in normal working conditions.
6. Curing:
Allow the mould to cure at least, overnight before making the back-up
mould. In cold conditions leave for at least 16 hours to ensure
complete curing. After the back-up mould is made and the silicone mould
is removed post curing of the mould for 4 to 6 hours at 60 degrees
celsius is an advantage and will increase the physical properties a
great deal.
7. Applying a
Back-Up
Mould: To ensure subsequent
castings are accurate the silicone mould must be held in the correct
shape by a support or back-up mould. This can be made of either plaster
or fibreglass. For the plaster back-up use an oil soap release agent
applied to the exterior of the silicone mould. For the fibreglass use
PVA release agent.
It is essential to locate any undercuts on the silicone mould that
would prevent its easy removal and fill them in with Plastelina Clay or
alternatively make a two piece back-up mould.
8: Using the
Silicone
Mould: If the mould is to be
used for concrete use a vegetable oil thinly applied as a release
agent. For plaster use an oil soap release. This should be applied to
the mould before each casting and any excess accumulating in undercuts
or detail should be removed with a cloth to remove pooling of the
release agent.
9: Mould Life and
Storage:
When fully cured these moulds are tough and have a high tear strength
if properly used and stored. The life depends on how you use it.
Abrasive materials such as concrete will eventually cause mould detail
to deteriorate while non abrasive material will not affect detail.
After use wash moulds with warm soap and water and let dry. Moulds
should be stored on a level surface in a dry non sunny, cool
environment.
Stan
Alderson.
Copyright.
2010 Aldax Enterprises Pty Ltd
Important:
The Material Safety Data Sheets (MSDS) should be read prior to using
any Industrial moulding making product and are available upon request
from Aldax or may be viewed on their website. Directions should be read
and followed carefully to ensure safety.
The information contained in this article is considered accurate.
However, no warranty is expressed or implied regarding the accuracy of
the data, the results to be obtained from the use thereof, or that any
such use will not infringe upon a patent. User shall determine the
suitability of the product for the intended application and assume all
risk and liability whatsoever in connection therewith.
For
Technical help: Tel. 02/9533 9555
Fax
02/9533 9955
Website:
www.aldax.com.au
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