A backup
mother mould is a rigid shell mould to support either your latex,
polyurethane or silicone mould, to prevent it from distorting
out of shape while casting. It can be made of plaster, plaster
of Paris bandage, fibreglass or one of the newer rigid polyurethane
plastic materials.
Plaster is often used to support smaller mould while fiberglass
is currently the material of choice for the concrete industry
because of its comparatively low cost and light finished weight. |
As
a rule of thumb, a latex mould of say a 200mm figure with
10 coats of latex could be a borderline case for the need
of a backup mother mould. We pour many of this type quite
successfully suspended from a hole cut in board or cardboard,
without distortion. On the other hand we have 100mm 3D tiles
that we would not think of pouring without a plaster backup
mother mould.
All backup mother moulds must be rigid to give proper support,
so it is essential that no undercuts are present on the outside
surface of your rubber mould. The easiest way to accomplish
this with a latex mould is to use small pieces of flexible
sponge material to fill in all areas around undercuts, covered
with additional coats of latex to yield a smooth surface.
Alternatively you could use Aldax
KwikMold #74 which is a filled latex paste to put the
final coats of latex on your mould which can easily be shaped
to fill undercuts and produce a smooth surface.
Backup mother moulds must be made while the model and covering
rubber mould is still attached to the mould board. |
 |
Making
a plaster backup over a latex mould
Firstly apply a thin coat of petroleum jelly over the exterior surface
of the latex mould to act as a release agent.
If the backup mother mould is to be two piece, and most would be,
a line should be drawn around the latex mould with a marking pen to
indicate the easiest parting line for the two halves.
Plastelina oil based clay is then applied to one side of the parting
line. The section of the clay on the parting line is thickened to
about 50mm, made vertical to the mould surface, smoothed and then
a grooved line cut into the plastelina about 5mm wide and 5mm deep
about halfway out from the mould. This groove is to act as a registration
and locking point for the backup mould to ensure it is located in
the correct position each casting.
Plaster can then
be applied to the first half of the backup mother mould. The
mixed plaster and water should be literally thrown against the
surface of the latex mould. This ensures that the surface is
covered evenly and eliminates most air pockets.
Small squares of hessian can then be soaked in the plaster mix
and laid over the thrown plaster to act as reinforcement. Overlap
the squares of plaster soaked hessian and apply several layers.
Build up to about 15 to 30mm thick and allow to dry.
Now remove the clay from the other side and prepare to make
the second side of the backup mould. First apply petroleum jelly
to all surfaces, taking care to cover the registration groove
that was previously cut into the plastelina clay. This groove
may require smoothing on the edges to ensure easy release.
Apply plaster and plaster soaked hessian as previously and then
allow to dry. Separate the two halves of the backup mother mould
and set aside, then remove the model from the latex mould. The
latex mould can now be nested inside the plaster backup mould
for casting.
Note: As the mould will be cast in the reverse position to which
it occupies now, we take the opportunity when making a backup
mother mould to ensure that the plaster backup is made with
a level flat surface on the top. This enables us to pour the
latex mould when the model has been removed by simply standing
on a level surface. The backup mould when held together with
elastic bands acts as a perfect vertical support of the latex
mould. |
 |
Making
a fiberglass backup Mother Mould.
The method is essentially the same as for plaster. Apply clay
to one half as previously and then a release agent of wax
or petroleum jelly to the latex mould. First coat the entire
latex surface with the resin and catalyst mix with a disposable
brush. Eliminate any air bubbles. When cured to a tacky state
apply a layer of fiberglass mat saturated with resin. Continue
applying overlapping squares of pre soaked mat until about
5mm thick. Allow to cure and then proceed with the second
half.
Remove the clay, apply release agent to all surfaces and complete
the second half. For large concrete backup moulds, do not
do the groove registration, simply drill through both halves
and use nuts and bolts to hold together.
|
You can find
all the materials you need for making backup moulds on our ebay store.
You can enter the store by clicking
here.
By
Stan Alderson |