Making Moulds for Castings
By David Collier (North West Model Shipwrights), Illustrated by Rich
Brayshaw
This article also appears in the Tutorials section at
Model Ship World
Introduction
How many times have you been put off building or modifying a
model because of the logistics of the task ahead? It's so easy not to
try to adapt a model, or to become restrained by what you think are your
own limitations. That project you have wanted to build could become a
step closer with the knowledge which will enable you to mass produce specific
items. Quite a lot of items on ship models are required in significant
numbers, for example, bollards, cannon, belaying pins, hatches and doors
etc. To guarantee uniformity between each item, a moulding process can
easily be employed to produce white metal or filled resin fittings. For
the purposes of this tutorial, we have selected a bollard in order to
illustrate this process.
Creating a Master
The first step is to produce a master of the item to be moulded.
This master is known as the 'plug'. It can be made from any suitable material
such as wood, plastic or metal. It can be an individual turned piece,
such as a cannon, or a composite plug, made from several different materials.
For the bollard, the main parts such as the upright pieces forming the
column can be cut from plastic tube, or turned to diameter from metal
stock. The top 'lip' of the column could be formed from a circular disc
of wood or plasticard. Again, the base could be made from thicker plasticard
sheet, or wood of the correct thickness. Remember that if you use wood,
you may need to fill any grain to ensure a smooth finish, as what you
are now creating, including any imperfections, will be replicated on every
cast part you eventually make. Take your time getting your plug right,
and it will pay dividends in the end.
Try using cellulose sanding sealer, such as the product which Humbrol
manufacture, and also H. Marcel Guest, in the UK. This is basically cellulose
dope with a quantity of talcum powder added. You can actually add a little
shop bought talc to your cellulose dope, and this will do the same job.
Try your local model shop for plasticard and brass in different sections
also.
Other detail, such as the simulated bolt heads on these bollards, could
be made from small rivet heads which are glued into place on the plug.
These pieces should now all be glued together to produce the highest standard
plug that you can achieve, as stated earlier. If it doesn't look right,
the chances are it's not. Throw it away, and remake the part until you
are satisfied with it's appearance. Check it against the finished model
in order to determine it's scale and to satisfy yourself its correct.
One of the attractive features of the moulding process is that fine detail
can be reproduced such as emblems, bolt heads and decor, but as I keep
saying, remember that the process will also faithfully reproduce any errors!
Creating the Mould
Once the master/plug is complete, a mould now has to be prepared.
The method which we will illustrate will show the creation of a split
mould which is dowelled together when the moulding material is poured.
A plastic box of suitable dimensions is required. This is filled with
pliable non-setting modelling clay such as 'Plasticine' which is sold
in the UK. Use this clay to fill the box, taking care that no air pockets
are introduced. Now level this off with a straight edge such as a steel
rule, or even a thin length of wire acting like a cheese cutter. Our master
part, the bollard is now pressed into the centre of the clay with slow,
even pressure until exactly half of it is submerged. This process is easier
if the clay has been 'worked' by hand for a little while first.
'Dowelling' holes should now be drilled into the clay in four places towards the corners of the box, to roughly half the depth of the clay. The diameter of these holes is not critical, but should not be less than 5mm (just over 3/16"). A funnel shaped pouring channel now has be cut into the clay with a craft knife, and in the same way, a number of air vent channels must be cut from those parts in the mould which will be at the bottom when pouring. Angle these vents upwards from the bottom of the mould so that molten material will simply not flow out of it when it is poured into the finished mould.
An alternative way to produce both the air vents and pouring funnel is
to use an appropriately shaped plastic or wooden plug, which again are
pushed into the clay halfway. You may find this an easier alternative,
and this is the method that Bernard Frolich advocates in the book "The
Art Of Shipmodelling" (Ancre)
At this point, the box now has to be extended upwards using pieces of rigid cardboard or plasticard and masking/insulation tape as a fixative. Ensure all the corners of this extension and the area where the extension meets the original box are well sealed with the tape to stop leakages Extend the box to twice its original depth. Our bollard, modelling clay, and inside surfaces of the extended box must now be sprayed with a release agent and allowed to dry.
The first half-mould is formed by pouring the moulding solution on top of the bollard and clay. The silicone rubber solution used must be suitable for the resin or metal chosen for the moulded item. For RESIN casts, rubber RTV 3325 should be used, and for METAL casts, rubber RTV 3255 is suitable. These rubbers, which belong to the Polycondensation moulding catagory can be expensive and are not re-useable, but they are easy to handle, require no heat or pressure, and will reproduce the finest finish. Also, you will usually find that these is enough rubber to produce several moulds. The smallest quantity that I have typically seen for sale is 1Kg (2.2lbs)
The manufacturer's instructions must be followed for mixing the rubber and catalyst, and care must be taken when mixing as NOT to cause air bubbles, as these can produce defective moulds. When mixed, the rubber must be poured into the extended box SLOWLY from one corner, at the same time making sure that the mould fills without trapping any air around the master. The mould should be filled as near as possible to the same depth as is the clay you are pouring upon. Now the rubber needs to be left for 24 hours to cure. Do not compromise on this time, but please be patient.
After the rubber has cured, the extended walls of the box have to be removed and, with the clay uppermost, the clay section is lifted from the newly formed silicone silicone half mould. Any particles of clay must now be cleaned off the half mould, and the extended box wall rebuilt around it again, adding again the same depth as the mould you have just created.
Places the bollard master into the mould along with the wooden pouring
funnel and vents if used, and spray well with the release agent and allow
to dry. The 2 halves of the mould will stick together if the release coat
is not complete, so this operation is vital.
More silicone rubber solution can now be mixed and poured as before, to form the second half of the mould and allowed to cure for another 24 hours. The mould can then be split and the master removed. The filling funnel slot should now be cut out and the air channels chased out, if you have not previously used the funnel and vent plugs. The mould is now ready for use, with the dowels ensuring perfect registration of the 2 mould halves every time.
If pouring metal, the mould should be lightly dusted with talc each time
in order to help disperse any gases. The metal should be heated to the
required temperature and poured into the mould which should be tightly
clamped together. With both metal and resin/filler mouldings, sufficient
time must be allowed for cooling/curing before splitting the mould to
extract the perfect replica of the master part.