Casting


The following article is part one of two sent to me by Ralph Brammer, a fantastic modeler in England, working in 7/8 scale. I am in his debt for these.


Casting by Ralph Brammer


Why cast?


There are two main reasons why we need to make a cast of an object, one is where the original is made from a fragile material, such as modelling clay and the other is when we need to duplicate an item many times.


The principle of casting is straight forward. You take an object, make a reverse shape in a pliable material such as clay or modelling clay, by pressing the original into the pliable material, removing the original, and filling the negative shape which is left with a liquid substance which then goes solid. When the liquid cures or sets, remove the outer material, and there you are!


The first casting I did was for a railway model. It was a simple axle-box. This was made by pushing the original into modelling clay and the impression made filled with a five minute epoxy resin. Primitive, but it worked.


The main materials we use today for casting are silicone rubber and resin. The silicone comes in a range of textures, from soft to hard. and the resins range from a setting time of a few minutes to several hours. They also cover a range of viscosity from those which are thin like milk to some which are syrup- like in consistency.


Most of the items we need to cast for model railways have undercutting on them which requires a fairly soft rubber so that it can be peeled away from the original or casting without tearing, At this stage it is worth remembering that any mould has a limited life. I find that thirty to forty cast is the number I expect to get from a small sized original. If I am casting a wagon body in 7/8” scale (this is a big lump of resin) I expect to get fifteen to twenty casts before the mould surface starts to break down. I put the difference in numbers down to the extra heat generated in the larger mould. The physical and chemical reasons need not bother us, only the results.


One Piece Moulds


Let’s get down to the practicalities of casting.
The list below is the equipment I use :


Silicone rubber from WACKER SILICONES. Elastosil M 4503.
Catalyst T35
Pipette (for drawing catalyst from bottle.)
Stainer Elastosil colour paste.
Weight scales (in grams)
Shallow mixing bowls.
Flexible spatula
Wooden spatula
Brass or aluminium rod (for stirring)
Cocktail sticks and brass wire (for helping flow of resin).
Rubber gloves.(Reject surgical ware).
Paper towels


Suppose you want to make a fleet of flat platform wagons with an inside chassis. This would be impractical to cast as one piece, so the wagon is broken down into two parts: a platform and a chassis. Each part can then be cast as a one piece mould. A one piece mould means that the original has a flat side which can be secured to a base. The object rising from the base must be capable of being removed from the silicone mould. A flat platform with low sides is ideal for a one piece mould.


With all casting the finished cast is only as good as the original master. Casting will not hide a blemish it will enhance it! So make the best master you can, remember you only have to make one! On completing the platform with all its strapping and nuts and bolts we now have our first master for making a mould


Ensure that the underside of the flat platform is as flat as possible. This area will be in contact with the base on which the casting will take place. The base can be any material which is not porous and is firm enough not to flex. We will need a base which is at least 1” bigger all round the platform. Materials I use for bases are 1/4 MDF or 60 thousand thick plastic sheet.


Right, we now have a perfect master with a perfectly flat underside to the master and a perfectly flat base 1” bigger all round than the original. We now glue the underside of the platform to the base making sure that there are no gaps between the platform and the base. Remember that silicone rubber is designed to flow into small spaces. Now that the platform and the base are joined together, what we need is something to keep the silicone rubber under control when it is poured onto the master. What we have to do is build a retaining wall around the master on the base. I use styrene sheet for this purpose. It is easy to cut and welds together well with solvent.


As the master of the wagon platform is square or rectangular the walls around can follow the same shape. I allow a gap of about ¼” to 3/8” away from the master for the wall. The four walls need to be about 1” taller than the top of the master, Ensure that that there are no gaps in any of the glued seams. Silicone rubber will find any gap. The reason for the extra height to the wall is to allow for the rocking of the silicone rubber in the mould to break air bubbles. Fill to 1/4 or 3/8” over the highest part of the master.


With most masters we don’t have to use a sealer before we start to cast. What I do is to spray the master with a car primer paint. This gives an even surface to the master and removes any finger prints etc. Silicone rubber will cast a finger-print left on a polished surface, I have done it! You can do the spraying of the master before or after glueing to the base and the erection of the walls. I do it after the walls are in place.


We are now ready to pour the silicone rubber into the mould. This is where care is needed.


A clear area to work on is needed. I use a large piece of MDF. Collect all your equipment together before you start.
With silicone rubber there is no need to work in a rush , you have plenty of time at each of the stages in the casting processes.


At this stage don the rubber gloves as it is time to open the can of silicone. Most tins have a clip seal and a rubber gasket to keep the can air tight. Remember this when you come to seal the tin after you have finished pouring. The rubber will oxidise over time if not properly sealed. Remove the lid and give the contents a good stir, some of the heavier elements settle to the bottom of the tin, over time. You now have the silicone rubber ready to pour into the bowl.


Switch on the scales, put the bowl with the two spatulas on the scales, set to grams and then zero the scales. Take the spatulas of the bowl and pour the silicone rubber into the bowl, judging the amount you think you need to fill the mould. Put the two spatulas onto the bowl and check the grams showing on the scales, note this. I have a pencil and paper ready at this stage. Write down the number of grams. The next stage is to add the catalyst to the silicone rubber. This is usually a percentage by weight . Check this with care, it is usually 4% or 5%. If you have 125grams of silicone rubber in the bowl at 4% you will need 5 grams of catalyst, 130 grams in total. If it is a 5% mix then you will need 6.25grams of catalyst a total of 131.25 grams. This can be rounded up to 132 grams..


Always check the percentage of base to catalyst as stated on the tin or check with your supplier.


This is the critical part of the process, mixing together the silicone rubber and the catalyst. To get a visual indication of a good and thorough mix I add a coloured stainer compatible with the silicone. Just dip about ¼” of the wooden spatula into the stainer and add this to the silicone rubber in the bowl. Use the flexible spatula to mix all the elements together. It will take a few minutes for all the ingredients to start to combine, but eventually you will get a smooth mix. The coloured stainer will help you to know that all the materials are blended together particularly getting into the corners of the bowl. When you are sure of the mix, it is time to start pouring the silicone rubber into the walls around the master. If the master has lots of nooks and crannies I use a cheap brush to push a layer of silicone rubber into the surface of the master making sure that no air bubbles exist on the surface or any air is left trapped in any holes, for example around rivet heads, or bolt heads. The cocktail sticks and the thin wire are useful for forcing silicone into nooks and crannies. Continue pouring the silicone into the mould, preferably into a corner until the master is covered with the correct amount of silicone rubber. Again a cocktail stick is useful to check this depth. Now keep tilting the mould gently from side to side to try to break any air bubbles which exist in the mix. Also gently tap the mould on a hard surface to release any bubbles so they float to the surface, and blow at the surface to pop the bubbles.


We now have almost finished the first part of the wagon, all that remains is to place the mould on a flat surface and leave to cure. Try to leave the mould for twenty four hours before considering removing the master from the silicone mould. It is a good idea to keep checking the walls around master in the first hour to make sure that there are no leaks through the walls. If you do spot a leak then clean off the leaked silicone rubber and using modelling clay push a wedge into the seam to stem the leak


The world is not perfect. Two situations which can exist to cause problems are, mixing too much or too little silicone rubber. If you mixed too little silicone rubber and the master is not covered all is not lost. Just repeat the process of mixing the silicone rubber, using clean equipment, and pour the new mix onto the old mix until you get the depth of silicone rubber you require on top of the master.


On the other hand , what if you mix up too much silicone rubber? The stuff is too expensive to waste. My solution is to have several masters ready for pouring. If you have several such masters ready you can choose the master best suited to the amount of silicone rubber you have left over from the first pouring.


The best way to clean the equipment after you have finished pouring the mould is to leave the silicone rubber on the equipment until it is cured. It is easy to rub the skin of rubber from equipment Don’t throw this rubber away, we will find a use for it at another stage in casting.


After the silicone rubber has cured now is the time to free the master from the mould.
First, remove the walls around the mould. This may need a little help by easing a knife between the walls and the base. Pull the walls away and start easing the silicone rubber off the master. Work all round the master to break the suction between the master and the mould.


When this process is complete you should now have the perfect mould and the master.


Save the master and the walls you have just taken from the base. You may wish to make another mould in the future.


There is just one more thing to do. What was the top of the mould when it was in the moulding wall becomes the base of the mould when we come to fill it with resin. The mould has frilly edges of silicone rubber on the four top edges. So that the mould will sit flat, for filling, these uneven bits of silicone rubber need cutting away. I use a pair of scissors for the job. Don’t throw these bits of rubber away but save them.


You now have a perfect mould of the flat platform and you want to start making the first part of the fleet of wagons,


Resin Casting


This is where we collect together all the items we need for the process of resin casting.


The resin normally comes as a two part pack which is mixed together as a 50%-50% mix by volume. The resin I use is SIKA BIRESIN G27 LV


The LV means low viscosity which is useful for pouring into thin gaps and small places. The better the flow the easier it is get the resin into difficult corners.


The equipment we need includes:
Rubber gloves
Resin
2. Plastic bottles for decanting resin from main containers
2. Jam jars
2. Teaspoons
Selection of 35mm film canisters
Selection of plastic drinks cups (Recycled)
Brass strip as a stirring rod for stirring resin
Brass strip for use as a squeezee.
Teflon Baking sheet (to be cut into pieces slightly bigger than each mould).
Brass wire and Cocktail sticks. For popping air bubbles


The most critical aspect of resin casting is to keep the two parts of the mix separate until you wish to mix them together ready for pouring. This means that equipment for each liquid must at all time be kept apart. The only equipment which meets the mixed liquids are the mixing vessel and the stirring rod. To fill different sized moulds requires different amounts of resin. I mix amounts as little as one teaspoon full of each liquid, up to a mix consisting of three film canisters of each liquid. Be careful not to cross contaminate any spoon or container, it is easy to do.
At this stage, a word of warning, Resin is Lethal . You will find it very difficult to remove from most surfaces and impossible to remove from others. Carpets just soak it up. I have lots of clothes with designer resin patches on where I have neglected to change into working clothes and thought I could get away with doing a little casting before doing another job which needed respectable clothes .So be warned!!


Let’s assume that the wagon platform mould will need one full film container of mixed resin to be filled. First we will need three 35mm film containers.


The first is for liquid number one
The second is for liquid number two.
The third is to be used as a mixing vessel for both liquids.


In the first instance I always decant the resin from the original tins into smaller plastic containers, The plastic containers are about 5” high. The reason for this stage is if I knock over a container I do not loose all the resin. I also us different shaped containers, so that I cannot get the lids mixed up. It is also easier to spoon out or pour the resin from the smaller container than the original tins.


For measuring the resin I use a clear plastic film container for the light coloured resin and a black plastic film container for the darker resin. If the film containers are the same colour wrap a coloured tape around one of them


Right now… half fill each container with its appropriate resin and pour these into the third film container. Stir the two liquids together with a brass stirring rod. Stir well for about fifteen seconds and then pour the mix into the mould. Always pour more resin into the mould than it needs to fill so that it flows onto the top surface of the mould. Then take a piece of Teflon parchment and place it with one end on the mould Gently roll it all over the mould pushing the resin in front of it. To complete the process use the other piece of brass as a squeegee moving it across the Teflon Parchment to flatten the surface of the resin to the surface of the mould. Put the mould on one side for about ten minutes. If you look at the resin that has been pushed off the end of the mould you will see it change colour and it will harden as the minutes pass. In ten minutes you can peel the Teflon off and when the resin has hardened totally peel the mould off the cast. Make sure that the cast does not deform as you start removing the mould. If it does start to bend, stop, and wait for a further ten minutes. I leave the cast for a couple of hours and them trim away any flash from the cast and gently rub it the flat side on piece of medium emery cloth. It is worth remembering that different resins have different curing times. The times given fit the resins I use.
There you are… a flat platform ready for its chassis.


Occasionally when you remove the Teflon from the mould you will see some air bubbles in the cast. These can be filled with resin from the next mix. Just drop a little resin in the hole using a cocktail stick. Leave to set and when it has set hard rub flat on the emery cloth.


If you have any resin left in the film container and you have another mould ready, you may be able to use the spare resin. Only do this if the resin is still fluid, if it is starting to change colour and thicken up,…ditch it. Any resin left in the film canister can be pulled out when it has set.



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