Following a request to have another demo to cast spoons using silicone moulds I tought it would be better to post a tutorial of the process.
Materials and equipment needed:
A model of the spoon to be copied.
A container in which to make the mould. I like to keep the packaging of bought lures since they are ideal for this purpose. Other materials such as metal strips, perspex glued together (or taped) will work equally well
A syringe to measure volume.
A homemade balance scale and 2 plastic containers
The silicone used is "Moldmax 30" obtainable in Dbn from AMT in the Power Centre, New Germay.
There are two grades of catalysts available, ie, "fast" and "standard" Silicone mixed with fast catalyst starts curing the moment its combined and it is better to use standard catalyst to give air bubbles time to escape before the silicone starts curing, With standard catalyst, curing will take about 5 - 10 hours, weather dependent.
For the purpose of this tut I placed the spoon on two pieces of cured silicone scrap pieces (5mm thick) to lift the spoon off the mould container. Normally one would prepare a small quantity of silicone and pour that into the container for the model to rest on.
Using the balance scale, the silicone is prepared as follows:
An adequate quatity of silicone is poured into the right hand plastic container on the scale.
Water (coloured for the pics) is then decanted into the left hand plastic container with the syringe until the scale balances. and the volume of the water is noted. Another 10% by volume of water is then added to the same container. Catalyst is then added to the silicone in the right hand until the scale balances again. The mixture is then mixed well.
To limit the forming of bubbles in the molud, a thin layer of silicone is spread over the model first. Ideally, the silicone should be degassed in a vacuum chamber but I found that this method helps to a large extent. When air bubbles are present in the silicone, they expand when the molten metal is poued in and this causes indentations on the surface of the casting.
Please note that a pouring flue made from clay was added to one end of the spoon.
The rest of the silicone is then added to fill the mould. The position of the flue should be noted for later reference
After curing (the mould should not be "tacky" to the touch) it is removed from the mould container and cut open for a short distance on the side where the flue is located.
Cure the mould further in an oven at about 60 degrees C for about 4 hours.
Pic of cut to show extent of cut. Cutting the mould for only a short distance eliminates leakages during the pouring process.
The silicone is flexible enough soo that the casting can be removed through this hole.
Let the mould cool down after curing and pour a liberal amount of baby powder inside to enhance the surface finish of the casting. Shake out the powder again.
LIGHTLY wind string around the cut section of the mould to seal it and position upright on a bed of sand for casting.
Melt the white metal and fill the cavity.
After cooling down the casting is removed and the flue part of the casting is sawn or snipped off.
Please note the small indentaions caused by air bubbles on the left hand side of the casting - I did not have standard catalyst which would have limited the forming of bubbles, but the indentations are relativly easily removed per following pics.
Using an orbital sander and 100 grid paper the casting is sanded to give a better surface finish. The metal is soft and this is a quick process. A drum sander can also be used but be careful not to sand flat areas on the surface.
The sanding marks are removed on a deburring wheel mounted on a drill press, but a sheet of 3M sanding material tied to a dowl stick on a drill will also work well.
If you want the spoon to have a mirror-like shine, it can now be polished with metal polish.
The finished spoon before drilling the holes for the split rings.
In this case I only have to drill one hole where the clay flue covered that area.
