Under the Choko Tree By Nevin Sweeney

The Moulder's Cookbook - Part 1

Miscellaneous Recipes & Formulas for Foundry Workers

The old formularies such as Henley’s or the Scientific American “Cyclopaedia” have heaps of materials on metals and alloys, but very little on the foundry trades as such. Yet apart from the basic techniques (discussed in the Backyard Foundrywork series) there are a lot of useful formulas involved in foundrywork, of interest and benefit to the small scale operator.

These recipes include sand mixtures for particular jobs, varnishes for patterns, pickles for castings, binders for cores etc.

The formulas below are compiled from old (pre-WWII) trade manuals supplemented by the old stand-by word of mouth (or as the pointy headed among us would have it “oral tradition”)

Patternmaking

Patterns likely to see a lot of use should be strong as well as light and easy to handle. An alloy which provides both of these qualities contains the following –

Zinc: 12.5% – 14.5%
Copper: 2.5% – 3%
Iron & silicon: 1.5%
Aluminium: the remainder

Screw head slots in wooden patterns should be filled, to avoid being reproduced in the casting. Beeswax was often used for this; it has the advantage of being easily removed if the pattern is to be dismantled later.

Plaster of Paris is often used to repair old patterns and is also used for building up fillets (cheaper than plastic wood). A fillet you will remember (?), is used to eliminate internal corners in a pattern.

Another compound for repairing patterns or for filling in any holes or gaps before applying varnish is made from 70% (by weight) ground chalk, 15% - 20% starch and 10% - 15% linseed oil.

A durable varnish for wooden patterns can be made by dissolving shellac in methylated spirits. To restore cloudy or dirty looking shellac varnish, add a small amount of powdered oxalic acid.

Epoxy resin is widely used in patternmaking, particularly in the production of plate patterns. The small scale foundryperson who wants to make a number of copies of a particular pattern to sell, swap or whatever might find the following method useful. The master pattern is first constructed in the usual manner, in wood, Make a plaster mould of this wooden pattern, allow to dry and paint with a cellulose based varnish. This plaster mould should be given a light coating of grease or oil before the epoxy is poured in.

Epoxy resin is expensive, so the bulk of the pattern is actually made up of a filler – metal powder or silica flour (powdered quartz). The filler has the additional property of imparting strength and wear resistance to the pattern.

A useful forma is as follows -:

Binder (epoxy resin) – 100
Plasticiser (oleic acid etc) – 15
Filler (iron powder) – 200
Hardener (polyethylene – polyamine) – 20

(Proportions here are by weight)

The mixture should be well stirred and poured into the moulds as soon as possible, as it thickens rapidly. The moulds are left for 24 hours to allow the plastic to solidify, and then heat treated at 50⁰C - 100⁰C for 4 – 5 hours. Any holes in the patterns can be patched with epoxy putty, and the moulds themselves can be re-used.

Another quite different formula gives a composition which has many advantages over wood for making small patterns. No doubt the enterprising experimenter could think of a lot of other uses for it too.

Mix three parts by volume of starch, two parts fine softwood sawdust and one part glue* into a thick paste with hot water (do not add the sawdust until the starch and glue have been dissolved). Heat to 190⁰F (88⁰C) and continue heating until the whole becomes a hard mass, then allow to cool and remove from the container.

The resulting composition is strong, hard and horn-like, and can be machined, sand papered and varnished the same as wood. The main advantage of this material over wood is that it has no grain, and therefore turned or complicated patterns do not have to be built up and glued together. For the same reason it is easier to turn and machine, and gives a smoother surface. It is also more fireproof than wood and less readily affected by moisture.

Moulding Sand

The backyard moulder should be able to get good results with small scale work using the basic sand mix described in the Metal Casting in the Backyard series. However, it is worth taking a look at a few different mixtures, including some used for special purposes.

For a start, there are fine and coarse sands. Without getting into the realm of sand testing, which is of little concern to commercial foundries (unless they use synthetic sand) and even less to the small scale operator. It would be safe to say that the good old Aussie beach sand is coarse, while red building sand (building sand) is medium to fine. As a general rule of thumb, light castings require finer grained sand than heavy castings. If course sand is used for light work, the castings will have a rough surface (although this may not matter). If fine sand is used in heavy moulding, the gas generated in the casting process will be unable to escape properly and is likely to rupture the mould. In practice though, a good medium grained sand should prove satisfactory for all normal applications provided –

a) That larger jobs are well vented to allow gas to escape.
b) That the surface of smaller moulds is dusted with fine facing sand or coated with a plumbago or china clay “wash” to ensure a smooth finish.

All these points have already been mentioned in the Metal Casting in the Backyard series.

An average moulding sand might contain 75% - 85% silica, 5% - 13% alumina, usually less than 2.5% lime and magnesia, less than 5% iron oxides, and up to 4% combined water. It is the alumina, chemically combined with the water and silica in the clay fraction of the sand which provides most of the “bond” referred to in earlier articles. Ferrous (iron) oxide is also a strong bonding agent, providing what is known as the “ferruginous” bond, which is only slightly affected by the burning action of the molten metal.

When a mould is cast the sand in closest contact with the molten metal becomes “burnt” – the water which is chemically combined with the clay particles is driven off, and much of the bond is destroyed. Naturally, because of the high temperatures involved in iron founding more sand is burnt than occurs in non-ferrous founding. Used sand, which incorporates a varying proportion of burnt sand depending on the type of casting (ferrous or non-ferrous) and the nature of the job (light, medium or heavy) is known as “floor sand”, possibly because that is where it generally sits. (this may explain why I talk about “backyard foundrywork” and not “kitchen foundrywork”).

Since the floor sand has had its bonding ability reduced to a greater of lesser extent by previous casts, it needs to be rejuvenated by the addition of new, unburnt sand from time to time. The following formulas come from an old British trade manual:

1. Light work

Floor sand – 12 parts
New sand – 2 parts
Coal dust – 1 part

2. General work

Floor sand – 16 parts
New sand – 8 parts
Coal dust – 3 parts

(Note that “General Work” actually refers to quite large castings, probably bigger than most backyard operators would attempt.)

3. Smooth Finish

Floor sand – 20 parts
New sand – 8 parts
Coal dust – 3 parts 
Carbon blacking – 3 parts
Talc – 1 part

Comments:  These recipes contain far more coal dust than is generally used today. From my experience the amount given here could easily be halved with no ill effects on the castings.

Coal dust is added to greensand for iron moulding. It is not used in non-ferrous moulding, or dry-sand moulding (of which more later). The coal gas generates gas in the presence of molten metal, which forms a thin film that prevents actual contact between the surface of the mould and the metal long enough to prevent impurities in the sand (slag) from being melted into the skin of the casting.

Brass, bronze and aluminium melt at a much lower temperature than iron, so as said above, less sand is burnt during casting. As a result, sand for non-ferrous work can be used repeatedly, with only small additions of new sand when necessary. (refer back to the “squeeze test” in part 2 of Metal Casting in the Backyard series). A finer grade of sand is generally used for brass, bronze and aluminium work – less water is also used.

The Metal Casting in the Backyard series did not discuss dry sand moulding, mainly because it really only comes into its own for large and intricate casting in iron, or occasionally brass or bronze. Moulding techniques are exactly the same as for greensand work, with two minor differences: less venting is needed and harder ramming may be used. This is because casting in a dry sand mould generates less gas. Hard ramming means that the finished casting is truer to pattern than a similar casting made in greensand when the pattern is large and unusually complicated.

Of course the sand is not dry during the moulding process – similar quantities are mix in as for the greensand. The finished moulds ae baked in an oven at 300⁰F - 400⁰F (150⁰C - 200⁰C) for several hours, depending on the size, until thoroughly dry and hard. For obvious reasons, metal moulding boxes must be used.

Dry-sand mixes are loamier than greensand mixes – basically so the sand binds together better. Although a lot of the old-timers swear by horse manure, well chopped chaff or sawdust is fine. A suitable mixture is as follows:

8 parts new sand
1 part chaff or sawdust.


*considering the age of the recipe, I think the term “glue” here is referring to powdered animal glue.

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