Moulding boxes, moulding sand and tools
This series of articles were originally published in the Australasian Survivor Magazine back in the early 1980s. They were written for the magazine by a small scale founder by the name of Paul.
This article will cover the preparations for moulding: tools, boxes and sand. Next time we will go through the process of moulding, step by step – and following articles (we hope) will cover the basics of core making, patternmaking and the construction of a “mini-cupola” furnace.
Moulding Boxes
There are a number of ways of making a mould, but advanced techniques such as “flaskless casting” need not concern the small-scale operator. The small foundryman can expect to make his moulds by ramming sand into a moulding box.
Boxes come in pairs, the “drag” or bottom and the top. Each is rammed up separately, and the two half-moulds brought together leaving a hollow in the middle where the molten metal goes…..but more about this later.
In commercial foundries moulding boxes are made of metal, to withstand rough treatment. It should be fairly easy to pick up cheap or free metal moulding boxes in a variety of sizes from your friendly local commercial foundry – they go through them at a rapid rate of knots. Often perfectly good boxes are dumped when all they need is a new set of handles bolted or welded on.
For small scale foundrywork you would be unlikely to need boxes bigger than, say, 18”x 18” x 6”. Below is an illustration of a typical metal moulding box.
If you prefer you can knock up your own metal boxes using sheet metal, angle iron and steel rods for your locating pins and handles.
An even simpler idea is the wooden box. Surprising enough until you see it, there is virtually no risk of scorching since moulding sand is such a good insulator – although it is good practice to use metal boxes when casting iron.
N.B. – Particularly with light wooden boxes, as well as with metal ones when you are making a large casting, it is most important that you weigh down the top of the mould before beginning the pour. This is to prevent the top lifting – not only would the casting be lost, but you would run the risk of injury as molten metal runs out the gap between the two halves of the moud.
Another tip for wooden boxes: if you’re making them out of planed wood it’s worth grooving the boards on the inside to help “catch aholt” of the sand.
Before starting off I would recommend you make, buy or otherwise obtain 3 or 4 pairs of boxes in different sizes (say from 5” x 5” x 2 1/2” each to 18” x 18” x 6” each).
Moulding Sand
Many a good casting and many a poor moulder have been ruined through a lack of sound sand sense. Sand, indeed, is the essence of moulding.
If possible it is well worth acquiring a half a yard or a yard of the genuine article from a commercial foundry. Most will part with small amounts like this for peanuts; in fact a few people I know regularly use old moulding sand for growing veggies. (repeated dollops of molten metal effectively sterilise the sand, and with the addition of some well-rotted compost you’ve got an ideal mix for anything you fancy growing.
The sand they give you will probably be “spent”. After repeated use the clay particles in the moulding sand get burnt and the sand loses its bonding ability.ba couple of shovelfuls (shovels full?) of ordinary clay to a wheelbarrow load of sand should be enough to remedy this. Mix well!
The best way to learn how to prepare your own foundry sand or revitalise somebody else’s is to start looking at what moulding sand is actually supposed to do.
- The sand must have fairly fine grain for accurate and smooth reproduction of the pattern;
- It must be refractory – that is able to withstand the high temperatures of the molten metal.
- It must be permeable enough to allow the gases generated during the casting to escape(and avoid your own baby Mt Vesuvius in the back yard).
- It must have a good “bond” , i.e. be able to hold together in the mould.
What sort of sand satisfies these requirements? If you live near natural deposits of foundry sand already utilised as such, look no further. This can be used as is, perhaps with a small shovelful of coal dust per wheelbarrow load to increase gas permeability (although this isn’t essential).
Otherwise fine grained red “building sand” or even sea sand (sieve out the shell grit) can be used as a base. These sands have little or no natural bond: this can be supplied by adding fuller’s earth (also known as diatomaceous earth) or bentonite at a rate of about one in twenty. Alternatively add ordinary clay (or fireclay) until you achieve the same consistency as for the 1:20 fuller’s earth mix. The actual amount of clay necessary will depend on the quality of the clay.
A word of caution about sea sand. I have heard of it being used, but have never seen it done, so I’d suggest the sea sand/clay mix should be a last resort.
The last step before the sand is ready for use involves “tempering it”, i.e. adding water. The result is known as “green sand” for its rich brown colour – and yes, there are a lot of Irish moulders around.
Water moistens the bonding agent and in fact the surface tension of the water itself forms part of the green bond.
The actual proportions are about a quart of water to a hundredweight of dry sand (or about 950mls of water per 50kg of green sand). Rake and/or sieve until thoroughly mixed. The proportion will, of course, vary depending on the quality of the sand , type of bonding agent etc. so this is where experience comes in.
Grab a handful of freshly tempered sand, squeeze and break the resulting lump in two. The break should be clean without “crumbiness” or “runniness”. Some moulders talk about “fat” sand, saying good moulding sand should have the consistency of animal fat when handled. I think they must have a better imagination than I do! Personally I think a handful of slightly moist garden soil gives a better idea of what a good green sand should be like.
More Tools
At this stage the backyard “foundryperson” will need a few more tools. A basic list follows. How to use them comes later.
- Three fettling trowels – a curved one for cutting ingates, a similar sized one for patching, smoothing etc. and a small flat one (use an artist’s spatula) for very fine work.
- Tubes and sticks for forming the runners. You will need an 8” length of ½” brass tube for iron castings, and an 8” length of 1” broomstick handle rounded at one end for brass and aluminium.
- A rammer, which can also be made form a length (10” – 12”) of broomstick handle – although I would suggest a 2”diameter rammer as well for the bigger jobs.
- An ordinary fine-mesh kitchen-type coffee strainer for dusting the mould with parting powder.
- A small coarse cloth bag for dusting with plumbago.
- A small (2”- 3” diameter at the mouth) plastic or metal funnel for forming the runner mouth.
- A rapping bar made from 10” of 1/4” or ½” steel rod (see illustration) and a 6” striker made from the same rod.
- A 12” length of pointed and straightened fencing wire for “venting” the mould.
