Practical Backyard Foundrywork - Part 5: Moulding Sand

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This is a reprint of an article published in the '80s in Australasian Survivor magazine.

Getting a decent batch of moulding sand is THE most important part of setting up a backyard foundry.

You can have an erratic (or even erotic) furnace, clapped out tools, shaky hands and broken down boxes – and I’ve seen all of these in various combinations – but if you have good sand you stand at least a sporting chance of producing passable castings. On the other hand, you might have the technical resources of BHP and have knocked back a professorship in Foundrywork at the University of Waikikamukau on the grounds that it is beneath your intellectual dignity…………………. But without good sand you are doomed to a lifetime of producing what are known in the trade as “shitters”.

Personally I prefer natural foundry sand. This does not necessarily mean it is better or produces cleaner castings – it is just that I am more familiar with it, as this is what we mainly used when I worked in the industry. If you can get hold of some natural stuff from a friendly foundry or as she’s dug out of the ground, I would say go for it, if only because what you see is what you get and you need only get the moisture content right without worrying about clay content, grainsize, bonding properties etc.

Most foundries, even small ones, use synthetic today. In fact, the majority did when I was in the industry (‘70s). The mob I used to work for switched over about 6 years ago.

“Synthetic” is actually a misleading term. Sand is a natural substance: synthetic foundry sand is natural (silica) sand with various things added to make it hold the shape of the pattern without crumbling. “Natural” foundry sand from the ground has a natural clay bond. Synthetic stuff has one big advantage – you have accurate control over the specifications and can whip up a batch to suit your particular needs of the moment.

Making an Impression

Moulding sand must be capable of taking an accurate impression of a pattern without breaking up when the pattern is withdrawn. It must be strong enough to withstand the flow and pressure of molten metal, yet must be able to give way as the casting solidifies or you will get what is known as “hot tearing” in the casting. It must be porous enough to allow the gases generated during casting to escape. Iron and particularly steel moulding sands must be very refractory, ie able to stand up to extremely high temperatures, but this needn’t concern us at the moment as we are looking at non-ferrous work.

Natural moulding sand consists mainly of silica, clay and iron oxide with smaller amounts of impurities such as lime, potash etc. which don’t really count unless you are making steel castings where (because their melting points are lower than that of steel) they can stuff up the surface of the casting. The “bond” imparted by the clay – either naturally occurring or added – and the size distribution of the sand grains. Are what makes a good moulding sand.

There are basically four types of moulding sand: green sand, dry sand, oil sand and “loam”.

Loam – actually a mortar-like high clay content sand often with such delectables as chopped straw, horsehair and manure added – is generally only used for large scale iron and steel castings upwards of many tons. So as someone in Queensland once said, “don’t you worry about that”.

“Oil” sand refers to sand with a chemical bonding agent added. The moulds are cast dry. The compounds used include linseed oil, which is often used in coremaking. Other chemical bonds such as carbamide-furan resins and phenol formaldehyde resins are suited to large scale production runs in modern commercial foundries, and they stink like billy-oh! Finally, although not really an oil sand, sand bonded with waterglass (sodium silicate) and “cured” ie dried, with carbon dioxide is used extensively for making both cores and moulds. Although it requires a bit of extra gear such as a sand mixer and CO2 tank, you can turn out good quality work quickly, which is ideal for medium to large production runs. I know a three man operation which uses this method to good effect, so it is suitable for smaller foundries. However, since this is a practical series, I won’t be going into CO2 moulding beyond this brief description.

It’s Not Easy Being Green

This leaves us with green sand and dry sand moulding. In foundry jargon “green” simple means “moist” – although it may be applied to inexperienced foundrypersons. Dry sand is actually moulded when green and dried out before casting. Low-clay green sand holds together better than dry sand of the same clay content: higher clay content sand is considerably stronger when dry than when green, so dry sand moulding is the preferred process when greater mould strength is required.

In practice, for small scale work the same sand mix is OK for both green and dry sand moulding, especially with synthetic sand which tends to have a greater dry strength than natural foundry sand anyway. Indeed, if you cast up some days after closing your moulds, as would be the case with most backyarders who aren’t at it full time, the moulds are likely to be partly dry anyway. So with small scale foundrywork it is a good idea to use slightly more clay than you would in a production foundry. Which of course leads on to what you have all been waiting for: the recipe. But first, a word about measurements.

Like Grains of Sand through the Hourglass

You can test foundry sand and its components (clay, silica etc) virtually down to the level of individual atoms, if that’s what turns you on. In a large commercial concern with bulk repetition work, it makes sense to have a sand lab. But we’re talking survival foundrywork, where things like compression strength, compactibility, plastic viscosity and shatter number probably sound like mere babblings from a padded cell.

The basic things a backyarder needs to have a working knowledge of are grain size (and more importantly size distribution) moisture content and clay content.

Foundry sand grain size is usually described by a grain fineness number. The main systems are based on either an imperial or metric sieve mesh size, which reflects the average grain size of a sand sample. This is useful but also misleading – like the bloke with his head in the oven and his feet in an ice bucket who reckons he’s just “warm”.

A better guide is the sand distribution graph, which shows the distribution grain sand for a particular sample. It is worked out by using a “nest” of different size sieves to divide the sample according to the proportion of different sized grains.

If you are making up your own mix, check the distribution graph for the sand you plan to buy. A good foundry sand will show a bell shaped curve – the bulk of the grains around the middle, tapering off at either side – ie fewer “large” or “small” grains.

Of course large and small are relative terms, depending on what you want the sand for. With heavy steel castings you might be looking at an average grain size of half a millimetre and a distribution from 0.1mm to 1.2mm. A facing sand for fine bronze artwork might have an average grain size of 0.1mm and a distribution between 0.05mm and 0.2mm. (facing sand gives a fine surface finish: because such fine sand is not very permeable, only enough is used to cover the face of the pattern and is “backed” with coarser moulding sand).

A Drop in the Bucket

Moisture content is usually in the range of 3% to 5% by weight – in practice you soon get a “feel” for what is too wet or too dry. Clay content is usually in the same range, depending on sand fineness and grain distribution (with finer sand, or a greater percentage of fine grains in “average” sand, you can use somewhat less clay). Most types of clay are suitable for synthetic sand but bentonite is often preferred because of its excellent green and dry bonding properties. However, id does make the sand rather “sticky” to work with, especially compared to natural foundry sand.

A General Purpose Mix for Brass, Bronze and Aluminium

1. 4 bags (160 kilos) medium fine silica sand (0.3mm average grain size: distribution between 0.2mm and 0.6mm).
2. 8kg bentonite: mix well before adding water. This gives 5% clay content.
3. 7.5 litres (about 4.5%) water.

Facing snad can be made up the same way: use 1.8kg of bentonite and 1.8 litres of water per 40kg of bag sand. Sand of 0.2mm average grain size (distribution 0.07mm to 0.4mm) should be adequate for general work; finer sands are available.

Specialty sands, bentonite and other clays can be obtained through industrial/construction sand suppliers and some bigger garden products suppliers.

For the rest of this series and other series by the same author, check out our site, here

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