Under the Choko Tree By Nevin Sweeney

Metal Casting in the Backyard - Part 4: Plate Patterns, Oddsides and Oilsand Moulding

By Paul

Last time we looked at how to mould a flat-based loose pattern. A similar type of pattern, especially common in industrial repetition moulding work, is the plate pattern. Here the flat pattern bas is actually glued or screwed onto the moulding board or “plate”. Sometimes with an “oddsided” pattern (more on this later) both sides of the plate are utilised.

Often a number of patterns are attached to the one plate, arranged in a grid or wheel plan to ensure each pattern has direct contact with the runner, to allow immediate access to the molten metal.

Symmetrical castings (2 equal halves) can be made with what’s known as a “turn-over” plate: a single half pattern is used to mould each half of the casting in turn. These must be accurately lined up! The second type of turn-over plate has two halves – symmetrical or not, as the case may be – of the one pattern attached to the same side of the plate. Thus the one plate is rammed up twice to provide two castings. A detachable ingate is added for the second ramming. Anyone using this method must make sure the correct halves are matched together. I’ve seen whole batches of machine-moulded castings ruined by a failure to do this.

You will note from the diagram that plate patterns generally have a built-in ingate. The runner former sits on this during the ramming of the cope.

The ramming technique is pretty much as I described it in the last article, although using a plate is somewhat easier than a loose pattern on a board. The plate is locked between the two moulding boxes by the locating pins, which fit through the corresponding holes in the plate. Ram the drag first. Turn the whole lot, boxes and all, over to ram the cope. The pattern is rapped while the box is still closed by striking the projecting rim of the pattern with a wooden mallet.

Oddsides

Another type of pattern is the “oddisde”. This is a loose pattern with an all-round contour, in contrast with the loose flat-based patterns that I talked about last time.

Some oddides are split along the “parting line” into not necessarily symmetrical halves. The parting line is simply that plane cut through a pattern which gives an equal contour or slope above and below.

The half without pins (usually the drag half) can be rammed as normal. The top half of the pattern is then “plugged into” the bottom before ramming up the cope.

The two sections come apart when the mould is opened after ramming, and each section is rapped out separately. Keep sand away from the dowel pins and the holes they fit into, or the two halves of the pattern will be unable to come apart freely when you try to separate the moulding boxes, which will result in a ruined mould.
The other sort of oddside isn’t split: this is the type likely to be encountered by the backyard moulder wanting a one-off casting using some common object as a pattern.

Prepare a rammed bed of sand in the top (cope) box, and strickle off flush with the sides. Dig out a cavity for the oddisde pattern, embed it up to its parting line, and finish off with a trowel to make a clean joint. Dust with parting powder, fit the drag box over the top (upside down, of course) and ram up.

Invert both boxes together, remove the cope, knock out the sand and replace it on top of the drag.

As the pattern will still be embedded in the drag while you ram up the cope, you will be unable to rap it out in the usual way once the cope is full of sand. So jab the rapping bar into the pattern before ramming the cope, and leave it there until the ramming is complete. Once the top box is full, enough of the bar will be protruding from the sand for you to loosen the pattern by rapping as usual. When the pattern has been loosened open the mould – the pattern will remain in the drag section and can be lifted out with the rapping bar or a screw hook.

Feeding Head and Runner Extensions

Small moulds are quite happy with just a single connection to the outside world, the runner. But molten metal, as I’ve said before, generates a lot of gas and pressure when it is contact with damp sand. So for heavier castings requiring deep cavities in the sand (especially iron), a “riser” is essential. In its simplest form you can make a riser just by pushing a length of fencing wire through the cope from bottom to top, connecting with the deepest part of the cavity left by the pattern. In principle this is not much different to “venting” the mould as described last time.

With a really bulky mould a “feeding head” might be necessary. The main reason for this is to counteract the problem of metal contraction – without some sort of reservoir of molten metal to replenish the slowly solidifying metal in the mould, a large mould can end up incomplete. Keep the feeding head open by pumping (gently) with a stirring rod.

A feeding head can be made in much the same way as the runner, except the top will probably need to be larger to hold the extra metal: use a funnel to mould the shape.

A runner extension accomplishes much the same purpose, and it is generally used with castings bulky overall, rather than in just one section, where a feeding head could be used. Line a 2 or 3 inch (50mm – 75mm) section of 3 or 4 inch (75mm – 100mm) metal pipe with about ½ inch (12mm) or so of sand and sit it on top of the runner hole.

Odds ‘n Sods ‘n Do’s ‘n Don’ts

With oddsides, double sided plate patterns, make sure you close up the boxes right way around, i.e. so the tops and bottoms of the moulds match.

Don’t forget your runner and ingate – I’ve seen it done. No matter how good your mould is, it’s all a waste of time if you can’t get the molten metal into it!

A rough guide to the weight of metal required is given by multiplying the weight of the pattern (in pine) by 14 for iron, 15 for brass and bronze, and 5 for aluminium.

Aluminium and copper alloy castings need a wider runner than iron castings because of differences in flowability, cooling rates, etc. Runners for aluminium, brass, and bronze should be about twice the diameter of those used for iron castings of a similar size.

Do weight the top of a box before casting.

A good test as to whether the box has been rammed up properly or not is to turn it over. If the sand falls out it has been rammed too lightly. However, if the surface of the sand near the pattern is cracked, it’s been rammed too hard. Be prepared for a bit of trial and error before you get the “feel” for ramming.

Do use parting powder and plumbago.
Don’t strain your back. Even a small box full of sand is heavy.
Do check for loose sand in the mould before closing it.
Don’t over-moisten moulding sand unless you get a bang out of explosions.

With large or precision castings, due allowance needs to be made in the patternmaking stage for metal shrinkage. More on this in a later article.

Oilsand Moulding

The small scale foundry person will find greensand moulding adequate for most needs. However, a knowledge of oilsand moulding is necessary for core-making (of which more next article). In addition oilsand comes into its own whenever really intricate moulding work is being attempted. An iron engine cylinder is a good example. This would be well-nigh impossible to mould in greensand, because of the fine cylinder fins.

Oilsand moulds must be baked before casting, as their advantage over greensand lies in their hardness. Before baking the oilsand must possess enough “green bond” (see previous articles) to take the impression of the pattern. After baking the shape is held by the sands “dry bond”.

Ordinary silica (sea) sand is used as the base. Having neither green nor dry bond of its own, plan silica sand needs the addition of various bonding agents or binders. In the case of the green bond, various “proprietary” gums are generally used, and the serious backyard foundry worker will find it worthwhile to obtain some, as they are sued in coremaking as well. “Dextrin” type powdered gums are available from good paint stores as well as from foundries and founder suppliers.

Another simpler alternative is plain old everyday molasses.

Linseed oil is still the usual agent used to add the dry bond (indeed that is why it is called “oilsand”) although various sophisticated and smelly chemicals such as furan/carbamide resins are becoming popular in the industry.

One hundredweight of dry sand, 2 pints of water, 1 pint linseed oil and two pounds of dextrin is a good standard oilsand formula. If you are using molasses or treacle instead of dextrin you will require less water.

Be sure the sand is not the ultrafine windblown variety, else it won’t vent i.e. air and gases won’t escape properly and your casting may end up spattered over the ceiling.

Oilsand must be thoroughly mixed. Hand mixing, unless you’ve got plenty of spare time, is not enough. A piece of ¼ inch mild steel rod bent into the shape f a sardine tin key makes an efficient stirring attachment for an electric drill. Mix in small (half bucketful) batches. The final product should a have a consistency somewhat firmer than brown sugar. If it seems to be lacking in green strength i.e. if it won’t hold the impression of your hand when you squeeze it and let it go, add some red moulding sand or bentonite to the mix. Bear in mind though, what you gain in green strength you will lose in dry strength once the mould is baked.

Moulding with oilsand is basically the same as with greensand. You don’t need to use oilsand for the whole mould anyway. For small castings it is sufficient to cover the pattern with oilsand to a depth of about half an inch then fill with greensand and ram as usual. Larger or more intricate castings will need a proportionately greater depth of oilsand.

A commercial liquid parting compound gives better results with oilsand than coal dust or gypsum parting powder, but you would have to get it from a foundry or foundry supplier. Spray or brush onto moulds before ramming to give a clean “strip”.

One formula suggests a 90% kerosene 10% oleic acid solution, but I’ve never seen it used and can’t comment on it.

With any especially fiddly pattern, whether you’re using oilsand or greensand, as stripping plate is a useful device to ensure a clean removal of the pattern from the sand. This is just a piece of thin ply with the outline of the pattern cut out of it. Hold the stripping plate flat on the sand after you’ve rapped the pattern, then draw the pattern up through its own outline. This should stop any sand from the sides of the mould being pulled away by the pattern.

The fiddlier the pattern, the more likely you are to need to do a minor repair or two before closing the mould. I have already mentioned several tools for this.

If a small piece of sand sticks to and comes out with the pattern, leaving a gap in the mould, don’t just replace and hope for the best. It won’t be properly bonded to the rest of the sand and is likely to crumble away when the metal is poured in. Instead, replace the missing piece with a slightly larger lump of sand than you have lost, put back the pattern, then rap and remove as before. (If a pattern tends to stick, it probably needs a touch up with fine grade sandpaper.

Baking your oilsand mould is a simple process – just stick in the oven at 400⁰F until deep brown and hard.

If this causes domestic problems (such as the chook and spuds having prior claim), the ambitious backyard foundry-person might want to buy a second-hand oven, or even build one for the exclusive use of moulds and cores.

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