What sand to use

The best sand to make moulds with is without doubt oil loaded or bonded sand with as fine a grain size as practicable, but it is difficult to pack as it will move and squeeze, this is particularly noticeable when loose patterns are used, however due to the small grain size it will take fine detail such as fingerprints, this sand will work with most metals but it is best for copper, and copper alloys.
This is remarkable sand can be used many times, with out the need to replenish the oils, if a little care is taken to remove the chard and blackened dusty chunks where all of the oil has burnt away each time the sand is used.

The used oil sand can then be but back with the main bulk of oil sand and mixed where the oil / resin will creep back into the used sand, and will be good for numerous uses with minimal loss of performance.
The little dusty chunks of burnt sand which are removed for the oil sand are very fine, and can be added to bulk green sand as filler.

Green sand is simply a mixture of fine refractory grade grit with an addition of clay of approximately 12% to form a bond, again it comes in many grain sizes with as many additives as you would care to imagine, horse manure being one of the most common, it is generally of a coarser grain size than a good quality oil sand but it will depend a great deal on where you obtain the sand and how much you are prepared to pay, mine is mostly second hand.

It will also require a great deal of mixing with the addition of a little water, but after the preparation it will pack well and the pattern will draw freely, providing the mould and pattern have been carefully prepared.
I have found that coarse grit green sand is best with metals that have a high surface tension, such as aluminium or iron as the metal will not follow each and every grain of sand, unlike copper.
This is not its only use however as it can be used as a backing sand when the more expensive oil sand is used as a face sand to take fine detail, but when the mould is broken it is much harder to reclaim the oil sand, care must also be taken to ensure that an adequate bond is made between the face and backing sand, or they will part and drop out.

Oil bonded powder

If you want really fine detail and a super smooth finish try oil impregnated powder, I have made some from a bag of powdered clay (bentonite) obtained from Bath Potters To prepare the powder just gently add a drip by drip of engine oil, I takes a long time to soak in but it is so fine, and will take every detail it is just perfect for casting medals, the downside is that you have to be just so good when drawing out the pattern, as any lateral movement will ruin your moul, your patterns have to be just perfect as every flaw will show like never before.

Parting powder

Parting powder is best but talcum powder can be used, every body uses talcum powder, but parting powder is better, I used talcum powder for two years before trying real parting powder and I was astonished at the difference.
Talcum powder will tend to absorb water causing it to stick to the sand, where it will break when parting the mould, the the correct grade of parting powder will not absorb moistur, for a parting bag, an old cotton sock can be used for a dusting bag, but again this is not as good a the correct cotton bag and parting powder, believe me I have done it, anyway the sock will smell more!!

Packing the mould

Always pack the mould using a moulding board on a solid bench, or base that is as firm as possible, with little or preferably no “bounce”.
Even when packing on a concrete path the moulding box will jump each time it is rammed causing the sand to be pushed through the moulding flask when ramming, this has the effect of having a raised rounded surface to the mould with the pattern on the top, and this prevents the two halves from fully meeting together, with the high probability of metal leakage.
The mould box and board should be clamped together to prevent the sand from working through when packing.

Pack the sand into each corner and crevice gently at first and with greater force as the box becomes full, with a density that is hard enough to prevent a finger being pushed into the mould body to ensure the sand has taken the pattern, and to be of consistent density throughout the mould as possible.
It is important to pack the first layers which are surrounding the pattern by pinching with your fingers to build up a mass that will compress further when the following layers of sand are added, this will help to prevent the second half of the mould impacting on the already packed half of the mould when it is time to pack it.
Packing the sand harder will cause much more work, take far longer, decrease permeability, and increase the need for vents.

Try when ever possible to sift the sand onto the pattern, this helps to fill the cavities on the pattern with a uniform sand mixture and density, then start packing by pinching the sand around the pattern by hand, this takes the detail. Ensure that you pack the first layer hard to prevent subsequent layers pushing the first layer around or cracks will appear. Rabble the surface of each layer to provide “KEY or bond” for the later layers of sand to bond to or they will split and most certainly drop out, to have a mould drop out just as you are placing them together is an absolute nightmlare! And of course will require that both the cope and the drag be remade

Always vent the mould on the reverse side of the mould (pattern) in as many places as possible by piercing through with a 16-gauge wire especially in the high spots that may not fill. A good tool here is an old knitting needle, but be sure to support the sand where you pierce or you will push out large craters making the sand at these points very thin and liable to fail with the metal pressure.

Never make the sand too wet or too dry unfortunately it is impossible teach this point as only experience will do here, you just have to get your hand in there to feel the sand but if you intend to use Green Sand (damp sand and clay) then a minimum of equipment is required.
Do not under any circumstances use a cement mixer to mix moulding sand and water, you will end up with a collection of sand balls of every size which are impossible to break down, having tried the experiment and discovered the effects of I ran the batch through sieves of various sizes, ending up with far more work and ruined a good half of the sand, ending up on the garden lawn, the grass just loves the stuff.
Build a Muller they are reasonably easy to make if you use bits ‘n’ bobs form junk sails I have posted a design on a page of this site and given links to others, build one they are essential.

Surface finish

Surface quality is a most desired attribute, and one of the hardest to achieve, as it is only possible to make as smooth finish in the mould as the finish of the pattern, and still suffer from surface blemishes.
The finish of the metal is the result of the choice of sand, its grade (fineness) the clay content, river washed, or milled sand.
Generally, river washed sand will not give as smooth a finish as milled sand due to the roundness of the grain, as when packed there is a larger gap between grains, this sand requires more binder.
Milled sand on the other hand will slot together more closely due to the odd grain shape however use what you can your finish will directly reflect the medium into which it was cast.
The type of sand must work with the type of metal being cast as the fluidity of the metal will vary, a good finish can be achieved with coarse grain sand when casting aluminium, on the other hand copper, and it’s alloys, are the most fluid of metals requiring the finest grades of sands and binders.

The more fluid the metal the more it will seep between the particles of sand, giving a rough finish to what would otherwise look to be a perfectly smooth mould,
The metals reacting with the sand and binder cause a similar effect and particles will adhere to the surface.
It is possible to smooth off the surface of the mould by adding finely ground clay or coal dust to the finished surface, this dust, held in a suspension of alcohol and brushed on using a fine camel hairbrush I use a good quality makeup brush, but don’t tell my wife.
An alternative is to spray on a fine clay slip on the sections that will contain metal, this is not a generally recognised trick but one I picked up from a pottery class I attended where the colour glaze was sprayed onto the finished pot, I instantly thought of improving the surface of a mould by the same process but a high temperature clay would be needed

Silica flour may be added in amounts up to 30%. The silica flour fills in the voids between the sand grains producing a nice smooth surface. However, gas defects and sand expansion defects are now a problem as well as more binder, which will mean more vents.

BUT BE WARNED SILIA SAND IS DANGEROUS STUFF!!

Always dry the mould surface prior to assembling the mould, as this will provide a barrier layer for the steam caused by damp (green sand) to escape as well as reducing the amount of steam created in the first place.
Every one knows that steam (water) does not mix with molten metal and as the metal is poured into a green mould, steam will rise out of the sand both cooling the metal as it flows through the mould and being absorbed as hydrogen gas into the metal this will cause porosity, pitting, and blisters to form in and on the metal surface despite having a large number of vents.
So I have the following suggestions on how to dry your mould they all require heat to evaporate away the water, so be careful.

Dry the mould by:

Heating the entire mould in an oven but this requires big oven but be careful as the mould may shrink out of the flask.
Burn the mould surface with a gas torch to dry the surface of the mould, but this will hasten the end of life of the wooden flask
Spray or paint methylated spirit onto the mould surface and burn it off, but again take care when wooden flasks are in use!
My favourite method it to have a set of 6mm steel plates cut to fit inside the mould box, heat to dull red and place onto the mould face.
An alterative to both drying and improving the mould is to paint on a mixture of alcohol and graphite dust or coal dust, the excess alcohol is then burnt off which both improves the mould finish and reduces the water content.

Core sand

Some times you will want holes or voids in a casting that are impossible to cut or machine, theses are made by adding a core to the mould, for an example look at the photo of the machine handle I cast, I wanted a square hole, I have drilled a round hole and filed it square but that would involve a lot of work which I did not want to do, it is far easier to cast the square hole in the first place and then touch it up to fit.

First you need to make a core box, a device that will enable the core sand to be rammed into to form the desires shape and then comes apart into two or more pieces to facilitate the removal of the core, but it must be made larger than the hole required to allow for shrinkage
Secondly you need some fine grain river washed sand, I have found the best sand is builders sand, passed through a fine sieve to remove the larger grains, this is then mixed with either linseed oil or sodium silicate, these additives linseed oil is available from local hardware stores and the sodium silicate is obtainable from a company called Bath Potters who are in radstock somerset

Mix the linseed oil into the sand so that it is just damp and sticking together, similarly mix the sodium silicate in to the sand so that there is an even distribution of the chemical, drop a little sand into the core box and press hard to form a solid pellet gently drill a 4mm hole in the centre and place a drinking straw into the hole, continue to fill the core box pressing a little harder than you would normally do when ramming an ordinary green sand mould, and extract the straw, now with the linseed oil mix remove the core and heat in an oven at low heat until hard, this core is now ready for immediate use and inclusion into the mould.
With the sodium silicate mix pass a gentle puff of CO2 through the core box to harden the sodium silicate, this core is now ready for immediate use and inclusion into the mould.
Do not however forget to modify your pattern to support the sand core by at least 1 inch at each end.
If the core is of an exceptionally long shape wire supporting reinforcing wire can be included to help avoiding the core collapsing during the pour.

Moulding defects and their causes

Gating

A common problem is that of gating and inrush, where the surface of the mould wears away under the force of the flow of molten metal, the most troublesome metal is copper, and copper-based alloys, these metals will “cut” the sand as they are poured.

Cold shuts

This is due to the metal freezing before it has fully run through out the mould or due to air pressure within the mould causing an airlock, mostly through low pouring temperature.

Hot shuts

Caused by the sand being too wet or poor gating, restricting metal flow through the mould, and by having too few vents to allow gasses to escape

Hot tears

Hot tears happen when mould is too strong. They occur in casting alloys that tend to have long freezing ranges during solidification. the metal tries to contract but cannot because the mould is too strong. High stresses develop at the corners while the metal at the centre is still liquid. When the stress is high enough, the casting cracks. Aluminium is the metal most likely to suffer this defect.

Collapsibility

One of the causes that lead to hot Tears is lack of collapsibility. The sand mould rammed too hard.
Collapsibility refers to how easily the sand breaks down and crumbles when the casting has cooled, and is ready to be shaken out.

One helpful sand additive that can improve collapsibility and minimize hot tears is cellulose.
Sand cores can also cause this effect by being too strong the addition of a hole through the core will improve the collapsibility of the core

Inclusions

This is simply where foreign material Slag, dross, ash & sand, along with what ever else comes out of the crucible along with the melt a casting that has sand included is impossible to machine as the metal is now highly abrasive and will quickly ruin the hardest tool edge

Blowholes

Blows occur when gas, either trapped or evolved, that produces voids,
Normally at the cope surface of the casting, some of the major factors that may contribute to or may alleviate blows include: