SAND PREPARATION
This thing is a life saver and has put the fun back into casting. Below is a rather long winded description of what a Muller is, and how it works but read on, it will help bring some meaning to the attached photo's.
The following description describes the process for tempering Green Sand
What does it do, and why do I need one?
Its main purpose is to firstly, mix powdered Bentonite (fine dry clay powder) evenly and thoroughly throughout the sand. It does this by the use of ploughs, as the unit rotates, the ploughs individually move the sand in and out as well as layering the mix. Secondly the Muller has a pair of weighted wheels, as the sand is moved in and out a small amount of water is added to dissolve out the Bentonite, the wheels force the sand into the Bentonite to coat each and every grain. This gives the mulled sand the ability to bind to itself allowing it to be moulded into the shape of the pattern it is rammed into. The ratio of the components basically comes down to a feel that is gained with experience. Too wet and you will have burnt holes in jumpers, shirts and skin when the resultant explosion of molten metal occurs. If the mix is too dry the edges of the mould will break away and the resultant casting resembles a lump of abstract art.
SOME TECHO STUFF
The geared motor is out of a scrap yard and was used to drive an industrial roller door. The motor was burnt out and had to be rewound, but the gearbox was in good shape. The motor used is only a 0.37 kw (½ horse) with an output speed from the box of 51 RPM. This is further reduced by a 5/8" sprocket and chain drive to give a speed of about 25 RPM at the centre shaft. The input torque to the shaft is around 141Nm (104 lb ft) constant. The wheels are diam 270mm (10.62") x 63mm (2 1/2") wide. When first completed and trialed under load the motor kicked like a mule on starting. I scrounged up a 5-ohm variable resistor and 240v timer and with the help of a good mate who put together a soft starter for the motor. It works a treat with a gentle start before the full power comes on (note the white box on the front). The only other bit of rework has been the blades (shape and position) and the addition of wheel scrapers. I may at a latter date have the wheels coated in polyurethane to prevent the sand from sticking to them.
I start off mulling with the full weight of the wheels on the mix. Towards the end I pack under the stops to raise the wheels and allow the sand to "fluff". At sometime in the future I'll add a quick lift jacking arr'gt to remove the need for packing under the stops. When the mull is complete the gate is slid open and the fluffed sand drops into a bin ready for moulding. I use a 15 micron graded silicon sand that I find gives extremely good definition on intricate parts and fine detail, although, there is still the little break a ways that sometimes occur when removing the pattern. The only change I would make if I had the money would be to use a 2hp motor. The existing motor gets reasonably hot after about 40 mins of mulling. That represents about three, 60 litre (13 gallon) drums which is around 70 kg (150 lb) total. This covers most of my needs for one day of moulding.
The other type of moulding sand that I have just started to use is Petro-Bond. The sand is mixed with Petro-bond powder and oil. This stuff is the absolute bee's knee's when it comes to moulding. It pulls away cleanly from the patterns after ramming leaving an extremely finely finished mould for pouring into. This has a red oxide colour as can be seen in the Muller barrel. I now use this Petro-bond sand exclusively for Aluminium and Bronze castings. My green sand will now be used for cast iron only.
For those who are starting or interested in stating up their own Hobby Cast Foundry there are other ways to mull your green sand. First you will need three components to prepare your sand for moulding
1st Silica sand, I obtain mine from a specialist minerals supplier but you can use sand from a garden supplier. Ask for river bed sand, it stands out from other sands as its white in colour. This sand will contain a lot of impurities so it will have to be washed, graded and dried before it can be used for green sand. Half fill a bucket with the sand, put the hose into the bucket and turn on hard, push the hose around under the sand and let the water flow over the lip of the bucket. You will have to stir it up with your hands to ensure all residual clay and other dissolvable impurities are washed out.
Next, it will need to be dried, I placed mine in an old pan and heated on the stove to drive all moisture.
Lastly it will need to be graded, I did mine through a fine flour sieve, this will be rather tedious but needs to be done properly if you want castings with no flaws and a fine finish.
2nd Bentonite, I can only suggest to look in the yellow pages of your telephone directory. Look for foundry suppliers and ring around. If you have a foundry near by ask if you can purchase a small amount. When purchased from a supplier it comes in a 22Kg (50 LB) bag.
3rd Water, no need to explain this one.
MULLING
There are a couple of ways that the tempering process can be done if you do not have access to a Muller, and as I have found they require far less effort than my "hand mulling" method. First lay some thick plastic sheet on a hard surface, preferably a concrete driveway. To start with add one cup of Bentonite to a standard bucket of dry sand and thoroughly mix with a shovel. Once it is mixed, lightly spray water onto the mix and continue turning with the shovel. A word of warning, be very sparing with the water, it needs to be damp not wet. Once mixed through, there are a number of ways to mull the mix.
1st Lay a second thick plastic sheet over the mix, now drive your car backwards and forwards over the sand to "mull" it. Stop and continue to turn with the shovel, add water if req'd. To determine the temper there is a standard test that can be performed, it's called the squeeze and break. Pickup a hand full of the sand and squeeze it in the palm of your hand it must not leave any moisture on your skin, now break it between two fingers and thumb. If it breaks in half cleanly, the bond is right, if it crumbles or breaks into a number of pieces more bentonite will need to be added. Repeat mulling until the temper feels right. Once complete, shovel it into a plastic bin and cover with damp towels to ferment overnight.
2nd Repeat as above but use a heavy grass roller over the mix to accomplish the mulling process.
As it will be the sand that is in direct contact with the molten metal, critical attention must be payed to its "temper' i.e. - its bond strength and wetness. The condition of your moulding sand will have a direct bearing on the quality of the casting.
BELOW IS A DESCRIPTION OF THE PROCESS I USE TO MULL MY GREEN SAND AND THEN CHECK IT'S TEMPER
Click the thumb nail to view full size
To one bucket of Green Sand add approximately a cup of Bentonite powder
Turn on Muller and let the plough's mix the Bentonite in whilst dry.
Add water a little at a time. I pour in around a small cup of water to start with, then add with a hand sprayer as needed.
Throughout the Mulling process use the squeeze test to determine the sands temper. Once the feel is right open the slide gate and empty into a bin.
To check the sands temper, grab a hand full.
Squeeze the sand hard into the palm of your hand.
Hold the compacted sand between your fingers and break in half with your thumb.
If the sand breaks cleanly in two, and does not leave your hand moist, the temper is right
Bins and more bins of moulding sand. The red coloured sand is Petro-bond, the black is green sand and the white is 15 micron silica sand that the two types of moulding sand is made up from.
The drawing above shows the basic envelope dimensions and arrangement.
The above photo shows the general arrangement, note the box in the front
housing the soft start.
View of the operators side
View from the top showing the Muller wheels and ploughs. The rotation is clockwise.
Close up view of the plough that moves the sand out. Note the height of the blade, to allow the sand to be layered not graded.
Close up view of the plough that moves the sand in. Note the extension on the outer edge to scrape the side of the barrel.
View showing the wheel and pivot assembly.
Gearbox mounted on the end of the motor. Note the guarding, very important if you have any little helpers around.
The soft starter consists of a variable 5 ohm resistor and a timer. When the motor start button is pressed current is passed through the resistor and is reduced, the resistor has a saddle that can be adjusted up and down the resistor body and clamped. The further down the body the saddle is clamped the higher the resistance, less current and softer the start. (You will note that I haven't used the connection to the variable slide on the resistor to fine tune the resistance, I found using the full resistance was perfect for my application.)
The resistor is approximately 12" long and 1" dia, it looks similar to the old resistance wire wound radiant heaters. The first resistor I tried was a 20 ohm, it glowed red and melted the plastic Rital box it was mounted in. Be careful if trying this arrangement to size the components to suit your motor rating.
The 240 V timer sets the time at which the resistor is cut out and full load current hits the motor giving full power. I have around a 5 sec soft start before full load current comes on. It is very effective, the acceleration is slow and gentle to get things moving and up to speed, there is no more kicking and wrenching of the motor mounts and less strain on the transmission.