Tuesday, 9 August 2011

CASTING INFO, TIPS & TRICKS

When making your pattern you need to allow for shrinkage of the metal once it cools. So a pattern should be about 2% larger then the final for aluminium.  You also might want to factor in extra material for machining of surfaces.


Average Shrinkage of Metal Castings
These average shrinkage figures are for metals cast with uniform sections under normal conditions. When using these figures, it should be remembered that the size and
the shape of the casting must be considered, since thick castings have less shrink and thin castings have more shrink than indicated by the figures given in the table below.

                               Typical Shrinkage

Metal                                                        (Inches per foot)
Cast Iron . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1/10
Ductile Iron . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1/10
Malleable Iron . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1/10
Steel. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3/16 or 1/4
Brass. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  3/16
Copper. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3/16
Aluminum. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1/8 or 5/32
Lead . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  5/16
Zinc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5/16



Some quick rules:

  • Cast iron will shrink 1/16" per foot from the template size. This means that the template has to be 5% larger on exterior dimensions and 5% smaller on interior dimensions versus the final product. The term for this is shrinkage.
  • Any area that will be machined after casting should have an extra 0.125" added because the iron will react with the silica to form a very hard layer and you'll want to machine past it.
  • In order to remove the template from the sand without breaking the sand mold, vertical lines are not allowed. A small angle (Draft) anywhere from 1-3 degree has to be made into the template. 
  • No sharp corners are allowed on the template.



The Following is from Flaming Furnace Blog

Sand Casting Terminology


Sand Casting Terminology: 



Flask: This is the container box that holds the molding sand. The box is opened 
from the top and the bottom and could be created in any shape and size. Most 
simple flasks contain two parts. Most flasks are made of alumimum, steel or 
wood and have different types of aligning "systems" such as pins or just timber 
parts aligining the cope and drag to the same position. 
For more commercial and multi use of  your flask hinges could be added to the flask parts in 
order to take the flask off and use it for another molding. It is optional to add ribs inside the flask 
for a better sand grip.



Cope: Is the top part of the flask.

Drag: Is the bottom part of the flask.

I made this simple sketch showing most parts of a basic sand casting structure:






















Parting line: Is the line where the drag and cope meet.

Pouring cap/basin: This is the hole on top of the cope part made to pour the molten aluminum into 

the flask.

Gas vent: This is so when pouring the molten metal the gas will escape from the vent. A gas vent can 

be easily made using a straight wire.

 Riser: The riser will feed more molten metal into the cavity mould when shrinking accurse.

Sprue: This is under the pouring cup connected to the runner channeling the molten metal into the 

mould cavity.

Runner: The horizental channel feeding the molten metal into the mould cavity.

Mould Cavity: This is where all the molten metal sets creating a duplicate of the pattern.

Pattern: Any objects placed in the casting sand creating an impression of the pattern to be created. 

A pattern could be made from steel, wood, plastic and other materials.

Green Casting Sand: Green sand is an aggregate of sand, (not green in color). Green sand is

 combined of very fine sand such as silica, olivine or even play sand mixed with bentonite clay
 and water. There are many recipes that can be found on the internet for the different types and 
quantities and mixing ratio. Adding to much water can lead to "porosity", casting defects as result 
of gas bubbles accouring while pouring the molten metal. Some metal casters use blow torches 
(heat-dry) over the finished moulded sand to achieve a more rigid mould. Making a good mix of
 green sand is a much cheaper solution then buying a ready mix.

 Oil Bonded Casting Sand: This type of sand could be bought as ready made. The sand is very 

good for high detail mouldings and is easy to work with. It's less likely to get high percentage of 
porosity because this type of sand is oil bonded and NOT mixed with water. The down side is it's    
 expensive and burnt sand smoke is not the best to inhale. I personally use petrobond and am 
amazed everytime at how easy it is to work with and the great results after every aluminum casting.

 Sand Muller: Mixing big quantities of sand by hand is quit a task and for that there is a great

 invention called a Sand Muller. The muller is a clean way of mixing the sand without getting 
unwanted contaminators into the sand and saving a lot of hard labour time. The muller mixes 
the sand, clay and water with its crushing wheels. There are many websites of homemade 
designes of big and small mullers whatever suits your needs just google "sand muller".

Lost Foam Casting: Another option of sand casting is to make a foam pattern with Polystyrene 

or styrofoam. The pattern dissolves when pouring the molten metal forming the desired object. 
The mould pattern can be easily carved out or cut with a hot wire Foam Cutter, the cutter could 
be made DIY or you can buy one. Molded foam is a cost effective solution. Burning foam fumes 
can be toxic always wear a respirator and work in a well ventilated area.

Sand casting tools: When working with casting sand the caster needs a few basic tools to achieve 

good casting results. Casting tools can be expensive, the hobby metal caster can use home objects 
or tools which are as good as professionals ones.

Sand Rammer: This is used to pack all the sand into the flask eliminating air pockets and lose 

sand that might ruin the final mould after pouring the molten metal. The rammer comes in different
 sizes. Most rammers are made of wood or aluminum, one side flat the other side narrow to fit into 
the flask corners. For bigger jobs and more commercial use the electric hand rammer is ideal. 
If one does not have a sand rammer a simple block of timber will do to pack the sand well into the 
flask.

Metal wire: can be used to create the gas vent holes.

Strike off bar: is used to strike all the extra sand from the flask top or bottom. I for example used 

L-shaped metal angle for leveling.

Molder Trowel: could be replaced with a pointy block layer finishing trowel.

Slick: this could be replaced with a simple small kitchen spoon to create for example the pouring 

basin or runner or to clear off unwanted sand particles etc'.

 Sand Sifter: A sifting tool is used for spreading very fine textured sand directly into contact with 

the pattern to get the best finishing results. Mesh size will determine how fine sand grain is sifted. 
I used a simple garden mesh with a deep wooden frame to prevent sand falling out of the sifting 
mesh.

 Parting Powder: is used to release cope from drag and over pattern preventing the molding sand 

from sticking and ruin the mould finshing. Talc powder can be used too and will be as effective.

Sprue and Riser: To create the Sprue and Riser can be done simply by cutting timber to fit the 

work flask. For example I used a rounded timber toy to create the Sprue adding the casting sand 
around it.

Another option and I think it is better is to use a hollow metal tube to make the sprue and riser by 

pushing the tube into the sand at the correct location. The metal tube should be of very small 
thickness which makes pushing it into the sand very easy without breaking the sand.

Sprue and riser extension: This is optional and has two purposes. First one is to pravent spilling 

molten metal all over the flask top. If you are working with wood flasks this will pronlong their 
lives.

Second the sprue and riser extension will act as a pressure head, feeding molten metal into the 

casting and limiting the aluminum shrinkage.









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