Metal Casting Zone Info Blog

Posts Tagged ‘mould’

Precision non-ferrous casting

Precision casting differs from sand casting and shell moulding in that the moulds they employ consist of only one part, while the pattern itself is expendable each time a casting is made. Precision casting processes offer much more freedom to the designer, and produce castings of a superior surface finish and a high degree of dimensional accuracy. Among other purposes, they are used for the casting of metals and alloys that are difficult to machine, since the castings usually require very little or no finishing treatment. These castings are used in precision engineering, clockmaking, and other fields of industrial production

Sand casting can be used for all common metals, and ther are many different sand-casting processes and special processes derived from this method. These are known by various names such as open sand moulding, pit moulding, box moulding and template moulding etc. The most commonly used method for making small castings is box moulding. With this method the pattern is embedded in the sand or other mould material, within a moulding box which is usually made up of an upper and lower part, the sand being compacted by ramming, vibration or pressure. The box is then opened and the pattern is removed, the cores inserted, the box closed again and the casting is carried out.

For casting very large, heavy or intricate parts the pit moulding process is employed. The pit method is where the mould is built up in a casting pit. To give the sand greater strength when used as a mould material for large castings, cement can be added. For symmetrically shaped castings the mould is sometimes formed by means of a template, a metal plate cut to the required profile for producing a certain shape when it is moved along a guide track or rotated on a pivot.

In dry sand moulding the mould is baked ; in green sand moulding and the mould is used with sand in the damp or “green” condition. The metal is poured from above into an open mould. The more usual kind of moulds are the closed moulds which are filled through a special system of channels known as gate runners, and are usually so contrived that the metal enters at a low point and rises in the mould. Once the metal has solidified and cooled, the casting is removed from the mould and the runners and risers are detached from the casting. The casting is then cleaned up by abrasive tumbling, blasting, cutting or grinding.

When casting with expendable moulds, the individual pattern parts are first made by hand or by mechanical means and then assembled. The moulding materials are those used for constructing the actual moulds in which the metal will be cast, are usually mineral substances such as cement, fireclay, plaster etc., in conjunction with bonding agents such as water glass, synthetic resins, oil, sulphite solution etc., which give the moulds the necessary strength and dimensional accuracy. The bond action is either achieved by drying or by chemical consolidation.

Precision non-ferrrous die casting, is the casting of metals such as zinc, aluminium and brass.

Manufacture of Sand Castings

Manufacture of Sand Castings

Engineers have required metal castings for over 5000 years.

As the technical ability of man has progressed, so have the techniques for making a Sand Casting.

Before a foundry can start producing Sand Castings, a number of decisions has to be made regards the Sand Casting Process :

a ) Metal Type

b) Contraction Rate. Ex if the finished product is to be 48″ long, then the pattern / mould cavity, will be approx. 49″long

c) Quality of the finished product, ie manhole cover or Nuclear Submarine part

1)    A pattern is required; this can be made from a cheap timber for single items, or metal for very large quantities.

2)     Cores will be required, if the casting is to have any internal shape

3)    A Foundry Engineer will determine how to ensure the casting is sound.

4)    His final determination is called the Runner System / Method

5)     To form the bottom half of the Mould, sand is now compacted around the bottom half of the pattern.

6)    A joint line is made between the top and bottom half of the mould. 3 dowels or pins are used to ensure the 2 halves of the mould can be put together exactly in the correct position.

7)    Sand is then compacted around the top half of the pattern.

8)    Split the 2 halves of the mould

9)    Carefully take the Pattern out of the sand.

Place any Cores in the mould.

10)   The top and bottom half moulds are clamped or  weighted down

leaving a cavity inside the mould

11) Molten metal is poured down the runner until the mould is full.

12) When cool the casting can be taken out of the mould

For further information on Sand Castings Tolerances please visit jade-trading.com

Introduction for Sand Casting Process with Philip White and Jenny Dunseath in the foundry at Camberwell college of Art London. Video by Chris Follows part of the Process Arts Project – process.arts.ac.uk NOTE: Because the sand is used in a damp condition there is minimal sand dust,and this occurs at the mould face immediately arround the hot metal casting ,clearly visible when the casting is knocked out. At this stage a small watering can or spray will help to stableize this dry material.A standard dust mask could be worn.It is always worth taking sensible precautions with all processes.With sand casting on the scale immagined here keep a water spray handy and use it when cleanig away any dry material. Text Information supplied by Jenny Dunseath: Pattern Making: The Pattern is a full size model of the part that makes an impression in the sand mold, with dimensional allocation for shrinkage and finishing. If the casting is hollow, additional patterns called cores are used to create these cavities in the finished product. Patterns are usually made of wood, plastic, metal, or plaster; however, other materials or combinations of materials are used if there are additional specific properties required of the pattern. Every Pattern must have a draft angle of approximately 2° – 3° to all walls parallel to the parting direction to facilitate removing the part from the mold. Paint the surface of the Pattern with Varnish or Shellac to make it water tight and to ensure that the sand …
Video Rating: 4 / 5

More Sand Casting Foundry Articles

using 80% lead &20%tin,what would be the melting tempretuer for this mix alloy?

The first of 14 videos showing how to make alloy castings in a home made foundry.
Part one shows how the mould is made and in part two the metal is melted and poured.

Making a greensand mould when there is no flat surface to make the parting line.

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Guidelines of making Plaster craft

Plaster craft is a complex process unless you are aware of its ins and outs. Given below are a few guidelines of making plaster craft.

Step – 1:

Cover your work surface with an old newspaper or a vinyl peace. As you may see from this photo plaster casting is a bit messy, so it is essential to protect your countertops.

Step – 2:

Check the mould to ensure it is clean and dry. Any dirt might show on the finished casting.

Step – 3:

Many moulds cannot sit flat on the counter, therefore, it is important to give them support while using. The most simple process is a zip lock bag filled with a few pounds of rice. Rice bags are convenient to pack and store when not being used and are made of common materials most people find handy. A box of sand will also work well but it is more difficult to store it when not being used.

Step – 4:

The surface tension of the water tends to trap air leading to pinholes in the finished casting. Airid is a product meant to break that surface tension, reducing the chances of trapped air. Spray or wipe a thin coat of Airid into the mould.

Step – 5:

Wiggle the mould down onto your rice/sand bag till it looks level. You are now ready to mix plaster.

Step – 6:

To find out how much plaster will it hold fill the mould with water. It is the exact amount of water you will need. Add a bit more and weigh it on scale.

Step – 7:

Plaster should always be added to water and never vice versa. Sprinkle it in slowly to allow it to absorb water.

Step – 8:

Let the mixture remain undisturbed for 2 minutes so the plaster absorbs all water.

Step – 9:

For coloring, pigments should be added now.

Step – 10:

Utilize a potato masher to mix thoroughly for about a minute. Small amounts can be mixed with a stick.

Step – 11:

Pour the plaster in a corner of mould and let it flow across the complete mould. On deeper moulds, pour it down the side of corner to avoid entrapped air.

Step – 12:

Once the mould is poured, wiggle it to dislodge any air which may have remained in the mould.

Step – 13:

Periodically feel the mould. When the mould is warm to touch, the plaster casting may be removed.

Step – 14:

Gently flex the edges of mould to break sides of casting loose.

Step – 15:

Hold the mould just above the rice/sand bag. Use pressure gently to take out the casting.

Step – 16:

After casting is out, set the mould to be cleaned.

Step – 17:

Utilize a knife to cut the sharp edges off the back of the casting. For fast drying it should be put where it gets air from all angles.

Step – 18:

When fully hardened, most of the plaster will flake off or breakout the plaster.

Step – 19:

Flex the blade of the plaster blender to flake the dried plaster off.

Step – 20:

Wipe off tools and moulds for final cleaning up.

Step – 21:

If you utilize a rice bag any drips may be removed by flexing the bag. For sand box, just pick out any plaster that has fallen on sand.

Step – 22:

Pack up your moulds, tools and plaster. Throw away the old newspaper with which you had covered the working area. If you had used vinyl piece, dust it and wipe with wet cloth.