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How we make castings

Introduction

We use traditional foundry techniques to produce a casting: We use a pattern to produce a sand mould into which we pour the molten metal. When the metal has solidified, we break up the mould to extract the casting. We reclaim the sand for reuse. We remove any excess metal and shotblast the casting to clean the surface of adhering sand and scale.

The pattern is made from wood, metal or resin and can be reused to make further castings.

The mould is generally made in two halves, in steel moulding boxes for support, and may include additional pieces of sand, known as cores, to form internal shapes. The joint line on the casting is chosen so that the pattern can be removed from the mould.

Molten metal enters the mould cavity by the running system consisting of downsprue, runner bar and ingates. Feeders are used to provide a reservoir of molten metal to compensate for the shrinkage in volume as the metal solidifies and changes from liquid to solid. The running system and the feeders have to be removed from the casting along with any excess metal flash on the joint line as part of the fettling process.

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A typical mould for a flanged pipe

Patterns

A pattern is the same basic shape as the component to be produced. An allowance is made in the size of the pattern because the casting contracts as it cools after solidification. This contraction allowance varies with the metal to be cast and for grey iron we allow about 1%. A casting may be made all in one half of the mould or split along one or more joint lines to allow the pattern to draw from the mould. These joint lines may be stepped to follow the shape of the casting.

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Pattern in one half of the mould

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Pattern in both halves of mould


Taper or draft has to be allowed on all vertical faces of the pattern to allow the pattern to release from the sand.

Cores are used to form internal shapes such as holes and bores. They may also be used to form undercuts which will not draw from the mould. A separate piece of pattern equipment known as a corebox is made to make the core. Cores locate in the mould in core prints which must be added to the pattern and allowed for in the corebox.


Sketch showing stages of moulding, pouring and knock out

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1.A moulding box is placed around the pattern on a board

2.The box is filled with sand and compacted around the pattern

3.When the mould has hardened the pattern is withdrawn from the mould

4.Cores locate in the core prints formed by the pattern and the top half of the mould is placed over the bottom

5.Molten metal is poured into the closed mould and allowed to solidify and cool

6.The mould is broken up to remove the casting and the running system is removed to leave the finished casting.

Pouring

When the finished mould is closed ready for pouring we place weights on the top surface to stop the pressure of the molten metal lifting the top of the mould and running out. Molten iron at about 1350°C is poured into the mould from a ladle. At this temperature the iron is very fluid and will fill all the small detail in the mould. As the iron cools to around 1200°C it begins to solidify in the thin sections first and then in the thicker sections. As the metal changes from liquid to solid the volume of the metal reduces so more liquid metal is drawn in from the feeders to compensate. Where there is insufficient liquid metal available, voids can form in the metal known as shrinkage porosity. This happens in the centre of a section or in the middle of a junction between two sections where the metal section is increased.

As the metal cools the resins in the sand burn away and the mould breaks down to allow the solid metal to contract. If the mould does not break down the metal is prevented from contracting and the metal section is put under stress and may even rupture causing a tear in the metal section.

Knock out, fettling and finishing

When the casting has cooled we break up the sand mould to remove the casting. We process the sand for reuse and the casting and runner and feeder system are shotblast to remove the adhering sand and scale. We use abrasive disc grinders to cut off the runner and feeder and to grind the surplus metal and flash from the casting

Some components require heat treatment to harden, stress relieve or to make the structure of the casting uniform.
Some customers require castings to be painted to prevent surface rusting and for some components we organise machining operations to deliver a finished component to the customer.

S. Walters
January 2002

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