Manufacturing Engineering - CASTING

The common materials of which the patterns are made are the following :

1) Wood : It is the most common material used for pattern making.

2) Metals : Metals are used with advantage, as pattern material, only when the number of castings to be made is very high and a closer dimensional accuracy is desired. They have a much longer life than wooden patterns and eliminate the inherent disadvantages of wood to a great extent.

3) Plastics : The plastics used as pattern materials are thermosetting resins. Phenolic resin plastic and foam plastic suit best for this purpose. For making the pattern, first the moulds are made, usually from plaster of Paris. The resin is then poured into these moulds and the two heated. At a specific temperature, the resin solidifies to give the plastic pattern.

4) Wax : Wax patterns are exclusively used in investment casting.

A pattern is always made larger than the required size of the casting considering the various allowances. These are the allowances which are usually provided in a pattern.

1.Shrinkage or contraction allowance:

The various metals used for casting contract after solidification in the mould. Since the contraction is different for different materials, therefore it will also differ with the form or type of metal.

2.Draft allowance

It is a taper which is given to all the vertical walls of the pattern for easy and clean withdraw of the pattern from the sand without damaging the mould cavity. It may be expressed in millimeters on a side or in degrees. The amount of taper varies with the type of patterns. The wooden patterns require more taper than metal patterns because of the greater frictional resistance of the wooden surfaces.

3.Finish or machining allowance

The allowance is provided on the pattern if the casting is to be machined. This allowance is given in addition to shrinkage allowance. The amount of this allowance varies from 1.6 to 12.5 mm which depends upon the type of the casting metal, size and the shape of the casting. The ferrous metals require more machining allowance than non ferrous metals.

4.Distortion or camber allowance

This allowance is provided on patterns used for casting of such design in which the contraction is not uniform throughout.

5.Rapping or shaking allowance

This allowance is provided in the pattern to compensate for the rapping of mould because the pattern is to be rapped before removing it from the mould.

The pattern can be classified in different types as per their design, some of those are mentioned below:

• Single Piece Pattern

• Two-Piece or Split Pattern

• Multipiece Pattern

• Match Plate Pattern

• Gated Pattern

• Sweep Pattern

• Loose Piece Pattern

• Skeleton Pattern

• Shell Pattern

• Segmental Pattern

Molding sands can be classified into various types according to their use - backing sand, core sand, dry sand, facing sand, green sand, loam sand, parting sand, system sand.

In order to make the casting core structurally strong to withstand operating pressures, binders are used. The core binder refers to the chemical mixture which adheres the filler to provide the core strength. Hence, the core represents a composite comprising the filler and the binder.

Sand additives are commonly used in molds and cores to reduce defects like veining, metal penetration and unacceptable surface finish. Historically, iron oxides have been the additives of choice, primarily red iron oxide (hematite, Fe2O3) and black iron oxide (magnetite, Fe3O4).

The basic properties required in moulding sand and core sand are adhesiveness, cohesiveness, collapsibility, flowability, dry strength, green strength, permeability, and refractoriness.

Various methods of mould making has been explained here.

The main elements needed for the gating system are as follows:

• Pouring basin or bush.

• Sprue or downspure.

• Sprue Well.

• Runner.

• Ingate.

• Ladle.

• Cope

• Drag

• Riser

Certain important accessories of gating system are :

• Splash core

• Core print

• Chaplets

• Skim bob

• Strainer

The do and don't (s) of the gating system.

The term gating ratio is used to describe the relative cross-sectional areas of the components of a gating system. It is defined as the ratio of sprue area to the total runner area to the total gate area.

The gating system is divided into two types, based on location and based on pressure. The scope of the course is limited to based on location.

Aspiration Effect is a phenomenon of providing an allowance for the release of air from the mold cavity during the metal pouring. The shape of the sprue is made with a little taper rather than straight, to allow the passage of the air inside the mold while metal fills in.

A core is a device used in casting and moulding processes to produce internal cavities and re-entrant angles (an interior angle that is greater than 180°). ... They are most commonly used in sand casting, but are also used in die casting and injection moulding.

Chaplet is a small metal insert or spacer used in molds to provide core support during the casting process.

The solidification time of a casting is a function of the volume of a casting and its surface area (Chvorinov's rule).

A riser, also known as a feeder,is a reservoir built into a metal casting mold to prevent cavities due to shrinkage. Most metals are less dense as a liquid than as a solid so castings shrink upon cooling, which can leave a void at the last point to solidify. Risers prevent this by providing molten metal to the casting as it solidifies, so that the cavity forms in the riser and not the casting.

We have 4 famous methods for designing a riser :

1. Caines method

2. Modulus method

3. Novel's research/Shape factor method

4. Shrinkage volume consideration method

A chill is an object used to promote solidification in a specific portion of a metal casting mold. Normally the metal in the mould cools at a certain rate relative to thickness of the casting.

The traditional casting (Sand casting) has following disadvantages :

1. Requirement of machining

2. Poor surface finish

Due to these two reasons, modern casting techniques are used.

Shell moulding, also known as shell-mould casting, is an expendable mold casting process that uses resin covered sand to form the mold. As compared to sand casting, this process has better dimensional accuracy, a higher productivity rate, and lower labor requirements.

Slush casting is a variation of permanent mold casting that is used to produce hollow parts. In this method neither the strength of the part nor its internal geometry can be controlled accurately. This metal casting process is used primarily to manufacture toys and parts that are ornamental in nature, such as lamp bases and statues.

The main principle of this casting process relies on the fact that when a metal casting hardens in a mold, it will solidify from the mold wall towards the inside of the casting. In other words a metal skin forms first, (as the external geometry of the part). This skin thickens as more of the metal casting's material converts to a solid state.

Investment casting is an industrial process based on lost-wax casting, one of the oldest known metal-forming techniques. The term "lost-wax casting" can also refer to modern investment casting processes. Investment casting has been used in various forms for the last 5,000 years.

Die casting is a metal casting process that is characterized by forcing molten metal under high pressure into a mould cavity. The mould cavity is created using two hardened tool steel dies which have been machined into shape and work similarly to an injection mould during the process.

Centrifugal casting or roto-casting is a casting technique that is typically used to cast thin-walled cylinders. It is typically used to cast materials such as metals, glass, and concrete. A high quality is attainable by control of metallurgy and crystal structure.

In this lecture we will understand how grains behave during the casting process.

There are several casting defects :

• Gas Porosity. Pinholes. Blowholes. Open Holes.

• Shrinkage Defects. Open. Closed. Warping.

• Mold Material Defects. Cuts and Washes. Swells. Drops. Metal Penetration. Rat Tails. Fusion. Run Out.

• Pouring Metal Defects. Cold Shot. Cold Shut. Misrun.

• Metallurgical Defects. Hot Tears. Hot Spots. Slag inclusion.

• Casting Shape Defects. Mismatches. Flash.

Do not forget to download the complete course in PDF format in the next lecture.

So this here is the last lecture of the course. I hope you understood everything. The complete contents are attached herewith.

All the very BEST !!