Casting Patterns: Functions, Material, Types, Allowances, Disadvantages [With PDF]

Introduction:

A pattern is an essential tool for the Casting Process. The main function of a Casting Pattern is to mold a cavity in the molding sand mixture such that the formed cavity looks the same as the casting product. The selection of material for casting pattern depends on different factors such as the number of casting, quality of casting and the degree of finish.

Colour of Pattern:

There are several colors used in the casting pattern which are as follows:

• Yellow used for Core Prints
• Red used for surfaces to be machined
• Black surfaces to be left unfinished
• Colorless used for parting surface

Functions of Pattern:

Following are the functions of Casting Pattern:

• It is used for preparing a mold cavity which is used for Casting any parts.
• Accurate pattern can minimize the production cost because no further machining is required.
• It reduces casting defects.

Materials Used for Pattern:

Points to be considered while selecting material for Casting Patterns:

• The materials used in the pattern should be cheap in cost and easily available in the market.
• The material should have a good surface finish.
• The material should withstand high temperatures and does not change its shape at high temperatures.

Generally, we use 5 different types of material to make the pattern and those are:

• Wood
• Metals
• Plaster of Paris
• Plastics
• Wax

Let us understand each material in detail.

Wood:

As we all know woods are easily available, and the price is quite low. There are some advantages of using wood in casting pattern which are as follows:

1. Wood is light in weight
2. Easily Available in the market
3. You can make any shape using wood
4. Woods gives good surface finish

However wood is attracted to moisture and sometimes it can change shape at high temperature or after dry out from moisture. Woods are very weak in strength, and it wears out quickly due to their low resistance to sand abrasion. Pines deodar, walnut, teak’s are used for making a pattern.

Metals:

In metals, cast iron, brass, aluminum are generally used in patterns. It gives smooth surface finish, this is the only reason that metals are used in large production casting workshops. Following are some advantages of using Metal Pattern:

1. Smooth surface finish can be obtained.
2. Deformation is less.
3. Closer dimensional tolerance.

Although there are some disadvantages of using this type of pattern like it is a little bit costlier, heavy, sometimes rusting effect occurs on the surfaces of the metals, etc.

Plaster of Paris:

It is generally used if you need to set up the pattern quickly. The main advantage of this casting pattern is that it can easily cast into intricate shapes. However, it is not recommended for repetitive usages as it is fragile.

Plastics:

Different types of plastics are nowadays used in casting patterns because of their lighter weight, strength, dimensionally stable properties, and also for cheap in cost.

Thermoplastics and polystyrene are commonly used for making patterns. Thermosetting plastics such as phenolics and epoxies are also used in a pattern.

There are few advantages of using Plastic Casting pattern which are as follows:

1. Light in weight.
2. Cheap in price.
3. It posses good compressive strength.
4. No tension of rusting or moisture absorbing.

However, they are a little weak and does not have good abrasion-resistant.

Wax:

A wax pattern is used in the investment casting process. By using this pattern we get a high degree of accuracy and can have an excellent surface finish.

However, it needs little care while handling otherwise it can break easily. It is generally used in small casting.

Types of pattern:

The patterns 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

Single Piece Pattern:

As the name denotes a single piece that means it has only 1 section, and inside this, the shape of the pattern is made.

Split Pattern:

It has two sections connected by a pin named dwell pin to attach the two-portion. When we join the two-block, the joining surface is called parting surface. Generally, it is used to make the hollow cylindrical casting.

Multipiece Pattern:

A multipiece pattern is made by more than two pieces, the upper one is called the cope, middle one is called cheek, and the bottom one is named as the drag portion.

Match Plate Pattern:

In match Plate Pattern, the cope and drag section of the split pattern are mounted on the opposite of metal or wood plate. This type of pattern is divided into two sections along the parting line formed by the metal or wood plate.

Gated Pattern:

A gated pattern consists of:

1. Gates
2. Runner
3. Riser

In this type of pattern, the molten metal pours through the runner. It passes through several gates and then finally to the pattern.

Sweep Pattern:

Sweep Pattern is designed for half shape of the casting products. It consists of a mold cavity, rotating spindle, and sweep pattern. Its edges are contours corresponding to the outer shape and size of the casting.

Loose Piece Pattern:

This type of pattern is used when a pattern has projecting parts that lie below or above the main parting line in a mold. This means when some parts of the product have a little outer or inner edge than to cast those parts we use loose piece patterns.

Skeleton Pattern:

It is generally used for casting very huge parts. This type of pattern is not totally solid. It looks like a rib cage. Here we make only the outer or inner surface structure. 

Shell Pattern:

This type of pattern is generally made of metals and fitted on a plate. It is like a split pattern, one half of the pattern is placed on the upper part of the plate and another one is fitted on the bottom of the plate.

Segmental Pattern:

It is used when we need to cast a large circular ring shape product. The final product is completed on a few repeated steps using this pattern. Firstly, we mold it in one place then we rotate the pattern to the next position and again the molding is done.

Pattern Allowances:

To make a casting perfect we need to consider the allowances because after cooling the molten metal, it can shrink and make any distortion.

There are 5-types of allowance that we need to consider when designing a pattern which are as follows:

• Contraction Allowances
• Draft Allowances
• Finishing or Machining allowance
• Shake allowance
• Distortion allowance

Contraction allowance / Shrinkage allowance:

As we know after cooling at room temperature all the metals shrink, that’s why we need to consider contraction allowances. 

The contraction allowance depends on the following:

• The type of casting method used.
• The type of material used for casting.

It can be also divided into two types:

• Liquid Shrinkage 

When the metal is in the liquid stage, the liquid shrinkage occurs. to encounter the shrinkage we generally provide a riser that supplies the extra molten metal to the cavity.

• Solid Shrinkage 

It’s occurs when the metal is in the solid stage, to encounter this situation we can provide the shrinkage allowances.

Draft allowance:

Due to the removal of the pattern, sometimes the edges of the casting breaks. To overcome this situation sometimes we provide a taper for easy removal of the pattern from the mold. Generally, a 1-3 degree draft is provided with the outer surface of the pattern to overcome this brokerage.

Finishing or Machining allowance:

After casting we need to do some machining operations, maybe grinding, or other surface finishing operations. All these operations are metal removal operations. So we need to keep the size of the pattern a little bigger than the actual size so that after the removal of some material, it will be of perfect shaped.

Shake allowance:

This is a negative allowance, used for easy removal of the pattern.

Distortion allowance:

Sometime within the pattern due to cooling, metal stress is generated. To overcome this situation we need to distort the pattern in the opposite direction.

Factors that must be considered when Designing a Pattern:

These are some points that should be considered while design a perfect casting pattern:

1. Selection of Proper Material for the pattern.
2. Select the perfect type of pattern as per requirement.
3. Allowances should be kept in mind while designing
4. Avoid sharp edges.
5. Core joints should be avoided.

FAQ’s

References (External Links):

• https://nptel.ac.in/content/storage2/nptel_data3/html/mhrd/ict/text/112107083/lec11.pdf
• https://patents.google.com/patent/US7185695B1/en