Patterns and Types of Patterns

 

Patterns and Types of Patterns

 

1.0           Brief introduction

In casting, a pattern is a replica of the object to be cast, used to prepare the cavity into which molten material will be poured during the casting process.

Patterns used in sand casting may be made of wood, metal, plastics or other materials. Patterns are made to exacting standards of construction, so that they can last for a reasonable length of time, according to the quality grade of the pattern being built, and so that they will repeatably provide a dimensionally acceptable casting.

The making of patterns, called patternmaking (sometimes styled pattern-making or pattern making), is a skilled trade that is related to the trades of tool and die making and moldmaking, but also often incorporates elements of fine woodworking. Patternmakers (sometimes styled pattern-makers or pattern makers) learn their skills through apprenticeships and trade schools over many years of experience. Although an engineer may help to design the pattern, it is usually a patternmaker who executes the design.

·      Introduction of Pattern

Patterns are required to make moulds. The mould is made by packing suitable moulding material, such as moulding sand around the pattern. When the pattern is withdrawn, the imprint provides the mould cavity, which is ultimately filled with molten metal to become casting.

A pattern may be defined as 'a full size model of the desired casting which when packed or embedded in a suitable moulding material, produces a cavity called mould'. This mould, when filled with molten metal forms the desired casting after solidification of the molten metal.

The pattern very closely conforms to the shape and size of the desired casting, except that it carries pattern allowances to compensate metal shrinkage, provide sufficient metal for machined surfaces, and facilitate moulding. The process of making a pattern is known as pattern making.

Patterns used in sand casting may be made of wood, metal, plastics or other materials. Patterns are made to exacting standards of construction, so that they can last for a reasonable length of time, according to the quality grade of the pattern being built, and so that they will repeatably provide a dimensionally acceptable casting.

·      Pattern Material

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 have withstood high temperatures and does not change its shape at high temperatures.

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

• a.     Wood
• b.    Metals
• c.     Plaster of Paris
• d.    Plastics
• e.     Wax

 

Wood:

As we all know woods are easily available, and the price is quite low so it is satisfied us some basic criteria which I mentioned above.

Some advantages using wood in pattern.

• a.     Wood is light in weight
• b.    Easily Available in the market
• c.     You can make any shape using wood
• d.    Woods gives good surface finish

However wood is attracted to moisture and sometimes it can change shape on high temperature or after dry out from moisture, this is an important con of using wood as a pattern.

Not only this reason woods are very week in strength, and it wears out quickly due to its low resistance to sand abrasion.

For these above reasons, it is not used for very big product casting.

Generally, 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.

These are some advantages of using Metal Pattern:

• a.     Smooth surface finish can be obtained by metal patterns.
• b.    Deformation is less.
• c.     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 occurred on the surfaces of the metals.

Plaster of Paris:

It is generally used if you need to set up the pattern quickly. The main advantage of this pattern is it can easily cast into intricate shapes.

However, it is not for repetitive usages as it is fragile.

Plastics:

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

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

There are few advantages of using pattern and those are:

• a.     Light in weight.
• b.    Cheap in price.
• c.     It posses good compressive strength.
• d.    No tension of rusting or moisture absorbing.

However, they are a little weak in strength and not good abrasion-resistant.

·      Colour Coding of Pattern 

Patterns are normally painted with different colors in such a way that mold maker would be able to understand the how to treat that particular surface. There is no universally accepted color code for a pattern. However, the common color codes used in pattern are

• 1.    Clear/ No color: denotes the parting face of the split pattern
• 2.    Yellow: the seats for loose core print painted yellow.
• 3.    Yellow strip on Red: indicates the seats for loose pieces.
• 4.    Red strips on yellow background: for supports/ stop-offs
• 5.    Black: the surface must be left as in the casting. This surface does not need a finishing operation.
• 6.    Red: the surface which painted red required to be machined.

·      Pattern Making Allowances

Although the pattern is used to produce a casting of the desired dimension, it is not dimensionally identical with the casting. For metallurgical and mechanical reasons, a number of allowances must be provided on the pattern to get dimensionally correct casting.

Shrinkage (Contraction) Allowances: When the molten metal is poured in the mould it cools and solidifies. The metal contracts in size when it cools and solidifies. To compensate this shrinkage the pattern must be made larger than the finished casting. Therefore, shrinkage allowance is the amount by which the pattern must be made larger than the desired casting.

To compensate this shrinkage, a shrink rule is used in laying out the measurement for the pattern. Different metals have different shrinkages and therefore a different shrink rule for each type of metal.

Machining Allowances: A casting may require machining all over or on certain specified portion. Therefore, additional metal must be provided on these surfaces so that there will be some metal contact to machine. The corresponding surface of the pattern must be made larger.

The amount of this allowance depends upon the metal of the casting, method of machining to be used, method of casting used, size and shape of the casting and the degree of surface finished required. Ferrous metals need more machining allowance than non-ferrous metals. Machining allowance may vary from 1.5 mm to 16 mm, but 3 mm allowance is quite common for small and medium size castings.

Distortion Allowances: This allowance is applicable only to the irregular shaped castings which are distorted in the process of cooling. It is a result of uneven metal shrinkage. For example, a casting in the form of a letter U will be distorted with the legs diverging, instead of remaining parallel. To compensate this, an opposite distortion is provided in the pattern so that the effect is neutralised and correct casting is obtained. For making the U shaped casting, the legs of the pattern are made convergent but as the casting cools after its removal from the mould, the legs straighten up and remain parallel.

Rapping Allowances: When the pattern is to be withdrawn from the mould, it is first rapped or shaken, by striking over it from side to side, so that its surfaces may be free from the adjoining walls of the mould. As a result of this, the cavity in the mould is slightly increased. To compensate this, a negative allowance is to be provided by making the pattern slightly smaller. In small and medium size casting, rapping or shake allowance is generally neglected.

 

·      Applications

• 1.    A pattern is essential for the production of any metal or alloy casting.
• 2.    A pattern is used to produce the cavity in the moulding sand for pouring the molten-metal.
• 3.    A properly manufactured pattern reduces the overall cost of casting.
• 4.    A properly manufactured pattern reduces the casting defects.
• 5.    A properly manufactured pattern provides good surface finish of the casting.
• 6.    A pattern may have projection known as core prints which helps in positioning of core.
• 7. A pattern establish the parting line and parting surfaces in the world.

·      Types of pattern

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

• a.     Single Piece Pattern
• b.    Two-Piece or Split Pattern
• c.     Multipiece Pattern
• d.    Match Plate Pattern
• e.     Gated Pattern
• f.      Sweep Pattern
• g.    Loose Piece Pattern
• h.    Skeleton Pattern
• i.      Shell Pattern
• j.      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 and this two-section is connected by pin named dwell pin to attach the two-portion. When we joined 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 on is called cheek, and the bottom one is named 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:

• ·       Gates
• ·       Runner
• ·       Riser

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

 Sweep Pattern:

Sweep Pattern is designed for the half shape of the casting products, it is consisted of a mold cavity, rotating spindle, and sweep pattern.

Its edges are contour 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. That means when some parts of the product have a little outer or inner edge than to casting those part 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. Where 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 in the upper part of the plate and another one is fitted on the bottom of the plate.

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, 1st we mold it in one place then we rotated the pattern to the next portion and again the molding is done