METAL CASTING ( PATTERN )

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A pattern is a shaped form of wood or metal around which sand is packed in the mold. When the pattern is removed the resulting cavity is the exact shape of the object to be cast.
The pattern must be designed to be easily removed without damage to the mold. It must be accurately dimensioned and durable enough for the use intended. Either one time use or production runs.PATTERN MAKING
Each different item we wish to cast presents unique problems and requirements. In a large foundry there is a close relationship between the pattern maker and the molder. Each is aware of the capabilities and limitations of his own field.
Throughout the industry, pattern making is a field and an art of it's own. The pattern maker is not a molder nor the molder a pattern maker. This is not to imply that the pattern maker cannot make a simple mold or the molder make a simple pattern but each may soon reach a point in the other's field beyond his own skill and experience.

In the hobby or one man shop, however, pattern and mold making are so closely interrelated as to become almost one continuous operation. This chapter will acquaint you with some of the various types of patterns and their requirements.

Types of Patterns

Patterns can be of different types depending on the shape and size of the part to be manufactured. Given below are some of the commonly used pattern types.

• A solid pattern is the most simple of all and is used to make simple shapes. As the name itself suggests, a solid pattern is a single solid piece without any subparts or joints.

• Shapes which are more intricate are manufactured using patterns which are made out of 2 or more pieces. These pieces are aligned together with the help of dowel pins, and such patterns are known as split patterns

• Sometimes it is convenient to produce multiple parts in one go and a single pattern is used to make the cavity for all the part spaces. There are runners between these pieces, which are also known as gates. Hence these patterns go by the name of gated patterns.

• In several cases it could be economical to save the money and efforts of making the full pattern because of symmetry. The cavity in such a case could be made by sweeping the pattern (which is a part of the full shape) around a central axis, hence these are known as sweeping patterns.

The above list is not exhaustive and there are several other types of patterns as well such as loose piece patterns, follow board and odd-shaped patterns which are used for different situations. Yet the description above should have given a broad idea to the reader about the patterns, their types and usefulness in the casting manufacturing process.
The two images show a solid pattern and a split pattern (both having same shape)


We will talk about wood pattern
Wood patterns used for sand casting are given several coats of Orange Shellac to which a pinch of oxalic acid has been added. This gives them a good waterproof smooth hard surface.
The majority of wood patterns are made of white pine (sugar pine) as it is easily worked and when shellacked properly will not warp under ordinary foundry use.
The approximate weight of a casting can be determined by weighing the wood pattern and multiplying by the appropriate factor indicated. Aluminum 8, cast iron 16.7, copper 19.8, brass 19.0, steel 17.0.
A white pine pattern weighing 1 lb, when cast in aluminum will weigh 8 pounds, in brass 19 pounds etc.

The first thing to do is to choose your PARTING LINE

Parting direction of a mold segment is the direction generally along the axis of the mold segments and is introverted from the adjoining mold segment. Draw direction is the other name of parting direction.

Parting surface is known as the surface of contact within any two segments of the mold.

Parting line is the line where parting surface meets with the casting surface of the mold. After choosing your parting line you now are going to make the pattern . When you are going to make the pattern you must add three allowances  to

the dimensions :

1* Machining allowance .
2* Draft allowance .
3*shrinkage allowance .

• . 1-Machining allowance : and we take it from this table :

This is a simple table

Metal	dimension (inch)	allowance (inch)
Cast iron	Up to 12 12 to 20 20 to 40	0.12 0.20 0.25
Cast steel	Up to 6 6 to 20 20 to 40	0.12 0.25 0.30
Non ferrous	Up to 8 8 to 12 12 to 40	0.09 0.12 0.16

The finish and accuracy achieved in sand casting are generally poor and therefore when the casting is functionally required to be of good surface finish or dimensionally accurate, it is generally achieved by subsequent machining. Machining or finish allowances are therefore added in the pattern dimension. The amount of machining allowance to be provided for is affected by the method of molding and casting used viz. hand molding or machine molding, sand casting or metal mold casting. The amount of machining allowance is also affected by the size and shape of the casting; the casting orientation; the metal; and the degree of accuracy and finish required.

and this a simple example:

The casting shown is to be made in cast iron using a wooden pattern. Assuming only machining allowance, calculate the dimension of the pattern. All Dimensions are in Inches

The machining allowance for cast iron for size, up to 12 inch is o.12 inch and from 12 inch to 20 inch is 0.20 inch .
For dimension 18 inch, allowance = 0.20 inch
For dimension 14 inch, allowance = 0.20 inch
For dimension 8 inch, allowance   = 0.12 inch
For dimension 6 inch, allowance   = 0.12 inch
The pattern drawing with required dimension is shown in Figure below

2-Draft allowance :


By draft is meant the taper provided by the pattern maker on all vertical surfaces of the pattern so that it can be removed from the sand without tearing away the sides of the sand mold and without excessive rapping by the molder. Figure  shows a pattern having no draft allowance being removed from the pattern. In this case, till the pattern is completely lifted out, its sides will remain in contact with the walls of the mold, thus tending to break it.



 Figure below is an illustration of a pattern having proper draft allowance. Here, the moment the pattern lifting commences, all of its surfaces are well away from the sand surface. Thus the pattern can be removed without damaging the mold cavity.

We take the draft allowance from this table :

Pattern material	Height of the given surface (inch)	Draft angle (External surface)	Draft angle (Internal surface)
Wood	1 1 to 2 2 to 4 4 to 8 8 to 32	3.00 1.50 1.00 0.75 0.50	3.00 2.50 1.50 1.00 1.00
Metal and plastic	1 1 to 2 2 to 4 4 to 8 8 to 32	1.50 1.00 0.75 0.50 0.50	3.00 2.00 1.00 1.00 0.75

3-Shrinkage allowance

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A shrinkage allowance for metal casting is something that must be figured into a design from the very beginning. As the molten metal cools and solidifies it will begin to contract. This means that although the molten metal completely filled up a mold, by the time the casting was cold, the casting is smaller than the mold.
What this mean is that a pattern must be made larger than the design drawing. The difference between the size or dimensions of the desired casting and the size of the pattern used to create the mold is called a shrinkage allowance.
The shrinkage allowance for metal casting varies by the type of metal. It takes experience in metal casting to be able to accurately judge the proper shrinkage allowance to be built into a pattern. Below is a basic shrinkage allowance and pattern oversize factor chart based on metal type.

Metal	Pattern Oversize Factor	Finish Allowance	Win Wall mm/(in)
Aluminum	1.08 – 1.12	0.5 to 1.0%	4.75 (0.187)
Copper Alloys	1.05 - 1.06	0.5 to 1.0%	2.3 (0.094)
Gray Cast Iron	1.10	0.4 to 1.6%	3.0 (0.125)
Nickel Alloys	1.05	0.5 to 1.0%	N/A
Steel	1.05 – 1.10	0.5 to 2%	5 (0.20)
Magnesium Alloys	1.07 – 1.10	0.5 to 1.0%	4.0 (0.157)
Malleable Irons	1.06 – 1.19	0.6 to 1.6%	3.0 (0.125)

The shrinkage allowance for metal casting is linear meaning that these allowances apply in every direction. In addition, the shrinkage allowances in the chart above are approximations. The actual size and shape of the casting actually determines the actual shrinkage allowance.
To complicate things further, different parts of a casting may require a totally different allowance. This is often found in the portion of the casting that is at the very top of the mold where impurities and air bubbles end up when the molten metal is poured.
If a casting is to be sand cast, then often the rough surface of the final casting will require surface finishing. Thus a machining allowance or “finish allowance” must also be configured into the casting pattern.
After taking all these allowances we make this table for the dimensions of our pattern .

Nominal Dimensions	Machining allowance	draft (Taper )allowance	Shrinkage allowance	Total pattern Dimensions
PARTING LINE Dim.
		0.0
		0.0
		0.0
		0.0
		0.0
		0.0
		0.0
		0.0

NOTE >>> We take add the draft allowance only to the parting line dimension .
               Fill the table with your pattern dimensions .
The new dimensions now that you will take to make the pattern is in the last column of the upper table .
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