A pattern is an essential tool for the Casting Process. It is used for molding a cavity in the molding sand mixture such that the formed cavity is looked the same as the casting product.
The selection of material for pattern depends on different factors that include the number of casting, quality of casting and the degree of finish.
Colour of Pattern:
There are several colors used in the pattern and those are like:
- Yellow used for Core Prints
- Red used for surfaces to be machined
- Black surfaces to be left unfinished
- Colour Less used for parting surface
Functions of Pattern:
These are some functions of a Pattern:
- It is used for preparing a mold cavity which is used for Casting any parts.
- Some accurate pattern can minimize the production cost of a product because of no further machining is required.
- It reduces casting defects.
Materials Used for Pattern:
Some key factors are in mind before choosing a material for the pattern:
- 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:
- Plaster of Paris
Let me go a little in-depth,
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.
Also, there are some advantages using wood in pattern and those are:
- Wood is light in weight
- Easily Available in the market
- You can make any shape using wood
- 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.
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:
- Smooth surface finish can be obtained by metal patterns.
- Deformation is less.
- 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.
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:
- Light in weight.
- Cheap in price.
- It posses good compressive strength.
- No tension of rusting or moisture absorbing.
However, they are a little weak in strength and not good abrasion-resistant.
A wax pattern used in the investment casting process. By using this pattern we get a high degree of accuracy and have an excellent surface finish.
However it needs little care handling otherwise it can be broken, and it is used in small casting.
Types of pattern:
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
Let me explain this pattern briefly.
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.
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.
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.
A gated pattern consists of:
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 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.
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.
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.
To make a casting perfect we need to consider the allowances because after cooling the molten metal it can shrink and makes any distortion.
So there are 5-types of allowance we are considering when designing a pattern and those are:
- Contraction Allowances
- Draft Allowances
- Finishing or Machining allowance
- Shake allowance
- Distortion allowance
Contraction allowance / Shrinkage allowance:
As we know after cooling in the room temperature all the metals shrink, that’s why we need to consider contraction allowances.
The contraction allowance depends on:
- 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 occurred. to encounter the shrinkage we generally provide a riser that supplied the extra molten metal to the cavity.
- Solid Shrinkage
It’s occurred when the metal is in the solid stage, to encounter this situation we can provide the shrinkage allowances.
When the casting is done, due to the removal of the pattern sometimes the edges of the casting brake, 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, may grinding, or surface finishing operation, these all operations are metal removal operation, so we need to keep the size of the pattern little bigger than the actual size so that after removal of some material, it will perfectly be shaped.
This is a negative allowance, used for easy removal of the pattern,
Some time within the pattern due to cooling of the metal stress is generated to overcome this situation we need to distort the pattern in the opposite direction.
Factors Should in Mind when Design a Pattern:
These are some points should be considered while design a perfect pattern:
- Selection of Proper Material for the pattern.
- Select the perfect type of pattern to do that casting.
- Allowances should be kept in mind while designing
- Avoid sharp edges.
- Core joints should be avoided.
This is all about Patterns used in casting, I hope I covered all the essential topics related to the pattern.
Now I want to hear from you feel free to share your thoughts on the comment section, and don’t forget to share it in your favorite social media handles. Cheers!
Some FAQ on Pattern:
What is the difference between pattern and casting?
What are the pattern allowances?
1. Contraction Allowances
2. Draft Allowances
3. Finishing or Machining allowance
4. Shake allowance
5. Distortion allowance
What are the 10 types of patterns?
1. Single Piece Pattern
2. Two-Piece or Split Pattern
3. Multipiece Pattern
4. Match Plate Pattern
5. Gated Pattern
6. Sweep Pattern
7. Loose Piece Pattern
8. Skeleton Pattern
9. Shell Pattern
10. Segmental Pattern