Casting is a very common manufacturing process to produce metal parts. It creates molds also called casting patterns where the manufacturers pour the molten metal next, and metal parts are finished after the cooling of shaped metal materials.
During the casting process, the casting pattern is very crucial because of its effect on final products. Casting manufacturers should consider the casting pattern design completely according to the final products. There will have an expensive changing cost after casting.
What Is Pattern in Casting?
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, is a skilled trade that is related to the trades of tool and dies making and mold making but also often incorporates elements of fine woodworking.
Patternmakers 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
Materials of Casting Pattern
Your casting pattern materials should have these properties:
- Lower cost and less weight: with the lower cost and less weight you are easier to find the balanced point of costs and returns.
- Resistance of water: choosing the material with resistance of water protects your casting pattern from rusting. Casting patterns with this kind of material will have a long lifetime and produce high quality patterns.
- Durable: durable material provides a long lifetime of your casting pattern, so you should consider the durable property of the material before you choose casting pattern materials.
- Versatile: various industries need casting patterns to create the casting process, so the versatile property of casting pattern material is very important. It ensures the pattern can be used in many kinds of industries, and repaired easier.
Casting Pattern Allowances
Other factors like the type of casting pattern and the properties of casting metal should also consider. Therefore, there have some casting pattern allowances you should pay attention to when you make a casting pattern.
- Draft allowances: draft allowances create a taper for removing casting patter without any distortion. And the accurate angle of taper depends on the type of mold and surface, and the complexity of the casting pattern.
- Shrinkage allowance: usually the casting pattern has a bigger size of the mold, because most metal material contracts when it cools. The shrinkage allowance compensates for the cooling shrink of metal, and the precise parameter of the allowance depends on the metal material.
- Distortion allowance: casting patterns have a special design for avoiding the expected cooling distortion. We call it a distortion allowance.
- Machining allowance: excess material in the finishing stage for compensating some loss materials.
Casting VS. Pattern
The main difference between pattern and casting is that a pattern is a tool while casting is a kind of process.
- Casting: It is a kind of process that manufacturers pour the molten metal into a mold, where the material cools and solidifies. The final shape of products was decided by the mold cavity, while the shape of the mold should be considered, and that is where the pattern appears.
- Pattern: Before you make a mold, you should design a pattern firstly. The pattern is the primary shape of the mold and finally, the product shapes according to the pattern.
Types of Patterns in Casting
As we discussed, casting objects are highly dependent on the pattern. According to the shape and size of casting and method of making cavity, the pattern can be classified as follow:
- Single piece pattern,
- Split pattern,
- Gated pattern,
- Multi piece pattern,
- Match plate pattern,
- Skeleton pattern,
- Sweep pattern,
- Lose piece pattern,
- Cope and drag pattern,
- Shell pattern.
There have more details about each of the casting patterns.
1. Single Piece Pattern
Single piece pattern, also called solid pattern is the lowest cost casting pattern. It is very suitable for a simple process, and small-scale production and the large casting manufacturers prefer it because this kind of casting pattern makes the casting process just needing simple shapes, flat surfaces like simple rectangular blocks. One flat surface is used to separate planes.
This is the simplest type of pattern, exactly like the desired casting. For making a mold, the pattern is accommodated either in cope or drag. Used for producing a few large castings, for example, stuffing box of a steam engine.
2. Split pattern
A two-piece pattern also called split piece pattern is a common casting pattern for intricate casting. This kind of pattern has parting planes which may have a flat or irregular surface, and the exact position of the plane was decided by the shape of the casting.
There are two pieces of the split-piece pattern. One of the parts is molded in drag and another is molded in cope. And the cope part always has dowel pins. With the dowel pins, the two halves of the split piece pattern can be aligned.
3. Multi Piece Pattern
A multi-piece pattern is a good solution for complex designs which is hard to make. This kind of pattern includes 3 or more patterns that help you achieve mold making.
Take the three-piece pattern as an example. The pattern is made of the top, bottom, and middle parts. The top part is cope, the bottom part drags, and the middle parts are called checkboxes.
4. Match Plate Pattern
A match plate pattern is a split pattern having the cope and drags portions mounted on opposite sides of a plate (usually metallic), called the “match plate” that conforms to the contour of the parting surface.
The gates and runners are also mounted on the match plate, so that very little handwork is required. This results in higher productivity. This type of pattern is used for a large number of castings.
5. Gate Pattern
A gated pattern is simply one or more loose patterns having attached gates and runners. Because of their higher cost, these patterns are used for producing small castings in mass production systems and on molding machines.
Gate pattern can consist of one or more patterns into a molding pattern. It is designed for the mold which makes multiple components at one casting process. The gates are used to combine the different patterns, and runners to create a flow way for the molten materials. When the gates and runners have already been attached, the patterns are losing. This kind of pattern is expensive, and it is usually used for small castings.
6. Skeleton Pattern
For large castings having simple geometrical shapes, skeleton patterns are used. Just like sweep patterns, these are simple wooden frames that outline the shape of the part to be cast and are also used as guides by the molder in the hand shaping of the mold. This type of pattern is also used in-pit or floor molding process.
This kind of casting pattern is expensive and not versatile. It is not the best choice from the aspect of economic, while is very efficient in extra sand removal. If you want to use this casting pattern you should highlight the wood frames when you casting. The skeleton pattern is widely used in the industries of the pit or floor welding.
7. Sweep Pattern
A sweep is a section or board (wooden) of proper contour that is rotated about one edge to shape mold cavities having shapes of rotational symmetry. This type of pattern is used when a casting of large size is to be produced in a short time. Large kettles of C.I. are made by sweep patterns.
This kind of casting pattern creates a cavity in the vertical direction and the base of it is attached with sand, and it also creates casting in a very short time, and it has consisted of three parts: spindle, base, and sweep which also called wooden board.
8. Loose Piece Pattern
When a one-piece solid pattern has projections or backdrafts which lie above or below the parting plane, it is impossible to withdraw it from the mold. With such patterns, the projections are made with the help of loose pieces. One drawback of loose feces is that their shifting is possible during ramming.
This kind of pattern needs extra skilled labor work, so it is an expensive casting pattern in castings.
9. Cope and Drag Pattern
A cope and drag pattern are a split pattern having the cope and drag portions each mounted on separate match plates. These patterns are used when in the production of large castings; the complete molds are too heavy and unwieldy to be handled by a single worker.
This kind of pattern has a bit similar to the two-piece pattern and is usually used in large casting.
10. Shell Pattern
The shell pattern is a good choice to create a hollow-shaped structure. It parts along the center and dowels the resultant halves.
Conclusion
During the casting process, casting patterns are very important which decide the casting molds and the quality of castings. Before choosing the casting pattern, you should consider some factors, such as the size, shape of your casting products, and choose the best pattern for your casting business.