Types of Pattern PDF

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All types of patterns are covered in this paper which is used in the casting process....

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Types of Pattern 1. Introduction In casting, pattern is the replica of the object to be casted, used to prepare the cavity into which the molten metal will be poured during the casting process. It provides the shape for object that need to be casted. Typically, patterns used in sand casting may be made of wood, metal, plastics or other material. Patterns are made to exacting standard 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 repeatedly provide a dimensionally acceptable casting.

2. Function 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. The patternmaker or foundry engineer decides where the sprues, gating systems, and risers are placed with respect to the pattern. Where a hole is desired in a casting, a core may be used which define a volume or location in a casting where metal will not flow into. Sometimes chills may be placed on a pattern surface prior to molding, which are then formed into the sand mold. Chills are heat sinks which enable localized rapid cooling. The chills can be reclaimed and reused.

Figure4. 1: Pattern

3. 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: 1. Wood 2. Metals

3. Plaster of Paris 4. Plastics 5. Wax

3.1. Wood As we all know woods are easily available, and the price is quite low so it is satisfied us some basic criteria which are 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, teaks are used for making a pattern.

3.2. Metals In metals, cast iron, brass, aluminum is 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.

3.3. 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.

3.4. 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: ➢ ➢ ➢ ➢

Light in weight Cheap in price It possesses good compressive strength No tension of rusting or moisture absorbing

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

3.5. Wax 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.

4. Types of Pattern Pattern is of ten types: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

Single/Solid Pattern Split Pattern Match Plate Pattern Cope and Drag Pattern Sweep Pattern Gated Pattern Loose Piece Patten Shell Pattern Skeleton Pattern Segmental Pattern

4.1. Single/Solid Pattern A single pattern or solid pattern is simplest. It is replica of the desired casting usually in a slightly larger size to offset the shrinkage of the intended metal. Gated patterns connect a number of loose patterns together with a series of runners that will detached after shake-out.

Figure4. 2: Single Pattern

4.2. Split Pattern Split pattern or two-piece pattern is most widely used type of pattern for complex casting. It is split along the parting surface, the position of which is determined by the shape of the casting. One half of the pattern is molded in drag and the other half is cope.

Figure4. 3: Split Pattern

4.3. Match Plate Pattern A match-plate pattern is similar to a split pattern, except that each half of the pattern is attached to opposite sides of a single plate. The plate is usually made from wood or metal. This pattern design ensures proper alignment of the mold cavities in the cope and drag and the runner system can be included on the match plate. Match-plate patterns are used for larger production quantities and are often used when the process is automated.

Figure4. 4: Match Plate Pattern

4.4. Cope and Drag Pattern A cope and drag pattern are similar to a match plate pattern, except that each half of the pattern is attached to a separate plate and the mold halves are made independently. Cope and drag patterns are often desirable for larger castings, where a match-plate pattern would be too heavy and cumbersome. They are also used for larger production quantities and are often used when the process is automated.

Figure4. 5: Cope and Drag Pattern

4.5. Sweep Pattern A sweep is a section or wooden board 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. The wooden rod is fixed to metal rod and rotated about the metal rod as the axis of rotation, for getting a complete mold cavity. It is economical.

Figure4. 6: Sweep Pattern

4.6. Gated Pattern A gated pattern consists of casting patterns, pattern and stripping plates, core boxes, patterns of parts of the gating, molding and control patterns, jigs, and drying plates. When large number of small castings are required, gated patterns are used.

Figure4. 7: Gated Pattern

4.7. Loose Piece Patten Loose piece pattern has more than two pieces of the patten. Small pieces of the pattern are attached to the main piece of pattern using wire. When molding is over, the main piece of pattern (large pieces) is removed from the mold, then loose and with draw the small pieces through the gap generated by the main piece. Loose piece pattern is expansive, and molding with these patterns requires highly skilled workers.

Figure4. 8: Loose Piece Pattern

4.8. 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.

Figure4. 9: Shell Pattern

4.9. Skeleton Pattern Pattern is the Skeleton of desired shape, generally mounted on the metal base. Skeleton is made from wooden strips and is filled with loam sand and rammed. Extra sand is removed by stickle. Cores are required if necessary. It is generally used for casting very huge parts.

Figure4. 10: Skeleton Pattern

4.10. Segmental Pattern It is used for preparing circular castings. In this type it does not revolve continuously like sweep pattern, instead prepares the mold by parts. It completes one portion of the mold and then moves to next position to make the next part of the mold and so on till the mold is completed.

Figure4. 11: Segmental Pattern

5. Pattern Allowances 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

6. Designing of 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....