Patterns are essential tools used in metal casting to create the desired shape and structure of the final metal object. They serve as a replica or model of the intended part and are made from materials such as wood, plastic, or metal.
Each pattern has its unique characteristics and is used in specific casting processes. The selection of the pattern type is important to ensure that the final casting is of the desired shape and quality.
This is why I’ll explain the various types of patterns used to create molds for metal objects. In this reading, we’ll explore what a pattern is, its applications, diagrams, types, and disadvantages.
What is Pattern?
A pattern in casting is an exact duplicate of the object to be cast; it is used to create the cavity in the sand mold that molten metal is poured into during the casting process. After the sand mould cavity has been formed using the pattern, the pattern is taken out and molten metal is poured into the cavity to create the casting. The design is non-consumable and almost repeatedly recyclable for creating further sand molds.
The function of patterns is to help reduce casting defects and produce an object with a smooth surface. Many kinds of patterns designs are typically made using materials like wood, metal, or plastic. Sometimes, wax patterns are required for specific applications. Wax patterns are used in the investment casting process.
Applications of Pattern
Patterns are extremely useful in a wide range of industries and are essential for the casting process. Among the noteworthy ways that patterns are used in casting are foundry sector, artistic and jewelry casting, production and model building.
1. Foundry Sector
In foundries, patterns are often used to create metal castings in a range of sizes and forms. They play a crucial role in forming the mold cavity, which is then filled with molten metal to produce the intended end result. Patterns are used in the manufacturing of components for industrial, automotive, and aerospace applications because they allow complicated geometries and small details to be replicated.
2. Artistic and Jewelry Casting
Artful castings and complex jewelry are made with the use of patterns. They make it possible to create original and eye-catching creations, from large sculptures to delicate jewelry. With the use of patterns, creative conceptions may be precisely duplicated, guaranteeing accurate reproduction in the finished metal castings.
3. Production and Model-Building
In the manufacturing sector, prototypes are created using patterns, which enables engineers and designers to test and assess product concepts before to mass production. Manufacturers are able to assess functionality, fit, and aesthetics by making patterns that closely resemble the desired components. This allows for more cost-effective production and changes to the design.
Diagram of Pattern
Types of Patterns in Casting
The various kinds of patterns used in the casting process are single-piece patterns, two-piece patterns, gated patterns, multi-piece patterns, match plate patterns, skeleton patterns, sweep patterns, loose-piece patterns, follow board patterns, cope and drag patterns, segmental patterns, and shell patterns. They are tailored to specific casting requirements.
1. Single-Piece Pattern
These types of patterns are less complex in design and can be placed either in cope or drag. Single-piece pattern types are used in large castings, e.g., stuffing boxes for steam engines. This pattern is exactly like the desired object. A single-piece pattern is also known as a solid pattern.
This pattern is economical because it simply requires straightforward shapes and flat surfaces. It is perfect for straightforward operations and small-scale manufacturing. This is why industries where large objects are made make use of it.
2. Split or Two-Piece Pattern
These types of patterns are used for intricate casting. It is also known as the split-piece pattern. It has cope and drag as its major components. The cope component utilizes dowel pins and is aligned with the drag.
It contains two dowel pins and two dowel holes that are used to align and attach the two sections. The shape of the casting determines the position of the separation planes (irregular or flat). The use of split patterns can be found in the production of steam valves and weapons.
3. Loose Piece Pattern
These types of patterns are obtained when a solid piece has a backdraft or projections that may lie above and below the parting plane. One disadvantage of loose patterns is that the adjustment of the projected pattern can be done during ramming. This may slow down the casting process.
4. Gated Pattern
Gate patterns contain one or more loose patterns attached to gates and runners. Gated patterns are used in producing small castings in mass-production systems. Just as earlier mentioned, casings are produced in bulk using multi-cavity molds.
The molds are made by joining a few patterns and gates and giving them a single runner for the melting metal. These patterns are composed of metal, and gates and runners are created by attaching metal pieces to the pattern.
5. Match Plate Pattern
These types of patterns are similar to split patterns. The cope and drag portions are mounted on opposite sides of a plate (usually metal), which adhere to the contour of the parting surface. Gates and runners are also mounted on the match plate. The match plate pattern is used for a large number of castings, resulting in higher productivity.
The other sides of a match plate are made of wood or metal. The plates also support the runners and gates. This kind of pattern is utilized in machine molding. It is widely used in industry for casting a variety of items, it is expensive, precise, and high-yield. The casting of metals like aluminum frequently employs this style of pattern.
6. Sweep Pattern
In a sweep pattern, the cavity is created by rotating a wooden board along one of its edges. These types of patterns are used when a large amount of casting is required in a short time. Its components include a spindle, a base, and a sweep or wooden board. It is known for producing castings in a short period of time.
7. Cope and Drag Pattern
These types of patterns are similar to split patterns. They contain a cope and drag portion, each mounted separately on a match plate. They are used in the production of large castings. The process cannot be done by a single worker due to the weight of the molds. The cope and drag portions of the mold are made separately. This is because the entire mold is too heavy for one operator to lift.
8. Segmental Pattern
Circular castings like wheel rims, gear blanks, and other similar items frequently use this kind of pattern. These patterns consist of pieces that are moved to create each piece of the mold to create the full mold.
9. Skeleton Pattern
This pattern is just like a sweep pattern made of wooden frames that carry the shape of the part to be cast. A skeleton pattern is for large castings with simple geometrical shapes. It is also used in pit and floor wood processes.
Skeleton patterns are a suitable option for castings with simple sizes and shapes because of their large size. This type of casting pattern is expensive and immobile. Although it is not the most economical choice, it is quite good at getting rid of additional sand.
10. Shell Pattern
These types of casting patterns are produced in a mold that outlines the shape of the part to be coated. A shell pattern type is a great option for building structures with hollow interiors. It divides in half, then dowels the two pieces together.
11. Follow Board Pattern
These types of patterns are used as devices. It is used for various purposes in foundry work. It is made of wooden board. As split or solid patterns become more challenging, a follow board with a contour matching the precise geometry of one-half of the pattern is created.
Pattern Materials
Wood, metal, or plastic are the most common materials used to create patterns. Other materials used are wax and plaster of Paris, although only in specific situations.
The most used wood for patterns is sugar pine, mostly because to its ease of working, light weight, and softness. Because Honduran mahogany is more durable than pine, it was used for more production parts. It is not prone to warping or curling once it has completely dried, making it nearly as sturdy as any wood available.
The foundry does not want the pattern to change shape after it has been constructed. Due to the destruction of the rain forests, authentic Honduran mahogany is becoming increasingly rare; instead, a range of woods are sold under the mahogany name.
Investment casting is an alternative casting method that makes use of wax patterns. For this, a blend of paraffin, beeswax, and carnauba wax is used. The wax “pattern” in this instance can only be used once as it is melted out of the mould cavity, which is often filled with a stiff substance similar to plaster rather than sand.
Because it sets rapidly and retains a great deal of elasticity, plaster of paris is typically used to create master dies and molds.
The durability and weather-resistant properties of fiberglass and plastic patterns have made them more and more popular in recent years. Metal patterns are more expensive, heavier, and more difficult to restore if they are damaged. However, they are long-lasting and resistant to dampness.
Pattern Designs and Allowances
The location of the risers, gating systems, and sprues in relation to the pattern is determined by the foundry engineer or patternmaker. A core, which specifies a volume or location in a casting that metal will not flow into, may be employed when a hole in the casting is required. Occasionally, chills are molded into a sand mold by first placing them on a pattern surface.
Heat sinks called chills provide for quick, aimed cooling. It can be desirable to cool the molten metal quickly in order to improve its grain structure or figure out how it cools in the mold. When the casting cools, they separate from it because they are frequently made of a different metal and are at a temperature that is significantly lower. The chills may then be taken back and used again.
Usually, tolerances are provided in the pattern to account for any dimensional changes that may occur during the (solid) cooling process. These pattern allowance include liquid shrinkage, draft allowance, finishing and machining allowance, shake allowance, and distortion allowance.
Liquid Shrinkage
Liquid shrinkage is the term for the volumetric shrinkage that occurs to almost all metals during solidification. take it another way, when a metal melts, or reaches the liquidus temperature, practically all metals experience a volume increase. Depending on the alloy, “volume shrinkage” typically ranges from 3.5% to 10.0%.
Draft allowance
There’s a chance that any leading edges will break off or sustain damage during the removal of the pattern from the sand mold. In order to prevent this, the pattern has a taper to make it easier to remove from the mold and lessen the chance of edge damage.
Shakes Allwowance
Typically, the pattern is wrapped around the faces after being removed from the mold cavity to make removal easier. This procedure enlarges the final cavity. Reducing the pattern dimensions is necessary to make up for this. This allowance does not have a set amount because it is mainly personnel-based.
Distortion Allowance
Cast deformities might result from stresses that have formed in the solid metal during the mould’s cooling process. This is more noticeable when the mold has a thinner width than it does length. By first warping the pattern in the other way, this may be avoided.
Finishing or Machining Allowance
Since sand castings usually produce poor surface finishes (dimensionally incorrect results), the cast product is frequently subjected to machining operations like turning or grinding to enhance the surface finish. A portion of the piece’s metal is removed during operations.
Limitations of Patterns
Below are the disadvantages of patterns in their various applications:
Expense and Duration
Creating patterns may be costly and time-consuming, particularly for intricate or large-scale castings. Pattern design, fabrication, and upkeep call for specific tools, supplies, and personnel. The entire cost of casting operations can be greatly impacted by the pattern creation cost.
Pattern Reduction and Dimensional Precision
The molten metal shrinks as it cools and hardens during the casting process. To guarantee precise dimensions in the finished casting, this shrinkage must be taken into consideration throughout the pattern design process. To accomplish the necessary casting parameters, pattern designers must take into account the relevant tolerances for shrinkage and dimensional changes.
Pattern Wear and Durability
Over time, patterns may wear out and degrade due to frequent use and contact with molding materials. The materials used for the pattern must be resilient to the pressures, abrasions, and chemical reactions that occur throughout the casting process. To guarantee their longevity and accuracy, regular maintenance and pattern repair could be necessary.
Limited Pattern Size
Casting might be limited by the size of patterns. Oversized or large-scale designs might be challenging to handle, ship, or create. They might complicate and increase the expense of the casting process by requiring larger resources, specialized tools, and additional structural support.