Application of Casting Process Equipment — Patterns and Pattern Plates

Patterns

A pattern is one of the essential tools in the molding process. Its function is to form the mold cavity, and therefore, the pattern directly determines the shape and dimensional accuracy of the casting. To ensure casting quality and improve molding productivity, the pattern must have sufficient strength and rigidity, an appropriate surface finish and dimensional accuracy in line with technical requirements. It should also be easy to use, simple to manufacture, and low in cost.

The external shape of a pattern is basically similar to that of the casting, but at places where holes in the casting are to be formed by sand cores, the pattern has no holes but instead protruding core prints (which form core seats in the sand mold).

The dimensions of a pattern must take into account not only the finished part dimensions, but also the process dimensions required for casting (including machining allowances, draft angles, and other process allowances) and the solidification shrinkage rate of the casting material. The dimensions of the pattern can be calculated using the following formula:

Pattern dimension = (Part dimension ± Process allowance) × (1 + Casting shrinkage rate)

In this formula, “＋” is applied to convex surfaces of the pattern, while “–” is applied to concave surfaces of the pattern.

Pattern dimensions refer to the dimensions directly forming the casting, excluding the structural dimensions of the pattern itself (such as wall thickness or stiffening ribs). Since core prints and gating/riser patterns do not form part of the casting body, shrinkage allowance is not required for them.

Depending on the structure of the casting, the molding method, and the production volume, patterns can be made from different materials. Based on material, patterns are generally classified into three types: wooden patterns, metal patterns, and plastic patterns.

1. Wooden Patterns

Patterns made of wood are called wooden patterns, and they are among the most widely used. Their advantages include light weight, ease of machining, short production cycle, and low cost. However, they have low strength and hardness, are prone to deformation and damage, and are generally suitable for single or small-batch production.

The wood used for making patterns must have straight grain, toughness, moderate hardness, fine texture, low moisture absorption, minimal swelling/shrinkage, and be free from knots and cracks. Common woods include red pine, white pine, and fir. Before making patterns, the wood should be dried to avoid shrinkage and deformation.

2. Metal Patterns

Patterns made of metal are called metal patterns. They have high strength, accurate dimensions, smooth surfaces, and good wear resistance. However, their production cycle is longer and cost higher, so they are mostly used in batch or mass production.

Common materials for metal patterns include aluminum alloys, cast iron, cast steel, and copper alloys. These materials vary in machinability and mechanical properties, and should be selected based on specific requirements.

Depending on size, patterns may be solid or hollow. Small patterns are usually solid, while medium and large patterns are typically hollow. Hollow metal patterns are usually designed with uniform wall thickness and stiffening ribs inside, ensuring sufficient strength and rigidity.

3. Plastic Patterns

Plastic patterns are made mainly from epoxy resins. Their advantages include smooth surfaces, good stripping performance, resistance to deformation, light weight, wear and corrosion resistance, simple manufacturing process, and short production cycle. However, they cannot be used under heating conditions and emit harmful gases during manufacture. Plastic patterns are generally used for medium and small castings produced in batches, especially those with complex shapes or difficult machining requirements.