How to determine the number of shell layers for products in precision casting

How to determine the number of shell layers for products in precision casting

For a newcomer in investment casting, confirming the number of layers for a product's production is a relatively difficult task. Because the number of shell layers is the key to determining whether the shell can withstand the impact of high-temperature metals and ultimately obtain a qualified casting. This qualified casting not only includes a smooth and flat surface, complete edges and contours, but more importantly, the dimensional accuracy of the casting meets the requirements. In general textbooks, the description we see only includes the surface layer, transition layer, back layer, and sealing layer, as well as the functions of each layer. However, there is little information on how many layers of shell are used in general products, and most of it is based on the experience of the designer. Today, precision casting manufacturer Tianyao Metal will talk to you about how the number of shell layers for general products is determined.

1、 We should understand the boundaries of designing the number of sales layers;

The design of the number of sales layers is not unlimited and must have a certain degree. This degree is sufficient, there is no need to design it too thick or with many layers, as it can bring many side effects. If a product is designed with 4 and a half layers or 5 and a half layers, then its degree is 4 and a half layers. You can design it as 5 and a half layers, but the material, labor, and production cycle costs of one layer, as well as the potential impact of poor heat dissipation (which may not necessarily) on the quality of the casting, may not be possible for you to consider. Therefore, it is recommended to use multiple options when sampling to determine the minimum number of layers in the product.

2、 The number of shell layers of a product is actually related to its structure, materials, and size;

2.1. A product with narrow grooves and fine holes may have more layers of shell making. Because it requires the use of fine sand to form a shell with narrow grooves and small holes.

2.2. The size of the product is definitely a key factor in determining the number of shell layers. The larger the size, the more layers the shell should have. The smaller the size, the less influence the number of shell layers. But it is not recommended to reduce the number of layers of the casting too much, as the dimensional accuracy of the casting also needs to be considered.

2.3. For products of the same size, thicker products have a slightly higher number of shell layers compared to relatively thinner products, because thicker products tend to accumulate heat, which puts more pressure on the thermal shock resistance of the shell.

2.4. The material of the casting is also an important factor in determining the number of shell layers for the product. Generally, steel parts use six and a half layers, while aluminum parts use five and a half layers. Relatively speaking, non-ferrous metal parts may have fewer layers, while black metal shells may have more layers.

3、 The number of shell layers is also related to the viscosity ratio of each layer.

In Mr. Jiang Buju's book "Practical Precision Casting Technology", two types of shell structures are proposed, as shown in the following figure. One is the overall structure, and the other is the layered structure. Generally speaking, using a slurry with low viscosity and good fluidity results in a relatively thin shell, while using a slurry with high viscosity results in a relatively thick shell. A thick shell naturally has greater strength than a thin one, so it may require fewer layers.

The above factors will all affect the shell making maturity of precision castings, hoping to be helpful to everyone.

Precision machining