Analyzes the cost of precision casting

1. Composition of process cost for precision castings

The entire silica sol investment casting process can be divided into four stages: wax mold manufacturing, shell making, melting and pouring, and post-processing. We have collected the inspection, equipment maintenance, and other costs that serve these four stages as auxiliary production costs. The costs incurred in the three stages of wax mold manufacturing, shell making, and melting and pouring in the four production processes are closely related to the process yield, and using pouring weight to calculate costs is more accurate than directly using casting weight. For example, if the cost of wax mold manufacturing is calculated based on the weight of the casting, the cost relationship between small and large parts is clearly not in line with reality. Therefore, a more reasonable method is to calculate the cost of wax mold manufacturing, shell making, melting and pouring process (referred to as the front cost in this article) based on the weight of molten steel poured (referred to as the pouring weight in this article), and the cost of post-processing and auxiliary production (referred to as the back cost in this article) based on the weight of the casting. The manufacturing cost and its composition calculated based on the weight of molten steel poured and the weight of castings are shown in Table 1, and their distribution structure proportions are shown in Figure 1 and Figure 2. It can be seen that the cost of shell making and melting stage accounts for over 60% of the process cost.

2. Main factors affecting the cost difference of precision castings

Strictly speaking, the manufacturing costs of different castings in each process are not completely the same, but some links have very small differences and can be calculated according to the average level. What we need to pay attention to are the factors that have a significant impact on the cost of castings. The main factors that lead to cost differences in the casting process include the following:

(1) Process yield

The process yield rate, also known as the yield rate, is the percentage of the actual casting weight obtained to the pouring weight. For specific castings, the process yield rate is equal to the percentage of the total weight of castings on the same tree to the tree weight. It is related to the casting structure and tree composition plan, and may vary from 30% to 60%, usually between 40% and 50%. The relationship between the initial cost and the process yield is

Front cost per kilogram of casting=

Cost per kilogram of pouring weight, production rate of the first stage process

The front-end cost per kilogram of castings is inversely proportional to the process yield. The lower the process yield, the higher the front-end cost per unit weight of castings, and the lower the process yield, the more significant the impact. The initial cost of pouring per kilogram of molten steel is 6 yuan. When the process yield is 45%, the initial cost of casting per kilogram is 13.33 yuan; When the production rate of the process is 30%, the cost of the front stage of the casting is 20 yuan/kg, which is 6.7 yuan higher than the average level, resulting in a 37.6% increase in process cost. The impact on the total cost of 304 stainless steel castings is about 17%; When the production rate of the process is 60%, the initial cost of the casting is 10 yuan/kg, which is 3.3 yuan lower than the average level, reducing the process cost by 18.5%, which is equivalent to a total cost reduction of about 7% for 304 stainless steel castings;

By taking the derivative of the cost of the front stage of the casting with the process yield, it can be concluded that the degree of influence of the process yield on the front stage cost per kilogram of the casting is inversely proportional to the square of the process yield. When the process yield is 45%, the front stage cost per kilogram of the casting increases by 0.3 yuan for every one percentage point decrease. When the process yield is 30%, the front stage cost per kilogram of the casting increases by approximately 0.67 yuan for every one percentage point decrease

It can be seen that the impact of process yield on cost is very significant. Just like the power factor in electrical engineering, reducing process production rate is equivalent to increasing reactive power consumption. Of course, the higher the production rate of the process, the better. It is not just a matter of wanting to increase it, as an excessively high production rate will reduce the feeding capacity of the pouring system, leading to insufficient feeding and resulting in shrinkage porosity or shrinkage defects; On the other hand, some castings, especially thin-walled castings with irregular shapes, are difficult to improve due to the limitations of casting structure and group tree schemes. This important factor should be taken into account when determining casting prices.

(2) Number of shell making layers

Due to differences in the shape and structure of castings, the number of shell making layers may vary. For example, castings with slender holes or narrow grooves require two or even three surface treatments; Generally, two layers of backing are sufficient for castings, while larger castings may require three or more layers. The shell making cost per kilogram of castings is approximately 5.9 yuan, with material costs accounting for 67.8%, fuel and power accounting for 23.9%, and wages accounting for 13.3%. In the cost of shell making materials of 4 yuan/kg, the consumption of zirconium sand and zirconium powder accounts for about 63%, accounting for 42.7% of the entire shell making cost, and the cost of silica sol accounts for about 12.2% of the shell making cost. Although zirconium sand and zirconium powder are only used for surface layers.