The accounting of casting cost is usually based on weight, and the distribution of production cost, especially the treatment of apportionment and accrued expenses and WIP cost, may also need to take into account the balance of various economic indicators. Therefore, the comprehensive kg cost calculated by accountants alone can not represent the actual cost of castings, and can not accurately reflect the impact of different casting structures and processes on the cost. The casting price can only be determined by multiplying the average price per kilogram by the corresponding floating coefficient. The determination of floating coefficient is usually based on the comparison of typical castings, and may even be determined according to the feeling. In fact, it is difficult to accurately evaluate the cost and profit and loss of specific castings.
Casting cost can be divided into direct cost and period cost. Direct manufacturing costs can be divided into direct materials, direct labor, fuel and power, manufacturing costs and so on. In order to analyze the cost according to the manufacturing process, the burden (including scrap steel and ferroalloy) which directly constitute the casting entity is classified as direct material, and the wax and shell material consumed in the manufacturing process are taken as auxiliary materials. In order to distinguish from the concept of manufacturing cost in accounting, the wage, fuel, power and auxiliary materials consumed in manufacturing process are counted as process cost. The depreciation, house rent and financial expenses are included in the enterprise management fee (cost accounting usually takes the depreciation and rent of production facilities as manufacturing expenses). In this way, the direct material and process costs constitute the direct cost of the casting, which is also the variable cost of the casting. The period cost such as management expense is the fixed cost of the enterprise. The fixed cost shared by castings is actually a part of the marginal profit of castings, which is affected by the scale of enterprises, the mode of operation and the structure of products. The direct material cost depends on the type of alloy and the cost of ingredients, which is a more transparent cost item in the society. Therefore, this paper focuses on the process cost.
1、 Composition of process cost of investment casting
The process of all silica sol investment casting can be divided into four stages: wax pattern manufacturing, shell making, melting and pouring and post-treatment. The costs of inspection and equipment maintenance for these four processes are classified as auxiliary production costs. In the four production processes, the costs of wax pattern manufacturing, shell making and melting casting are closely related to the process yield. It is more accurate to calculate the cost by pouring weight than by casting weight directly. For example, if the wax pattern manufacturing cost is calculated according to the casting weight, the cost relationship between small and large parts is obviously not in line with the actual situation. Therefore, the more reasonable method is that the cost of wax pattern manufacturing, shell making, melting and pouring process (referred to as the front stage cost in this paper) is calculated according to the weight of molten steel poured (referred to as pouring weight in this paper), and the cost of post-treatment and auxiliary production (referred to as the cost of later stage in this paper) is calculated according to the weight of casting.
2、 Main factors influencing cost difference of investment casting
Strictly speaking, the manufacturing cost of different castings in each process is not exactly the same, but some links have very small differences, which can be calculated according to the average level. We should pay attention to the factors that have a greater impact on the casting cost. The main factors leading to the difference of casting process cost are as follows:
1. Process yield
Process yield, also known as yield, is the percentage of the actual casting weight in the pouring weight. For a specific casting, the process yield is equal to the percentage of the total weight of the castings on the same group number to the total weight of the whole group. It is related to the casting structure and the group tree scheme, which may vary from 30% to 60%, generally between 40% and 50%. The lower the process yield is, the higher the front stage cost per unit weight of casting is, and the lower the process yield is, the more significant the influence is. The cost of the first stage of casting per kilogram of molten steel is 6 yuan. When the process yield is 45%, the cost of the first stage is 13.33 yuan per kilogram of casting; when the process yield is 30%, the cost of the front section of the casting is 20 yuan / kg, which is 6.7 yuan higher than the average level, making the process cost increase by 37.6%, and the influence range on the total cost of 304 stainless steel castings is about 17%; When the process yield is 60%, the front stage cost of the casting is 10 yuan / kg, which is 3.3 yuan lower than the average level, which reduces the process cost by 18.5%, which is equivalent to the total cost reduction of about 7% for 304 stainless steel castings;
When the process yield is 45%, the front stage cost of each kilogram of casting will increase by 0.3 yuan for every one percentage point reduction. When the process yield is 30%, the front stage cost per kilogram of castings will increase by 0.3 yuan for every one percentage point reduction. When the process yield is 30%, the front stage cost per kilogram of castings will be approximately reduced by one percentage point 67 yuan
Therefore, the effect of process yield on cost is very significant. Like the power factor in Electrotechnics, reducing the process yield is equivalent to increasing the reactive power consumption. Of course, the higher the process yield is, the better it is, and it is not the higher the process yield is. Too high process yield will reduce the feeding capacity of the gating system, resulting in shrinkage porosity or shrinkage cavity defects due to insufficient feeding; On the other hand, some castings, especially the thin-walled castings with irregular shape, are difficult to improve the process yield due to the limitations of casting structure and composition scheme. This important factor should be taken into account when determining the casting price.
2. Shell making layers
Due to the different shape and structure of castings, the number of shell layers will be different. For example, for castings with long and thin holes or slots, two or even three surface layers are required; for general castings, two back layers are enough, while for larger castings, three or more times are required