1.Composition of process cost of precision casting
The whole silica sol investment casting process can be divided into four stages:wax mold manufacturing,shell making,smelting and pouring,and post-treatment.We classify the inspection,equipment maintenance and other costs serving these four processes as auxiliary production costs.In the four production processes,the costs incurred in the three stages of wax mold manufacturing,shell making,smelting and pouring 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 mold manufacturing cost is calculated according to the casting weight,the cost relationship between small parts and large parts is obviously not in line with reality.Therefore,a more reasonable method is to calculate the cost of wax mold manufacturing,shell making,smelting and pouring process(referred to as the front cost in this paper)according to the weight of molten steel pouring(referred to as the pouring weight in this paper),and the cost of post-treatment and auxiliary production(referred to as the back cost in this paper)according to the weight of castings.The manufacturing cost and its composition calculated by the weight of molten steel poured and the weight of castings are shown in Table 1,and its distribution structure proportion is shown in Figure 1 and Figure 2.It can be seen that the cost of shell making and smelting accounts for more than 60%of the process cost.
2.Main factors affecting the cost difference of precision castings
Strictly speaking,the manufacturing cost of different castings in each process is not exactly the same,but some links have little difference,which can be calculated according to the average level.We should pay attention to those factors that have a greater impact on the casting cost.The main factors that lead to the difference of casting process cost are as follows:
(1) Process yield
The process yield,also known as the yield,is the percentage of the actually obtained casting weight in the pouring weight.For specific castings,the process yield is equal to the percentage of the total weight of castings in the same tree in the tree weight.It is related to the casting structure and tree assembly scheme,and may vary from 30%to 60%,generally between 40-50%.The relationship between front-end cost and process yield is
Cost of front section per kilogram of casting=
Cost per kilogram of pouring weight at the front stage process yield
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 front cost of pouring molten steel per kilogram is 6 yuan.When the process yield is 45%,the front cost of casting per kilogram is 13.33 yuan;When the process yield is 30%,the front-end cost of castings is 20 yuan/kg,which is 6.7 yuan higher than the average level,which increases the process cost by 37.6%,and the impact on the total cost of 304 stainless steel castings is about 17%;When the process yield is 60%,the cost of the front section 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 deriving the front-end cost of castings from the process yield,it can be concluded that the influence of the process yield on the front-end cost per kilogram of castings is inversely proportional to the square of the process yield.When the process yield is 45%,the front-end cost per kilogram of castings increases by 0.3 yuan for each percentage point reduction.When the process yield is 30%,the front-end cost per kilogram of castings increases by about 0.67 yuan for each percentage point reduction
It can be seen that the influence of process yield on cost is very significant.Like the power factor in Electrotechnics,reducing the process yield is equivalent to increasing reactive power consumption.Of course,the process yield is not the higher the better,nor is it the higher you want.Too high process yield will reduce the feeding capacity of the gating system,resulting in insufficient feeding and shrinkage porosity or shrinkage defects;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 the casting structure and tree assembly scheme.This important factor should be taken into account when checking the casting price.
(2) Number of shell layers
Due to the different shape and structure of castings,the number of shell layers will be different.For example,castings with slender holes or slots need to be coated twice or even three times;Generally,it is enough to make two back layers for castings,while larger castings may need to make three or more layers.The shell making cost per kilogram of castings is about 5.9 yuan,of which material cost accounts for 67.8%,fuel and power account for 23.9%,and wages account for 13.3%.In the shell making material cost of 4 yuan/kg,the consumption of zirconium sand and zirconium powder accounts for about 63%,accounting for 42.7%of the whole 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 layer.
At present,many factories use zircon powder,mullite powder and silica sol shell for the first and second layers.The original water glass shell process shall be used for the back layer.It is an improved scheme combining the excellent surface quality of silica sol shell with the advantages of low cost and short cycle of sodium silicate.Compared with sodium silicate shell,the surface quality of its castings has been greatly improved,and the surface roughness,surface defects and repair rate have been reduced.The production cycle is similar to that of sodium silicate shell.
Main problems
(1) Because the water glass binder is retained in the back layer,the overall high-temperature strength and creep resistance of the shell are lower than those of the silica sol shell,and the dimensional accuracy and geometric tolerance of the cast are not as good as those of the silica sol shell.
(2) The permeability is not as good as that of water glass shell or silica sol shell.The high temperature strength of the shell is not as good as that of silica sol shell,which is easy to cause waste products.
(3) The quality stability of composite shell castings is better than that of sodium silicate,but far less than that of silica sol shell castings.
(4) Because the composite shell uses the expensive zircon powder as the surface layer,its shell cost is 4.5 times that of the sodium silicate shell.If the back layer uses mullite sand powder,its shell cost is almost the same as that of the silica sol shell,and its advantage of low cost is not obvious.
(5) Composite shell cannot use medium temperature wax.Medium temperature wax cannot be dewaxed with hot water.When dewaxing in autoclave,due to high temperature and high pressure,the medium temperature wax liquid will have a violent saponification reaction with water glass and residual hardener in the back layer,and cannot be reused without recycling.