Dimensional stability and accuracy of investment castings
It is always one of the main goals of investment casting workers to continuously improve the dimensional accuracy of investment castings and reduce the waste caused by dimensional aberration.
1.Dimensional stability of wax mold and its influencing factors
Figure 1 shows the results obtained by Professor Robert C. Voigt of the University of Pennsylvania after tracking and measuring 29 kinds of investment castings. It can be seen that in most cases, when the size of the wax mold fluctuates greatly, the size of the casting fluctuations are also large, and the exceptions are few. From the overall point of view, the size fluctuation of wax mold accounts for 10% to 70% of the size fluctuation of casting.
Figure 1 Comparison of size fluctuation between precision casting and wax mold
Note: σ- standard deviation
The process parameters have a decisive influence on the dimensional stability of wax mold. The main factors are as follows:
1)Wax pressing temperature
The influence of wax pressing temperature is different for different moulds (see Figure 2). As can be seen from FIG. 2, when wax base mold is used, the influence of wax pressing temperature on the dimensional stability of wax mold is very sensitive, while the influence of resin base mold is small.
2)Injection pressure
As can be seen from FIG. 3, the shrinkage rate of the wax mold significantly decreases when the pressure is small and the pressure increases. However, when the pressure increases to a certain extent (>1.6MPa), the pressure has almost no effect on the size of the wax mold.
Wax pressing temperature /℃
FIG. 2 Relationship between wax pressing temperature and wax mold shrinkage
Injection pressure /MPa
FIG. 3 Relation between injection pressure and shrinkage of wax mold
3)Flow speed
The flow speed of the mold can be changed in the following two ways, but the impact on the size of the wax mold is not the same: By changing the flow speed of the wax press setting this method has less effect on the shrinkage rate of the wax mold. However, it has important influence on the filling and surface quality of wax mold with complex thin wall parts or with core. By changing the cross-sectional area of the wax injection mouth, this method has a greater impact, because increasing the cross-sectional area of the wax injection mouth can not only reduce the wax pressing temperature, but also extend the solidification time of the mold material at the wax injection mouth, thereby increasing the degree of wax mold compaction, reducing the shrinkage rate and surface shrinkage.
4)Injection time
The so-called pressure injection time here includes three periods of filling, compaction and retention. Filling time refers to the time when the mold is filled with the squeezes.
Compaction refers to the time from full pressure to the closing of the wax nozzle; Hold refers to the time from the closing of the nozzle to the drawing of the mold.
The injection time has a significant effect on the shrinkage rate of the wax mold (Figure 4), because increasing the injection time may result in more mold material being squeezed into the cavity, and the wax mold will be compacted to a greater extent, thereby reducing the shrinkage rate. This can be demonstrated by the increase in the weight of the wax mold with the extension of the compaction time (see Figure 5). The compaction time should be appropriate, if the compaction time is too long, the mold material at the wax injection mouth has been completely solidified, and the compaction will not work. It can also be seen from Figure 4 that when the injection time is relatively short (15-25s), the wax pressing temperature rises and the shrinkage rate increases, but when the injection time is extended to 25-35s (in fact, the compaction time is extended on the premise that the filling time remains constant), the influence of the wax pressing temperature becomes smaller. When the injection time is increased to more than 35s, the opposite situation will occur, that is, as the wax pressing temperature rises, the shrinkage rate of the wax mold will become smaller (see Figure 5). This phenomenon can be explained by the fact that increasing the mold temperature and extending the compaction time have the same effect as increasing the compaction degree of the wax mold.
5)Molding temperature and wax pressing equipment
The wax mold cools slowly and the shrinkage rate increases with high molding temperature. This is because the wax mold is still in the press before the mold is drawn, and the shrinkage is limited, and after the mold is turned into free shrinkage. Therefore, if the mold temperature is high, the final shrinkage rate is large, and vice versa, the shrinkage rate is small.
Similarly, the cooling system of the wax press may have an impact of about 0.3% on the size of the wax mold.
Finally, it is worth emphasizing that when the wax base molding material is used, the wax paste is a solid, liquid and gas three-phase coexistence system. The volume ratio between the three phases has a great influence on the size of the wax mold. The proportional relationship between the three can not be controlled in the actual production, which is also an important reason for the poor dimensional stability of the wax mold pressed with wax base mold.
FIG. 4 Comprehensive effects of injection time and wax temperature on shrinkage of wax mold
Injection time /s
FIG. 5 Influence of pressing time on compaction degree of wax mold
2.Influence of shell material and shell making process on dimensional stability of castings
The influence of shell on the size of castings is mainly caused by the thermal expansion, thermal deformation (creep at high temperature) and the restraint (hindrance) of shell on the cooling shrinkage of castings.
1)Shell thermal expansion
Mainly depends on the shell material. The expansion rate of different refractory materials is different. Among the commonly used refractory materials, the expansion rate of fused quartz is the smallest, followed by aluminum silicate, and silica is the largest and uneven. After testing to determine the aluminum silicate shell from room temperature heating to 1000℃, the shell can produce about 0.25% expansion, the proportion of the overall shrinkage of the size of the casting is not large, so if such refractories are used, the shell has good dimensional stability, such as the use of fused quartz will undoubtedly be better. However, if silica is used, the size of the shell fluctuates greatly.
2)Thermal deformation
For example, the creep degree of the shell with water glass as the binder is significantly greater than that of the silica sol and ethyl silicate shell at high temperature above 1000℃.
Although fused corundum itself has a high refractoriness, due to the presence of impurities such as sodium oxide, the shell roasting temperature above 1000℃ may also produce creep, resulting in poor dimensional stability.
3)The constraints of the shell on the shrinkage of the casting-the conciliation and collapsibility of the shell mainly depend on the shell material.
In summary, the influence of the shell on the size fluctuation of the casting, the refractory material plays a main role, but the role of the binder can not be ignored. In contrast, the impact of the shell making process is less.
3.The influence of stress on dimensional stability caused by uneven cooling of castings
The cooling speed of each part of the casting (including the pouring system) is different and the thermal stress causes the deformation of the casting, which affects the dimensional stability. This is often encountered in actual production. Reducing the cooling rate of castings and improving the sprue combination are effective preventive measures.