Practice of Investment Casting for Water Glass Shell

Whether it is the water glass shell making process or the silica sol shell making process, before describing the investment casting shell making practice, it is necessary to first talk about designing a reasonable casting plan. The casting plan involves the position of the inner gate on the casting, the shape and size of the inner gate, the type of die head to be used, and what kind of the filling method to be used.

The most important thing is to adopt different melting module tree schemes according to the structural characteristics of different castings. The casting plan design to ensure coating and sanding operations. he casting plan should also be conducive to the hardening and drying of the mold shell, and to avoid coating accumulation. The design of casting schemes for defects such as bubble beans should also ensure smooth wax removal from the mold shell and a high production rate of the casting process.

Firstly, the assembly of wax molds should facilitate the shell making operation of making the sand shell. For narrow gaps on the wax molds, it is easy to cause coating accumulation and difficulty of sand attaching, and there is a risk of sand layer stacking and bridging. During pouring melt metal, break and steel leakage often occur in this area, and a disposal plan needs to be prepared.

Secondly, for large flat castings, sand coating accumulation on the flat surface should be prevented. When coating, sand shell layer accumulation is prone to occur on large flat surfaces, and the mold shell where the coating accumulates becomes hardened and impermeable, causing the casting to bulge. Therefore, when assembling this type of casting, it should be considered that excess sand layer on a large surface is easy to drain, and a certain amount of brushing space should be left.

Thirdly, ensure the hardening and drying of the shell to prevent the accumulation of moisture, which is beneficial for drying and hardening. During assembly, a certain interlayer spacing should be ensured. Due to chemical hardening, the interlayer spacing of water glass shells can be appropriately small, usually 8-12mm. For silicone shells, due to drying, dehydration, and hardening, the interlayer spacing should be appropriately larger, and it is recommended not to be less than 12.7mm. The handling of interlayer spacing should be flexible depending on the structure and size of the components.

The interlayer spacing is small, and the wax mold is not easy to fall off, but the coating is easy to accumulate and form a bridge, and it is easy to cause steel leakage. The casting cools slowly. For thin and slender parts, in order to ensure sufficient filling of the metal liquid, the shell needs to be slowly cooled, and the interlayer spacing should be smaller. For some isolated hot spots that are difficult to shrink, in order to ensure good heat dissipation in this area, the layer spacing should be larger. For castings with larger planes, to avoid coating accumulation, the interlayer spacing should be larger. On the premise of ensuring the quality of castings, it should be said that the smaller the interlayer spacing, the more conducive it is to improving the production rate of casting processes.

Fourthly, in order to prevent the formation of bubble beans (iron beans) in grooves, blind holes, and corners, it is necessary to avoid suffocation when dipping the slurry. During operation, the module should be tilted, slowly, and rotated to dip the slurry. If necessary, the rotation of the slurry bucket should be paused before dipping.

Fifthly, there are blind holes and elongated holes on the wax mold, making it difficult to coat the sand. It is easy to hold air during coating, and easy to build bridges during sand removal, and leak steel during pouring. Therefore, when painting sand, it is often necessary to blow air, and after brushing sand, it is often necessary to blow sand and poke bridges.

After applying a certain number of layers, sometimes dry sand is needed to seal after filling, which is not easy to produce bubble beans and defects such as steel leakage after operation.
Sixth, if the wax mold is damaged or falls off during shell making, it not only reduces the production rate of the casting process, but also causes steel leakage during poor shell repair. Therefore, the inner runner should have a certain strength and should not be too small. In addition, reinforced auxiliary runners or inclined assembly wax molds can be added, and the number of internal gates can be appropriately increased if necessary.

The wax in the module must be removed after shell making and before pouring. The usual wax removal methods are hot water dewaxing and high-pressure steam kettle dewaxing. If the wax is not completely discharged, there is a possibility of carbonization of the wax during shell roasting, resulting in rough surface of the cast after casting, commonly known as carbon contamination defect.

For silica sol shells, resin based molds are commonly used, and static precipitation method is used for recycling. When the resin based mold material has high viscosity, low static temperature, and insufficient static time, impurities such as zirconium powder may be mixed in the mold material. When the wax is not completely discharged, impurities such as zircon powder will remain in the mold shell after calcination, which will contaminate the surface of the casting after pouring. In addition, during dewaxing, the wax will expand due to heat. If the dewaxing is not smooth, the mold shell will expand and crack, leading to defects such as running fire, steel leakage, cracks, and flow lines in the casting. The dewaxing index of the silica sol mold shell is directly related to the tendency of the mold shell to crack.

The smaller the dewaxing index, the less likely the mold shell is to crack during dewaxing. Therefore, in the design of the casting scheme, an appropriate dewaxing index should be ensured. In order to improve the dewaxing performance of large castings, auxiliary wax discharge ports are often directly set up in the concave parts of the castings that are difficult to remove wax. After dewaxing, refractory clay and other materials are used to block the auxiliary wax discharge ports. During pouring, the auxiliary wax discharge ports can also serve as gas storage.

Silicon sol shell dewaxing is a high-pressure steam dewaxing process, unlike hot water dewaxing which involves a wax pouring operation. The wax liquid flows out of the sprue cup solely by gravity, making it difficult to remove wax that is lower than the inner runner in the direction of dewaxing. This not only increases wax consumption, but also easily causes defects such as “carbon pollution” and “powder pollution”. Therefore, in casting schemes, thin gap strips are often added on the side of the inner runner to ensure that the wax is completely discharged.

The yield of casting process is an important evaluation index of casting schemes, which has a significant impact on the cost of castings. For every 1% increase in process yield, casting costs decrease by 1-2%. A new investment casting gating system design method, the gate cup filling and shrinking capacity method, can achieve a production rate of 60% for castings.

The Hengjin method is a representative design method in the investment casting gating system design. This method uses the sprue as the source of shrinkage to provide the molten metal. Therefore, the sprue is thick, and the equivalent diameter of the sprue is generally 1.5-2.5 times that of the inner sprue. It should be said that the safety factor of the Hengjin method is relatively high, but it has been found that when the gating cup is not sufficient using the Hengjin method, there is still a possibility of shrinkage (loosening) defects in the upper layer of the casting group of the sprue type gating system. After dissecting the sprue cup and the sprue, it was found that the sprue cup is the concentrated area for filling and shrinking holes, indicating that the sprue cup is the true source of filling and shrinking metal liquid.

Therefore, a complete set of sprue cup selection diagram and computer application program were established while ensuring the safe filling and shrinking amount of the sprue cup. And reduce the function of the sprue to the same pouring and contraction channel as the inner sprue, thereby reducing the equivalent diameter of the sprue to only 1.1-1.2 times that of the inner sprue. The reduction of the sprue not only reduces the self consumption shrinkage of the sprue, but also fully utilizes the shrinkage resources of the pouring shrinkage system, greatly improving the production rate of the casting process. And because the sprue is quickly filled, an effective pressure head is quickly established, which actually ensures filling.

For large batches of perennial casting products, dedicated pouring and shrinkage systems should be used as much as possible. The design of dedicated pouring and shrinkage systems should minimize useless parts, reduce quality, and improve process yield. At present, the height of most commonly used sprues in China is 250-320mm. The higher the sprue, the more castings are assembled, and the higher the process yield. Try to expand to 400mm as much as possible. When using multiple straight runners for small parts, the more supports and the more assemblies, the higher the yield of the process.