Investment casting process for long hole and narrow groove castings

Investment casting process for long hole and narrow groove castings

The investment casting shell is a repetition of the process of "coating sand drying". However, for castings with structures such as long holes and narrow grooves, due to the difficulty of coating and sanding inside the long holes and narrow grooves, it is difficult to cast when the ratio of hole depth to aperture H/d is greater than 5, and the ratio of slot depth or length to width of narrow grooves is greater than a certain proportion. Mechanical processing is often used to achieve this. For slightly larger diameter long holes, although the shell can be made, it has been filled with slurry and sand, increasing the probability of shrinkage and porosity defects. Today, precision casting manufacturer Tianyao Metal will discuss several methods of investment casting for this type of casting.

1、 Soaking method

1. Immersion method: After the second or third layer of shell making, immerse the shell into the slurry to fill the remaining space of the shell with long holes or narrow grooves. Wait for the slurry to dry thoroughly before proceeding to the next layer of shell making. The purpose is to increase the strength of the inner shell of the hole and prevent the occurrence of fire.

2. Key points of the process

① Soaking slurry: silica sol+mullite powder. High viscosity, poor flow during soaking, and difficulty in filling; The viscosity is low, and it appears full when soaked, but after drying, there will be many voids left due to the evaporation of water, which affects the strength.

② Blow out the floating sand in the long hole before soaking.

③ After soaking, the shell must be completely dry to ensure strength.

2、 Local insulation method for molded shells

1. Local insulation method for shell: Before baking, insulation material is applied locally on the shell to delay the cooling and solidification time, open the shrinkage channel of the molten steel, and achieve the goal of eliminating shrinkage and porosity defects.

2. Process analysis

After casting, due to the quenching effect of the shell quenching water, the casting solidifies first and obtains a dense structure; The shrinkage of the molten steel near the top is supplemented by hot steel from the gate, which also results in a dense structure; Located in the narrow groove in the middle, the steel replenishment in the upper part and the quenching effect in the lower part cannot reach the lower part. After the shell making is completed, the flow hole is basically filled with slurry and molding sand, and the heat dissipation and cooling conditions are extremely poor, resulting in shrinkage defects in the middle of the flow hole. After applying insulation material to the outer shell of the flow hole, the solidification time was delayed, and a top-down shrinkage channel was opened to allow hot steel from the gate to flow from top to bottom, solving the problem of shrinkage and porosity inside the hole.

3、 Self hardening core method

1. After 2-3 layers of shell making, before the narrow groove is filled with paint and molding sand, pour the prepared patented core making material J × R-2 slurry with water-soluble phosphate as the binder. The method of self hardening and forming hydrated phosphate crystals after slurry injection, followed by normal shell casting, and then removing the core to obtain narrow grooves.

2. Key points of the process

① As the powder to liquid ratio decreases, the fluidity of the slurry improves, but the curing time increases.

② After the slurry is prepared, it should be poured as soon as possible. Over time, due to solidification reasons, the fluidity will sharply decrease, affecting operation.

③ Before pouring the slurry, blow out the floating sand in the tank to reduce the flow resistance of the slurry, while also improving the density and strength of the self hardening core.

④ When using grouting materials, it is advisable to tilt the shell to avoid gas entrapment.

⑤ Due to the uneven depth of the narrow groove, two rounds of grouting were used to fill the groove as much as possible, in order to reduce the difficulty of shell making and avoid the occurrence of fire defects.

⑥ Strictly control the curing time of the slurry after pouring. Due to the volume expansion during the solidification process of the slurry, the shell may expand and crack over time, resulting in the shell being scrapped.

4、 Conclusion

Long hole and narrow groove castings are one of the common structures in investment casting. During production, they are often repaired or even scrapped due to defects such as fire and shrinkage in the holes and grooves. The several process methods described in this article can to some extent solve these problems and produce castings that meet the requirements.

Precision machining