Precision Casting Methods: Water Glass Ammonium Chloride and Silica Sol

In order to facilitate operators in familiarizing themselves with and mastering the process characteristics and technical features of precision casting using the water glass method, and to better apply them in production, this guideline is specially formulated.

This process guideline is applicable to the entire process from wax pattern preparation to shell mold pouring.

The Technical Department is the department responsible for formulating and overseeing this guideline. All workers in each process should operate according to this guideline. The molds for pressing wax patterns should meet the requirements of the product drawings and be used only after passing inspection. The wax material should be mixed in a ratio of paraffin wax to stearic acid at 1:1, melted, and mechanically stirred into a paste with wax crumbs before being injected into the mold with a wax injection machine.

Wax patterns must be allowed to cool under pressure in the mold before removal. Deformed wax patterns should be corrected promptly and cooled in cold water before removing burrs and trimming. After the wax patterns are properly trimmed and inspected, the surface grease should be cleaned off before assembling with gating systems. Once the assembled mold is welded, wax residues from both the inner and outer surfaces need to be thoroughly removed before coating with shell material.

For the facing slurry, use 320-mesh zirconium silicate powder, while for the backing slurry, use aluminum oxide powder with a mesh size of 200 or higher, or silicon carbide powder and quartz powder, with a binder having a modulus of 3.1 to 3.4 and a density of 1.30 to 1.40, using 40# water glass.

For the facing layer, use 80 to 100 mesh brown fused alumina, for the second layer, use 40 to 70 mesh quartz sand, for the third layer, use 20 to 40 mesh quartz sand, and for subsequent layers, use 10 to 20 mesh quartz sand.

Slurry Preparation: For the facing layer and the second layer: Dilute water glass with water to a density of 1.28 to 1.30, then add zirconium silicate powder at a ratio of 1:1.1 to 1.2 (adjust the ratio according to temperature changes), mechanically stir, then add 0.05% cleaning agent and 0.05% defoamer, continue stirring for no less than 6 hours, let it stand for 4 hours to mature, and then mix evenly before use. For the transition layer (third layer), use water glass with a density of 1.30 to 1.32, mixed with high-alumina powder and quartz powder at a ratio of 1:0.5:0.5. The reinforcing layer is prepared similarly to the third layer, with slightly thicker proportions.

The viscosity of the slurry is determined using a 100ml flow cup. The viscosity for the facing layer, the second layer, and the third layer should be 28 to 35 seconds, while for the reinforcing layer, it should be 45 to 50 seconds.

After the module passes inspection, immerse it in uniformly stirred slurry, move it up and down twice, then lift it out. Use a brush to pierce the bubbles at the letters and corners and brush off excess slurry. The entire module must be uniformly coated with slurry, without omissions or accumulation. Then, it can be coated with sand. The entire slurry coating process should not be too long to avoid natural hardening of the surface, which would prevent sand coating.

The sanding process should be swift to prevent slurry from dripping and accumulating. Sanding should be uniform without any gaps. After sanding, the module should be placed in a ventilated area for natural drying before being immersed in ammonium chloride solution for hardening. Starting from the third layer, there is no need for natural air drying before immersion in the ammonium chloride solution for hardening.

The surface layer should harden for 15 minutes, the second and third layers for 15 to 20 minutes, and the reinforcement layer for 40 to 60 minutes. Adjustments should be made based on actual conditions, ensuring the principle of "not wet or white." During hardening, it's essential to release air and turn over the module to ensure thorough hardening of every part.

The hardening agent is prepared by using industrial-grade ammonium chloride with a purity of over 95%. It is dissolved in water to achieve a concentration ranging from 22% to 28%. The pH value should not exceed 7.5, and the temperature should be maintained between 15°C to 25°C. In winter, the temperature should preferably be above 10°C. If the concentration of the hardening agent solution decreases over time, additional ammonium chloride should be added promptly to increase the concentration. If the pH value exceeds 7.5, concentrated hydrochloric acid should be added to adjust it.

After coating and scraping, the assembled modules can undergo wax removal after being left to stand for three to four hours. Once the debris at the sprue of the module is cleaned, it is placed into the wax removal frame. When the wax removal water temperature reaches 90°C, the frame is lowered into the wax removal pot, ensuring that the frame is approximately 100mm above the bottom of the tank. The gas valve is then partially closed, maintaining the water temperature at 90-95°C to prevent boiling and splashing of water, which may carry debris into the mold cavity. To replenish the hardening of the shell during the wax removal process, an appropriate amount of ammonium chloride should be added to the water.

When the wax no longer floats upwards at the sprue of the shell, the wax removal frame is lifted from the water surface. The water inside the mold cavity is quickly poured out manually, and then the shell is inverted and placed on a clean surface.

The cleaned wax-free shell should be left to dry for 4-6 hours before being placed in the kiln for firing. During firing, the sprue should be facing downwards. The shell is initially subjected to low-temperature baking, followed by temperature elevation firing, with the temperature controlled at 800-850°C. It should be kept at this temperature for 1-2 hours before the furnace is allowed to cool to below 500°C. At this point, it can be removed from the kiln and left to wait for pouring. The shell should have sufficient strength at 400°C, and during pouring, careful observation and control are necessary.

During transportation, the shell should be positioned with the sprue facing downwards to prevent sand particles or other foreign objects from falling into the mold cavity, which could cause defects such as sand inclusions.