Avoiding Process Complexity

The process requirements for producing casting molds using low-temperature wax-stearic acid blend and manufacturing mold shells using precision casting silica sol as a binder are studied. The main focus is on the wax mold manufacturing process, silica sol shell-making process, and firing process. Over the past five years, production practices have demonstrated that the precision casting silica sol shell-making process is mature, providing technical assurance for producing qualified mold shells and precision castings.

Using low-temperature wax-stearic acid blend to produce wax molds (commonly known as "lost wax molds") for turbine impellers, nozzle rings, and other disc-type precision castings; using precision casting silica sol as a binder to manufacture mold shells for turbine impellers, nozzle rings, and other castings. The main focus is on researching the wax mold manufacturing process, silica sol shell-making process, and firing process, thereby providing technical assurance for producing qualified mold shells and precision castings.

The composition and performance of the mold material should meet the requirements of raw material inspection. When melting the wax material, all kinds of waxing tools must be cleaned properly. The waxing equipment should be cleaned at least once a month, and it is strictly prohibited for water or dirt to enter the wax liquid. Overheating of the wax liquid is strictly prohibited. When preparing paste wax, slowly add the shredded wax flakes into the continuously stirred wax liquid, and the amount added depends on the specific situation. The temperature of the paste wax ranges from 45 ℃ to 48 ℃. Mix the paste wax thoroughly and evenly to prevent air intake during the mixing process from causing bubble defects in the wax mold, and to avoid cracking and deformation caused by insufficient pressure, wrinkling, or excessive cooling time due to the viscosity of the paste wax.

The wax material ratio: 50% paraffin, 50% stearic acid (by weight); After melting, the wax material should be kept at a temperature of 70°C to 75°C; The temperature of the wax liquid should not exceed 90°C.

Temperature range during wax pattern pressing: 18°C to 27°C; Wax material temperature: 45°C to 48°C; Pressing temperature: 20°C to 25°C; Injection pressure: 0.1 MPa to 0.3 MPa; Preheating temperature of the wax injection gun: 35°C to 45°C; Cooling water temperature for the wax pattern: 15°C to 25°C; Demolding time: approximately 1 minute.

Before wax pressing, check whether the mold model meets the requirements. Before wax pressing, it is necessary to clean the residual wax, water droplets, and other attachments on the surface of the mold as much as possible, avoiding the use of steel knives to prevent damage to the mold. Keep the mold and die clean. After wax pressing, the mold should be wiped clean and oiled according to regulations for storage. During the wax pressing process, the surfaces of the mold and die should be promptly coated with a release agent for quick and convenient demolding. Transformer oil is used as the release agent. After dipping a dry gauze or cloth in the release agent, wipe it on the mold die, striving for thin and uniform application, and no oil traces visible to the naked eye. After removing the wax pattern, it should be placed in cooling water for no more than 20 minutes.

Remove excess flash and burrs from the wax pattern, as well as trim the wax injection port. No other areas are allowed to be altered during trimming, and care must be taken to avoid damaging the working surface of the casting and to avoid welding or altering the machining reference plane.

Wax patterns must undergo self-inspection and specialized inspection before welding can be conducted. Welding should be performed according to the requirements of the mold assembly, with a gap of more than 5mm between wax patterns. The welding temperature for wax patterns and sprues should be around 80℃. Welding must be smooth, ensuring a smooth transition between wax patterns and sprues, with no wax pimples or depressions. The sprues must be flattened with a hot knife and not scraped flat, ensuring there are no uneven surfaces. No voids in the welds are permitted, ensuring welding strength.

Prepare a neutral soap solution with a concentration of 0.2% to 0.3%, with a temperature of 20°C to 25°C. Clean the mold assembly in the soap solution using a soft brush to remove surface oil contamination, then rinse thoroughly in water at 20°C to 25°C to remove excess soap solution, ensuring proper coating performance on the wax pattern surfaces. 

Self-inspection takes precedence over specialized inspection; wax patterns that do not pass inspection are not allowed to be welded into mold assemblies. Inspectors examine the wax pattern assembly, conducting both visual quality inspections (100% visual inspection of wax patterns, with no shrinkage, cracks, wrinkles, or blisters allowed on the surface) and dimensional inspections (sampling wax pattern dimensions), and control the wax making process accordingly.

There are no national standards for silica sol used in investment casting, only the aviation industry standard HB5346-1986 "Technical Requirements for Silica Sol Used in Investment Casting" (see Table 1). Silica sol is a water-based binder used in investment casting for layers (surface layers and back layers). Silica sol can be easily formulated into a high-solid content coating, providing stable performance. Silica sol-based shell molds do not require chemical hardening and do not cause air pollution during the shell-making process.

Table 1 Technical Requirements for Silica Sol Used in Investment Casting

Grade Chemical Composition / % Physical Properties Others    

SiO2    Na2O    Density / g·cm-3    PH Value    Kinematic Viscosity / mm2·s-1    SiO2 Particle Diameter / mm    Appearance    Stability Period    

GRJ-26    24～28    ≤0.3    1.15～1.19    9～9.5    ≤6    7～15     Milky white or light blue without impurities    ≥1 year

GRJ-30    29～31    ≤0.5    1.20～1.22    9～10    ≤8    9～20    

Correctly selecting the proportions of various raw materials to ensure that the coating has the appropriate specific gravity and viscosity is the key to controlling the coating (see Table 2). In silica sol coatings, in addition to silica sol and refractory powder, there is also a certain amount of surfactant (JFC, a fatty alcohol polyoxyethylene ether produced by Jiangsu Haian Petrochemical Factory) and defoamer (n-octanol).

Table 2 Silica Sol Coating Liquid Formulation

CategoryUnitSurface Layer (1)Layers 2-3Layers 4-5Outer Layer  

Silica Sol    kg    10    10    10    10    

Zirconium Silicate Powder    kg    38    0.3    

Coal Gangue Powder    kg    30    20    1.2    

Wetting Agent    g    20    

Defoamer    g    20    

Viscosity    s    30～35    30    10～15    15～20    

To control the viscosity of the coating and achieve stable shell quality, adjust the viscosity based on the specific conditions at the time. For thin-walled, complex, or deeply recessed parts, use the lower limit of viscosity, and vice versa for the upper limit.

Following the coating formula, add a certain amount of refractory material, wetting agent, and defoamer. First, add the wetting agent and defoamer to the coating bucket, then add the silica sol, and start stirring. While continuously stirring, add the refractory material. After all materials are added, continue stirring for 6 to 12 hours. Measure the viscosity after stabilization. If it is too high, dilute it with silica sol; if it is too low, add a certain amount of refractory powder until the viscosity meets the requirements.

Coating room temperature: 22°C to 25°C; Relative humidity: 45% to 65%; Ventilation: Good.

Slowly immerse the cleaned wax pattern (after drying) into the L-shaped coating mixing bucket and rotate it up and down to ensure thorough and uniform wetting of the mold assembly with the coating. Remove it slowly, rotating until there is no accumulation or dripping of coating, then proceed to sand coating. Ensure that sand grains are evenly attached to the coating surface. The viscosity and sand particle size requirements for each layer of coating are specified in Table 3. When applying the surface layer coating, use a dedicated sieve to filter out impurities such as sand particles to prevent sand inclusion in the shell, which could lead to casting defects.

表3  硅溶胶涂料的性能要求Table 3 Performance Requirements of Silica Sol Coating

Number of Shell Layers/FaceCoating Viscosity/sDensity/g·cm-3Sand Particle Size/Mesh    

1    30～35    1.81～2.03    80/100（Zirconium Silicate Powder）    

2    30    1.84～1.85    30/60（Coal Gangue Powder）    

3    30    1.83～1.84    30/60（Coal Gangue Powder）    

4    15～20    1.81～1.82    16/30（Coal Gangue Powder）    

5    15～20    1.80～1.81    16/30（Coal Gangue Powder）    

6（Outer Layer）    15～20    1.79～1.80    

Silica sol must be strictly controlled for temperature, relative humidity, and air velocity during the drying process. The specific process parameters are as follows (refer to Table 4).

Table 4: Process Parameters for Silica Sol Shell

Time (h)   Number of Layers/Coats   Drying Temperature (℃)   Relative Humidity (%)   Air Velocity (m/s) 

≥4    1～2    20～25    50～60    Not applicable    

≥6    3～5    ≥30    30～50    ≥3    

≥8    6    ≥30    30～50    ≥3    

When the environmental temperature and relative humidity are difficult to adjust, they can be controlled to certain relatively stable values: temperature between 22°C and 25°C; humidity between 45% and 65%.

For dewaxing, a water bath is used with a temperature of 90°C to 95°C, and the dewaxing time should be at least 20 minutes to prevent boiling water from causing sand grains or coating to enter the mold shell.

Self-inspection of the mold shell is carried out by the operator. If there are delamination, blistering, cracking, or loose ends, it is considered unqualified. Any excess coating in the sprue or uneven adhesion must be removed; otherwise, it is considered unqualified.

After each dewaxing process, the wax water is poured out to form wax ingots. When pouring the wax ingots, use a 160 to 200 mesh sieve to filter out impurities from the wax liquid. Drain all the water from the tank and clean the dewaxing tank thoroughly. Clean water should be added for the next dewaxing process.

Place the mold shell facing downward in the furnace and bake at a temperature between 900°C and 1050°C. After reaching the temperature, maintain the temperature for 1 to 2 hours before starting pouring. When it needs to be packed, wait for it to cool to below 450°C before removing it from the furnace and packing it.

Carefully inspect the quality of the mold shell. Any cracks, peeling, or flaking in the cavity are not suitable for packing. Remove any sprue flash and floating sand. Place the mold shell facing downwards, tap and shake the mold shell evenly on all sides to remove any trapped dust, sand, or debris on the cavity surface. Use compressed air to blow out any trapped dust and surface debris inside the cavity, then rinse the mold shell cavity thoroughly with water to remove internal debris. After gently shaking the mold shell again, pack it. After filling with sand, cover the sprue with asbestos board and cover the top of the sand mold with two halves of asbestos board with a gap (the size of the asbestos board should cover the entire top of the sand mold and extend beyond the edge to prevent sand mold burst skin from falling into the mold cavity during baking and casting). If there is any damage to the sprue and sand grains are exposed at the cup mouth, stop using it. After repair and inspection, it can be used again. Pack when the temperature is below 450°C. Bake in a box resistance furnace to 900°C, and then cast after 2 hours of insulation.