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Analysis of Six Defects in Investment Casting Shell

Edit:Xiangyang Liqiang Machinery Co., Ltd.UpDate:2019-10-01

The extensive defects of investment casting shells include cracks, deformation, swelling, ant holes, worm holes and vents, etc., which are now analyzed.

1. Shell cracks

There are two cases of shell cracks, one is that the rubber inlet cup causes cracks, as shown in the figure below; the other is that the surface of the shell causes cracks.

The crack characteristics of the rubber inlet cup: the rubber inlet cup of the mold shell has cracks, and the rubber inlet cup cracks when the situation is serious.

Cracks on the shell surface: There are curved cracks on the surface of the shell with different depths.

1. Causes of cracks in investment casting mold shells

(1) The displacement coefficient and bulk density of sodium silicate in industrial coatings are too high or too low; the powder-to-liquid ratio in industrial coatings is too low; or the mass concentration, temperature and hardening time of lamellar are improper, hard bottom Insufficient chemicalization; or the natural drying time of the shell before the hard bottoming is not enough, which will adversely affect the re-penetration of cinnamyl alcohol to the hard bottom, which damages the continuity and compactness of the silicone gel; or the accumulation of the shell Insufficient layers, etc., resulted in low tensile strength of the shell and cracks.

(2) Uneven coating of industrial coatings, or uneven thickness of the sanding layer; especially in the parts where no sand is sprinkled after dipping the industrial coatings, the support plate is uneven when the silicone gel shrinks, causing cracks in the shell.

(3) The temperature of the dewaxing solution is low and the dewaxing time is too long. Since the thermal expansion coefficient of the wax material exceeds the thermal expansion coefficient of the shell, the slow dewaxing will cause the shell to be subjected to various ground stresses during the dewaxing process; if it exceeds the strength limit of the shell at this time, it will cause Cracks, even cracking.

(4) During firing, the temperature of the shell entering the furnace is increased, the temperature rises too fast, or the high temperature releases a chill; or the shell is repeatedly burned, resulting in micro cracks or even cracks, which reduces the tensile strength; or the high temperature of the shell The tensile strength is low, which causes cracks when the shell is burnt.

(5) When eliminating the glue inlet cup, industrial equipment will harm the glue inlet cup.

2. Preventive Measures against Cracks in Investment Casting Shell

(1) With the following precautions, the high temperature tensile strength of the shell will be high. ①The modification coefficient of sodium silicate is M=~, and the bulk density ρ=~3 g/cm3 is equipped with engineering reinforcement layer industrial coating. ② Adopt reasonable industrial coating equipment production technology, and promote the "configuration-temperature-viscosity" trend chart of industrial coatings. ③ Use a reasonable hard bottoming production process to control the "mass concentration-temperature-hard bottoming time" of polycinnamyl alcohol; or use aluminum chloride instead of ammonium chloride hard bottomed shell. ④ Reasonable shell making process, such as the reasonable coordination of industrial coating viscosity and sand particle size distribution, the basic parameters of the hard bottoming production process should ensure that the shell is sufficiently hard bottomed. ⑤ Take measures to improve the tensile strength of the shell, such as generally increasing the number of stacked layers of the shell, or using a composite shell; when necessary, the shell of large objects can be reinforced with fine iron wire engineering.

(2) The wax mold penetrates into the qualified industrial paint, moves back and forth and continues to rotate. After mentioning it, drip the excess industrial paint to make the industrial paint evenly cover the surface of the mold; no industrial paint can appear. Part of the industrial coating is accumulated or lacking (missing coating); and sand immediately and evenly.

(3) Properly increase the temperature of the dewaxing solution and control it at 95-98℃; reduce the dewaxing time, 15-20 minutes, not more than 30 minutes; the key link of sodium silicate shell dewaxing: high temperature and fast. When necessary, improve the dewaxing method.

(4) Choose a reasonable sintering production process, the ammonium chloride hard-bottomed shell sintering temperature T=850~900℃, time~2h; and strictly enforce it; when necessary, use stage heating or cooling water; Shell cooking cannot exceed 2 times.

The well-fired shells are milky white, powdery white or light pink; the poorly-fired shells are light brown or social, indicating that the shells have more carbon residue.

(5) Be careful when eliminating the glue inlet cup to avoid industrial equipment hazards; when necessary, improve the structure of the glue inlet cup.

2. Shell deformation

Features: The specifications and models of the cavity do not meet the requirements of the sample drawings

1. Cause

(1) The high temperature tensile strength of the shell is low, and its professional ability to resist high temperature deformation is also low. Most of the shell deformation is caused by dewaxing, firing or concrete pouring. When the concrete is poured, the high temperature near the glue inlet will deform the shell.

(2) The high viscosity of the industrial coating, the coating is too thick, and the industrial coating is accumulated; or the quality of the hard bottoming solution is low, the temperature is low, the hard bottoming is faster, the hard bottoming is insufficient, etc., resulting in the tensile strength of the shell low.

(3) The temperature of the dewaxing solution is low and the dewaxing time is too long. Since the thermal expansion coefficient of the wax material exceeds the thermal expansion coefficient of the shell, the slow dewaxing will cause the shell to be subjected to various ground stresses during the dewaxing process, and the shell will deform during the dewaxing process.

(4) The heating temperature of the shell of medium and large objects is too high, the heating time is too long; or the cooling water is too fast; or the improper placement during the heating (stacking, extrusion processing) promotes the imbalance of the fulcrum support plate And other reasons, causing part of the deformation.

(5) The roasting furnace cannot consider the requirements of the production process.

(6) Improper cooperation between firing and concrete pouring, the tensile strength of the shell in the hot state after firing is higher; the temperature decreases, the tensile strength decreases. If the shell is poured into concrete in a hot state, the shell will heat up greatly and the coefficient of thermal expansion will be large, causing the shell to deform.

2. Preventive measures

(1) Reasonable matching and control of the following factors can appropriately improve the professional ability of high temperature deformation of the shell: ①The deformation coefficient of sodium silicate M=~, the bulk density ρ=~3 g/cm3 Engineering reinforcement layer equipped Industrial coatings. ②Using polycinnamyl alcohol; the aluminum chloride hard-bottomed shell is harder than the ammonium chloride shell. ③Al-Si series powder and sand replace silica powder and sand. ④ Shell making process. For example, the viscosity of the industrial coating and the particle size distribution of the sand are reasonably matched, and the basic parameters of the hard bottoming production process must ensure that the shell is sufficiently hard bottomed. ⑤ The firing and concrete pouring are coordinated reasonably to prevent the shell from pouring concrete in hot state. ⑥Use composite shell, such as sodium silicate and silica sol shell, sodium silicate and ethyl silicate shell, etc.

(2) Master, use and control the trend chart of the relationship between "configuration-viscosity-temperature" of industrial coatings; pay attention to the coating process to make the coating evenly covered; control the mass concentration, temperature and hard bottoming of the hard bottoming solution Time makes the shell ample and hard.

(3) The shell is parked for 12-24 hours and then dewaxed; a reasonable dewaxing production process is selected, the temperature of the dewaxing liquid is controlled at 95-98℃, and the dewaxing time is 15-20 minutes, not exceeding 30 minutes; and the shell is in Placement in the dewaxing process, etc.

(4) Choose a reasonable firing process, the firing temperature of the ammonium chloride hard-bottomed shell is T=850~900℃, and the time is ~2h; pay attention to the placement of the shell during the firing process to avoid mutual extrusion of the shells Press processing; control the cooling water speed issued by the burning and so on.

The high-quality grilled shell is creamy white, powdery white or light pink.

(5) Improve the on-time inspection and normal maintenance of the roasting furnace, so that it takes into account the requirements of the production process.

Third, the shell swelling

Characteristics: The whole shell swells, or part of the coating swells. Core swelling

1. Cause

(1) Various reasons for the shell level: ① High water glass modulus, or high viscosity of industrial coating; improper operation process or improper control of the sanding time, causing a part of the industrial coating or sand to accumulate; and hard bottoming Poor and other reasons. ②The natural drying time before the hard bottoming of the base cushion is too long, and the two layers of industrial coatings are wet and not well combined. ③After the industrial coating is sprinkled with sand, there is floating sand on the surface; or the particle size distribution of the foundation cushion sand is too fine, too much dust in the sand or too much moisture and water content of the small stones; resulting in a firm combination of the two layers of industrial paint and the foundation cushion sand. ④After the foundation cushion is hard-bottomed, it dries faster, and there is too much hard-bottoming liquid left on part of the shell.

(2) Reasons for the low professional ability of the shell to resist high temperature deformation: ①The high temperature tensile strength of the shell is low, and its professional ability to resist high temperature deformation is also low. Most of the shell swelling is caused by dewaxing, burning or concrete pouring. ②The high viscosity of industrial coatings, the coating is too thick, the accumulation of industrial coatings, or the concentration of hard bottoming liquid is low, the temperature is low, the hard bottoming is faster, the hard bottoming is insufficient, etc., resulting in low tensile strength of the shell. Professional ability to resist high temperature deformation is also low.

(3) The temperature of the dewaxing solution is low and the dewaxing time is too long.

(4) The heating temperature of the shell of medium and large objects is too high, the heating time is too long; or the cooling water is too fast; or the heating is not placed properly (stacking, extrusion processing) and other reasons.

(5) Improper coordination between firing and concrete pouring. For example, if the mould shell is placed in a hot state, the mould shell will heat up and the coefficient of thermal expansion will be large, resulting in swelling of the mould shell.

2. Preventive measures

(1) Various precautions to prevent the shell level: ① It is advisable to control the temperature of the shell making site at 22~26℃; appropriately reduce the viscosity of industrial coatings and improve their fluidity; when necessary, use industrial coatings Add appropriate surfactants to improve the coating and coverage of industrial coatings; pay attention to the operation process, that is, the mold shell penetrates into the industrial coatings, move back and forth and continue to rotate, and then drop the excess industrial coatings. , So that the industrial coating is evenly coated and covered on the surface of the mold shell, and sand is immediately scattered, and a part of the industrial coating or molding sand cannot be accumulated, and there is sufficient hard bottoming. ②The necessary time for the shell to be dried in nature before hard bottoming, generally 15-40min is used, and the shell should be "not wet or white"; after hard bottoming, it should be air-dried to make the shell penetrate to the hard The bottoming, hard bottoming is more abundant. ③Inspect the particle size distribution and dust of the molding sand (dust moisture content ≤), control the relative humidity of the air in use of the molding sand should be ≤; and immediately remove the excess floating sand on the shell; the particle size distribution of the foundation cushion need not be too fine to 40/70 mesh is appropriate.

(2) Various preventive measures to prevent the deformation of the shell: ①Choose the appropriate water glass modulus and bulk density, and select the reasonable "powder-to-liquid ratio, viscosity and temperature" of industrial coatings; aluminum-silicon-based powder and sand, instead of Silica powder and sand; select appropriate polycinnamyl alcohol and reasonable basic parameters of hard bottoming production process to control the mass concentration, temperature and hard bottoming time of the hard bottoming solution, so that the shell can be fully hardened. ② Pay attention to the coating process, so that the coating room is evenly covered. ③The shell is parked for 12-24 hours and then dewaxing; the temperature of the dewaxing liquid is controlled at 95~98℃, and the dewaxing time is 15~20min, not more than 30min. ④The heating temperature of the ammonium chloride hard-bottomed shell is T=850~900℃, and the time is ~2h; the cooling water speed of the burning is controlled.

In other words, select reasonable industrial coating production technology, shell-making production technology, firing and concrete pouring production technology, or use conforming shells, etc., to improve the professional ability of the shells to resist high temperature deformation


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