Coated sand casting materials and technology knowledge

Coated sand casting materials and technology knowledge  

our country is in1950s began to study the application of coated sand and shell (core) technology. Until the middle of the 1980s, only a few factories used self-made coated sand for shell core production. Since then, coated sand has been introduced to the market as a commodity. With the continuous improvement of raw materials, manufacturing equipment and manufacturing technology, the quality of coated sand has been continuously improved and the production cost has decreased. Since 1990s, the application of coated sand has developed more rapidly and the types of products have been increasing, and has formed a serialization. At present, the annual output of coated sand for casting in China has reached more than 500,000 tons, with nearly 100 professional manufacturers. With the rapid development of my country's automobile industry and the export needs of mechanical products, as well as the development of the international casting market, the requirements for the quality of castings are getting higher and higher, and the application of coated sand will be in the short term the internal growth is rapid.

First, the selection of raw materials for coated sand

coated sand is generally composed of aggregates, binders, curing agents, lubricants and special additives.

1. Aggregate

aggregate is the main body of coated sand. The requirements for aggregate are: high fire resistance, less volatile matter, relatively round particles and high strength. Generally, natural scrubbing silica sand is selected, which is mainly due to its abundant reserves and low price, which can meet the casting requirements. Only steel castings or iron castings with special requirements are zirconium sand or chromite sand. The general requirements for silica sand are:

(1) high SO2 content. Sand for cast iron and nonferrous casting requires SO2>90%, and steel casting requires> 97%;

(2) silt content ≤ 0.2%;

(3) The particle size distribution should be 3~5 sieve dispersion;

(4) AFS fineness: different fineness should be selected according to the requirements of casting surface roughness, generally AFS50 ~ 65;

(5) grain shape: choose silica sand with good roundness as much as possible, and the angular factor should be <1.3;

(6) ph value <7;

(7) Silica sand needs to be scrubbed with water. If there are special requirements, silica sand can be pickled or subjected to high-temperature active treatment (roasting at 900 ℃)

2. Adhesive

at present, phenolic resins are widely used as binders. Phenolic resins are divided into solid and liquid, thermosetting and thermoplastic. At present, thermoplastic solid phenolic resin is usually used to make coated sand. The performance requirements are: 

(1) polymerization speed (hot plate method):25~27s;

(2) softening point (global method):90~105 ℃;

(3) Liquidity (oblique plate method):60 ~ 110mm;

(4) Free phenol content (bromination method):≤ 4%.

The performance of the binder has a great influence on the quality of coated sand. People have been working on how to improve the performance of phenolic resin and find alternatives to phenolic resin. Foreign countries have developed special resins with different properties, such as high-strength low-gas resin, easy-to-break resin, and there are also reports of using modified polyester resin, but it has not been fully promoted, the modified phenolic resin is still widely used so far. In recent years, the research and development of phenolic resins for coated sand has developed rapidly in China, and different modified phenolic resins have been developed for production.

3. Curing agent, lubricant, additive

the curing agent usually uses urotropine, namely hexamethylenetetramine, and the molecular formula is ( CH2) 6N4, required to be an industrial first-class product. Lubricants generally use calcium stearate. Its function is to prevent clumping of coated sand, increase fluidity, make the surface of the mold and core dense and improve the demoulding of the sand mold (core). The main function of the additive is to improve the performance of the coated sand. The additives currently used mainly include: high temperature resistant additives (such as carbon-containing materials or other inert materials), Yiguisan additives (such as manganese dioxide, potassium dichromate, potassium permanganate, caprolactam, etc.), reinforcement and toughening additives (such as ultra-short glass fiber materials, organosilane KH-550, etc.), anti-sticking sand additives and anti-aging additives, etc.

II. Classification of Coated Sand 

with the continuous expansion of the application range of coated sand, different casting processes, different material requirements and different structures of castings put forward different performance requirements for coated sand. Since since the 1990s, coated sand with different properties has been developed in China. According to its performance characteristics, the coated sand in the domestic market can be divided into two categories: dry and wet. Among them, dry categories include: common category, high temperature resistant category, high strength and low gas generation category, easy to collapse category, centrifugal casting category, etc. Wet coated sand includes mechanical and manual. The main components, characteristics and scope of application of various types of coated sand are as follows:

1. Ordinary coated sand

its composition is usually composed of quartz sand thermoplastic phenolic resin, urotropine and calcium stearate, without relevant additives. Its resin addition is usually relatively high under certain strength requirements, and does not have the characteristics of high temperature resistance, low expansion, and low gas generation. It is suitable for the production of some iron castings with low requirements and simple structure. 

2. High-strength and low-gas coated sand

on the basis of ordinary coated sand, by adding relevant characteristics of "additives" and adopting a new preparation process, the amount of resin is greatly reduced, the gas generation is significantly reduced, and the gas generation speed can be delayed. The coated sand has the characteristics of high strength, low expansion, low gas generation, slow gas generation, etc., and is especially suitable for sand cores requiring low gas generation in iron castings and small and medium steel castings, such as valve body sand cores. 

3. High temperature resistant coated sand

the sand generally refers to the coated sand with high thermal strength, long heat resistance time, high temperature deformation, rather than its high refractoriness. It is through a special process formulation technology (usually a certain amount of inert materials such as milling sand, high chromium ore sand, carbon-containing materials or other inert materials are added when silicon sand is coated), A new type of coated sand with excellent high temperature performance and comprehensive casting performance is produced. It has the characteristics of high temperature resistance, high strength, low expansion, low gas generation, slow gas generation, etc. It is especially suitable for complex thin-walled precision iron castings (such the production of automobile engine block, cylinder head, etc.) and high-demand steel castings (such as container corners and train brake buffer housings, etc.) can effectively eliminate casting defects such as sand sticking, deformation, hot cracking and pores. 

4. Coated sand easy to collapse

it is a kind of coated sand developed for non-ferrous metal (especially aluminum alloy) castings that are not easy to clean sand. The sand has good strength and excellent low-temperature collapsibility. Use it to produce non-ferrous metal (aluminum alloy, copper alloy, etc.) castings, no need to reheat the castings to clear the sand, just cool the castings after pouring after 24 hours, the sand can be vibrated.

5. Centrifugal casting of coated sand

the coated sand is suitable for centrifugal casting process and can be used to replace paint to produce centrifugal cast pipes. According to the different material, diameter and technical requirements of the cast pipe, it can be processed into centrifugal cast coated sand with different properties. The difference with other types of coated sand is that the coated sand has higher density, lower gas generation and slow gas generation speed.

6. Wet mechanical coated sand

it is a kind of coated sand that is wet at room temperature. Long-term storage at room temperature will not cure naturally (can be stored more than 1 year), according to the technical requirements of different castings, coated sand with various characteristics such as wet high strength and low gas generation can be used, the coated sand can completely replace the hot core box sand (the shot head of the core shooting machine does not need to be modified to produce various materials and various types of castings).

7. Wet handmade coated sand

the wet compressive strength of the coated sand can be any adjustment within 0.1 ~ 0.3MPa. The storage period is more than 3 months, suitable for manual modeling or core making (similar to tung oil sand and grease sand). After demoulding at room temperature, put the mold (core) into an oven or oven for heating, the heating temperature is 250~350 ℃, the heating time is 3~10min, and the cured type (core) has the characteristics of high strength, low gas generation, small creep deformation, no moisture absorption, etc. The coated sand is suitable for the production of single piece, small batch and high precision castings, especially suitable for township enterprises. It is worth noting that when the type (core) is heated and solidified, it must wait for the furnace temperature to reach the required temperature before heating, and do not heat with the furnace temperature.

Three, coated sand core (type) process 

in recent years, the application of coated sand has become more and more extensive. It is used in almost all casting methods. The process methods are different, but whether it is core making or modeling, its basic process requirements are (except wet manual): heating temperature200~300 ℃; Curing time 30~150s; Sand shooting pressure 0.15 ~ 0.6MPa. The specific parameters should be adjusted according to the equipment model, type, core quality and complexity, and the type of coated sand. The principle is: simple-shaped sand core, good fluidity (or coarse particle size) the coated sand can choose a lower sand shooting pressure, a lower heating temperature for the thin sand core, and the curing time can be appropriately extended when the heating temperature is low; And vice versa. Several different application examples are described below.

1. Make solid core

in theory, coated sand can produce solid cores of almost all types, especially high-precision castings. When wet coated sand is used, the core can be directly made of hot core box equipment and tooling without any changes to the original equipment and tooling. When dry coated sand is used, due to its good fluidity, it is necessary to carry out special treatment on the exhaust mode of the core box, the nozzle and the seal of the core box. Can use safety the principle of rest angle (the rest angle of coated sand is about30o ) , to solve the problem of coated sand entering the shooting cavity when the sand is discharged after shooting and the coated sand automatically falls when the sand is not shot. At present, there are many manufacturers using dry coated sand hot core box core making process.

2. Making shell type

the shell type is made of coated sand, which is commonly found in shaft parts such as pouring camshafts or disc parts such as brake pads and some valve parts with high surface requirements. The relevant parameters are as follows: sand-iron ratio1:(1.5)~ 4; Draft inclination 0.5~1, etc., shell wall thickness 8~12mm.

Uniform shell wall thickness can reduce the amount of coated sand and obtain uniform casting structure. The shell type is made of coated sand, which can be fixed core shooting machine or flip core shooting machine. The former can be dry coated sand or wet coated sand, while the latter can only be dry coated sand.

3. Making shell core

the shell core made of coated sand is usually made of swing shell core machine (such k85, K87, K89, etc.), through sand shooting, crust, put out the residual sand, curing, film removal and other links to complete the core process.

In order to ensure the quality of the shell core, in addition to ensuring the quality of the coated sand, a reasonable core-making process must be selected according to the specific conditions of each sand core.

(1) Core box temperature Core box temperature is one of the main factors affecting shell thickness, generally controlled at 230-300 ℃, and selected according to the following principles: (1) to ensure that the resin on the coated sand softening and curing required heat; (2) to ensure the formation of the required shell thickness and shell core surface is not coking; (3) to shorten the crust and hardening time as far as possible to improve productivity.

(2) Sand shooting pressure and time The sand shooting pressure should be determined according to the shape and complexity of the sand core, so that the core sand can be tightly filled with the core box. When the pressure is too high, it is easy to cause the sand core to shrink and run sand. When the pressure is too low, the sand core is prone to looseness. The sand shooting time is too long, which is easy to make the crusted shell slip, causing the crust to be very thin; If the time is too short, the sand core cannot be formed. The sand shooting pressure is generally 0.15 ~ 0.4MPa, and the sand shooting time is generally controlled at 3~10s.

(3) Crust time The length of crust time depends on the thickness of the sand core shell, which depends on the strength requirements of the sand core during transportation, core assembly and pouring. On the premise of certain quality of coated sand, the thicker the shell layer, the higher the strength. When the shell thickness is 6 ~ 8mm, the crust time is 10~30s, and when the shell thickness is 10mm, the crust time is 20~65s.

(4) After the end of the crust stage, the core shell machine will swing the sand shooting opening of the core box downward and swing left and right at an angle of 45o to pour out the remaining sand that has not been crusted. This period of time can be determined according to the complexity of the sand core shape and the arrangement of the sand shooting port, and it is appropriate to reverse the net, generally 5-10s.

(5) Hardening time In order to fully harden the sand shell, the sand core should continue to harden in the heated core box. The hardening time can be determined according to the thickness of the shell and the temperature of the core box. If the time is too short, the strength of the shell is lower if it is not completely cured. If the time is too long, the surface layer of the sand core is easy to burn. Generally controlled at 20-100s.

4. Thermal centrifugal casting

the process is a traditional centrifugal casting process in a rough metal type (iron type), which is mainly used to produce tube-type rotary castings, first, apply a layer of paint on the surface of the inner cavity of the iron mold to facilitate mold removal. In recent years, it has been developed to replace coating production with coated sand suitable for centrifugal casting. Due to its poor exhaust effect, it is required to have a thin wall (approximately3mm), and usually double-layer sand is used, the contact surface with the casting is fine sand (70/140 mesh), and the outer layer is coarse sand (50/100 mesh).

5. Iron type coated sand process

the process is based on rough forming the inner cavity of the metal type (iron type) is coated with a thin layer of coated sand (3-8mm) to form a mold, as shown in Figure 1. It uses various means, such as process tests, computer simulations, experience charts, etc., to reasonably set the iron wall thickness and coated sand thickness, gating system and coated sand layer materials, in order to achieve the purpose of filling, solidifying and cooling cast iron under a relatively favorable condition and improve its internal quality and surface quality.

China's iron type sand coating technology was first developed and successfully applied in small and medium-sized casting enterprises. At the end of the 1970s, China built the first iron-shaped sand-clad casting production line. After 1980s, the basic research team and application enterprises of iron-type sand-coated casting process in China have been formed and developed rapidly. At present, nearly 100 enterprises in China have applied iron-type sand-coated casting to production, with an annual output of about 100,000 tons of iron-type sand-coated castings (mainly crankshafts, camshafts, gears and export castings, etc.), the annual consumption of coated sand is about 20,000 tons.

Many small and medium-sized foundry enterprises in our country have poor casting quality, low economic benefits and poor casting production conditions to varying degrees, and they have little financial resources to carry out fundamental technological transformation, therefore, some small and medium-sized foundry enterprises consider adopting iron-type sand-coated casting technology with less investment and quick results to improve their competitiveness. Because the average accuracy of castings produced by iron-type sand-clad casting is CT7 or so, the surface roughness is 12.5um, and the production capacity can be basically formed by investing hundreds of thousands of yuan. If large enterprises adopt this process, production costs can be effectively reduced.

Main process parameters of iron-type sand-clad casting: iron-type wall thickness25mm, sand layer thickness 5-8mm (suitable for small diameter ductile iron crankshaft); The quality of iron mold is generally about 5 times that of mold casting; the iron-sand ratio (the ratio of casting quality to the amount of coated sand consumed) is 6-7. The iron-type sand-clad molding method (I. E. The acquisition of the sand-clad layer) includes sand shooting molding and compression molding. The former is currently used more, and the sand-clad layer is ordinary coated sand, especially suitable for mass production; The latter is suitable for single piece of large quality (iron size up to several meters long) crankshaft, etc., the sand coating is a variety of flow of self-hardening sand. The choice of sand shooting pressure in the former is related to the thickness of coated sand. When the sand shooting pressure is 0.2-0.6MPa and the thickness of coated sand is 4-5mm, the density is the largest.

Due to the better rigidity of the iron-type sand-clad mold, the compactness of the casting can be improved, and the density is increased on average.0.86%. Many enterprises have realized the as-cast production of four-cylinder and below ductile iron crankshafts. The casting performance is more than QT800-2, and the casting rejection rate is 3% ~ 5%.

IV, regeneration and Quality Control of Coated Sand

with the continuous improvement of human requirements for environmental protection, the recovery and regeneration of coated sand becomes more and more important and urgent. At present, the commonly used regeneration methods are mechanical regeneration and thermal regeneration. 

The basic principle of mechanical regeneration is to use the grinding wheel of the high-speed rotary wheel (about3500r /min) and the gap (5 ~ 10mm) between the slow rotating rotary frame rub the old sand, in order to achieve the purpose of removing the resin on the surface of the old sand and shaping the old sand, the generated dust is treated by ventilation and dust removal. The basic principle of thermal regeneration is to use old sand pretreatment and boiling thermal regeneration furnace equipment to first vibrate and magnetically separate the old sand, and then roast it in a boiling regeneration furnace, after sand production, it can be used back after heat preservation, cooling and screening. The performance of recycled sand regenerated by thermal method can reach: ignition reduction (mass fraction)<0.3%; Particle size concentration rate (mass fraction) 80% ~ 90%; Gas generation <3 mL/g.

In order to ensure the quality of the coated sand core, the quality of the coated sand must first be ensured. The various performance indicators of the coated sand must meet the production requirements. The inspection method can refer to the machinery industry standards ( JB/T8583.1997 Coated Sand for Casting).

In addition to the geometry, dimensional accuracy, surface quality and shell thickness of the coated sand core, the curing of the core sand should be checked by observing the color of the core. The normal situation is that the appearance of the sand core should be uniform yellowish brown, the center is light yellow, the appearance is brown or even black when it is over-burned, the appearance is yellow when it is not hardened, and the center is white.

V. Outlook 

the key issue of the advancement of modeling and core-making technology in the 21st century should be the development of non-polluting, less pollution, low production costs, energy-saving and material-saving and good working conditions. Therefore, the future development direction of coated sand technology should be:

(1) Reduce the environmental pollution of coated sand, develop phenolic resin with low free phenol, low gas and less pollution, and protect the natural environment on which human beings depend.

(2) Research on phenolic resin substitutes. Because of the high price of phenolic resin, it is urgent to develop a new type of binder with low price and good quality.

(3) Further study and improve the rheology of phenolic resin, and continuously improve the reaction, coating and curing characteristics of the resin.

(4) To study the performance control in the resin synthesis process, the appropriate gradation of macromolecules and small molecules in the resin also needs further research to improve the strength and toughness of the resin and the resistance to shelling.

(5) Further study the reuse of old sand. With the continuous increase of the amount of coated sand in my country, the amount of many manufacturers has reached more than thousands of tons. It is of great practical significance to study the recycling of coated sand and old sand, which can not only improve the performance of coated sand, but also can save costs and reduce the pollution of waste materials to the environment.

(6) Further reduce the curing temperature of the coated sand and reduce the curing time, which can not only reduce energy consumption, improve work efficiency, but also reduce the deformation of the mold and improve the size accuracy of the mold and core. The author is convinced that with the increasing development of my country's national economic construction, the technology of coated sand will become more and more advanced in my country's casting industry, and its application will become more and more extensive.

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