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1. Knowledge and understanding of coated sand (coated sand belongs to organic binder type, core sand)

1. The characteristics of coated sand: It has suitable strength function; good fluidity, the prepared sand molds and cores are clearly summarized, and the structure is fine, which can produce messy sand cores; the sand molds (cores) have good appearance and rough appearance. The degree can reach Ra=6.3～12.5μm, the dimensional accuracy can reach CT7～CT9 level; the collapsibility is good, and the castings are easy to arrange.
2. Applicable scale: coated sand can be used to manufacture molds and sand cores. The molds or cores of coated sand can be used in conjunction with other sand casting molds (cores); not only can be used for metal gravity Casting or low-pressure casting can also be used for iron sand-coated casting and thermal centrifugal casting; it can be used not only for the production of cast iron and non-ferrous alloy castings, but also for the production of steel castings.

2. Preparation of coated sand

Composition of coated sand

It is generally composed of refractory materials, binders, curing agents, lubricants and special additives.

1. Refractories are the main body of coated sand. The requirements for refractory materials are: high refractoriness, less volatile matter, rounder particles, and firmness. Generally choose natural wiping silica sand. The requirements for silica sand are: high SiO2 content (die casting/cast iron and non-ferrous alloy castings are required to be greater than 90%, steel castings are required to be greater than 97%); mud content is not greater than 0.3% (for wiping sand)--[Mud content rule of water-washed sand Smaller than; particle size ① scattered on adjacent 3 to 5 sieve sizes; rounded grain shape, angular elements should not be greater than 1.3; acid consumption value is not less than 5ml.
2. Phenolic resin is widely used as the binder.
3. Urotropine is generally used as the curing agent; calcium stearate is generally used as the lubricant, whose function is to avoid agglomeration of the coated sand and add fluidity. The primary role of additives is to improve the function of coated sand.
4. The basic proportion of coated sand is the proportion of distribution (mass fraction, %) Description: 100 rubbing sand for raw sand, phenolic resin 1.0～3.0 accounting for the weight of raw sand, urotropine (aqueous solution 2) 10-15 accounting for resin The weight of calcium stearate is 5-7 for the weight of resin, and the weight of additive 0.1-0.5 is for the raw sand. 1:2) 10～15 account for resin weight, calcium stearate 5～7 account for resin weight, additive 0.1～0.5 account for raw sand weight.

Production process of coated sand

The preparation process of coated sand mainly includes three types: cold coating, warm coating, and thermal coating. Nowadays, the production of coated sand simply uses the hot coating method. The thermal coating process is to first heat the raw sand to a certain temperature, then mix it with resin, urotropine aqueous solution and calcium stearate, then cool, crush and sieving. Because of the difference in formula, the mixing process is different. At present, there are many varieties of coated sand production lines in China. There are about 2000-2300 semi-automatic production lines for technical feeding, and nearly 50 automatic production lines controlled by computers have effectively improved the production power and product stability. For example, the automatic visual production line of cnc operating and Casting Co., Ltd., its feeding time is controlled to 0.1 second, the heating temperature is controlled to 1/10°C, and the sand mixing situation can be observed from time to time through video, and the output power reaches 6 tons/hour.

The primary product type of coated sand

1. General coated sand. The general coated sand is the traditional coated sand. Its composition is generally composed of quartz sand, thermoplastic phenolic resin, urotropine and calcium stearate. No additives are added, and the resin content is average. It is relatively high under certain strength requirements, and does not have the characteristics of high temperature resistance, low swelling, and low gas evolution. It is suitable for the production of castings with low requirements.
2. Features of high-strength and low-gassing coated sand: high strength, low swelling, low gassing, slow gassing, and oxidation resistance. Introduction: high-strength, low-gassing coated sand is an updated product of general coated sand After participating in the "additives" of relevant characteristics and the selection of new preparation processes, the amount of resin is greatly reduced, and its strength is more than 30% higher than that of ordinary coated sand, and the amount of gas generation is significantly reduced, and the gas generation speed can be delayed, which is better To adapt to the needs of casting production. This kind of coated sand is mainly suitable for the production of iron castings, small and medium steel castings and alloy steel castings. Now there are three series of this kind of coated sand: GD-1 high-strength low-gassing coated sand; GD-2 high-strength, low-swelling, low-gassing coated sand; GD-3 high-strength, low-swelling, low-gassing and anti-oxidation Laminated.
3. High temperature resistant (type) coated sand (ND type) 　Features: high temperature resistance, high strength, low swelling, low gassing, slow gassing, easy to collapse, anti-oxidation Introduction: high temperature resistant coated sand is specially Process formula technology produces new coated sand with excellent high-temperature functions (high strength at high temperatures, long heat-resistance time, low thermal expansion, low gas generation) and comprehensive casting functions. This type of coated sand is especially suitable for the production of messy thin-walled fine cast iron parts (such as car engine cylinder blocks, cylinder heads, etc.) and high-demand steel castings (such as container corners and train brakes; middle shell parts, etc.) , Can effectively eliminate casting defects such as sand sticking, deformation, hot cracking and pores. Now the coated sand has four series: VND-1 high temperature resistant coated sand. ND-2 high temperature resistant, low swelling and low gassing coated sand ND-3 high temperature resistant, low swelling, low gassing and oxidation resistant coated sand ND -4 High temperature resistant, high strength bottom, low swelling and low gassing film.
4. Easily collapsible coated sand has good strength and excellent low-temperature collapsibility function, which is suitable for the production of non-ferrous metal castings.
5. Other special requirements coated sand In order to meet the needs of different products, a series of special coated sands have been developed, such as: coated sand for centrifugal casting, chilled coated sand, wet coated sand, anti-sticking sand, and Veining, anti-orange peel coated sand, etc.

3.The primary process of core making with coated sand

The heating temperature is 200-300℃, the curing time is 30-150s, and the sand shooting pressure is 0.15-0.60MPa. For sand cores with simple shapes and coated sand with good fluidity, lower sand injection pressure can be selected, lower heating temperature for thin sand cores, and curing time can be appropriately extended when the heating temperature is low. The resin used in coated sand is phenolic resin. The advantages of the core-making process: suitable strength function; good fluidity; good surface quality of the sand core (Ra=6.3-12.5μm); strong moisture absorption resistance of the sand core; good collapsibility and easy finishing of castings.

Mold (mold) temperature

The mold temperature is one of the primary factors affecting the thickness and strength of the shell. It is generally controlled at 220-260°C and selected according to the following criteria:

• Ensure that the resin on the coated sand is softened and cured to meet the heat required;
• Ensure that the thickness of the shell that constitutes the requirement and the appearance of the shell (core) are not coked;
• Shorten the time of crusting and hardening as much as possible to improve the yield.

Sand shooting pressure and time

The sand shooting time is generally controlled within 3~10s. If the time is too short, the sand mold (core) cannot be formed. The sand shooting pressure is generally about 0.6MPa; when the pressure is too low, insufficient shooting or looseness is likely to occur. 3. Hardening time: The length of the hardening time mainly depends on the thickness of the sand mold (core) and the temperature of the mold, generally around 60-120s. If the time is too short, the strength of the shell layer is not completely solidified; if the time is too long, the outer surface of the sand mold (core) is easy to scorch and affect the quality of the casting. Examples of process parameters of coated sand modeling (core): serial number drawing number shell thickness (㎜) weight (㎏) mold temperature (℃) sand shooting time (s) hardening time (s) 1 (guide sleeve) DN80-05 8～ 10 2.5～2.6 220～240 2～3 60～80 2 (valve body) DN05-01 10～12 3.75～3.8 240～260 3～5 80～100.

Problems and countermeasures in the application of coated sand

There are many types of core-making methods, which can be divided into two categories: thermosetting methods and cold-setting methods. Coated sand core-making methods belong to the category of thermosetting methods. Any kind of core-making method has its own advantages and disadvantages, which mainly depends on the product quality requirements, degree of disorder, production batch, production cost, product price, etc. It is necessary to decide which core-making method to choose. It is often effective to use coated sand for sand cores with high requirements on the quality of the inner cavity and appearance of the castings, high dimensional accuracy, and messy shapes. For example: sand cores for intake and exhaust passages, water passage sand cores, and oil passage sand cores of car engine cylinder heads, water passage sand cores and oil passage sand cores for cylinder blocks, shell core sand cores for intake manifolds and exhaust manifolds, Runner sand cores for hydraulic valves, air passage sand cores for car turbochargers, etc. However, some problems are often encountered in the application of coated sand. Here, I will only talk about the understanding of the work.

The method of confirming the strength and gas volume of coated sand

On the premise that the quality of the raw sand and the quality of the resin are certain, the key element that affects the strength of the coated sand is mainly taken from the amount of phenolic resin. If the amount of phenolic resin is large, the strength will be improved, but when the gas evolution is also added, the collapsibility will decrease. Therefore, in the production and application, the strength of the coated sand must be controlled to reduce the amount of gas and improve the collapsibility. A balance point must be found when the strength standards are formulated. This balance point is to ensure the surface quality of the sand core and its strength under the premise of no deformation and core breakage during pouring. In this way, the appearance quality and dimensional accuracy of the castings can be guaranteed, and the amount of gas generation can be reduced, the shortcomings of the casting pores can be reduced, and the sand production function of the sand core can be improved. For the storage of sand cores, during the transfer process, the station tools and sand core carts can be selected, and a 10mm-15mm thick sponge can be laid on them, which can reduce the loss rate of sand cores. .

Storage period of coated sand core

Any sand core will absorb moisture, especially in the southern area where the relative humidity is high. It is necessary to regulate the storage period of the sand core in the process documents, and use the lean production first-in first-out production method to reduce the storage volume and storage period of the sand core. Each enterprise should confirm the storage period of sand cores in combination with its own plant conditions and local climatic conditions.

Control the supply quality of coated sand

When the coated sand enters the factory, it is necessary to attach the supplier's quality assurance information, and the company checks it according to the sampling standard, and can only be put into the warehouse after the inspection is qualified. The quality assurance and technical department will make the processing result when the enterprise sampling test is unqualified, which is to accept the concession or return the goods to the supplier.

Qualified coated sand was found to be cracked and deformed during core making

The cracking and deformation of the sand core during core making is generally considered to be caused by the low strength of the coated sand. In fact, many production processes are involved in sand core cracking and deformation. In the event of an abnormal situation, it is necessary to find the real cause before it can be completely dealt with. The specific reasons are as follows:

1. The temperature of the mold and the time of mold retention during core making are related to whether the hardening thickness of the sand core crust meets the process requirements. The process parameters that are regulated in the process require a scale, and this scale needs to be adjusted by the skills of the operator. At the upper limit of the mold temperature, the lower limit can be taken at the time of mold retention, and the upper limit of the mold temperature is taken at the time of mold retention. Continuous training and improvement of operating skills are required for operators.
2. Phenolic resin and sand particles will stick to the mold during core making. It is necessary to arrange it in time and spray the release agent, otherwise the sand core will be broken or deformed when the mold is opened.
3. The spring ejector rod on the static mold of the hot core box mold will cause elastic failure due to long-term operation under high temperature conditions and cause the sand core to crack or deform. It is necessary to replace the spring in time.
4. The movable mold and the static mold are not parallel or on the same centerline. When the mold is closed, under the pressure of the oil cylinder or the air cylinder, the front end of the positioning pin has a slope, and the mold is still tight, but it moves when the mold is opened. The mold and the static mold will still return to their original condition, causing the sand core to crack or deform. Under such conditions, sand will run off when sand is shot, and the size of the sand core will become larger. The countermeasure is to adjust the parallelism and coaxiality of the mold in time.
5. When producing hollow sand cores on the shell core machine, pour out the unhardened coated sand from the core. When it needs to be used from scratch, it is necessary to screen the unused coated sand and mix it in a ratio of 3:7 Application, in this way, can ensure the appearance quality of the core sand core and the strength of the sand core.