Application of Cladding Sand in Casting

Clad sand casting in the foundry field has a fairly long history, casting production is also quite large; but the use of clad sand casting production of precision cast steel parts face many problems: sticky sand (scarring), cold segregation, porosity. How to solve these problems to be further explored.

 

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

 

(1)Characteristics of coated sand: suitable strength properties; good fluidity, clear contour of the sand type and sand core, dense organisation, and the ability to produce complex sand core; good surface quality of the sand type (core), surface roughness up to Ra = 6.3 ~ 12.5 μm, and dimensional accuracy up to the level of CT7 ~ CT9; good routing, and the casting is easy to clean up.

 

(2) Scope of application: coated sand can be used to make both casting moulds and sand cores, and the moulds or cores of coated sand can be used in conjunction with each other as well as with other sand moulds (cores); not only can it be used for metal-type gravity casting or low-pressure casting, but also can be used for iron sand casting and centrifugal casting in the hot method; not only can it be used in the production of cast iron and non-ferrous alloy casting, but also can it be used in the production of cast steel parts.

 

The preparation of coated sand

 

1. Composition of coated sand

Generally consists of refractory materials, binder, curing agent, lubricant and special additives.

 

(1) Refractory material is the main body of the film sand. Requirements for refractory materials are: high refractoriness, low volatiles, more rounded particles, solid and so on. Generally use natural scrubbing silica sand. Requirements for silica sand is: SiO2 content is high (cast iron and non-ferrous alloy castings require more than 90%, steel castings require more than 97%); mud content of not more than 0.3% (for scrubbing sand) - [water-washed sand mud content is less than the provisions of; particle size ① distribution of the adjacent 3 ~ 5 sieve number; rounded particle shape, angular factor should be no more than 1.3; acid consumption value of not less than 5ml.

 

(2) The binder commonly adopts phenolic resin.

 

(3) The curing agent usually adopts urotropin; the lubricant usually adopts calcium stearate, whose function is to prevent the clumping of coated sand and increase the fluidity. The main function of additives is to improve the performance of coated sand.

 

(4) The basic proportion of coated sand Ingredients Proportion (mass fraction, %) Description: raw sand 100 scrubbing sand, phenolic resin 1.0 ~ 3.0 of the weight of the raw sand, urolithiopharmine (aqueous solution 2) 10 ~ 15 of the weight of the resin, calcium stearate 5 ~ 7 of the weight of the resin, additives 0.1 ~ 0.5 of the weight of the original sand. 1:2) 10 ~ 15 of the weight of the resin, calcium stearate 5 ~ 7 of the weight of the resin, additive 0.1 ~ 0.5 of the original sand. ～Additives 0.1 to 0.5 % of the weight of the original sand.

 

2. Production process of coated sand

 

There are mainly three kinds of preparation process for coated sand, namely, cold method coating, warm method coating and hot method coating, and at present the production of coated sand almost always adopts the hot coating method. The hot laminating process is to heat the raw sand to a certain temperature, then mix and stir with resin, aqueous urotropin and calcium stearate respectively, and then crushed and sieved after cooling. The mixing process is different due to the difference of formula. At present, there are many kinds of domestic laminating sand production lines, there are about 2,000~2,300 semi-automatic production lines with manual charging, and there have been nearly 50 computer-controlled automatic production lines, which have effectively improved the production efficiency and product stability. For example, the automated visual production line of XX Foundry Co., Ltd. has the feeding time control accurate to 0.1 second, the heating temperature control accurate to 1/10 ℃, and can observe the sand mixing status through the video, and the production efficiency reaches 6 tonnes/hour.

 

3. Main product types of coated sand

 

(1) Ordinary type of coated sand Ordinary coated sand, i.e. traditional coated sand, is usually made up of quartz sand, thermoplastic phenolic resin, urolithiophosphate and calcium stearate, with no relevant additives, and the amount of the resin added is usually relatively high under a certain requirement of strength, and it does not have the characteristics of high temperature resistance, low expansion and low outgassing, which is suitable for casting production of less demanding requirements.

 

(2) High-strength and low-gassing laminating sand Characteristics: high strength, low expansion, low-gassing, slow-gassing, antioxidant Introduction: High-strength and low-gassing laminating sand is a newer product of ordinary laminating sand, by adding the relevant characteristics of the ‘additives’ and adopting a new formulation process, so that the resin dosage has dropped considerably, and the strength of its strength is 30% or more higher than that of the ordinary laminating sand. Its strength is higher than that of ordinary coated sand by more than 30%, and the amount of gassing is significantly reduced and the speed of gassing can be slowed down, so that it can be better adapted to the needs of casting production. This type of coated sand is mainly suitable for cast iron parts, small and medium-sized cast steel, alloy cast steel production. At present, there are three series of this kind of coated sand: GD-1 high strength and low outgassing coated sand; GD-2 high strength and low expansion and low outgassing coated sand; GD-3 high strength and low expansion and low outgassing antioxidant coated.

 

(3) High-temperature resistant (type) coated sand (ND type) Characteristics: high-temperature resistance, high strength, low expansion, low outgassing, slow outgassing, easy to collapse, oxidation resistance Introduction: High-temperature resistant coated sand is a new type of coated sand with excellent high-temperature performance (high strength at high temperature, long heat resistance, low thermal expansion, low outgassing) and comprehensive casting performance, produced by a special process formula. This kind of coated sand is especially suitable for complex thin-walled precision cast iron parts (such as automobile engine block, cylinder head, etc.) and high requirements of the production of cast steel parts (such as the container angle and train brake retardation; in the vessel shell parts, etc.), can effectively eliminate sticky sand, deformation, thermal cracking and porosity and other casting defects. At present, there are four series of this film sand: VND-1 high temperature resistant film sand. ND-2 high temperature resistant low expansion and low outgassing coated sand ND-3 high temperature resistant low expansion and low outgassing antioxidant coated sand ND-4 high temperature resistant high strength low expansion and low outgassing coated sand.

 

(4) Easy to collapse type of coated sand has good strength, and at the same time has excellent low-temperature collapse performance, suitable for the production of non-ferrous metal castings.

 

(5) Other special requirements of coated sand In order to meet the needs of different products, a series of special coated sand has been developed, such as: centrifugal casting coated sand, cold coated sand, wet coated sand, anti-sticking sand, anti-vein pattern, anti-orange peel coated sand, and so on.

 

The main process of coating sand core making

 

Heating temperature 200-300 ℃, curing time 30-150s, sand pressure 0.15-0.60MPa. Simple shape of the core, good fluidity of the laminated sand can be used for lower sand pressure, thin sand core to choose a lower heating temperature, low heating temperature can be appropriately prolonged curing time and so on. The resin used for coated sand is phenolic resin. Advantages of the core making process: suitable strength properties; good fluidity; good surface quality of the sand core (Ra = 6.3-12.5μm); sand core anti-hygroscopicity; good routing, easy to clean the castings.

 

1. Casting (mold) temperature

 

Casting temperature is one of the main factors affecting the thickness and strength of the shell layer, generally controlled at 220 ~ 260 ℃, and selected in accordance with the following principles: (1) to ensure that the resin on the softening and curing of sufficient heat.

 

(1) Ensure sufficient heat for softening and curing the resin on the laminated sand;

(2) Ensure that the required shell thickness is formed and the surface of the shell (core) is not scorched;

(3) Try to shorten the shell and hardening time, in order to improve productivity.

 

2. Sand injection pressure and time 

 

Sand injection time is generally controlled in 3 ~ 10s, too short a time, the sand mold (core) can not be molded. Sand injection pressure is generally about 0.6MPa; when the pressure is too low, it is easy to cause insufficient shot or loose phenomenon.

 

3. Hardening time: the length of the hardening time depends mainly on the thickness of the sand mold (core) and the temperature of the casting, generally in 60 ~ 120s. Time is too short, the shell layer is not fully cured low strength; time is too long, the sand (core) surface layer is easy to burn affect the quality of castings. Examples of process parameters of coated sand molding (core): No. Shell thickness (㎜) Weight (kg) Casting Temperature (℃) Injection 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 kinds of core-making methods, which can be divided into two categories: thermosetting methods and cold-setting methods, and coated sand core-making belongs to the category of thermosetting methods. Any kind of core making method has its own advantages and disadvantages, which mainly depends on the quality requirements of the product, complexity, production batch, production cost, product price and other factors to decide what kind of core making method. High surface quality requirements for the casting cavity, high dimensional accuracy requirements, the shape of the complex sand core using coated sand core is very effective. For example: car engine cylinder head intake and exhaust sand core, waterway sand core, oilway sand core, cylinder block waterway sand core, oilway sand core, intake manifold, exhaust manifold shell core sand core, hydraulic valve runners sand core, automotive turbocharger airway sand core and so on. But in the use of coated sand often encountered some problems, here only on the work of the experience to talk about.

 

1. The method of determining the strength and gassing amount of laminated sand

Under the premise that the quality of raw sand and resin is certain, the key factor affecting the strength of laminated sand is mainly taken from the amount of phenolic resin added. If more phenolic resin is added, the strength will be increased, but the amount of gas also increases, and the collapse will be reduced. Therefore, in the production of applications must control the strength of the sand to reduce the amount of gas, improve the collapse, in the development of strength standards must find a balance. This balance is to ensure that the surface quality of the sand core and in the pouring does not produce deformation, does not produce broken core under the premise of strength. So as to ensure the surface quality of castings and dimensional accuracy, but also to reduce the amount of gas, reduce the casting of porosity defects, improve the sand core sand performance. Sand core storage, handling process can be used in the workstation equipment, sand core trolley, and on top of it lay 10mm ~ 15mm thick sponge, so as to reduce the loss rate of sand core.

 

2. Storage period of sand core for coated sand

Any sand core will absorb moisture, especially in the southern region of the air relative humidity, sand core storage period must be stipulated in the process documents, the use of lean production first-in-first-out production mode to reduce the amount of sand core storage and storage cycle. Each enterprise should combine their own plant conditions and local climate conditions to determine the storage period of sand core.

 

3. Control the quality of supply of coated sand

Laminating sand into the factory must be accompanied by the supplier's quality assurance information, and the enterprise according to the sampling standards for inspection, inspection of qualified before entering the warehouse. Enterprises sampling test failed by the quality assurance and technical departments to make the results of the treatment, is a concession to accept or return to the supplier.

 

4. Qualified coated sand is found to be fractured and deformed during core making.    

The fracture and deformation of the sand core during core making is usually thought to be caused by the low strength of the laminated sand. In fact, the sand core breakage and deformation will be involved in many production processes. The abnormal situation must be investigated to the real cause in order to completely solve the problem. Specific causes are listed below:

 

(1) The temperature of the mold and the retention time of the mold during core making are related to whether the thickness of the hardened sand core crust meets the process requirements. The process parameters specified in the process need to have a range, the range depends on the operator's skills to adjust. In the upper limit of the mold temperature can take the lower limit of the time to stay in the mold, mold temperature in the lower limit of the time to stay in the mold to take the upper limit. Operators need to be constantly trained to improve operating skills. 

 

(2) When making core, there will be phenolic resin and sand particles sticking on the mold, which must be cleaned up in time and sprayed with mold releasing agent, or else it will accumulate more and more and when the mold is opened, the core will be pulled off or deformed.

 

(3) The spring top bar on the static mold of the hot core box mold will cause the core to break or deform due to elasticity failure when working under high temperature for a long period of time. The spring must be replaced in time. 

 

(4) The moving and static molds are not parallel or not in the same centerline, when the mold is closed under the pressure of the cylinder or cylinder, the front end of the locating pin has a slope, the mold will still be closed tightly, but in the opening of the mold, the moving and static molds will still be restored to the original state of the sand core fracture or deformation. In this case, the sand will run away when shooting sand, and the size of the sand core will become larger. The solution is to adjust the parallelism and coaxiality of the mold in time.

 

(5) In the shell core machine on the production of hollow sand core, from the sand core out of not yet hardened coated sand need to be reused, must be sifted and unused coated sand according to the ratio of 3:7 mixed with the use of the core, so as to ensure that the shell core sand core surface quality and strength of the sand core.