Coated sand casting has a long history in the field of casting, and the output of castings is also quite large; however, there are many problems in the production of precision steel castings by coated sand casting: sticky sand (crust), cold insulation, and pores. How to solve these problems needs to be further explored.
First, the understanding and understanding of coated sand (coated sand belongs to organic binder type, core sand):
(1) Characteristics of coated sand: It has suitable strength and properties; good fluidity, clear sand core and sand core, compact structure and ability to manufacture complex sand core; sand type (core) surface with good quality and rough surface The degree can reach Ra=6.3~12.5μm, the dimensional accuracy can reach CT7～CT9 grade; the collapsibility is good, and the casting is easy to clean.
(2) Scope of application: Coated sand can be used to make both cast and sand cores. The type or core of coated sand can be used together with other sand types (cores); not only for metal gravity Casting or low-pressure casting, can also be used for iron-type sand casting, but also for thermal centrifugal casting; not only for the production of cast iron, non-ferrous alloy castings, but also for the production of steel castings.
Second, the preparation of coated sand: 1) coated sand is generally composed of refractory materials, binders, curing agents, lubricants and special additives.
(1) The refractory material is the main body constituting the coated sand. The requirements for refractory materials are: high refractoriness, less volatiles, rounded and solid particles. Naturally scrubbed silica sand is generally used. The requirements for silica sand are: high SiO2 content (more than 90% for cast iron and non-ferrous alloy castings, greater than 97% for cast steel parts); mud content not greater than 0.3% (for scrubbing sand)--[Specification of mud content in washing sand Less than; particle size 1 is distributed on adjacent 3 to 5 sieve numbers; the grain shape is rounded, the angular factor should be no more than 1.3; the acid consumption value is not less than 5 ml.
(2) Phenolic resins are commonly used as binders.
(3) The curing agent is usually urotropine; the lubricant is generally calcium stearate, which is used to prevent the agglomeration of the coating sand and increase the fluidity. The main role of the additive is to improve the properties of the coated sand.
(4) Basic proportion of coated sand Composition (mass fraction, %) Description Original sand 100 Scrubbing sand Phenolic resin 1.0～3.0 占原重重 Urotropine (Aqueous solution 1:
1) 10~15 accounted for the weight of resin, calcium stearate 5~7, which accounted for the weight of the resin. Additive 0.1~0.5
2) Production process of coated sand The preparation process of coated sand mainly includes cold coating, warm coating and thermal coating. At present, the production of coated sand is almost always by thermal coating. The thermal coating process firstly heats the raw sand to a certain temperature, and then mixes with the resin, the aqueous solution of urotropine and calcium stearate, and is cooled and crushed and sieved. The mixing process varies depending on the formulation. At present, there are many types of coated sand production lines in China. There are about 2,000 to 2,300 semi-automatic production lines for manual feeding, and nearly 50 automatic computer-controlled production lines have been used to effectively improve production efficiency and product stability. For example, the automated visual production line of xx Foundry Co., Ltd. has a feeding time control accurate to 0.1 second, a heating temperature control accurate to 1/10 ° C, and the ability to observe the sand mixing state through video, and the production efficiency reaches 6 tons / hour.
3) Main product types of coated sand
(1) Ordinary coated sand Introduction: Ordinary coated sand is the traditional coated sand. Its composition is usually composed of quartz sand, thermoplastic phenolic resin, urotropine and calcium stearate. Without adding additives, the resin is added. The amount is usually relatively high under certain strength requirements, and does not have the characteristics of high temperature resistance, low expansion, low gas generation, etc., and is suitable for the production of castings which are not required to be required.
(2) High-strength and low-flammable coated sand features: high strength, low expansion, low gas generation, slow gas generation, and anti-oxidation. Introduction: High-strength low-gas-coated sand is a new generation of ordinary coated sand. By adding the "additives" of the relevant characteristics and adopting a new preparation process, the amount of the resin is greatly reduced, the strength is more than 30% higher than that of the ordinary coated sand, the gas generation amount is remarkably lowered, and the gas generation speed can be retarded, and the gas can be better. Adapt to the needs of casting production. This type of coated sand is mainly suitable for the production of medium and small cast steel and alloy steel castings in cast iron parts. At present, there are three series of coated sands: GD-1 high strength low gas-coated sand; GD-2 high strength low expansion low gas sand; GD-3 high strength low expansion low gas anti-oxidation film .
(3) High temperature resistant (type) coated sand (ND type) Features: high temperature resistance, high strength, low expansion, low gas generation, slow gas generation, easy to collapse, anti-oxidation Introduction: High temperature resistant coated sand is through special process Formulated technology produces new coated sand with excellent high temperature performance (high strength at high temperature, long heat resistance, low thermal expansion, low gas generation) and comprehensive casting properties. This kind of coated sand is especially suitable for the production of complex thin-walled precision cast iron parts (such as automobile engine block, cylinder head, etc.) and high-demand steel castings (such as container angle and train brakes; middle case parts, etc.). It can effectively eliminate casting defects such as sand, deformation, hot crack and pores. At present, there are four series of coated sand: VND-1 high temperature resistant coated sand. ND-2 high temperature resistant low expansion low gas mulch coated sand ND-3 high temperature resistant low expansion low gas and anti-oxidation coated sand ND-4 High temperature resistant high strength bottom low expansion low gas mask
(4) Easy-to-break-type coated sand has good strength and excellent low-temperature collapse performance, and is suitable for producing 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 such as: coated sand for centrifugal casting, chilled coated sand, wet coated sand, anti-stick sand, anti-proof Vein, anti-orange peeling sand, etc. At the same time, the fast-curing coated sand introduced in 2004 can increase the curing speed by 30-70% on the basis of various properties, effectively improve production efficiency and save energy.
Third, the main process of coated sand core:
The heating temperature is 200-300 ° C, the curing time is 30-150 s, and the sand shooting pressure is 0.15-0.60 MPa. A sand core with a simple shape and a good fluidity coated sand can be used with a lower sanding pressure, a thin sand core with a lower heating temperature, and a lower curing temperature when the heating temperature is low. The resin used for the coated sand is a phenolic resin.
Advantages of core making process: It has suitable strength performance; good fluidity; good surface quality of sand core (Ra=6.3-12.5μm); sand core has strong anti-hygroscopicity; good collapsibility, easy to clean casting.
1. Mold (mold) temperature: The mold temperature is one of the main factors affecting the thickness and strength of the shell. It is generally controlled at 220-260 °C and is selected according to the following principles:
(1) ensuring sufficient heat for softening and solidifying the resin on the coated sand;
(2) ensuring that the required shell thickness is formed and the shell (core) surface is not coked;
(3) Minimize the crust and hardening time to increase productivity.
2, sand injection pressure and time: sand shooting time is generally controlled at 3 ~ 10s, when the time is too short, the sand type (core) can not be formed. The sand injection pressure is generally about 0.6 MPa; when the pressure is too low, it is easy to cause insufficient or loose shot. 3, hardening time: the length of hardening time depends mainly on the thickness of the sand type (core) and the temperature of the mold, generally around 60 ~ 120s. If the time is too short, the shell layer is not fully cured, and the strength is low; if the time is too long, the surface layer of the sand type (core) is easily burnt to affect the quality of the casting. Example of process parameters of coated sand molding (core): serial number, shell thickness (mm), weight (kg), mold temperature (°C), sanding time (s), hardening time (s) 1 (guide sleeve) DN80-05 8～ 10 2.5 to 2.6 220 to 240 2 to 3 60 to 80 2 (valve body) DN05-01 10 to 12 3.75 to 3.8 240 to 260 3 to 5 80 to 100.
Fourth, the working principle of the core shooting machine: When shooting sand, the compressed air quickly enters into the sanding cylinder, pushing the sand through the gap of the sand layer to form the sand airflow, so that the core sand is injected into the core box at a higher speed, filling the core box Get tight at the same time.
V. Problems in the application of coated sand and solutions There are many kinds of methods for core making. The total can be divided into two categories: thermosetting method and cold setting method. The coated sand core is a thermosetting method. Any method of core making has its own advantages and disadvantages, which depends on the quality requirements, complexity, production volume, production cost, product price and other factors to determine which core making method to use. It is very effective to use a coated sand core for the sand core with high quality requirements, high dimensional accuracy and complex shape. For example: the intake and exhaust sand core of the car engine cylinder head, the water channel sand core, the oil channel sand core, the water channel sand core of the cylinder block, the oil channel sand core, the inlet core tube, the shell core of the exhaust manifold, Flow channel sand core for hydraulic valves, air cores for automotive turbochargers, etc. However, there are still some problems encountered in the use of coated sand. Here, I will only talk about the experience at work.
1 Determination method of strength and gas generation of coated sand Under the premise of the quality of raw sand and the quality of resin, the key factor affecting the strength of coated sand is mainly taken from the amount of phenolic resin added. When the amount of the phenol resin is increased, the strength is increased, but the amount of gas is also increased, and the collapsibility is lowered. Therefore, in production applications, it is necessary to control the strength of the coated sand to reduce the amount of gas generated, improve the collapsibility, and find a balance point in the formulation of the strength standard. This balance point is to ensure the surface quality of the sand core and the strength without deformation during casting and without breaking the core. In this way, the surface quality and dimensional accuracy of the casting can be ensured, the amount of gas generated can be reduced, the porosity of the casting part can be reduced, and the sanding performance of the sand core can be improved. For the storage of the sand core, the station tool and the sand core car can be used in the transportation process, and a sponge of 10 mm to 15 mm thick is placed on the sand core to reduce the loss rate of the sand core.
2 Storage period of coated sand core: Any sand core will absorb moisture, especially the relative humidity of the air in the south. It is necessary to specify the storage period of the sand core in the process documentation, and use the lean production first-in, first-out production method to reduce the sand core. Storage and storage period. Each company should determine the storage period of the core in combination with its own plant conditions and local climatic conditions.
3 Control the quality of the coated sand: When the coated sand enters the factory, it must be accompanied by the supplier's quality assurance information, and the enterprise will check according to the sampling standard, and the inspection can be completed before entering the warehouse. When the enterprise fails to pass the sampling test, the quality assurance and technical department will process the result, which is a concession to accept or return to the supplier.
4 Qualified coated sand is found to break the core during core making. The fracture deformation of the core during core making is usually considered to be caused by the low strength of the coated sand. In fact, sand core breakage and deformation can involve many production processes. In the event of an abnormal situation, the real cause must be found to be completely resolved. The specific reasons are as follows:
1) The temperature of the mold and the mold retention time during core making are related to whether the hardening thickness of the sand core shell meets the process requirements. The process parameters specified in the process need to have a range, which needs to be adjusted by the skill of the operator. The mold retention time can be taken as the lower limit at the upper limit of the mold temperature, and the upper limit of the mold retention time when the mold temperature is at the lower limit. Operators need constant training to improve operational skills.
2) When making the core, there will be phenolic resin and sand on the mold. It must be cleaned and sprayed with the release agent in time. Otherwise, the more the more the mold is opened, the core will be broken or deformed.
3) The spring ejector on the static mold of the hot box mold will break or deform the sand core due to the elastic failure caused by working in the high temperature state for a long time. The spring must be replaced in time.
4) The moving mold and the static mold are not parallel or not on the same center line. Under the pressure of the cylinder or the cylinder, the front end of the positioning pin has a slope, and the mold will still be tight, but the mold is static when the mold is opened. The mold will still return to its original state to break or deform the core. In this case, the sand will run when the sand is shot, and the size of the core will become larger. The solution is to adjust the parallelism and coaxiality of the mold in time.
5) When the hollow core is produced on the core-core machine, the un-hardened coated sand is poured out from the sand core. When it needs to be reused, the coated sand that has not been used and sieved must be mixed in a ratio of 3:7. This will ensure the surface quality and core strength of the core of the shell core.