Key points of foundry technology of coated sand (2)

Main process of core making with resin coated sand

Heating Temperature 200-300, curing time 30-150s, shot pressure 0.15-0.60 MPA. The sand core with the simple shape and the resin coated sand with good fluidity can select lower shooting pressure, lower heating temperature for thin sand core, longer curing time when the heating temperature is low. The resin used in coated sand is phenolic resin. The advantages of the core-making process are Proper Strength Property; good fluidity; good surface quality of sand cores (Ra 6.3-12.5 m); strong moisture absorption of sand cores; good collapsibility; easy cleaning of castings.

1.Mold temperature: Mold temperature is one of the main factors affecting the thickness and strength of the shell layer, generally controlled at 220 ~ 260, and selected according to the following principles:

(1) ensure that the resin on the coated sand is softened and cured with sufficient heat;

(2) ensure that the required shell thickness is formed and that the surface of the Shell (core) is not charred;

(3) shorten the time of crusting and hardening as far as possible to improve productivity.

2.Sand shooting pressure and time: Sand shooting time is generally controlled at 3 ~ 10s, too short time can not molding sand (core). The shooting sand pressure is generally about 0.6 MPA when the pressure is too low, it is easy to cause the phenomenon of insufficient firing or loose. 3. Hardening time: the length of the hardening time depends mainly on the thickness of the sand mold (core) and the temperature of the mold, generally around 60 ~ 120 S. If the time is too short and the shell layer is not fully solidified, the strength is too low, if the time is too long, the surface layer of the sand mold (core) will be scorched easily, which will affect the casting quality.

The working principle of the core shooting machine

when shooting sand, the Compressed air quickly enters the sand shooting cylinder, pushes the sand through the sand layer gap, forms the sand airflow, causes the core sand to shoot into the core box with a higher speed, at the same time filling the core box to obtain the compactness.

Two methods to solve the problems in the application of coated sand

There are many kinds of methods to solve the problems in the application of coated sand, which can be divided into two kinds: thermosetting method and cold-setting method. Any core-making method has its own advantages and disadvantages, which mainly depends on the product quality requirements, complexity, production batch, production cost, product price, and other factors to decide which core-making method to use. It is very effective to use coated sand to make sand cores with the high quality of inner cavity surface, high dimensional precision and complex shape. For example INLET AND OUTLET CHANNEL SAND core, waterway sand core, oil channel sand core, water channel sand core of cylinder body, inlet manifold, exhaust manifold shell core, flow channel sand core of hydraulic valve, automotive turbocharger, air duct, sand cores, etc… But in the use of coated sand often encounter some problems, there is only a brief talk about the work of the experience.

  • The determination method of strength and gas evolution of coated sand is based on the quality of the original sand and resin, and the key factors affecting the strength of coated sand are mainly determined by the addition of Phenol formaldehyde resin. The more Phenol formaldehyde resin is added, the higher the strength is, but the more gas is emitted, the lower the collapsibility is. Therefore, it is necessary to control the strength of coated sand in production to reduce gas emission and improve collapsibility. The balance point is to ensure the surface quality of the core and the strength without deformation or core breaking during pouring. In this way, the surface quality and dimensional accuracy of the casting can be ensured, the air emission can be reduced, the air hole defect of the casting can be reduced, and the sand-producing performance of the sand core can be improved. The sand cores can be stored and transported with working position instruments, sand core trolley and 10 mm ~ 15 m thick sponge, which can reduce the core loss rate.
  •  Storage period of resin coated sand cores: any sand cores can absorb moisture, especially in the south area where the relative humidity is high, the storage period of sand cores must be specified in the process document Reduce the storage quantity and cycle time of sand cores by using the first in first out mode of production in lean production. Every enterprise should determine the storage cycle of sand cores according to the conditions of its own plant and local climate.
  • control the supply quality of coated sand: coated sand into the factory must be accompanied by the supplier’s quality assurance information, and enterprises according to the sampling standard inspection, inspection qualified before storage. When the sample is not qualified, the quality guarantee and the technical department will deal with the result to accept or return the goods to the supplier.
  • The fracture deformation of sand core is found in the qualified resin coated sand during core making, and the fracture deformation of the sand core in core making is usually considered to be caused by the low strength of resin coated sand. In fact, the fracture and deformation of sand cores involve many production processes. In case of an abnormal situation, the real cause must be found out before it can be solved thoroughly. The reasons are as follows:1) the temperature of the mold and the retention time of the mold are related to whether the hardening thickness of the sand core can meet the technological requirements. Process parameters specified in the process need to have a range, the scope of the skills of operators to adjust. At the upper limit of mold temperature, the lower limit of mold retention time can be taken, and at the lower limit of mold temperature, the upper limit of mold retention time. Operators need continuous training to improve operational skills.

    2) when making core, there will be Phenol formaldehyde resin and sand on the mold, which must be cleaned and sprayed in time, otherwise, the sand core will be broken or deformed when more and more mold is opened.

    3) the spring ejector rod on the static die of the hot core box mold will cause the sand core to fracture or deform due to the elastic failure of the spring ejector rod under the condition of high temperature for a long time.

    4) the dynamic die and the static die are not parallel or not on the same center line. When the die is closed, under the pressure of the oil cylinder or the air cylinder, the front end of the locating pin has a slope, and the die will close tightly, but the dynamic die and the static die will still return to the original state to fracture or deform the sand core. In this case, the shot sand will run sand, sand core size will increase. The solution is to adjust the parallelism and coaxially of the mold in time.

    5) when producing hollow sand-core on shell-core machine, when the resin-coated sand which is not hardened is poured from the sand-core and needs to be used again, the resin-coated sand which has not been used through sieving must be mixed according to the ratio of 3:7, so as to ensure the surface quality and core strength of shell-core sand-core.

    6) thickness of sand coating: thickness of sand coating is very important to casting quality and production cost. The thickness of sand coating is too thick, which not only affects the cooling effect but also increases the production cost. The thickness of coated sand is too thin, too heavy, and the hardness of casting is too high, which is inconvenient for finishing. In general, the finish machining surface is covered with thick sand, the non-processing surface is covered with thin sand; the casting of pearlite Matrix material is covered with thin sand; the casting of ferrite Matrix material is covered with thick sand; the hot node is covered with thin sand (as low as 3 ~ 4) , the non-hot node is covered with thick sand; the sand near the shooting sand port is covered with thick, far from the perforation hole is covered with thin sand; when the casting is large and complex, the sand is covered with thick, otherwise it will affect the flow rate of sand, causing the lower part of the mold to be filled with sand. The thickness of sand coating is generally controlled from 5 to 8. If the casting is high (deep), in the lower to set up “air plug”, to prevent the lower part of the vacuum from starting can not air, suction injury casting.

    7) when heating iron mold and Sand Mould, the uniformity of heating temperature and whole temperature should be controlled.

    8) when designing the parting surface, make the height of the upper mold and the lower mold (sandbox) as much as possible, reduce the sand filling distance, and make the resin coated sand filling the casting mold easily.

    9) sand blasting and air exhaust: When the cavity is shot between the tank and the Iron Model, the gas in the cavity should be discharged smoothly, otherwise, the phenomenon of incomplete sand coating or sand coating will occur. For example Sandbox and the iron mold bottom plate contact surface, sandbox to open exhaust duct, exhaust duct size, only full exhaust and cannot run sand as a rule (generally can be slotted with a hand saw).

    10) sandbox design: the thickness of the sandbox is good, but the hardness of casting is high, the thickness of the sandbox is poor.

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