Teach you a few coated sand core making process

In recent years, the application of coated sand is more and more widely, almost used in all casting methods, the process method is not the same. However, the basic technological requirements for core making and modeling are (except for wet manual work): heating temperature 200~300 ℃; Curing time 30~150s; The sand shooting pressure is 0.15~0.6MPa, and the specific parameters shall be adjusted according to the equipment type, type, core quality and complexity, type of coated sand, etc. The principle is that the sand core with simple shape and the coated sand with good fluidity (or coarse particle size) can choose lower sand shooting pressure, and the thin sand core can choose lower heating temperature, and the curing time can be appropriately prolonged when the heating temperature is low; and vice versa. Several different application examples are listed below.

1. Making solid cores In theory, coated sand can produce almost all types of solid cores, especially high-precision castings. When wet coated sand is used, hot core box equipment and tooling can be directly used to make cores without any changes to the original equipment and tooling. When using dry coated sand, due to its good fluidity, the exhaust mode of the core box, nozzle and core box sealing need to be specially treated. The principle of angle of rest (angle of rest of coated sand is about 30o) can be used to solve the problems of coated sand entering the injection cavity during exhaust after sand shooting and automatic falling of coated sand when not shooting. At present, the use of dry state coated sand hot core box core-making process more manufacturers.

2. Thermal centrifugal casting This process is a traditional centrifugal casting process in rough-formed metal (iron). It is mainly used to produce tubular rotary castings. First, a layer of paint is applied to the surface of the inner cavity of the iron mold to facilitate mold taking. In recent years, coated sand suitable for centrifugal casting has been developed to replace the production of coatings. Due to its poor exhaust effect, it requires a thin wall (about 3mm), and usually uses double-layer sand, fine sand (70/140 mesh) is used on the contact surface with the casting, and coarse sand (50/100 mesh) is used on the outer layer.

3. Iron type coated sand process The process is to form a casting mold by coating a thin layer of coated sand (3-8mm) in the inner cavity of the rough forming metal type (iron type). Through various means, such as process test, computer simulation, experience chart, etc., it reasonably sets the iron type wall thickness and coated sand thickness, pouring system and coated sand layer material, in order to make the cast iron in a more favorable conditions to complete the filling solidification cooling, improve its internal quality and surface quality.

The iron mold sand-coating technology in China was first developed and successfully applied in small and medium-sized foundry enterprises. In the late 1970 s, China built the first iron-type sand-coated casting production line. After the 1980 s, the basic research team and application enterprises of iron mold coated sand casting process in China have been formed and developed rapidly. At present, nearly 100 enterprises in China have applied iron mold sand-coated casting to production, with an annual output of about 100000 tons of iron mold sand-coated castings (mainly crankshafts, camshafts, gears and export castings, etc.) and an annual consumption of about 20000 tons of coated sand.

Many small and medium-sized casting enterprises in China have poor casting quality, low economic efficiency and poor casting production conditions, and have little financial resources to carry out fundamental technological transformation, so some small and medium-sized casting enterprises will consider the use of less investment, see 1 effect fast iron sand casting technology to improve their competitiveness. Because the average precision of castings produced by iron-type sand-coated casting is about CT7 and the surface roughness is 12.5um, the production capacity can be basically formed by investing hundreds of thousands of yuan. If large enterprises adopt this process, the production cost can be effectively reduced.

The main process parameters of iron mold sand-coated casting are as follows: the wall thickness of iron mold is 25mm, and the sand-coated layer thickness is 5-8mm (applicable to ductile iron crankshafts with small cylinder diameter); The quality of iron mold is generally about 5 times that of mold castings. The iron-sand ratio (the ratio of casting quality to the amount of coated sand consumed) is 6-7. There are two kinds of iron-type sand-coated molding methods (I .e. obtaining sand-coated layer): sand injection molding and compression molding. The former is widely used at present, and the sand-coated layer is ordinary coated sand, which is especially suitable for mass production. The latter is suitable for crankshafts with large single piece quality (iron type size up to several meters long), and the sand-coated layer is self-hardening sand in various fluid states. The selection of the former sand shooting pressure is related to the thickness of the coated sand. When the sand shooting pressure is 0.2-0.6MPa and the thickness of the coated sand is 4-5mm, its density is the largest.

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

4. Making shell type Coated sand is used to make shell type, which is common in pouring shaft parts such as camshafts or disc parts such as brake pads and some valve parts with high surface requirements. The relevant parameters are as follows: sand-iron ratio 1 :( 1.5)~ 4; draft angle 0.5 ° ~ 1 °, etc., shell wall thickness 8~12mm.

Uniform shell wall thickness can reduce the amount of coated sand and obtain uniform casting organization. To make shell type with coated sand, fixed core shooting machine or flip core shooting machine can be used. The former can use dry coated sand or wet coated sand, while the latter can only use dry coated sand.

5. Coated sand is used to make shell cores. Swing shell core machines (such as K85, K87, K89, etc.) are usually used to make shell cores. The core making process is completed through sand shooting, encrusting, residual sand removal, curing, film removal and other links.

In order to ensure the quality of the shell core, in addition to ensuring the quality of the coated sand, it is also necessary to select a reasonable core-making process according to the specific conditions of each sand core. Core box temperature Core box temperature is one of the main factors affecting the shell thickness, generally controlled at 230-300 ℃, and selected according to the following principles:

Ensure that the required shell thickness is formed and the surface of the shell core is not coking;

Try to shorten the crusts and hardening time to improve productivity. The sand injection pressure and time shall be determined according to the shape and complexity of the sand core, so that the core sand can be tightly filled into the core box. When the pressure is too high, it is easy to make the sand core neck and run sand; when the pressure is too low, the sand core is easy to loose. If the sand shooting time is too long, it is easy to make the encrusted shell slip, resulting in a very thin encrusted shell. 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. The duration of the crust time depends on the shell thickness of the sand core, which is determined by the strength requirements of the sand core during transportation, core assembly and pouring. Under the premise of a certain quality of coated sand, the thicker the shell, 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. After the encrusting stage is finished, the shell core machine will face down the sand injection port of the core box and swing it left and right at an angle of 45o to pour out the unencrusted residual sand. This period of time can be determined according to the complexity of the sand core shape and the arrangement of the sand injection port, and it is appropriate to pour the net, generally 5-10s. 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 shell thickness and the temperature of the core box. If the time is too short, the shell is not fully cured, the strength is lower; if the time is too long, the surface of the sand core is easy to burn. General control in 20-100s.

Ensuring sufficient heat required for softening and curing of the resin on the coated sand;

This article is finished by coated sand, the point of view of this article does not represent the point of view of this site.