Classification and characteristics of sand casting

The mold used in sand casting is generally composed of an outer sand mold and a core. In order to improve the surface quality of castings, a layer of paint is often applied to the surface of the sand mold and core. The main components of the coating are powdery materials and binders with high refractoriness and high chemical stability at high temperature. In addition, carriers (water or other solvents) and various additives are added to facilitate application.

1. Raw materials and techniques of sand casting
The basic raw materials for making sand molds are foundry sand and molding sand binder. The most commonly used foundry sand is siliceous sand. When the high temperature performance of silica sand cannot meet the requirements, special sands such as zircon sand, chromite sand and corundum sand are used. In order to make the produced sand mold and core have a certain strength, and not deform or damage during handling, molding and pouring of liquid metal, a molding sand binder is generally added to the casting to bond the loose sand particles into molding sand. The most widely used molding sand binder is clay, and various drying oils or semi-drying oils, water-soluble silicates or phosphates, and various synthetic resins can also be used as molding sand binders. The outer sand mold used in sand casting is divided into three types: clay wet sand mold, clay dry sand mold and chemically hardened sand mold according to the binder used in the molding sand and the way of building its strength.

2. Clay wet sand mold
Clay and an appropriate amount of water are used as the main binder of the molding sand, and the sand mold is directly formed and poured in the wet state after it is made. Wet casting has a long history and is widely used. The strength of green sand depends on the clay slurry made by mixing clay and water in a certain proportion. Molding sand - it has a certain strength after being mixed, and after being pounded into a sand mold, it can meet the requirements of molding and pouring. Therefore, the amount of clay and moisture in the molding sand are very important process factors.

The advantages of clay green sand casting are:
2.11 Clay is rich in resources and cheap.
2.12 Most of the used viscous green sand can be recycled after proper sand treatment.
2.13 The cycle of making the mold is short and the efficiency is high.
2.14 The mixed molding sand can be used for a long time.
2.15 After the sand mold is pounded, it can still tolerate a small amount of deformation without damage, which is very beneficial to drafting and core lowering.

weakness is:
2.21 When mixing sand, it is necessary to coat the viscous paste on the surface of sand grains, and it is necessary to use high-power sand mixing equipment with kneading effect, otherwise it is impossible to obtain molding sand of good quality.
2.22 Since the molding sand has quite high strength after mixing, the molding sand is not easy to flow during molding, and it is difficult to spring. It is laborious and requires certain skills when molding by hand. When molding with a machine, the equipment is complicated and huge.
2.23 The rigidity of the mold is not high, and the dimensional accuracy of the casting is poor.
2.24 Castings are prone to defects such as sand flushing, sand inclusion, and pores.

3. In recent years, Sanzhuji has mainly produced rotor-type sand mixers according to the characteristics of green sand mixing. Efficient rotor-type sand mixers can greatly improve the quality of clay green sand. The new high-power sand mixer can
Make the sand mixing work high efficiency and high quality. The appearance of the shock compaction molding machine mainly based on shock compaction has significantly improved the compactness and uniformity of the casting mold. In order to improve the dimensional accuracy and surface quality of castings, Sanzhuji has launched a high-pressure molding machine mainly based on compaction. Using a high-pressure molding machine to make a sticky green sand mold can not only improve the dimensional accuracy of the casting, improve the surface quality, but also simplify the action of compacting the mold, shorten the cycle, and realize high-speed and automation throughout the molding and molding process. The new molding machine developed by Sanzhuji with gas shock pressure utilizes the thixotropy of clay slurry to obtain a very tight mold by instantaneously applying a pressure of 0.5 MPa. These advances are important conditions for clay green sand casting to adapt to modern industrial requirements.

4. Chemical hardening sand mold
The molding sand used for this type of sand is called chemically hardened sand (furan resin sand/phenolic resin sand). The binders are generally sub-polymerized under the action of the hardener and then become stand-alone.
The most commonly used materials are various synthetic resins and water glass. There are basically 3 ways of chemical hardening.

4.1 Self-hardening: Both the binder and the hardener are added during sand mixing (using a Resin sand mixer). After the sand mold or core is made, the binder is guided by the action of the hardener.
Cause the sand mold or core to harden on its own. The no-bake method is mainly used for modeling, but it is also used to manufacture larger cores or cores with small production batches.

4.2 Aerosol hardening: Add binder and other adjuncts when mixing sand, without adding hardener first. After molding or core making, blow in the gaseous hardener and blow the liquid hardener atomized in the gaseous carrier to disperse it in the sand mold or core, resulting in the hardening of the sand mold. Aerosol hardening is mainly used for core making, and sometimes for making small sand molds.

4.3 Heat hardening; add binder and latent hardener that does not work at room temperature when mixing sand. After the sand mold or core is made, it is heated, and the latent hardener reacts with some components in the binder to generate an effective hardener that can harden the binder, thereby hardening the sand mold or core. The heat-hardening method is mainly used for core making except for the manufacture of small thin-shelled sand molds.

5. The characteristics of chemical hardening sand casting process are:
5.1 The strength of the chemically hardened sand mold is much higher than that of the clay sand mold, and after the sand mold is hardened to a fairly high strength, the film is released, and it does not need to be repaired. Therefore, the mold can more accurately reflect the size and contour shape of the pattern, and it is not easy to deform in the subsequent process. The obtained casting has high dimensional accuracy.
5.2 Because the viscosity of the binder and hardener used is not high, it is easy to mix with the sand, the sand mixing equipment is light in structure, low in power and high in productivity, and the sand processing part can be simplified.
5.3 The mixed molding sand has good fluidity before hardening, and the molding sand is easy to be pounded during molding, so a large and complicated molding machine is not required.
5.4 When modeling with chemically hardened sand, materials such as wood, plastic and metal can be selected according to production requirements.
5.5 The content of binder in chemically hardened sand is much lower than that of clay sand, and there is no powdery auxiliary material. For example, if the original sand with the same grain is used, the gap between sand grains is much larger than that of clay sand. In order to prevent the metal from infiltrating between the sand grains during casting, the surface of the sand mold or core should be coated with high-quality paint.
5.6 Bonding with water glass
The chemical hardening sand of the agent is low in cost, and the working environment is odorless in use. However, the molding sand is not easy to disintegrate after the metal is poured into the mold; the used old sand cannot be directly recycled and must be regenerated, and the regeneration of sodium silicate sand is more difficult.
5.7 The cost of chemically hardened sand with resin as a binder is high, but the casting is easy to separate from the molding sand after pouring, the workload of casting cleaning is reduced, and most of the used sand can be recycled and reused.

6. Core
In order to ensure the quality of castings, the cores used in sand casting are generally dry cores. According to the different binders used in the core, the core is divided into clay sand core, oil sand core and resin sand core. Clay Sand Cores Simple cores made from clay sand. Oil sand core The core made of core sand with dry oil or semi-dry oil as binder is widely used. Oil viscosity mixed core sand flow It has good performance and is easy to tighten when making cores. However, the strength of the newly made core is very low, and it is generally carried by a profiled core board, and then baked in an oven at 200 ~ 300 ° C for several hours, and the oil is oxidized and hardened by air. The disadvantage of this core-making method is that the core is easily deformed during demolding, handling and baking, resulting in a reduction in the dimensional accuracy of the casting, a long baking time and high energy consumption.

7. Resin sand core
Various cores made of resin sand. The core is hardened in the core box and then taken out to ensure the correct shape and size of the core. According to different hardening methods, the manufacture of resin sand cores is generally divided into two methods: hot core box core making and cold core box core making.
7.1 Core making by hot box method:
Furan resin is usually used as core sand binder, and latent hardener (such as ammonium chloride) is also added. When making the core, keep the core box at 200~300°C. After the core sand is injected into the core box, the cymbal chloride reacts with the formaldehyde in the resin at a higher temperature to generate acid, so that the core hardens quickly. It takes about 10 to 100 seconds to build demold strength. Using the hot core box method to make cores is economical and economical, and the dimensional accuracy of the core is relatively high, but it consumes a lot of energy, emits irritating gases, and the working conditions of workers are also very poor.
7.2 Core making by cold box method:
Use urethane resin as the core sand binder. When the core is made by this method, the core box is not heated, and the core can be hardened by blowing amine steam into it for a few seconds. This method is superior to the hot box method in terms of energy, environment, and production efficiency. At present, the most widely used method is the furan resin cold box method which is hardened by blowing sulfur dioxide. The hardening mechanism is completely different from the urethane cold box method, but the process characteristics, such as fast hardening and high core strength, are roughly the same as the urethane cold box method.

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