Pre-coated sand casting (also known as Resin-coated Sand Casting) refers to a casting method in which molding sand or core sand is used to cover a solid resin film before molding. There are two kinds of coating processes: cold method and hot method: the cold method is to dissolve the resin with ethanol, and adds additives during the sand mixing process, so that both are coated on the surface of the sand grains, and the ethanol volatilizes to obtain the coated sand; this method is to Preheat the sand to a certain temperature, add resin to melt, stir to coat the surface of the sand, add aqueous solution and lubricant, cool, crush, and sieve to obtain coated sand. Generally used for steel castings and iron castings.

Pre-coated sand casting mainly uses natural quartz sand as raw sand, and thermoplastic phenolic resin and reinforcing agent as raw materials. According to the different technical needs of users, properly adjust the curing speed, mold release, fluidity, collapse, the surface finish of castings, storage, and other ratios. Pre-coated sand castings are one of molding materials for automobiles, tractors, and hydraulic parts.

Using high pressure to inject the resin-coated sand into the steel mould.

Using high temperature to sinter sand to get the complete shells and cores.

(Heating temperature was set depending on the different wall thicknesses and volume of the sand shell).

This way, the shells and cores have high strength, better cavity surface, more exact dimensions and exact geometrical tolerance.

Pre-coated Sand Casting Part(Resin-coated Sand Casting)

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The feature of the precoated sand casting part

1. It has suitable physical strength performance. It can be made into high-strength shell core-coated sand, medium-strength hot core box-coated sand, and low-strength non-ferrous alloy-coated sand.

2. Good fluidity, the mold (core) produced has clear outline and compact structure, and can manufacture complex sand cores, such as cylinder heads and cylinder water jacket cores.

3. The surface quality of the sand core is good, with little or no coating, you can get a better surface quality of the casting, the surface roughness can reach Ra=12.5~18μm, and the dimensional accuracy can reach CT8~CT10.

4. Good collapsibility, easy-to-clean castings.

5. The sand core has strong moisture absorption resistance and a long storage time, which is convenient for storage, transportation, and use.

6. Coated sand has a high cost and high energy consumption. It will produce an irritating odor during the molding, core-making, and pouring process, and may cause agglomeration when stored for a long time in high temperature and high humidity seasons.

The pre-coated sand casting part preparation process

The preparation process of coated sand mainly includes cold coating, warm coating and thermal coating. Among them, thermal coating is the most commonly used method. This method has the characteristics of less numerical consumption and high production efficiency.

1. Cold lamination process. Add the raw sand and thermosetting powdery phenolic resin into the sand mixer and mix evenly, then add an appropriate amount of solvent and continue mixing until the powdery resin is melted and adhered to the surface of the sand grains, the solvent evaporates, and the sand is crushed and sieved That's it. This method takes a long time to mix sand, consumes a lot of solvent, and the volatilized solvent gas is prone to explosion, so it is rarely used.

2. Warm coating process. Mix thermosetting liquid phenolic resin with raw sand at 60-80°C, mix for 2-3 minutes, then produce sand, then cool, crush and sieve.

3. Thermal lamination process. First, the raw sand is heated to a certain temperature, then mixed with resin, urotropine aqueous solution and calcium stearate respectively, cooled, crushed and sieved. Due to the difference in equipment and proportioning, the mixing process is different.

The pre-coated sand should be stored in a well-sealed container or bag under dry conditions at room temperature (≤20°C), and the storage time is 0.5 to 1 year. If the coated sand is stored for too long, local agglomeration may easily occur. If the agglomeration is not serious, it can be crushed and continue to be used after screening.

Coated sand core making process

Pre-coated sand has a wide range of applications, and its process methods are different. Whether it is core making or molding, the basic process requirements are (except wet manual): heating temperature 200~300 ℃, curing time 30~150s, sand injection pressure 0.15~0.6Mpa. The specific parameters should be adjusted according to the equipment model, type (core) weight and complexity, and the type of coated sand. Low sand shooting pressure, low heating temperature for fine and thin sand cores, and low heating temperature can properly prolong the curing time, etc.; and vice versa.

Main types of pre-coated sand

1. Ordinary pre-coated sand. It is composed of quartz sand, thermoplastic phenolic resin, urotropine and calcium stearate, without adding relevant additives. It is suitable for the production of general iron castings.

2. High-strength and low-gas-forming coated sand. It is an updated product of ordinary coated sand. It is prepared by adding relevant additives and adopting new technology. Its strength is more than 30% higher than ordinary coated sand, and the gas generation is also significantly reduced. It is suitable for the production of complex precision iron castings.

3. High-temperature pre-coated sand. It has the characteristics of high strength and long heat resistance at high temperature and is suitable for the production of complex thin-walled iron castings such as automobile engine cylinder blocks, cylinder heads, and container corners.

4. Easy-to-collapse coated sand. It has good strength and excellent low-temperature collapse performance and is suitable for the production of non-ferrous metal castings.

5. Other special requirements for coated sand. In order to meet the needs of different products, a series of specially coated sands have been developed, such as coated sand for centrifugal casting, chilled coated sand, wet coated sand, anti-sticking sand, anti-veining, anti-orange peel coated sand wait.

Raw and auxiliary materials of coated sand

Coated sand is generally composed of raw sand, binder, curing agent, lubricant, and special additives.

1. Raw sand.

The raw sand is the main body of the coated sand. The requirements for the raw sand are: high refractoriness, less volatile matter, relatively round particles, and high self-strength. , generally choose natural silica sand, which is mainly due to its abundant reserves and cheap price, which can meet the requirements of most castings. Only castings with special requirements use special sand (such as zircon sand, chromite, ceramsite sand, magnetite sand, etc.) as the raw sand of coated sand.

2. Binder

Currently, thermoplastic phenolic resins are commonly used as binders. The performance requirements of thermoplastic phenolic resin are: A. Polymerization speed (hot plate method): 25-75S; B. Softening point (ring and ball method): 90-105°C; C. Fluidity (slant plate method): 60- 110mm; D. Free phenol content (bromination method): ≤4%.

3. Curing agent, lubricant, additive.

The conventional curing agent for coated sand is urotropine, and its addition amount is 5% to 20% of the resin. In order to meet some special requirements (such as anti-shelling, fast polymerization speed, etc.), some companies have developed special curing agents. The role of the lubricant is to prevent agglomeration and improve the fluidity and mold release of the coated sand. There are mainly calcium stearate and ammonium stearate. The lubricant should try to use a product with a high melting point to prevent the shelling of the coated sand during use. The main function of additives is to improve the performance of coated sand. At present, the widely used additives mainly include high temperature resistant additives, easy-to-collapse additives, strengthening and toughening additives, anti-sticking sand additives and wet additives. Many coated sand factories have unique additive formulations. The performance of additives plays an important role in the production of special coated sand and is one of the core competitiveness of the coated sand market.

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