Common Knowledge About Foundry Sand

Tidal sand, clay sand, water glass, coated sand, and resin sand all belong to sand casting in casting, but they are differences in different binders. Tidal sand, clay sand, and green sand are the same, and the binder is ordinary clay or bentonite. Due to the addition of water during the sand mixing process, it is called tidal sand. The binder for sodium silicate sand is sodium silicate, which is generally hardened by blowing carbon dioxide or adding a hardening agent to self harden. The binder for coated sand is generally phenolic resin, which is slightly toxic and is commonly used for sand cores. The binder for resin sand is furan resin, and the resin sand molding has a relatively large gas generation after pouring. At present, the quality of castings molded with resin sand is in sand casting. Water glass sand is a material used to make molds and can also be used as a binder, and is not an independent casting method.

There are primarily two methods of application:

1. Form sand molds using a mixture of quartz sand and sodium silicate (as a binder), then inject carbon dioxide for rapid curing.

2.When carrying out the shell making process in lost wax casting, add sodium silicate as a binder.

The wax used with sodium silicate is soft, while that used with silica sol is hard. The surface of products made with silica sol is superior to those made with sodium silicate. The difference between the two lies primarily in the quality of the molds produced. Silica sol molds are more suitable for precision casting, resulting in better surface smoothness, less deformation, lower shrinkage rates, and precise dimensions without the need for secondary processing. However, molds made with silica sol are generally used for small products, whereas those made with sodium silicate are used for relatively larger ones. Both methods differ in mold quality; silica sol molds are more suitable for precision casting, resulting in superior surface smoothness, minimal deformation, lower shrinkage rates, and precise dimensions without requiring secondary processing. The wax, facing sand, and adhesives used for both are different, resulting in noticeable differences in product quality. Silica sol technology is predominantly used abroad, while sodium silicate technology is more prevalent domestically. Additionally, cleaning sand treated with sodium silicate is more challenging because sodium silicate can sinter on the surface of the casting.Sodium silicate solution, commonly known as water glass or "pearl alkali", has advantages such as low price, high strength, and non-toxicity, making it widely used in foundry production. However, due to its poor collapsibility, it cannot replace other binders. Various measures have been taken to improve the collapsibility of sodium silicate sand, but none have achieved satisfactory results. Carbon dioxide sodium silicate sand, due to its high molding efficiency and the ability to pour without baking the mold, has been widely used in the production of steel castings and some cast iron parts. However, this type of sand has long faced two major challenges: poor collapsibility and difficulties in recycling old sand, which significantly limits its broader application. In recent years, with technological advancements, methods such as modifying sodium silicate and adding additives to improve collapsibility have emerged, gradually addressing the issue of poor collapsibility. Several solutions have also been proposed for recycling old sand. The author adopts wet reclamation technology, which requires less investment, has a high recycling rate, is environmentally friendly, and provides a low-cost solution to this problem. Characteristics include simple equipment, low investment, high recycling rate, environmental friendliness, and low cost.

1: Simple Equipment: One simple magnetic separator, crusher, stirring washing machine, clean water tank, wastewater tank, and wastewater treatment tank, each, with two water pumps.

2: Low Investment: For a foundry producing thousands of tons annually, only tens of thousands of yuan need to be invested.

3: High Recycling Rate: The recycling rate of old sand exceeds 90%.

4: Good Environmental Performance: After treatment, wash sand wastewater is recycled, achieving zero wastewater discharge. The old sand recycling process is operated using wet methods, with virtually no dust emissions.

5: Low Cost: The cost of recycling and reclaiming one ton of old sand is less than 30 yuan.

The solidification process of sodium silicate sand

In the past decade, significant breakthroughs have been made in both the deepening understanding of the basic composition and the "aging" phenomenon of sodium silicate, as well as the development of hardening technology. While maintaining sufficient process strength in core sand, the addition of sodium silicate can be reduced to 2.5% to 3.5% by mass fraction. This has effectively addressed the long-standing issues of poor collapsibility and the inability to reuse old sand. The hardening methods of sodium silicate sand can be divided into two types: CO2 gas hardening and self-hardening, with thermal hardening being rarely used.

CO₂ gas hardening method

This method is one of the earliest rapid forming processes used in the field of sodium silicate binder. Due to its convenient operation, flexible use, non-toxicity, and odorlessness, it has been widely applied in the production of steel castings in most domestic and international settings.

(1) Hardening Principle and Characteristics

Sodium silicate has a history of over three hundred years. Due to its complex and variable composition, its basic composition has not been fully understood, and research on sodium silicate has mainly remained at a macroscopic level. In recent years, the development of various testing methods has allowed for analysis and research at the molecular level, revealing that newly prepared sodium silicate is a true solution. However, during storage, the silicic acid in sodium silicate undergoes condensation, gradually transforming from a true solution into large-molecule silicic acid solution, which later becomes silicic acid gel particles. Therefore, sodium silicate is actually a heterogeneous mixture composed of polymeric silicic acid with different degrees of polymerization, and it is susceptible to the influence of its modulus, concentration, temperature, electrolyte content, and storage time.

When sodium silicate sand is hardened by CO2 gas, the surface layer of sodium silicate absorbs CO2, increasing its modulus and dehydrating. Under the dual action of acidification and dehydration, it rapidly hardens to form initial strength. The hardened surface layer of sodium silicate hinders CO2 from penetrating into deeper layers, and the inner layer of sodium silicate can only increase its strength through dehydration. The disadvantage of this method is that the strength of the core sand is low, with high moisture content and susceptibility to moisture absorption. It also exhibits poor collapsibility and is mostly used in the production of medium and small steel castings.

(2) Modification of Sodium Silicate

During storage, sodium silicate molecules undergo condensation to form gel particles, which can reduce its bonding strength by 20% to 30%, a phenomenon known as sodium silicate aging. To eliminate aging, sodium silicate must be modified. Currently, there are two methods of modification: physical modification and chemical modification. Physical modification involves supplying energy to sodium silicate through methods such as magnetic fields, ultrasonic waves, high frequency, or heating, to break apart the already polymerized gel particles and redistribute the polymeric silicic acid molecules evenly. This type of modification is effective for high-modulus sodium silicate but may lead to re-aging issues. Chemical modification involves adding small amounts of compounds to sodium silicate. These compounds contain functional groups such as carboxyl, amide, carbonyl, hydroxyl, ether, and amino groups. They adsorb onto the surface of silicic acid molecules or gel particles through hydrogen bonds or electrostatic forces, altering their surface energy and solvation capacity, thereby improving the stability of polymeric silicic acid and preventing aging. For example, adding polyacrylamide, modified starch, or polyphosphate to sodium silicate has achieved good results.

(3) Development Prospects

Using modified sodium silicate to improve its bonding ability often increases production costs and complicates processes. In recent years, Japan has developed the VRH method, which first removes air from the intergranular gaps of sand particles and then blows in CO2 gas to rapidly harden the mold. This process can reduce the amount of sodium silicate added (by mass fraction) to below 3.0%, while the amount of CO2 used is only one-tenth of the original amount. Additionally, some authors have proposed adding an inorganic substance to sodium silicate sand, which, after high-temperature treatment, forms a large number of holes on the bonding bridges at room temperature. This creates a new process where the core sand collapses on its own without external forces.

2. Ester Hardening Method

(1) Hardening Principle and Characteristics

This method uses liquid organic esters as hardeners for sodium silicate. Organic esters hydrolyze into alcohol and acid in alkaline sodium silicate solution. Alcohol has strong hydrophilicity and can remove water from sodium silicate, forming its solvation water. The acid reacts with sodium silicate, precipitating sodium acetate, which also has some hydrophilicity and can remove water from sodium silicate, forming its crystalline water. Under the dual action of acidification and dehydration, sodium silicate sand hardens. This hardening process gives the core sand very high strength. Not only can the amount of sodium silicate added be reduced to below 3.0%, but the hardened sand also exhibits good hardness and moisture resistance, making it suitable for the production of various large steel castings. The drawbacks are slow hardening speed, high brittleness, and poor flowability of the core sand.

(2) Main Raw Materials and Process Control of Core Sand

The raw materials for ester-hardened sodium silicate sand include silica sand, sodium silicate, and liquid organic esters. The quality and proper selection of these materials directly affect the success of the process, the quality of castings, and production costs. For ester-hardened sodium silicate sand, although the requirements for silica sand are not as strict as those for resin sand, in order to reduce the amount of sodium silicate added, silica sand should meet the following requirements: clay mass fraction ≤ 1.0%, water mass fraction ≤ 0.5%, fine powder mass fraction < 1.0%, and angularity coefficient ≤ 1.3. Sodium silicate should meet the requirements of national professional standard ZBJ31003—88. Strictly controlling the modulus of sodium silicate is the key to the successful application of this process and should be adjusted according to the season and room temperature: in summer, M = 2.2~2.4, and in other seasons, M = 2.4~2.6. When conditions permit, it is best to modify sodium silicate to eliminate aging. Currently, organic esters used in casting production include propylene glycol acetate ester, ethylene glycol acetate ester, diethylene glycol acetate ester, and propylene glycol carbonate ester, with an addition amount (by mass fraction) of 8% to 12%. Organic esters are the key materials that determine the process performance and production cost of ester-hardened sodium silicate sand and must be carefully selected. Different types (fast esters, slow esters, or mixed esters) of organic esters should be selected reasonably based on the size of the mold or core and the modulus of sodium silicate. For large castings, sand mixing can be done using a continuous sand mixer, while molding and coring are done manually.

(3) Development Prospects

By adding certain additives to form composite sodium silicate, which acts as an auxiliary binder, the bonding properties of sodium silicate can be further improved. Additives such as phosphates, borates, or aluminates can be added. In addition, the widespread use of organic ester hardeners has been hindered by their high price. Some have proposed a hardener composed of inorganic acid and organic substances, mainly phosphoric acid, phosphates, and urea. This type of hardener not only has low production costs but also results in core sand with good collapsibility.

The Chemical Composition of Sodium Silicate

Sodium silicate, also known as water glass, is a soluble alkali metal silicate material formed by the combination of alkali metal oxides and silica dioxide. It can be classified into sodium water glass and potassium water glass based on the type of alkali metal present. Their molecular formulas are Na2O.nSiO2 and K2O.nSiO2, respectively, where the coefficient n is called the modulus of sodium silicate. It represents the molar ratio of silica dioxide to alkali metal oxide in sodium silicate. The modulus of sodium silicate is an important parameter, typically ranging from 1.5 to 3.5. A higher modulus indicates that solid sodium silicate is less soluble in water. When n is equal to 1, it dissolves readily in cold water, while higher values of n require hot water or even steam pressure above 4 atmospheres for dissolution. As the modulus increases, the silica dioxide content in sodium silicate also increases, leading to higher viscosity and greater bonding strength, making it easier to decompose and harden.

There are two main methods for producing sodium silicate: the dry method and the wet method. In the dry method, quartz rock and pure alkali are ground and mixed, then melted at temperatures between 1300-1400°C in a furnace to produce solid sodium silicate, which is subsequently dissolved in water to obtain liquid sodium silicate. 

In the wet method, quartz rock powder and caustic soda are used as raw materials. They undergo a pressurized steam reaction in a high-pressure steam boiler at 2-3 atmospheres to directly produce liquid sodium silicate.