Recently, some customers have inquired about sodium silicate sand casting equipment and sodium silicate sand recovery equipment. Compared with furan resin sand casting, sodium silicate sand casting has a lower cost and is suitable for steel castings. Qingdao sanzhuji combines the casting characteristics of sodium silicate sand A summary is made.

1. Factors affecting the "aging" of water glass, and eliminating the "aging" of water glass
The freshly prepared water glass is a true solution. However, during the storage process, the silicic acid in the water glass will undergo polycondensation, which will gradually polycondense from the true solution into a macromolecular silicic acid solution, and finally become a silicic acid gel. Therefore, water glass is actually a heterogeneous mixture composed of polysilicic acid with different degrees of polymerization, which is easily affected by its modulus, concentration, temperature, electrolyte content and storage time.
During storage, the molecules of water glass undergo condensation polymerization to form a gel, and its bonding strength gradually decreases with the extension of storage time. This phenomenon is called "aging" of water glass.
The "aging" phenomenon can be explained by the following two sets of test data: high modulus water glass (M=2.89, ρ=1.44g/cm3) stored for 20, 60, 120, 180, 240 days, then blow CO2 hardened water glass The dry tensile strength of sand drops correspondingly by 9.9%, 14%, 23.5%, 36.8% and 40%; low modulus sodium silicate (M=2.44, ρ=1.41g/cm3) is stored for 7, 30, 60 and 90 days after being dried The tensile strength decreased by 4.5%, 5%, 7.3% and 11% respectively.
The storage time of water glass has little effect on the initial strength of ester-hardened water glass self-hardening sand, but it has a significant effect on the later strength. According to the measurement, it is reduced by about 60% for high modulus water glass, and 15-20% for low modulus water glass. . The residual strength also decreases with the extension of storage time.
The polycondensation and depolymerization reactions of polysilicic acid proceed simultaneously during the storage process of water glass, and the molecular weight is disproportionated, and finally a multi-dispersion system in which monoorthosilic acid and colloidal particles coexist are formed. The degree of polymerization of silicic acid is disproportionated, and the content of monoorthosilicic acid and polysilicic acid increases with the extension of storage time. As a result of condensation polymerization and depolymerization reaction of water glass during storage, the bonding strength is reduced, that is, "aging" occurs.
The main factors affecting the "aging" of water glass are: storage time, modulus and concentration of water glass. The longer the storage time, the higher the modulus and the greater the concentration, the more serious the "aging".
The long-existing water glass can be modified by various methods to eliminate "aging" and restore the water glass to the performance of fresh water glass:
1.1 Physical modification
The aging of water glass is a spontaneous process that slowly releases energy. The physical modification of "aged" water glass is to use magnetic field, ultrasonic, high frequency or heating to provide energy to the water glass system to promote high polymerization polysilicate glue The particles re-depolymerize and promote the homogenization of the molecular weight of polysilicic acid, thereby eliminating the aging phenomenon, which is the mechanism of physical modification. For example, after treatment with a magnetic field, the strength of sodium silicate sand is increased by 20-30%, the amount of sodium silicate added is reduced by 30-40%, CO2 is saved, collapsibility is improved, and there are good economic benefits.
The disadvantage of physical modification is that it is not durable, and the bonding strength will decrease when stored after treatment, so it is suitable for use as soon as possible after treatment in the foundry. Especially for water glass with M>2.6, the concentration of silicic acid molecules is high, and after physical modification and depolymerization, it will polycondensate relatively quickly. It is best to use it immediately after treatment.
1.2 Chemical modification
Chemical modification is to add a small amount of compounds to the water glass, these compounds all contain carboxyl, amide, carbonyl, hydroxyl, ether, amino and other polar groups, which are adsorbed on silicic acid molecules or colloidal particles through hydrogen bonding or static electricity Surface, change its surface potential energy and solvation ability, improve the stability of polysilicic acid, thereby preventing "aging" from proceeding.
For example, adding polyacrylamide, modified starch, polyphosphate, etc. to the water glass can achieve better results.
Incorporating organic matter into ordinary water glass or even modified water glass can play a variety of functions, such as: changing the viscous flow properties of water glass; improving the modeling performance of water glass mixtures; improving the bonding strength, so that the absolute addition of water glass The amount is reduced; the plasticity of the silicic acid gel is improved; the residual strength is reduced, so that the water glass sand is more suitable for cast iron and non-ferrous alloys.
1.3 Physical-chemical modification
Physical modification is suitable for "aged" water glass, and it can be used immediately after modification. Chemical modification is suitable for processing fresh water glass, and the modified water glass can be stored for a long time. The combination of physical modification and chemical modification can make the water glass have a lasting modification effect. For example, adding polyacrylamide to the autoclave to modify the "aged" water glass has a good effect, and the pressure and pressure of the autoclave Stirring is a physical modification, and adding polyacrylamide is a chemical modification.

2. Prevent CO2 blowing hardened sodium silicate sand mold (core) surface chalking
After soda sodium silicate sand is blown CO2 hardened and left for a period of time, sometimes a substance like hoar frost will appear on the surface of the lower mold (core), which will seriously reduce the surface strength of the place and easily cause sand washing defects during pouring. According to analysis, the main component of this white substance is NaHCO3, which may be caused by excessive moisture or CO2 in the sodium silicate sand. The reaction is as follows:
Na2CO3+H2O→NaHCO3+NaOH
Na2O+2CO2+H2O→2NaHCO3
NaHCO3 easily migrates outwards with water, causing powdery like frost on the surface of the mold and core.
The solution is as follows:
2.1 Control the water content of sodium silicate sand not to be too high (especially in the rainy season and winter).
2.2 The time for blowing CO2 should not be too long.
2.3 The hardened mold and core should not be placed for a long time, and the mold should be timely cast.
2.4 Adding about 1% (mass fraction) of the syrup with a density of 1.3g/cm3 to the sodium water glass sand can effectively prevent surface powdering.

3. Improve the moisture absorption resistance of sodium silicate sand mold (core)
The soda water glass sand core hardened by CO2 or heating method is assembled in the wet clay mold. If it is not poured in time, the strength of the sand core will decrease sharply, not only may creep, even collapse; it is stored in a humid environment The strength of the sand core is also significantly reduced. The strength value of the CO2 hardened sodium water glass sand core placed in an environment with a relative humidity of 97% for 24 hours. The reason for the loss of strength when stored in a humid environment is the re-hydration of sodium water glass. The Na+ and OH— in the soda water glass binder matrix absorb water and etch the matrix, and finally break the silicon-oxygen bond Si—O—Si, resulting in a significant decrease in the bonding strength of the sodium water glass sand.
Influence of Sodium Sodium Silicate Sand Core Stored in High Humidity on Its Strength
The measures to solve this problem are:
3.1 Adding lithium water glass to the sodium water glass, or adding inorganic additives such as Li2CO3, CaCO3, ZnCO3, etc. to the sodium water glass, because relatively insoluble carbonates and silicates can be formed, and free sodium ions can be reduced. Therefore, the moisture absorption resistance of the sodium water glass binder can be improved.
3.2 Add a small amount of organic material or organic matter with surfactant effect to the sodium water glass. When the binder is hardened, the hydrophilic Na+ and OH- ions in the sodium water glass gel may be substituted by organic hydrophobic groups, or each other In combination, the exposed organic hydrophobic base improves moisture absorption.
3.3 Improve the modulus of water glass, because the moisture resistance of high modulus water glass is stronger than that of low modulus water glass.
3.4 Add starch hydrolysate to sodium silicate sand. A better method is to use starch hydrolyzate to modify sodium water glass.

4. CO2 blowing hardened water glass-alkaline phenolic resin sand composite process characteristics
In recent years, in order to improve the quality of steel castings, some small and medium-sized enterprises urgently need to adopt the resin sand process. However, due to limited economic capacity, they are unable to purchase resin sand regeneration equipment, and the old sand cannot be recycled, resulting in high production costs. In order to find an effective way to improve the quality of castings without increasing the cost, the process characteristics of CO2 blowing hardened sodium silicate sand and CO2 blowing hardened alkaline phenolic resin sand can be combined, and CO2 blowing hardened sodium silicate-alkaline phenolic resin The resin sand compounding process uses alkaline phenolic resin sand as the surface sand and water glass sand as the back sand, while blowing CO2 for hardening.
The phenolic resin used in CO2-alkaline phenolic resin sand is made by polycondensation of phenol and formaldehyde under the action of a strong alkaline catalyst and adding a coupling agent. Its PH value is ≥13, and its viscosity is ≤500mPa. The amount of phenolic resin added to the sand is 3% to 4% (mass fraction). When the CO2 flow rate is 0.8~1.0m3/h, the best blowing time is 30~60s; if the blowing time is too short, the hardening strength of the sand core will be low; if the blowing time is too long, the strength of the sand core will not increase, and it is wasted gas.
CO2—alkaline phenolic resin sand does not contain harmful elements such as N, P, S, etc., so casting defects such as pores, surface microcracks, etc. caused by these elements are eliminated; harmful gases such as H2S and SO2 are not released during pouring, which is beneficial to environmental protection ; Good collapsibility, easy to clean; high dimensional accuracy; high production efficiency.
The CO2 blowing hardened water glass-alkaline phenolic resin sand composite process can be widely used in steel castings, iron castings, copper alloys and light alloy castings.
The composite process is a simple and convenient process. The process is as follows: firstly mix the resin sand and water glass sand separately, and then put them into two sand hoppers; then add the mixed resin sand as surface sand to the sand box and pound solid, the thickness of the surface sand layer is generally 30~50mm; then add water glass sand as the back sand to fill and compact; finally blow CO2 gas into the mold for hardening.
The diameter of the blowing tube is generally 25mm, and the hardenable range is about 6 times the diameter of the blowing tube.
The blowing time depends on the size, shape, gas flow, and area of ??the exhaust plug of the sand mold (core). Generally, the blowing time is controlled within 15~40s.
Mold can be taken after blowing hard sand mold (core). The strength of the sand mold (core) rises quickly. Brush the paint within half an hour after taking the mold, and close the box for pouring after 4 hours.
The composite process is particularly suitable for steel casting plants that do not have resin sand regeneration equipment and need to produce high-quality castings. The process is simple and easy to control, and the quality of the produced castings is comparable to that of other resin sand castings.
CO2 blowing hardened sodium silicate sand can also be compounded with CO2 blowing hardened sodium polyacrylate resin sand for the production of various high-quality castings.

The above is all the explanation of sodium silicate sand casting. Qingdao Sanzhuji Equipment Manufacturing Co., Ltd. can customize the used sand recovery plan according to the characteristics of the casting sand. At the same time, it can provide sand reclamation equipment, no-bake resin sand mixer, molding machine, pneumatic conveyor, pouring machine Welcome guests to visit the factory.

Qingdao Sanzhuji Equipment Manufacturing Co., Ltd. specializes in the production of sand reclamation equipment,foundry machines,Sand casting equipment,GS high efficiency rotary mixer,no-bake resin sand mixer,Jolt squeeze molding machine/Jolt-squeezing moulding machines,Multi- Piston Moulding Machine/Hydraulic multi-piston moulding machine,foundry molding machine,flaskless moulding machine,shot blasting machine,dust collector,according to the amount of old sand recovered Carry out plan customization, and provide sand reclamation equipment installation, commissioning, and training. Welcome guests to visit the factory.