阅读说明：本技术 一种快速固化的低温溃散型覆膜砂的制备方法 (Preparation method of fast-curing low-temperature collapsible precoated sand ) 是由 余俊 潘是凯 于 2021-08-31 设计创作，主要内容包括：本发明公开了一种快速固化的低温溃散型覆膜砂的制备方法,属于覆膜砂制造技术领域。本发明采用水洗砂、宝珠砂混合砂作为骨料,液晶环氧树脂改性的脲醛树脂作为粘结剂,六亚甲基四胺作为固化剂,硬脂酸钙作为润滑剂,高岭土、己内酰胺、二氧化锰和重铬酸钾混合物作为溃散性添加剂,缓慢降温加入各添加剂分步混砂制得一种快速固化的低温溃散型覆膜砂。本发明制备得到的覆膜砂常温固化速度快,低温溃散性能极佳。(The invention discloses a preparation method of fast-curing low-temperature collapsible precoated sand, and belongs to the technical field of precoated sand manufacture. The invention adopts the mixed sand of washed sand and baozhu sand as aggregate, urea resin modified by liquid crystal epoxy resin as binder, hexamethylenetetramine as curing agent, calcium stearate as lubricant, and the mixture of kaolin, caprolactam, manganese dioxide and potassium dichromate as collapsibility additive, and the fast-curing low-temperature collapsibility type precoated sand is prepared by slowly cooling, adding each additive and mixing sand step by step. The precoated sand prepared by the invention has high normal-temperature curing speed and excellent low-temperature collapsibility.)

1. A preparation method of fast-curing low-temperature collapsible precoated sand is characterized by comprising the following steps:

1) mixing the washed sand and the Baozhu sand to obtain aggregate for later use;

2) heating the aggregate obtained in the step 1) to 130-165 ℃, transferring the aggregate into a sand mixer, adding liquid crystal epoxy resin modified urea-formaldehyde resin sand mixer, and coating the sand for 1.5-2 min;

3) when the temperature of the aggregate mixture obtained in the step 2) is cooled to 88-105 ℃, adding a hexamethylenetetramine solution and part of collapsibility additive sand mixture for 1-1.5min, continuously cooling to 75-85 ℃, adding calcium stearate and the rest of collapsibility additive sand mixture for 0.5-1min, immediately discharging sand, crushing and screening after the mixture is finished, and obtaining precoated sand;

the preparation method of the liquid crystal epoxy resin modified urea-formaldehyde resin comprises the following steps: mixing urea, formaldehyde solution, oxalic acid, sodium citrate and liquid crystal epoxy resin, heating to 85-95 ℃ for reflux, and vacuumizing reactants to obtain the urea-formaldehyde resin.

2. The method for preparing the fast-curing low-temperature collapsible precoated sand according to claim 1, wherein the mixing operation of step 1) is as follows: calcining the washed sand in a calcining furnace at the temperature of 640-720 ℃ for 3.5-5h, and cooling to room temperature; soaking the Baozhu sand in a sulfuric acid solution with the mass fraction of 75%, keeping the temperature overnight, and filtering and drying; the mass ratio of the obtained two is (4.4-8.2): 1 stirring and mixing uniformly.

3. The preparation method of the rapidly-cured low-temperature collapsible precoated sand according to claim 2, wherein SiO in the washed sand₂The content of (A) is not less than 97%; the granularity of the washing sand and the granularity of the pearl sand are both 110-220 meshes.

4. The preparation method of the fast-curing low-temperature collapsible precoated sand according to claim 1, wherein the mass ratio of urea, formaldehyde solution, oxalic acid, sodium citrate and liquid crystal epoxy resin in the raw materials for preparing the liquid crystal epoxy resin modified urea-formaldehyde resin is 100: (160-190): (5-10): (1-2): (3-6).

5. The method for preparing the fast-curing low-temperature collapsible precoated sand according to claim 4, wherein the liquid crystal epoxy resin is a thermotropic liquid crystal epoxy resin.

6. The preparation method of the fast-curing low-temperature collapsible precoated sand according to claim 1, wherein the preparation method of the collapsible additive in step 3) is as follows: the kaolin is taken and calcined for 1.5 to 2.5 hours at the temperature of 560 ℃ and 600 ℃, and then the obtained kaolin is mixed with caprolactam, manganese dioxide and potassium dichromate and ground to obtain the kaolin.

7. The method for preparing the fast-curing low-temperature collapsible precoated sand according to claim 6, wherein the mass ratio of calcined kaolin to caprolactam to manganese dioxide to potassium dichromate is 1: (0.8-1.5): (0.2-0.4): (0.4-0.5).

8. The preparation method of the fast-curing low-temperature collapsible precoated sand according to claim 1, wherein the raw materials in the step 3) comprise, by weight: 1000 parts of aggregate, 26-54 parts of liquid crystal epoxy resin modified urea-formaldehyde resin, 1.6-4.2 parts of hexamethylenetetramine solution, 2.5-4.8 parts of calcium stearate and 4-6 parts of collapsibility additive.

Technical Field

The invention belongs to the technical field of precoated sand manufacturing, and particularly relates to a preparation method of fast-curing low-temperature collapsible precoated sand.

Background

The precoated sand molding method was invented by J.Croning in Germany in 1944, and introduced and popularized in 1953 in Japan. In the initial stage of the invention, powdered phenolic resin and a hardening agent are mixed into silica sand to form the phenolic resin-silica sand; then, the precoated sand is prepared by thermoplastic phenolic resin, latent curing agent (hexamethylenetetramine) and lubricant through a certain coating process, when the precoated sand is heated, the resin coated on the surface of sand grains is melted, and the melted resin is quickly converted from a linear structure into a non-melted body structure under the action of methylene decomposed from the hexamethylenetetramine, so that the precoated sand is cured and molded.

Due to the rapid development of the automobile industry and the demand for export of mechanical products, higher demands are made on the quality of castings and correspondingly higher demands are made on precoated sand for casting, and the conventional technology obviously cannot keep up with the development. The precoated sand prepared by the conventional thermal method has a low curing speed at normal temperature, and the curing speed and time directly influence the core-making productivity and the shelling phenomenon generated when the tipping bucket is reset; the large shell mold has large deformation, is easy to generate peripheral warping phenomenon, and can reduce the phenomenon by short curing time of the precoated sand. In addition, the pouring temperature of the alloy casting is low, and the thermal decomposition temperature of the phenolic resin is relatively high, so that the collapsibility of the precoated sand is poor. In conclusion, the development of a precoated sand system which is fast in curing and excellent in collapsibility is of great significance.

Disclosure of Invention

Aiming at the problems in the background art, the invention provides a preparation method of fast-curing low-temperature collapsible precoated sand.

The invention is realized by the following technical scheme:

a preparation method of fast-curing low-temperature collapsible precoated sand comprises the following steps:

1) mixing the washed sand and the Baozhu sand to obtain aggregate for later use;

2) heating the aggregate obtained in the step 1) to 130-165 ℃, transferring the aggregate into a sand mixer, adding liquid crystal epoxy resin modified urea-formaldehyde resin sand mixer, and coating the sand for 1.5-2 min;

3) and (3) when the temperature of the aggregate mixture obtained in the step 2) is cooled to 88-105 ℃, adding a hexamethylenetetramine solution and 60-70% of collapsibility additive sand-mixing for 1-1.5min, continuously cooling to 75-85 ℃, adding calcium stearate and the rest collapsibility additive sand-mixing for 0.5-1min, immediately discharging sand, crushing and screening after the completion, and obtaining the precoated sand.

Further, the mixing operation of step 1) is: calcining the washed sand in a calcining furnace at the temperature of 640-720 ℃ for 3.5-5h, and cooling to room temperature; soaking the Baozhu sand in a sulfuric acid solution with the mass fraction of 75%, keeping the temperature overnight, and filtering and drying; the mass ratio of the obtained two is (4.4-8.2): 1 stirring and mixing uniformly.

Further, SiO in the washed sand₂The content of (A) is not less than 97%; the granularity of the washing sand and the granularity of the pearl sand are both 110-220 meshes.

The washed sand (silica sand) is hard in texture, wear-resistant, stable in chemical property, wide in distribution and low in cost, is very suitable for castings, but also has partial defects, such as large expansion coefficient and easy sand adhesion on the surfaces of the prepared castings; while the Baozhu sand grains are spherical, the surface is smooth, the structure is compact, and the thermal expansion coefficient is small. According to the invention, the calcined washed sand and the pickled Baozhu sand are mixed according to the mass ratio of (4.4-8.2): 1, the aggregate is prepared by mixing, and castings prepared by using the aggregate do not have expansion defects, have high yield of the castings, lower cost and high cost performance.

Further, the preparation method of the liquid crystal epoxy resin modified urea-formaldehyde resin comprises the following steps: mixing urea, formaldehyde solution, oxalic acid, sodium citrate and liquid crystal epoxy resin, heating to 85-95 ℃ for reflux, and vacuumizing reactants to obtain the urea-formaldehyde resin; the formaldehyde solution is a formaldehyde water solution with the mass fraction of 37%.

Further, the mass ratio of urea, formaldehyde solution, oxalic acid, sodium citrate and liquid crystal epoxy resin in the raw materials for preparing the liquid crystal epoxy resin modified urea-formaldehyde resin is 100: (160-190): (5-10): (1-2): (3-6).

Further, the liquid crystal epoxy resin is thermotropic liquid crystal epoxy resin.

The performance of the binder has a great influence on the quality of the precoated sand, and people are constantly working to find a substitute of phenolic resin. Aiming at the problems that the thermal decomposition temperature of phenolic resin is higher than that of an alloy casting and the curing speed of precoated sand prepared from the phenolic resin is low, the invention uses a liquid crystal epoxy resin modified urea-formaldehyde resin as a binder instead. The liquid crystal epoxy resin is a cross-linked polymer material with a network structure, wherein the geometrical shape of liquid crystal molecules and spherical molecules have good mechanical property and adhesive property and lower thermal decomposition temperature. The cured urea-formaldehyde resin is semitransparent, has good insulation property, good wear resistance and low cost, but is not resistant to strong acid and strong alkali and has poor initial adhesion, so that the urea-formaldehyde resin is modified by adopting the liquid crystal epoxy resin, the curing speed of the modified urea-formaldehyde resin is high, the thermal decomposition temperature is obviously reduced compared with that of the phenolic resin, and the collapsibility of the prepared casting is improved.

Further, the preparation method of the collapsibility additive in the step 3) comprises the following steps: the kaolin is taken and calcined for 1.5 to 2.5 hours at the temperature of 560 ℃ and 600 ℃, and then the obtained kaolin is mixed with caprolactam, manganese dioxide and potassium dichromate and ground to obtain the kaolin.

Further, the mass ratio of the calcined kaolin, caprolactam, manganese dioxide and potassium dichromate is 1: (0.8-1.5): (0.2-0.4): (0.4-0.5).

The collapsibility of the precoated sand can be improved by using caprolactam, manganese dioxide and potassium dichromate as additives, but the collapsibility of the precoated sand is poorer in dispersibility in the liquid crystal epoxy resin modified urea-formaldehyde resin, the kaolin is calcined at the temperature of 560 ℃ and 600 ℃, and is mixed and ground with the caprolactam, the manganese dioxide and the potassium dichromate in proportion, and the prepared collapsibility additive is good in dispersibility and strong in activity in the liquid crystal epoxy resin modified urea-formaldehyde resin, so that the collapsibility of the finally prepared precoated sand can be greatly improved, and the strong acid and alkali resistance of the precoated sand can also be improved. In addition, the inventor discovers through research and experiment that the collapsibility and the chemical stability of the prepared precoated sand can be further improved by mixing the collapsibility additive component prepared by the invention with the curing agent and the lubricant in a step-by-step manner.

Further, the raw materials in the step 3) comprise the following components in parts by weight: 1000 parts of aggregate, 26-54 parts of liquid crystal epoxy resin modified urea-formaldehyde resin, 1.6-4.2 parts of hexamethylenetetramine solution, 2.5-4.8 parts of calcium stearate and 4-6 parts of collapsibility additive.

Calcium stearate is used as a lubricant to prevent the precoated sand from caking, increase the fluidity of the precoated sand, make the surfaces of the mold and the core compact and improve the mold release property of the sand core.

Further, the hexamethylenetetramine solution is a hexamethylenetetramine aqueous solution with a mass fraction of 40%.

Compared with the prior art, the invention has the beneficial effects that:

the invention adopts the mixed sand of washed sand and baozhu sand as aggregate, urea resin modified by liquid crystal epoxy resin as binder, hexamethylenetetramine as curing agent, calcium stearate as lubricant, and the mixture of kaolin, caprolactam, manganese dioxide and potassium dichromate as collapsibility additive, and the fast-curing low-temperature collapsibility type precoated sand is prepared by slowly cooling, adding each additive and mixing sand step by step. The precoated sand prepared by the method has high normal-temperature curing speed, and can greatly improve the core-making production rate; in addition, the low-temperature collapsibility is excellent, and the manufactured metal casting can be vibrated and shaked after being cooled after being cast.

Detailed Description

In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. The examples, in which specific conditions are not specified, were conducted under conventional conditions or conditions recommended by the manufacturer. The reagents or instruments used are not indicated by the manufacturer, and are all conventional products available commercially.

Example 1

A fast-curing low-temperature collapsible precoated sand:

1. calcining the washed sand in a calcining furnace at 680 ℃ for 4.5h, and cooling to room temperature; soaking the Baozhu sand in a sulfuric acid solution with the mass fraction of 75%, keeping the temperature overnight, and filtering and drying; the mass ratio of the obtained two is 6.6: 1 stirring and mixing uniformly to obtain the aggregate.

2. According to the mass ratio of 100: 175: 8: 1.5: 4.2 mixing urea, formaldehyde solution, oxalic acid, sodium citrate and liquid crystal epoxy resin, heating to 90 ℃ for reflux, and vacuumizing reactants to obtain the adhesive.

3. Taking kaolin to calcine for 2.2h at 585 ℃, and then mixing the obtained kaolin with caprolactam, manganese dioxide and potassium dichromate according to the mass ratio of 1: 1.2: 0.3: 0.5 mixing and grinding to obtain the collapsibility additive.

4. Weighing 1000 parts of aggregate, 40 parts of binder, 3.0 parts of curing agent, 3.6 parts of lubricant and 5 parts of collapsibility additive according to parts by weight; heating the aggregate to 145 ℃, transferring the aggregate into a sand mixer, adding a binder into the sand mixer for sand mixing and film covering for 2min, adding a curing agent and 64% of collapsibility additive into the sand mixer for sand mixing for 1.5min when the temperature of the mixture is cooled to 95 ℃, continuously cooling the mixture to 82 ℃, adding a lubricant and the rest 36% of collapsibility additive into the sand mixer for 0.5min, immediately discharging the sand after the mixture is cooled, crushing and screening the discharged sand, and obtaining the coated sand.

Example 2

1. Calcining the washed sand in a calcining furnace at 645 ℃ for 3.5h, and cooling to room temperature; soaking the Baozhu sand in a sulfuric acid solution with the mass fraction of 75%, keeping the temperature overnight, and filtering and drying; the mass ratio of the obtained two is 4.8: 1 stirring and mixing uniformly to obtain the aggregate.

2. According to the mass ratio of 100: 175: 8: 1.5: and 4, mixing urea, formaldehyde solution, oxalic acid, sodium citrate and liquid crystal epoxy resin, heating to 90 ℃, refluxing, and vacuumizing reactants to obtain the binder.

3. Calcining kaolin at 580 ℃ for 2.2h, and then mixing the obtained kaolin with caprolactam, manganese dioxide and potassium dichromate according to the mass ratio of 1: 1.2: 0.3: 0.5 mixing and grinding to obtain the collapsibility additive.

4. Weighing 1000 parts of aggregate, 40 parts of binder, 2.8 parts of curing agent, 3.6 parts of lubricant and 5 parts of collapsibility additive according to parts by weight; heating the aggregate to 145 ℃, transferring the aggregate into a sand mixer, adding a binder into the sand mixer for sand mixing and film covering for 2min, adding a curing agent and 64% of collapsibility additive into the sand mixer for sand mixing for 1.5min when the temperature of the mixture is cooled to 98 ℃, continuously cooling the mixture to 80 ℃, adding a lubricant and the rest 36% of collapsibility additive into the sand mixer for 0.5min, immediately discharging the sand after the mixture is cooled, crushing and screening the discharged sand, and obtaining the coated sand.

Example 3

1. Calcining the washed sand in a calcining furnace at 710 ℃ for 4.8h, and cooling to room temperature; soaking the Baozhu sand in a sulfuric acid solution with the mass fraction of 75%, keeping the temperature overnight, and filtering and drying; the mass ratio of the obtained two is 8: 1 stirring and mixing uniformly to obtain the aggregate.

2. According to the mass ratio of 100: 170: 6: 1: and 4, mixing urea, formaldehyde solution, oxalic acid, sodium citrate and liquid crystal epoxy resin, heating to 86 ℃, refluxing, and vacuumizing reactants to obtain the adhesive.

3. Calcining kaolin at 570 ℃ for 1.8h, and mixing the obtained kaolin with caprolactam, manganese dioxide and potassium dichromate according to a mass ratio of 1: 0.85: 0.38: 0.4 mixing and grinding to obtain the collapsibility additive.

4. Weighing 1000 parts of aggregate, 28 parts of binder, 4 parts of curing agent, 4.6 parts of lubricant and 4 parts of collapsibility additive according to the parts by weight; heating the aggregate to 135 ℃, transferring the aggregate into a sand mixer, adding a binder into the sand mixer for sand mixing and film covering for 1.5min, adding a curing agent and 64% of collapsibility additive into the sand mixer for sand mixing for 1min when the temperature of the mixture is cooled to 90 ℃, continuously cooling the mixture to 76 ℃, adding a lubricant and the rest 36% of collapsibility additive into the sand mixer for 1min, immediately discharging the sand, crushing and screening the discharged sand, and obtaining the film-coated sand.

Example 4

1. Calcining the washed sand in a calcining furnace at 680 ℃ for 4.5h, and cooling to room temperature; soaking the Baozhu sand in a sulfuric acid solution with the mass fraction of 75%, keeping the temperature overnight, and filtering and drying; the mass ratio of the obtained two is 6.6: 1 stirring and mixing uniformly to obtain the aggregate.

2. According to the mass ratio of 100: 190: 5: 2: 6 mixing urea, formaldehyde solution, oxalic acid, sodium citrate and liquid crystal epoxy resin, heating to 90 ℃, refluxing, and vacuumizing reactants to obtain the binder.

3. Calcining kaolin at 600 ℃ for 2.2h, and mixing the kaolin with caprolactam, manganese dioxide and potassium dichromate according to a mass ratio of 1: 1.2: 0.3: 0.5 mixing and grinding to obtain the collapsibility additive.

4. Weighing 1000 parts of aggregate, 40 parts of binder, 3.0 parts of curing agent, 3.6 parts of lubricant and 5 parts of collapsibility additive according to parts by weight; heating the aggregate to 145 ℃, transferring the aggregate into a sand mixer, adding a binder for sand mixing and coating for 1.5min, adding a curing agent and 70% of collapsibility additive for sand mixing for 1min when the temperature of the mixture is cooled to 95 ℃, continuously cooling the mixture to 82 ℃, adding a lubricant and the rest 30% of collapsibility additive for sand mixing for 1min, immediately discharging the sand after the completion, crushing and screening to obtain the coated sand.

Comparative example 1

The procedure was as in example 1 except that no liquid crystal epoxy resin was used.

Comparative example 2

Directly mixing kaolin with caprolactam, manganese dioxide and potassium dichromate according to a mass ratio of 1: 1.2: 0.3: 0.5 blend grinding gave the collapsibility additive except that the same parameters as in example 1 were used.

Comparative example 3

All the collapsibility additives are independently subjected to sand mixing by the curing agent and the lubricant, then the sand mixing is added for 0.5min, and then the materials are discharged, except that the parameters are the same as the parameters of the step 1.

The results of the performance tests of the precoated sand obtained in examples 1 to 4 and comparative examples 1 to 3 are shown in Table 1.

TABLE 1

The detection standard is as follows: JB/T8583-2008.

The above-described embodiments are only preferred embodiments of the present invention and are not intended to limit the present invention. Various changes and modifications can be made by one skilled in the art, and any modification, equivalent replacement, improvement and the like made within the spirit and principle of the present invention should be included in the scope of the present invention.