JPS6364260B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing resin-coated sand grains for shell molds (hereinafter simply referred to as resin-coated sand grains) for manufacturing low-expansion molds used in casting castings. In recent years, casting products for vehicles in particular have become thinner in order to reduce weight, and are also becoming more complex in shape to take measures against exhaust gas. As a result of these changes in cast products, it has naturally become necessary to improve the dimensional accuracy of molds, and in particular to develop molds with low thermal expansion during pouring. The thermal expansion of the mold is mainly caused by the type of sand grains and the phenolic resin that binds them.
The industry is focusing on the development of low thermal expansion type phenolic resins. Known types of phenolic resins include modified novolac type phenolic resins (novolac resins) and modified solid ammonia resol type phenolic resins, especially solid resol type phenolic resins containing SR acid as the main component (solid resol resins). be. Among these, the above-mentioned novolak resin has excellent dimensional accuracy, but has extremely poor sand removal properties (core disintegration properties) after casting, and sand residue, such as in cast iron water jacket cores, can cause serious defects. In other words, it cannot be used in areas where sand deposits may occur. on the other hand,
Although the solid resol resin has excellent dimensional accuracy and disintegrability, its mold strength is inferior to that of the novolak resin, so the amount of resin added to the sand grains increases, resulting in an increase in the cost of the resin-coated sand grains. SUMMARY OF THE INVENTION An object of the present invention is to eliminate the above-mentioned drawbacks, provide a method for producing resin-coated sand grains that is excellent in dimensional accuracy, collapsibility, and mold strength, and is inexpensive. The purpose of the present invention is to knead sand grains and solid resol resin to produce resin-coated sand grains, and to add 2 to 20 parts by weight of hexamethylenetetramine (hereinafter also referred to as hexamine) to 100 parts by weight of solid resol resin. achieved by things. The blending ratio of hexamine is 2 to 20 parts by weight, preferably 3 to 10 parts by weight, based on 100 parts by weight of the solid resol resin. Also, when adding, it is preferable to dissolve it in water and add it as an aqueous solution, and the concentration of the aqueous solution is 10 to 50.
It is best to adjust it to %. In addition, the solid resol resin of the present invention is annealed with a resin containing SR acid as a main component, which is subjected to addition condensation with this SR acid alone or a mixture with phenols, formaldehyde, and an ammonia catalyst, concentrated and dehydrated, and then cooled and solidified. This is the obtained solid resol resin. Here, SR acid is bisphenol A residue, which is composed of unreacted carbolic acid, bisphenol A, its various isomers, and derivatives. This type of solid resol resin has the effect of suppressing thermal expansion of the mold. Next, the intention of adding hexamine will be explained in detail. It has been known to use hexamine as a curing agent for novolak resin, which is a thermoplastic resin. However, resol resin, which is a thermosetting resin, does not require a curing agent. When solid resol resin is added to heated sand grains and coated, the solid resol resin is temporarily melted and used for coating, but when hexamine is added, the viscosity of the solid resol resin when melted decreases and the coating is complete. This has the effect of improving the strength of the mold when the resin-coated sand grains are used to make a mold. Furthermore, during mold making, the addition of hexamine reduces the melt viscosity of the resin coated on the sand grains, thereby improving mold strength. Further, the coated solid resol resin acts as a binder for the sand grains, but if crosslinking is insufficient in this case, the strength will be insufficient. Hexamine not only lowers the melt viscosity, but also actively participates in intermolecular reactions when the resol resin is cured, acts as a crosslinking agent, and fixes nitrogen between molecules. The cured product thus obtained has sufficient crosslinking, which in turn improves mold strength. As described above, the improvement in mold strength due to the addition of hexamine makes it possible to reduce the amount of solid resol resin added, and improves the disintegrability. Reference example 1 In the separable flask from 1, 500g of SR acid and 37
Prepare 360g of % formalin, heat to 40℃, add 20g of 20% soluble soda aqueous solution, and heat to 85-90℃.
The methylolation reaction was carried out for 40 minutes. Then, after cooling the contents to 50℃, add 25% ammonia water 51
g was added thereto, heated again to 70-75°C, and reacted for 80 minutes. Bisphenol S20 to the resulting product
g, add 3 g of aminosilane, -60 to -65 cmHg
Concentration and dehydration was carried out under reduced pressure, and as soon as the liquid temperature reached 95°C, the contents were discharged, cooled and solidified to obtain a solid resol resin with a weight average molecular weight of 1480. This solidified resin was crushed into 6 to 20 meshes, mixed with 3 phr of wax, and powdered. Reference example 2 Into the three-neck flask from 1, add 500 g of SR acid and 37
% formalin, heated to 70°C, added 2.1g of oxalic acid, and heated to 110% at reflux temperature.
After reacting for a minute, monoethanolamine was added to adjust the pH of the contents to 3.0-4.0. Next, concentrate and dehydrate under reduced pressure of -60 to -65 cmHg, and when the temperature of the contents reaches 180°C, reduce the pressure and stop heating, and add 16 g of wax and 1.5 g of aminosilane.
After melting, discharge and cool to solidify, melting point 58℃
A novolak resin was obtained. Reference example 3 600g of phenol in the three-necked flask from 1.
Prepare 414g of 37% formalin, heat to 70℃, add 2.1g of oxalic acid, and then heat at reflux temperature.
The reaction was allowed to proceed for 150 minutes. Next, concentrate and dehydrate under reduced pressure of -60 to -65 cmHg, and when the temperature of the contents reaches 160°C, stop reducing the pressure and heating, and remove the wax.
After adding and dissolving 3 phr, the mixture was discharged, cooled and solidified to obtain an unmodified novolak resin with a melting point of 68°C and an average molecular weight of 2490. Reference example 4 Phenol 500 in 1 separable flask
g, 85% paraformaldehyde 338g, 25%
Prepare 75g of ammonia water and heat at a temperature of 60 to 65℃.
The reaction was allowed to proceed for 120 minutes. Then, concentration and dehydration were carried out under reduced pressure of -60 to -65 cmHg, and when the liquid temperature reached 100°C, the contents were immediately discharged and cooled to solidify to obtain an unmodified solid resol resin with a weight average molecular weight of 1730.
Crush this into 6 to 20 pieces and wax 3phr.
We had a mixed discussion. Example 1 Put 350 kg of recycled sand heated to 120-140°C into a speed maller, add 6.5 kg of solid resol resin obtained in Reference Example 1 while stirring, stir and mix for 20 seconds, and then cool with 30% hexa water to 1.1 kg. I added 3.2 of water. After that, stir and knead for 60 seconds, add 0.175Kg of calcium stearate, and knead for another 20 seconds.
The sand was removed to obtain resin-coated sand grains. Example 2 The amount of 30% hexa water is 2.2 and the amount of cooling water is 2.4.
Resin-coated sand grains were obtained in the same manner as in Example 1 except for the following. Example 3 The amount of 30% hexa water is 3.3, the amount of cooling water is 1.6
Resin-coated sand grains were obtained in the same manner as in Example 1 except for the following. Comparative example 1 Without adding 30% hexa water, the amount of cooling water was 3.9
Resin-coated sand grains were obtained in the same manner as in Example 1 except for the following. Comparative Example 2 Resin-coated sand grains were prepared in the same manner as in Example 3, except that the novolak resin obtained in Reference Example 2 was used instead of the solid resol resin obtained in Reference Example 1, and the amount of cooling water was changed to 2.7. I got it. Comparative Example 3 Resin-coated sand grains were obtained in the same manner as in Comparative Example 3, except that the unmodified novolak resin obtained in Reference Example 3 was used. Comparative Example 4 Resin-coated sand grains were obtained in the same manner as in Comparative Example 1, except that the unmodified solid resol resin obtained in Reference Example 4 was used. As described above, a test was conducted according to JlS K6901 using the resin-coated sand grains obtained in Example 1-3 and Comparative Example 1-4. The results are listed in Table 1 below.

[Table] In Table 1 above, the disintegration test method is to first heat the mold to 250°C in the shape shown in the first figure and then heat it to 350°C for 60 seconds.
A test piece was created by firing it in a furnace for 60 seconds. After that, the molten cast iron was melted to a temperature of 1300 to 1330℃, and the core/casting was poured under the conditions of 125g/1370g.
After 10 minutes, the mold was taken apart and left to stand for one day, and then its disintegration properties were tested. The test was carried out using the apparatus shown in Figure 2, where impact was applied with a rammer, and the number of times the rammer was used until all the cores fell was taken as the collapsibility index. However, for those that have been used more than 1000 times, the disintegration index was calculated as the sand remaining ratio to core weight x 1000 + 1000. As is clear from Table 1, the present invention satisfies the required values of room temperature bending strength, warm bending strength, rapid thermal expansion coefficient, and disintegration properties in a well-balanced manner.
Moreover, an inexpensive method for producing resin-coated sand grains is provided.

[Brief explanation of drawings]

Both Figures 1 and 2 are diagrams for illustrating the collapsibility test. FIG. 2 is a cross-sectional view of the sex testing device. 1: sprue, 2: weir, 3: core, 4: rammer,
5: Drive unit, 6: Fixing bolt, 7: Sand receiving can.

Claims (1)

[Claims] 1. In a method for producing resin-coated sand grains for shell molds by kneading heated sand grains and solid ammonia resol type phenolic resin mainly composed of SR acid, hexane is added to 100 parts by weight of the solid ammonia resol type phenolic resin during kneading. A method for producing resin-coated sand grains for shell molding, which comprises adding 2 to 20 parts by weight of methylenetetramine.