1. Why is phosphoric acid used as a curing agent for high-nitrogen furan self-hardening resins, and rarely as a curing agent for low-nitrogen furan self-hardening resins?
This is because the low-nitrogen sterol resin, when using an acid as a curing agent, has a slow curing rate, a long demolding time, and a low strength. The high nitrogen low sterol resin uses phosphoric acid as a curing agent to obtain the necessary hardening speed. Moreover, the high nitrogen low sterol resin uses phosphoric acid as a catalyst to obtain a good final strength.
The reason for this result is mainly due to the poor mutual solubility of phosphoric acid and sterol, and the affinity with water is large, so that the water contained in the resin and the catalyst and the water generated by the resin in the polycondensation reaction are not easily diffused and discharged, and the phosphoric acid is used as a core. The water droplets remain in the resin film, and the density of the resin film is destroyed, so the strength is low. The high-nitrogen resin and the water have good mutual solubility, and various moistures are not easily concentrated as water droplets with phosphoric acid as the core, and the resin film has a good structure and high strength.
2. Why is the hard permeability of phenol urea-formaldehyde self-hardening resin better than that of furan self-hardening resin sand? Because the curing process of the phenol urea-ruthenium resin is a polymerization reaction, no small molecule water is generated during the curing process, and there is no problem that the internal and external curing speeds are inconsistent due to the different volatilization speeds of the internal and external moisture, and the curing process of the furan self-hardening resin is The polycondensation reaction produces water during the reaction, and the evaporation rate of the water inside and outside the core is different, resulting in different curing speeds inside and outside, so the hard permeability is poor. This is also the reason why the curing speed of furan self-hardening resin sand is greatly affected by the relative humidity of air.
3. Why can we use high nitrogen furan resin for the production of cast aluminum and cast copper?
Since the aluminum and copper metal liquids hardly dissolve nitrogen, even if nitrogen is used in the casting process, the nitrogen generated by the decomposition of the resin is not absorbed by the aluminum or copper metal liquid, and is not precipitated during the solidification process. Nitrogen forms a nitrogen pore. Therefore, the production of cast aluminum, cast copper parts can be used for high-nitrogen resin.
4. Why do self-hardening resin sands produce heavy castings, and the casting system should be made of ceramic tubes?
Using self-hardening resin sand molding, when casting a heavy casting, due to the long pouring time, the casting system is prone to premature collapse of the resin sand under the long-term heat of the high-temperature molten metal, resulting in sandblasting defects. For heavy castings, the casting system should be made of ceramic pipes, and at the same time the problem of the gating system, especially the sprue, is difficult to apply.
5. When using self-hardening resin sand molding and core making, how to determine whether the usable time of resin sand meets the requirements?
When using a batch sand mixer, the resin sand can be used for a longer period of time than when the resin sand is mixed and unloaded until it is used up; when a continuous sand mixer is used for molding and core making, the resin sand can be used. The time is greater than the time from the initial sanding position of the sand outlet of the sand mixer to the continuous sanding back to the position.
6. Why is the slope of the pattern of self-hardening resin sand larger than that of clay sand?
Resin sand core has certain hardening strength when starting the mold, and has no repellent property. Generally, the sand type or sand core wall can not be loosened by tapping the pattern and the core box frame, and the friction force is large when the mold is pulled; the resin sand type and The repairability of the sand core is poor. When the mold is released, the sand type is incomplete and difficult to repair.
7. Why should the resin sand process produce cast iron instead of shrinking the riser, and set up the gas riser?
Resin sand type has good rigidity and high sand strength at the initial stage of casting. This conditionally utilizes the graphitization expansion of the solidification process of cast iron, effectively eliminates shrinkage and shrinkage defects, and realizes less riser and no riser casting of gray cast iron and ductile iron castings. . However, in order to facilitate the smooth discharge of a large amount of gas in the cavity during the pouring process, it is necessary to set up a gas riser in principle, and in principle, it is required to be ∑F.
8. Why does the sterol-containing 70%-80% furan self-hardening resin have the highest room temperature final strength?
Theoretically, the higher the ratio of urea-formaldehyde resin in the furan resin, the greater the final strength at room temperature. Therefore, when the sterol content is more than 80%, the higher the sterol content, that is, the lower the nitrogen content, the lower the room temperature final strength. However, as the urea-formaldehyde resin increases, the water content of the furan resin increases relatively, which in turn reduces the hardening speed and the final strength. When the sterol content is less than 70%, the final strength of the resin decreases more than the upward trend. Therefore, in general, when the sterol contains 70% to 80%, the furan self-hardening resin sand has the highest room temperature final strength.
9. Why use excessively active catalytically active curing agents and excessive amounts of curing agent, which will result in a decrease in the final strength of the resin sand?
The use of a catalytically active curing agent or an excessive amount of curing agent may result in a pre-cure stage that is too short, and the unreacted roll-formed prepolymer in the resin is not fully stretched and ordered, and is pre-polymerized by cross-linking. The substance is clogged in the bulk structure, so that a large number of active groups in the coil are not involved in the reaction, and a polymer having a large degree of polymerization cannot be formed. Therefore, although the resin sand hardens faster and the initial strength is higher, it will inevitably lead to a significant decrease in the final strength.
10. Why is it not necessary to use phosphoric acid as a curing agent when the old sand of self-hardening resin sand needs to be recycled?
After the sand type and core of the curing agent are made of phosphoric acid, the phosphoric acid cannot be decomposed and destroyed under the action of the molten metal, and the phosphate is deposited on the surface of the sand, which is difficult to remove by regeneration, resulting in a decrease in the strength of the resin sand prepared by the reclaimed sand. A lot, and increase the expansion of the mold, so that the tendency to sand is increased.
11. Why is the acid-hardened resole phenolic resin self-hardening sand suitable for use as a curing agent with a low free acid content, a high acid acid, and a non-universal inorganic acid as a curing agent?
Due to the high water content of the acid-hardened resole phenolic resin, in addition to the condensation of the resin itself to produce water, a large amount of water will be released, which will act as a dilution hardener and slow the reaction. . Increasing the free acid content of the hardener will make the hardening speed faster, but the strength of the self-hardening sand will be greatly reduced. Therefore, to ensure the hardening speed without increasing the strength of the resin sand, the curing agent can only be increased. Total acid value. Since the free acid of the inorganic acid is generally high, for the acid-hardened resole phenolic resin, an organic acid having a high total acidity and a low free acid content should be used as a curing agent.
12. Why is the amount of curing agent for acid-cured resole phenolic resin sand to be expressed as a percentage of the resin?
This is because the acid-cured resole phenolic resin is formed by the condensation of benzoic acid and formaldehyde under the action of a basic catalyst, and the alkaline catalyst is neutralized with an acid before leaving the factory to make it weakly acidic. Therefore, the resin is not as sensitive to the acidic hardener as the furan resin, and the crosslinking reaction occurs when the acid concentration is relatively high. In addition, such resins have a high water content and are generally about 15% or higher in some respects. When the cross-linking reaction occurs, in addition to the water produced by the resin itself, a lot of water which is miscible with the resin is released. These waters are diluted with a hardener, and the more the amount of resin added from the hard sand, the stronger the dilution, so that the same curing speed is maintained, and the amount of hardener added has to be increased. Therefore, for the resin phenolic resin self-hardening sand, the amount of the hardener added is preferably calculated as a percentage of the resin, that is, the amount of the acid-hardened resol phenolic resin curing agent is increased as the amount of the resin added increases.
13. Why should the newly-formed core and the newly repaired area not be painted immediately?
Because the hardening reaction of the resin in the newly-formed and newly repaired type and core is still in the initial stage, if the water (solvent) in the water-based paint is encountered, the hardening will be affected normally (for the phenol urethane self-hardening resin, The unreacted two-component polyisocyanate can be deactivated by reaction with moisture; if an alcohol-based coating is used, it needs to be ignited immediately after application of the coating, and the unreacted resin is also overfired. These will affect the surface stability of the core.
14. Why is it difficult to regenerate old sand from alkaline phenolic resin sand?
Since the alkaline phenolic resin has a high basicity (the base pH is usually 11 to 13), the resin contains a large amount of alkali (generally potassium hydroxide). When poured, the alkali in the resin easily reacts with silica sand to form a low melting point. The silicate, the molten silicate firmly adhered to the sand is not easy to remove, making the old sand regeneration more difficult.
15. When choosing the type of resin, what factors should be considered for the casting?
The following factors should be considered primarily for castings:
(1) The material of the casting. When the material of the casting (casting steel, high-alloy cast iron) has a tendency to generate nitrogen pores, it is preferable to select a low-nitrogen or nitrogen-free resin; when the casting (the ductile iron casting, the gray iron casting) is made of a small nitrogen gas, the tendency is small. For the ferrous metal, the medium nitrogen resin may be selected; for the copper and aluminum parts which are insoluble in nitrogen, a high nitrogen resin is selected.
(2) The weight of the casting, the wall thickness, and the pouring temperature. When the weight of the casting is large, the wall is thick, and the pouring temperature is high, the resin with high sterol content and low urea-formaldehyde content should be selected to meet the needs of the high-temperature strength of the core; the casting temperature is low, the weight is light, and the thin-walled casting is The required high temperature strength is low, and a resin having a higher urea-formaldehyde content can be selected to reduce the cost.
(3) The structure of the casting. Castings are prone to thermal cracking. It is advisable to use a resin with a low temperature and low temperature as a binder. For steel castings prone to hot cracking, alkaline phenolic resin should be used, and furan nitrogen-free resin should not be used as binder; castings are prone to cold cracking, and it is better to use binders with good collapsibility.
July 08, 2023
