1. Common defects and prevention of furan resin sand casting
Furan resin sand castings are of good quality and have a low rejection rate. However, if the selection of raw and auxiliary materials, process design, modeling (core) operation, production management and other aspects are not properly controlled, the castings will also have many defects, or even be scrapped in batches. Based on Sanzhuji’s years of experience, this article talks about common defects in furan resin sand castings and their preventive measures.Stoma and pinhole
Furan resin sand has good air permeability, but it has a higher gas output than various inorganic molds, and it seems that gas defects are more likely to occur on the whole. The gas source has the following aspects:
1.11 At present, the foreign advanced index resin content (mass fraction) is 0.6%～0.8%, and domestic is 0.8%～1.0%. However, many manufacturers add a lot of resin and curing agent, and the resin w(N) is too high. The upper mold (core) is not completely hardened, and then pouring and other factors will increase the amount of gas and cause pore defects.
1.12 The particle size of the raw sand is too fine, and the air permeability is reduced (the particle size is required to be 30/70, and the moisture content of the raw sand is less than 0.2%).
1.13 Poor quality or insufficient drying of the paint makes the sand mold (core) high in residual moisture.
1.14 Poor regeneration of old sand makes the loss of sand in the system out of control, and the content of fine powder exceeds the standard, resulting in increased gas generation and decreased air permeability.
1.15 The mold (core) is not fully hardened during pouring.
1.16 Improper process design. For example, the pouring system is poorly designed, the pouring speed is slow, the pressure head is too low, and the sand core air outlet scheme is improper.
1.17 Caused by improper operation. If the sand core exhaust is not considered when molding (core), the sand core outlet is not connected to the sand mold vent during molding, or the air outlet is not sealed, the molten iron is drilled in during pouring, and the local accumulation of paint is serious. As a result, the drying is insufficient, the core head is sealed by the paint, which affects the exhaust of the sand core, the pouring speed is too slow or the flow is cut off, so that the pouring system is not filled with metal. Sanzhuji combined with Self-hardening Sand Reclamation Systems to give the following preventive measures:
1.21 The amount of resin and curing agent added should meet the standard, and low-N resin with high furfuryl alcohol content and low urea content should be selected. According to different seasons, choose suitable curing agent varieties, and do everything possible to reduce the amount of binder added. After fully hardened, mold casting, especially under the condition of old sand recycling, the resin w(N) of steel castings should be controlled below 1%, and iron castings should also be controlled below 2%. Adding 2% to 0.3% of the resin mass silane in the resin before sand mixing can effectively improve the bonding strength, thereby reducing the amount of resin.
1.22 Use low-concentration volatile paint, after ignition and drying, there is a danger of residual alcohol in the mold. In short, the concentration of the paint is adjusted to "30" Baume or more. In addition, there may be deteriorating alcohol or alcohol with high water content in the volatile paint solvent, and the ignition and drying are not complete, resulting in residual water in the mold. This is an important cause of pores. Therefore, it is required that the water content of the solvent in the paint should not exceed 5 %. In addition to paying attention to the solvent quality of the coating, the mold should be spray-fired and dried before closing the box.
1.23 The time required for hardening after modeling varies with factors such as temperature, humidity, amount of curing agent, type of curing agent and other factors. Generally, it takes more than ten hours to complete the reaction. The unreacted mold has a higher gas volume. In winter, it should be noted that the sand core should be used the next day, and the appearance should be kept at least 6-8 hours before pouring. The mold release time is preferably not less than 20 minutes. difficult.
1.24 The ignition loss of organic reclaimed sand is proportional to the gas evolution of the mold and is closely related to gas defects. The ignition loss of cast iron should be less than 3%, and the content of fine powder should be controlled below 0.8%. In addition, reduce the sand-to-iron ratio "S/M" as much as possible. For this reason, special sand boxes can be used for castings that are repeatedly produced in batches to reduce the amount of sand eaten, or use steel plates to weld various shapes of hollow box frames or partitions, and place them in Fill the corner of the sand box or replace the molding sand in the box. You can also bury old sand blocks or foam blocks at a certain distance from the cavity. Generally, the "S/M" ratio should be controlled below 3.
1.25 The speed should be mastered during pouring, and the flow must not be interrupted in the middle. After pouring, the air should be ignited and so on.
1.26 The molding process should pay attention to the following conditions: the macroscopic air permeability of resin sand is very good, but the organic matter gasses faster during pouring, and it is too late to disperse the gas. In order to eliminate the cavity gas in time, the upper mold air outlet should be appropriately increased. The large plane and raised parts should be provided with air outlets (which can be drilled with an electric drill before closing the box, or air outlet pins can be placed on the wooden mold before modeling, (flat size 80 mm×10 mm, 80 mm×8 mm , Round size φ 20 mm). At the same time, the design of the pouring system should pay attention to the sealing of the straight, horizontal and in-runners, and pay attention to the shape of the straight runners to prevent turbulent flow of molten metal, entrain air, and burn furan resin To produce more gas, it is usually better to choose a closed gating system with F straight: F horizontal: F inner = 1.5: 1.25:1. In principle, it is safer and reasonable to use bottom injection. The gating system should be carefully painted, if the sprue is more Deep, replaced by ceramic tubes.
2. Mechanical sticky sand
2.1 The raw sand has a relatively coarse grain size and too concentrated distribution, resulting in large gaps in sand particles, and the molten metal easily penetrates into the sand mold, which is a mechanically sticky sand in the state of "iron sandwiched".
2.2 Caused by poor paint layer. The Baume concentration of the paint should be greater than "30". Double-layer coatings can be used for thick and large parts that are severely heated. The bottom coating has a certain penetration depth and the surface coating builds a certain coating thickness.
For cast iron, graphite coating is still a coating with excellent anti-sticking properties, but in order to improve the resistance to thermal shock, it is best to mix more than 20% zircon powder in the aggregate.
2.3 The compactness of the type (core) is not enough, so that the surface of the type (core) is loose, the stability is poor, and the resistance to mechanical sanding is poor.
2.4 When the proportion of new sand is high, the anti-sticking ability is worse than that of recycled sand.
2.5 Other factors that affect the surface stability of sand molds, such as the use of molding sand that exceeds the use time, and excessive sand temperature, all reduce the ability to resist mechanical sand sticking.
3. Veining
Due to the high SiO2 content of quartz sand, its expansion coefficient is high. The thermal expansion coefficient of new sand is different from the low thermal expansion coefficient of the coating. When pouring is heated, the thermal expansion of the sand mold will "crack" the coating, causing the molten iron to drill into the sand and solidify. Together, the formation of vein-like stripe defects, and often accompanied by mechanical sand, the veining is easier to remove. The solution is to recycle the old sand with a higher proportion, reduce the expansion coefficient, and make the thermal expansion coefficient of the sand close to the coating layer.
4. Cracks
The thermal cracking tendency of furan resin sand castings is greater than that of sodium silicate sand and clay sand castings. This may be due to the good rigidity of the resin sand, the large thermal expansion coefficient, and the slow cooling rate of the castings. The hot cracking tendency of steel castings is more serious. Castings with complex structures and large wall thickness differences and parts with large shrinkage resistance are more likely to appear cracks. In addition, the use of sulfonic acid curing agents can cause surface vulcanization and cause microcracks on the surface of the castings to become the characteristics of cracks. Sanzhuji combined with Self-hardening Sand Reclamation Systems to give the following preventive measures:
4.1 Improve the type (core) concession, such as adding 2% to 3% wood powder and other disintegrating agents in the molding sand.
4.2 When modeling (core), embed expanded polystyrene blocks in the back sand, try to reduce the amount of sand eaten by the thinner (core), and make hollow cores.
4.3 The use of zirconium sand and chromite sand instead of silica sand in the parts prone to cracks can significantly reduce the cracks, because these two materials have low thermal expansion coefficients.
4.4 Change the gating system to make the castings solidify at the same time.
4.5 Under allowable conditions, make reasonable modifications to the casting structure.
4.6 Appropriately reduce the pouring temperature, which has a significant effect on reducing cracks.
4.7 Install anti-cracking ribs at the place where cracks are prone to occur.
4.8 Under special circumstances, use phosphoric acid instead of sulfuric acid curing agent.
4.9 Reasonable use of cold iron and other chilling measures.
5. Slag inclusion
It is mainly the slag produced by the reaction between the molten metal and the binder, and the scars caused by the long casting time (core) top being roasted by the high temperature molten metal. The former mostly occurs in the part of the early molten iron that begins to flow into the mold, and is often accompanied by stomatal defects, while the latter often occurs in the upper and upper side of the cavity. Sanzhuji combined with Self-hardening Sand Reclamation Systems to give the following preventive measures:
5.11 When designing the pouring system, follow the principle of "fast, stable, closed, bottom pouring, ensure head pressure, and handle molten iron", and set up an overflow riser on the top surface of the castings with conditions to prevent cold and dirty castings. The molten iron is introduced into the overflow riser and overflows.
5.12 Use paint with high strength, good heat resistance and low gas evolution.
5.13 When pouring large flat plate castings, it is best to use inclined pouring, and set an appropriate number of overflow risers on the opposite side of the sprue cup, which can effectively prevent slag inclusion defects on the large surface.
Insufficient hardness
Furan resin sand casting mold has poor thermal conductivity and slow cooling and solidification rate of molten metal, which will lead to low hardness of adze parts. It should be noted that the hardness of the surface layer (3 mm) of resin sand castings is 10-15 HB lower than that of the inside. Therefore, when measuring the hardness, the surface layer (3 mm) needs to be removed before the correct hardness value can be measured. Sanzhuji combined with Self-hardening Sand Reclamation Systems to give the following preventive measures:
5.21 Appropriately reduce the CE value of molten iron to prevent the production of ferrite.
5.22 Add a small amount of stable pearlite elements, such as: Cr, Cu.
5.23 Improve the cooling rate of castings, such as using suitable cold iron and using tellurium powder coating.
5.24 Appropriately lower the pouring temperature.
5.25 Properly advance the unpacking time.
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.
