Lost foam casting defects: How to deal with sand intake problem

Slag imperfection of lost foam casting

Slag inclusion defect refers to the defect caused by dry sand, paint other inclusions accompanied by molten iron entering the casting during pouring.On the surface layer of the machined casting, white black-gray inclusion spots can be seen, which are distributed singly in pieces. White is quartz sand particles, while black-gray is residue other inclusion after pyrolysis of slag, paint, foam model.This defect is commonly known as "sand entrainment" "slag entrainment". It is a very common defect in epc production.Nearly vanishing mold casting plants are common difficult to eradicate completely.Only in each process to take a variety of measures careful operation in order to reduce "slag" to a very low, achieve more satisfactory results.

According to the surface condition of the casting pouring system, it is possible to determine whether there is any defect of sand inlet slag entrainment before the cooling punching box of the lost die casting is cleaned.If the surface joint of the sprue cup, runner, runner, inner runner gate the surface of their castings are heavily cracked, it is almost certain that there are slag inclusion sand entry defects.If the runner bar runner tendon is broken, white spots can be seen on the fracture, a circle of white spots will be produced when the fracture is serious.Such castings, especially plate-like pancake castings, will have white black gray spots on the machining surface.If the operation procedure is strictly controlled, the casting quality order completion schedule will be seriously affected.Causes of slag inclusion sand intake defects

It has been proved by long term observation in production practice that all parts the runner cup, straight runner, cross runner, inner runner to the casting are likely to cause sand feeding, especially the joint part between the casting system the casting.In the whole production process, the main causes of slag inclusion sand entry defects are the shedding cracking of the coating on the white mold surface of the casting system, the shedding cracking of the coating on the white mold joint part, the shedding cracking of the coating on the white mold surface of the foam plastic, the untight closure of the straight gate.Secondly, the selection of process parameters, such as the size of pouring net head, pouring temperature, negative pressure degree, dry sand grain size, as well as the model transport process packing operation, have a great impact on the slag inclusion sand feeding defects of casting.Only by taking systematic measures careful operation in these links can the slag defects of casting be reduced basically eliminated, high quality castings be obtained.It is a systematic project to overcome the defect of slag inclusion.

The method measure to reduce overcome the slag inclusion defect is a difficult problem in the production of lost mold casting.At present, there are three types of products that have been successfully produced in epc production, namely, anti-wear parts, pipe parts box castings, which are all castings with little no processing.For castings with many machining surfaces high requirements, slag inclusion defect is a key problem to be solved.Our experience is to take measures the following aspects to reduce eliminate slag inclusion defects:

1. The functions of lost foam coating are as follows: 1) It can improve the surface finish of casting, reduce the roughness of casting by 2-3 grades, improve the surface quality performance of casting.2) Reduce prevent sand adhesion sand hole defects.3), is conducive to sand clearing falling.4) The liquid gas that have disappeared during pouring shall be discharged smoothly into the casting sand through the coating layer, the metal liquid shall be prevented infiltrating into the sand mold, the casting shall be prevented producing pores, metal penetration carbon defects.5) To improve the shape strength stiffness to prevent deformation damage during transportation sand filling vibration modeling, which is conducive to improving the dimensional accuracy yield of casting.The coating for lost mold casting is required to have a series of properties, such as strength, permeability, fire resistance, thermal insulation, crack resistance, quick quick heat resistance, moisture absorption, clarity, coat hanging, suspension, flowing, etc., to prevent slag defects first requires the coating to have high strength fire resistance.It is required that the coating layer hanging on the surface of the white mold shall crack crack during drying transportation, that is, the coating shall have sufficient room temperature strength; in the pouring process, in the high temperature of the metal under the action of a long time scour coating layer is off, do produce crack cracking, that is, there is a high temperature strength.In liquid metal into the casting sprue closed tightly, casting coating layer on the surface of the pouring system don't fall off, do produce crack craze is the first condition of preventing slag defects, if the runner seal is tight, coating layer falls off, crack craze, a large number of sand, coating inclusion defects, would be the formation of slag metal.Strength air permeability are two important properties of the coating, sometimes the casting system coating is required to have higher fire resistance than the casting coating, in order to resist the erosion of high temperature metal for a long time without falling off cracking.Operators must ensure the uniformity of the coating in the process of painting.

2.During packing, the coating layer on the surface of the pattern group (model + gating system) is allowed to fall off, crack crack, especially at the junction of the straight runner the cross runner, the junction of the cross runner the inner runner, the junction of the inner runner the casting mold. As long as there is loosening, crack unsound connection, sand may enter.This requires joint strength to be high, coating to be relatively thick, casting system to have enough rigidity, when necessary, need to set tensile reinforcement reinforcement sleeve.Shape group placed on the bottom of the sand box sand should be stable, do allow to be suspended when the start of sand molding, in order to avoid shock crack coating layer.Do add sand vigorously to the appearance, should first add sand with a hose, vibration real-time reuse the rain shower equipment sand.Start vibration modeling vibration to be slight, amplitude to be small, such as dry sand buried appearance then large vibration.In the shaking molding gating system, especially when the straight runner is allowed to bend, so as to break the coating layer, to close the straight gate to avoid sand.The entire packing, sanding, vibration, molding operation process must be very careful, must ensure that before pouring the pattern group coating layer does have any shedding, cracking cracks.The spout cup should be cleaned again before pouring to ensure that there is no floating sand, dust debris. 

3. Casting head, temperature time During casting, the higher the head is, the greater the scouring of the casting system mold, the greater the possibility of sand-feeding caused by the washing out of the paint. The casting head of different sizes should be different.To choose the appropriate capacity of the ladle, ladle to reduce the pouring height as far as possible, the nozzle as close to the ladle cup, should avoid using large ladle pouring small work.The higher the casting temperature, the higher the performance requirements on the coating, the more likely to produce sand slag other defects, should choose the appropriate casting temperature.For gray cast iron, the temperature of discharging can be about 1480℃, the pouring temperature is 1380-1420℃;Ductile iron casting temperature should be above 1500℃, casting temperature 1420-1450℃;The casting temperature of steel castings is 1480-1560℃.The casting time of a box of cast iron with molten iron of 300-500 kg can be controlled in 10-20 seconds so.

4. The casting process of epc under negative pressure is generally carried out under the vacuum condition. The effect of negative pressure is to tighten the dry sand, accelerate the exhaust improve the filling capacity, the casting under the vacuum sealing condition improves the working environment.The negative pressure degree has a great influence on casting quality. The excessive negative pressure degree increases the possibility of drawing in dry sand inclusions when metal liquid flows through cracks cracks, also increases the sand adhesion defects of castings.Too fast filling speed increases the flushing ability of the metal to the runner mold, which makes the paint fall off enter the metal easily, also damages the paint layer easily, causing sand to enter.For cast iron, the appropriate negative pressure is generally 0.025-0.04mpa.

5. Setting slag, skimming slag collecting riser Setting slag, skimming riser on casting system setting slag, skimming casting setting adopting slag skimming measures are helpful to improve the defects of sand inlet slag inclusion.

6. The grain size of molding sand is too coarse too fine, which affects the occurrence of slag inclusion sand bonding defects. The grain size is too coarse, which increases the slag inclusion defects of molding sand.The cast iron is generally made of 30/50 dry quartz sand (washed sand).

7. The problem of molten iron purification should be considered in the whole forming process of lost foam casting, which is one of the key technologies of lost foam casting.The whole process of molten iron smelting, overheating pouring into the mold should be considered. Filtration technology is one of them.