The root cause preventive measures of slag inclusion defects in lost foam casting

Methods  measures to reduce  overcome the disadvantages of slag inclusion

Difficulties in entering sand  shortcomings of slag inclusion are a difficulty in lost foam casting production, especially for mass-produced auto parts processing. At this stage, the production  manufacture of lost foam casting are mainly successful in three products, namely anti-wear parts, pipe fittings  box castings, all of which are unprocessed  rarely processed castings. For castings with many processing surfaces  high requirements, the disadvantage of slag inclusion is a key difficulty that needs to be solved. Taking measures  the following aspects can reduce  overcome the disadvantages of slag inclusion:

1. Paint

The effects of lost foam coatings are:

1) Improve the surface smoothness of castings, reduce the roughness of castings by 2-3 levels, achieve Ra=3.2-6.3μm,  improve the surface quality  performance indicators of castings.

2) Reduce  avoid the disadvantages of sand sticking  sand holes.

3) It is beneficial for sand removal  falling sand.

4) The liquid  vapor melted by the lost foam during pouring are smoothly discharged into the casting sand according to the architectural coating layer,  the metal material liquid is prevented  penetrating into the sand mold, so as to prevent the casting  causing pores, metal material penetration  carbon defects.

5) Improve the compressive strength  rigidity of the shape, avoid deformation  destruction during transportation  sand filling vibration modeling design, which is beneficial to improve the dimensional accuracy  yield of castings. Its main function is to improve the compressive strength  rigidity of the plastic foam,  to avoid damage  deformation during the design of sand filling vibration during transportation  loading. When pouring, the architectural coating layer separates the metal material  the dry sand, to prevent the metal material  penetrating into the dry sand,  also to prevent the dry sand  entering the metal material. When plastic foam is pyrolyzed, a large amount of pyrolysis products (vapor  liquid) are also discharged into dry sand according to the architectural coating layer  discharged to mold.

Architectural coatings for lost foam casting are required to have compressive strength, air permeability, refractoriness, thermal insulation, explosion resistance, crack resistance, rapid heat resistance, moisture absorption, cleaning, coating, suspension,  non-flowing. To avoid the shortcomings of slag inclusion, it requires architectural coatings to have high compressive strength  fire resistance. It is required that the architectural coating layer coated on the surface of the white mold does  cause cracks  cracks during the drying  transportation process, that is, the architectural coating should have sufficient room temperature compressive strength;  during the pouring process, the high-temperature metal material is washed for a long time Under the effect, the architectural coating layer should  fall off  cause cracks, that is, it has high high temperature compressive strength.

When the liquid metal enters the mold, the direct gate is tightly closed, the architectural coating layer on the surface of the casting  the casting system does  fall off, does  cause cracks  cracks, which are the first conditions to avoid the disadvantages of slag inclusion. If the runner is  tightly sealed, the architectural coating layer As a result of shedding, cracking  cracking, a large amount of sand, architectural coatings  inclusions will enter the metal material to form the disadvantage of slag inclusion. Compressive strength  air permeability are two important properties of architectural coatings. Sometimes the architectural coatings used in the pouring system are required to have higher fire-resistant compressive strength than the architectural coatings for castings to resist the long-term erosion effect of high-temperature metal materials. Without falling off  cracking.

2. Packing operation

When loading the vehicle, the architectural coating layer (model + pouring system) of the surface layer is  allowed to have any shedding, cracks  cracks, especially at the junction of the sprue  the runner, the junction of the cross runner  the inner runner, If there is a crack  a bad connection between the inner gate  the mold, sand will enter. This requires high compressive strength at joints, thick architectural coatings,  sufficient rigidity for the pouring system,  if necessary,

3. Pouring head, temperature  time

The higher the pressure head during casting, the greater the flushing of the casting system  the casting shape,  the greater the probability of sand ingress due to damage to the paint. The pressure heads for castings of different sizes should be different. To choose a ladle with a suitable volume, the ladle needs to reduce the casting height as much as possible,  the ladle mouth is as close as possible to the pouring cup,  it is necessary to prevent the use of a large ladle to pour small jobs.

The higher the casting temperature, the higher the paint performance index standard,  the easier it is to cause some defects such as sand sticking  slag inclusion. The appropriate casting temperature should be selected. For gray iron castings, the furnace temperature can be around 1460℃,  the casting temperature can be 1360-1420℃; the furnace temperature of ductile iron castings can be around 1500℃,  the casting temperature is 1380-1450℃; the casting temperature of steel castings is 1450-1700 ℃. The casting time of a box of iron castings requiring 300-500 kilograms of molten iron can be adjusted within 10-20 seconds.

4. Negative pressure The casting operation process of lost foam casting is usually carried out under the premise of vacuum. The effect of negative pressure is to compact the dry sand, accelerate the exhaust,  improve the filling capacity. Under the premise of vacuum sealing, the casting improves the environment. The magnitude of the negative pressure has many adverse effects on the quality of castings. Excessive negative pressure will increase the probability of absorbing dry sand  inclusions when the metal material fluid ruptures  internal cracks,  also causes slag inclusions in castings. Some defects grow. Too fast filling speed increases the ability of metal materials to flush sprues  castings,  it is easy to cause paint to fall into the metal material,  it is also very easy to damage the paint layer  cause sand. For iron castings, the suitable negative pressure is usually -0.02-0.04MPa.

5. Set slag collecting gate, slag blocking  slag skimming facilities Setting slag-collecting gates  selection of slag-retaining  slag-skimming countermeasures on the sprue  castings is beneficial to improve some defects of sand  slag inclusion.

6. molding sand Type sand grains that are too coarse  too fine will adversely affect the defects of slag inclusion. If the particle size distribution is too coarse, some defects of slag inclusion will increase. Iron castings usually use dry quartz sand (washed sand) with a particle size distribution of 30/70.

7.  hot metal treatment technology

The entire forming operation of lost foam castings must take into account the treatment of molten iron, which is also one of the key technologies of lost foam casting. The whole process including molten iron smelting, overheating,  pouring into the mold must consider the purification problem,  filtration technology is one of them.

in conclusion

1. Some defects of slag inclusion are caused by sand, paint  inclusions entering the casting with the liquid metal during the casting operation. They are common in the production of lost foam casting,  serious adverse effects affect the quality  produce.

2. In the whole process of lost foam casting production, the falling, cracking  internal cracking of the paint layer, as well as the lack of tightness of the sprue  the casting system, are the main reasons for the defects of sand  slag inclusion.

3. Improve paint strength, refractoriness  other performance indicators, careful packing operation, selection of correct negative pressure casting process parameters, slag collection, slag slag skimming technology, hot metal filtration, purification technology  other countermeasures can reduce  overcome slag inclusion Some defects.