Defects and solutions of lost-wax castings
Lost wax castings may have wrinkles, carbon black, assimilation and slag inclusions, pores, sand sticking and other problems. The following solutions are proposed.
1. Wrinkle defect
The wrinkle defect is because polystyrene foam plastics are pyrolyzed by high-temperature metal to form a large amount of carbon that cannot be expelled from the mold cavity in time and remains on the grinding ball. It often appears at the cold end of the metal liquid or the liquid flow, usually on the top of the grinding ball.
Solution:
1) Select suitable materials when foaming the grinding ball. Low-density casting special foam EPS vaporizes quickly and leaves less residue.
2) Increase the pouring temperature and pouring rate. Increasing the pouring temperature of the grinding ball by 50%-80% can accelerate gasification. Increasing the pouring rate can reduce heat loss.
3) Improve the vacuum degree. It is beneficial to exhaust smoke and gas, improve the gasification conditions, and make the residue and gasification products escape quickly.
4) Improve the air permeability of the casting. Use coarse sand and paint with good air permeability.
2. Carbon black defect
The indirect cause of carbon black is the high carbon content in ductile iron grinding balls. The C% of ordinary grinding balls is 3.3%~3.8%. This defect will occur.
Solution:
Under the same conditions for casting and molding, the process should adopt coatings with good wettability and air permeability, increase the amount of air extraction during casting and use expandable polymethacrylate resin beads EPMMA raw materials, strictly control the amount of shape binder, which can effectively avoid the formation of carbon black.
3. Assimilation and slag inclusion
The formation of assimilation and slag inclusion defects is due to:
1) The residual magnesium content in ductile iron is high, which increases the formation temperature of the oxide film on the surface of the ductile iron liquid, and the oxide film is formed at a higher temperature. During the casting process, the molten iron is secondary oxidized to form slag inclusions.
2) The original iron liquid contains high sulfur, and many sulfides (MgS, Ce3S4, CeS) are generated, which promotes assimilation.
3) The pouring temperature is low, which is not conducive to the floating and removal of slag inclusions.
Solution:
1. Ensure that the pouring temperature is >1300℃, which is conducive to the accumulation and floating of assimilates and is convenient for removal.
2. Under the condition of ensuring spheroidization, reduce the amount of residual magnesium as much as possible and reduce the sulfur content of the original iron liquid.
3. Use rare earth magnesium alloy as a spheroidizer to control the rare earth to reduce the conjunctival temperature of the iron liquid oxide film.
Defects of lost foam castings and solutions
4. Add a filter mesh to collect slag bags in the pouring channel during pouring.
4. Porosity defects
① During the gas generation process of the lost foam cracking, the gas enters the casting and generates pores. During the filling process, turbulence occurs, or in the case of top pouring or side pouring, part of the sample is surrounded by the metal liquid and then cracked. The generated gas cannot be discharged from the metal, and pores will occur. Such pores are large and numerous and accompanied by carbon black.
Prevention and control measures:
Improve the process to replace layer by layer during the pouring and filling process, without turbulence, increase the pouring temperature; improve the permeability of the coating and sand mold.
②Poor drying of the sample and coating layer causes pores. The sample contains water, the coating is poorly dried or the foaming agent content is too high. A large amount of gas is generated during pouring and it is very easy to generate back-spray. In this case, pores are most likely to occur; if the coating is not dried enough and the water content is too high, it is very easy to form invasive pores.
Prevention and control measures:
The sample must be dried; the coating must be dried.
③ Excessive amount of adhesive in the shape causes pores. The amount of adhesive used for the combination bonding of the shapes is too large. When the liquid metal is poured and filled, a large amount of gas is generated locally, which can cause the molten metal to churn during the filling process. At this time, the gas cannot be discharged in time, and the casting will produce pores.
Prevention and control measures:
Use low-emission shape adhesive; under the condition of ensuring strong adhesion, the less adhesive is used, the better.
④ Air is involved during pouring to form pores. During the pouring process of the evaporative die casting, the sprue cannot be filled, and air will be involved. If the involved air cannot be discharged in time, the casting will produce pores.
Prevention and control measures:
Use a closed pouring system. During pouring, keep a certain amount of molten metal in the cross-mouth cup to ensure that the sprue is filled.
5. Sand sticking defect
There are two reasons for the occurrence of sand sticking defect in castings:
a. The coating falls off or cracks, and the molten metal penetrates into the molding sand, which is easy to cause mechanical sand sticking; when the coating is not matched with the molten metal, and there are small sand particles in the dry sand, chemical sand sticking will occur. b. The influence of the negative pressure during pouring on the flow ability of the molten metal. The greater the negative pressure, the better the flow of the molten metal, and the easier it is to cause sand sticking.
Solution:
For the sand sticking defect of expendable mold castings, it can generally appear in various parts of the casting under different conditions: in the absence of negative pressure, sand sticking mostly appears at the bottom or side of the casting, and the hot zone of the casting and the area where the molding sand is not easy to compact; in negative pressure casting, it can appear on all sides, especially at the corners of the casting and the overheated area during the pouring of the string casting.