Causes of porosity in lost foam casting：
1. The gasification and decomposition of the foam model produces a large amount of gas and residues that cannot be discharged in time. The foam and coating layer are filled with dry sand. The drying is not good. Under the high temperature of the liquid alloy, a large amount of hydrogen and oxygen are cracked to invade the casting. The main cause of stoma.
2. Poor deoxidation of molten steel, dirty slag removal in the furnace base, furnace and ladle, too short sedation time, poor slag retention during the pouring process, and unreasonable pouring process resulting in slag holes.
3. Due to the unreasonable design of the gating system, the filling speed of the molten metal is greater than the foam gasification retreat and the gas discharge speed, causing the filling front to entrap the gasification residue in the metal liquid and vaporize again to form smoke-black decomposition pores on the inner wall .
4. The pouring temperature is low, and the molten metal at the front of the filling can not fully vaporize the foam, and the undecomposed residual material has no time to float to the riser and solidify to form pores in the casting.
5. The location of the inner runner is unreasonable, and a dead corner area is formed during filling. Due to the gas pressure in the cavity, gasification residues accumulate in the dead corner to form pores. The cross-sectional area of the inner runner is too large, making the filling speed faster than Foam gasification retreat speed, swallow foam, decomposition and gasification inside the alloy, but the gas cannot be discharged to form pores.
6. The air permeability of the coating is poor or the negative pressure is insufficient, and the air permeability of the filling sand is poor, and the gas and residues in the cavity cannot be discharged in time, and pores are formed under the filling pressure.
7. The pouring speed is too slow to fill the sprue cup, exposing the sprue, entraining air, sucking in the slag, and forming enveloping air holes and slag holes.
8. The volume of the sprue cup is too small, and the molten metal forms a vortex, which intrudes into the air to form air holes.
9. The connection between the sprue cup and the sprue and the pouring system is not well sealed, especially the connection between the sprue and the sprue cup is not well sealed, and it is easy to form sand and pores under the action of negative pressure. This phenomenon can be calculated and explained by Bernoulli equation.
10. The particle size of the molding sand is too fine, the dust content is high, the air permeability is poor, and the negative pressure is distorted due to the blockage of the negative pressure pipe. The negative pressure value around the cavity is far lower than the indicated negative pressure, and the vapor cannot be discharged from the coating in time. Form pores or wrinkled skin.
Sanzhuji made the following summary based on the experience of investment sand recycling:

1. Choose a suitable model material and use copolyfoam as the model material. Because the copolymer is zipper-type decomposition, the degree of one-time gasification is high, and the liquid phase ratio is small. Small molecule gases are easy to overflow from the coating. The density of the white mold is as small as possible under the premise of ensuring the strength and smoothness to reduce the total amount of gas generation. The pouring system adopts a hollow sprue and an additional filter to mechanically block the slag of the liquid alloy and absorb the comb flow to make the filling More stable.
2. The use of the gating system and the insulation riser The opening of the inner runner should be conducive to the smooth, stable and rapid filling of the alloy, without forming a dead corner area. An atmospheric pressure insulation riser is set on the top of the casting to integrate slag feeding and shrinking. The relevant parts are equipped with conformal sand-insulating chilled iron to realize the sequential solidification of castings, and float the gas, slag quality and gasification residues in the alloy to the riser part, reducing the probability of pores, slag holes and shrinkage holes. Practice has proved that the feeding effect of pulp riser and floating bead riser is better than that of foam spherical riser, and the riser covering agent can be used comprehensively.
3. Improve the air permeability of the coating. The particle size of the refractory aggregate should be appropriate. The 70-100 mesh sand-like substance in the aggregate should contain 10-15%. The composite suspending agent binder is used to prepare the coating with good high and low temperature strength and resistance. Abrasion, easy to paint, no cracking, large exhaust ability, good air permeability, uniform sintering, easy to peel off out of the box, no wetting and chemical reaction with alloy, general castings are painted twice, coating thickness is 1-1.2 mm.
The quality management of lost foam coatings is critical. In mass production, periodically check the air permeability of the slag coating and adjust the aggregate particle size in time. Since the binder and suspending agent of the paint contain organic substances, special attention should be paid to the fermentation and enzyme changes of the paint in summer and autumn. Qualified castings cannot be separated from the best paint, and the quality of the paint cannot be controlled lightly.
4. The pouring temperature should be suitable. Because of the exothermic gasification foam during the filling process of the lost foam, it is recommended that the pouring temperature of the black alloy is 30-50℃ higher than that of the sand casting. The thermal effect of the molten metal and the foam is restricted by the pouring temperature. The pouring temperature is suitable for sufficient pyrolysis. The pyrolysis products of the appearance are mainly in the gaseous state of small molecules, which are easily discharged from the cavity under the action of the negative pressure field. When the pouring temperature is low, the appearance Insufficient pyrolysis, liquid residues will block the coating layer, the discharge of pyrolysis gas will be blocked, the back pressure is formed in the cavity, and the filling fluidity will be reduced. In addition, the solidification rate is fast, and the gas slag in the alloy is too late to float. Collected in the riser, the probability of generating pores increases. The pouring temperature must match the smelted alloy to avoid other casting defects caused by high temperature.
5. The final deoxidation of molten steel and the sedation of the lost foam in the ladle follow the principle of high temperature and low temperature quick pouring. High temperature tapping is to leave a suitable time for the sedation in the ladle. The final deoxidation and slag removal of the molten steel is a prerequisite for reducing the three-hole defect. Before tapping, take a sample to observe the shrinkage of the molten steel. If the sample does not shrink, it is not allowed to leave the furnace. The stove should be kept clean and hygienic.
6. Reasonable pouring position The best pouring position for castings is to obtain good gasification conditions and exhaust conditions during filling, which is convenient for gasification and slag removal. Choose vertical pouring or inclined pouring, that is, follow the principle of maximizing side surface area. The gating system should have the functions of blocking slag, collecting gas and feeding.
7. Reasonable pouring process and negative pressure lost foam pouring process is based on the principle of filling the pouring cup to close the straight gate. If the pouring speed is suddenly fast or slow, the filling will cause violent disorder and violent boiling in the cavity. In the future, the foam that will not vaporize is entrapped in the alloy liquid to form pores. In particular, it is not allowed to expose the sprue to invade the slag gas. The ideal pouring speed is that the filling speed of the liquid alloy is equal to or slightly lower than the gasification speed of the model. The main points of the process are as follows:
7.1 Use a hollow sprue and install a filter or use a dedicated person to strengthen the slag retention.
7.2 Pouring speed, especially at the moment when the vehicle is lifted and stopped, strive to balance and stop flow. The pouring process is always full of the sprue cup, and the pouring enters the end of the pouring period to slowly close the bag, so that the slag, gas and gasification residue have sufficient time Float into the riser.
7.3 There must be rich molten iron in the molten iron ladle, or the molten molten iron containing slag must be poured into the casting. Shaking the ladle is not as good as the bottom leaking ladle. If funds permit, using an automatic pouring machine will greatly reduce the defects caused by manual pouring.
7.4 Determine a reasonable vacuum degree. Too high a vacuum aggravates the permeability of the liquid alloy and the sticky sand, and causes the wall effect, which is not conducive to the adsorption of the liquid phase foam by the coating to generate many pores. Appropriate negative pressure is a guarantee for exhaust and a measure to prevent sand sticking. A reasonable negative pressure field is established during the pouring process to make the amount of air extraction larger than the amount of air generated. Fasten dry sand to ensure the geometric size of the casting, shorten the solidification time, and prevent pores. Diffusion and growth, because the decomposition of the liquid phase coating at the interface between the metal and the coating is still going on after the pouring is stopped. Negative pressure is the main control factor for lost foam casting. In recent years, many companies mistakenly believe that the Pressure can discharge the gas in molten steel (especially nitrogen-containing steel), negative pressure exhaust can only discharge the gas from the front of molten steel to the bubble gap. The liquid phase substance and coating are first wetted and then gasified: hydrogen nitrogen in molten steel The negative pressure cannot be raised during the pouring process.
7.5 Foam mold. The coating layer must be thoroughly dried and the molding sand must be dry. Take the model with you, reduce the exposure time in the air to prevent the coating from absorbing moisture. Pay special attention to the rainy season and the high humidity areas in the south and coastal areas. Visible and invisible moisture have negative effects on lost foam casting. If the drying is not complete, pores, sand sticking, and deformation will be reflected on the same batch of castings at the same time.
7.6 In the process of mold bonding, special hot melt adhesive or cold glue should be used. The less the amount is, the better if it is firmly bonded. Enterprises with conditions recommend using mechanical bonding and try to avoid using white latex that vaporizes slowly.

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