Solution for lost foam casting porosity

Process measures to control stomatal defects

1. Choose suitable model materials Copolymer foam is used as the model material. Because the copolymer is decomposed by a zipper, the degree of one-time gasification is high, the liquid phase ratio is small, small molecule gases are easy to escape the coating. The density of the white mold should be as small as possible under the premise of ensuring the strength smoothness to reduce the total amount of gas. The pouring system adopts a hollow sprue an additional filter to mechanically block the slag of the liquid alloy absorb the comb flow to make the filling More stable.

 2. The use of the pouring system the insulation riser The opening of the inner runner should be conducive to the smooth, stable 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, the relevant parts are equipped with conformal sand cold iron to achieve The sequential solidification of the castings floats the gas, slag quality gasification residues in the alloy to the riser, reducing the chance of pores, slag holes shrinkage holes. Practice has proved that the feeding effect of pulp riser floating bead riser is better than that of foam spherical riser, 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. The formulated coating has good high low temperature strength, wear resistance, easy painting. No cracking, large exhaust capacity, good air permeability, uniform sintering, easy to peel off when out of the box, no wetting chemical reaction with alloy, general castings are painted twice, the coating thickness is 1-1.2mm. The quality management of lost foam coatings is critical. In mass production, periodically check the air permeability of the slag coating adjust the aggregate particle size in time. Since the binder suspending agent of the paint contain organic substances, special attention should be paid to the fermentation enzyme changes of the paint in summer autumn. Qualified castings cannot be separated the best paint, the quality of the paint cannot be controlled lightly.

4.Pouring temperature should be appropriate Due to 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 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 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, the filling fluidity will be reduced. In addition, the solidification speed is fast, the gas slag in the alloy will have time to float. Collected in the riser, the probability of generating pores increases. The pouring temperature should match the alloy being smelted to avoid other casting defects caused by high temperature.

5. Final deoxidation of molten steel sedation in the ladle Lost foam follows the principle of high temperature low temperature quick pouring. High temperature tapping is to leave a suitable time for the sedation in the ladle. The final deoxidation slag removal of the molten steel is a prerequisite for reducing three-hole defects. Before tapping, take a sample to observe the shrinkage of the molten steel. If the sample does shrink, it is allowed to leave the furnace. The stove should be kept clean hygienic.

6. Reasonable pouring position The best pouring position for castings is to obtain good gasification exhaust conditions during filling, which is convenient for gasification slag removal. Choose vertical pouring oblique pouring, that is, follow the principle of maximizing side surface area, the pouring system should have slag blocking Gas gathering feeding function.

7. Reasonable pouring process negative pressure degree The pouring process of the lost foam is based on the principle of filling the pouring cup to close the straight gate. If the pouring speed is suddenly fast slow, the filling will cause violent turbulence violent boiling in the cavity, the foam that will be vaporized in the future will be entrapped in the alloy Pores are formed in the liquid, in particular, it is allowed to expose the sprue to cause slag gas to invade. The ideal pouring speed is that the filling speed of the liquid alloy is equal to slightly lower than the gasification speed of the model. The main points of the process are as follows:

(1) Use a hollow sprue add a filter use a dedicated person to strengthen the slag retention. …

 (2) Pouring speed, especially at the moment when the vehicle is lifted stopped, strive to balance stop the flow. The pouring process is always full of the sprue cup, the pouring enters the end of the pouring period to slowly close the bag, so that the slag, gas gasification residue are sufficient The time floats into the riser.

(3) There must be a wealth of molten iron in the molten iron ladle, otherwise the molten iron containing slag must be poured into the casting. Shaking the ladle is as good as the bottom leaking ladle. If funds allow, using an automatic pouring machine will greatly reduce the defects caused by manual pouring . …

(4) Determine a reasonable vacuum degree. Too high a vacuum aggravates the permeability of the liquid alloy the sticky sand, causes the wall effect, which is conducive to the adsorption of the liquid phase foam by the coating to generate many pores. Proper negative pressure is a guarantee for exhaust 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 geometry of the casting, shorten the solidification time, prevent pores Diffusion growth, because after the pouring is stopped, the decomposition of the liquid phase coating at the interface between the metal the coating is still going on. 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 the front of molten steel to the foam gap. The liquid phase substance coating are first wetted then vaporized: hydrogen nitrogen in molten steel The negative pressure cannot be raised during the pouring process.

(5) Foam mold. The coating layer must be thoroughly dried the molding sand must be dry. Take the model with you, reduce the exposure time in the air to prevent the coating absorbing moisture. Pay special attention to the rainy season the high humidity areas in the south coastal areas. Both visible invisible moisture are negative for lost foam casting If the drying is complete, pores, sand sticking, deformation will be reflected on the same batch of castings at the same time. To In the mold bonding process, special hot melt adhesive cold glue should be used. The less the amount is, the better when the adhesion is guaranteed. If possible, enterprises recommend mechanical bonding try to avoid using white latex that vaporizes slowly.