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Casting surface appears "orange peel" prevention and control measures:
Castings "orange peel" is a recurring casting defects in production, it has a greater impact on the quality of castings, defects appear in the casting fat parts, hot joints and near the internal sprue and the concentration of heat and slow cooling parts. Casting surface with slightly convex small round spots, "eye ring", these rough surfaces, looks like "orange peel" spots, recurring in a variety of castings, sometimes the whole batch of castings have, the number of castings in each of a few, as much as the entire plane; small round spots, some larger, some look like "orange peel" spots. According to xinyuanzhu group statistics, 15% of the scrap is "orange peel" defects caused by, and the carbon steel parts produce orange peel defects The chances are more.
Analysis of the causes of orange peel defectsThe most fundamental reason for "orange peel" is the accumulation of paint surface and insufficient hardening. After baking, the surface of the shell forms yellow or yellow-green glass body, which reacts with the steel and forms silicate tumor adhering to the surface of the casting after pouring. Simply extend the hardening time, will not help to finally solve the "orange peel" problem. xinyuanzhu group through practice, there are several reasons.
1. The influence of raw materials
The powder-liquid ratio of water glass coating is low, and the powder is not uniformly distributed. The higher the modulus of water glass, the greater the density, the lower the powder-liquid ratio of the coating, the more uneven the distribution of powder, but also the most difficult to fully harden.
1.1 The influence of water glass: the modulus of water glass, density and the number of impurities have a great impact on the fluidity of the coating. With the increase of modulus, the proportion of water glass sub-gel particles increases, its viscosity will increase, the rheology of the coating deteriorates, and when the module is hung in the surface layer is very easy to cause local accumulation of paint. The impact of inconsistent water glass parameters on the performance of the coating is great, which is easy to be ignored. The inconsistency of parameters is manifested in two aspects.
1.11 inconsistency of modulus, just into the factory of water glass only after a long period of static diffusion (dispersion) to make the same batch of modulus convergence, to achieve a stable state of dispersion; this process takes more than a week, if used in a hurry is not possible to obtain the ideal coating rheological properties.
1.12 The inconsistency of the solution density, before matching the coating usually has to adjust the density of the water glass solution, should pay special attention to the density measured immediately after adding water and stirring is not true, because the process of liquid dispersion and stabilization has not been completed, there is a certain error with the desired density, according to which the preparation of the coating, its viscosity and fluidity have errors.
1.2 The influence of refractory powder: the distribution and shape of refractory powder particles have a greater impact on the rheology of the coating, bimodal powder coating has a better rheology is recognized; but even if the particle size distribution is basically the same bimodal powder, when the shape of refractory powder particles are polygonal, sharp-angled and lamellar powder formulated coating, in the powder-liquid ratio and water glass modulus under the same conditions of its rheology will also have a great difference. When the shape of the powder is closer to the flake, its specific surface area is also larger, the friction and force between the particles increases, and the viscosity of the coating will be greater than that of the polygonal powder.
1.3 The comprehensive influence of water glass density and powder-liquid ratio: the change of water glass density and powder-liquid ratio on the surface coating rheology is very intuitive, water glass density and powder-liquid ratio is too large when the coating viscosity increases, the rheology becomes poor, the coating becomes thicker will cause the local accumulation of coating on the surface of the shell, the shell hardening is bad eventually lead to "orange peel". problem.
2. The influence of the process
2.1 Insufficient air-drying of surface layer: air-drying of surface layer is the process of re-uniformization of coating, at the same time, it is also the process of water-glass dehydration and curing, such as air-drying time is too short, the surface layer coating is not uniformly distributed on the surface of the molten mold, resulting in insufficient hardening afterwards, and after dewaxing, it will form agglomerates on the inner surface of the shell and local formation of sodium salt impurities.
2.2 Excessive drip control: excessive drip control means that when the surface layer is dipped in the coating, the unidirectional flow fails to stick to the sand in time, which will lead to the accumulation of coating in the local direction on the surface of the molten mold, causing the subsequent hardening to be incomplete.
2.3 Poor inter-layer hardening of the shell: Due to the existence of unhardened part of the coating layer especially in the first two layers, the unhardened coating in the dewaxing and baking caused by the collection of sodium salts on the inner surface of the shell, and the reaction of the steel to generate "orange peel" defects.
3. Environmental impact
In the cold winter, the low room temperature makes the paint flow poorly and causes the paint to pile up, and the thick pile of paint cannot be completely hardened; in addition, the temperature of the hardening liquid decreases with the room temperature, which also causes the hardening process to be slow and incomplete. The influence of environmental humidity mainly occurs in the rainy season, the increase of air humidity will affect the air-drying process, often because of insufficient air-drying and "orange peel" problem.
4. Avoid "orange peel" defective measures
4.1 Raw material selection
4.11 water glass in the case of suitable modulus, must strictly control the content of impurities; should be adjusted according to the temperature of the environment, humidity, structural characteristics of the casting and the characteristics of the matching powder water glass density.
4.12 powder in the particle size in line with the use of the requirements of the conditions, its particle shape is crucial, spherical and polygonal powder is more desirable, and flaky powder can not be used.
4.2 Process countermeasures
4.21 adjustment of water glass density: the density of the appropriate or not will directly affect the surface quality of the casting, too much density will lead to poor flow of paint and cause delamination and "orange peel" defects, too little density will form the surface of the casting cucumber spur; suitable density is usually related to the ambient temperature, particle size of the powder, microscopic shape and the structure of the casting characteristics of the casting. Density is generally controlled between 1.27-1.29g/cm3.
The adjustment principle is:
Increase the density when the ambient temperature is high and decrease it when it is low.
Increase density when the powder is coarse and the proportion of flakes is small, decrease density when the powder is fine and the proportion of flakes is large.
Simple structure of the coating easy flow of the casting can be appropriate to increase the density, and vice versa to reduce the density.
4.22 Determination of powder-liquid ratio: powder-liquid ratio is also one of the important factors affecting the surface quality of the casting, the ratio is too large will lead to uneven coating hanging due to the poor fluidity of the coating and produce delamination and coating accumulation; while too small will produce cucumber spurs on the surface of the casting. The principle of the ratio is to ensure the liquidity of the coating under the premise of trying to improve the powder-liquid ratio.
4.23 Concentration, temperature and hardening time of hardening liquid: in general, the hardener with ammonium chloride mass fraction above 25% will have better hardening effect; if the ammonium chloride content is low, the hardening effect cannot be improved by extending the time.
4.24 operation method of coating hanging: In actual production, there is a considerable part of "orange peel" problem is caused by improper operation, the unidirectional flow of paint is very easy to produce accumulation and cause inadequate hardening, so in the wax mold dip hanging paint after the drop control until the completion of the whole process of spreading sand, the direction of the module must be constantly changed.
4.25 Dewaxing process: In the dewaxing hot water supplemented with appropriate hardener, due to the heat absorption and reaction of the hardener, will further make the surface layer of the reaction products retained in the dewaxing water NaCl dissolved in the dewaxing water and most of the removal, at this time, the surface of the shell is formed by a layer of low sodium silica layer, which is conducive to prevent the "orange peel" defects.
4.26 ambient temperature: low ambient temperature will lead to poor flow of paint, resulting in uneven coating and the formation of orange peel and other shell defects, the ambient temperature of the shell making process should be controlled at 15 ℃ or above.
Xinyuanzhu Group specializes in the production of Ductile iron castings, Pig iron gray iron casting, Steel casting, Brass Casting, Malleable Iron Casting, and undertakes batch casting processing of castings. Welcome guests to cooperate with pictures or samples.
