Prevention and Control of Orange Peel Defects in Precision Casting

Prevention and Control of Orange Peel Defects in Precision Casting

What is precision casting orange peel? It has a significant impact on the quality of castings, with defects occurring in the thickened areas of castings, near hot spots and internal runners, and in areas where heat is concentrated and cooling is slow. There are slightly convex small circular spots on the surface of the casting, which are in the shape of "eye circles". These rough surfaces look like "orange peel" spots, which repeatedly appear in various castings, sometimes throughout the entire batch of castings. According to our company's statistics, 15% of the waste is caused by "orange peel" defects. Today, precision casting manufacturer Tianyao Metal will talk to you about how to prevent the occurrence of "orange peel".

1、 The causes of orange peel defects

The fundamental cause of "orange peel" is the accumulation and insufficient hardening of the coating surface. After calcination, the shell forms yellow or yellow green glass on its surface, which reacts with the molten steel after pouring to form silicate nodules that adhere to the surface of the casting.

2、 Impact on craftsmanship

2.1 Insufficient air drying of the surface layer. Surface layer air drying is a process of re homogenization of coatings, as well as a dehydration and solidification process of water glass. If the air drying time is too short, the distribution of surface layer coatings on the surface of the mold is uneven, resulting in insufficient hardening afterwards. After dewaxing, cluster like aggregates will form on the inner surface of the mold shell, and sodium salt impurities will be locally formed.

2.2 Excessive drip control. Excessive drip control refers to the failure of unidirectional flow to timely adhere sand when the surface layer is coated with coating, which will lead to the accumulation of coating in local directions on the surface of the investment mold, resulting in incomplete hardening afterwards.

2.3Poor interlayer hardening of the shell. Due to the presence of unhardened parts in the coating layer, especially in the first two layers, the unhardened coating causes sodium salt accumulation on the inner surface of the mold shell after dewaxing and calcination, which reacts with molten steel to form "orange peel" defects.

3、 Environmental impact

In the cold winter, the low room temperature reduces the fluidity of the coating, causing the coating to accumulate, and the coating that accumulates too thick cannot fully harden; In addition, the temperature of the hardening solution can also cause a slow and incomplete hardening process as the room temperature decreases. The impact of environmental humidity mainly occurs during the rainy season, and an increase in air humidity can affect the air drying process, often resulting in "orange peel" problems due to insufficient air drying.

4、 Measures to avoid "orange peel" defects

4.1 Determination of powder to liquid ratio. The powder to liquid ratio is also one of the important factors affecting the surface quality of castings. If the ratio is too large, it will cause uneven coating due to poor fluidity of the coating, resulting in layering and coating accumulation; If it is too small, it will produce cucumber thorns on the surface of the casting. The principle of mixing ratio is to increase the powder liquid ratio as much as possible while ensuring the fluidity of the coating.

4.2 The concentration, temperature, and hardening time of the hardening solution. In general, hardeners with a mass fraction of ammonia chloride above 25% will have better hardening effects; If the ammonium chloride content is low, extending the time cannot improve the hardening effect.

4.3 Coating and hanging operation method. In actual production, a considerable part of the "orange peel" problem is caused by improper operation. The unidirectional flow of the coating is prone to accumulation, resulting in insufficient hardening. Therefore, the direction of the mold must be constantly changed throughout the entire process from the dripping control after the wax mold is immersed in the coating until the sanding is completed.

4.4 Dewaxing process. Adding an appropriate hardener to the dewaxing hot water will further dissolve the reaction product NaCl retained on the surface layer in the dewaxing water and remove most of it due to the heat absorption and reaction of the hardener. At this time, a low sodium silicone layer is formed on the surface of the mold shell, which is conducive to preventing the occurrence of "orange peel" defects.

Precision machining