Why Is the Scrap Rate of Castings So High? — Cause Analysis and Countermeasures for Iron Inclusions on the Surface of Silica Sol Castings

Since China introduced the silica sol shell production process, the investment casting (lost-wax precision casting) production in China has achieved rapid development and significant progress. In response to the development of the casting industry in recent years, competition within the industry has become increasingly fierce, with comparisons made across raw materials, equipment, processes, and other aspects. Meanwhile, customers have put forward higher and higher requirements for casting quality.

Why is the scrap rate of castings so high? In this article, we will start by analyzing the causes of iron inclusions on the surface of silica sol castings and then propose corresponding countermeasures.

There are many reasons for iron inclusions in castings, which can be analyzed from three aspects: welding, shell making, and pouring. Among these three, shell making is the most critical. Let’s first analyze the causes of iron inclusions from the perspective of shell making:

① Improper preparation of raw materials: If a large amount of gas is generated due to chemical reactions in the surface layer slurry, it will adhere to the wax pattern along with the coating. In addition, if the slurry is too thick, its fluidity will decrease, making it impossible to cover some grooves and corners of the wax pattern, leaving air holes. After pouring, iron beads will appear in these areas.

② Proper selection of sand grain size: For castings with complex structures (such as deep and narrow gaps or holes), if the sand used for sprinkling is too coarse, these areas will be blocked, resulting in insufficient slurry dipping. This leads to an uncompact shell, with thinner walls and reduced strength in some parts.

Corresponding measures: Before dipping the slurry, first use an air gun to blow off the floating sand on the upper layer. Then, pre-coat the wax pattern with a layer of silica sol before dipping it into the main slurry—this enhances the fluidity of the slurry and prevents blockages. After removing floating sand, operators can use thin rods to clear the sand accumulated in round holes and gaps, which is also beneficial for subsequent slurry dipping and sanding. In addition, for products with deep holes, after the third layer of shell making, grout and fill sand into the holes to prevent iron leakage from the inner walls of the deep holes.

③ Inadequate drying of the mold shell: This directly leads to a significant decrease in shell strength. The root cause is poor temperature and humidity control in the workshop, resulting in insufficient drying time.

Corresponding measures: Control the temperature and humidity in the workshop. The specific values are shown in the table below, and adjustments should be made according to actual conditions:

Next, let’s discuss welding. Welding does not directly cause iron inclusions in products, but unreasonable welding processes will increase the difficulty of shell making, thereby raising the iron inclusion rate of products. Often, to maximize profits, factory owners will fully assemble products on a square mold head. Although this increases the yield rate, the products are arranged too densely, which prevents the slurry and sand from entering. As a result, the shell thickness between the gate and the wax rod is insufficient and lacks adequate strength—molten steel will cause iron leakage once it impacts this area. Therefore, it is necessary to reasonably arrange the gaps between products.

Secondly, for products with deep holes and grooves, their complex surfaces must face upward or away from the wax rod. This facilitates slurry draining, air blowing, and drying. Shell-making workers are paid based on piece rates; they cannot afford to waste too much time and energy on a single string of products. For them, the more products they make in the same time, the higher their income. Therefore, we should also improve the welding method to help them reduce operation difficulty, improve work efficiency, and ensure product quality.

In summary, to address the iron inclusion defect in castings, we should mainly identify the causes from the aspects of welding, shell making, and pouring. As long as these three links are well controlled, most iron inclusion problems can be solved.