Specifications and Requirements for Lost - Foam Casting

There are certain norms and requirements for lost - foam casting. In the molding process, the following aspects should be noted:

Lost - foam casting generally uses silica sand, and the silica dioxide content is required to be above 85% - 90%. Considering air permeability and refractoriness, silica sand with a particle size of 0.850 - 0.300mm is generally selected for steel castings and iron castings. First, lay a layer of dry sand about 100mm thick at the bottom of the sand box, then place the EPS mold and gating and riser systems coated with paint in the center of the sand box. Fill the sand while vibrating to compact it. Stop adding sand when the dry sand is about 50mm away from the top surface of the sand box. Cover the dry sand with a plastic film, then place the pouring cup at the top of the straight runner, and sprinkle a layer of loose sand on the film to prevent the molten iron from splashing during pouring, burning through the film and causing air leakage and sand box collapse.

Pouring

In the lost - foam casting process, the entire technological process includes seven links, and each link has a great influence on the quality of lost - foam castings. Of course, the pouring process is also a crucial part of the entire lost - foam casting process. Mastering the precautions of the pouring process and strictly controlling every detail in the pouring process is the key to producing high - quality lost - foam castings.

The pouring process of lost - foam casting is the process of molten steel filling the mold while the styrofoam mold vaporizes and disappears. The following points need to be noted during the entire process:

1. The runner should be filled with molten steel from beginning to end. If it is not full, due to the limited strength of the coating layer, sand collapse and air intake are likely to occur, causing casting defects.

2. When pouring molten steel, it is necessary to ensure stability, accuracy, and speed. Fill the pouring cup instantaneously and pour continuously and quickly. About one ton of molten steel should be poured in about one minute. If the molten steel of the castings in the same box breaks the flow, air will be sucked in, which may cause sand collapse or an increase in the number of pores in the castings, leading to the scrapping of the castings.

3. Lost - foam casting adopts a negative - pressure closed - type system, and it must be closed below the pouring cup. When the molten steel enters the mold, it begins to liquefy, burn, and vaporize and disappear. A temporary cavity is formed in a short distance at the front end of the molten steel. Therefore, the filling speed of the molten steel should be designed to be approximately the same as the disappearance speed of the styrofoam model. To prevent the high - temperature radiation of the molten steel from melting other models in the same box, the runner should be appropriately far away from the casting model. The position of the vertical runner should be selected at the lowest position of the entire box of castings. During pouring, pay attention to adjusting and controlling the negative - pressure vacuum degree within a certain range. After pouring, maintain a certain negative - pressure state for a period of time. After the negative - pressure stops and the molten steel solidifies, take out the casting from the box.

Prevention of Back - Spray

Lost - foam casting is one of the casting processes. There are various casting defects in the casting process, which will lead to defects in the castings, and lost - foam casting is no exception. Back - spray is a common phenomenon in lost - foam casting. When the back - spray is severe, it may endanger the personal safety of the pourer and must be taken seriously. To reduce the back - spray phenomenon, the following measures can be taken:

First, the density of the foam model should be small. On the premise of ensuring the surface quality and strength of the foam, the foam should be made as light as possible to reduce the gas evolution during pouring.

Second, the foam model must be dried thoroughly before coating. The weight change of each batch of foam models during the drying process should be sampled and inspected, and a baking weight - change curve should be made. The coating can only be applied when the weight no longer changes.

Third, the gating system, especially the straight runner and the horizontal runner, should not be coated. This can enable the gas generated during pouring to be quickly and fully evacuated. Moreover, not coating saves the consumption of paint. Finally, cover a baffle at the pouring cup to block the back - sprayed molten steel and prevent it from splashing out and endangering the on - site operators.

Using the above methods can effectively prevent the back - spray phenomenon in the lost - foam casting process, better prevent casting defects, improve the quality of castings, and also improve the safety of production.

Factors Affecting Precision

In lost - foam casting, generally, the factors affecting the dimensional accuracy of castings are multifaceted, such as the structure, shape, and size of the lost - foam casting itself, the influence of the quality of the foam plastic, the influence of the coating layer, and the influence of the molding and pouring links during the casting process.

In the entire lost - foam casting process, the pouring process is one of the crucial links. The four factors of the gating system, pouring speed, pouring temperature, and pouring process are the key processes to improve the quality of castings. Only by continuous improvement and perfection can high - precision and high - quality castings be produced.

The coating layer also has a great influence on the accuracy of lost - foam castings. The properties and thickness of the coating can affect the size of the pattern, and thus affect the dimensional accuracy of the castings. Therefore, in lost - foam casting, the coating should have good properties and sufficient strength, and the coating - hanging operation process should be reasonable.