Abstract: The common casting defects in lost foam casting of steel castings, analyzes the causes of these defects, and summarizes the defects and prevention measures based on practical experience.
Keywords: lost foam casting; steel casting; defects; prevention measures
1. Introduction
Lost foam casting, also known as EPC (Expendable Pattern Casting), was invented by Americans in 1956. By the 1980s, it had developed to a considerable scale and was successfully applied to industrial production. From small-batch, large-piece production to mass production of small pieces on assembly lines, and from the production of simple non-machined parts to complex machined parts, the United States, Italy, Germany, and Japan are representative of industrial production. In the 1990s, automotive castings gradually adopted the lost foam casting method. To date, there are over 3,000 factories using lost foam casting technology in China, but most are small-scale, producing single or small-batch castings. Currently, domestic lost foam casting technology is still in the stage of technological maturity, and the characteristics and laws of casting need to be further understood, especially theoretical research on the mold filling characteristics of molten metal, the properties of coatings, and the pyrolysis characteristics of patterns, which are not yet fully mastered. This leads to some defects in castings, such as carburization, sand inclusion, scabbing, deformation, collapse of the mold during pouring, difficulty in removing the coating after pouring, and so on. Since 2009, I have been dedicated to the research and development of the lost foam casting process for carbon steel castings. This paper analyzes the defects such as carburization, porosity, slag inclusion, back spray, and negative pressure cutting that are prone to occur during trial production, providing some guidance for better application of this process in the production of steel castings.
2. Common Defects and Prevention Measures
Table 1: Summary of Common Defects and Prevention Measures in Lost Foam Casting of Steel Castings
| Defect Type | Cause | Prevention Measures |
|---|---|---|
| Carburization | The foam pattern material mainly consists of carbon and hydrogen, which decompose rapidly under high-temperature molten steel to produce hydrogen and free carbon. | 1. Use high-quality foam plastics with low carbon content and high molecular weight. 2. Design a reasonable pouring process to accelerate pattern gasification and reduce contact and reaction time between decomposition products and molten steel. 3. Set risers at the farthest end from the ingate or at the highest point of the casting to allow contaminated steel water to enter the riser. 4. Adopt the “burn first, pour later” method. |
| Porosity | – Foam pattern pyrolysis products entrapped in molten metal. – Inadequate drying of foam patterns and coatings. – Excessive binder in the pattern. – Air entrapped during pouring. – Inadequate deoxidization in molten steel. | 1. Improve the process to ensure smooth mold filling without turbulence. 2. Adequately dry foam patterns and control the addition of foaming agents. 3. Use a low-gas-emitting binder and minimize adhesive usage. 4. Design a reasonable pouring system to avoid air entrapment. 5. Strictly follow melting and deoxidization processes. |
| Slag Inclusion | Dry sand grains, coatings, and other impurities entering the casting during pouring. | 1. Use coatings with good overall performance, including high strength, high refractoriness, and good crack resistance at high temperatures. 2. Carefully handle packing operations to avoid cracking the coating layer. 3. Reasonably set pouring head pressure, temperature, and time. 4. Set an appropriate negative pressure. 5. Incorporate slag baffles, slag skimming, and slag-collecting risers in the pouring system design. 6. Adopt steel water purification techniques. |
| Back Spray | Excessive gas emission during pattern thermal decomposition, which cannot be timely removed, causing a sharp increase in pressure in the casting cavity. | 1. Control the density of EPS patterns between 0.015~0.020 g/cm³ and ensure they are dry. 2. Use coatings with good permeability and adjust the coating thickness to 1.0~2.0 mm. 3. Control sand permeability and particle size; design sand boxes scientifically and reasonably. 4. Control pouring temperature and speed. 5. Design a reasonable pouring system to ensure smooth and balanced mold filling. |
| Negative Pressure Cutting | Damage to the mold during pouring, causing outside air to be sucked into the mold; strong airflow creates a cutting phenomenon in the unsolidified molten metal. | Determine process parameters based on factors such as casting structure, size, and quality. Generally, maintain a suitable negative pressure of -0.020~-0.035 MPa during pouring, ensure a coating thickness of 1.0~2.0 mm, lower the pouring height as much as possible, keep the pouring ladle nozzle close to the pouring cup, and control the post-pouring pressure holding time within 3~7 minutes. |
2.1 Carburization Defect
Carburization is one of the common defects in the lost foam casting process of steel castings. The foam pattern material, mainly composed of carbon and hydrogen, decomposes rapidly under high-temperature molten steel to produce hydrogen and free carbon. Due to hydrogen’s stronger affinity for oxygen than carbon, the hydrogen first combines with oxygen in the pattern gaps to form water vapor and is expelled, while a large amount of free carbon residue remains in the mold and reacts with the surface of the molten steel, resulting in surface carburization of the casting, known as carburization.
According to relevant literature and experiments, carburization shows a certain regularity: it mainly occurs on the surface of the casting, with almost no carburization in the core, and no carburization near the ingate. Carburization becomes more severe farther from the ingate. To control the carbon content of castings within the process requirements based on the mechanism of carburization, the following measures can be taken:
- Selecting high-quality foam plastics: The quality of foam plastics directly affects the gasification speed and pyrolysis product morphology of the pattern during pouring. High-quality foam plastics have low carbon content, high molecular weight, and lower density under the same foam strength. Selecting foam with low gas emission, low carbon content, and low bulk density is the preferred condition for producing steel castings using lost foam casting and is currently the most effective way to solve the problem of steel carburization.
- Choosing a reasonable pouring process: The design of the pouring process for lost foam casting should accelerate pattern gasification, reduce and stagger the contact and reaction time between the liquid and solid phases of decomposition products and molten steel, and reduce or avoid steel carburization. This mainly includes controlling pouring temperature and speed, ensuring good coating permeability and dry sand mold permeability, selecting an appropriate negative pressure on the mold wall, and using anti-carburization coatings.
2.2 Porosity Defect
Based on their causes, pores in steel castings produced by lost foam casting can be divided into the following four categories:
- Pores caused by foam pattern pyrolysis products entrapped in molten metal: Turbulence during mold filling or partial encapsulation of the pattern by molten metal during gasification can cause the pattern to pyrolyze, and the resulting gas cannot escape from the molten metal, forming pores. These pores are large and numerous, with carbon black on the inner surface.
Prevention Measures: Improve the process to ensure smooth mold filling without turbulence; reasonably increase pouring temperature and negative pressure (if turbulence causes pores, reduce negative pressure); improve coating and sand permeability.
- Pores caused by inadequate drying of foam patterns and coatings: Inadequate drying of foam patterns or excessive foaming agent content can produce a large amount of gas during pouring, causing pores.
Prevention Measures: Adequately dry foam patterns according to the molding process characteristics of the pattern material; ensure the coating is thoroughly dry and strictly control the addition of foaming agents.
- Pores caused by excessive binder in the pattern: Binders with high gas emission and excessive use can slow down gasification, making it easy for gas to be entrapped in the molten steel, forming pores.
Prevention Measures: Use low-gas-emitting model binders and minimize adhesive usage while ensuring adhesion.
- Pores caused by air entrapped during pouring: An unfilled sprue can easily entrap air, which, if not promptly removed, can cause pores.
Prevention Measures: Design a reasonable pouring system to ensure smooth metal flow during mold filling without entrapping air; use a closed pouring system to ensure the pouring cup contains a certain amount of molten metal, ensuring the sprue is filled; use a hollow sprue mold to reduce gas emission, helping to prevent pore formation.