Lost foam casting (LFC), also known as negative pressure full mold casting, has been developed and applied in industrial production since its invention by Americans in 1956. By the 1980s, it had reached a considerable scale and was successfully used in industrialized production, ranging from single-piece and small-batch production of large parts to large-batch and small-piece production on assembly lines, and from the production of simple non-machined parts to that of complex machined parts. Countries such as the United States, Italy, Germany, and Japan are representative in industrialized production. By the 1990s, the use of lost foam casting for automotive castings had gradually developed. Up to now, there are more than 3000 manufacturers in China using the lost foam casting process, but most of them are small-scale production of single-piece or small-batch castings. At present, the lost foam casting technology in China is still in the stage of technological maturity, and the characteristics and laws of casting still need to be deeply understood. In particular, the theoretical research on the filling characteristics of molten metal, the properties of coatings, and the pyrolysis characteristics of patterns has not been fully mastered, resulting in some defects in castings, such as carbon increase, sand inclusion, sticking sand, deformation, collapse of the mold during pouring, and difficulty in removing the coating after pouring. Since 2009, I have been committed to the research and development of the lost foam casting process for carbon steel castings. This article will analyze the defects such as carbon increase, porosity, slag inclusion, back spraying, and negative pressure cutting that are prone to occur during the trial production process, to provide some guidance for the better application of this process in the production of steel castings.
1. Common Defects and Prevention Measures
1.1 Carbon Increase Defect
Carbon increase is one of the common defects in the lost foam casting process of steel castings. The foam pattern material is mainly composed of carbon and hydrogen, which decomposes rapidly under high-temperature molten steel, producing hydrogen and free carbon. Since the affinity of hydrogen for oxygen is stronger than that of carbon, the hydrogen decomposed at high temperature first combines with the oxygen in the gaps of the pattern to form water vapor and is discharged, while a large amount of free carbon decomposed remains in the mold and infiltrates the surface of the molten steel, resulting in carburization on the surface of the casting, which is called carbon increase. According to relevant literature and experiments, there is a certain regularity in carbon increase, that is, carbon increase mainly occurs on the surface of the casting, and the core hardly increases carbon, and there is no carbon increase near the inner gate, and the farther away from the inner gate, the more serious the carbon increase. In view of the mechanism of carbon increase, the following measures can be taken to basically control the carbon content of the casting within the process requirements:
- Select High-Quality Foam Plastics
The quality of foam plastics directly affects the gasification speed of the pattern and the form of pyrolysis products during the pouring process. High-quality foam plastics have low carbon content, large molecular weight, and lower pattern density under the same foam strength. The selection of raw bead particles with less gas generation and low carbon content and molded foam with low bulk density and light weight is the first choice for the production of steel castings by lost foam casting, and it is also the most effective way to solve the problem of carbon increase in steel castings at present. - Choose a Reasonable Pouring Process
The design of the pouring process for lost foam casting should be able to accelerate the gasification of the mold material, reduce and stagger the contact and reaction time between the liquid and solid phases in the decomposition products and the molten steel, and reduce or avoid carburization of the steel castings. It mainly includes controlling the pouring temperature and pouring speed, good coating permeability and dry sand mold permeability, appropriate negative pressure of the mold wall, and the use of anti-carburization coatings, etc. - Set Risers at the Furthest Point from the Inner Gate or at the Highest Point of the Casting
Utilizing the characteristic that the farther away from the inner gate, the more serious the carbon increase, a riser is set at the farthest end from the inner gate or at the highest point of the casting, so that the steel water with serious carbon increase pollution that enters the mold cavity first enters the riser, and the casting body obtains purer molten steel. - “Burn First and Pour Later”
That is, the foam plastic in the mold cavity is burned to form a cavity, and then the molten metal is poured in.
1.2 Porosity Defect
According to the causes, the porosity generated in steel castings by lost foam casting is divided into the following four categories:
- Porosity Caused by the Entrapment of Pyrolysis Products of the Foam Pattern in the Molten Metal
During the pouring and filling process, turbulence may occur, or during the gasification process, part of the pattern may be surrounded and pyrolyzed by the molten metal, and the generated gas cannot be discharged from the molten metal, resulting in porosity. This kind of porosity is large and numerous, and the inner surface has carbon black.
Prevention measures: Improve the process to ensure smooth filling of the molten steel without turbulence; reasonably increase the pouring temperature and increase the negative pressure (if porosity is caused by turbulence, reduce the negative pressure); improve the permeability of the coating and the molding sand. - Porosity Caused by Poor Drying of the Foam Pattern and Coating
When the foam pattern coating is not dried sufficiently or the foaming agent content is too high, a large amount of gas will be generated during pouring, causing porosity.
Prevention measures: Fully dry the foam pattern (operate according to the molding process characteristics of the mold material foaming); the coating must be completely dried, and the addition amount of the foaming agent must be strictly controlled. - Porosity Caused by Excessive Amount of Pattern Binder
When the amount of the pattern binder is too large and the gasification is slow, it is easy to entrain gas into the molten steel to form porosity.
Prevention measures: Select a low-gas-emitting model binder, and use as little adhesive as possible while ensuring a firm bond. - Porosity Caused by the Entrapment of Air during Pouring
When the sprue is not filled during pouring, it is easy to entrap air. If these gases cannot be discharged in time, it is easy to cause porosity.
Prevention measures: Design a reasonable pouring system to ensure smooth filling of the molten metal without entraining air; when using a closed pouring system, ensure that there is a certain amount of molten metal in the pouring cup to ensure that the sprue is in a filled state; use a hollow sprue mold to reduce the gas generation and is beneficial to prevent the generation of porosity. - Porosity Caused by Quality Problems of Molten Steel Smelting
During the smelting process of carbon steel, gas is contained, and pre-deoxidation treatment and final deoxidation treatment are required. If the deoxidation is not sufficient, oxygen will remain in the molten steel, forming porosity.
Prevention measures: During the smelting process, strictly follow the smelting process, perform deoxidation treatment before pouring, and purify the molten steel.
1.3 Slag Inclusion Defect
Slag inclusion defect refers to the defect formed by dry sand grains, coatings, and other inclusions entering the casting with the molten iron during the pouring process, which is a relatively common defect in the production of lost foam castings. On the machined surface of the casting, white or grayish-black inclusion spots can be seen, distributed individually or in patches. The white ones are quartz sand particles, and the grayish-black ones are slag, residue after the pyrolysis of the coating, foam model, and other inclusions.
After the lost foam castings are cooled and removed from the box, the presence of sand ingress and slag inclusion defects can be determined based on the surface condition of the casting and the pouring system. In production practice, if the pouring cup, sprue, runner, inner gate, and the surface or connection of the gate, as well as the surface of the casting, have serious sand sticking or cracked sand sticking, then sand ingress may occur in all parts from the pouring cup, sprue, runner, inner gate to the casting. Among them, the unsealed sprue is the main reason for sand ingress or slag inclusion defects. In addition, process parameters such as the net pressure head of the pouring system, the pouring temperature, the negative pressure, the particle size of the dry sand, the transportation process of the pattern, and the operation of boxing are all important reasons for the slag inclusion and sand ingress defects of the castings.
Slag inclusion defect is a major problem in lost foam casting production and is a systematic project. Only by taking systematic measures and careful operation in these links can the slag inclusion defect of the castings be reduced and basically eliminated to obtain high-quality castings.
- Select a Coating with Good Comprehensive Performance
In addition to the general requirements for performance, the coating for steel castings also requires high strength, high refractoriness, good thermal shock resistance, coating adhesion, suspension, and non-flowability. To prevent slag inclusion defects, the coating is first required to have high strength and refractory properties, and the sprue should be tightly sealed when the molten steel enters the mold, and the coating layer on the surface of the casting and the pouring system should not fall off or crack. In addition, the coating applied to the surface of the white mold is required to have sufficient room temperature strength and not crack during drying and transportation. Furthermore, the coating layer should have good high-temperature strength, that is, during the pouring process, the coating does not fall off or crack under the long-term scouring action of the high-temperature metal, and the coating layer should have a certain thickness. - Standardize the Boxing Operation
During boxing, when the pattern assembly is placed on the bottom sand of the sand box, it should be stable, and sand vibration molding should not be started when it is suspended, to avoid cracking the coating layer. Do not add sand violently to the pattern, but first use flexible sand addition to prevent the rupture of the sand layer, and seal the sprue tightly to avoid sand ingress. The entire boxing and molding operation process must be very careful to ensure that there is no shedding, cracking, or crack in the coating layer of the pattern assembly before pouring, and the pouring cup should be checked again before pouring to ensure that there is no floating sand, dust, and debris. - Reasonably Set the Pouring Pressure Head, Temperature, and Time
The higher the pouring pressure head, the greater the scouring of the pouring system and the mold by the molten metal, and the greater the possibility of damaging the coating and causing sand ingress. The pressure head should also be different for castings of different sizes. Select a ladle with an appropriate capacity, reduce the pouring height as much as possible, and keep the nozzle of the ladle as close to the pouring cup as possible to avoid using a large ladle for small castings. Select an appropriate pouring temperature, as the higher the pouring temperature, the higher the requirements for the performance of the coating, and the easier it is to produce defects such as sand sticking and slag inclusion. - Reasonably Determine the Negative Pressure
The pouring process of lost foam casting is generally carried out under vacuum conditions, and the negative pressure is used to compact the dry sand, accelerate the exhaust, and improve the filling ability. The magnitude of the negative pressure has a great influence on the quality of the castings. Excessive negative pressure increases the possibility of the molten metal inhaling dry sand and inclusions when passing through cracks and cracks, and also increases the sand sticking defect of the castings. At the same time, if the negative pressure is too large, the filling speed is too fast, which increases the scouring intensity of the metal on the sprue and the mold, and it is easy to make the coating fall off and enter the metal, and it is also easy to damage the coating layer and cause sand ingress. For steel castings, the appropriate negative pressure is generally between -0.030 and -0.045 MPa. - Set Up Slag Blocking, Slag Skimming, and Slag Collecting Risers
When designing the pouring system, the functions of slag blocking and slag skimming should be considered, and the setting of slag collecting risers on the casting is helpful to improve the sand ingress and slag inclusion defects. - Use Molten Steel Purification Technology
Molten steel smelting must pay attention to the purification problem, which is one of the key technologies of lost foam casting, including the whole process from molten steel smelting, overheating, deoxidation until pouring into the mold.
1.4 Back Spraying
During the pouring process, due to the excessive amount of gas generated by the thermal decomposition of the gas mold and the inability to be discharged in time, the pressure in the casting cavity rises sharply, which easily causes flame or molten metal spraying, resulting in the ζ₯εΊ of the casting. The prevention measures are:
- Control the Density of the EPS Pattern within 0.015 – 0.020 g/cm3, and Ensure the Pattern is Dry. After Applying the Coating, it Should also be Dried to Reduce the Moisture Content and Gas Generation.
- Select a Coating with Good Permeability and Adjust the Coating Thickness Appropriately (1.0 – 2.0 mm is Appropriate) to Facilitate the Escape of Gas Generated by the Pyrolysis of the Pattern in Time.
- Control the Permeability and Particle Size of the Dry Sand; the Design of the Sand Box Should be Scientific, Reasonable, and Applicable. Control the Negative Pressure (Vacuum Pump Suction) to Ensure that the Pattern Gasifies under Vacuum and Oxygen-Deficient Conditions with Little Combustion and Reduced Gas Generation.
- Control the Pouring Temperature and Pouring Speed, and Use the Heat of the Molten Steel to Ensure the Gasification of the Pattern. At the Same Time, Control the Pouring Speed when a Large Amount of Gas is Generated by the Pattern to Avoid Excessive Speed, which May Cause a Large Burst of Pyrolysis Gas.
- Design a Reasonable Pouring System to Ensure that the Molten Metal Fills the Mold Cavity Smoothly, Balancedly, and Rapidly, so that the Gas Generated by the Pyrolysis of the Pattern Can Escape from the Cavity and be Absorbed and Drained.
1.5 Negative Pressure Cutting
The main reason: In the vacuum state of the sand box, due to the damage of the mold during the pouring process, the outside air is inhaled into the mold; the strong airflow penetration process has a penetrating and scouring force on the un-solidified molten metal, forming a cutting phenomenon, which we call the “negative pressure cutting” phenomenon. The main reasons for this phenomenon are:
The negative pressure during pouring is too high, the coating thickness is too thin or damaged, and the holding time after pouring is too long. Determine the process parameters according to the factors such as the structure, size, and quality of different castings. Generally, the negative pressure in the sand box during pouring is within the appropriate negative pressure range of -0.020 to -0.035 MPa, the coating thickness is ensured to be 1.0 – 2.0 mm, the pouring height of the ladle should be reduced as much as possible during pouring, and the nozzle of the ladle should be as close to the pouring cup as possible, and the holding time after pouring is controlled within 3 – 7 minutes.
2. Summary
Lost foam casting of steel castings has great development space in China’s lost foam casting industry. As long as effective production operation control methods are adopted for production and operation management, the melting process of the castings is strictly controlled, the process conditions for carbon increase during the pouring process are reduced and eliminated, combined with the actual production conditions, the training of skilled technicians is strengthened, the overall level of the team is improved, and every process in the process is strictly controlled, completely qualified steel castings can be produced.
In the production of lost foam casting of steel castings, various defects may occur, such as carbon increase, porosity, slag inclusion, back spraying, and negative pressure cutting. These defects are related to many factors, including the quality of the foam pattern, the pouring process, the coating, the sand, and the vacuum system. To prevent these defects, corresponding measures should be taken, such as selecting high-quality foam plastics, choosing a reasonable pouring process, using a suitable coating, ensuring the dryness of the pattern and the coating, controlling the pouring temperature and speed, and setting a reasonable negative pressure.
In addition, the operation and management of the production process are also very important. The production process should be strictly controlled, and the quality of each process should be ensured. At the same time, the training of skilled technicians should be strengthened to improve the overall level of the team. Only by taking comprehensive measures can the quality of lost foam castings of steel castings be improved and the production of high-quality castings be realized.
| Defect | Causes | Prevention Measures |
|---|---|---|
| Carbon Increase | Decomposition of foam pattern material, resulting in the presence of free carbon in the mold and infiltration with the surface of the molten steel. | Select high-quality foam plastics with low carbon content and low gas generation. Choose a reasonable pouring process, such as controlling the pouring temperature and speed, ensuring good coating and mold permeability, and using anti-carburization coatings. Set risers at the appropriate location to allow the carbon-increased steel water to enter the riser. Burn the foam plastic in the mold cavity before pouring the molten metal. |
| Porosity | Entrapment of pyrolysis products of the foam pattern, poor drying of the foam pattern and coating, excessive amount of pattern binder, entrapment of air during pouring, and quality problems of molten steel smelting. | Improve the pouring process to ensure smooth filling of the molten steel without turbulence. Increase the pouring temperature and negative pressure appropriately (reduce the negative pressure if porosity is caused by turbulence). Ensure the thorough drying of the foam pattern and the coating, and strictly control the addition amount of the foaming agent and the binder. Design a reasonable pouring system to avoid entrapment of air. Perform deoxidation treatment before pouring to purify the molten steel. |
| Slag Inclusion | Entry of dry sand grains, coatings, and other inclusions into the casting during the pouring process. | Select a coating with high strength, high refractoriness, and good performance to ensure the tight sealing of the sprue and the integrity of the coating layer. Standardize the boxing operation to avoid damage to the coating layer. Set the pouring pressure head, temperature, and time reasonably. Determine the negative pressure appropriately to reduce the possibility of sand and inclusions being drawn i |