Gravity casting

First, the basic production requirements of lost foam casting technology

As a method of near-static forming of castings, lost foam casting technology has developed rapidly in recent years. In foreign countries, the lost foam casting technology has shown great vitality due to the mechanized and automated lost foam casting production lines being put into operation and the significant economic and social benefits. Although the application of lost foam casting technology in China has been slow in the past, it has developed rapidly in recent years. In particular, due to the low investment in lost foam casting equipment and short process routes, many of the original small and medium-sized foundry companies are increasingly adopting this technology. However, some companies have failed to pay attention to some operational problems, which has caused some problems in the production process, which has a great impact on the quality of castings.

1, model making
Model making is a very important part in the lost foam casting process. The selection of EPS raw materials, the processing technology of the model, the dimensional accuracy, the density of the model, and the amount of pyrolysis products during casting are the prerequisites for obtaining high-quality castings. There are several ways to make existing SME models:

(1) Cut and bonded with packaging EPS sheets.
(2) Self-made molds, commissioned by foreign factories.
(3) Self-made simple pre-forming equipment.

Using the above method to make the model, there is a widespread phenomenon that does not pay attention to the change of the density of the pattern. Especially when the model is entrusted to the outside factory, the moisture is not easy to control. When the pouring occurs, the molten iron is sprayed back from the gate or the casting has cold separation and insufficient pouring. . Therefore, in the production process, the test of the density of the model should be strengthened, and the drying time of the model should be increased. After the EPS beads are selected by the process experiment, the raw material manufacturer cannot be arbitrarily changed; when the pre-fabrication is used, the weighing tool is used to control the beads. Grain density, a method of controlling the density of beads by artificial experience; after the above method is adopted, the problem is solved.

2. Problems with vibration
Vibration compaction is one of the four key technologies for lost foam casting. The function of vibration is to make the dry sand generate dynamic flow in the flask, improve the filling and density of the dry sand, and prevent casting defects. In the dry sand vibration filling, the ideal situation is that the dry sand is in an orderly flow during the vibration process, and under the premise of ensuring that the model is not deformed, it is uniformly filled into various parts of the model, so that the sand in the sand box is obtained higher. And a more uniform packing density. The lost foam casting vibration table of small and medium-sized enterprises is mostly self-made equipment. When vibrating, the most common phenomenon is due to improper vibration operation, resulting in deformation of the pattern, cracking of the coating layer, etc., resulting in corresponding casting defects. Some shakers themselves are prone to deformation due to excessive excitation force and unbalanced polarization blocks of the same set of motors. For this reason, the excitation force, amplitude and vibration time should be adjusted mainly; for large-sized and simple-structured castings, the three-dimensional vibration of the six motors can be changed to the vertical or horizontal vibration of the two-motor; in particular, the vibration of the detecting instrument The parameters of the station are tested and adjusted to meet the design requirements.

3. There is a problem with the use of paint
In the lost foam casting process, the use of paint can improve the stiffness and strength of the pattern, so that the EPS pattern is isolated from the mold to prevent sticking sand and mold collapse; during the casting process, the sample pyrolysis product is allowed to pass through the coating in a timely and smooth manner. The coating is generally composed of a refractory material, a binder, a suspending agent, etc., and the ratio of each composition has a great influence on the performance of the coating. However, some companies are not very clear about the role of the coating composition, arbitrarily modify the coating formulation and preparation process, or continue to use the composition due to the lack of a certain composition, resulting in greatly reduced coating performance; some enterprises have problems in the pattern dip coating drying process, Sometimes in order to shorten the time, the next dip coating is carried out in the case where the first coating is not dried, resulting in insufficient drying inside the mold, and moisture is present therein; in summer, only the drying method is used, and the process is unstable, resulting in instability. Back spray or void formation during casting; coating thickness is not noticed to vary depending on the casting, the casting temperature and the change in the hot metal head.
Only by paying attention to and solving the above problems, and working hard on the details of the operation, there will be no casting defects due to the paint.

4. There is a problem with the pouring process
In the lost foam casting, in order to discharge the gas and the sample gasification residue, the sprue should have sufficient height so that the molten metal has sufficient pressure head to promote the stable and rapid filling of the metal liquid flow, and ensure the complete and clear surface of the casting. In practice, some enterprises adopt the original pouring cup for sand casting. Due to the small size, it is easy to cause the workpiece to be scrapped due to the unstable liquid flow. In order to ensure that there is enough flow to keep the pouring process flowing, and quickly establish the starting head, you can use a larger pouring cup; the sprue is made hollow to reduce the gas and back spray, and increase the pressure when starting the pouring. head.
The lost foam casting adopts negative pressure dry sand vibration modeling. When this method is used, the casting strength is much greater than the wet sand strength. The suction negative pressure method can improve the stability of the mold and timely remove the pyrolysis gasification products generated during the gasification of the pattern. However, in the production process, some factories only pay attention to the negative pressure before pouring, but the negative pressure changes are often ignored during the casting process, which causes casting defects. This problem can be well solved by taking a method of adjusting the negative pressure during the casting process according to the size of the casting and the amount of pyrolysis products.

Second, what factors should be considered when choosing a casting method?

Casting methods are commonly used for sand casting, followed by special casting methods such as metal casting, investment casting, gypsum casting, and the like. Sand casting can be divided into clay sand type, organic binder sand type, resin self-hardening sand type, lost mold and the like.

Principles of casting method selection:

1 Sand casting is preferred, mainly because sand casting has lower cost, simple production process and shorter production cycle than other casting methods. When the wet type does not meet the requirements, consider using a clay sand table dry sand type, dry sand type or other sand type. Clay wet sand castings can weigh from a few kilograms up to tens of kilograms, while clay dry castings can weigh up to tens of tons.

2 The casting method should be compatible with the production batch. Casting methods such as low-pressure casting, die-casting, and centrifugal casting are only suitable for mass production because of the high cost of equipment and molds.

3 The modeling method should be suitable for the factory conditions.
For example, it is also the production of castings such as large machine bed, generally adopting the core forming method, not making the pattern and the sand box, and assembling the core in the pit; while the other factory adopts the sand box modeling method to make the appearance. Different production conditions (including equipment, site, staff quality, etc.), production habits, and accumulated experience are different. It should be considered according to these conditions what products are suitable and what is not suitable (or not).

4 To balance the accuracy requirements and costs of castings.


Third, the defects and prevention in the heat treatment of the mold

1. The surface of the mold has soft spots
After the heat treatment of the mold, there are soft spots on the surface, which will affect the wear resistance of the mold and reduce the service life of the mold.
(1) Reasons for production
The mold has scale, rust and local decarburization on the surface before heat treatment. After quenching and heating, the cooling and quenching medium is improperly selected, and the impurities in the quenching medium are excessive or aged.
(2) Preventive measures
Before the heat treatment of the mold, the scale and rust should be removed. When the mold is heated during quenching, the surface of the mold should be properly protected. Vacuum furnace, salt bath furnace and protective atmosphere furnace should be used for heating. When quenching and heating, select a suitable cooling medium, and filter the long-term cooling medium frequently, or replace it regularly.

2. The mold is poorly organized before heat treatment
The final spheroidized structure of the mold is coarse and uneven, and the spheroidization is imperfect. The structure has mesh, band and chain carbides, which will cause the mold to crack after quenching, resulting in scrapping of the mold.

(1) Reasons for production
There is serious carbide segregation in the original structure of the die steel material. Poor forging process, such as forging heating temperature is too high, deformation is small, stop forging temperature is high, and the cooling speed after forging is slow, so that the forged structure is coarse and there are mesh, band and chain carbides, so that spheroidizing annealing It is difficult to eliminate. The spheroidizing annealing process is not good, such as the annealing temperature is too high or too low, the isothermal annealing time is short, etc., which may result in uneven spheroidizing annealing or poor spheroidization.

(2) Preventive measures
Generally, according to the working conditions of the mold, the production batch and the toughening performance of the material itself, try to select a good quality mold steel material. Improve the forging process or use normalizing heat treatment to eliminate the non-uniformity of the network and chain carbides and carbides in the raw materials.

High-carbon die steel with severe segregation of carbides that cannot be forged can be subjected to solution heat treatment. The correct spheroidizing annealing process specification for the forged blank can be tempered heat treatment and rapid uniform spheroidizing annealing. The furnace is properly installed to ensure the uniformity of the temperature of the mold blank in the furnace.

3. The mold produces quench cracks
The cracking of the mold after quenching is the biggest defect in the heat treatment process of the mold, which will cause the finished mold to be scrapped, causing great loss in production and economy.

(1) Reasons for the cause
There is severe network carbide segregation in the mold material. There are mechanical or cold plastic deformation stresses in the mold. Improper heat treatment (heating or cooling too fast, improper selection of quenching cooling medium, low cooling temperature, too long cooling time, etc.).

The shape of the mold is complicated, the thickness is uneven, the sharp corners and the threaded holes are used, so that the thermal stress and the structural stress are excessive. Excessive heating of the quenching heat causes overheating or overheating. After quenching, the tempering is not timely or the tempering time is insufficient. When the quenching is heated, the quenching is performed again without intermediate annealing. Heat treatment, improper grinding process. When subjected to electrical discharge machining after heat treatment, high tensile stress and microcracks are present in the hardened layer.

(2) Preventive measures
Strictly control the intrinsic quality of the mold raw materials, improve the forging and spheroidizing annealing process, eliminate the network, ribbon, and chain carbides, and improve the uniformity of the spheroidized structure. The mold after mechanical processing or after cold plastic deformation shall be subjected to stress relief annealing (>600 ° C) and then subjected to heat quenching. For molds with complex shapes, asbestos should be used to block threaded holes, and the dangerous section and thin wall should be wrapped, and graded quenching or austempering should be used.

Annealing or high temperature tempering is required for rework or refurbishment of the mold. Preheating should be adopted during quenching heating, pre-cooling measures should be taken during cooling, and suitable quenching medium should be selected. The quenching heating temperature and time should be strictly controlled to prevent overheating and over-burning of the mold.

After the mold is quenched, it should be tempered in time, the holding time should be sufficient, and the high alloy complex mold should be tempered 2-3 times. Choose the right grinding process and the right grinding wheel. Improve the mold EDM process and perform stress relief and tempering.

4. The microstructure of the mold after quenching
After the mold is quenched, the coarse structure will seriously affect the mechanical properties of the mold. When used, the mold will be broken, which will seriously affect the service life of the mold.

(1) Reasons for the cause
The mold steel is confused, and the actual steel quenching temperature is much lower than the quenching temperature of the required mold material (such as GCr15 steel as 3Cr2W8V steel). The steel was not subjected to the correct spheroidization process before quenching, and the spheroidized structure was poor. The quenching heating temperature is too high or the holding time is too long. Improper placement in the furnace and overheating in the vicinity of the electrode or heating element area. For molds with large cross-section changes, the quenching heating process parameters are not properly selected, and overheating occurs at thin sections and sharp corners.

(2) Preventive measures
Before the steel is put into storage, it should be strictly inspected to prevent the steel from being confused. Proper forging and spheroidizing annealing should be performed before the mold is quenched to ensure good spheroidization. Correctly formulate the mold quenching heating process specification and strictly control the quenching heating temperature and holding time. Regularly check and calibrate the temperature measurement instrument to ensure the normal operation of the instrument. Keep an appropriate distance from the electrode or heating element when heating in the furnace.