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The density of the lost foam pre-expanded beads determines the density and degree of fusion of the various parts of the die, and the quality of the die determines the quality of the casting. The main factors affecting the density of the pre-expanded beads of the lost foam are the quality of the raw materials and the performance of the pre-foaming machine. The use of raw materials that meet the requirements of the lost foam process and the high-performance pre-expanding machine for the lost foam can achieve the same density disappearance. Mold pre-expanded beads.

Loss-mode pre-expanded beads of uniform density are the basis for the production of high quality molds. As the first process of lost foam casting, the pre-issue t-package is the most important part of lost foam casting.

1. The importance of the quality of the lost mode die

1. 1 The importance of the quality of the lost mode die
As one of the most advanced casting processes in the world, lost foam casting can achieve precision casting precision.

The lost foam casting line consists of three parts: self-region, yellow zone and black zone. From the pre-fabrication of raw materials to the casting of castings, after a series of production processes, there are many factors that affect the quality of the chess pieces. Kind. According to the statistical analysis of the defects of the lost foam casting, 70% of the casting defects are caused by the poor quality of the die. Once the die is manufactured, its quality is difficult to adjust in the subsequent process. The quality of the die determines the quality of the lost die casting. The high quality die is the primary prerequisite and guarantee for the production of high quality castings.

1. 2 Main features of high quality lost foam die

1) The die is evenly filled and the density of each part is consistent.
2) The consistency of the beads is consistent
3) The surface of the die is smooth and flat
4) Accurate die only inch

1.3 How to get high quality lost mode die

1) Use suitable materials that meet the lost foam production process.
2) Using a high-performance foaming pre-expansion machine to obtain pre-expanded beads of uniform density, thus ensuring uniform density and uniformity of the die.
3) Using a high-performance automatic mold and mold for the lost-foam mold, the scientific mold forming process is designed to ensure that the parts of the mold are evenly filled, and the precise size and perfect bead fusion degree are obtained.

2. Effect of pre-expanded bead density on die and casting

2.1 Effect on density, fusion and surface finish of the die
The pre-expanded beads have the same density and uniform particle size, and the density of each part of the produced die is uniform, the degree of fusion is uniform, especially the surface finish is high, the casting defects can be reduced, and the quality and surface finish of the casting can be improved.

2.2 Influence on castings of multiple pieces
In the lost foam production process, more than 85% of the models need to be sliced ​​and then bonded into a complete model. Generally, the die is divided into 2-3 pieces, and the complex one needs to be divided into 4- 5 pieces. If the density of the pre-expanded beads is inconsistent when the sheets are formed, the density of each part of the mold after bonding may be deviated. The density deviation of each sheet exceeds 0.6 g / liter, which will cause scratch casting defects. The density deviation of each part of the model is large. When the temperature and speed of the casting are the same, the burning speed, combustion decomposition and gas generation of the model are constantly changing. It is easy to bonfire (back spray) during casting, and the casting is easy to cause wrinkles ( Defects such as carbon deposits, pores, oversize and deformation.

3. Analysis of the influence of the quality of raw materials on the density of pre-expanded beads

3.1 Quality of raw materials
At present, there are no EPS materials for the lost molds in China, and most of the lost foam companies use EPS materials for packaging. Compared with the EPS material used in the lost mold of Styrochem, the stability of the raw material is slightly worse.

3. 1. 1 Feng Li Trail
The particle size consistency of the domestic EPS raw materials is poor, and the particle size fluctuation of the beads after pre-expansion is large, which will affect the degree of fusion and surface roughness of the various parts of the die, and reduce the quality of the casting.

3. 1. 2 amylose content
The content of pentamidine is an important factor affecting the quality of pre-expansion of beads. The content of pentamidine in EPS is most suitable for pre-expansion at 5.9%-6.5%. The content of pentamidine in the EPS is inconsistent, and it is necessary to constantly adjust the parameters of the pre-foaming machine to stabilize the density during pre-carrying. There are many factors affecting the change of the content of pentane, such as: the fluctuation of the content of pentamidine in the domestic EPS itself is large. The volatilization of the amylose v. caused by the change of the temperature during storage and transportation of the raw material manufacturer. The pentylene is volatilized.

After the same batch of EPS raw materials arrived at the factory for 7 days, 3 bags were randomly selected for testing. The content of pentamidine in the 3 bags of raw materials was inconsistent (9%, 7.8%, 7.3%, respectively), and the results of 14 days, 21 days… 54 days later It shows that in the same batch of EPS raw materials, the content of pentamidine and the rate of volatilization of amylose are also different. Therefore, it is difficult to produce a high-quality die using a material having a large particle diameter error and an inconsistent EP material.

4. Analysis of the effect of foaming machine performance on the density of pre-expanded beads

After long-term application, domestic lost foam enterprises have realized the importance of bead pre-expansion and die-forming in lost foam casting.

4.1 Performance analysis of domestic pre-expander

1) Steam pressure control is inaccurate, the degree of drying of the beads is inconsistent, and the curing time cannot be accurately controlled. The pre-expanded beads have a density fluctuation of 1.3 – 2.4 g.
2) The accuracy of the feed weighing is low, the weighing deviation is 8 – 9%, so that each time
The weight of the beads entering the pre-expansion chamber varies, and at the same vapor pressure and pre-expansion time, the higher the feed, the higher the density, and the lower the feed, the lower the density.
3) No density detection and feedback control device, manual adjustment of pre-fire gun parameters based on manual experience.
4) Sometimes pre-expanded beads are agglomerated to form a dead material.
5) The beads remaining in the pre-expansion chamber after each pre-expansion cannot be removed. When the content of pentamidine in these beads volatilizes to a lower level, a dead material is formed, and the dead material cannot be in the molding machine. Perform secondary foaming. When the dead material is mixed into the next batch of pre-expanded beads and transported to the molding machine to form the die, they will shrink, resulting in inconsistent bead density and degree of fusion inside the die, and formation of holes in the surface of the die. , causing scratch casting defects, causing loose casting and slag holes.

5. Germany Teubert lost mode special pre-expansion hunger

Germany Teubert intermittent foaming mold pre-foaming machine, fully automatic pre-foaming production, the operator only needs to input the target density of the beads into the control computer, the system will automatically adjust the parameters and control the density.

Main performance characteristics:

1) Fully automatic production, suitable for EPS and copolymer pre-expansion, foam chamber temperature can be adjusted from 70 °C -13O”C, steam chamber can be adjusted at any time, steam pressure control accuracy O.Olbar The density fluctuation after pre-expansion of domestic beads is: 1:0.3 g, the density fluctuation after pre-expansion of imported beads is: 1:1 g

2) Proprietary density control device, fully automatic detection of the density of pre-expanded beads per beat, and feedback to the PLC to adjust various parameters, full-closed detection and adjustment parameters, reducing external factors on the pre-foaming machine Interference ensures the accuracy of density control.

3) Automatic weighing and adjusting the weight of the beads entering the pre-expansion chamber each time, the feeding weighing accuracy is high, and the weighing deviation is less than 2%.

4) The proprietary pre-foaming chamber design facilitates the cleaning of residual beads and avoids the mixing of dead materials into the next batch of pre-expansion operations, ensuring the quality of the die and castings.

5) The fluidized drying bed with adjustable temperature ensures the uniform drying degree of the beads and better controls the curing time.

6. How to get the density – honey i Xiao lost mold pre-package rough recommendations

6.1 Selection of raw materials
It is preferred to select the special materials for the lost foam with uniform particle size and stable pentane content; or to meet the requirements of the lost foam process, select raw materials with smaller particle size and relatively stable pentamidine content, and reduce the raw material to the die and casting. The impact of quality.

6. 2 Storage of raw materials
When the raw materials are shipped from the factory, they are packed and transported in a sealed container. When the raw materials arrive at the factory, they are stored in a special constant temperature storage room. The temperature of the storage chamber is stabilized at 15″C -20 .C, which is as slow as possible. The volatilization rate keeps the content of pentamidine stable.

6.3 Selection of pre-foaming machine
The performance of the domestic pre-foaming machine is not good, but the price is straight, the imported pre-foaming machine has superior performance, but it is expensive. The performance of the pre-explosive gun is an important factor affecting the uniform density of pre-expanded beads. Try to use a high-performance lost-mode special pre-emphasis machine to obtain stable high-quality pre-expanded beads. After selecting the right raw materials and the pre-emphasis machine for the lost foam, the pre-expanded beads with uniform density can be obtained, which makes it possible to produce high-quality molds and lays a foundation for the production of high-quality castings.

With the rapid development of the automobile industry, the production and use of large-scale stamping parts has increased sharply, and the demand for medium and large-sized molds has also increased significantly. In recent years, a large number of researches and practices have shown that the yield strength of nodular cast iron is higher than that of ordinary steel. The matrix structure is the same as that of steel. The spherically distributed graphite is not easy to generate stress concentration. In addition, ductile iron can be immersed in lubricating oil, and the graphite in the structure has self-lubricating. The effect can effectively reduce the friction in stretching. In theory, the heat treatment process that can be applied to steel is basically applicable to ductile iron. Therefore, the microstructure of the ductile iron can be changed by appropriate heat treatment to improve its performance. The ductile iron material is the preferred material to replace the alloy mold steel, and the high-strength gray cast iron material can also replace part of the carbon steel.

The FM method is a solid self-hardening sand casting method, which is different from the dry sand binderless vacuum molding method. It has many years of production history and mature experience in China. In the production of cast iron parts, furan resin is usually used to fill the mold to cover the vaporized foam mold. After the mold is hardened, the molten iron is poured into the cavity, and the foam mold is burned out in the sand mold to form a casting. This method eliminates the process of demoulding, tumbling, lower core and boxing in general sand casting, which improves the overall dimensional accuracy of the casting, reduces the labor intensity and shortens the production cycle, and is suitable for casting large and medium single piece cast iron molds. method.

How to choose the right coating in the real casting process is a crucial step. In the past, in the process of coating configuration, the different materials and particle size distribution of the aggregates were adjusted according to the requirements of the castings to obtain the technical indexes of different refractoriness, coating strength, sintering performance and gas permeability of the coating. Considering the different mold casting size and wall thickness, the different characteristics of the paste-like solidification of ductile iron and the layered solidification of gray iron, the most suitable coatings often need to be customized. This method is difficult to achieve under the current commercial conditions of coatings. To this end, I have tested the use of two or more different types of coatings, in the use of the site according to the needs of the composite configuration of the process to meet the needs of the field. This not only takes into account the commodification, standardization, but also meets the individual needs.

1, paint selection and performance testing

According to the general requirements of cast iron lost molds, we chose two coatings as the basic coating. One is a graphite alumina-based paint specially used for cast iron parts (hereinafter referred to as E5), and the other is a high-refractory, high-breathing paint (hereinafter referred to as E2) with a brown corundum base as the main aggregate, as a follow-up test. And tested the base paint.

1.1, refractoriness
Refractoriness, also known as refractory degree, refers to the temperature at which a material reaches a certain degree of softening under high temperature. It is an important technical index for evaluating refractory materials. The degree of refractoriness is not a physical constant of a substance, but a technical index whose height is determined by the chemical composition of the material, the degree of dispersion, the proportion of the liquid phase in it, and the viscosity of the liquid phase.

The meaning of refractoriness is different from the melting point. The melting point is the temperature at which the liquid phase and the solid phase of the crystal are in equilibrium. The degree of refractoriness is the temperature at which a multiphase body reaches a certain degree of softening. For most common refractory materials, they are heterogeneous heterogeneous materials, without a certain melting point, and the liquid phase to complete melting is a gradual process. In a fairly wide range of high temperatures, the solid and liquid phases coexist. Therefore, the characteristics of softening and melting of such materials at high temperatures can only be measured in terms of refractoriness.

The test standard for refractoriness is in accordance with GB/T 7322-2007 refractory test method for refractory materials. The test principle is to plant the test cone of refractory material on the frustum together with the standard temperature measuring cone of known refractoriness under the specified conditions. The degree of refractoriness of the test cone is expressed by heating and comparing the bending of the test cone with the standard temperature measuring cone.

The test parameters are: rated temperature 1800 °C, heating rate 0-20 °C / min, cone table rotation speed 3r / min, control mode is microcomputer automatic measurement and control.

It has been found through experiments that the refractoriness of the coating E5 is 1550 ° C, and the refractoriness of the E 2 is 1680 ° C. The refractoriness of the coatings of the two materials is greater than the general pouring temperature of the cast iron parts. Considering that the actual wall thickness and the thermal joint of the casting have different requirements on the refractoriness of the coating, the two coatings can be compounded according to the actual use requirements of the casting, and the ideal state has been achieved.

1.2, gas permeability
There are many factors that affect the permeability of the coating. In the case of selecting the same bonding system and the same coating thickness, the change in the type and composition of the refractory aggregate in the coating has an important influence on the gas permeability. It was verified by making three kinds of representative different aggregates into test samples and measuring the gas permeability of the samples. The test method adopts the coating of the current common metal mesh sheet, and after drying, the gas permeability data is measured in the sand permeability tester.

The larger the particle size of the powder, the higher the gas permeability. This is because as the particle size of the powder becomes larger, the gap between the powders becomes larger, and the gas permeability of the coating naturally increases. And among the three aggregates of brown fused alumina, zirconium powder and bauxite, brown corundum has the best gas permeability. When the average particle size is 0.038mm, the permeability of brown corundum is better than that of the other two aggregates of 0.075mm. When the particle size of the three aggregates reaches 0.075mm, the permeability of brown corundum reaches the highest. It is twice as much as the other two aggregates. It can be seen from the results that the aggregate structure of the brown corundum can improve the gas permeability of the coating.

The structure of the corundum powder is an irregular polygonal crystal, and the powder has a large hardness, and this property easily forms voids between the powders. Moreover, the brown fused alumina has small thermal expansion and uniformity, so that the volume under high temperature is stable and cracked, and the size of the gas permeable pores is not changed, so that the gas permeability of the coating is higher than that of other powders.

Through the gas permeability test of aggregates, we should choose brown fused alumina as the main aggregate to make coating E2 to meet the high gas permeability requirements of customers. However, the one-sided improvement of gas permeability is at the expense of reducing the surface quality of the coating. In order to ensure the gas permeability and also the surface quality of the coating, we have chosen E5 coating made of bauxite as the main aggregate. The two coatings are used in combination according to actual needs.

2, the use of paint

For the production of cast iron parts, graphite is an ideal choice for refractory aggregates. Graphite is easy to produce a reducing atmosphere at the mold interface, inert to iron oxide, and produces a joint between the mold and the casting interface at high temperatures. A bright carbon film that is not wetted by metals and metal oxides, allowing the sand layer to be easily peeled off. Finally, the surface of the casting is smooth, and the graphite has good thermal stability and a small expansion coefficient. The coating can withstand extreme temperature changes at high temperatures without cracking.

According to the experience of our comprehensive majority of customers. Unless the special complicated parts are used, the castings can be directly selected from the E5 type below 3 tons, which can basically meet the requirements of refractoriness and gas permeability; the casting ratio of the castings in the range of 3 to 10 tons of E5 and E2 is 6: 5, that is, 6kgE5 and 5kgE2 are used in combination; if the casting is more than 10 tons, the mixing ratio of paint E5 and E2 is generally required to be 3:10. Of course, it can also be adjusted according to the actual process of the customer when using it on site.

2.1 brushing
In some smaller companies, the brushing process is generally used in view of the site and turnover. Brushing method, as its name suggests, is mainly done by hand through the brush. Relatively speaking, the technical requirements of the coating method for the coating are not as strict as the spraying method and the dip coating method, but since it is mainly done manually, for the operation The technical and experience requirements are relatively high. The biggest disadvantage of the brushing method is the high labor intensity and low production efficiency. Almost all work is done manually.

When using the brushing process, the general control of the composite paint is between 90 and 100, and the paint is applied with 2 to 3 layers. It is important to note that since it is hand-painted, it is inevitable that there are more or less in some places, so it is necessary to prevent a large amount of paint from accumulating in corners and deep pits. Figure 4 shows the mold casting used for manual brushing.

For most large-scale companies, the flow coating process is generally recommended. At this time, the Baume degree of the composite coating is generally controlled between 65 and 70, and the coating is applied to 2 to 3 layers. Since the paint at this time is relatively thin, after the flow coating is completed, the excess paint will naturally flow away when the model is turned over, and there is no accumulation in some places as in the case of brushing. Figure 5 is a view of the case where the mold casting is flow coated.

3, field application practice

3.1 automotive stamping die castings
A large-scale foundry enterprise in Anhui uses furan resin sand as a mold, and uses a lost foam casting process to mass produce automobile stamping die cast iron parts. The coating process is to apply a composite coating with a weight ratio of 6:5 (E5:E2) on the pattern. The coating has a Baume degree of 68, the coating flow is applied twice, the total thickness of the coating is about 0.8 mm, and the pattern after drying is shown in Fig. 6. The casting material is HT300, the casting temperature is 1390 °C, the casting weight is 7.5 tons, the casting result is shown in Figure 7. After the casting is cleaned, the surface is smooth and the size is accurate.

3.2 Large machine tool castings
A foundry enterprise in Jiangsu Province also uses a furan resin sand lost foam process to produce large machine tool castings. The coatings are brushed, the Baume degree is controlled at around 90, and the weight ratio of the coating is 3:10 (E5:E2). The coating is twice, the coating thickness is about 1mm, the pattern after brushing the composite coating is shown in Figure 8; the casting material is HT250, the casting weight is 13.2 tons, and the casting temperature is 1450 °C.

4 Conclusion

For the use of the lost foam process for large mold cast iron parts, the choice and rational use of the paint is particularly critical. Through the research in this paper and the experience of similar factories, we can draw the following conclusions:

1) By selecting the composite use of two different properties of the lost foam coating, the field can be flexibly adjusted according to the process requirements, and the most suitable use effect has been achieved;
2) The refractoriness and permeability of the coating are important indicators for measuring the performance of the lost foam coating;
3) The use of graphite as a refractory aggregate for cast iron parts is an ideal choice, considering its refractoriness can be added as part of the refractory material;
4) The structure of brown corundum powder is irregular polygonal crystal, and the hardness of the powder is also large. This property is easy to form voids between the powders, so that the permeability of the coating is high;
5) The coating process can significantly improve the working efficiency by using the flow coating process, and the uniformity of the coating can be better ensured.

The production of aluminum alloy castings by the lost foam process has become an unstoppable trend. Cylinder top cover, new energy vehicle gearbox, heavy truck gearbox, etc., have gradually realized mass production in China, especially the mass production of 12-speed truck gearboxes. It marks the arrival of a new era of China’s lost foam aluminum alloy casting. The successful development of the 0.12 gearbox gearbox’s lost-mould mold has laid the foundation for its mass production.

The basic structural shape of the gearbox, the product is a cavity structure, the outer part is full of reinforcing ribs, and the middle partition plate technical requirements: wall thickness 8 mm, casting material ZL101A, white mold material: EPS, castings must not have sand holes, pores, shrinkage , cracks and other defects, non-machined surface should be smooth and smooth, surface shot peening; castings using T6 solution heat treatment plus full artificial aging, casting hardness of 90-100HB castings dimensions: 589X589X404 (unit: mm), weight: 48.9 kg .

12-speed truck gearbox lost foam process casting difficulties:

1. The material is aluminum alloy, the casting temperature is 720-750 °C, the casting temperature is low, and the white mold is not easy to be gasified and liquefied.
2. The shape has many reinforcing ribs, the spacing of the reinforcing ribs is small, and it is easy to stick sand.
3, the internal partition and ribs more than oil holes, rib thickness 9.7mm, hole size: 30X27 (unit: mm), the ribs are easy to tear when forming.
4. The product is open on both sides, the internal cavity, the white mold is difficult to demould, and the anti-deformation measures should be taken during casting.
5. The shape of the box body is multi-shaped, and there are many side core inserts when the white mold is formed.
6. The coaxiality of the three bearing holes of the intermediate partition and the end face must be guaranteed.
7. The 4 sides of the box are closed, and it needs to be formed by two pieces of bonding.

In order to solve the above problems, it is necessary to introduce a lost foam mold structure of a 12-speed truck gearbox. The mold process should be consistent with the molding equipment of the foundry, conform to the lost foam casting production line of the foundry, and the complete set of lost molds of the foundry manufacturer. The production process is combined, and the mold structure adopts the following technical solutions:

Firstly determine the segmentation scheme, the structural analysis of the gearbox body, combined with the molding machine, the vibrating table, the coating, the bonding process, etc. of the casting manufacturer, the gearbox body should consider the white mold automatic molding process and the automatic bonding in the white zone section. Process. This process should focus on:

First: the white mold segmentation helps to automatically release the mold. The white mold adopts the cylinder automatic ejection mechanism during molding. The cylinder acts on the ejector rod and the stripper plate, so that the white mold is evenly forced to automatically push out 25mm, white mold and convex. The mold body is automatically separated, and the white mold is not deformed during the demolding process and is not displaced. Forming is stable.

Second: the white mold segmentation helps the automatic core pulling, and the inner and outer shape of the box determines its multi-side core pulling mechanism. After the piece is divided, all the side core inserts can be cylinder-assisted, so that the white mold is formed. The appearance quality is good, the efficiency is high, and it is not deformed.

Third: the white mold segmentation helps the automatic filling, and the white mold after the segmentation can effectively use the φ30mm straight-through automatic gun, which is dense in packing and uniform in density, and improves the filling speed of the white mold.

Fourth: the white mold segmentation helps to auto-heat and cool. When the white mold is formed, it can adopt automatic heating and spray cooling for a large area, so that the white mold is mature and the surface is smooth.

Fifth: The white mold segmentation helps the robot to operate. After the white mold is automatically ejected 25mm, the robot uses the suction cup to take out the white mold, and the white mold reserves enough position to place the suction cup.
Sixth: The white mold segment helps to automatically bond, the split surface is straight, no groove or inverted buckle, easy to use with automatic glue glue, easy to bond.

(Luoyang Liu’s Mould Co., Ltd. Liu Zhonghua, Zhang Guangbo)

If you are looking for technical issues related to lost foam casting, design points, or how to apply them on castings, then the following is exactly what you want to see. From the choice of materials to the production of white molds, the process is ultimately.

First of all, about the white mold

In the production of lost foam casting, in addition to those who have a white mold mechanized production foundry, it is a very common production process to use a bonded white mold to enter the lost mold mold in the production of white mold. In production, there are many precautions that require special attention.

1. Selection of materials such as EPS and STMMA.
The quality of the material directly affects the quality of the white mold. Poor quality materials will cause serious carbonation in the production of castings and directly increase the rate of scrap. Poor quality materials, gasification rate during gasification, air pollution is also worse than good quality materials. The gasification speed of the mold and the amount of gas generated will affect the operation of pumping negative pressure. Air pollution directly affects the environmental assessment of foundry companies. These chain reactions are combined and often become unaffordable costs for lost foam foundries.
Therefore, in the selection of materials, we must pay great attention. It is necessary to visit the problem of carbon increase caused by the white mold, and also to avoid the problem of slow gasification of the white mold.

2. In the production of lost foam molds, the mold stress problem should be fully considered.
The mold stress is not only the structural stress of the white mold itself but also the stress generated during the bonding process. There is also an increase in coating stress after brushing the coating. The heat generated during the mold drying process affects the stress.
Solving the problem of mold stress is the primary condition for ensuring the accuracy of lost foam castings.

3, the choice of white mold adhesive
White mold adhesive is an important material for hand-splicing lost molds. It is also the most easily overlooked material in production. Qualified adhesive can not only ensure the bonding of the module, but also reduce the amount of water and reduce the excessive water molecules during casting, thus affecting the quality of the casting.

Five design points of the lost mold

The key to the success of the lost mold design is to understand the importance of close cooperation between the casting manufacturer, the user and the mold designer. The mold designer first meets the requirements of the mold and the needs of the casting factory, and secondly meets the requirements of mold manufacturing and mold forming.

1, casting holes
In other types of casting methods, the casting holes can be obtained by sand cores or by machining (eg drilling). For each casting, whether the casting holes have their corresponding characteristics, but in general, the casting holes The cost of subsequent machining is reduced. The casting holes can be made directly on the mold by special mold design such as using pins, mandrels or other mechanisms.

2, reduce machining allowance
Standardizing the correct EPC mold design can reduce the weight of the casting by reducing the draft and machining allowance. This flexible design is not possible in many other casting methods. It is one of the greatest advantages of the EPC process.

3. Eliminate complex and expensive sand cores
The EPC process eliminates the indispensable sand core in ordinary sand casting. The fixed and movable blocks in the EPC mold form a shape similar to the sand core in the foam model, both in the shape of the sand core.

4. Comprehensive casting design
The parting surface is determined according to the shape of the casting. Generally, the parting surface of the lost foam casting is similar to other casting processes, but in some details it is slightly different: the production designer must work with the mold designer to determine the parting surface. The position of the glued surface and the desired characteristics can reduce the number of model blocks.

5. Sand core characteristics
For castings with grooves, internal cavities, connecting channels or other holes, sand cores are required for other casting methods: in EPC processes, they can be solved by using live blocks or by gluing together the block models.

Process flow

In the lost foam casting process, the whole process includes seven links, each of which has a great influence on the quality of the lost foam casting. Of course, the casting process is also a vital part of the entire lost foam casting process. Precautions for the casting process, strict control of each detail in the casting process is the key to producing high quality lost foam castings.

The casting process of lost foam casting is a process in which the molten steel is filled and the vaporization of the foam mold disappears. The whole process needs to pay attention to:
(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, the sand collapse and the phenomenon of air intake are easy to occur, resulting in casting defects.
(2) When casting molten steel, be sure to grasp it to ensure stable, accurate and fast. Instantly fill the sprue cup and do it quickly and continuously, and the ton of molten steel is about a minute or so. If the molten steel in the same box castings will suck in the air, it may cause sand-slipping phenomenon or increase the porosity of the casting, resulting in the scrapping of the casting.
(3) Lost foam casting is closed with a negative pressure and must be closed below the pouring cup. The molten steel enters the molding die, and it begins to liquefy and burn, and the gasification disappears. The front end of the molten steel forms a temporary cavity at a short distance, so the speed at which the molten steel is filled is almost the same as the speed at which the foam model disappears. In order to prevent the high temperature radiation of molten steel from melting other models in the same tank, the runner is appropriately farther away from the casting model. The position of the sprue is selected from the lowest position of the full box casting. When pouring, pay attention to adjusting and controlling the vacuum of vacuum in a certain range. After pouring, keep it under a certain negative pressure for a period of time. The negative pressure stops and the molten steel condenses out of the box.

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.