Optimal control of the most popular lost foam cast

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Optimization control of lost foam casting production system

abstract the optimization control of key technical points in lost foam casting production system is discussed, such as the control of various raw materials, the control of model and coating preparation process, the control of dry sand molding and the control of various process conditions. By controlling the key technical points, the stability of process conditions is taken as the basic element of stable production. Especially with the popularization of lost foam casting technology, many small and medium-sized enterprises have been put into production one after another, and they do not fully meet the standard conditions for lost foam casting production for the time being. In this situation of international standardization in the field of long and short Electromechanical, strengthening other conditions and using corresponding compensation measures can also produce qualified castings

key words: Lost Foam Casting, production system, optimal control

the advantages of lost foam casting technology are becoming more and more obvious, and many small and medium-sized enterprises have been put into production. Especially in recent years, EPC enterprises have sprung up, which can be clearly realized from the order demand of EPC equipment manufacturers. However, there are also many lost foam casting enterprises that have dismounted soon after they were put into production. The reason is that many enterprises do not have a deep understanding of the lost foam casting process. At first, it seems that the lost foam casting process is very simple, and they did not fully estimate its regularity and complexity, and rushed to the horse. As a result, they encountered unexpected problems in actual production, and they were not prepared for them. They were helpless to deal with the problems, and they got off the horse in a hurry. The result is not only a waste of time, money and energy, but also the loss of the market

in fact, EPC is a systematic engineering, which can be used to produce various castings in theory, and has achieved extensive success in the laboratory. However, due to the widespread use of foam polystyrene (EPS) for packaging as the model material of lost foam casting in China, and the lack of technical advantages of special coating technology, as well as the immature dry sand vibration technology and lost foam casting process, many early-stage enterprises, especially small and medium-sized enterprises, do not have a deep understanding of these, so there will be a passive situation of rushing up and down

does EPC technology really have no development prospects in China? The answer is No. Through more than ten years of production practice, it is a strong proof that the number of enterprises that adopt EPC technology for casting production in China has grown from less than ten at the beginning to hundreds at present. There are many successful examples of lost foam casting production in China, and there are also a considerable number of enterprises that fail to achieve the expected results. Analyzing the successful experience and the lessons of failure, for the development prospect of EPC in China, the key lies in the cognition of this process and the optimal control of the production system of this process, including the optimal control of raw materials, coating technology, dry sand compaction technology and EPC process technology [1]

I. optimization control of raw materials

the raw materials required for EPC production are roughly divided into model raw materials, dry sand raw materials, coating raw materials, alloy smelting raw materials and so on. As the EPC process is a systematic project, the selection of raw materials is particularly important. Therefore, controlling the quality and parameters of various raw materials has become the basis for the success or failure of EPC

model materials are usually called beads. Beads used in casting are generally divided into two types, namely polystyrene beads and PMMA polymethyl methacrylate beads, both of which belong to polymer materials. There is also a polymer of ps+pmma. For low carbon steel castings, the carbon in the model material is easy to cause Carburization on the casting surface, resulting in various carbon defects. Among them, the influence of PS (containing 92% carbon), PS + PMMA copolymer and PMMA (containing 60% carbon) on the carburization of castings decreases in turn [2]. In addition, the density of the model is an important control parameter of its gas evolution. The gas evolution of the above three materials from small to large is PS, PS + PMMA copolymer, PMMA. At the same time, the size of beads should be selected according to the wall thickness of the produced castings. Generally, the thicker and larger castings choose the beads with coarser particle size, on the contrary, the thinner castings choose the beads with finer particle size, so that the thinnest part of the castings should keep more than three beads

in addition, the pre delivery and forming control of model materials is also a key to the success of technology. Generally, the density of pre released beads is controlled at about 0.024 ~ 0.03g/cm3, and its volume is about 30 times that of the original beads. The density of the forming model is controlled at about 0.02 ~ 0.025g/cm3

dry sand is the molding material of lost foam casting. Due to the characteristics of this process, the selection of dry sand should be related to the casting material produced. Dry sand with high fire resistance and coarse particles is used for superalloys. At present, natural quartz sand is mainly used for dry sand. Iron slag, dust and moisture in the sand should be removed, and the service temperature should not be higher than 50 ℃

coating is an essential raw material in lost foam casting. Now many foundries use self-made coating. The main function of coating is to improve the strength and stiffness of the model and prevent damage or deformation; Isolate molten metal and mold; Exclude model gasification products; Ensure the surface quality of castings, etc. The refractory aggregates in EPC coating mainly include zircon powder, bauxite, brown corundum powder, quartz powder, talc powder, mullite powder, mica powder, etc. Its particle size grading should take into account the prevention of sand sticking and high-temperature permeability. The particle shape is conducive to improving the permeability. Usually, a certain number of spherical particles are selected, which is conducive to the escape of gas after model gasification or the elimination of liquefaction products of incomplete decomposition of the model

II. Control of coating preparation

the carrier of EPC coating is mostly water-based, which is conducive to environmental protection. Its adhesives mainly include clay, water glass, syrup, pulp waste liquid, white latex, silica sol, etc. The following factors should be considered in the selection of adhesives: high temperature gas generation; Coating property; Coating strength and stiffness; Etching model, etc. The suspending agent is used to prevent the coating from deposition, layering and caking, making the coating thixotropic. Generally, bentonite, attapulgite clay, organic polymer compounds and their complexes can be used. In addition, surfactants need to be added to the EPC coating to increase the coating hanging property and improve the affinity and bonding strength between the coating and the model surface. In addition, other additives are often added, such as defoamer, water reducer, preservative, pigment, etc. [3]

for this reason, the coating is required to have good strength, air permeability, fire resistance, thermal insulation, rapid cooling and heating resistance, moisture absorption, cleaning, coating and hanging, suspension, etc. In general, it mainly includes working performance and process performance

the working properties of the coating include strength, air permeability, fire resistance, thermal insulation, resistance to rapid cooling and heating, etc., which are mainly the properties that should be possessed during pouring and cooling, of which the most important are strength and air permeability. The process performance of the coating includes coating and suspension, which is mainly the performance required in the coating and hanging operation

generally, water-based coating is used in lost foam casting, and the coating is generally not wet with the model, so it is required to improve the coating properties of water-based coating. Coating property refers to that the model generally needs to be hung and dried after coating. It is hoped that the coating will not drip as soon as possible after coating, so as to ensure the uniformity of the coating layer and reduce environmental pollution. Suspension refers to that the coating maintains the uniformity of density without deposition during the use of the coating

the preparation process control of coating is the key link of coating technology. Domestic coatings mostly adopt the process of grinding, roller mixing or mixing. For example, iron quality is heavy. According to production practice, the quality of grinding and roller mixing is better than that of mixing. It is suggested that qualified enterprises can prepare coatings by grinding or roller mixing

because different alloys have different effects on coatings, it is suggested to develop corresponding coatings according to different types of alloys, such as cast iron coatings, cast steel coatings, non-ferrous alloy coatings, etc. In the process of coating preparation and mixing, reasonable aggregate grading should be used as far as possible to make the aggregate mix evenly with adhesives and other additives

in addition to meeting the requirements of coating performance, the coating and drying process also have a certain impact on production. Dip coating is often used in production, and it is best to complete it at one time. It can also be applied twice, but it should be dried after each application. During drying, pay attention to the uniformity of drying temperature and drying time to ensure that the coating is completely dry and does not crack

III. control of dry sand molding process

dry sand molding is to embed the model in the sand box, vibrate and compact on the shaking table, ensure that the dry sand around the model is filled in place and obtain a certain degree of compactness, so that the molding sand has sufficient strength to resist the impact and pressure of liquid metal

the first step of dry sand modeling is to add dry sand to the sand box. In order to ensure that the dry sand is filled in place, first add a certain thickness of bottom sand to the sand box and vibrate it tightly, then put it into the model cluster, then add a certain thickness of dry sand, bury the model cluster to one-third to one-half, and then vibrate appropriately to promote the dry sand to fill the cavity of the model. Finally, fill the sand box for vibration. The vibration time should not be too long to ensure that the model will not be damaged and deformed, and that the coating layer will not fall off and crack

vibration parameters should be selected according to the casting structure and model cluster form. For most castings, vertical unidirectional vibration should be generally adopted. For castings with complex structure, unidirectional horizontal vibration or two-dimensional and three-dimensional vibration can be considered. The magnitude of vibration intensity has a great influence on dry sand molding, and the vibration intensity is expressed by vibration acceleration. For castings and model clusters with general complexity, the vibration acceleration is between 10 ~ 20m/S2. The amplitude is an important vibration parameter that affects the model to maintain a certain stiffness. The amplitude of EPC is generally 0.5 ~ 1mm [4]. The choice of vibration time is delicate, which should be combined with the structure of castings and model clusters. But in general, the vibration time should be controlled within 1 ~ 5min. At the same time, the vibration time when the bottom sand and the model cluster are buried in half should be as short as possible, which can be selected as 1 ~ 2min. The vibration time after the model cluster is completely buried is generally controlled within 2 ~ 3min

IV. control of casting process

EPC process includes design of gating and riser system, pouring temperature control, pouring operation control, negative pressure control, etc

gating system plays a very important role in lost foam casting process and is a key to the success of casting production. In the design of gating system, the particularity of this process should be taken into account. Due to the existence of model clusters, the behavior of molten metal after pouring is very different from that of sand computer automatically collecting data and calculating results. Therefore, the design of gating system must be different from sand casting. When designing the section size of each part of the gating system, the resistance caused by the existence of the mold during the molten metal pouring of EPC should be considered, and the minimum flow resistance area should be slightly larger than that of sand casting

due to the variety of castings 4 The vibration and shape caused by the universal experimental machine itself are different, and the specific production process of each casting has its own characteristics, which are very different. These factors directly affect the accuracy of gating system design results. For this purpose, castings can be classified in some way. For small and medium-sized castings, they can be classified according to the characteristics of casting production process, as shown in Table 1 [2]. The combination mode of model clusters can basically reflect casting

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