1, problems and solutions in design and production of casting production lines at home and abroad.

I. Overview

With the continuous development of national economy, the requirements for castings have become higher and higher in recent years, especially for automobile engine cylinder blocks and cylinder heads, which require not only good materials, but also high dimensional accuracy, high surface finish and light weight. Therefore, as a key component affecting the quality of castings, the molding department has adopted new technologies and equipment to meet the requirements of casting quality and output. According to incomplete statistics, there are about 60 high pressure molding lines, air impact molding lines and static pressure molding lines imported from abroad. There are more than 70 high pressure molding lines and pneumatic molding lines designed and manufactured by ourselves in China.

From the application situation, these molding lines have indeed played an important role in improving the output and quality of castings in China, but compared with our hopes, they are far from enough. The actual production capacity of the import line is generally 50.80% of the designed production capacity, and the domestic line is now estimated to account for only 50%, and the operating rate of this 50% is also low. There are many reasons for the above phenomenon, which can be summarized as follows.

Two. existing problems

Problems in the design of 1.

Due to the complex equipment, many actions and strong logic of the molding line, there are inevitably some inadequacies in the design, especially in the initial stage of the molding line design, there are many problems such as unreasonable material selection, improper component selection and weak logical relationship. This determines that most of the early high-voltage lines in China did not work well. For example, a large factory designed a high pressure molding line in the early 1970s, but it has not been used since its manufacture and installation. The main reasons are: the original position of vertical hydraulic cylinders on many auxiliary machines is set in the middle, and many auxiliary machines cannot be in the original position due to the leakage of domestic hydraulic valves; Running parts do not consider manufacturing errors and hydraulic leakage, and often collide. There is no interlock on the interlocking appliances, which has brought great economic losses to the factory for so many years. It is said that it will be taken off the shelf recently. At home and abroad, the molding line also has design deficiencies. For example, when a factory introduced high pressure molding Line, the inspection and cleaning of the sand box was not considered in the design, so that the sand box was often stuck when entering the dumper, and even the dumper was damaged. In addition, the improper design of the static pressure molding line imported from another factory caused the upper box to turn over and the sand to fall into the inner cavity of the finished lower box, resulting in casting defects.

2. Poor equipment reliability

The factors that affect the design reliability mainly include design, manufacture, installation, production management and maintenance.

Improper selection of parts and unreasonable selection of materials in design is one of the important reasons that affect reliability. In the past, localization and cost reduction were emphasized, so all components were made in China. However, due to the poor quality of domestic equipment, the operating rate of the molding line has been seriously affected. For example, due to the error of mechanical transmission, the track of the transfer car will be misaligned with the track of the cooling channel, which will lead to the derailment of the transfer car and the sand box and cause long-term parking; Seals of the same specification can only be used for 3 ~ 6 months in China, and imported ones can be used for 12 years; The same pipe joint, domestic will leak oil, imported will not leak oil. Only this one, when a molding line is severely punished, it leaks more than 200 tons of oil every year, worth more than 654.38+0 million yuan; Because the proximity switch is not allowed to signal, it often leads to misoperation and parking; The leakage and low accuracy of hydraulic valves and pneumatic valves are also the main factors affecting the operating rate of the molding line. For example, the molding machine in a factory has not only electrical interlocking, but also pneumatic interlocking. The pipe material of pneumatic control pipeline is φ 8× 1, and there are many pipe joints connected. Due to the air leakage of the pipe joint and the air valve, the pressure of the control air path is often reduced, and the air valve cannot be actuated. Therefore, some interlocks have to be cancelled at risk.

Manufacturing quality will also affect the running rate of molding line, including internal quality and dimensional accuracy. For example, because the machining accuracy can not meet the requirements, the moving part and the fixed part of the equipment collide and the positioning is not accurate; The service life and reliability of the equipment will be affected because the material or heat treatment of the parts can not meet the requirements; Because the hydraulic system is not clean, the oil is polluted and the valve is stuck. I have been to a construction site, and the workbench of two main engines is also made of ductile iron. One ball melted for several years, and the other one soon broke down, with a cross section like a hornet's nest. For another example, since the gas flushing line imported from a factory was put into production in 1996, the oil cylinder of the main engine workbench has been replaced three times. The first time it broke down before the warranty period, resulting in a claim for one, and it has been replaced every two or three years since then. In addition, the loose wire connector after long-term use also leads to two or three bad circuits. The installation does not conform to the specifications, and cutting corners is also one of the important reasons for poor reliability. For example, the pipeline was not cleaned according to the specifications during installation, and argon arc welding was replaced by ordinary welding, resulting in welding slag in the pipeline; The place where the pipe clamp is installed is not installed or installed less, which leads to pipeline vibration, loose pipe joint and oil leakage after a long time; KVV should be used where RVV cord should be used, which will lead to open circuit; Welding at the bolted place, and so on.

3. Maintenance difficulties

Due to the lack of on-site experience of designers, the designed equipment often only pays attention to functionality, but not to the convenience of maintenance. For example, some wearing parts or wear-resistant parts can be assembled in the factory in turn, but if they are worn during use and need to be replaced, they must be disassembled into eight pieces to be replaced. This is time-consuming and affects the accuracy of the whole equipment. For another example, the previous filter screen was placed at the oil suction port of the pump and buried in the oil tank. Because of oil pollution, the filter screen is often cleaned, but before cleaning the filter screen, the oil must be drained first, instead of putting the filter screen on the oil return pipe, which is convenient for cleaning and replacement. There is not enough maintenance space when installing the pipe clamp in the valve box or where many pipes are parallel. Once one pipe leaks oil, the other pipes must be removed and tightened, just like planting onions. In the manufacturing process, the quality is neglected, and the parts are seriously out of tolerance, which is also an important reason for the difficulty in maintenance. For example, one part and another part are over-matched, and the assembly becomes an interference fit due to machining errors. Once the parts have problems and need to be replaced, it is difficult to take them out. In addition, the cylinder end pipe joint that is often disassembled does not use ball joint, but uses end right-angle joint, which brings difficulties to maintenance. It is also common to pay attention to the direction of pipes and wires during installation and ignore the possibility of maintenance. For example, some equipment was originally used as maintenance space at a certain distance from the trench wall, but it was not noticed when it was installed, and it was convenient to take pipes or cable troughs, so it was installed, but it was bitter when it was repaired.

4. Insufficient production tasks and high cost.

In today's market economy, the cost of castings is becoming more and more important. In recent years, due to the vigorous development of township and private casting enterprises and the requirements of urban environmental protection, coupled with the low cost and flexible operation of township and private casting enterprises, the castings of these enterprises have become more and more important in the market, resulting in insufficient production capacity of some large and medium-sized enterprises with molding lines. For example, many factories now love to "open three times and stop four times". In the first half of the month, the shift was changed from two or three shifts to a single shift, and long holidays were often taken. The high running cost of molding line is also a factor affecting its use. If the molding line is started, all equipment must be started, including the equipment of relevant engineering departments. This consumes a lot of electricity and all the staff are on duty at the same time. Coupled with the loss of oil leakage, it will be uneconomical to start the machine at low output. For example, there is a factory with a large output and went to an air injection molding line. Later, the output dropped sharply, and it was obviously not cost-effective to open a molding line. In addition, cost accounting was implemented, so the modeling line had to be sealed and changed to ground modeling.

5. Poor management

Without overall planning, fragmentation is serious, which leads some enterprises to launch blindly without considering their actual situation. However, due to lack of funds, the products are not suitable, and although a lot of idle equipment has been invested, it has not yet formed production capacity.

Poor internal management of the enterprise is mainly manifested in: unclear responsibilities of maintenance personnel, lack of clear equipment maintenance system, disconnection between spare parts procurement and maintenance, low quality of maintenance personnel, poor salary and treatment, etc. We often see this phenomenon: when the operator goes to work, the repairman is resting, and when the operator leaves work, the repairman also leaves work. At the very least, whether the equipment needs spare parts and whether it works in spite of illness is bound to be maintained, and no one cares. Equipment can only be repaired if it really fails, and the replacement spare parts are often inappropriate. For example, spare parts on the molding line of a factory are organized by the equipment department. What to prepare online, how much to prepare, basically do not communicate with maintenance personnel, and the purchased spare parts are not compared with the physical objects used on the modeling line. Therefore, valves that used to be 24 volts often appear, but they were changed to 220 volts when they were replaced; It should be an internal control internal leakage valve. If it is changed, it will become an internal control internal leakage valve. Only when the processed spare parts are replaced can the out-of-tolerance phenomenon be found, which will affect production.

6. The ministries don't match.

Due to the great difference between the calibrated productivity of foundry equipment at home and abroad and the actual situation, it often leads to the mismatch between the parts of foundry workshop, thus affecting the starting rate of molding line. According to incomplete statistics, the downtime caused by the mismatch of various parts of the general molding line accounts for about 30-50%. For example, in a factory, a molding line was introduced into the workshop design, but other engineering departments used domestic equipment. After it was put into use, two problems appeared: first, the high failure rate of other engineering departments seriously affected the start-up rate of the molding line, which made the molding line in a semi-shutdown state; Second, the sand mixing capacity is not enough. The mixing capacity of domestic sand mixer can only reach about half of the nominal capacity, but it is considered according to the nominal capacity in design. So, this has caused such consequences. This workshop has been put into production for about three or four years, and the factory has made up its mind to transform the sand treatment department. At present, it is in good use.

Three. Solution to the problem

If you want to use a molding line well, you must do a good job of "prevention" and "preparation". "Prevention" is to prevent problems, and "preparation" is impossible to prevent. When problems arise, you should be prepared to solve them as soon as possible. But to achieve these two points, we must work hard in the following aspects.

1. Strengthen learning, attract advanced technology and experience at home and abroad, and nip in the bud.

The quality of designers directly affects the level of modeling line. Only when the design level is improved can a good molding line be made. Therefore, designers must master the advanced technology and equipment at home and abroad, from "primary school" to "university", constantly sum up experience and gradually improve their design level. In recent years, China foundry equipment designers have fully realized this point. Through their efforts, production practice and absorption of foreign advanced technology and technology, the design level of foundry equipment in China has been greatly improved. They not only have the ability to design high-level modeling lines, but also have the ability to do it on the spot. Through continuous improvement, they designed a number of automatic modeling lines with reasonable layout and reliable performance. These improvements are as follows: in technology, pneumatic microseismic molding is changed into high pressure molding, and then it is developed into air impact molding, static pressure molding and contact dynamic impact molding. Make the equipment more and more simple and the manufacturability better and better. Reliability: In the past, the control of molding line was controlled by sequential controller, which had huge equipment, many faults and difficult maintenance. However, with the PC, we immediately used it in modeling line control. At present, almost no one has said that there is something wrong with electrical appliances. In the past, in order to realize slow-fast-slow motion, auxiliary machines and transfer vehicles were controlled by primary and secondary motors or travel valves. Now with FM motor and proportional valve, it is easy to solve the problem and the reliability is improved. In the past, travel switches used for motion detection and signal are now used as proximity switches or encoders; In the past, due to the high oil temperature, the seal was easy to age, which led to oil leakage and other phenomena, which seriously affected the start of the molding line. Therefore, the cooling area of hydraulic oil is increased, and the model of overflow valve is changed, so that the load is discharged without load instead of overflow, which reduces the cause of heating and temperature drop. Piston accumulator is changed into bladder accumulator, which has reliable performance and sensitive action; Replace unreliable domestic components with imported components. Maintenance: No matter how good a molding line is, it is impossible to have no problems at all, but it is difficult to solve the problems, so the design level cannot be said to be very high. Designers have also made great efforts to this end. It's good: the hydraulic system was broken before, so the system should be removed first and repaired after the oil return is finished. Now there is a valve box on the equipment, and a stop valve is added to the oil inlet and the oil outlet, so it is very convenient to close the valve during maintenance. In addition, for larger parts that are often disassembled, two hanging holes are directly designed in the design, which is very convenient to maintain. Professional design: In the past, many workshops in large factories were designed by their own technicians. However, due to the limitation of specialty and practical experience, there are many problems after the design is completed, especially the mismatch between departments. Therefore, it is best not to invite non-professional technicians to design casting projects, but to invite professional design institutes to design them, so that there will be fewer or no mistakes and no detours.

2. Strengthen quality awareness and improve product quality.

We are all familiar with the phrase "quality is life", but in practice, the understanding of quality is still insufficient, and it should be strengthened to make every employee realize that there is no survival without quality. All operations are carried out in accordance with the specifications, and it is absolutely forbidden to cut corners for a little profit. It used to happen that drawings belonged to drawings and processing belonged to processing. The people who process them don't look at the requirements of the drawings and what the equipment is made of. For example, because the threaded hole of the pipe joint at the opposite end is sealed with a combination pad, the verticality between the threaded hole and the plane of the end face countersunk head is required on the drawing, but the processing workers don't care, or even flatten it, which is easy to cause oil leakage. There are also many screw fixing devices, and often several screw holes are misaligned. So, grind the screw into a tap and screw it or not. Of course, after so many years of production practice, many factories have realized the importance of quality and the processing methods have been improved a lot. For example, many factories now use special planes or machining centers to process sand boxes, and the oil cylinders they made in the past are now outsourced to professional oil cylinder factories. In addition, the quality of basic parts must be improved. In the past, hydraulic valves were fixed with screws of the same grade as 12.9, and imported ones did not leak oil, while domestic ones did. The gear in the reducer requires hard tooth surface, soft tooth surface and large practical resistance, which will be broken if not used much, and so on.

3. Strengthen management, improve the maintenance system and get ready.

First of all, the higher authorities should decide whether to go to the modeling line according to the specific situation of the enterprise, and do the first pass well to avoid dismounting halfway and causing economic losses to the country and enterprises. If you go to the modeling line, you must strengthen the internal management of the enterprise, and the personnel related to the modeling line must be clear about their responsibilities and rights. Whoever has a problem, who is responsible, who solves it. We should have a strict management system, pay attention to the cooperation between departments, and pay attention to the cultivation and rational utilization of talents. No matter how good the line is, it won't work well if the management and maintenance can't keep up. Therefore, we must attach importance to the quality of maintenance personnel. Maintenance personnel must know the molding line very well, understand the purpose of each part, and carry out pre-inspection, pre-repair and inspection at ordinary times, so as to judge the fault quickly and correctly and eliminate the fault in time. I have been to a site, and the maintenance personnel have never seen the hydraulic schematic diagram of the molding line, and they are not clear about the action principle of the whole line. So, when something went wrong, they were in a hurry and finally they could use it. What is the problem and how to solve it are not clear. Therefore, the boot rate is greatly affected. This situation must be improved in the future. The management of spare parts is particularly important for auto production line, and it is suggested that this work should be managed by special personnel. The spare parts list should be provided according to the requirements of the molding line, checked with the molding line after purchase, and classified and saved. Storage conditions should meet the requirements of materials, spare parts should be inspected regularly, and expired parts should be cleaned up in time and new parts should be added. It is necessary to ensure that spare parts used in the molding line can be provided accurately at any time, so as to ensure the normal operation of the molding line. In a word, it is not a simple matter to use a molding line well. Dozens of devices, one or two hundred points, run without errors every day. Efforts should be made not only from the design, manufacture, installation, debugging, maintenance, spare parts and other aspects of the molding line itself, but also from the aspects of production management, coordination of various departments, and correct determination of process parameters. With the continuous improvement of technical level, manufacturing level, designer design level and user management level, the domestic molding line will be manufactured and used well.

Xiaolong Liu

2. How to prolong the service life of die casting die?

The defects of the material itself and the methods of maintenance will affect the life of the die-casting die. This paper introduces how to improve the service life of die-casting die from the latter, and lists the common fault causes and troubleshooting methods of die-casting die.

Because of the long production cycle, large investment, high manufacturing accuracy and high cost, the die-casting mold is expected to have a long service life. However, due to the influence of a series of internal and external factors, such as materials and processing, the die is prematurely invalid and scrapped, resulting in great waste.

The main failure forms of die casting die are: sharp corner, corner crack, splitting, hot cracking (cracking), wear and erosion, etc. The main reasons for the failure of die casting die are: the defects of the material itself, processing, use, maintenance and heat treatment.

1, defects of the material itself

As we all know, the use conditions of die-casting dies are extremely bad. Take the aluminum die casting die as an example, the melting point of aluminum is 580-740℃, and the temperature of molten aluminum is controlled at 650-720℃ during use. In die casting without preheating the die, the cavity surface temperature rises from room temperature to liquid temperature, and the cavity surface bears great tension. When the ejector is opened, the surface of the cavity bears great compressive stress. After thousands of die casting, defects such as cracks will appear on the die surface.

Therefore, the use conditions of die casting belong to rapid heating and rapid cooling. The die material should be hot-working die steel with high cold and hot fatigue resistance, fracture toughness and thermal stability. H 13(4Cr5MoV 1Si) is a widely used material at present. According to reports, 80% of the cavities abroad use H 13, and 3Cr2W8V is still widely used in China. However, 3Cr2W8VT_ has poor artistic performance, poor thermal conductivity and high linear expansion coefficient, which leads to large thermal stress in work and leads to mold cracking. Martensitic aging steel is suitable for dies that are resistant to thermal cracking and have low requirements on wear resistance and corrosion resistance. Heat-resistant alloys such as tungsten and molybdenum are limited to small inserts with serious thermal cracking and corrosion. Although these alloys are fragile and sensitive to notches, they have the advantage of good thermal conductivity, and they have good adaptability to die casting dies for thick die castings that need cooling and cannot be equipped with water passages. Therefore, under reasonable heat treatment and production management, H 13 still has satisfactory performance.

The materials used to manufacture the die-casting die should meet the design requirements in all aspects, so as to ensure that the die-casting die can reach the designed service life under its normal use conditions. Therefore, a series of inspections should be carried out before putting into production to prevent the premature scrapping of the mold caused by defective products and waste of processing costs. Common inspection methods include macroscopic corrosion inspection, metallographic inspection and ultrasonic inspection.

(1) Macroscopic corrosion inspection. Mainly check the pores, deviations, cracks, cracks, nonmetallic inclusions, hammer cracks and joints on the surface of the material.

(2) Metallographic examination. Mainly check the segregation, distribution, grain size and intergranular inclusions of carbides on the grain boundaries of materials.

(3) Ultrasonic inspection. Mainly check the defects and dimensions inside the material.

2, die-casting mold processing, use, repair and maintenance.

The problems that should be paid attention to in the design of die-casting mold have been introduced in detail in the mold design manual, but when determining the injection speed, the maximum speed should not exceed100 m/s. Too high speed will lead to mold corrosion and increase the deposits on the cavity and core. However, if it is too low, defects will easily occur in the casting. Therefore, for magnesium, aluminum and zinc, the corresponding minimum injection speeds are 27, 18 and 12m/s, the maximum injection speed of cast aluminum should not exceed 53m/s, and the average injection speed is 43m/s. ..

In the process of machining, the thicker template thickness cannot be guaranteed by superposition method. Because the thickness of steel plate is 1 times, the bending deformation is reduced by 85%, and lamination can only play a superposition role. The bending deformation of two plates with the same thickness as the veneer is four times that of the veneer. In addition, when processing cooling water channels, special attention should be paid to ensuring concentricity in double-sided processing. If the corners of the head are not concentric with each other, the connected corners will crack during use. The surface of the cooling system should be smooth, preferably without machining marks.

The application of EDM in mold cavity machining is more and more extensive, but there is a hardened layer on the surface of the machined cavity. This is due to the self-carburization and quenching of the die surface during machining. The thickness of hardened layer is determined by the current intensity and frequency during machining, which is deeper during rough machining and shallower during finishing. No matter how deep it is, there is great stress on the surface of the mold. If the hardened layer or stress is not removed, cracks, pits and cracks will appear on the surface of the die during use. The following methods can be used to eliminate the hardened layer or stress: ① remove the hardened layer with oilstone or grinding; (2) Without reducing the hardness, reducing the stress below the tempering temperature can greatly reduce the surface stress of the die cavity.

The casting process should be strictly controlled during the use of the mold. Within the range permitted by the process, the casting temperature and injection speed of molten aluminum should be reduced as much as possible, and the preheating temperature of the mold should be increased. The preheating temperature of aluminum die casting die is increased from 100 ~ 130℃ to 180 ~ 200℃, and the die life can be greatly improved.

Welding repair is a common means in mold repair. Before welding, the model of die steel to be welded should be mastered, and surface defects should be eliminated by machining or grinding. The welding surface must be clean and dry. Covered electrode used should be consistent with the composition of die steel, and must be clean and dry. The mold is preheated together with covered electrode (H 13 is 450℃), and after the surface and center temperatures are consistent, it is repaired by shielding gas welding. In the welding process, when the temperature is lower than 260℃, it should be reheated. After welding, when the mold is cooled to touch, it is heated to 475℃ and kept at 25 mm/h, and finally completely cooled in still air, and then the cavity is trimmed to complete. Heating and tempering after welding is an important part of welding repair, that is, eliminating welding stress and tempering the thin layer under the welding layer which is heated and quenched during welding.

After the mold is used for a period of time, there will be deposits in the cavity and core due to the fast injection speed and long use time. These deposits are composed of release agent under high temperature and high pressure, impurities in cooling liquid and a small amount of die-casting metal. These deposits are very hard and firmly attached to the surface of the core and cavity, which is difficult to remove. When removing deposits, you can't use a blowtorch to remove them, which may lead to local hot spots or decarbonization points on the surface of the mold, thus becoming the birthplace of hot cracking. Grinding or mechanical removal shall be adopted, but other profiles shall not be damaged, resulting in dimensional change.

Regular maintenance can keep the mold in good condition. After the new mold is tested, whether it is qualified or not, it should be tempered and not cooled to room temperature. When the new dies reach the design life of 1/6 ~ 1/8, that is, aluminum die-casting dies 10000, magnesium die-casting dies 5000, and copper die-casting dies 800, the die cavity and die frame should be tempered at 450-480℃, and the die cavity should be polished and nitrided. After that, do the same maintenance every 12000 ~ 15000 module. The mold can be used for 50,000 times and maintained every 25,000 ~ 30,000 times. The above method can obviously slow down the cracking speed and time caused by thermal stress.

In the case of serious erosion and cracking, the die surface can be nitrided to improve the hardness and wear resistance of the die surface. However, the hardness of the nitrided matrix should be 35-43HRC. When it is lower than 35HRC, the nitrided layer can not be firmly combined with the matrix, and it will fall off after a period of use. When it is higher than 43HRC, it is easy to break the convex part of the cavity surface. When nitriding, the thickness of nitrided layer should not exceed 0. 15mm, if it is too thick, it will fall off at parting surface and sharp corners.

3. Heat treatment

Whether the heat treatment is correct or not is directly related to the service life of the die. Due to the incorrect heat treatment technology and technical regulations, the mold is deformed, cracked and scrapped, and the residual stress of heat treatment leads to the failure of the mold in use, accounting for about half of the failure ratio of the mold.

Die casting die cavities are all made of high-quality alloy steel. These materials are expensive and the processing cost is high. If the heat treatment is improper or the quality of heat treatment is not high, it will lead to scrapping or the life can not meet the design requirements, and the economic loss is great. Therefore, the following points should be paid attention to during heat treatment:

Forgings (1) are spheroidized and annealed before cooling to room temperature.

(2) After rough machining and before finishing, add quenching and tempering treatment. In order to prevent the hardness from being too high, resulting in machining difficulties, the hardness should be limited to 25-32HRC, and stress relief tempering should be arranged before finishing.

(3) Pay attention to the critical points Ac 1 and AC3 of steel and the holding time during quenching to prevent austenite coarsening. When tempering, the temperature should be kept at 20 mm/h, and the tempering times are generally 3 times. When nitriding, the third tempering can be omitted.

(4) During heat treatment, attention should be paid to decarburization and carburization of the cavity surface. Decarburization will quickly cause damage and high-density cracks; Increasing carbon will reduce cold and hot fatigue resistance.

(5) When nitriding, it should be noted that there should be no oil stain on the nitriding surface. The cleaned surface is not allowed to be touched directly by hand, and gloves should be worn to prevent uneven nitriding layer caused by oil pollution on the nitrided surface.

(6) Between two heat treatments, when the temperature of the first heat treatment process drops to contact, the next heat treatment process is carried out, and cooling to room temperature is not allowed.