Keywords : model making, vibration problems, coating, pouring, lost foam
Abstract : Lost foam casting technology as a casting near static forming method has been developed rapidly in recent years. In foreign countries, the lost foam casting technology has shown strong vitality due to the mechanized and automated lost foam casting production lines and the significant economic and social benefits.
1 Model making
Model making is a very important part in 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 number of pyrolysis products during pouring are the prerequisites for obtaining high-quality castings. The existing models of small and medium-sized enterprises have the following ways :
(1) It is cut and bonded with packaging EPS plate.
(2) Self-made mold, commissioned factory processing.
(3) Self-made simple preforming equipment.
Using the above method to make the model, there is a widespread phenomenon that the density change of the model is not paid attention to. In particular, the moisture content of the model is not easy to control when it is commissioned to the outside factory, and there are often phenomena such as the back injection of molten iron from the gate during pouring or the cold shut and insufficient pouring of the castings. Therefore, in the production process, the test of model density 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 changed at will ; the bead density is controlled by weighing tool in advance, and the method of controlling bead density by manual experience is changed. After adopting the above method, the problem is solved.
2 The problems of vibration
Vibration compaction is one of the four key technologies of EPC. The role of vibration is to make dry sand flow dynamically in the sand box, improve the filling and density of dry sand, and prevent casting defects. In the vibration filling of dry sand, the ideal condition is that dry sand orderly flow in the vibration process, under the premise of ensuring that the model does not deform, evenly filled to all parts of the model, so that the sand in the sand box to obtain higher and more uniform filling density.
The vibration table of lost foam casting in small and medium-sized enterprises is mostly self-made equipment. In vibration, the most common phenomenon is due to improper vibration operation, resulting in mold deformation, coating cracking, etc., resulting in corresponding casting defects. Some shaking table itself due to excessive excitation force, the same group of motor polarization block imbalance is also easy to cause pattern deformation. Therefore, the excitation force, amplitude and vibration time should be adjusted ; for castings with large size and simple structure, the three-dimensional vibration of six motors can be changed to the vertical or horizontal vibration of two motors ; in particular, the parameters of the shaking table are detected and adjusted by testing instruments to meet the design requirements.
3 Problems in the use of coatings
In lost foam casting process, the use of coating can improve the stiffness and strength of the mold, isolate EPS mold from the mold, and prevent sand sticking and mold collapse. During the casting process, the high temperature decomposition products of the sample are allowed to be discharged smoothly and timely through the coating. Coatings are generally composed of refractory materials, binders, and suspending agents. The proportion of each component has a great influence on the performance of coatings.
However, some enterprises are not very clear about the role of coating composition. The coating formulation and preparation process are changed arbitrarily, or the coating performance is greatly reduced due to the lack of a component. Some enterprises have problems in the sample dip drying process. Sometimes in order to shorten the time, the next dip coating is carried out in the case of the first coating is not dried, resulting in insufficient drying inside the model, where there is water ; in summer, only the sun drying method is used, and there is instability in the process, resulting in back spray or porosity during pouring ; the coating thickness did not notice changes depending on the casting, pouring temperature and iron pressure head.
Only by paying attention to and solving the above problems and working hard on the operation details will not produce casting defects due to coating.
4 Problems in pouring process
In lost foam casting, in order to discharge gas and foam gasification residue, the straight runner should have enough height to make the metal liquid have enough pressure head to promote the stable and rapid filling of metal liquid flow and ensure the complete and clear surface of the casting. In practice, some enterprises use the original gate cup for sand mold casting, because of the small size, it is easy to appear the phenomenon of liquid flow instability leading to workpiece scrap. In order to ensure sufficient flow rate to make the pouring process flow continuously and quickly establish starting pressure head, large gate cup can be used instead; straight runners are made into hollows to reduce blowback and increase the pressure head at the start of pouring.
The negative pressure dry sand vibration molding is used in lost foam casting, and the strength of the mold is much higher than that of the wet sand. The negative pressure extraction method can improve the stability of the mold and timely extract the pyrolysis gasification products generated during the gasification of the model. However, in the production process, some factories only pay attention to the surface negative pressure before pouring, but the negative pressure change is often ignored in the pouring process, resulting in casting defects. This problem can be well solved by adjusting the negative pressure in the pouring process according to the size of the casting and the amount of pyrolysis products.
The common casting method is sand casting, followed by special casting methods, such as metal casting, investment casting, gypsum casting and so on. Sand casting can be divided into clay sand mold, organic binder sand mold, resin self-hardening sand mold, lost foam and so on.
4.1 Principle of casting method selection:
4.1.1 Sand casting is preferred due to its low cost, simple production process and short production cycle compared with other casting methods. When the wet mold cannot meet the requirements, consider using clay sand surface dry sand mold, dry sand mold or other sand mold. The casting weight of clay green sand casting can range from a few kilograms to tens of kilograms, while the casting weight of clay dry mold can reach tens of tons.
4.1.2 The casting method shall be compatible with the production batch. Low pressure casting, die casting, centrifugal casting and other casting methods are only suitable for mass production.
4.1.3 The molding method shall be suitable for the factory conditions
For example, for the same production of large machine tool bed and other castings, the core assembly molding method is generally adopted, and the core is assembled in the pit without making patterns and sand boxes; Other factories use sand box modeling method to make patterns. Different enterprises have different production conditions (including equipment, site, staff quality, etc.), production habits and accumulated experience. According to these conditions, we should consider what products are suitable for and what products are not (or cannot) suitable for.
4.1.4 Give consideration to the accuracy requirements and cost of castings.
4.2 Defects and prevention in die heat treatment
4.2.1 There are soft spots on the die surface
There are soft spots on the surface of the die after heat treatment, which will affect the wear resistance of the die and reduce the service life of the die.
（1）Cause
There are scale, rust and local decarburization on the surface of the die before heat treatment. After quenching and heating, the cooling quenching medium is improperly selected, and there are too many impurities or aging in the quenching medium.
（2）Preventive measures
Oxide scale and rust spots shall be removed before mold heat treatment, and the mold surface shall be properly protected during quenching and heating. Vacuum electric furnace, salt bath furnace and protective atmosphere furnace shall be used for heating as far as possible. When cooling after quenching and heating, appropriate cooling medium shall be selected, and the cooling medium used for a long time shall be filtered or replaced regularly.
4.2.2 Poor organization of mould before heat treatment
The final spheroidizing structure of the die is coarse and uneven, the spheroidizing is imperfect, and the structure has network, band and chain carbides, which will make the die easy to produce cracks after quenching and cause the die to be scrapped.
（1）Cause
There is serious carbide segregation in the original structure of die steel. Poor forging process, such as too high forging heating temperature, small deformation, high stop forging temperature and slow cooling speed after forging, makes the forging structure coarse and has network, strip and chain carbides, which is difficult to eliminate during spheroidizing annealing. Poor spheroidizing annealing process, such as too high or too low annealing temperature and short isothermal annealing time, can cause uneven spheroidizing annealing structure or poor spheroidizing.
（2）Preventive measures
Generally, die steel materials with good quality shall be selected as far as possible according to the working conditions of the die, production batch and the strength and toughness of the material itself. Improve forging process or normalize preparation heat treatment to eliminate network and chain carbides and non-uniformity of carbides in raw materials.
Solution refining heat treatment can be carried out for high carbon die steel with severe carbide segregation which can not be forged. The correct spheroidizing annealing process specification for the forged die blank can be formulated by quenching and tempering heat treatment and rapid uniform spheroidizing annealing. Charge the furnace reasonably to ensure the uniformity of mold blank temperature in the furnace.
4.2.3 Quenching crack of die
The crack of die after quenching is the biggest defect in the process of die heat treatment, which will scrap the processed die and cause great losses in production and economy.
（1）Causes
There is serious network carbide segregation in die material. There is machining or cold plastic deformation stress in the die. Improper heat treatment operation (too fast heating or cooling, improper selection of quenching cooling medium, too low cooling temperature, too long cooling time, etc.).
The die has complex shape, uneven thickness, sharp corners and threaded holes, resulting in excessive thermal stress and structural stress. Overheating or overburning occurs when the quenching heating temperature is too high. Tempering after quenching is not timely or tempering holding time is insufficient. During repair quenching, reheat and quench without intermediate annealing. Heat treatment, improper grinding process. During EDM after heat treatment, there are high tensile stress and microcracks in the hardened layer.
（2）Preventive measures
Strictly control the internal quality of die raw materials, improve forging and spheroidizing annealing process, eliminate network, strip and chain carbides, and improve the uniformity of spheroidizing structure. The die after machining or cold plastic deformation shall be subject to stress relief annealing (> 600 ℃) before heating and quenching. For the mold with complex shape, asbestos shall be used to block the threaded hole, wrap the dangerous section and thin wall, and adopt graded quenching or isothermal quenching.
Annealing or high temperature tempering is required when repairing or refurbishing the die. Preheating shall be taken during quenching heating, precooling measures shall be taken during cooling, and appropriate quenching medium shall be selected. The quenching heating temperature and time shall be strictly controlled to prevent mold overheating and overburning.
The die shall be tempered in time after quenching, and the holding time shall be sufficient. The high alloy complex die shall be tempered for 2-3 times. Select the correct grinding process and appropriate grinding wheel. Improve the die EDM process and carry out stress relief tempering.
4.2.4 Coarse structure of die after quenching
The coarse structure of the die after quenching will seriously affect the mechanical properties of the die. When in use, the die will break and seriously affect the service life of the die.
（1）Causes
The die steel is confused. The quenching temperature of the actual steel is much lower than that of the required die material (for example, GCr15 steel is regarded as 3Cr2W8V steel). The steel was not spheroidized correctly 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 will easily lead to overheating in the area close to the electrode or heating element. For dies with large cross-section changes, improper selection of quenching and heating process parameters will cause overheating at thin cross-section and sharp corners.
（2）Preventive measures
The steel shall be strictly inspected before warehousing to prevent confusion and disorderly placement of steel. Correct forging and spheroidizing annealing shall be carried out before die quenching to ensure good spheroidizing structure. Correctly formulate the quenching and heating process specification of die, and strictly control the quenching heating temperature and holding time. Regularly detect and calibrate the temperature measuring instrument to ensure the normal operation of the instrument. When heating in the furnace, keep an appropriate distance from the electrode or heating element.
