In the lost foam casting, the foam coated with refractory coating is placed in a sand box, and the surrounding of the pattern is filled with dry sand. The microseismic and negative pressure are used to compact the liquid metal without casting the core, in the casting and A new casting process that continues to maintain a constant negative pressure during the solidification process to vaporize the foam and then replace it with metal to form a casting.
1. The process flow of lost foam casting is as follows:
1) Pre-expansion
Model production is the first process of the lost foam casting process. Complex castings, such as cylinder heads, require several foam models to be separately fabricated and then glued into a single overall model. Each block model requires a set of molds for production. In addition, a set of tires may be needed in the gluing operation to maintain the accurate positioning of each block. The molding process of the model is divided into two steps. The first step is to Polystyrene beads (EPS) are pre-expanded to a suitable density, typically by rapid steam heating, which is referred to as pre-expansion.
2) Model molding
The pre-expanded beads are first stabilized, then sent to the hopper of the molding machine, and fed through the feeding hole. After the mold cavity is filled with the pre-formed beads, steam is introduced to soften the beads. Expanding, squeezing all the voids and bonding them together completes the manufacturing process of the foam model, which is called autoclaving.
After molding, the model is cooled by a large flow of water in the water-cooling cavity of the mold, and then the mold is opened to take out the model. At this time, the model temperature is high and the strength is low, so care must be taken during demolding and storage to prevent deformation and damage. .
3) Model cluster combination
Before the model is used, it must be stored at the appropriate time to make it mature. The typical model storage period is up to 30 days. For the model formed by the uniquely designed mold, it only needs to be stored for 2 hours. After the model is matured, it can be divided. The block model is glued together. The mass-produced castings must be bonded to the automatic bonding machine using a hot-melt adhesive to ensure the bonding accuracy. Castings produced in small and medium-sized batches can be used, hand-bonded with cold glue, and the joints of the glued surfaces should be tightly sealed to reduce the possibility of casting defects.
4) Model cluster dip coating, drying
In order to produce more castings per box casting, sometimes many models are glued into clusters, the model clusters are immersed in refractory coatings, and then dried in air circulating ovens of about 30~60C (86-140F) for 2~3 After the hour, after drying, put the model cluster into the sand box, and fill the dry sand to make the vibration tight. The internal cavity of all the model clusters and the dry sand of the periphery must be tight and supported.
5) Pouring
After the model cluster is filled in the sandbox by the dry sand vibration, the vacuum is vacuumed to form a negative pressure to strengthen the compactness. The mold can be poured. After the molten metal is poured into the mold, the model gasification is replaced by metal to form a casting. In the lost foam casting process, the casting speed is more critical than conventional hollow casting. If the casting process is interrupted, the sand pattern may collapse and cause waste. Therefore, in order to reduce the difference between each casting, it is best to use an automatic pouring machine.
6) Falling sand cleaning
After the pouring, the vacuum is released for a while, the vacuum is released, the casting is solidified and cooled in the flask, and then the sand is dropped. Casting sand is quite simple, and the tilting sandbox castings fall out of the loose dry sand. The castings are then automatically separated, cleaned, inspected and placed in a casting box for transport. The dry sand can be reused after being treated and cooled by the sand treatment system, and other additional processes are rarely used, and the metal scrap can be remelted in production.
2. Advantages of the lost foam casting process
1) High precision of castings Lost-die casting is a new process with almost no allowance and precise molding. This process does not require mold taking, no parting surface, no sand core, so the casting has no flash, burr and draft angle, and The dimensional error caused by the core combination is reduced, the surface roughness of the casting can reach Ra3.2 to 12.5μm; the dimensional accuracy of the casting can reach CT7 to 9; the machining allowance is at most 1.5 to 2mm, which can greatly reduce the machining The cost can be reduced by 40% to 50% compared to conventional sand casting methods.
2) Flexible design: Provides sufficient freedom for the structural design of the casting. Highly complex castings can be cast by foam molding.
3) There is no sand core in traditional casting, so there is no uneven wall thickness of the casting due to inaccurate sand core size or inaccurate core position in traditional sand casting.
4) There is no chemical binder in the clean production sand. The foam is harmless to the environment at low temperature, and the recovery rate of the old sand is over 95%.
5) Reduce investment and production costs to reduce the weight of casting blanks, and the machining allowance is small.
Therefore, lost foam casting technology is in line with the general trend of casting development: it has broad development prospects.
3. Disadvantages and limitations of the lost foam casting process
The lost foam casting process and other casting processes have their shortcomings and limitations. Not all castings are suitable for production by the lost foam process, and specific analysis is required. The use of this process is mainly based on the following factors.
1) The batch size of castings is larger, and the economic benefits are more impressive.
2) The order of good and poor applicability of casting materials is roughly: gray cast iron – non-ferrous alloy – ordinary carbon steel – ductile iron – low carbon steel and alloy steel, because the foam is burned and decomposed during the casting process The effect of the substance on the alloy solution is different. For example, for steel castings with low carbon content, the use of lost foam casting may cause carbonation problems in the casting skin. Therefore, it is necessary to make necessary preparations before production to prevent the process experiment and debugging cycle from being too long.
3) The size of the casting mainly considers the scope of use of the corresponding equipment.
4) Casting structure The more complicated the structure of the casting, the better the economical and economic benefits of the lost foam casting process. For the case of a narrow internal cavity channel and interlayer, it is necessary to carry out the experiment before the lost foam process before it can be put into production. . For some simple conditions, the sand casting method can also produce high quality castings, and the production efficiency and casting cost are lower than that of the lost foaming process. In this case, the lost foam casting method is not necessarily used.
4. Development status of the lost foam casting process in China:
Through years of production practice, China’s enterprises using the lost foam technology for casting production have grown from less than ten in the initial stage to hundreds of current ones. There are many successful examples of lost foam casting production in China, and there are quite a few companies that have not achieved the expected results. Analysis of successful experience and failure lessons, the key to the development prospects of lost foam casting in China is the degree of awareness of this process, which lies in the optimal control of the production system of the process, including raw materials, coating technology, dry sand tight Real technology and optimized control of lost foam casting process technology.
