Introduction to Lost Foam Casting
In 1958, H.F. Shroyer invented and patented the technology of manufacturing metal castings using expandable foam plastic molds. The initial model used was made of polystyrene (EPS) sheets and molded with binder-containing molding sand. The German company Grunzweig and Harrtmann purchased this patent and developed and applied it. The technology of using binder-free dry sand to produce castings was later patented by T.R. Smith in 1964. Before 1980, the use of the binder-free dry sand process had to be approved by Full Mold Process, Inc. After that, the patent was invalid.
The most common and practical method is to put the model coated with refractory material into a sand box, fill it tightly with dry sand around the model, and pour liquid metal to replace the foam plastic model. This casting process is called It is called: lost foam casting (EPC), gasified mold casting and solid mold casting, etc. The Lost Foam Casting Committee of the American Foundry Association adopted "lost foam casting" as the name for the process.
Lost foam casting is an innovative casting process that can be used to produce non-ferrous and ferrous metal power system parts, including: cylinder block, cylinder head, crankshaft, gearbox, intake pipe, exhaust pipe And castings such as brake hubs. The process flow of lost foam casting is as follows:
1) Pre-foaming
Model production is the first step in the lost foam casting process. Complex castings such as cylinder heads require several pieces of foam. The models are made separately and then glued together into a single overall model. Each block model requires a set of molds for production. In addition, a set of molds may be required during 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 are pre-expanded to an appropriate density, usually by rapid heating with steam. This stage is called pre-expansion.
2) Model molding
The pre-foamed beads must be stabilized first, and then sent to the hopper of the molding machine, where materials are added through the feeding holes, and the mold cavity After being filled with pre-expanded beads, steam is introduced to soften and expand the beads, filling all the gaps and bonding them together. This completes the manufacturing process of the foam model. This stage is called autoclave molding.
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 temperature of the model increases and the strength is low, so caution must be taken during demoulding and storage. operation to prevent deformation and damage.
3) Model cluster combination
Before use, the model must be stored for an appropriate period of time to mature and stabilize. The typical model storage period is up to 30 days, and for molds with unique designs The formed model only needs to be stored for 2 hours. After the model is matured and stabilized, the segmented model can be glued together.
The gluing of the block model is carried out on an automatic gluing machine using hot melt glue. The joints of the glued surfaces should be tightly sealed to reduce the possibility of casting defects
4) Dip coating of model clusters
In order to produce more castings per box of casting, sometimes Many models are glued into clusters, immersed in refractory paint, and then dried in an air circulation oven at about 30 to 60C (86-140F) for 2 to 3 hours. After drying, put the model clusters into a sand box. Fill the dry sand with vibration and compaction, and the dry sand inside and outside the cavities of all model clusters must be compacted and supported.
5) Pouring
After the model clusters are filled solidly in the sand box through dry sand vibration, the mold can be poured. After the molten metal is poured into the mold (the pouring temperature is about At 760C/1400F, for cast iron around 1425C/2600F), the mold gasification is replaced by metal to form the casting. Figure 1 is a schematic diagram of the sand box and pouring of the lost foam process.
In the lost foam casting process, the pouring speed is more critical than traditional hollow casting. If the pouring process is interrupted, the sand mold may collapse and become scrap.
Therefore, in order to reduce the difference between each pouring, it is best to use an automatic pouring machine.
6) Shakout cleaning
After pouring, the casting is solidified and cooled in the sand box, and then shaken out. Casting shakeout is fairly simple, the casting falls out of the loose dry sand by tipping the flask. The castings are then automatically separated, cleaned, inspected and placed in casting bins for transport.
Dry sand can be reused after cooling, and other additional processes are rarely used. Metal scraps can be remelted and used in production.
1.2 Advantages of the lost foam casting process
The lost foam casting process has three main advantages in technology, economy and environmental protection.
1.2.1 Technical aspects
1) Increased degree of freedom in model design
The new process makes it possible to carry out modeling design, and it is completely possible to start from the first stage You can add some additional functions to the model. For example: the diesel preheater has a special functional component that can be manufactured using the lost foam casting process but cannot be produced using traditional casting methods.
2) Eliminate the need for sand cores used in casting production
3) Many castings do not need riser feeding
4) Improve casting accuracy
It can obtain complex shapes and structures, can produce high-precision castings 100 times repeatedly, and can control the wall thickness deviation of castings between -0.15~0.15mm
5) No flash will be produced on the model joint surface
6) It has the advantage of reducing the weight of castings by about 1/3
7) Reduces machining allowances
It can reduce machining allowances, for some The parts may not even be machined. This greatly reduces machining and machine tool investment (for example, investment can be reduced by half for different situations).
8) Compared with traditional cavity casting, mold investment is reduced.
9) Completely eliminates the traditional shakeout and core processes
1.2.2 Economic aspects
1) Complex castings can be produced as a whole
Adopting a new process design, the block model can be glued to form an overall model and cast into complex integral parts. Compared with the original assembly of multiple castings (such as a diesel preheater), the benefits can be 1 to 10 times.
2) Reduce workshop personnel
When establishing a lost foam casting factory, the number of employees employed is smaller than that of a traditional casting factory, so this factor should be considered.
3) Flexible casting process
The flexibility of the casting process is very important, because the new process may simultaneously produce a large number of similar or different castings and gating systems in the sand box. It is therefore very flexible. In summary, we can say that each advantage is consistent with economic benefits while also improving working conditions.
1.2.3 Environmental Protection
Polystyrene and PMMA produce carbon monoxide, carbon dioxide, water and other hydrocarbon gases when burned, and their contents are all lower than the European standards. Dry sand can use natural silica sand, which can be recycled 100 times and does not contain binders. The paint used in the model is composed of water with binders and other auxiliary materials added, which does not cause pollution.
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