3. Binding the model into a cluster:
It is to combine and glue the foam model processed by yourself (or purchased) with the injection and riser model to form a model cluster. This combination may be done before painting, or during buried box modeling after painting preparation. It is an essential process in lost foam (solid mold) casting.
Currently used adhesive materials: rubber latex, resin solvent, hot melt adhesive, adhesive tape.
4. Model coating:
The surface of the casting foam model needs to be coated with a certain thickness of paint to form the inner shell of the casting. The role of the coating is to improve the strength and rigidity of the EPS model, improve the resistance of the model surface to sand wear, prevent damage to the model surface during sand addition, and prevent deformation of the model during vibration molding and negative pressure molding, thereby improving the dimensional accuracy of the casting.
The purchased special coating agent for lost foam casting is mixed with water in a coating mixer to achieve the appropriate viscosity. The mixed paint is put into a container and painted on the model set by dipping, brushing, pouring, spraying, etc. Generally, two coats are applied, with a coating thickness of 0.5-2mm. It is selected according to the type of casting alloy, the structural shape and size. The coating is dried at 40-50℃.
5. Vibration modeling:
Introduces a commonly used modeling method (one of two methods).
The process includes the steps of preparing the sand bed, placing the EPS model, filling the sand, sealing and molding.
① Preparation of the sand bed
Place a sand box with a vacuum chamber on the vibration table and fix it with a clamp. Put a certain thickness of bottom sand on the bottom (generally the thickness of the sand bed is 50-100mm or more) and vibrate it to solidify.
Casting sand is dry quartz sand without binders, fillers and water. Ferrous metals can use coarse sand because of the high temperature, but fine sand is used for aluminum alloys. After processing, the molding sand needs to be reused.
A sand box refers to a sand box with a one-side opening, an exhaust chamber or exhaust pipe, and a lifting mechanism or walking mechanism.
② Place the EPS model
After compaction, the EPS model group is placed on it according to the process requirements and fixed with sand.
③ Sand filling
Add dry sand (there are several ways to add sand) and apply vibration (X, Y, Z directions) at the same time. The time is usually 30-60 seconds, and the molding sand fills all parts of the model, improving the filling rate. The density of the molding sand.
④ Sealing and molding
The surface of the sand box is sealed with a plastic film, and a certain vacuum is drawn into the sand box using a vacuum pump, and the difference between the atmospheric pressure and the pressure in the mold is used to "bond" the sand grains. By putting them together, it is possible to prevent the mold from collapsing during the casting process. This is called "negative pressure molding". It is more commonly used.
6. Replacement of the casting
EPS models usually soften at about 80°C and decompose at 420-480°C. The decomposition products include gas, liquid and solid. The three have different thermal decomposition temperatures and different contents.
During full mold casting, the EPS model undergoes thermal decomposition and gasification due to the heat of the liquid metal, generating a large amount of gas, which is continuously discharged through the coating sand, forming a certain gas pressure in the gap between the mold, the model and the mold. The metal continuously occupies the position of the EPS model and moves forward, and a displacement process occurs between the liquid metal and the EPS model. The end result of the displacement is casting.
The pouring operation process adopts low speed-high speed-low speed. Continuous pouring is maintained to ensure that no interruptions occur during the pouring process. After pouring, the vacuum state of the mold is maintained for 3-5 minutes, and then the pump is stopped. The pouring temperature is 30-50°C higher than sand casting.
7. Cooling and cleaning
After cooling, it is easiest to remove the sand from the mold. You can tilt the sand box to lift the casting, or lift the casting directly from the sand box and take it out. The casting and the dry sand will naturally separate. The separated dry sand will be processed and reused.
Advantages
Compared with traditional sand casting, lost foam casting has the following great advantages:
1. Lost foam casting does not require parting or coring, so it is especially suitable for box, shell and bobbin castings with complex geometric shapes that are difficult to achieve with traditional casting.
2. Lost foam casting uses dry sand to fill the pattern, so it can be used repeatedly, resulting in less industrial waste and significantly reducing costs.
3. Lost foam casting is free of burrs and burrs, and can reduce cleaning time by more than 80%.
4. Lost foam casting is versatile. It can produce not only cast iron and ductile iron, but also steel castings at the same time, making it flexible to switch and versatile.
5. Lost foam casting is not only suitable for mass casting and mechanized operations, but also suitable for manual assembly of models for small volume products.
6. If investment in lost foam casting is properly made, it can achieve dust-free air, sand-free ground, low labor intensity and good working environment, and transform industries dominated by male workers into industries dominated by female workers. .
7. Lost foam casting does not require a molding process, and anyone with a certain level of education can become a skilled worker with short training, so it is especially suitable for areas and enterprises with a lack of technical capabilities.
8. Lost foam casting is suitable for group casting. It is easy to remove sand from the dried sand-embedded mold. For castings of certain materials, residual heat treatment can also be performed according to the purpose.
9. Lost foam casting is suitable not only for small and medium-sized parts, but also for large castings such as machine tool beds, large-diameter pipes, large cold stamping molds, and accessories for large mining equipment. It has a short production cycle, low costs, and a short production cycle, so it is particularly popular.
