Lost foam casting, also called solid casting, is a model of a foam model with the same shape and shape as the casting. After brushing the refractory paint and drying it, it is buried in the dry sand and is molded under vacuum and under vacuum. The gasification of the model, the liquid metal occupying the position of the model, and the emerging casting method of forming the casting after solidification and cooling, the idea of the process as the core, is still the soul of the industry, which affects the future of the lost foam casting industry and the mold manufacturing industry.
Lost foam casting process
1. Make foam plastic white mold, combined pouring system, gasification mold surface brush, spray special high temperature resistant paint and dry.
2. Place the special compartment sand box on the vibrating table, fill in the bottom sand (dry sand), tap it, scrape the flat, put the dried gasification mold on the bottom sand, fill the dry sand, and shake the appropriate time. Scrape the box mouth.
3. Cover with plastic film, put on the pouring cup, connect the vacuum system to vacuum, dry sand is formed and then cast, the white mold gasification disappears, and the molten metal replaces its position.
4. Release the vacuum. After the casting is condensed, turn the box and remove the casting from the loose dry sand.
Lost foam casting process control
First: the choice of foam beads
1. There are three types of foam beads for foam casting.
(1) Expandable polystyrene resin beads (abbreviated as EPS);
(2) Expandable methyl methacrylate and styrene copolymer resin beads (STMMA for short);
(3) Expandable polymethyl methacrylate resin beads (abbreviated as EPMMA).
2. Commonly used expandable polystyrene resin beads (EPS) for casting non-ferrous metals, gray iron and general steel casting.
3. bead characteristics: translucent beads, pre-expansion multiple 40 ~ 60, particle size of 0.18 ~ 0.80 亳 m (6 sizes).
4. The particle size of the original bead generally selected is less than or equal to 1/9 to 1/10 of the minimum wall thickness of the casting.
Second: About the production of models
1. Made of foamed beads: pre-foaming – curing – foam molding – cooling out of the mold.
(1) pre-foaming
Before the EPS beads are added to the mold, they are first foamed to expand the beads to a certain size. The foaming process determines the density, dimensional stability and accuracy of the model and is one of the key links. There are three methods for pre-expansion of EPS beads: hot water pre-expansion, steam pre-expansion and vacuum pre-expansion. The vacuum pre-expanded beads have a high foaming rate, and the beads are dried and used more.
(2) ripening
The pre-foamed EPS beads are placed in a dry, ventilated silo for a certain period of time. In order to balance the external pressure in the beads, the beads have elasticity and re-expansion ability to remove the moisture on the surface of the beads. The ripening time is between 8 and 48 hours.
(3) Foam molding
The foamed and matured EPS beads are filled into the cavity of the metal mold, heated, and the beads are expanded again to fill the voids between the beads, and the beads are fused to each other to form a smooth surface, that is, a mold. Cooling must be carried out before the mold is released to cool the model below the softening temperature. After the model is hardened and shaped, the mold can be released. After the mold is released, there should be time for the model to be dry and dimensionally stable. The equipment has two kinds of steaming cylinders and automatic forming machines.
2, made of foam plastic sheet: foam plastic sheet – resistance wire cutting – bonding – model. For a simple model, a foam wire cutting device can be used to cut the foam sheet into the desired model. For complex models, first use a resistance wire cutting device to divide the model into several parts and then glue them into a whole model.
Third: the model is combined into clusters
It is a combination of a self-processed (or purchased) foam model and a pouring riser model to form a model cluster, which is sometimes carried out before the coating, sometimes in the case of a buried shape after the coating is prepared. It is an indispensable process for lost mold (real) casting. Currently used bonding materials: rubber emulsion, resin solvent and hot melt adhesive and tape paper.
Fourth: model coating
The surface of the solid casting foam model must be coated with a certain thickness of paint to form the inner shell of the mold. The role of the coating is to improve the strength and rigidity of the EPS model, improve the surface erosion resistance of the model surface, prevent the surface damage and vibration modeling of the model during sanding and the deformation of the model during negative pressure setting, and ensure the dimensional accuracy of the casting. A commercially available special coating for lost foam casting, which is stirred with water in a paint mixer to obtain a suitable viscosity. The agitated paint is placed in a container, and the model group is coated by dipping, brushing, pouring, and spraying. It is usually applied twice to make the coating thickness 0.5~2mm. It is selected according to the type, structure and size of the casting alloy. The coating is dried at 40 to 50 °C.
Fifth: vibration modeling
We introduce a commonly used modeling method (one of two methods). The process includes the following steps: sand bed preparation – placing the EPS model – sand filling – sealing and setting.
1. Sand bed preparation
Place the flask with the pumping chamber on the shaker and clamp it. Put a certain thickness of bottom sand at the bottom (generally the thickness of the sand bed is above 50~100mm), and the vibration is tight. The molding sand is dry quartz sand without binder, no filling, and no water. The ferrous metal has a high temperature and can be made of coarser sand. The aluminum alloy is made of fine sand. The sand is used repeatedly after being treated. The sand box is a sand box with a single opening, a pumping chamber or an air suction pipe, a lifting or walking mechanism.
2. Place the EPS model
After the tapping, the EPS model group is placed according to the process requirements, and the sand is fixed.
3. Sand filling
Add dry sand (several sanding methods) and apply vibration (X, Y, Z directions) for 30~60 seconds, so that the sand fills all parts of the model and increases the bulk density of the sand.
4. Sealed stereotypes
The surface of the sand box is sealed with a plastic film. The vacuum pump is used to draw a certain vacuum into the sand box. The sand is “bonded” together by the difference between the atmospheric pressure and the pressure inside the mold to keep the casting process from collapsing. “Negative pressure setting. More commonly used.
Sixth: casting replacement
The EPS model generally softens around 80 °C and decomposes at 420-480 °C. The decomposition products are gas, liquid and solid. The thermal decomposition temperature is different, and the contents of the three are different. In solid casting, under the action of liquid metal, the EPS model undergoes pyrolysis gasification, generates a large amount of gas, continuously discharges through the coated sand, and forms a certain pressure in the mold, model and metal gap. The metal constantly occupies the position of the EPS model and advances, causing a replacement process between the liquid metal and the EPS model. The end result of the displacement is the formation of a casting. The pouring operation is slow-fast-slow. And keep pouring continuously to prevent the pouring process from breaking. After pouring, the mold vacuum is maintained for 3 to 5 minutes and then the pump is stopped. The pouring temperature is 30 to 50 ° C higher than the temperature of the sand casting.
Seventh: cooling and cleaning
After cooling, the real casting sand falling is the simplest. The sand box can be slanted out of the casting or the casting can be directly hoisted from the sand box. The casting is naturally separated from the dry sand. The separated dry sand is treated and reused.
The above is the introduction of the lost foam casting process.
