Squeeze Casting

The process is suitable for components with relatively thick wall sections with high mechanical properties, as for example required of safety components in automotive engineering. The cast components can be welded and heattreated, and they can be produced with near net shape. Aluminum alloys can be used which are difficult or impossible to produce by standard die casting.

Buhler selectively utilizes the advantages of the horizontal shot sleeve system.
Advantages using Buhler machines

• The velocity and pressure intensification profile matched to the component geometry can be programmed in very many discrete steps. Real time control maintains these parameters constant.
• Depending on the type of shot unit selected, it is possible to generate high pressure intensification values during the solidification phase.

Your benefits
• Low capital investment, as no special-purpose machines are required.
• Entering of future-oriented market segments using existing SC machines.
• Low maintenance and training requirements thanks to unified machine and die ranges.

Plaster Mold Casting

Plaster mold casting is a manufacturing process having a similar technique to sand casting. Plaster of Paris is used to form the mold for the casting, instead of sand. In industry parts such as valves, tooling, gears, and lock components may be manufactured by plaster mold casting.

The Process

Initially plaster of Paris is mixed with water just like in the first step of the formation of any plaster part. In the next step of the manufacture of a plaster casting mold, the plaster of Paris and water are then mixed with various additives such as talc and silica flour. The additives serve to control the setting time of the plaster and improve its strength. The plaster of Paris mixture is then poured over the casting pattern. The slurry must sit for about 20 minutes before it sets enough to remove the pattern. The pattern used for this type of metal casting manufacture should be made from plastic or metal. Since it will experience prolonged exposure to water from the plaster mix, wood casting patterns have a tendency to warp. After striping the pattern, the mold must be baked for several hours, to remove the moisture and become hard enough to pour the metal casting. The two halves of the mold are then assembled for the casting process.

Properties and Considerations of Manufacturing by Plaster Mold Casting

• When baking the casting mold just the right amount of water should be left in the mold material. Too much moisture in the mold can cause metal casting defects, but if the mold is too dehydrated, it will lack adequate strength.
  


• The fluid plaster slurry flows readily over the pattern, making an impression of great detail and surface finish. Also due to the low thermal conductivity of the mold material the casting will solidify slowly creating more uniform grain structure and mitigating casting warping. The qualities of the plaster mold enable the process to manufacture parts with excellent surface finish, thin sections, and produces high geometric accuracy.
  

Castings of high detail and section thickness as low as .04 - .1 inch, (2.5 - 1 mm), are possible when manufacturing by plaster mold casting

• There is a limit to the casting materials that may be used for this type of manufacturing process, due to the fact that a plaster mold will not withstand temperature above 2200F (1200C). Higher melting point metals can not be cast in plaster. This process is typically used in industry to manufacture castings made from aluminum, magnesium, zinc, and copper based alloys.
  


• Manufacturing production rates for this type of metal casting process are relatively slow, due to the long preparation time of the mold.
  


• The plaster mold is not permeable, which severely limits the escape of gases from the casting. 

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Green Sand Molding

The most common method used to make metal castings is green sand molding. In this process, granular refractory sand is coated with a mixture of bentonite clay, water and, in some cases, other additives. The additives help to harden and hold the mold shape to withstand the pressures of the molten metal.
The green sand mixture is compacted through mechanical force or by hand around a pattern to create a mold. The mechanical force can be induced by slinging, jolting, squeezing or by impact/impulse.
The following points should be taken into account when considering the green sand molding process:

• for many metal applications, green sand processes are the most cost-effective of all metal forming operations;
• these processes readily lend themselves to automated systems for high-volume work as well as short runs and prototype work;
• in the case of slinging, manual jolt or squeeze molding to form the mold, wood or plastic pattern materials can be used. High-pressure, high-density molding methods almost always require metal pattern equipment;
• high-pressure, high-density molding normally produces a well-compacted mold, which yields better surface finishes, casting dimensions and tolerances;
• the properties of green sand are adjustable within a wide range, making it possible to use this process with all types of green sand molding equipment and for a majority of alloys poured.

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Unbonded Sand Processes

Unlike the sand casting processes that use various binders to hold the sand grains together, two unique processes use unbonded sand as the molding media. These include the lost foam process and the less common V-process.
Lost Foam Casting—In this process, the pattern is made of expendable polystyrene (EPS) beads. For high-production runs, the patterns can be made by injecting EPS beads into a die and bonding them together using a heat source—usually steam. For shorter runs, pattern shapes are cut from sheets of EPS using conventional woodworking equipment and then assembled with glue. In either case, internal passageways in the casting, if needed, are not formed by conventional sand cores but are part of the mold itself.
The polystyrene pattern is coated with a refractory coating, which covers both the external and internal surfaces. With the gating and risering system attached to the pattern, the assembly is suspended in a one-piece flask, which then is placed onto a compaction or vibrating table. As the dry, unbonded sand is poured into the flask and pattern, the compaction and vibratory forces cause the sand to flow and densify. The sand flows around the pattern and into the internal passageways of the pattern.
As the molten metal is poured into the mold, it replaces the EPS pattern, which vaporizes. After the casting solidifies, the unbonded sand is dumped out of the flask, leaving the casting with an attached gating system.
With larger castings, the coated pattern is covered with a facing of chemically bonded sand. The facing sand is then backed up with more chemically bonded sand.
The lost foam process offers the following advantages:

• no size limitations for castings;
• improved surface finish of castings due to the pattern’s refractory coating;
• no fins around coreprints or parting lines;
• in most cases, separate cores are not needed;
• excellent dimensional tolerances.

V-process—In the V-process, the cope and drag halves of the mold are formed separately by heating a thin plastic film to its deformation point. It then is vacuum-formed over a pattern on a hollow carrier plate.
The process uses dry, free-flowing, unbonded sand to fill the special flask set over the film-coated pattern. Slight vibration compacts the fine grain sand to its maximum bulk density. The flask is then covered with a second sheet of plastic film. The vacuum is drawn on the flask, and the sand between the two plastic sheets becomes rigid.
The cope and drag then are assembled to form a plastic-lined mold cavity. Sand hardness is maintained by holding the vacuum within the mold halves at 300-600 mm/Hg. As molten metal is poured into the mold, the plastic film melts and is replaced immediately by the metal. After the metal solidifies and cools, the vacuum is released and the sand falls away.

 

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METAL CASTING PROCESSES

•Sand Casting

•Other Expendable Mold Casting Processes

•Permanent Mold Casting Processes

•Foundry Practice

•Casting Quality

•Metals for Casting

•Product Design Considerations

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