PERMANENT MOLD CASTING
At least three families of molding and
casting processes can be categorized as permanent mold processes. These include diecasting (high-pressure diecasting), low-pressure permanent mold casting and permanent mold casting. Unlike sand casting processes, in which the mold is destroyed after pouring to remove the casting, permanent mold casting uses the mold repeatedly. Diecasting—Diecasting is used to
produce small- to medium-sized castings at high-production rates. The metal molds are coated with a mold surface coating and preheated before being filled with molten metal. Premeasured amounts of molten metal are forced from a shot chamber into the permanent mold or die under extreme pressure (greater than 15,000 psi). Castings of varying weights and sizes
Automated vertically parted molding machines, normally used for high-production runs, compact molding sand by squeezing.
Lost Foam Casting—In the lost foam
process, the pattern is made of expendable polystyrene (EPS) beads. For high-pro- duction 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 as- sembled 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 is then 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 bond-
2011 CASTING SOURCE DIRECTORY
ed sand. The facing sand is then backed up with more chemically bonded sand. The lost foam process offers the follow-
ing advantages: • no casting size limitations; • improved casting surface finish; • no fins around coreprints or parting lines;
• in most cases, no separate cores are 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. The mold is then 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 vibra- tion compacts the fine grain sand to its maximum bulk density. The flask is then covered with a second plastic sheet. The vacuum is drawn on the flask, and the sand between the two 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 by the metal. After the metal solidifies and cools, the vacuum is released and the sand falls away.
can be produced. Nearly all die castings are produced in nonferrous alloys with limited amounts of cast iron and steel castings produced in special applications. Die castings and the diecasting process
are suitable for a wide variety of applica- tions in which high part volumes are needed. Benefits include: • excellent mechanical properties and surface finish;
• dimensional tolerances of 0.005-0.01 in.; • recommended machining allowances of 0.01-0.03 in.;
• thin-section castings. Permanent Mold Casting (Gravity
Diecasting)—Another form of permanent mold casting involves molten metal being poured into the mold either directly or by tilting the mold into a vertical position. In this process, the mold is made in two halves from cast iron or steel. If cores are to be used, they can be metal inserts, which operate mechanically in the mold, or sand cores, which are placed in the molds before closing (semi-permanent molding). The mold halves are preheated, and the
internal surfaces are coated with a refrac- tory. If static pouring is to be used, the molds are closed and set into the vertical position for pouring; thus, the parting line is in the vertical position. In tilt pour- ing, the mold is closed and placed in the horizontal position, at which point molten metal is poured into a cup(s) attached to the mold. The mold then is tilted to the vertical position, allowing the molten metal to flow out of the cup(s) into the mold cavity. The various permanent mold tech-
niques—static pour and tilt pour—offer a variety of advantages for a range of met- alforming applications. Benefits include:
METAL CASTING DESIGN & PURCHASING 9
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