PERMANENT MOLD CASTING
At least three families of molding
and casting processes can be categorized as permanent mold processes. Tese include diecasting (high-pressure di- ecasting), 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 cast- ings at high-production rates. Te 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.
process, the pattern is made of expendable polystyrene (EPS) beads. For high-produc- tion runs, the patterns can be made by in- jecting 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 conven- tional 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. Te 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 vi- brating 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. Te 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- ed sand. Te facing sand is then backed up
2013 Casting sourCe DireCtory
with more chemically bonded sand. Te lost foam process offers the fol-
lowing advantages: • no casting size limitations; • improved casting surface finish; • no fins around coreprints or part- ing 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. Te mold is then vacuum-formed over a pat- tern on a hollow carrier plate. Te 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. Te flask is then covered with a second plastic sheet. Te vacuum is drawn on the flask, and the sand between the two sheets becomes rigid. Te cope and drag then are assembled
to form a plastic-lined mold cavity. Sand hardness is maintained by hold- ing 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 applications 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 per- manent mold casting involves molten metal being poured into the mold either directly or by tilting the mold into a ver- tical 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 mechani- cally in the mold, or sand cores, which are placed in the molds before closing (semi-permanent molding). Te mold halves are preheated, and
the internal surfaces are coated with a refractory. 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 pouring, 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. Te mold then is tilted to the vertical position, allowing the molten metal to flow out of the cup(s) into the mold cavity. Te various permanent mold tech- niques—static pour and tilt pour—offer
Metal Casting Design & PurChasing 9
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