For high-volume production, the manual drives are replaced by two -way hydraulic mechanisms. These can be programmed to open and close in a preset cycle. Thus, except for pouring of the metal and removal of castings, the operation is automatic.
A method of permanent mold casting has been developed in which the metal is not ladled by hand. This is called the Wessel Process . The equipment for which is shown in fig. 3. In this method, the permanent mold is mounted on rails against the end face of the tilting reverberate furnace. As the furnace is tiled about an axis bear its center of gravity, metals flows through a pouring hole in ht wall of the furnace in the mold. The assembly
remains in its tilted position for a predetermined interval, then returns to the s tarting
position. Tilting is done by means of a hydraulic cylinder.
Molds are parted vertically, parallel to the direction of metal flow. One mold half slides on the mounting rails; the other, which is hinged, swings away from the mounting rails, pulling the casting and sprue with if to leave the pouring hole clear for the next cycle. Core manipulation and casting ejection are the same as in conventional practice.
B1.1.7
SELECTION OF MOULD MATERIALS
Four principle factors affect the selection of material s for permanent molds and
cores: -
§ The pouring temperature of the metal to be cast § The size of the casting
§ The number of castings per mold § Cost of the mold material
Mould Materials: -
As indicated in Table A, gray iron is the most commonly usedmold material. Aluminum or graphite molds are sometimes used for the small -quantity
production of aluminum and magnesium castings, and graphite or carbon linear on steel are sometimes used for molds for casting copper alloys ( see also the section “Solid Graphite Molds” in this article).
With aluminum or magnesium casting alloys, it is not unusual to obtain 100,000 castings, or more, per mold; however, molds for copper or gray iron casting alloys have a shorter life because of the higher pouring temperatures required. Pouring temperatures for specific metals are as follows:
Gray iron molds without tool steel inserts are satisfactory for long production runs of aluminum and magnesium castings that will be magnesium casting that be machined extensively and for which s urface finish is not a major consideration. In the casting of zinc, well over 00.000 pours are possible in a gray iron mold (die casting is usually selected to produce zinc castings such large quantities).
Mold Inserts: -
Full or partial mold cavities ins erts of the same material as the mold , or of a different material, are sometimes used to obtain longer mold life, or to simplifymachining handling or replacement. Inserts can also be used for venting, cooling thin walls, and heating portions of the mold o r the full cavity area . Inserts made of cast -to-shape gray iron are used for casting complex alu minum and magnesium parts that r ange in surface
area from 320 to 2900 cm 2 (50 to 50 in.2). Tolerance on these parts range from 0.76 to ±
1.5 mm (± 1.5 mm ± 0. 030 to ± 0.060 in.). Inserts last for 5000 to 20.000 pours, depending on casting complexity.
Core Materials: -
Core materia ls are recommended in Tables B and C on the basis of performance over a wide range of coring requirement for small and large cores. Anexpendable core is used when the location or shape of the core does not permit its removal from the casting or when an in tricate design can be obtained at less cost with materials for such cores. These materials are listed below in order of increasing preference:
§ Sand (oil – bonded or resin –bonded, shell, car bon dioxide-silicate) § Plaster
§ Graphite and carbon
Table A recommended permanent mold Materials Casting alloy No. of Pours
1000 10,000 100,000
for small Castings (25 mm or 1inch Maximum dimension)
Zinc Gray Iron: 1020steel Gray Iron: 1020steel Gray Iron: 1020steel Aluminum Mg. Gray Iron: 1020steel Gray Iron: 1020steel Gray Iron with AISI-H14
inserts: 1020steel Copper Gray Iron Gray Iron Alloy Cast Iron
Gray Iron Gray Iron (a)Gray Iron (a) Quantity not Poured
for medium and large castings (upto 915mm or 36Inch maximum dimension)
Zinc Gray Iron:AISI-H11 (b)Gray Iron: AISI-H11 (b) Gray Iron: AISI-H11 (b) Aluminum Mg. Gray Iron Gray Iron Gray Iron with AIS-H11/H1(c)
inserts: 1020steel Copper Alloy Cast Iron Alloy Cast Iron Alloy Cast Iron (d)
Gray Iron Gray Iron (a) Gray Iron (a) Quantity not Poured
METAL/ALLOY TEMPERATURE, °C (°F) Zinc Aluminum Magnesium Copper Gray Iron 465-620 (870-1050) 675-790 (1250-1450) 705-790 (1300-1450) 980-1230 (1800-2250) 1275-1355 (2325-2475)
Table B Recommended Materials for small cores (<75mm or 3In in Diameter & 255mm or 10In. long) for Permanent moulds
Casting alloy Recommended core materials (a)
Zinc Sand Plaster, Gray Iron 1020steel
Aluminum Mg. 1010 or 1020 steel Sand Plaster H11 Die steel or Equivalent (b) carbon (c) Copper Sand 1020steel Gray Iron Plaster (d) Graphite9(c)
Gray Iron Sand Graphite Carbon and Gray Iron (a) Materials are listed descending order of preference (b) Hardened to 40-45HRC (c) for use with relatively few Pours (d) for Castings of Aluminum Bronze.
Table B Recommended Materials for Large cores (<75mm or 3In in Diameter & 255mm or 10In. long) for Permanent moulds
Casting alloy No. of Pours
1000 10,000 100,000
Zinc Gray Iron: 1020steel Gray Iron: 1020steel Gray Iron: 1020steel Aluminum Mg. Gray Iron: Gray Iron with Gray Iron: Gray Iron with Gray Iron: Gray Iron with
1020steel insert (b) 1020steel or H11 Insert (b) H11 Insert (b), Sand Plaster (b) Gray Iron: 1020steel H11 Die steel,
Copper Sand Sand Quantity not Poured
Gray Iron Sand, Graphite, Sand, Graphite,Quantity not Poured
Carbon Gray Iron Carbon Gray Iron
(a)Material listed in descending order of preference, (b) except for openings with complex shape, which required expandable sand cores.
B1. 1.8
GRAVITY DIE CASTING MOULD LIFE
Mold life can vary from as few as 100 pours as many as 250,000 pours (or even more), depending on the variables discussed later in the section. A mold for an aluminum piston for example, can be accepted to produce 250,000 casting before requiring repair. After the production of 250,000 more castings, the repaired mold will require a major overhaul.
Mold life is likely to be longer in the casting of magnesium alloys than in the casting of aluminum alloys of similar size and shape; this is because molte n magnesium does not attack ferrous metal molds however the difference the difference in mold life for magnesium alloys depends to a great Extent on the effectiveness of the mold coating used
in the casting of gray iron, mold coating used in the casting of gray iron, mold life is
Molds are often fabricated from cast iron is cause casting the mold close to the finished shape can decrease machining costs. In ad dition cast iron is much more resistant to attack by molten aluminum than steel; however is weld able and easier to repair than cast iron. Therefore steel molds are often used for high-production castings.