Permanent mold casting is a
metal casting
In metalworking and jewelry making, casting is a process in which a liquid metal is delivered into a mold (usually by a crucible) that contains a negative impression (i.e., a three-dimensional negative image) of the intended shape. The metal is ...
process that employs reusable
molds ("permanent molds"), usually made from
metal
A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typicall ...
. The most common process uses gravity to fill the mold, however gas pressure or a
vacuum
A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often dis ...
are also used. A variation on the typical gravity casting process, called slush casting, produces hollow castings. Common casting metals are
aluminium
Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. I ...
,
magnesium
Magnesium is a chemical element with the symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 of the periodic ta ...
, and
copper
Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...
alloys. Other materials include
tin
Tin is a chemical element with the symbol Sn (from la, stannum) and atomic number 50. Tin is a silvery-coloured metal.
Tin is soft enough to be cut with little force and a bar of tin can be bent by hand with little effort. When bent, t ...
,
zinc
Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodi ...
, and
lead
Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cu ...
alloys and
iron
Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in f ...
and steel are also cast in
graphite
Graphite () is a crystalline form of the element carbon. It consists of stacked layers of graphene. Graphite occurs naturally and is the most stable form of carbon under standard conditions. Synthetic and natural graphite are consumed on large ...
molds.
[.]
Typical products are components such as
gear
A gear is a rotating circular machine part having cut teeth or, in the case of a cogwheel or gearwheel, inserted teeth (called ''cogs''), which mesh with another (compatible) toothed part to transmit (convert) torque and speed. The basic pr ...
s,
splines,
wheel
A wheel is a circular component that is intended to rotate on an axle Bearing (mechanical), bearing. The wheel is one of the key components of the wheel and axle which is one of the Simple machine, six simple machines. Wheels, in conjunction wi ...
s,
gear housing
The gear housing is a mechanical housing that surrounds the mechanical components of a gear box.
It provides mechanical support for the moving components, a mechanical protection from the outside world for those internal components, and a fluid-ti ...
s,
pipe fittings,
fuel injection housings, and automotive engine
piston
A piston is a component of reciprocating engines, reciprocating pumps, gas compressors, hydraulic cylinders and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder and is made gas-tig ...
s.
Process
There are four main types of permanent mold casting: gravity, slush, low-pressure, and vacuum.
Gravity process
The gravity process begins by preheating the mold to 150–200 °C (300–400 °F). to ease the flow and reduce thermal damage to the casting. The mold cavity is then coated with a
refractory
In materials science, a refractory material or refractory is a material that is resistant to decomposition by heat, pressure, or chemical attack, and retains strength and form at high temperatures. Refractories are polycrystalline, polyphase, ...
material or a
mold wash, which prevents the casting from sticking to the mold and prolongs the mold life. Any sand or metal
cores are then installed and the mold is clamped shut.
Molten metal
Melting, or fusion, is a physical process that results in the phase transition of a substance from a solid to a liquid. This occurs when the internal energy of the solid increases, typically by the application of heat or pressure, which inc ...
is then poured into the mold. Soon after solidification the mold is opened and the casting removed to reduce chances of
hot tear
Hot or the acronym HOT may refer to:
Food and drink
*Pungency, in food, a spicy or hot quality
*Hot, a wine tasting descriptor
Places
* Hot district, a district of Chiang Mai province, Thailand
**Hot subdistrict, a sub-district of Hot Distric ...
s. The process is then started all over again, but preheating is not required because the heat from the previous casting is adequate and the refractory coating should last several castings. Because this process is usually carried out on large production run work-pieces automated equipment is used to coat the mold, pour the metal, and remove the casting.
[.][.]
The metal is poured at the lowest practical temperature in order to minimize cracks and porosity.
The pouring temperature can range greatly depending on the casting material; for instance zinc alloys are poured at approximately , while Gray iron is poured at approximately .
Mold
Molds for the casting process consist of two halves. Casting molds are usually formed from gray cast iron because it has about the best
thermal fatigue Thermo-mechanical fatigue (short TMF) is the overlay of a cyclical mechanical loading, that leads to fatigue of a material, with a cyclical thermal loading. Thermo-mechanical fatigue is an important point that needs to be considered, when constructi ...
resistance, but other materials include steel, bronze, and graphite. These metals are chosen because of their resistance to erosion and thermal fatigue. They are usually not very complex because the mold offers no collapsibility to compensate for shrinkage. Instead the mold is opened as soon as the casting is solidified, which prevents hot tears. Cores can be used and are usually made from
sand
Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural class of s ...
or metal.
As stated above, the mold is heated prior to the first casting cycle and then used continuously in order to maintain as uniform a temperature as possible during the cycles. This decreases thermal fatigue, facilitates metal flow, and helps control the cooling rate of the casting metal.
Venting usually occurs through the slight crack between the two mold halves, but if this is not enough then very small vent holes are used. They are small enough to let the air escape but not the molten metal. A
riser must also be included to compensate for shrinkage. This usually limits the yield to less than 60%.
Mechanical ejectors in the form of pins are used when coatings are not enough to remove casts from the molds. These pins are placed throughout the mold and usually leave small round impressions on the casting.
Slush
''Slush casting'' is a variant of permanent molding casting to create a ''hollow casting'' or ''hollow cast''. In the process the material is poured into the mold and allowed to cool until a shell of material forms in the mold. The remaining liquid is then poured out to leave a hollow shell. The resulting casting has good surface detail but the wall thickness can vary. The process is usually used to cast
ornamental products, such as
candlestick
A candlestick is a device used to hold a candle in place. Candlesticks have a cup or a spike ("pricket") or both to keep the candle in place. Candlesticks are less frequently called "candleholders".
Before the proliferation of electricity, candl ...
s,
lamp
Lamp, Lamps or LAMP may refer to:
Lighting
* Oil lamp, using an oil-based fuel source
* Kerosene lamp, using kerosene as a fuel
* Electric lamp, or light bulb, a replaceable component that produces light from electricity
* Light fixture, or li ...
bases, and
statuary
A statue is a free-standing sculpture in which the realistic, full-length figures of persons or animals are carved or cast in a durable material such as wood, metal or stone. Typical statues are life-sized or close to life-size; a sculpture t ...
, from low-melting-point materials.
A similar technique is used to make hollow chocolate figures for
Easter
Easter,Traditional names for the feast in English are "Easter Day", as in the '' Book of Common Prayer''; "Easter Sunday", used by James Ussher''The Whole Works of the Most Rev. James Ussher, Volume 4'') and Samuel Pepys''The Diary of Samuel ...
and
Christmas
Christmas is an annual festival commemorating Nativity of Jesus, the birth of Jesus, Jesus Christ, observed primarily on December 25 as a religious and cultural celebration among billions of people Observance of Christmas by country, around t ...
.
The method was developed by
William Britain in 1893 for the production of lead
toy soldier
A toy soldier is a miniature figurine that represents a soldier. The term applies to depictions of uniformed military personnel from all eras, and includes knights, cowboys, American Indians, pirates, samurai, and other subjects that involve ...
s. It uses less material than solid casting, and results in a lighter and less expensive product. Hollow cast figures generally have a small hole where the excess liquid was poured out.
Similarly, a process called ''slush molding'' is used in automotive dashboard manufacture, for soft-panel interiors with artificial leather, where a free-flowing (which behave like a liquid) powder plastic compound, either PVC or TPU, is poured into a hot, hollow mold and a viscous skin forms. Excess slush is then drained off, the mold is cooled, and the molded product is stripped out.
Low-pressure
![Low pressure permanent mold casting schematic](https://upload.wikimedia.org/wikipedia/commons/6/62/Low_pressure_permanent_mold_casting_schematic.svg)
Low-pressure permanent mold (''LPPM'') casting uses a gas at low pressure, usually between 3 and 15 psi (20 to 100 kPa) to push the molten metal into the mold cavity. The pressure is applied to the top of the pool of liquid, which forces the molten metal up a refractory pouring tube and finally into the bottom of the mold. The pouring tube extends to the bottom of the ladle so that the material being pushed into the mold is exceptionally clean. No risers are required because the applied pressure forces molten metal in to compensate for shrinkage. Yields are usually greater than 85% because there is no riser and any metal in the pouring tube just falls back into the ladle for reuse.
[.]
The vast majority of LPPM casting are from aluminum and magnesium, but some are copper alloys. Advantages include very little turbulence when filling the mold because of the constant pressure, which minimizes
gas porosity and
dross
Dross is a mass of solid impurities floating on a molten metal or dispersed in the metal, such as in wrought iron. It forms on the surface of low- melting-point metals such as tin, lead, zinc or aluminium or alloys by oxidation of the metal. For ...
formation. Mechanical properties are about 5% better than gravity permanent mold castings. The disadvantage is that cycles times are longer than gravity permanent mold castings.
Vacuum
Vacuum permanent mold casting retains all of the advantages of LPPM casting, plus the dissolved gases in the molten metal are minimized and molten metal cleanliness is even better. The process can handle thin-walled profiles and gives an excellent
surface finish
Surface finish, also known as surface texture or surface topography, is the nature of a surface as defined by the three characteristics of lay, surface roughness, and waviness.. It comprises the small, local deviations of a surface from the perf ...
. Mechanical properties are usually 10 to 15% better than gravity permanent mold castings. The process is limited in weight to .
Advantages and disadvantages
The main advantages are the reusable mold, good surface finish, good dimensional accuracy, and high production rates. Typical tolerances are 0.4 mm for the first 25 mm (0.015 in for the first inch) and 0.02 mm for each additional centimeter (0.002 in per in); if the dimension crosses the
parting line
A parting line, in industrial casting of molds, is the border line in which draft angles change direction. One can check the parting line in the mould or product which divides the two half, i.e; the core and the cavity of a molded part. It is so ...
add an additional . Typical surface finishes are 2.5 to 7.5 μm (100–250 μin)
RMS. A draft of 2 to 3° is required. Wall thicknesses are limited to . Typical part sizes range from 100 g to 75 kg (several ounces to 150 lb). Other advantages include the ease of inducing
directional solidification
Directional solidification (DS) and progressive solidification are types of solidification within castings. Directional solidification is solidification that occurs from farthest end of the casting and works its way towards the sprue. Progressiv ...
by changing the mold wall thickness or by heating or cooling portions of the mold. The fast cooling rates created by using a metal mold results in a finer
grain
A grain is a small, hard, dry fruit (caryopsis) – with or without an attached hull layer – harvested for human or animal consumption. A grain crop is a grain-producing plant. The two main types of commercial grain crops are cereals and legum ...
structure than sand casting. Retractable metal cores can be used to create
undercuts while maintaining a quick action mold.
[.]
There are three main disadvantages: high tooling cost, limited to low-melting-point metals, and short mold life. The high tooling costs make this process uneconomical for small production runs. When the process is used to cast steel or iron the mold life is extremely short. For lower melting point metals the mold life is longer but thermal fatigue and erosion usually limit the life to 10,000 to 120,000 cycles. The mold life is dependent on four factors: the mold material, the pouring temperature, the mold temperature, and the mold configuration. Molds made from gray cast iron can be more economical to produce but have short mold lives. On the other hand, molds made from H13 tool steel may have a mold life several times greater. The pouring temperature is dependent on the casting metal, but the higher the pouring temperature the shorter the mold life. A high pouring temperature can also induce shrinkage problems and create longer cycle times. If the mold temperature is too low
misrun
A casting defect is an undesired irregularity in a metal casting process. Some defects can be tolerated while others can be repaired, otherwise they must be eliminated. They are broken down into five main categories: ''gas porosity'', ''shrinkage ...
s are produced, but if the mold temperature is too high then the cycle time is prolonged and mold erosion is increased. Large differences in section thickness in the mold or casting can decrease mold life as well.
[.]
References
Bibliography
*
*
*.
External links
Efunda: Engineering Fundamentals
Casting (manufacturing)
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