Jump to content

Permanent mold casting: Difference between revisions

From Wikipedia, the free encyclopedia
Browse history interactively
← Previous edit
Content deleted Content added
VisualWikitext
the heading is wrong so i added a correct one
m Apply Template:Convert and tidy
 
(21 intermediate revisions by 17 users not shown)
Line 1: Line 1:
{{short description|Metal casting process that employs reusable molds}}
== Permanent MOULD casting ==
[[Image:Castingtinsoldiers.jpg|thumb|upright|Permanent mould casting]]
[[Image:Castingtinsoldiers.jpg|thumb|upright|Permanent mold casting]]


'''Permanent mould casting''' is a [[metal casting]] process that employs reusable [[Moulding (process)|moulds]] ("permanent moulds"), usually made from [[metal]]. The most common process uses gravity to fill the mould, however gas pressure or a [[vacuum]] are also used. A variation on the typical gravity casting process, called '''slush casting''', produces hollow castings. Common casting metals are [[aluminium]], [[magnesium]], and [[copper]] alloys. Other materials include [[tin]], [[zinc]], and [[lead]] alloys and [[iron]] and steel are also cast in [[graphite]] moulds.<ref name="todd">{{harvnb|Todd|Allen|Alting|1994|pp=258–262}}.</ref><ref name="degarmo327"/>
'''Permanent mold casting''' is a [[metal casting]] process that employs reusable [[Molding (process)|molds]] ("permanent molds"), usually made from [[metal]]. The most common process uses gravity to fill the mold, however gas pressure or a [[vacuum]] are also used. A variation on the typical gravity casting process, called '''slush casting''', produces hollow castings. Common casting metals are [[aluminium]], [[magnesium]], and [[copper]] alloys. Other materials include [[tin]], [[zinc]], and [[lead]] alloys and [[iron]] and steel are also cast in [[graphite]] molds.<ref name="todd">{{harvnb|Todd|Allen|Alting|1994|pp=258–262}}.</ref><ref name="degarmo327"/>


Typical products are components such as [[gear]]s, [[spline (mechanical)|splines]], [[wheel]]s, [[gear housing]]s, [[pipe fitting]]s, [[fuel injector|fuel injection housings]], and automotive engine [[piston]]s.<ref name="todd"/>
Typical products are components such as [[gear]]s, [[spline (mechanical)|splines]], [[wheel]]s, [[gear housing]]s, [[Piping and plumbing fitting|pipe fitting]]s, [[fuel injector|fuel injection housings]], and automotive engine [[piston]]s.<ref name="todd"/>


==Process==
==Process==
There are four main types of permanent mould casting: gravity, slush, low-pressure, and vacuum.
There are four main types of permanent mold casting: gravity, slush, low-pressure, and vacuum.


===Gravity process===
===Gravity process===
The gravity process begins by preheating the mould to 150–200&nbsp;°C (300–400&nbsp;°F) to ease the flow and reduce thermal damage to the casting. The mould cavity is then coated with a [[refractory]] material or a [[mould wash]], which prevents the casting from sticking to the mould and prolongs the mould life. Any sand or metal [[core (casting)|cores]] are then installed and the mould is clamped shut. [[Molten metal]] is then poured into the mould. Soon after solidification the mould is opened and the casting removed to reduce chances of [[hot tear]]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 mould, pour the metal, and remove the casting.<ref name="degarmo325">{{harvnb|Degarmo|Black|Kohser|2003|p=325}}.</ref><ref name="Kalpakjian">{{harvnb|Kalpakjian|Schmid|2006|pp=303–304}}.</ref><ref name="degarmo326"/>
The gravity process begins by preheating the mold to {{convert|150|–|200|C|F}}. to ease the flow and reduce thermal damage to the casting. The mold cavity is then coated with a [[refractory]] material or a [[mold wash]], which prevents the casting from sticking to the mold and prolongs the mold life. Any sand or metal [[core (casting)|cores]] are then installed and the mold is clamped shut. [[Molten metal]] is then poured into the mold. Soon after solidification the mold is opened and the casting removed to reduce chances of [[hot tear]]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.<ref name="degarmo325">{{harvnb|Degarmo|Black|Kohser|2003|p=325}}.</ref><ref name="Kalpakjian">{{harvnb|Kalpakjian|Schmid|2006|pp=303–304}}.</ref><ref name="degarmo326"/>


The metal is poured at the lowest practical temperature in order to minimize cracks and porosity.<ref name="Kalpakjian"/> The pouring temperature can range greatly depending on the casting material; for instance zinc alloys are poured at approximately {{convert|370|C|abbr=on}}, while Gray iron is poured at approximately {{convert|1370|C|abbr=on}}.<ref name="todd"/>
The metal is poured at the lowest practical temperature in order to minimize cracks and porosity.<ref name="Kalpakjian"/> The pouring temperature can range greatly depending on the casting material; for instance zinc alloys are poured at approximately {{convert|370|C|abbr=on}}, while Gray iron is poured at approximately {{convert|1370|C|abbr=on}}.<ref name="todd"/>


====Mould====
====Mold====
Moulds for the casting process consist of two halves. Casting moulds are usually formed from gray cast iron because it has about the best [[thermal fatigue]] 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 mould offers no collapsibility to compensate for shrinkage. Instead the mould is opened as soon as the casting is solidified, which prevents hot tears. Cores can be used and are usually made from [[moulding sand|sand]] or metal.<ref name="Kalpakjian"/><ref name="degarmo326"/>
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]] 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 [[molding sand|sand]] or metal.<ref name="Kalpakjian"/><ref name="degarmo326"/>


As stated above, the mould 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.<ref name="degarmo326"/>
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.<ref name="degarmo326"/>


Venting usually occurs through the slight crack between the two mould 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 (casting)|riser]] must also be included to compensate for shrinkage. This usually limits the yield to less than 60%.<ref name="degarmo326"/>
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 (casting)|riser]] must also be included to compensate for shrinkage. This usually limits the yield to less than 60%.<ref name="degarmo326"/>


Mechanical ejectors in the form of pins are used when coatings are not enough to remove casts from the moulds. These pins are placed throughout the mould and usually leave small round impressions on the casting.{{Citation needed|date=November 2009}}
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.{{Citation needed|date=November 2009}}


===Slush===
===Slush===<!-- [[Hollow casting]], [[hollowcasting]], [[hollow cast]], [[hollowcast]] & [[slush casting]] redirect here -->
<!-- [[Hollow casting]], [[hollowcasting]], [[hollow cast]], [[hollowcast]] & [[slush casting]] redirect here -->
''Slush casting'' is a variant of permanent moulding casting to create a ''hollow casting'' or ''hollow cast''. In the process the material is poured into the mould and allowed to cool until a shell of material forms in the mould. 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 [[ornament (architecture)|ornamental]] products, such as [[candlestick]]s, [[lamp (fixture)|lamp]] bases, and [[statuary]], from low-melting-point materials.<ref name="degarmo327"/> A similar technique is used to make hollow chocolate figures for [[Easter]] and [[Christmas]].<ref>http://www.worldcollectorsnet.com/magazine/issue9/iss9p1.html</ref>
''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 [[ornament (architecture)|ornamental]] products, such as [[candlestick]]s, [[lamp (fixture)|lamp]] bases, and [[statuary]], from low-melting-point materials.<ref name="degarmo327"/> A similar technique is used to make hollow chocolate figures for [[Easter]] and [[Christmas]].<ref>{{cite web |url=http://www.worldcollectorsnet.com/magazine/issue9/iss9p1.html |title=A Dinky Life, the illustrated history of a passion by a long-time collector of die-cast |access-date=2009-11-04 |url-status=dead |archive-url=https://web.archive.org/web/20100115131801/http://worldcollectorsnet.com/magazine/issue9/iss9p1.html |archive-date=2010-01-15 }}</ref>


The method was developed by [[W._Britain|William Britain]] in 1893 for the production of lead [[toy soldier]]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.{{Citation needed|date=November 2009}}
The method was developed by [[W. Britain|William Britain]] in 1893 for the production of lead [[toy soldier]]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.{{Citation needed|date=November 2009}}


Similarly, a process called ''slush moulding'' is used in automotive thermoplastic dashboard manufacture, where a liquid resin is poured into a hot, hollow mould and a viscous skin forms; excess slush is drained off, the mould is cooled, and the moulded product is stripped out.<ref name="define">{{cite web
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.<ref name="define">{{cite web
| title = Slush Moulding
| title = Slush Molding
| publisher = McGraw-Hill Dictionary of Scientific & Technical Terms, 2003, McGraw-Hill Companies, Inc.
| publisher = McGraw-Hill
| work = Dictionary of Scientific & Technical Terms
| year = 2003
| url = http://encyclopedia2.thefreedictionary.com/slush+moulding}}</ref>
| url = http://encyclopedia2.thefreedictionary.com/slush+molding}}</ref>


===Low-pressure===<!-- [[LPPM]], [[Low-pressure permanent mould casting]] & [[Low pressure permanent mould casting]] redirects here -->
===Low-pressure===<!-- [[LPPM]], [[Low-pressure permanent mold casting]] & [[Low pressure permanent mold casting]] redirects here -->
[[File:Low pressure permanent mould casting schematic.svg|thumb|300px|upright|Schematic of the low-pressure permanent mould casting process]]
[[File:Low pressure permanent mold casting schematic.svg|thumb|300px|upright|Schematic of the low-pressure permanent mold casting process]]


Low-pressure permanent mould (''LPPM'') casting uses a gas at low pressure, usually between 3 and 15&nbsp;psi (20 to 100&nbsp;kPa) to push the molten metal into the mould 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 mould. The pouring tube extends to the bottom of the ladle so that the material being pushed into the mould 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.<ref name="degarmo327"/><ref name="degarmo328">{{harvnb|Degarmo|Black|Kohser|2003|p=328}}.</ref>
Low-pressure permanent mold (''LPPM'') casting uses a gas at low pressure, usually between 3 and 15&nbsp;psi (20 to 100&nbsp;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.<ref name="degarmo327"/><ref name="degarmo328">{{harvnb|Degarmo|Black|Kohser|2003|p=328}}.</ref>


The vast majority of LPPM casting are from aluminum and magnesium, but some are copper alloys. Advantages include very little turbulence when filling the mould because of the constant pressure, which minimizes [[gas porosity (casting)|gas porosity]] and [[dross]] formation. Mechanical properties are about 5% better than gravity permanent mould castings. The disadvantage is that cycles times are longer than gravity permanent mould castings.<ref name="degarmo328"/>
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 (casting)|gas porosity]] and [[dross]] 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.<ref name="degarmo328"/>


===Vacuum===<!-- [[Vacuum permanent mould casting]] redirects here -->
===Vacuum===<!-- [[Vacuum permanent mold casting]] redirects here -->
Vacuum permanent mould 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]]. Mechanical properties are usually 10 to 15% better than gravity permanent mould castings. The process is limited in weight to {{convert|0.2|to|5|kg|abbr=on}}.<ref name="degarmo328"/>
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]]. Mechanical properties are usually 10 to 15% better than gravity permanent mold castings. The process is limited in weight to {{convert|0.2|to|5|kg|abbr=on}}.<ref name="degarmo328"/>


==Advantages and disadvantages==
==Advantages and disadvantages==
The main advantages are the reusable mould, good surface finish, good dimensional accuracy, and high production rates. Typical tolerances are 0.4&nbsp;mm for the first 25&nbsp;mm (0.015&nbsp;in for the first inch) and 0.02&nbsp;mm for each additional centimeter (0.002&nbsp;in per in); if the dimension crosses the [[parting line]] add an additional {{convert|0.25|mm|abbr=on}}. Typical surface finishes are 2.5 to 7.5&nbsp;μm (100–250&nbsp;μin) [[Root mean square|RMS]]. A draft of 2 to 3° is required. Wall thicknesses are limited to {{convert|3|to|50|mm|abbr=on}}. Typical part sizes range from 100&nbsp;g to 75&nbsp;kg (several&nbsp;ounces to 150&nbsp;lb). Other advantages include the ease of inducing [[directional solidification]] by changing the mould wall thickness or by heating or cooling portions of the mould. The fast cooling rates created by using a metal mould results in a finer [[crystallite|grain]] structure than sand casting. Retractable metal cores can be used to create [[undercut (manufacturing)|undercuts]] while maintaining a quick action mould.<ref name="degarmo327">{{harvnb|Degarmo|Black|Kohser|2003|p=327}}.</ref><ref name="degarmo325"/>
The main advantages are the reusable mold, good surface finish, good dimensional accuracy, and high production rates. Typical tolerances are 0.4&nbsp;mm for the first {{convert|25|mm|in|abbr=on}} for the first inch) and 0.02&nbsp;mm for each additional centimeter (0.002&nbsp;in per in); if the dimension crosses the [[parting line]] add an additional {{convert|0.25|mm|abbr=on}}. Typical surface finishes are 2.5 to 7.5&nbsp;μm (100–250&nbsp;μin) [[Root mean square|RMS]]. A draft of 2 to 3° is required. Wall thicknesses are limited to {{convert|3|to|50|mm|abbr=on}}. Typical part sizes range from 100&nbsp;g to 75&nbsp;kg (several&nbsp;ounces to 150&nbsp;lb). Other advantages include the ease of inducing [[directional solidification]] 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 [[crystallite|grain]] structure than sand casting. Retractable metal cores can be used to create [[undercut (manufacturing)|undercuts]] while maintaining a quick action mold.<ref name="degarmo327">{{harvnb|Degarmo|Black|Kohser|2003|p=327}}.</ref><ref name="degarmo325"/>


There are three main disadvantages: high tooling cost, limited to low-melting-point metals, and short mould life. The high tooling costs make this process uneconomical for small production runs. When the process is used to cast steel or iron the mould life is extremely short. For lower melting point metals the mould life is longer but thermal fatigue and erosion usually limit the life to 10,000 to 120,000 cycles. The mould life is dependent on four factors: the mould material, the pouring temperature, the mould temperature, and the mould configuration. Moulds made from gray cast iron can be more economical to produce but have short mould lives. On the other hand, moulds made from H13 tool steel may have a mould life several times greater. The pouring temperature is dependent on the casting metal, but the higher the pouring temperature the shorter the mould life. A high pouring temperature can also induce shrinkage problems and create longer cycle times. If the mould temperature is too low [[misrun]]s are produced, but if the mould temperature is too high then the cycle time is prolonged and mould erosion is increased. Large differences in section thickness in the mould or casting can decrease mould life as well.<ref name="degarmo326">{{harvnb|Degarmo|Black|Kohser|2003|p=326}}.</ref>
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]]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.<ref name="degarmo326">{{harvnb|Degarmo|Black|Kohser|2003|p=326}}.</ref>


==References==
==References==
Line 52: Line 55:


===Bibliography===
===Bibliography===
*{{Citation | last = Degarmo | first = E. Paul | last2 = Black | first2 = J. T. | last3 = Kohser | first3 = Ronald A. | title = Materials and Processes in Manufacturing | publisher = Wiley | year = 2003 | edition = 9th | isbn = 0-471-65653-4 | postscript =.}}
*{{Citation | last1 = Degarmo | first1 = E. Paul | last2 = Black | first2 = J. T. | last3 = Kohser | first3 = Ronald A. | title = Materials and Processes in Manufacturing | publisher = Wiley | year = 2003 | edition = 9th | isbn = 0-471-65653-4 | postscript =.}}
*{{Citation | last = Kalpakjian | first = Serope | last2 = Schmid | first2 = Steven | title = Manufacturing Engineering and Technology | publisher = Pearson | year = 2006 | edition = 5th | isbn = 0-13-148965-8 | postscript =.}}
*{{Citation | last1 = Kalpakjian | first1 = Serope | last2 = Schmid | first2 = Steven | title = Manufacturing Engineering and Technology | publisher = Pearson | year = 2006 | edition = 5th | isbn = 0-13-148965-8 | postscript =.}}
*{{Citation | first1 = Robert H. | last1 = Todd | first2 = Dell K. | last2 = Allen | first3 = Leo | last3 = Alting | year = 1994 | title = Manufacturing Processes Reference Guide | publisher = Industrial Press Inc. | url = https://books.google.com/books?id=6x1smAf_PAcC | isbn = 0-8311-3049-0}}.
*{{Citation | first1 = Robert H. | last1 = Todd | first2 = Dell K. | last2 = Allen | first3 = Leo | last3 = Alting | year = 1994 | title = Manufacturing Processes Reference Guide | publisher = Industrial Press Inc. | url = https://books.google.com/books?id=6x1smAf_PAcC | isbn = 0-8311-3049-0}}.


==External links==
==External links==
*[http://www.efunda.com/processes/metal_processing/permanent_mould.cfm Efunda: Engineering Fundamentals]
*[http://www.efunda.com/processes/metal_processing/permanent_mold.cfm Efunda: Engineering Fundamentals]
*[http://batesvilleproducts.com/process.html Article on Permanent Mould Process]
*[http://batesvilleproducts.com/process.html Article on Permanent Mold Process]
*


⚫
[[Category:Casting (manufacturing)]]
{{Metalworking navbox|castopen}}
{{Metalworking navbox|castopen}}

⚫
[[Category:Casting (manufacturing)]]

Latest revision as of 11:13, 1 July 2024

Permanent mold casting

Permanent mold casting is a metal casting process that employs reusable molds ("permanent molds"), usually made from metal. The most common process uses gravity to fill the mold, however gas pressure or a vacuum are also used. A variation on the typical gravity casting process, called slush casting, produces hollow castings. Common casting metals are aluminium, magnesium, and copper alloys. Other materials include tin, zinc, and lead alloys and iron and steel are also cast in graphite molds.[1][2]

Typical products are components such as gears, splines, wheels, gear housings, pipe fittings, fuel injection housings, and automotive engine pistons.[1]

Process

[edit]

There are four main types of permanent mold casting: gravity, slush, low-pressure, and vacuum.

Gravity process

[edit]

The gravity process begins by preheating the mold to 150–200 °C (302–392 °F). to ease the flow and reduce thermal damage to the casting. The mold cavity is then coated with a refractory 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 is then poured into the mold. Soon after solidification the mold is opened and the casting removed to reduce chances of hot tears. 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.[3][4][5]

The metal is poured at the lowest practical temperature in order to minimize cracks and porosity.[4] The pouring temperature can range greatly depending on the casting material; for instance zinc alloys are poured at approximately 370 °C (698 °F), while Gray iron is poured at approximately 1,370 °C (2,500 °F).[1]

Mold

[edit]

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 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 or metal.[4][5]

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.[5]

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%.[5]

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.[citation needed]

Slush

[edit]

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 candlesticks, lamp bases, and statuary, from low-melting-point materials.[2] A similar technique is used to make hollow chocolate figures for Easter and Christmas.[6]

The method was developed by William Britain in 1893 for the production of lead toy soldiers. 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.[citation needed]

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.[7]

Low-pressure

[edit]
Schematic of the low-pressure permanent mold casting process

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.[2][8]

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 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.[8]

Vacuum

[edit]

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. Mechanical properties are usually 10 to 15% better than gravity permanent mold castings. The process is limited in weight to 0.2 to 5 kg (0.44 to 11.02 lb).[8]

Advantages and disadvantages

[edit]

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 (0.98 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 add an additional 0.25 mm (0.0098 in). 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 3 to 50 mm (0.12 to 1.97 in). Typical part sizes range from 100 g to 75 kg (several ounces to 150 lb). Other advantages include the ease of inducing directional solidification 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 structure than sand casting. Retractable metal cores can be used to create undercuts while maintaining a quick action mold.[2][3]

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 misruns 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.[5]

References

[edit]
  1. ^ a b c Todd, Allen & Alting 1994, pp. 258–262.
  2. ^ a b c d Degarmo, Black & Kohser 2003, p. 327.
  3. ^ a b Degarmo, Black & Kohser 2003, p. 325.
  4. ^ a b c Kalpakjian & Schmid 2006, pp. 303–304.
  5. ^ a b c d e Degarmo, Black & Kohser 2003, p. 326.
  6. ^ "A Dinky Life, the illustrated history of a passion by a long-time collector of die-cast". Archived from the original on 2010-01-15. Retrieved 2009-11-04.
  7. ^ "Slush Molding". Dictionary of Scientific & Technical Terms. McGraw-Hill. 2003.
  8. ^ a b c Degarmo, Black & Kohser 2003, p. 328.

Bibliography

[edit]
[edit]
Retrieved from "https://en.wikipedia.org/enwiki/w/index.php?title=Permanent_mold_casting&oldid=1231996395"