Manufacturing Technology II(ME-202)

Special MouldingProcesses

Dr. Chaitanya Sharma

PhD. IIT Roorkee

Title of slide

Lesson ObjectivesIn this chapter we shall discuss the following:1. Carbon dioxide molding process2. Investment casting process3. Shell molding process4. Die casting process

5. Full molding process6. Vacuum-Sealed casting process.

Learning Activities1. Look up

Keywords2. View Slides; 3. Read Notes, 4. Listen to

lecture5. http://www.custom

partnet.com/wu/Sa

ndCasting

Keywords: Carbon dioxide molding process, Investment castingprocess, Die casting process, Shell molding process, Full moldingprocess, Vacuum-Sealed casting process

Other Expendable Mold Processes

• Shell Molding

• Vacuum Molding

• Expanded Polystyrene Process

• Investment Casting

• Plaster Mold and Ceramic Mold Casting

Here is a good reference web site:

http://www.custompartnet.com/wu/SandCasting

Carbon Dioxide Moulding

• Carbon dioxide moulding is a sand casting process thatemploys a moulding mixture of sand and liquid silicatebinder such as sodium silicate (Na2SiO3).

• Moulding mixture is hardened by blowing CO2 gas throughit.

• In this process, the CO2 gas forms a weak acid whichhydrolyzes the sodium silicate, thus forming anamorphous silica that becomes the bond.

• There is also a bonding action from the sodium silicateitself. The use of CO2 gives an almost instantaneous set.

• Mold is fully hardened before pattern is drawn from moldsections.

Advantages & Disadvantages

Advantages: • Good dimensional tolerances through strong core & mould

• Increased accuracy & Excellent casting surface finish

• Accommodates wide range of core and mould sizes

• Co2 core making can be automated for large production.

• Reduced production time, fuel cost and no of mould boxes

• All sands can be used as base aggregate for silicate sand mixture.

Disadvantages• More alkaline the binder, longer it takes to gas and the greater

the tendency for the core to remain rubbery instead of firm.

Application:• Ideal for casting application where speed and flexibility is

paramount

Shell Mould Casting

• Shell mould casting is similar to sand casting, where in moltenmetal is poured into an expendable mould.

• The mould is a thin-walled shell created from applying asand-resin mixture around a pattern.

• Shell mould casting requires the use of a metal pattern, oven,sand-resin mixture, dump box, and molten metal.

• A reusable pattern allows for higher production rates, whiledisposable moulds enable complex geometries to be cast.

• Shell mould casting allows the use of both ferrous and non-ferrous metals, most commonly using cast iron, carbon steel,alloy steel, stainless steel, aluminum alloys, and copper alloys.

• Typical parts are small-to-medium in size and require highaccuracy, such as gear housings, cylinder heads, connectingrods, and lever arms.

Steps In Shell Moldings

• Casting process in which the mold is a thin shell of sandheld together by thermosetting resin binder.

1. Heating of (a match-plate or cope-and-drag) metalpattern which is placed over a box containing sand mixedwith thermosetting resin.

2. Inverting of the box: x is inverted so that sand andresin fall onto the hot pattern, causing a layer of themixture to partially cure on the surface to form a hardshell;

3. Repositioning of the box so that loose uncured particlesdrop away;

4. Curing of sand shell, shell is heated in oven for severalminutes to complete curing;

5. Shell mold is stripped from the pattern;

Shell Molding Steps

1. Pattern heating

2. Inverting Box

3. Repositioning of box

Shell Molding Steps

4. Curing sand shell

5. Shell mold stripping from pattern;

6. Assembling of shell mold halves & pouring

7. Finished casting after sprue etc.

Steps In Shell Moulding

• The shell mold casting process consists of the following steps:

• Pattern creation - A two-piece metal pattern is created in the shape of the desiredpart, typically from iron or steel, aluminum or graphite for casting reactive materials.

• Mold creation - First, each pattern half is heated to 175-370°C and coated with alubricant to facilitate removal. Next, the heated pattern is clamped to a dump box,which contains a mixture of sand and a resin binder. The dump box is inverted, allowingthis sand-resin mixture to coat the pattern. The heated pattern partially cures themixture, which now forms a shell around the pattern. Each pattern half andsurrounding shell is cured to completion in an oven and then the shell is ejected fromthe pattern.

• Mold assembly - The two shell halves are joined together and securely clamped toform the complete shell mold. If any cores are required, they are inserted prior toclosing the mold. The shell mold is then placed into a flask and supported by a backingmaterial.

• Pouring - The mold is securely clamped together while the molten metal is poured froma ladle into the gating system and fills the mold cavity.

• Cooling - After the mold has been filled, the molten metal is allowed to cool andsolidify into the shape of the final casting.

• Casting removal - After the molten metal has cooled, the mold can be broken and thecasting removed. Trimming and cleaning processes are required to remove any excessmetal from the feed system and any sand from the mold.

•

Advantages & Disadvantages

• Advantages of shell molding:

– Smoother cavity surface permits easier flow of molten metal and better surface finish

– Good dimensional accuracy - machining often not required

– Mold collapsibility minimizes cracks in casting

– Can be mechanized for mass production

• Disadvantages:

– More expensive metal pattern

– Difficult to justify for small quantities

• Applications:

• Cylinder heads, connecting rods

Full-Mould Process

• When only a few casting are required the expense ofmaking a pattern can be an important factor.

• In this process the pattern is made of polystyrenearound which the sand mold is formed.

• The sand may be bonded or unbounded. Then, withoutremoving the pattern, the molten metal is poured intothe mold, this immediately vaporizes the polystyrenethus forming the mold cavity.

• Complex casting of various sizes can be madeeconomically by this process.

Investment Casting (Lost Wax Process)

• A pattern made of wax is coated with a refractorymaterial to make mold, after which wax is meltedaway prior to pouring molten metal.

• "Investment" comes from a less familiar definition of"invest" - "to cover completely," which refers tocoating of refractory material around wax pattern.

• It is a precision casting process - capable of producingcastings of high accuracy and intricate detail .

• This process is beneficial for casting metals with highmelting temperatures that can not be molded inplaster or metal such as turbine blades or firearmcomponents., parts for the automotive, aircraft, andmilitary industries.

• The mold is formed by using a wax pattern - a disposable piece in theshape of the desired part.

• The pattern is surrounded, or "invested", into ceramic slurry thathardens into the mold.

• Investment casting is often referred to as "lost-wax casting" becausethe wax pattern is melted out of the mold after it has been formed.

• Lox-wax processes are one-to-one (one pattern creates one part), whichincreases production time and costs relative to other casting processes.

• However, since the mold is destroyed during the process, parts withcomplex geometries and intricate details can be created.

• Investment casting can make use of most metals, most commonly usingaluminum alloys, bronze alloys, magnesium alloys, cast iron, stainlesssteel, and tool steel.

• Investment casting requires the use of a metal die, wax, ceramic slurry,furnace, molten metal, and any machines needed for sandblasting,cutting, or grinding

Steps In Investment Casting

Fig:

1. Wax patterns are produced,

2. Several patterns are attached to a sprue to form a pattern tree

3. Pattern tree is coated with a thin layer of refractory material,

4. The full mold is formed by covering the coated tree with sufficient refractory material to make it rigid

Steps In Investment Casting

Fig:

5. Inverting and heating the mold to melt the wax and permit it to drip out of the cavity,

6. Mold preheating (high temperature), pouring and solidification,

7. the mold is broken away from the finished casting and the parts are separated from the sprue

Application of Investment Casting

Fig: A one-piece compressorstator with 108 separateairfoils made by investmentcasting (photo courtesy ofHowmet Corp.).

Advantages and Disadvantages

• Advantages of investment casting:– Parts of great complexity and intricacy can be cast– Close dimensional control and good surface finish – Wax can usually be recovered for reuse – Additional machining is not normally required - this

is a net shape process

• Disadvantages– Many processing steps are required– Relatively expensive process

Plaster Mold Casting

• Similar to sand casting except mold is made of plaster of Paris (gypsum - CaSO4-2H2O)

• In mold-making, plaster and water mixture is poured over plastic or metal pattern and allowed to set – Wood patterns not generally used due to extended contact

with water

• Plaster mixture readily flows around pattern, capturing its fine details and good surface finish

Advantages and Disadvantages

• Advantages of plaster mold casting:– Good accuracy and surface finish

– Capability to make thin cross-sections .

• Disadvantages:– Mold must be baked to remove moisture, which

can cause problems in casting

– Mold strength is lost if over-baked

– Plaster molds cannot stand high temperatures, so limited to lower melting point alloys

Permanent Mold Casting Processes

• Economic disadvantage of expendable mold casting: a new mold is required for every casting

• In permanent mold casting, the mold is reused many times

• The processes include:

– Basic permanent mold casting

– Die casting

– Centrifugal casting

Basic Permanent Mold Process

• Uses a metal mold constructed of two sectionsdesigned for easy, precise opening and closing

• Molds used for casting lower melting point alloys arecommonly made of steel or cast iron

• Molds used for casting steel must be made ofrefractory material, due to the very high pouringtemperatures

Steps In Permanent Mold Casting

Fig:

1. Mold is preheated and coated

2. Cores (if used) are inserted & mold is closed,

3. Pouring of molten metal & solidification

Advantages and Limitations

• Advantages of permanent mold casting:– Good dimensional control and surface finish

– More rapid solidification caused by the cold metal mold results in a finer grain structure, so castings are stronger

• Limitations:– Generally limited to metals of lower melting point

– Simpler part geometries compared to sand casting because of need to open the mold

– High cost of mold

Applications of Permanent Mold Casting

• Due to high mold cost, process is best suited to high volume production and can be automated accordingly

• Typical parts: automotive pistons, pump bodies, and certain castings for aircraft and missiles

• Metals commonly cast: aluminum, magnesium, copper-base alloys, and cast iron

Die Casting

• A permanent mold casting process in which molten metal is injected into mold cavity under high pressure

• Pressure is maintained during solidification, then mold is opened and part is removed

• Molds in this casting operation are called dies; hence the name die casting

• Use of high pressure to force metal into die cavity is what distinguishes this from other permanent mold processes

Die Casting Machines

• Designed to hold and accurately close two mold halves and keep them closed while liquid metal is forced into cavity

• Two main types: 1. Hot-chamber machine

2. Cold-chamber machine

Hot-Chamber Die Casting

• Metal is melted in a container, and a piston injects liquid metal under high pressure into the die

• High production rates - 500 parts per hour not uncommon

• Applications limited to low melting-pointmetals that do not chemically attack plunger and other mechanical components

• Casting metals: zinc, tin, lead, and magnesium

©2007 John Wiley &

Sons, Inc. M P

Groover,

Hot-Chamber Die Casting

Figure 11.13 Cycle in hot-chamber casting: (1) with die closed and plunger withdrawn, molten metal flows into the chamber (2) plunger forces metal in chamber to flow into die, maintaining pressure during cooling and solidification.

Cold-Chamber Die Casting Machine

• Molten metal is poured into unheated chamberfrom external melting container, and a pistoninjects metal under high pressure into diecavity

• High production but not usually as fast ashot-chamber machines because of pouring step

• Casting metals: aluminum, brass, and magnesiumalloys

• Advantages of hot-chamber process favor itsuse on low melting-point alloys (zinc, tin, lead)

©2007 John Wiley &

Sons, Inc. M P

Groover,

Cold-Chamber Die Casting

Figure 11.14 Cycle in cold-chamber casting: (1) with die closed and ram withdrawn, molten metal is poured into the chamber

Cold-Chamber Die Casting

Figure 11.14 Cycle in cold-chamber casting: (2) ram forces metal to flow into die, maintaining pressure during cooling and

solidification.

Molds for Die Casting

• Usually made of tool steel, mold steel, or maraging steel

• Tungsten and molybdenum (good refractory qualities) used to die cast steel and cast iron

• Ejector pins required to remove part from die when it opens

• Lubricants must be sprayed into cavities to prevent sticking

Expanded Polystyrene Process

Fig: Expanded polystyrene casting process: pattern of polystyrene is coated with refractory compound;

• Uses a mold of sand packed around a polystyrene foam pattern which

vaporizes when molten metal is poured into mold.

• Other names: lost-foam process, lost pattern process, evaporative-foam

process, and full-mold process

• Polystyrene foam pattern includes sprue, risers, gating system, and

internal cores (if needed)

• Mold does not have to be opened into cope and drag sections

From www.wtec.org/loyola/casting/fh05_20.jpg

Expanded Polystyrene Process

Fig 1: Expandedpolystyrene castingprocess: (2) foam patternis placed in mold box, andsand is compacted aroundthe pattern;

Fig 2: Expanded polystyrene casting process: (3)molten metal is poured into the portion of thepattern that forms the pouring cup and sprue. Asthe metal enters the mold, the polystyrene foam isvaporized ahead of the advancing liquid, thus theresulting mold cavity is filled.

Advantages and Disadvantages

• Advantages of expanded polystyrene process:

– Pattern need not be removed from the mold

– Simplifies and speeds mold-making, because two moldhalves are not required as in a conventional green-sandmold

• Disadvantages:

– A new pattern is needed for every casting

– Economic justification of the process is highlydependent on cost of producing patterns

Vacuum Sealed Moulding Process

• Vacuum sealed moulding process (VSMP) was developed in Japan in1971 & used to produce dimensionally accurate and smooth castings.

• In VSMP moulds are made utilizing dry sand, plastic film and aphysical means of binding using negative pressure or vacuum.

• In VSMP, a vacuum, of the order of 250 – 450 mm hg, is imposed tobind the dry free flowing sand encapsulated in b/w two plastic films.

• Mold cavity is formed by vacuum forming of a plastic film over thepattern, backed by unbounded sand, which is compacted by vibrationand held rigidly in place by applying vacuum.

• When the metal is poured into the molds, the plastic film first meltsand then gets sucked just inside the sand voids due to imposedvacuum where it condenses and forms a shell-like layer.

• The vacuum must be maintained until the metal solidifies, afterwhich the vacuum is released allowing the sand to drop away leaving acasting with a smooth surface.

• No shakeout equipment is required and the same sand can be cooledand reused without further treatment.

©2007 John Wiley &

Sons, Inc. M P

Groover,

Expanded Polystyrene Process

• Applications:– Mass production of castings for automobile

engines

– Automated and integrated manufacturing systems are used to

1. Mold the polystyrene foam patterns and then

2. Feed them to the downstream casting operation

Ceramic Mold Casting

• Similar to plaster mold casting except that mold is made of refractory ceramic material that can withstand higher temperatures than plaster

• Can be used to cast steels, cast irons, and other high-temperature alloys

• Applications similar to those of plaster mold casting except for the metals cast

• Advantages (good accuracy and finish) also similar