Special Casting Processes - Manufacturing Engineering - Mechanical Engineering

Special Casting Processes

Special casting processes are groups of casting methods developed to produce components with higher dimensional accuracy, better surface finish, superior metallurgical properties, or geometries and thin sections that are difficult to obtain by conventional sand-mould casting. These processes use specialised mould materials or techniques (thin-shell moulds, expendable wax patterns, reusable metal dies, rotating moulds, vacuum support or pressure assistance) to meet the needs of modern engineering components.

Shell moulding

Shell moulding is a casting process in which the mould is a thin shell of sand bonded by a thermosetting resin. The shell is typically about 9 mm thick and is produced by coating a heated metal pattern with a sand-resin mixture that cures to form the shell. The shell thickness is controlled accurately by varying the contact time between the molten pattern and the sand mixture.

Typical steps in shell moulding:

1. Preheat the metal pattern to the required temperature.
2. Coat the pattern with a mixture of sand and thermosetting resin; allow the resin to cure and form a thin shell.
3. Remove the uncured loose sand; the remaining shell is stripped from the pattern.
4. Assemble two or more shell halves around cores (if needed) and clamp them to make the complete mould.
5. Pour molten metal into the shell mould and allow solidification.
6. Break the shell (shakeout), clean, and perform final machining or finishing as required.

Advantages of shell moulding include improved dimensional accuracy and better surface finish compared with green-sand moulds, and the ability to produce more intricate shapes with reduced fettling. Typical applications are precision parts such as gear housings, valve bodies and small engine components. Limitations include higher tooling and material costs versus simple sand casting, making the process most economical for medium to high volumes.

Vacuum moulding

Vacuum moulding uses sand held together by atmospheric pressure created by a vacuum rather than by a chemical binder. The term "vacuum" refers to the method of forming the mould, not the casting of the metal itself. A porous pattern or flask wall is used and a vacuum draws the sand against the pattern to hold the shape while the casting is poured.

Key points about vacuum moulding:

• Produces moulds with good dimensional stability and surface finish.
• Eliminates chemical binders and reduces gas defects related to binders.
• Useful when improved metal surface quality or reduced contamination from binders is required.
• Suitable for applications where metal cleanliness and fine detail are important.

Investment casting

Investment casting (also called lost-wax casting) is a precision casting process in which a pattern made of wax or other low-melting material is coated with successive layers of refractory material to build a ceramic shell; the wax is then melted or vapourised out before pouring the metal. The name investment comes from the older meaning "to cover completely," referring to the coating of the refractory around the pattern.

Typical sequence of steps in investment casting:

1. Produce patterns: make wax patterns individually or assemble many patterns on a gating sprue to form a pattern tree.
2. Attach patterns to a central sprue and carry out any touches-up of the wax patterns.
3. Dip the pattern assembly in a fine refractory slurry, then stucco (coarse refractory particles); repeat several coating and drying cycles to build the shell thickness required.
4. Dry and cure the shell; then remove the wax by heating (dewaxing) or melting to leave the hollow ceramic shell.
5. Fire the shell to develop strength and remove any residual organics.
6. Pour molten metal into the shell mould, allow solidification and cooling.
7. Break the ceramic shell (shakeout), cut off gating, and finish the casting by machining and heat treatment as required.

Advantages of investment casting:

• Capability to cast parts of great complexity and intricacy.
• Close dimensional control-tolerances as fine as ± 0.075 mm are possible on many features.
• Excellent surface finish, reducing or eliminating the need for secondary machining.
• Ability to produce thin sections and delicate details.

Disadvantages of investment casting:

• Relatively high cost because many process steps and skilled handling are involved.
• Best suited to small- and medium-sized components; however, complex parts weighing up to 75 lb (≈34 kg) have been produced.

Applications of investment casting:

• All types of metals can be investment cast, including carbon steels, stainless steels and high-temperature alloys (nickel and cobalt-based superalloys).
• Typical parts include intricate machinery components, turbine engine blades and vanes, jewellery, dental fixtures and aerospace parts where precision and surface quality are critical.

Permanent-mould casting processes

Permanent-mould casting refers to a set of processes that use reusable metal moulds (dies) rather than expendable sand or ceramic moulds. The metal dies are usually made of steel or cast iron, providing dimensional accuracy and a long production life.

Processes that fall within this group include die casting, centrifugal casting and other variations such as slush casting, low-pressure permanent-mould casting and vacuum permanent-mould casting.

Variations of the basic permanent-mould method:

• Slush casting
• Low-pressure casting
• Vacuum permanent-mould casting

Slush casting

Slush casting is used to make hollow castings. The mould is filled with molten metal and allowed to cool just enough to form a solidified skin at the mould surface; the mould is then inverted to drain the remaining liquid metal from the interior, leaving a hollow shell of metal. This method is commonly used for non-structural hollow articles such as decorative items and some light fittings.

Low-pressure permanent-mould casting

Low-pressure casting forces liquid metal into the mould from below under a relatively low positive pressure-roughly 0.1 MPa-so that the metal fills the die by upward flow. This produces sound castings with good directional solidification and reduces turbulence, gas entrapment and oxide formation.

Vacuum permanent-mould casting

Vacuum permanent-mould casting is similar in general configuration to low-pressure casting. Instead of forcing metal into the die by positive pressure, a reduced air pressure inside the mould draws the molten metal into the cavity. This reduces gas entrapment and improves the ability to cast thin sections and complicated shapes with improved surface quality.

Die casting

Die casting is a high-pressure permanent-mould process in which molten metal is injected into a metal die (tool) under high pressure. Typical die-casting pressures range from 7 to 350 MPa, depending on the process and part geometry.

Two main machine types are used:

• Hot-chamber die casting
• Cold-chamber die casting

1. Hot-chamber die casting
   In a hot-chamber machine the metal is melted in a container that is an integral part of the machine. A plunger or piston forces the molten metal through a gooseneck and into the die under high pressure. Typical injection pressures for hot-chamber machines are 7 to 35 MPa. The process is suitable for alloys that have low melting points and do not severely attack the plunger or gooseneck, such as zinc, tin, lead and sometimes magnesium.
2. Cold-chamber die casting
   In a cold-chamber machine molten metal is ladled from an external furnace into an unheated injection chamber; a piston then injects the molten metal into the die. Cold-chamber machines are typically used for casting aluminium, brass and magnesium alloys. Low-melting-point alloys (zinc, tin, lead) can also be cast in cold-chamber machines, but hot-chamber machines often have advantages for those alloys.

Advantages of die casting:

• Very high production rates are possible.
• Economical for large production quantities due to long die life and rapid cycles.
• Close tolerances are achievable; for small parts tolerances on the order of ± 0.076 mm are common.
• Good surface finish reduces the need for secondary finishing.

Limitations of die casting:

• Shape restrictions: part geometry must allow for removal from the die (drafts, parting lines, and undercuts require special features such as slides or lifters).
• Initial tooling costs are high, so die casting is most economical for high-volume production.

Centrifugal casting

Centrifugal casting encompasses several methods that use rotation of the mould to distribute the molten metal by centrifugal force, concentrating metal at the outer regions of the mould cavity. The group includes true centrifugal casting, semi-centrifugal casting and centrifuge casting.

• True centrifugal casting is used for casting symmetric cylindrical parts (pipes, rings, cylinders) where the mould is rotated about its axis and molten metal is poured into the rotating mould; the metal is forced outward to form the part.
• Semi-centrifugal casting produces castings that combine a central hub and outer rim with centrifugal force used to improve metal soundness in the rim.
• Centrifuge casting refers to arrangements in which cavities or cores are located away from the rotation axis so that molten metal is distributed into those cavities by centrifugal force.

Centrifugal casting produces dense, fine-grained castings with reduced impurities and directional solidification, making it suitable for pipes, bearings, bushings and rings.

Some common defects in castings

Casting defects common to special casting processes

The following defects can occur in special casting processes; brief causes and typical remedies are noted.

• Misruns - incomplete filling of the mould; caused by low pouring temperature, thin sections, or slow fill speed. Remedy: raise pouring temperature, improve gating, increase metal fluidity.
• Cold shuts - lines where two metal fronts fail to fuse; caused by low metal temperature or poor flow. Remedy: increase temperature, change gating to reduce turbulent meeting of fronts.
• Shrinkage cavity - internal voids from solidification shrinkage; remedy: improve feeding, add risers or chills, control solidification pattern.
• Microporosity - fine porosity due to dissolved gases or interdendritic shrinkage; remedy: reduce gas pickup, modify alloy/solidification rate, apply vacuum or pressure-assisted filling.
• Hot tearing - cracks formed during solidification under restraint; remedy: redesign to reduce restraint, adjust cooling rates, use modified alloys.
• Sand blow - gas pockets from moisture or volatile binders in sand; remedy: dry moulds, use appropriate binders, control heating.
• Pinholes - small surface gas pores; remedy: degas melt, improve venting in mould.
• Sand wash - erosion of sand from mould walls into metal by turbulent flow; remedy: modify gating to reduce turbulence, use coatings on mould walls.
• Scabs - rough projections on surface where sand has detached and been fused; remedy: improve sand strength or coating, reduce metal temperature at mould wall.
• Penetration - metal penetrating between sand grains producing a rough surface; remedy: increase mould strength, use finer sand or coating, reduce pouring velocity.
• Mould shift - misalignment of mould halves causing mismatch; remedy: improve mould clamping and registration features.
• Core shift - displacement of internal cores leading to dimensional errors; remedy: better core supports and accurate core seating.
• Mould crack - cracking of mould under thermal stresses causing leaks or defects; remedy: use stronger mould materials, control heating and cooling rates.

Advantages of special casting processes

• Greater dimensional accuracy compared with conventional sand casting.
• Higher metallurgical quality: denser microstructure and fewer internal defects in many processes.
• Lower unit production cost in high-volume production (especially die casting and centrifugal casting).
• Ability to cast extremely thin sections and intricate details (investment, die, and shell casting).
• High production rates with reduced cycle times (die casting).
• Better surface finishes on the castings so less machining and finishing labour is required.
• Minimum need for further machining for many features, saving time and cost.
• Castings may possess a finer grain structure and slightly higher strength and ductility than some solid-mould castings.

Selection guidelines and typical applications

Choice of a special casting process depends on part geometry, material, required tolerances and surface finish, production volume and cost targets. General guidance:

• Use investment casting for complex shapes, excellent surface finish and tight tolerances, especially for aerospace, turbine, jewellery and medical parts.
• Use die casting for high-volume production of non-ferrous parts (aluminium, zinc, magnesium) where speed and repeatability are priorities.
• Use permanent-mould and low-pressure casting for medium-volume aluminium parts requiring good mechanical properties.
• Use centrifugal casting for symmetric parts such as pipes, cylinders, bearings and bushings where density and soundness are critical.
• Use shell moulding when improved surface finish and dimensional accuracy are required for small to medium-sized iron or steel castings.

Summary: Special casting processes extend the capability of traditional casting by providing improved dimensional control, surface finish and metallurgical quality. Each process has distinct strengths and limitations; selection should be based on component requirements, alloy choice and production economics.