Fig. 9.1 Diagram illustrating components of an investment mould.

of the mould all have an effect on the quality of the casting. For large castings, two or more sprues may be necessary in order to ensure that molten alloy is able to reach all parts of the mould cavity before solidifying.

9.3 Casting machines

Numerous types of casting machines are available, the aim of each being to cause molten alloy to completely fill the investment mould cavity. The three main variables which characterise the machines are as follows:

(1)The alloy may be melted in the mould sprue base (mould crucible) or in a separate crucible located in the casting machine.

(2)The alloy may be melted by one of several methods including gas-air torch, oxyacetylene torch, electrical induction heating or electrical resistance melting.

(3)The molten alloy may be driven into the mould by gravity, air pressure, steam pressure or by centrifugal force. The arm of the centrifugal casting machine is rotated either by a spring or by means of an electric motor.

Possibly the most popular system in current use is that in which the alloy is melted in a separate crucible using electrical induction heating and forced into the mould using centrifugal force.

The classical approach to centrifugal casting was to have the casting ring attached to a chain. The alloy was melted in the mould sprue base and forced into the mould cavity by swinging the casting ring around at the end of the chain.

9.4 Faults in castings

The faults which can occur in casting may be of four types.

(1)Finning and bubbling.

(2)Incomplete casting.

(3)Porosity in casting.

(4)Oversized or undersized casting.

Finning and bubbling: Finning occurs when the investment is heated up too rapidly in the furnace. This causes the investment to crack. Molten alloy flows into the cracks forming thin ‘fins’ on the casting in regions where the cracks have been located.

Bubbling effects on casting appear as spheres of excess material attached to the surface of the casting. These reflect the presence of surface porosities in the investment, a problem which can be overcome by vacuum investing.

Finning and bubbling increase the time required to finish a casting and if the defects occur in critical areas (e.g. near a crown shoulder) can result in a need to re-cast.

Incomplete castings: There are many possible causes of incomplete castings. In any casting the greater the number and thickness of the sprues, the more readily the metal will fill the mould. Against this, the sprues must be severed from the completed casting and an excessive number of sprues creates more work in finishing. Also, a larger weight of alloy is required for the casting and this presents difficulties in melting. It will be seen that the point of attachment of the sprues is a common site for defects and therefore an excessive number should be avoided.

The sprues should be attached at points of greatest bulk within the casting. This helps create a larger heat sink in these areas and prevents premature solidification which would cause incomplete filling of the mould. Placing sprues near to the bulky areas of the casting also aids the process of sprue removal and finishing without damaging the casting.

If the alloy is not properly melted, or if the mould temperature is too low, solidification occurs

before the mould can be properly filled. The balance between molten alloy temperature and mould temperature plays an important part in ensuring complete filling of the mould as discussed in the previous section.

If there is insufficient thrust created during casting the alloy may not flow to all parts of the mould cavity. For centrifugal casting machines the thrust depends on the rotational speed of the casting arm, the length of the arm and the density of the alloy. The problem is therefore more significant for base metal alloys which have lower density and create less thrust.

Back pressure effects are caused by an inability of air or other gases within the mould to escape, making way for the alloy. To assist the escape of gases, the investment material between the casting and the end of the ring should be as thin as is consistent with strength, (distance X in Fig. 9.1). Also, the end of the ring should not be completely covered by any part of the casting apparatus. In all cases the plate of metal which supports the end of the ring must be perforated.

Permeability of investments varies with particle size distribution, but generally it decreases in the order of gypsum-, phosphateand silica-bonded. A rather dense layer of investment material is often created at the base of the ring, particularly when the base of the ring has been closed temporarily by a sheet of metal or glass. This dense layer should be scraped away to facilitate the escape of gases. When using silica-bonded or fine-grained, phosphate-bonded investments a vent, 0.5 mm in diameter, should be provided to allow escape of gases towards the crucible end of the mould. A casting which has been subjected to back pressure is rounded at the edges and lacking in detail.

Defects may also be caused by cooling shrinkage. On solidification, the alloy contracts but the outer portions of the casting remain in contact with the internal walls of the mould.

The thinner sections, or those portions which are less effectively insulated against heat loss by the investment material, freeze first. As they solidify they contract and draw molten metal from the remaining portions. Voids will be formed unless more metal can enter the mould. Local shrinkage defects are commonly seen in the casting at the base of the sprue (Fig. 9.2). It is preferable, therefore, that the casting should freeze by a wave of solidification traversing its mass, moving towards the sprue. A reservoir of metal is then present

Fig. 9.2 Diagram illustrating a casting fault occurring at the base of the sprue.

within the sprues if these are of sufficient thickness. One method is to thicken up a section of each sprue as near to the casting as possible. These round sprue reservoirs should freeze last of all and any shrinkage porosity will be found in them, and not in the casting.

Porosity: Porosity may be seen as surface pitting on the casting or may be revealed within the cast metal on finishing and polishing. Broken pieces of investment, or particles of dirt which have fallen down the sprue, may become embedded in the casting and produce pitting of the surface. For this reason all casting moulds should be handled with the sprue downwards.

Gaseous porosity in castings is produced by gases which become dissolved in the molten alloy. Copper, gold, silver, platinum, and particularly palladium, all dissolve oxygen in the molten state. On cooling, the alloys liberate the absorbed gases but some remain trapped when the alloy becomes rigid. This type of porosity may affect all parts of the casting. Its effects can be reduced by avoiding overheating of the alloy or casting in the atmosphere of an inert gas or vacuum.

Undersized or oversized castings: The final fit of a casting depends on a balancing out of expansions and contractions which occur during its construction. The major dimensional changes involved are the casting shrinkage of the alloy which should be compensated for by the setting expansion,

thermal expansion and inversion of the investment. Faults in technique, for example not heating the investment mould to a high enough temperature, may produce insufficient compensation for casting shrinkage.

It should be remembered, however, that other factors such as the choice of impression material

and impression technique may also influence the final result.

9.5 Suggested further reading

Asgar, K. & Arfaei, A.H. (1985) Castability of crown and bridge alloys. J. Prosthet. Dent. 54, 60.

10.1 Introduction

The previous chapters have dealt with casting alloys and casting techniques. Casting, however, is not the only way in which metals can be shaped. An alternative approach is to use cold working, in which the metal is hammered, drawn or bent into shape at temperatures well below the recrystallization temperature of the metal, often at room temperature. Metal or alloy structures produced in this way are said to have a wrought structure and often rely for their special properties on the work hardening which takes place during shaping.

Examples of the use of wrought alloys in dentistry include materials for making instruments and burs, wires, and occasionally, denture bases. Steel and stainless steel are the most widely used wrought alloys and are therefore worthy of some detailed discussion.

10.2 Steel

Steel is an alloy of iron and carbon in which the carbon content is less than 2%. Greater quantities of carbon produce a very brittle alloy which is unsuitable for cold working. In the solid state, steel is able to adopt a variety of structures depending on the carbon content and temperature. Above 723ºC an interstitial solid solution of carbon in a face-centred cubic iron matrix is formed. This solid solution, termed austenite, is unstable below 723ºC (the critical temperature) and the facecentred cubic iron matrix breaks down to form two phases. One phase consists of a very dilute solid solution of carbon in iron (up to 0.02% C), called ferrite. The other phase is a specific compound of iron and carbon with formula Fe3C, called cementite. The mixture of ferrite and cementite is termed pearlite. These transitions are

illustrated in the iron-carbon phase diagram given in Fig. 10.1.

It can be seen that certain aspects of the ironcarbon phase diagram resemble that for a eutectic alloy shown in Fig. 6.10. The critical temperature (Tc) for the iron-carbon system is equivalent to the eutectic temperature which characterised the eutectic alloys. In both cases, the temperature in question indicates the point at which the alloys undergo phase separation. Eutectic refers to the behaviour of an alloy of two mutually insoluble metals during crystallization. In the case of the iron–carbon system the transitions occur within the solid state. An alloy containing 0.8% carbon (corresponding to point X in Fig. 10.1) is known as the eutectoid alloy. Alloys with greater concentrations of carbon are called hypereutectoid alloys and those with smaller carbon contents, hypoeutectoid alloys. Both hypereutectoid and hypoeutectoid alloys consist of a mixture of ferrite and cementite at room temperature. The hypereutectoid alloys contain relatively greater amounts of cementite whilst the hypoeutectoid alloys contain greater amounts of ferrite.

Cementite is a very hard, brittle material whilst ferrite is softer and more ductile. Hence the hypereutectoid steels which contain greater quantities of cementite are commonly used to produce cutting instruments such as burs. The hypoeutectoids are used for the construction of non-cutting instruments such as forceps.

Steel can be further hardened by heat treatment. If an alloy is heated to a temperature above the critical temperature but below the solidus temperature it forms an austenitic solid solution as shown in Fig. 10.1. If the alloy is then quenched there is insufficient time for the alloy to undergo the transition from the austenitic structure to the pearlite structure. Instead, a very hard and brittle