Casting procedures &

defects

Aisha HabeebSVS Institute of Dental Sciences

ContentsIntroductionHistorySteps in casting procedureSprue formersCrucible formersCasting rings and ring liners Investing procedure

Wax burnoutCasting of alloys into moldCasting of titanium alloysCleaning of castingCasting defectsConclusion

Introduction

ACCORDING TO GPT (7th edition): Casting is defined as something that has been cast in a mold, an object formed by the solidification

of a fluid that has been poured or injected into a mold

ACCORDING TO CRAIG (13th Edition): Casting is the process by which a wax pattern of a restoration is converted to a

replicate in a dental alloy

ACCORDING TO WILLIAM J. OBRIEN (3rd edition): Casting is a process of forming objects by pouring molten metals in molds that

are cooled to cause solidification.

Copper was cast in Mesopotamia in about

3000 B.C

lost wax moulding process was first developed.

Benevento Cellini (1500-1571) in his autobiography described his method of casting in both gold and bronze by coating his finely detailed wax model

with a reinforced refractory shell

The oldest dental castings were gold inlays

W.H. Taggart

1907

described a casting process which launched the cast restoration as it is known today.

direct wax pattern of the lost tooth structure

His process was

sprue

Plaster of Paris 37.5%silica 57.5%

graphite 5%. Invest

To remove

wax

cast gold into space formed using air pressure

setting expansion - 0.45% hygroscopic expansion -

1% thermal expansion- 0.77%

Investment

Taggart was unable to overcome the problem of casting shrinkage which resulted in restorations which were under size.

Lane believed that all castings made by Taggart method were undersize and attributed this to the shrinkage of gold.

Van Horn introduced a different method of compensation recommending that the wax pattern be invested at a temperature equal to mouth

temperature

the idea of casting into an enlarged mould which he achieved by using an investment containing high percentage of silica (75% approx) plus plaster of

Paris, heated to about 650ºC.

Lane

1910

High silica content

investment

warm water (46ºC)

sealed inlay ring in a water bath held at 43ºC until the investment had set

The low heat casting technique was finally abandoned in 1928 when Coleman published his research paper No.32 for the United States National Bureau of Standards in which he demonstrated the great shrinkage which occurred when investment is cooled after heating.

Weinstein in 1929 found that adding boric acid to customary investment mixtures prevented the shrinkage which occurred when the investments were heated to about 370ºC and with the addition of this chemical quartz based investments expanded approximately 0.9% when heated to 700ºC.

Moore(1993) discovered by the addition of chlorides to investments he could obtain a thermal expansion of as much as 1.1% from quartz based investment.

Two important developments occurred in the next few years were

use of Cristobalite in investment

Hygroscopic setting expansion technique

Sweeney, Paftenbarger (1930-33)

use of a “Control powder” mixed with a cristobalite investment in varying proportions designed to give different thermal expansion to the investment.

Phillips (1935)

In 1932 Scheu presented a end technique which used the setting expansion of investment to compensate totally for the gold alloy shrinkage.

Dr. Wilmer souder recognized that the thermal expansion of the investment was greatly inhibited by the rigid metal casting ring and advocated lining the ring with soft asbestos to act as a heat resisting cushion which would permit the investment to

undergo its full thermal expansion.

Lost wax technique

Dimensional changes in the Lost wax Technique

Can be summarized by the equation

Wax shrinkage + metal shrinkage = wax expansion + setting expansion + Hygroscopic expansion + Thermal expansion

Goldberg (1937) recommended the use of the wax with 0.1% Contraction for the direct technique and the wax with 0.38% contraction for the a patterns

made on a die at room temperature in the indirect technique

Casting shrinkage

The shrinkage occurs in 3 stages:

• The thermal contraction of the liquid metal between the temperature to which it is heated and the liquidus temperature.

• The contraction of the metal inherent in its change from the liquid to the solid state.

• The thermal contraction of the solid metal that occurs on further cooling to room temperature.

Coleman used the expression ‘casting shrinkage’ to include the contraction which occurred during all the three stages already described and employed the term ‘net casting shrinkage’ to describe the casting contraction measured in his experiments.

Earnshaw (1957) suggested the following nomenclature to give a more precise meaning for the term “casting shrinkage”.

Inherent casting shrinkage

Actual casting shrinkage

Net casting shrinkage

Effect of alloy composition on casting shrinkage: varies with alloy composition, the variation amongst gold alloys normally used for ordinary cast inlays, crowns and bridges is not large enough to warrant changes in investment expansion (Morey 1991).

Effect of shape of the casting on casting shrinkage

Interferences could have a greater effect in castings of more complex shapes, especially if those shapes allow interlocking of the solidified alloy and the mould.

Size and shape of the casting effect the linear casting shrinkage (Paffenbarger).

An investment with high compressive strength at the casting temperature will be more effective than a weaker one in restricting casting shrinkage of a complex casting

and the effect will be greatest when interlocking of casting and mould in maximal (Earnshaw, R) 1969.

least in buccolingual and occluso-gingival directions

MOD inlay casting

greatest in the mesiodistal direction

Restraint imposed by the walls of the mould

Effect of alloy shrinkage on the fit of MOD inlay casting

masked by the marginal bevel

For a given alloy the inherent casting shrinkage is determined by its coefficient of thermal expansion and its solidus temperature, the actual or observed casting shrinkage will be lower by a varying amount depending on the size and shape of the casting and the compressive strengths of the investment mould at its burnout temperature (Moorey 1991)

Linear solidification shrinkage of casting alloys

Type I (Au-based) 1.56% Type II (Au-Based) 1.37% Type III (Au-based 1.42%

Type IV (Ni-Cr based) 2.30% Type IV (Co-Cr) 2.30%

Theory of compensation

Compensation for the shrinkage of wax and gold by investment expansion was studied scientifically in the early

1920s when Weinstein and Coleman at the United States National Bureau of Standards.

Contraction occurs both during cooling of the liquid alloy and during solidification.

Price, Coleman, Sonder

Setting expansion occurs as a result of normal crystal growth

Hygroscopic expansion

Effect of Added water

Effect of confinement

The following factors tend to increase hygroscopic expansion: Increased silica content of the investment & hemihydrate as the binder. Increased setting expansion of the binder Thicker mix of investment Increasing mixing time (spatulation) Immersion of the investment at or before its initial set length of time it is

immersed. Temperature of water bath Lining the ring with asbestos liner Using a split ring or one made of flexible rubber Storage at 100% humidity Hygroscopic technique with the pattern in the upper apart of the ring.

Wax pattern expansion

while the investment is still fluid occurs when the wax is warmed above the temperature at which it was formed

Heat may come from

chemical reaction of the investment warm water bath

Sprue Direction

Should be directed away from thin or delicate parts of the pattern, because the molten

metal may abrade or fracture investment in this area and resulting in casting failure.

It should be directed away from or at 45º to these details.

Length of the sprue

(1/4”) 6 mm

3.25 mm (1/8”)

Major factor governing the sprue length is the length of the ring

Number of Sprues

Patterns may be sprued

directly or indirectly

Reservoir

Wax is added around the sprue former 1-2 mm from the pattern in order to create an area in the mold (a reservoir) with dimensions far exceeding that of the thickest portion of the pattern.

Crucible former

It is cone shaped

Casting ring

Holds the investment in place during setting and restricts the expansion of mold with the

use of solid metal rings

To overcome this

Split ring flexible rubber ring ceramic paper liner

• Stainless steel has been found to produce the most acceptable rings.

• The thermal expansion of SS is 1.20% at 700ºC which is compatible with the expansion of investments provided a liner is used (Ray 1933).

29mm

38mm

Ringless casting system

• This method uses paper or plastic casting ring and is designed to allow unrestricted expansion (Engelman 1989)

• Also called as power cast ringless system consisting of three sizes of rings and formers, preformed wax sprues and shapes, investment powder, and a special investment liquid.

• The crucible former and plastic ring are removed before wax elimination, leaving the invested wax pattern.

to provide a buffer of pliable material against which the investment can expand to enlarge the mould

Ring liners

Need

Liner

semi hygroscopic expansion of the investment

Asbestos liner

• no longer be used because its carcinogenic potential

can cause• asbestosis• bronchogenic lung cancer• mesothelioma

The currently accepted threshold limit value for asbestos fibers range from 2 x 105 – 20×105 fibres/m3

Material of liner

manufactured by standard paper making technique from fibers of

alumino silicate glass derived from Kaolin

Cellulose fibers readily absorb water when immersed and

therefore like asbestos wetted before use

absorbent cellulose

non absorbent ceramic

Alternatives to asbestos liners have been introduced

How to use liner

can be used dry or wet

The maximum thickness of liner is 1 mm

Preparing the wax pattern for investment

wax pattern should be cleaned of any debris, grease or oils

thin films of cleanser left on the pattern reduces the surface tension of the wax

The distortion of the wax pattern after its removal from the die is a function of the temperature and time interval before investing.

The nearer the room temperature approaches the softening point of the wax

the more readily internal stresses are released

a pattern should not stand for more than 20 to 30 minutes before being

invested

Investment Techniques

Hand investing

Double investing

Vacuum investing

INVESTMENT MATERIALS

A heat resistant or a refractory material used to form a mold into which a metal or alloy is cast . (OR) A molding material that surrounds the pattern & subsequently hardens & forms the mold after the wax pattern is eliminated.

The operation of forming the mold is called Investing .

Types – Gypsum bonded investments

Phosphate bonded investments

Ethyl silicate bonded investments

Casting procedure

Once the investment has set for approximately 1 hour for most gypsum and phosphate bonded investments it is ready for burnout

The crucible former and any metal sprue former are carefully removed

If the burnout procedure does not immediately follow the investing procedure, the invested ring is placed in a humidor at 100% humidity

Dehydration of set investment that has been stored for an extended period in 100% humidity may not replenish

all of the lost water

Casting Machines

The alloys are melted

separate crucible by a torch flame and cast into mold by centrifugal force

electrically by resistance heating or induction furnace then cast into the mold by motor or spring action

vaccum arc melted and cast by pressure in an argon atmosphere

induction heating then cast into the mold centrifugally by motor or spring action

All casting machines accelerate the molten metal into the mold either by centrifugal force or air pressure

Centrifugal machines

Each of these machines depends on the centrifugal force applied to the molten metal to cause it to completely fill the mold with properly melted metal

Advantages

• Simplicity of design and operation• the opportunity to cast both large and small castings on the same machine

Operative Dentistry Modern theory and practice-Marzouk

Dental materials – Philiphs – Anusavice

Restorative Dental materials – Craig

Fixed prosthodontics – Rosenstiel

Fundamentals of fixed prosthodontics – Schillinburg.

REFERENCES

Dental Materials – Ferra caneDental Materials –AndersonAustralian Dental Journal 1991 36 (5): 391-61992 37(1) 93-541991 36(4): 302-91992 37 (2): 91-7Notes on Dental Materials – E.C. CoombeBritish Dental Journal 1972: 428-435.

CASTING DEFECTS

To be continued in PART II