Casting procedure & defects

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Good morningGood morning

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Casting Casting

procedures

procedures

AndAnd

Casting defects

Casting defects

Presented by:

Bhanuchandar. DDept of Prosthodontics

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Contents • Introduction• History• Steps in casting procedure• Sprue formers• Crucible formers• Casting rings and ring liners • Investing procedure• Wax burnout• Casting of alloys into mold• Casting of titanium alloys• Cleaning of casting• Casting defects• Conclusion

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Introduction

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History

• This meticulous procedure of casting was used to produce jewelry and ornaments.

• 3500 B.C. – Egyptians first manufactured refractories which in the form of glass vessels around a refractory core of mud, sand, and animal dung.

• 11th Century Theophilus Described lost wax technique, which was a common practice in jewelry.

• 1558 B. Cellini haveattempted use of wax and clay for preparation of castings.

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• 1897 Phillibrook described a method of casting metal filling

• 1907 – William. H .Taggart introduced the lost wax technique in dentistry.

• 1949 – Moore and Walt developed phosphate bonded investment.

• 1959 - Asgar & Peyton stated that flaring should occur at the sprue/wax pattern junction

• 1959 - Strickland et al stated the importance of the type, shape location & direction other than the size of the sprue

• 1959 - Morrison and Warmick reported the findings of ethyl silicate refractory material for dental use.

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Steps involved in the casting

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Spruing

Sprue former:

A wax, plastic or metal used to form the channel or channels which allows molten metal

to flow into a mold to make casting.

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• Purpose of the sprue former• Requirements of the sprue

former

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Sprue materials:

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Advantages of hollow sprue former:

• It increases the contact area.• It holds less heat than the solid sprue

former.

“Care must be taken to examine the orifice for small particles of investment that may break off while removing the metal sprue former.”

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Sprue former diameter:

• The diameter and length of the sprue former depends on:- The type and size of the pattern.- The type of casting machine to be used.- The dimensions of the casting rings in which casting is made.

• Pre fabricated sprue former are available in a wide variety of gauge from 6 to 18.

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• Usually for molar and metal ceramic restoration - 10-gauge (2.5mm)

Premolars and partial coverage restoration - 12-gauge (2.0mm)

• A narrow sprue may be useful in air pressure casting procedure where the metal is melted in conical depression formed by crucible former. so narrow sprue prevention premature metal flow into mold.

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Reservoir:

• Reservoir is a small amount of additional wax added to the sprue former 1mm below the wax pattern.

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Sprue former length:

• The length of the sprue former - it keeps the wax pattern 6mm from the

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Sprue former position:• The ideal area- point of greatest bulk in the

pattern.

• The point of attachment should permit stream of metal to be directed to all parts of the mold without having to flow opposite the direction of casting force

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Sprue former direction:

Attached 45 degrees to the walls of mold, which decreases the turbulence of molten alloy.

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Attachment morphology:

• The attachment of sprue former to the wax pattern should be smooth and do not posses pits or irregularities.

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Spruing

Direct Indirect

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Vents

• Vents are the additional sprues placed at thin or thick wax patterns to improve the quality of the casting.

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Auxillary sprue

• For large casting an additional auxillary sprue may be placed for filling the mold.

• Usually 14 to 16 gauge sprue are used.

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Crucible formerThey are available as:

Rubber, Metallic and Plastic

They are of two types:1) Steep sided cone- to cast metal

using centrifugal casting force2) Shallow cone- cast metal using

stream or air pressure

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Casting rings

• Casting rings are used to confine the fluid investment around the wax pattern while the investment sets. And …….

• Considerations in selection of casting rings:

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They are available as: 1) Shapes - Round

- Oval 2) Complete rings –

Rigid - Metal - Plastic Flexible - Rubber

3) Split rings - Metal - Plastic

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Ring less casting system:- plastic rings which is conical in shape

with tapering walls are used.- Used for traditional gold-base alloys.

Flexible rings Split casting rings

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Casting ring liner

Materials used are:- Asbestos liner- Cellulose liner- Ceramic liner- Combination of

ceramic and cellulose liner

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Function of casting ring liner:

• Allow uniform expantion.• In case of wet liner

technique- hygroscopic expansion.

• Thickness of the liner should be less than 1mm.

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Investing

.

Investment materials are:

1. Gypsum bonded investment2. Phosphate bonded investment 3. Ethyl silicate bonded investment

The process of covering or enveloping an object such as a denture, tooth, wax form, crown,

with a suitable investment material before processing, or casting

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-Thin film of cleaner on pattern reduces surface tension of wax better “wetting” of wax pattern

by the investment.

-Some of the commercially available debubblizing agents can be used.

•The wax pattern should not stand for more than 20-30 min

before being invested. So, it is best to invest

the wax pattern as soon as possible

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• Investment mixing:1. Hand mixing 2. Vacuum mixing

Bubble free casting with different technique-

17% - open investment95% - vacuum investment

• Advantage of vacuum mixing:

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Setting of investment:

• After mixing the investment is poured in to the casting ring up to its rim.

• For hygroscopic expansion technique • For controlled water added technique• For Thermal expansion technique

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Wax elimination or burn out

• It is advisable to begin the burnout procedure while the mould is still wet.

Water trapped in the pores of the investment reduces the absorption of wax.

As the water vaporizes it flushes wax

from the mold.

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• Purpose of burn out:• Heat application:• Heating of ring should be done

slowly. - Hygroscopic low-heat

technique- High-heat thermal

expansion technique

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Hygroscopic low-heat technique

• The temperature used in this technique is 500°c for 60-90 mins.

• Obtain compensation expansion from three sources:

1. Immersion of investment in 37°c water bath.

2. The warm water entering the investment mold from the top adds some of the expansion.

3. The thermal expansion at 500°c

• This technique causes 0.55% of expansion.

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High-heat thermal expansion technique

Gypsum bonded Investment :

• The investment is slowly heated to 650°c - 700°c in 60mins. Then maintained for 15-30 mins at this temperature.

• Above 700°c sulfur dioxide

- Contaminates gold castings and makes them extremely brittle

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Phosphate bonded investment :

• They need higher 2nd stage temperature for -

- total elimination of wax

- and Prevent premature solidification of higher melting alloys.

• After initial slow raise of temp to 315°c, the temperature is rapidly raised to 750-900°c and maintained for 30 mins.

• The technique cause 1.33-1.58 % of Thermal expansion

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Accelerated casting method (J Prost dent. 66: 155,1991)

• To reduce the total time, Alternative Accelerated casting technique is proposed that uses phosphate bonded investment which sets in 15 mins and then 15 min burn out is done at 815°c.

• This method is used for preparing post and core restorations

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HEAT SOURCES: two basic modes-

• Torch flame - Gas air torch

- Gas oxygen torch

- Oxy acetylene torch

- hydrogen oxygen generator• Electricity

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

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Two type of torch tips:1. Multi-orifice2. Single-orifice

Zones of the blow touch flame:

• Zone 1 - colorless zone • Zone 2 – Combustion

zone• Zone 3 - Reducing

zone • Zone 4 - oxidizing zone

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Two types of flame can be obtained with a casting torch:

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The changes seen in metal during fusing are :

• Initially appear spongy later small globules of fused metal appear later molten metal flows assuming a spherical shape at proper casting temperature the molten alloy is light orange and tend to spin or follow.

• At this stage the temperature of molten alloy is 38°c above its liquidus temperature.

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• During melting of the gold alloys flux may be added-

- Minimizing porosity- To increase fusing of metal- Prevent oxidation

• Commonly used fluxes are fused borax powder ground with boric acid power.

• Charcoal

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Gas air torch: -• To melt conventional noble metal

alloys (used for inlays, crown and bridge) whose melting points less than 1000°c

Gas–oxygen torch: • Used to melt metal ceramic alloys of

higher temperature up to 1200°c.Oxy-acetylene torch : • One volume of acetylene and two and

half volume of oxygen are needed.

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They are:• Oxidizing flame - 6000°F • Carburizing flame- 5400°F • Neutral flame - 5600°F

If distance is reduced to - 7.5 mm slight porosity

5 mm increased porosity due to occluded H2 gas.

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Casting machines1. Air pressure casting

machines:Alloy is melted in situ in crucible hollow of the ring, followed by applied air pressure.

2. Centrifugal casting machine:Alloy is melted in a crucible, and forced in to mold by centrifugal force.

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3. Electrical resistance - heated casting machine :-

• It is used to melt ceramic alloys. Here the alloy is automatically melted in graphite crucible.

• The crucible in the furnace is always against the casting ring. So the metal button remain molten slightly longer and ensures complete solidification.

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Direct-current arc melting machine:-• Produce between two electrodes: the

alloys and the water cooled tungsten electrode.

• > 4000°C – alloy melts very quickly.• High risk of over heating of the alloy.

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4. Induction melting machine:• Metal is melted by an induction field

that developed with in the crucible surrounded by water- cooled metal tubing.

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• The electric induction furnace is a transformer in which an alternating current flows through the primary winding coil and generates a variable magnetic field in the location of the alloy to be melted in a crucible

• It is more commonly used for melting base metal alloys not been used for noble alloy casting as much as other machines

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Casting force:

• Casting force > Surface tension of alloy + Resistance offered by gas in the mold.

• This can be done by use of following different type of force

• Vacuum force • Air or Gas Pressure • Centrifugal force

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Sufficient mass of alloy must be present to sustain adequate casting pressure

• 6g is typically adequate for premolar and anterior casting

•  10g is adequate for molar casting • 12 g is adequate for pontic

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CASING OF TITANIUM ALLOYS

• Titanium offers lot of advantages and can be used for casting.

Problems associated are:• High melting point of titanium of

1671°C (when other dental casting alloys have liquidus temperature below 1500° C).

• Tendency for the molten metal absorbs several gases in molten state.

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• 1977 – Walter start :- casted Ti alloy as crown and bridges and R. P. D. frame work.

• A new pressure / vacuum casting machine was developed. With argon

– arc system for melting alloy. • Molten alloy drawn into the mold by

gravity or vacuum & subjected to additional pressure to force the alloy into the mold

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Casting Crucible

They are of 3 types:• Clay Crucibles• Carbon Crucibles• Quartz Crucibles (zircon-alumina)

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• Traditionally a wet lining of asbestos sheet was used on casting crucible. The moistened asbestos sheet provides a clean and good surface on which the alloy could be melted.

• Advantages is, prevent alloy contamination with oxides and residuals that may be present in the crucible

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Cleaning of the casting

• Consider the gold crown & bridge alloys.

• After casting has been completed, ring is removed & quenched in water.Advantages:

1. Noble metal is left in an annealed condition for burnishing & Polishing.

2. When water contacts hot investment, violent reaction ensues. Investment becomes soft, granular & casting is more easily cleaned.

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PICKLING:

• Surface of the casting appears dark with oxides and tarnish. Such a surface film can be removed by a process called Pickling.

• Best method for pickling is to place a casting in a dish & pour acid over it.

• Heat the acid but don't boil it.

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Hydrochloric acid Sulfuric acid Ultrasonic devices• Gold and palladium based metal

ceramic alloys and base metals, these alloys are not generally pickled.

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• Casting is both an art and science governed by numerous rules, or “laws”.

• Based on earlier work of Ingersoll & Wandling (1986), W. Patrick Naylor formulated an expanded set of 17 separate recommendations for Spruing, investing, burnout, and melting and casting procedures.

• Collectively these guidelines are referred to as the laws of casting.

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Attach the pattern Sprue former to the thickest part of the wax pattern

Orient the wax pattern so all the restoration margins will face the trailing edge when the ring is positioned in the casting machine

Position the wax pattern in a “cold zone” of the investment mold and the reservoir in the “heat centre” of the casting ring

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A reservoir must have sufficient molten alloy to accommodate the shrinkage that occurs within the restorations

Do not cast a button if a connector (runner) bar, or other internal reservoir, is used

Turbulence must be minimized, if not totally eliminated

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Select a casting ring of sufficient length and diameter to accommodate the patterns to be invested

Increase the wettability of the wax patterns

Weigh any bulk investment and measure the investment liquid for a precise powder-liquid ratio

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Eliminate the incorporation of air in the casting investment and remove the ammonia gas by product of phosphate-bonded investments

by mixing under vacuum

Allow the casting investment to set completely before initiating the burnout procedure

Use a wax elimination (burnout) technique that is specific for the type patterns involved (wax versus plastic)

and recommended for the particular type of casting alloy selected

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When torch casting, use the “reducing zone” of the flame to melt the alloy and not the oxidizing zone

Provide enough force to cause the liquid alloy to flow onto the heated mold

Adequate heat must be available to properly melt and cast the alloy

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Cast toward the margins of the wax patterns

Do not quench the ring immediately after casting

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Classification

According to Anusavice• Distortion

• Surface roughness and irregularities

• Porosity

• Incomplete or missing details

Based on location• Internal

• external

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According to Rosensteil

• Roughness

• Nodules

• Fins

• Incompleteness

• Voids or porosity

• Marginal discrepancy

• Dimensional inaccuracies

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Distortion • Distortion of the casting probably related to

distortion of the wax pattern.Causes: • Can occur from the time of pattern preparation

to the time of investing due to stress relaxation.

• Distortion of the wax pattern occurs during the investment procedure.

Minimized by:

Application of minimum pressure Manipulation of wax at high temperature Investing pattern immediately If storage is necessary, store in refrigerator

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Surface Roughness and Irregularities

surface roughness Defined as relatively finely spaced surface imperfections whose height, width and direction establish the predominant surface pattern.

Surface irregularities Isolated imperfections such as nodules that are not characteristic of the entire surface area

• The surface roughness of the casting is > wax pattern - the particle size of the investment and its ability to reproduce the pattern in microscopic detail

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Air bubbles:• Small nodules on the casting

Prevented By:– Proper investment technique– Vibration of mix or by vacuum mixing – Application of wetting agent properly and

correctly – important that it be applied in a thin layer.

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Water films:• If the Investment becomes separated from the

wax pattern, a water film may form irregularly over the surface.

• Appears as minute ridges or veins on the surface.

Prevented By: 1.Use of wetting agent2.Correct L/P ratio (Too high L/P ratio may

produce these irregularities)

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Rapid Heating RatesCauses:

• Fins or spines

• Flaking of the investment

Prevented by:

• Heat gradually at least 60min from room temperature to 700°c.

• Greater the bulk – more slowly heated.

Under heating• Incomplete elimination of wax residues.

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Prolonged heating• Decomposition of the investment

Liquid/Powder Ratio• The amount of water and powder measure

should be accurate.

Casting pressure• To high pressure – rough surface of the casting

• To low pressure – incomplete casting

• Average – 0.01 to 0.14 Mpa and 3 to 4 turns of the spring.

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Foreign bodies • Any casting that shows sharp, well- defined

deficiencies indicates the presence of some foreign particles in the mold. They may be:

- Pieces of the investment - Bits of the carbon from the flux- Sulfur components from –

decomposition of the gypsum investment and high sulfur content torch flame.

Pattern position• Should not place too close together• Should not place many patterns in same planeSpace between the pattern is atleast 3mm

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Impact of metal alloyCause:

• The direct impact of molten alloy may fracture or abrade the mold surface regardless of its bulk. It results ……..

Prevented by:

• This type of surface roughness or irregularities can be avoided by proper spruing.

Carbon inclusions• Carbon from- carbon crucible, carbon

containing investment – absorbed by the alloys during casting results in formation of carbides or visible carbon inclusion.

Other causes

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porosity

Classified as follows:

I. Solidification defectsA. localized shrinkage porosityB. Micro porosity

II. Trapped gases A. pin hole porosity B. gas inclusion porosity C. sub surface porosity

III. Residual porosity

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Localized shrinkage porosityCause:

• By premature termination of the molten metal during solidification.

• Porosity in the pontic is caused by- retain heat because of its bulk and located in the center of the rings.

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Suck back porosity – Interior of the crown near the area of the sprue create a hot spot on the mold wall

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Pin hole and Gas inclusion porosity

• Characterized by spherical contour, but gas inclusion porosities are much larger than pin hole porosity.

• Occur primarily because most metals dissolve gases when molten these gases expelled during solidification..

• Also be caused by gas occluded from a poorly adjusted torch flame or use of oxidizing zone rather than reducing zone.

• Casting is usually black, do not clean easily on pickling

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Sub surface porosity• Caused by simultaneous nucleation of solid

grains and gas bubbles at the first moment that the alloy freezes at mold walls

• Prevented by controlling the rate at which the molten metal enters the mold.

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Back pressure porosity• Some times referred to as entrapped-air

porosity.

• The entrapment is frequently found in a “POCKET” at the cavity surface of a crown or MOD casting.

• Also found on the outer surface of the casting when the casting or mold temperature is low, that solidification occurs before the trapped air can escape.

• Thickness of the investment• Incomplete elimination of wax

residues.

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Prevented by:• Proper burnout

• Sufficiently high casting pressure

• Investment of adequate porosity

• Adequate L/P ratio

• Adequate mold and casting temperature.

• Thickness of investment between tip of pattern and end of ring is not greater than 6mm.

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In complete casting• Factors that inhibit the mold filling is:

1. In sufficient venting

2. In sufficient casting pressure

3. Incomplete elimination of wax

4. Lower L/p ratio

5. Viscosity of the fused metal

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Viscosity of the fused metal:• In complete casting resulting from too greater

viscosity is due to insufficient heating of the alloy.

• However, both the surface tension and the viscosity of the molten alloys are reduced by increased in temperature higher than its liquidus temperature

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Reference • K. J. Anusavice, Phillips Science of Dental Materials, 11th

edition

• Introduction to metal ceramic technology, W.P. Naylor

• Craig R.G, Restorative Dental Materials;10th edition

• Rudd and morrow, Dental laboratory procedures- removable partial denture, 2nd edition

• Rudd and morrow, Dental laboratory procedure- fixed partial denture,

• John j. manappallil, basic dental materials, 2nd edition

• Rosensteil, Contemporary fixed Prosthodontics

• Shillinburg, fundamentals of fixed Prosthodontics

• JPD- 1978; 3, 137-14

• internet

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