Casting procedures

CONTENTS
 INTRODUCTION
 PREPARATION OF MASTER CAST AND DIE
 WAX PATTERN
 SPRUE DESIGN
 CASTING RI...
Casting procedures

INTRODUCTION
According to GPT 8th edition,
Casting is defined as something that has been cast in a mol...
Casting procedures

According to ADA specify No. 25
Advantages –
1) Easy to use
2) Easily available
3) Compatible with mos...
Casting procedures

Materials used –
 Model paint
 Colored nail polish
 Thermoplastic polymers dissolved in volatile so...
Casting procedures

-

It forms the outline of the mold into which molten metal is poured.

-

The casting can be no more ...
Casting procedures

-

Should not be larger than that of pattern, because it could cause
distortion.

-

Should not be sma...
Casting procedures

-

It is best for molten alloy to flow from thick areas to thin areas (e.g.
margins) this minimizes th...
Casting procedures

5) Sprue direction
Should be directed away from thin or delicate areas because molten
alloy may abrade...
Casting procedures

Procedure –
-

Liner should be cut to cover the inside diameter of the walls of ring
without overlap.
...
Casting procedures

4) Adequate strength to resist casting forces
5) Sufficient porosity to allow gas escape.
6) Easy reco...
Casting procedures

3) Silica bonded and investment material
Advantages – Withstands high temperature
Indications – Titani...
Casting procedures

Investing –
Before the actual investing –
-

The wax pattern is cleaned with commercial wax pattern cl...
Casting procedures

c. Additives such as boric acid or sodium chloride to eliminate
shrinkage of set investment because of...
Casting procedures

a) Preferred material is cristoballite or quartz (75-90%).
Binder

is

mono

ammonium

phosphate

(7-1...
Casting procedures

Indications –
Casting titanium alloys for cast partial restorations.
Factors responsible for Compensat...
Casting procedures

6) In case of thermal expansion, increase in burnout temperature, increases
the expansion and vice ver...
Casting procedures

-

Investment powder

-

Water or, colloidal silica

-

Spatula

-

Brush

-

Casting ring and liner

...
Casting procedures

-

Ideally should be kept in oven when mold is still little wet as water
trapped prevents absorption o...
Casting procedures

Laws of casting:
Ingersoll & Walding (1986) formulated an expanded set of 17 separate
recommendations ...
Casting procedures

1st Law of Casting
 Attach the pattern sprue former to the thickest portion of the wax
pattern:
• Thi...
Casting procedures

4th Law of Casting
 A reservoir must have sufficient molten alloy to accommodate the
shrinkage occurr...
Casting procedures

• A button can draw available molten alloy from the bar, shift the
heat centre and reduce the feed of ...
Casting procedures

• Wetting agent should be brushed or stained on the patterns and
dried before investing
• Too much wet...
Casting procedures

11th law of casting
 Allow the casting investment to set completely before initiating the
burn out pr...
Casting procedures

• Penalty- cold shuts, short margins and cold welds (too little heat)
or rough castings and investment...
Casting procedures

• Penalty- cold shuts, short margins and otherwise incomplete
castings
17th law of casting
 Do not qu...
Casting procedures

2) Casting machines
Alloys are melted in one of the following ways, depending upon the
casting machine...
Casting procedures

The other method for melting most base metal alloys like Ni-Cr, cobalt
chromium, cobalt nickel chromiu...
Casting procedures

-

An asbestos paper base is used in the crucible to direct the molten metal
into mold.

-

When a tor...
Casting procedures

For gold crown and bridges –
Flux can be used. A flux is a substance, which is used to increase the
fl...
Casting procedures

-

Create more pressure on metal in mold
Typically 6gms(4dwt) is adequate for premolar and anterior ca...
Casting procedures

o If overheated, produces changes in composition and excessive
oxidation, which can interfere with met...
Casting procedures

4) Vacuum or, pressure assisted casting machine.
Here, molten alloy is heated to casting temperature, ...
Casting procedures

-

This investment is removed by carving and breaking it off the casting.

-

Residue is eliminated wi...
Casting procedures

3) Contraction occurs when the metal cools to room temperature.
The casting shrinkage differ for vario...
Casting procedures

(“freezing”) or mould, insufficient casting force, not enough metal,
or metal spillage.
4. Voids or po...
Casting procedures

Some commonly used abrasives are diamond, silicone carbide, emery,
aluminium oxide, garnet, sand, Trip...
Casting procedures

finishing bur to smooth out the occlusal grooves, cusp ridges are finished with
a small rubber disc.
F...
Casting procedures

4. Dental laboratory procedure- rudd and marrow
5. Dental materials and their selection-willian .j.o’ ...
Upcoming SlideShare
Loading in...5
×

Casting procedures /certified fixed orthodontic courses by Indian dental academy

1,006

Published on

The Indian Dental Academy is the Leader in continuing dental education , training dentists in all aspects of dentistry and offering a wide range of dental certified courses in different formats.


Indian dental academy provides dental crown & Bridge,rotary endodontics,fixed orthodontics,
Dental implants courses.for details pls visit www.indiandentalacademy.com ,or call
0091-9248678078

1 Comment
1 Like
Statistics
Notes
No Downloads
Views
Total Views
1,006
On Slideshare
0
From Embeds
0
Number of Embeds
1
Actions
Shares
0
Downloads
32
Comments
1
Likes
1
Embeds 0
No embeds

No notes for slide

Casting procedures /certified fixed orthodontic courses by Indian dental academy

  1. 1. Casting procedures CONTENTS  INTRODUCTION  PREPARATION OF MASTER CAST AND DIE  WAX PATTERN  SPRUE DESIGN  CASTING RING LINER  INVESTING PROCEDURE  LAWS OF CASTING  CASTING PROCEDURE  CLEANING THE CASTING  CASTING DEFECTS AND CAUSES  SUMMARY  REFERENCES Page 1
  2. 2. Casting procedures INTRODUCTION According to GPT 8th 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. The lost wax casting technique was first described at the end of 19th century as a means of making dental castings. The process consists of surrounding the wax pattern into a mold made up of heat resistant investment material, eliminating the wax by heating and then pouring the molten metal into the mold through a channel called sprue. In dentistry, the resultant casting should produce an accurate replica of wax pattern with surface details and accurate dimensions. Small variations in the investing or casting can produce significant defects in final restoration. Successful casting depends on accuracy and consistency of technique. We are going to study it in detail to know the exact influence of each variable in the technique and to make rational changes to modify the technique according to need. PREPARATION OF MASTER DIE: Most commonly used materials –  Certified type IV (dental stone with high strength) with expansion of 0.1%  Certified type V dental stone with high strength high expansion of 0.3% Page 2
  3. 3. Casting procedures According to ADA specify No. 25 Advantages – 1) Easy to use 2) Easily available 3) Compatible with most of the impression materials. This expansion helps in compensation of relatively larger solidification shrinkage of metal alloys. Methods of altering die dimensions – - Addition of accelerator (Potassium sulphate) and/or retarder (borate) can be added to type IV stone to decrease expansion below 0.7% to avoid increase in diameter of die. Die hardner –  To increase resistance to abrasion and thickness. Die spacers  To produce relief space for cement  Films are coated upto 20mm of thickness  Applied within 0.5mm of finish line Page 3
  4. 4. Casting procedures Materials used –  Model paint  Colored nail polish  Thermoplastic polymers dissolved in volatile solvents. Die lubricants – To avoid sticking of wax pattern to die. Indications – Easy removal of casting from die and thickness Other die materials like – Epoxy resin, electroplated dies (silver, copper etc). Flexible dies can also be used. WAX PATTERN Inlay wax it is a specialized dental wax which is applied in the die surface for preparation of direct and indirect patterns in lost wax technique to fabricate metal castings and heat pressed ceramics. The technique used to prepare pattern on dies is called indirect wax technique. According to ADA Specification No. 4 dental inlay casting wax used for wax pattern on die is soft inlay wax. When the pattern is prepared – - It should be accurate reproduction of missing tooth structure. Page 4
  5. 5. Casting procedures - It forms the outline of the mold into which molten metal is poured. - The casting can be no more accurate than the wax pattern, regardless of the care taken in subsequent casting procedures. - Therefore the pattern should be properly adapted to the prepared tooth surface, properly carved and with minimal distortion. - Before the wax pattern is prepared, the die surface should be painted with separating medium e.g., dilubricators for easy removal. - Then the prepared wax pattern is ready for investing. This procedure is called investing the pattern. SPRUE DESIGN – Purpose of sprue former The purpose of sprue former is to provide a channel through which the molten alloy can be poured when the wax is removed from the investment. The sprue former should be attached to the pattern on the master die provided the pattern can be removed from the die in line with its path of placement with a small heated instrument. Any instruments tip movement that will distort the pattern should be avoided. Principles for optimal performance – 1) Sprue diameter – - Should be approximately of same diameter as that of thickest portion of wax pattern. Page 5
  6. 6. Casting procedures - Should not be larger than that of pattern, because it could cause distortion. - Should not be smaller than that of pattern, because it will solidify first forming a not spot resulting in localized shrinkage porosity or, “Suckback” porosity. - Reservoir sprues can be used to overcome these problems. Avulsed 2.5mm (10 gauge) sprue for molar and metal ceramic patterns – 2mm (12 gauge) sprue for premolars and inlay or onlay restorations. 2) Sprue position with plastic sprues - Matter of individual judgment. - Some prefer occlusal surface or, proximal areas or, first below the nonfunctional cusps to avoid grinding of occlusal surface and contact points. - Ideal area is area of greatest thickness of wax pattern to avoid distortion of thinnest areas of wax pattern during attachment and to allow easy flow of alloy into mold cavity. 3) Sprue attachment – Flare – - Generally flared for high density gold alloys, but often used for low density alloys. - Acts as reservoir and facilitates entry of fluid alloy into pattern area. - If possible, sprue former should be attached to the area of pattern with largest cross-sectional area. Page 6
  7. 7. Casting procedures - It is best for molten alloy to flow from thick areas to thin areas (e.g. margins) this minimizes the distortion. 4) sprue length – Should be long enough to position the pattern properly from cutting ring within 3-6 mm of trailing end to provide pathway for escape and short enough so that molten alloy does not solidify before it fills the mold. Whenever plastic sprues are used to wax sprues it is advisable to used two-stage wax burnout technique as it ensures complete carbon removal, because plastic gets softened above the temp of that of melting point of inlay waxes. It is used in ‘gate’ technique for long span multiple techniques thickness – 6 gauge or larger. Method of spruing – Pattern can be sprued directly or, indirectly. In direct method sprue provides direct passage between pattern and crucible former. Indication: single crown units. In indirect technique – there is a reservoir or connector bar between the pattern and crucible former. Indication – multiple crown units or FPDS. A reservoir should be added to a sprue network to prevent localized shrinkage porosity, when the molten alloy fills the heated casting ring, the pattern area should solidify first and the reservoir last, because of its large mass of alloy and position in heat center or ring, the reservoir remains molten to flow liquid alloy into mold as it solidities venting. A thin auxiliary sprue also can be used, which helps gases to escape and ensure casting solidification in critical areas. This procedure is called venting. Page 7
  8. 8. Casting procedures 5) Sprue direction Should be directed away from thin or delicate areas because molten alloy may abrade or fracture this area and there could be casting failure. Correct direction – 45° to the thickest areas of proximal surface. Incorrect position – - 90° to the flat surface or, occlusal surface. - Causes turbulence and localized porosity. - Poor mold filling since fluid alloys is not reached to cusp tips by centrifugal action. CASTING RING LINERS Materials used – 1) Asbestos – But can no longer used because of its carcinogenic effects which cause biohazards. 2) Cellulose (paper) liner 3) Alluminosilicate ceramic liner Other methods like using split ring or flexible rubber ring are in use. Purpose – - To allow the setting expansion of investment and to avoid the mold to became smaller because of reverse pressure. Page 8
  9. 9. Casting procedures Procedure – - Liner should be cut to cover the inside diameter of the walls of ring without overlap. - Positioned with sticky wax and then used dry or wet Wet liner technique – - Lined ring should be immersed in water for sometime - Excess water in shaken off. - Spruing of liner should be avoided to prevent a considerable amount of water removal and hence allow for expansion. - Wet liner provides semi hygroscopic expansion as this water is absorbed during setting. - Approximately 1mm thick. - 3.5mm short of the ring ends to produce more uniform expansion. INVESTING PROCEDURE: 1. Selection of investment material The ideal investment should have following properties – 1) Controllable expansion to compensate shrinkage of casting alloys during cooling. 2) Ability to produce smooth surface details on casting without nodules. 3) Chemical stability during high casting temperature. Page 9
  10. 10. Casting procedures 4) Adequate strength to resist casting forces 5) Sufficient porosity to allow gas escape. 6) Easy recovery of casting. Classification by binder material – 1) Gypsum bonded investment materials - Satisfy most of the requirements of ideal material. - Not suitable for casting of metal ceramic alloys because it is unstable at high temperature (not more than 650 to 700°C) - During casting of complete crowns, obtaining additional setting expansion may be difficult. Indications – Gold alloys (Type II, III and IV) fuses at 700°C 2) Phosphate bonded investment material – Advantages – - More stable at high (around 1200°C) temperature. So material of choice for casting metal ceramic alloys as they fuse around 1200°C. - Expands rapidly. Disadvantages – Short working time than gypsum bonded. Indications – Cobalt chromium alloys for crowns and copings. Page 10
  11. 11. Casting procedures 3) Silica bonded and investment material Advantages – Withstands high temperature Indications – Titanium alloys for cast partial castings. Factors that increase expansion in: 1. Gypsum bonded investment material - Prolonged spatulation (so, vacuum mixing is preferred) - Storage at 100% humidity - Lower w/p ratio - Use of dry liner - Use of two liners 2. Use of full width ring liner Hygroscopic technique with pattern in upper part of ring Phosphate bonded - Heat from setting reaction softens wax allows free set expansion. - Increased strength of material at high temperature restricts shrinkage of alloy as it cools. - Powder mixed with colloidal silica decrease surface roughness of casting and increase expansion. Page 11
  12. 12. Casting procedures Investing – Before the actual investing – - The wax pattern is cleaned with commercial wax pattern cleaner or, synthetic detergent to remove oily layer of die lubricant. - Acts as surface tension reducer and - Aids in complete wetting of pattern and covering all the areas. - Then it is air dried. Investing – - While the wax pattern cleaner is air dried, depending upon the binder used appropriate amount of distilled water or colloidal silica special liquid(13mm) and powder are mixed. - Liquid is dispensed in a clean dry bowl and powder (60gm) is gradually added - Mixing is performed gently with spatula until powder is fully wet. Properties of Gypsum investments and phosphate bonded investment: Gypsum investments – 1) Composition a. Binder is gypsum and refractory material is silica (cristobaccile or quartz) b. Liquid used is distilled water Page 12
  13. 13. Casting procedures c. Additives such as boric acid or sodium chloride to eliminate shrinkage of set investment because of dehydration on heating. 2) Low heat investment i.e., stable at lower temperature (can be heated to 700°C) (If heated to a higher temperature, binder breaks down and releases sulphur dioxide which may contaminate alloys). 3) Three types of expansions are associated – a. Setting expansion caused by normal crystal growth and recrystallization (0.3 to 0.4%0 b. Hygroscopic expansion (1.2 to 1.4%) caused by water being added to investment c. Thermal expansion (1-1.2%) when silica is subjected to high temperature (Cristoballite has high thermal expansion than quartz) (Thicker the mix, more is thermal expansion). 4) Finer the particle size of binder, smoother will be surface texture of mold. 5) Strength characteristics depend upon type of gypsum used and initial water powder ratio. 6) Indication for Inlay gold (Type II, III, IV) Phosphate bonded investment – 1) According to Aan and Asgar (1966) and Mabie (1973) Page 13
  14. 14. Casting procedures a) Preferred material is cristoballite or quartz (75-90%). Binder is mono ammonium phosphate (7-10%) magnesium oxide (3-5%). b) Liquid used is colloidal silica (2%) with water, which gives high strength to the mass. c) Additives such as carbon, to reduce oxidation of alloys and other ionic salts. 2) High heat investment i.e., stable at high (can be heated upto 1425°C) fusion of investment components, there by sealing escape routs for gases released during passing of molten alloy. May cause backpressure porosity. 3) Thermal expansion – it is during wax burnout. 4) Same 5) Indications a. Precious (High noble) alloys b. Non precious (base metal) alloys for ceramics c. Semiprecious (low noble) alloys metal rests. Silica bonded investment material are also available, which are used in casting titanium alloys. Another technique is divestment where the wax pattern along with die is invested. Page 14
  15. 15. Casting procedures Indications – Casting titanium alloys for cast partial restorations. Factors responsible for Compensation for shrinkage – Occasionally it is important to alter the size of the mold depending upon the size of casting. Factors influencing mold size – 1) Two liners allow greater setting and thermal expansion than one liner. 2) Varying l/p ratio – lower l/p ratio – greater expansion; greater l/p ratio – lower expansion. But if the mixture is too thick it may distort the wax pattern during application and there are chances of air entrapment during investing. If it is too thin, it may cause rough casting surface. 3) In case of cristoballite investment, too much of expansion may take place (Setting expansion of 0.3 to 0.4% plus thermal expansion of 1.4%, total 1.7%). This could cause thick castings. 4) In case of hygroscopic expansion in gypsum bonded investments, greater the delay in investing the filled ring in water bath, lesser will be expansion. 5) Increase in temperature of water bath increase the expansion and vice versa. Page 15
  16. 16. Casting procedures 6) In case of thermal expansion, increase in burnout temperature, increases the expansion and vice versa. 7) Controlled water added techniques – here in a flexible rubber ring, pattern is invested, allowed to set specified amount of water is added on the surface of set investment as room temperature depending upon the expansion required. Investing – - Vacuum mixing of investment material is highly recommended for 1545 sec according to manufacturers instructions. - Recommended w/p 60gms of powder and 13ml of liquid and first the mixture is hand mixed to make it thoroughly wet. - Consistent results in casting with minimal surface defects. - In case of phosphate bonded investment, the mixture is held under vacuum for 14 to 35 seconds for release of ammonia. - Satisfactory results are possible with brush application of the investments or, when the investment is poured into ring under vacuum. - The ring is slightly tilted to allow the trapped air bubbles to appear on surface. Armamentarium is – - Vacuum mixer and bowl. - Vibrator Page 16
  17. 17. Casting procedures - Investment powder - Water or, colloidal silica - Spatula - Brush - Casting ring and liner After the ring is filled to the rim allow it to set for 1 hour. In cast of gypsum investments – If the hygroscopic technique is used the filled ring is placed in a 37°C water bath for 1 hour with crucible former side down. The longer the delay in keeping the invested ring in water bath, lesser will be the expansion. For thermal expansion, the invested ring is allowed to bench set. Investment rings should be placed in humidor to prevent drying if the pattern is not going to be burned out for several hours or, overnight. Plastic vials serve as well as humidors when damp piece of paper towel is placed at the bottom. Wax burnout procedure (lost wax process) - Introduced by Taggart in 1907. - Elimination of wax pattern from the mold of set investment material is referred as burnout. - Should not be started until investment is set for minimum 1 hour, if preheated causes crack in investment. Page 17
  18. 18. Casting procedures - Ideally should be kept in oven when mold is still little wet as water trapped prevents absorption of wax and when water vaporizes it flushes out the wax. - If the burnout procedure is delayed for several hours or overnight, it should be kept in humidor to avoid excessive drying as molten wax during burnout may get absorbed by dry investment. - If the rubber crucible former is used, it is removed to facilitate a rapid and clean burnout; the ring can be placed on a raised object within the oven, allowing free release of molten wax and gases. For gypsum bonded investment – - The mold should be placed in an oven pre heated to 480°C and held for 20 minute for low heat hygroscopic expansion then temperature is raised to 700°C slowly for high thermal and held for 30minutes. It can be kept in oven at room temperature and then furnace temperature increased slowly to 700°C. - If rapidly heated, it causes cracks in investment. - And if overheated, investment will break down and cause release of sulphate which can contaminate molten alloy. For phosphate bonded investment – From room temperature to maximum 200 to 1030° C, depending upon alloy used for 30 minutes. Once the burnout is completed, casting should be completed soon after; otherwise significant variation from desired casting dimension would occur. Page 18
  19. 19. Casting procedures Laws of casting: Ingersoll & Walding (1986) formulated an expanded set of 17 separate recommendations for spruing, investing, burnout, melting & casting procedures. Collectively these guidelines are referred to as “the laws of casting”. Page 19
  20. 20. Casting procedures 1st Law of Casting  Attach the pattern sprue former to the thickest portion of the wax pattern: • This provides the molten metal to flow from larger diameter to thinner sections • Penalty: cold shuts, short margins and incomplete casting 2 nd Law of Casting  Orient wax patterns so all the restoration margins will face the trailing edge when the ring is positioned in the casting machine: • Add a wax dot to the crucible so that, it will guide us in placing the ring in casting machine • Penalty: cold shuts and short margins 3rd Law of Casting  Position the patterns in the “cold zone” of the investment and reservoir in the “heat center” of the casting ring: • Adherence to this law causes porosity in the reservoir • Penalty: shrinkage porosity Page 20
  21. 21. Casting procedures 4th Law of Casting  A reservoir must have sufficient molten alloy to accommodate the shrinkage occurring within the restorations: • Molten metal shrinks and creates a vacuum, for complete casting vacuum must be able to draw additional metal from adjacent source. • Penalty: shrinkage porosity and/or suck-back porosity. 5th law of casting  Do not cast a button if a connector bar or another internal reservoir is used: • With indirect spruing the largest mass of metal should be the reservoir Page 21
  22. 22. Casting procedures • A button can draw available molten alloy from the bar, shift the heat centre and reduce the feed of the metal to the restorations • Wax patterns should not be larger than the connector bar • Weigh the sprued patterns and use the wax pattern-alloy conversion chart Penalty: shrinkage porosity and suck back porosity (potential distortion during porcelain firing). 6th law of casting  Turbulance must be minimized, if not totally eliminated • Eliminate sharp turns, restrictions, points or impingements that might create turbulance and occlude air in the casting • Restrictions or constrictions can accelerate the metals flow and abrade the mold surface (mold wash) • Penalty- voids and /or surface pitting 7th law of casting  Select a casting ring of sufficient length and diameter to accommodate the patterns to be invested • The casting ring should permit the patterns to be ¼ inch apart and ¼ inch from the top of investment and 3/8 inch of investment between pattern and ring liner • Penalty- mold fracture, casting fins and shrinkage porosity 8th law of casting  Increase the wettability of wax pattern Page 22
  23. 23. Casting procedures • Wetting agent should be brushed or stained on the patterns and dried before investing • Too much wetting can weaken the investment and produce bubbles or fins on the casting • Penalty- bubbles (due to air entrapment) 9th law of casting  Weigh any bulk investment and measure the investment liquid for precise powder liquid ratio • A thick mix of investment increases investment expansion and produces loose fitting castings • Thinner mix yields less expansion with tighter fitting castings • Penalty- ill fitting casting 10th law of casting  Eliminate the incorporation ofair in the casting investment and remove the ammonia gas by –product of phosphate bonded investments by mixing under vacuums • Vacuum mixing removes air and gas providing an uniform mix without large voids • Entrapped air can affect the expansion at various sites of the investment • Penalty- small nodules, week mold and distortion of the casting Page 23
  24. 24. Casting procedures 11th law of casting  Allow the casting investment to set completely before initiating the burn out procedure • The mold may not withstand steam expansion during burnout if the mold is not set • Advise to wait till the recommended setting time by the manufacturer • Penalty- mold cracking/ blowout or fins on the casting 12th law of casting  Use a wax elimination technique that is specific for the type of patterns involved and recommended for the particular type of casting alloy selected • Plastic sprues should be heated slowly, so they can soften gradually and not exert pressure, so use a two stage burnout • Recommended atleast a 30 min heat soaking at 800F for the first burnout • Penalty- cold shuts, short margins, cold welds, mold cracks and/or casting fins 13th law of casting  Adequate heat must be available to properly melt and cast the alloy • Selected heat source should be capable of melting the alloy to the point of sufficient fluidity • Too much heat or high temperature can burn off minor alloying elements and /or oxidation (burned metal) Page 24
  25. 25. Casting procedures • Penalty- cold shuts, short margins and cold welds (too little heat) or rough castings and investment breakdown (too much heat) 14th law of casting  When torch casting, use the ‘reducing zone’ of the to melt the alloy and not the oxidizing zone • Melt achieved by the exclusive use of reducing zone minimizes the likelihood of metal oxidation and gas absorption and ensures the proper melt • Penalty- gas porosity and/ or a change of alloys quotient of thermal expansion 15th law of casting  Provide enough force to cause the liquid alloy to flow into the heated mold • Low density metal generally needs four winds of a centrifugal casting arm as compared to higher density, gold based alloys • Do not over wind • Penalty- cold shuts, short margins, cold welds (insufficient force), or mold fracture and fins (too much force) 16th law of casting  Cast towards the margins of wax patterns • Place the heated ring in the casting cradle using the orientation dot, so the pattern margins face the trailing edge Page 25
  26. 26. Casting procedures • Penalty- cold shuts, short margins and otherwise incomplete castings 17th law of casting  Do not quench the ring immediately after casting: • Uneven cooling and shrinkage between alloy and investment can apply tensile forces to the casting dot • It can reduce strength • Penalty- hot tears CASTING PROCEDURE – 1) Casting crucibles Following types available – a) Clay – Appropriate for high noble and noble alloys. b) Carbon – High noble alloy and high fusing gold faced metal ceramic alloy. c) Quartz and d) Zirconia alumina (Are high fusing metal ceramic all like titanium, alloys palladium silver for metal ceramic and nickelcobalt alloys. Page 26
  27. 27. Casting procedures 2) Casting machines Alloys are melted in one of the following ways, depending upon the casting machines. a) Alloys are melted in a separate crucible by torch flame, and are cast into the mold by centrifugal force by motor or, spring action. b) Alloys are melted electrically by a resistance heating or, induction furnace and then cast into the mold by centrifugal force by spring or motor action. Torch melting - Used to melt the alloys in crucible - Fuel used is mixture of natural or artificial gas like (propane / acetylene) air or oxygen. - Temperature of gas air flame is greatly influenced by mixture of gas and air or oxygen. - Four zones are differentiated as: o Combustion zone o Reducing zone deeply blue and hottest used to melt alloys. o Oxidizing zone with should not be used as it oxidizes the alloys. For noble alloys – mixture of gas and air is used. For high fusing alloys – Here mixture of gas and oxygen is used. Page 27
  28. 28. Casting procedures The other method for melting most base metal alloys like Ni-Cr, cobalt chromium, cobalt nickel chromium, commercially pure titanium and titanium aluminum valium require special melting equipments such a indication melting machine, vacuum melting devices or melting units. The parts of the flame are identified by conical areas – 1) The inner most cone is the zone where gas and air get mixed, no heat is produced here. 2) Combustion zone – it is green in color and definitely oxidizing, gas and air get partially burned, should not be used. 3) Reducing zone – Dimly blue and located just beyond the tip of combustion zone. a. Hottest part of flame b. Should be kept constantly on the alloy during melting 4) Oxidizing zone a. Outer zone b. Combustion occurs with oxygen in air c. Lower temperature d. Courses oxidization of alloys Procedure for torch melting of gold alloys – - With little practice the clinician can detect the proper zone of flame in contact by observing the condition of alloy surface. Page 28
  29. 29. Casting procedures - An asbestos paper base is used in the crucible to direct the molten metal into mold. - When a torch is used with compressed air and gas valves should be adjusted until the cone is approx 40 minutes. - When the reducing zone is contact, the surface of molten alloys is bright and mirror like. - When oxidizing zone in contact, there is thin film of “dross” on the surface of molten alloy. - Care should be taken not to over heat the alloy. - If under heated, the alloy first appears spongy and nodules of fused alloy appear on the surface. - At a proper casting temperature, the alloy is spherical in shape, bright orange in colour and tends to spin, follow the flame when it is slightly moved. - At this point the alloy should be about 38 to 66°C above it melting temperature. 3) The alloy is melted by induction heating and then cast into the mold by centrifugal force by spring or motor action. 4) The alloy is vacuum are melted and cast by pressure an argon atmosphere. In addition to this molten metal can be casted by air or, vacuum pressure. Page 29
  30. 30. Casting procedures For gold crown and bridges – Flux can be used. A flux is a substance, which is used to increase the fluidity and to prevent or, reduce the oxidation of molten alloy – Added when alloy is completely melted. Examples: 1. Fluxes containing charcoal. Advantages- Excellent for cleaning old alloys Disadvantages – Small bits of carbon may be carried into the mold and cause a deficiency at critical margin. 2. Fluxes made from equal parts of fused borase powder ground with boric acid powder. Boric acids aids in retaining boran on the surface. Care to be taken – The alloy should be preheated before the mold is removed from burnout oven. - This will reduce the time for cooling of investment. - If it is cooled before casting, mold size reduces because of cooling contraction (eg., quartz investment). Amount of gold used – For a 2 penny wt (dwt) of casting, 8 penny wt (dwt) alloy is advisable (is abbreviation of denarins, Roman copper coin) - Produce better detail and definition. Page 30
  31. 31. Casting procedures - Create more pressure on metal in mold Typically 6gms(4dwt) is adequate for premolar and anterior casting, 9gms (6dwt) for molars and 12gms (8gdwt) for pontics is sufficient. When previously cast alloy is used, two different types should not be used. If a pressure machine is used, casting pressure should be between 7-9 kgs. Torch melting of metal ceramic alloys – The alloys used here are - All alloys can be cast wt gas – oxygen or, oxyacetylene torch. - It differs from casting low fusing gold alloys in following respectso No asbestos o is used in high heat crucibles; it may react with alloys at high temperature. o The technician should have on high heat crucible for each brand of alloys to avoid cross contamination. o No flux is used, since it may interfere with bond strength and composition. o High fusing gold alloys are heated while hot before they are released into mold. o Base metal alloys should not “Pull” like gold alloys, rather, at casting temperature, the ingot is slump and round over at corner. Page 31
  32. 32. Casting procedures o If overheated, produces changes in composition and excessive oxidation, which can interfere with metal porcelain bond and color of porcelain. o Greater casting pressure i.e. (number of turns in centrifugal machine is required (5 turns). o Gas oxygen flame should be 20 to 25 mm long for single orifice tip and for multi orifice tip; it should be about 13mm long. o Oxygen pressure should be 2 Ps for single tip and 4 to 10 psi for multi orifice tip. Other methods of melting – 1) Electrical resistance heated casting machine – In this device, current is passed through a resistance heating conductor, and automatic melting of alloy occurs in a graphite ceramic crucible. Indications – Alloys for metal ceramic prosthesis. 2) Induction melting machine. In this unit, alloy is meted by an induction field that develops within a crucible surrounded by water cooled metal tubing. Indications – Base metal alloys. 3) Direct current arc melting machine – here the direct current arc is produced between two electrodes; the alloys and water cooled tungsten electrode. Temperature tin arc 4000°C and the alloy melts quickly. Disadvantages – Risk of overheating alloy and damage. Page 32
  33. 33. Casting procedures 4) Vacuum or, pressure assisted casting machine. Here, molten alloy is heated to casting temperature, drawn into the evaluated mold by gravity or, vacuum and subjected to additional pressure to force the alloy into mold. Indications – titanium and titanium alloys. Casting technique – Armamentarium - Broken arm centrifugal casting machine. - Crucible - Blow torch - Protective coloured goggles. - Tongs - Casting alloy - Flux (for gold alloys) RECOVERY OF CASTING – - After casting the machine should be allowed to spin freely until it has nearly stopped. - The ring is removed and promptly eventuated in water. - Gypsum bonded investments burst apart when quenched. - Phosphate bonded investment remain warmer longer. Page 33
  34. 34. Casting procedures - This investment is removed by carving and breaking it off the casting. - Residue is eliminated with tooth brush. - Final traces can be removal ultrasonically. Pickling – - Often the surface of gold alloy casting, appears dark with oxides and tarnish. - This can be removed by process called pickling. - Best pickling solution for gypsum bonded insvestment is 50% HCl solution. Disadvantages – fumes are likely to corrode, into rotary metal furnishing. Health hazard. - Solution of sulphuric acid can also be used. - The best method of pickling is placing the casting in a test tube or, dish and to pour acid over it. - Gold based and palladium based metal ceramic alloys and base metal alloys arc bench cooled to room temperature before casting is removed. The casting shrinkage occurs in three stages – 1) Thermal contraction occurs between the temperature to which metal is heated and its liquidious temperature. 2) Contraction of metal, which is inherent when metal, changes from its liquid state to solid state. Page 34
  35. 35. Casting procedures 3) Contraction occurs when the metal cools to room temperature. The casting shrinkage differ for various alloys because of their composition. Linear solidification shrinkage of casting alloys – Type Casting shrinkage (%) Type I (Au based) Type II (Au based) 1.37 Type III (Au based) 1.42 Type IV (Ni-Cr based) 2.30 Type V (Co-Cr based) - 1.56 2.30 Casting shrinkage that occurs when metal shrinks to room temperature is greatest. - It can be eliminated particularly with well designed sprue system into with reservoir sprue that forces ample amount of liquid metal to the sites of solidification particularly at the walls and corners. - Nobel metal alloys have les casting shrinkage than base metal alloys, which contain platinum or, palladium. DEFECTS IN CASTING 1. Nodules – due to bubbles of gas trapped in investment. 2. Fins – caused by cracks in investment from weak mix, excessive casting force, too rapid burnout, reheating the ring, pattern too close to edge of ring or rough handling of ring after investing. 3. Incompleteness in areas of wax thinner than 0.3mm, inadequate heating of metal, incomplete wax elimination, excessive cooling Page 35
  36. 36. Casting procedures (“freezing”) or mould, insufficient casting force, not enough metal, or metal spillage. 4. Voids or porosity – debris trapped in mould, too narrow or too long or incorrectly located sprue, absence of a chill vent in a large casting, dissolution of gases in metal, back pressure porosity where investment is not porous enough. 5. Marginal discrepancies – due to distortion of wax pattern on removal from die, increased setting expansion (hygroscopic technique). 6. Dimensional inaccuracies – standardized procedures not followed. Metal Finishing The metal should be smooth and polished before cementation. A rough surface attracts plaque. Finishing utilizes coarser to fine abrasive particles. Minute amounts of the abraded surface material are filled into surface irregularities, resulting in a microcrystalline surface layer known as the Beilby layer. Cutting discs, mounted points, or stones, flexible paper-backed discs, rubber discs impregnated with abrasives, finishing burs and polishing compounds applied with rotary brushes are used in the finishing procedure. Abrasives These are exceptionally hard materials that develop sharp cutting edges when they are chipped. The abrasive must be harder than the material on which used. The binder must be tough. Page 36
  37. 37. Casting procedures Some commonly used abrasives are diamond, silicone carbide, emery, aluminium oxide, garnet, sand, Tripoli and rouge. Finishing gold castings Use a separating disc to cut the sprue. Diagonal cutting pliers may be used, but the stress could distort a thin casting. After cutting, the contour in the area of the sprue should be continuous with the surrounding area. Inspect the internal portions of the casting. Remove any small nodules of gold with a No.330 bur in a high-speed hand piece. Trace all the negative angles on the inside of the occlusal surface with the bur tip. There should be no remnants of investments. Seat the casting gently on the die. A blue die relief agent is painted on the die, which leaves blue marks on the inside of the casting where it binds; relieve such areas. A No.2 Craytex wheel is used to smooth the area in which the sprue was attached. A5/8 inch Burlew wheel is used to smooth the surfaces. Care is taken so that the wheel does not run over the margin. The disc is rotated parallel to the margin rather than perpendicular to it. Place the casting on the die and the die into the working cast. Check for complete seating. Adjust interproximal areas. Mark occlusal prematurities with articulating paper and remove them with a green stone. Use a No.0 “bud” Page 37
  38. 38. Casting procedures finishing bur to smooth out the occlusal grooves, cusp ridges are finished with a small rubber disc. Finishing base metal castings Coarser and harder abrasives are used. Diagonal cutting pliers may not cut the sprues easily. If the alloy contains beryllium, it is ground only where there is adequate exhaust ventilation or the technician is wearing an approved respirator. SUMMARY The wax pattern is the precursor of the finished cast restoration. As it will be duplicated exactly through investing and casting, the final restoration can be no better than its wax pattern. A successful result depends on careful handling of the wax. Strict adherence to standardized procedures ensure a good and long lasting cast restoration obliterating for costly remakes and annoyance to the patient and the operator. REFERENCES 1. Anusavice “Skinners science of dental materials”. Tenth Edition. 2. Fundamentals of fixed prosthodontics- shillinburg 3. Contemporary fixed prosthodontics- roesensteil Page 38
  39. 39. Casting procedures 4. Dental laboratory procedure- rudd and marrow 5. Dental materials and their selection-willian .j.o’ brien 6. Restorative dental materials-craig 7. Removable prosthodintics- stewart 8. Earnshaw R. ‘The effects of additives on the thermal behaviour of gypsum bonded casting investments part I. Aus. Dent. J. 20 : pp27, 1975. 9. Neiman R. and Sarma A.C.: ‘Setting and thermal reactions of phosphate cements’. J. Dent. Res. 9: pp1478, 1980. 10. Mabie C.P.: ‘Petrographic study of the refractory performance of high-fusing dental alloy investments: II silica-bonded investments. J. Dent. Res. 52: pp758, 1973. Page 39

×