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Failures in casting

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Failures in casting

  2. 2. Good Morning…
  3. 3. FAILURES IN CASTING PROCEDURES Presented By:- Dr Nishant Khurana PG Student Guided By:- Dr M.P. Singh
  4. 4. TOPICS INCLUDED IN SEMINAR Introduction Classification of different types of defects Etiology of casting defects Its prevention Conclusion References
  5. 5. INTRODUCTION • Casting is defined as the act of forming an object in a mold. The object formed from this procedure is called as casting. • Any irregularity in the metal casting process that is undesired is called Casting Defect. • Casting is one of the most common procedures in dentistry and is mostly done by dental technicians. • By understanding the cause of the defect, a dentist as well as technician can prevent the defect.
  7. 7. DISTORTION • Distortion of the casting is usually due to distortion of wax pattern Some distortion of wax occurs when the investment hardens or due to hygroscopic setting expansion. It does not cause serious problems Some distortion of wax occurs during manipulation, because of the release of internal stresses
  8. 8. • Minimize distortion by : 1. Manipulation of wax at high temperature 2. Investing pattern within one hour after finishing 3. If storage is necessary, store in refrigerator DISTORTION
  9. 9. SURFACE ROUGHNESS • Surface roughness can usually be traced to : 1) Air bubbles on wax pattern cause nodules on the casting. it can be avoided by :- - Proper mixing of investment - vibration of mix or by vacuum investing -Application of wetting agents
  10. 10. 2) Too rapid heating, it may cracks the investment resulting in fins it can be avoided by :- - Heat the ring gradually to 7000 C (in at least 1 hour) 3) Higher W/P ratio gives rougher casting. Larger particle size of investment has the same effect it can be avoided by :- - Using correct W/P ratio & select investment of correct particle size
  11. 11. 4) Prolonged heating causes disintegration of the mould cavity it can be avoided by :- - complete the casting as soon as the ring is heated & ready - Overheating of gold alloy has the same effect. It disintegrates the investment. 5) Too high or too low casting pressure it can be avoided by :- - Using 15 lbs /sq inch of air pressure or three to four turns of centrifugal casting machine
  12. 12. Rounded , incomplete margins are evidence of insufficient casting pressure
  13. 13. 6) Composition of the investment. Proportion of the quartz & binder influences the surface texture of casting. Coarse silica will give coarse casting 6) Foreign body inclusion shows sharp, well defined deficiencies. Inclusion of flux shows bright concavities
  15. 15. POROSITY • Porosity may be internal or external • External porosity can cause discoloration of the casting • Internal porosity weakens the restoration • Severe porosity at the tooth restoration junction can cause secondary caries
  16. 16. • Porosities are Classified as 1) Those caused by solidification shrinkage  Localized shrinkage porosity  Suck back porosity  Microporosity 2) Those caused by gas  Pin hole porosity  Gas inclusions  Sub-surface porosity 3) Those caused by air trapped in the mould Back pressure porosity Acc. To Phillips
  17. 17. Localized Shrinkage Porosity Or Shrink Spot • These are large irregular voids usually found near the sprue-casting junction. • It occurs when cooling sequence is incorrect & the sprue freezes before the rest of the casting. • During a correct cooling sequence, the sprue should freeze last. • This helps the molten metal to flow into the mould to compensate for the shrinkage of the casting as it solidifies.
  18. 18. • If the sprue solidifies before the rest of the casting no more molten metal can be supplied from the sprue. • The subsequent shrinkage produces voids or pits known as shrink- spot porosity it can be avoided by :-  using sprue of correct thickness Attach sprue to thickest portion of wax pattern Flaring the at the point of attachment or placing a reservoir close to the wax pattern
  20. 20. SUCK BACK POROSITY • It is a variation of the shrink spot porosity • This is an external void usually seen in the inside of a crown opposite the sprue • A hot spot is created by the hot metal impinging on the mould wall near the sprue
  21. 21. • The hot spot causes this region to freeze last • The sprue has already solidified, no more molten material is available & the resulting shrinkage causes a peculiar type of shrinkage called suck back porosity • It is avoided by reducing the temperature difference between the mould & the molten alloy
  22. 22. MICROPOROSITY • These are fine irregular voids within the casting • It is seen when the casting freezes too rapidly • Rapid solidification occurs when the mould or casting temperature is too low
  23. 23. PIN HOLE POROSITY • Many metals dissolve gases when molten • Upon solidification the dissolved gases are expelled causing tiny voids, e.g.. platinum & palladium absorb hydrogen • Copper & Silver dissolve oxygen
  24. 24. GAS INCLUSION POROSITY • These are also spherical voids but are larger than pin hole type • They may also be due to dissolved gases, but are more likely due to gases carried in or trapped by the molten metal • A poorly adjusted blow torch can also occlude gases
  25. 25. BACK PRESSURE POROSITY • This is caused by inadequate venting (air escape) of the mould • The sprue pattern length should be adjusted so that there is not more than 1/4th thickness of the investment between the bottom of the casting ring & the wax pattern • When the molten metal enters the mould, the air inside is pushed out through the porous investment at the bottom
  26. 26. • If the bulk of the investment is too great, the escape of air becomes difficult causing increased pressure in the mould • The molten metal will then solidify before the mould is completely filled resulting in a porous casting with rounded short margins
  27. 27. • Avoid by 1) Using adequate casting force 2) Use investment of adequate porosity 3) Place pattern not more than 6 to 8 mm away from the end of the ring 4) Providing vents in large castings
  28. 28. CASTING WITH GAS BLOW HOLES • If there is any wax residue remaining in the mould, it gives off a large volume of gas, as the molten alloy enters the mould cavity • This gas can cause deficiencies in the casting & blow holes in the residue button • To help the elimination of wax completely from the mould, the burn out should be done with the sprue hole facing downwards for the wax to run down
  29. 29. INCOMPLETE CASTING • An incomplete casting may result when 1) Insufficient alloy used 2) Alloy not able to enter thin parts of mould 3) When mould is not heated to casting temperature 4) Premature solidification of alloy 5) Sprues are blocked with foreign bodies 6) Back pressure due to gases in mould cavity 7) Low casting pressure 8) Alloy not sufficiently molten or fluid
  30. 30. INCOMPLETE CASTING Wax pattern too thin Incomplete wax elimination
  31. 31. To bright and shiny casting with short and rounded margins • When the wax is not completely eliminated, it combines with oxygen or air in the mould cavity forming carbon mono-oxide which is a reducing agent. The gas prevents the oxidation of the surface of the casting gold with the result that the casting which comes out from the investment is bright and shiny. The formation of gas in the mould is so rapid that it also has a back pressure effect.
  32. 32. Incomplete casting
  33. 33. Incomplete casting resulting from incomplete wax elimination is characterized by rounded margins & shiny appearance
  34. 34. SMALL CASTING • If compensation for shrinkage of alloy is not done by adequate expansion of mould cavity, then a small casting will result. Another reason is, the shrinkage of the impression material.
  35. 35. CONTAMINATION 1. Due to oxidation when molten alloy is over heated 2. Use of oxidizing zone of the flame 3. Failure to use flux 4. Due to formation of Sulphur compounds it can be avoid by :- • Not overheating alloy • Use reducing zone of the flame • Use of flux
  36. 36. BLACK CASTING • Can be due to two reasons 1. Overheating the investment above 7000c causes it to decompose liberating Sulphur or Sulphur compounds. They readily combine with the metals in gold alloy forming a sulphide film. This gives a dark casting which cannot be cleaned by pickling.
  37. 37. 2. A black casting, can be also due to incomplete elimination of the wax pattern, as a result of heating the mould at too low temperature. A carbonized wax remains which sticks to the surface of the casting. It can be removed by heating over a flame.
  38. 38. CONCLUSION • An unsuccessful casting results in considerable trouble & loss of time • In almost all instances, defects in castings can be avoided by strict observance of procedures governed by certain fundamental rules & principles. • Seldom is a defect in a casting attributable to factors other than carelessness or ignorance of the operator • With present technique the casting failure should be the exception, not the rule.
  39. 39. REFERENCES • Fundamentals of fixed prosthodontics- Herbert T. Shillingburg • Dental laboratory procedures – FPD kenneth D.Rudd , Robert M. Morrow • PHILIPS’ Science of Dental Materials Eleventh Edition Anusavice • Essentials of Dental Materials SH Soratur • Contemporary fixed Prosthodontics, Rosenstiel
  40. 40. THANK YOU

Notes de l'éditeur

  • Today I will be bothering u all with the topic
  • Casting procedure is technique sensitive procedure. There are certain rules and principles for casting process and
    both dentist as well as dental technicians should be aware of.
  • This final concluding chart is taken from Contemporary FPD by Rossenstiel
  • Lastly I would say with present technique