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Casting defects

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Casting defects

  2. 2. CASTING DEFECTS  Error in the procedure often results in defective casting, these defects are known as casting defects Dental casting system aims to provide a metallic copy of the wax pattern as accurate as possible. Nevertheless, a wide range of variables may influence the final result and predictable outcomes are hardly achievable. While casting dental prostheses, problems frequently observed are incomplete casting and internal porosity 2
  3. 3. Classification According to philips Distortion Porosity Incomplete or missing details Surface roughness and irregularities- 1. Air bubbles 2. Water films 3. Rapid heating rates 4. Under heating 5. Liquid/powder ratio 3
  4. 4. 1. Prolonged heating 2. Temperature of alloy 3. Casting pressure 4. Composition of investment 5. Foreign bodies 6. Impact of molten alloy 7. Pattern position 8. Carbon inclusion Based on location Internal external 4 Anusavice, Kenneth J., et al. Phillips' science of dental materials.11th edition.
  5. 5. According to Rosensteil  Roughness  Nodules  Fins  Incompleteness  Voids or porosity  Marginal discrepancy  Dimensional inaccuracies 5 Contemporary Fixed Prosthodontics STEPHEN F. ROSENSTIEL, 4th edition.
  6. 6. Distortion  Distortion of the casting is probably related to distortion of the wax pattern. Causes:  Can occur from the time of wax pattern preparation to the time of investing due to stress relaxation.  during the investment procedure Distortion of the wax pattern can occur . Minimized by:  Application of minimum pressure  Manipulation of wax at high temperature  Investing pattern immediately  If storage is necessary, store in refrigerator 6
  7. 7. Hardening of the investment Distortion of the wax pattern Setting and hygroscopic expansion of investment produce non-uniform expansion on the walls of the pattern 7
  8. 8.  Distortion thickness of the wax pattern 8
  9. 9. Surface Roughness and Irregularities Surface roughness Defined as relatively fine 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 9
  10. 10.  The surface roughness of the casting is greater than the wax pattern from which it is made, because - the particle size of the investment and -its ability to reproduce the pattern in microscopic detail 10
  11. 11. A-AIR BUBBLE  Small nodules on the casting are caused by air bubbles, that become attached to the surface during or subsequent to the investing procedure. 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. 11
  12. 12. B-Water films: Prevented By: 1.Use of wetting agent 2.Correct L/P ratio (Too high L/P ratio may produce these irregularities) Wax is repellent to water, If Investment becomes separated from the wax pattern water film may form irregularly over the surface. Appears as minute ridges or veins on the surface. 12
  13. 13. C-Under heating If the heating time is too short Incomplete elimination of wax residues Hot alloy come in contact with carbon residue Voids or porosities may occur in casting from gas formed  Occasionally casting is covered with TENACIOUS carbon coating-impossible to remove by PICKLING 13
  14. 14. D-Prolonged heating High heat technique, Decomposition or disintegration of the investment Walls of the mold are roughened. Product of decomposition are sulfur compounds, which contaminates the casting, Reason -surface of the casting does not respond to pickling sometimes.  Prevented by- when thermal expansion technique is used, the mold should be heated to the casting temperature & NEVER HIGHER. 14
  15. 15. E-Liquid/Powder Ratio  The amount of water and powder should be measure accurately.  Too little water- investment too thick & cannot be applied to the wax pattern  Too much water- reproduces rough casting. 15
  16. 16. E-Casting pressure  To high pressure – rough surface of the casting  To low pressure – incomplete casting  Average – 0.10 to 0.14 Mpa in an air pressure machine - 3 to 4 turns of the spring in centrifugal casting machine is sufficient for small casting. 16
  17. 17. F-Foreign bodies Foreign body into mold surface roughness  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. 17
  18. 18. G-Pattern position Expansion of wax is higher than that of investment Causing breakdown or cracking of investment PREVENTION-3mm spacing should be given 18 PATTERN SHOULD NOT PLACED TO CLOSED TOGETHER
  19. 19. H-Impact of metal alloy Cause:  The direct impact of molten alloy on the weak portion of the mold surface, may fracture or abrade the mold surface regardless of its bulk. This can lead to improper seating of the casting. Prevented by:  Proper spruing by directing the molten alloy at an <90o to investment surface. 19
  20. 20. Carbon inclusions  Carbon from- carbon crucible, - carbon containing investment, - improperly adjusted torch– Can be absorbed by the alloys during casting results in formation of carbides or visible carbon inclusion. 20
  21. 21. Porosity The presence of voids or pores within a structure.gpt 9 This can be internal/external. Surface roughness Internal porosity not only weaken the casting-lead to 2o caries by accumulation of plague 21
  22. 22. Porosity Classified as follows: I. Solidification defects A. localized shrinkage porosity B. Micro porosity II. Trapped gases A. pin hole porosity B. gas inclusion porosity C. sub surface porosity III. Residual air 22
  23. 23. I. Solidification defects  A. localized shrinkage porosity-  By premature termination of the molten metal during solidification.  Linear contraction of noble metal alloy changing from liquid to solid is at least 1.25%  Molten metal through sprue compensates this shrinkage  Metal in sprue freezes incomplete casting. localized shrinkage void 23
  24. 24.  Porosity in the pontic is caused by- retain heat because of its bulk and located in the center of the rings.  Prevented-adding one or more gauge sprues(18gauge) To area which is far from the main sprue. & extending laterally within 5mm of edge of ring. Localized shrinkage –occur near the sprue casting junction 24
  25. 25. Suck back porosity  Interior of the crown near the area of the sprue create a hot spot on the mold wall Hot spot Created by hot metal impinging from sprue channel on a point on mold wall. Causes local region to freeze last Result SUCK BACK POROSITY 25
  26. 26.  SUCK BACK POROSITY-occurs at inciso axial line angle  Metal at this surface creates higher localized mold temperature k/A HOT SPOT.  This area retain the localized molten metal for lone after the casting have solidified  Eliminated by- flaring the point of sprue attachment  Lowering the casting temprature by 300 26
  27. 27. Microporosity  Occurs from solidification shrinkage.  This is not detectable unless the casting is sectioned 27
  28. 28. Pin hole and Gas inclusion porosity  Related to the gas entrapment.  Characterized by spherical contour.  On solidification the absorbed gases are expelled & pinhole porosity result.  Eg- copper & silver dissolves oxygen platinum & palladium dissolves hydrogen  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 28
  29. 29. 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. 29
  30. 30. Back pressure porosity  Some times referred to as entrapped-air porosity. Causes  Inability of the air in the mold to escape through the pores in the investment 30
  31. 31. 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. 31
  32. 32. INCOMPLETE CASTINGS: -  Partially complete casting or perhaps no casting at all  Two factors –that inhibit the ingress of the liquefied metal are insufficient venting of the mold and high viscosity of the fused metal. 32
  33. 33. Insufficient Venting  Due to the back pressure exerted by the air in the mold.  Molten alloy does not fill the mold before if solidifies  Overcome back pressure  Continuous pressure is applied for 4sec  The mold is filled and the metal is solidified in 1 second or less, yet it is quite soft during the early stages point. These appear as rounded incomplete margins. 33
  34. 34. According to Rosensteil • Excess surfactants • Improper water powder ratio • Excessive burnout temperature Rough casting • Air trapped during investing procedure • Inadequate vacuum during investing • Lack of surfactants Nodules 34
  35. 35. 35 • Excessive vibration Nodules on occlusal surface • Wax pattern too thin • Cool mold • Inadequate metal Incomplete casting
  36. 36. • Increase w/p ratio • Pattern too near to investment • Premature heating(wet mold) • Too rapid heating Fins • Incomplete wax elemination Incomplete casting with shiny,round defect • Improper w/p ratio • Improper mixing time • Improper burnout temperature Inadequate or excessive expansion 36
  37. 37. • Wax pattern distortion • Uneven expansion Marginal discrepancy • Suck-back-improper pattern positin & norrow,long sprue • Inclusion-particle of investment & dislodge during casting porosity 37
  38. 38. Casting made simple using modified sprue design: An in vitro study  Indian Journal of Dental Research, Vol. 25, No. 3, May-June, 2014, pp. 340-345  Abstract  Background: Success in dental casting restorations for fixed partial dentures (FPDs) depends on the castability. Castability is described as the ability of an alloy to faithfully reproduce sharp detail and fine margins of a wax pattern. The goal of a prosthodontist is to provide the patient with restorations that fit precisely. Regardless of the alloy used for casting, the casting technique should yield a casted alloy, which should possess sufficient mass, surface hardness and minimal porosity after casting. Materials and Methods: Twenty patterns for casting were made from three-dimensional printed resin pattern simulating a 3 unit FPD and casted using modified sprue technique. Later test samples were cemented sequentially on stainless steel model using pressure indicating paste and evaluated for vertical marginal gap in eight predetermined reference areas. Marginal gap were measured in microns using Video Measuring System (VMS2010F-CIP Corporation, Korea). A portion of the axial wall of the cast abutments depicting premolar and molar were sectioned and embedded in acrylic resin and tested for micro hardness using Reichert Polyvar 2 Met Microhardness tester (Reichert, Austria) and porosity using Quantimet Image Analyzer (Quantimet Corporation London, England). Results: The results obtained for marginal gap, micro hardness, and porosity of all test samples were tabulated, descriptive statistics were calculated and the values were found to be within the clinically acceptable range. Conclusion: The new sprue technique can be an alternative and convenient method for casting which would minimize metal wasting and less time consuming. However, further studies with same technique on various parameters are to be conducted for its broad acceptance 38
  39. 39. CONCLUSION  An unsuccessful casting result in considerable trouble and loss of time, in almost all instances, defects in castings can be avoided by strict observance of procedures governed by certain fundamental rules and principles.  Seldom is a defect in a casting attributable to other factors than the carelessness or ignorance of the operator. With present techniques, casting failures should be the exception, not the rule 39
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  41. 41. REFRENCES  K. J. Anusavice, Phillips Science of Dental Materials, 11th edition  Rosensteil, Contemporary fixed rosthodontics  Shillinburg, fundamentals of fixed prosthodontics  Indian Journal of Dental Research, Vol. 25, No. 3, May-June, 2014, pp. 340-345 41
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