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Casting procedure & casting defects


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casting procedure, casting defects, casting machines, finishing & polishing of casting

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Casting procedure & casting defects

  4. 4.  11th Century Theophilus Described lost wax technique, which was a common practice for making jewellery  1558 – B. Cellini - claimed to have attempted use of wax and clay for preparation of castings  1884 – A. de saran used 24K gold to form Inlay
  5. 5.  1897 - Phillibrook described a method of casting metal filling  1907 -Taggart -devised a practically useful casting machine.  1959 - Strickland et al stated the importance of the type, shape location & direction other than the size of the sprue  Apart from this various studies conducted on the properties of investment materials and casting alloys have lead a path for a better, practical and useful processing methods.
  6. 6.  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.(GPT-8)
  7. 7. 1) To heat the alloy as quickly as possible to a completely molten condition. 2) To prevent oxidation by heating the metal with a well adjusted torch. 3) To produce a casting with sharp details by having adequate pressure to the well melted metal to force into the mold
  8. 8.  The process of attaching a sprue former/sprue pin to the wax pattern is called as spruing Purpose:-  To provide a channel through which molten alloy can reach the mold in an invested ring after the wax has been eliminated..
  9. 9.  1) Must allow the molten wax to escape from the mould.  2) Sprue must enable the molten metal to flow into the mould with as little turbulence as possible.  3) Metal must remain molten slightly longer than the alloy that has filled the mould
  10. 10. Principles of selecting an appropriate sprue
  11. 11.  uses single stage burnout.  More preferred because it melts at the same rate as the pattern, & allow for easy excape for molten wax.
  12. 12.  used for castings of alloys which use 2 stage burn out with Phosphate bonded investment.  Their main disadvantage is its softening temperature, which is higher than wax pattern. And may block escape of wax.  They may be used for casting FPD’s because of their high rigidity, which minimizes distortion.  Plastic sprues may be completely solid (or) hollow plastic.
  13. 13.  should be a non-rust metal to avoid contamination of wax. Hollow metallic sprue increase contact surface area and strengthen the attachment between the sprue and pattern. They are removed from the investment at the same time as the crucible former.
  14. 14. A d v a n t a g e s o f h o l l o w s p r u e f o r m e r :  I t i n c r e a s e s t h e c o n t a c t a r e a .  I t h o l d s l e s s h e a t t h a n t h e s o l i d s p r u e f o r m e r . “C a r e m u s t b e t a k e n t o e x a m i n e t h e o r i f i c e f o r s m a l l p a r t i c l e s
  15. 15.  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.  The diameter of sprue should be equal to the thickest portion the wax pattern.
  16. 16.  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 prevents premature metal flow into mold.
  17. 17.  Large diameter sprue: this improves the flow of molten metal into the mould.  less diameter sprue: causes localized shrinkage porosity
  18. 18.  The length of the sprue former - it keeps the wax pattern 6mm from the end of the ring
  19. 19. Very short sprue : porosity in casting at the junction of sprue and pattern. Very long sprue : sprue solidifies first leading to casting shrinkage and incomplete casting
  20. 20.  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
  21. 21.  Full veneer crown - sprue is attached to Maxillary buccal and mandibular lingual cusp.  Partial veneer crown - sprue is attached to cusp that encompasses the preparation.  If attached to cusp tips near margins of wax pattern, distortion and restriction of flow of molten metal into mold occurs.
  22. 22. Attached 45 degrees to the walls of mold, which decreases the turbulence of molten alloy.
  23. 23.  The attachment of sprue former to the wax pattern should be smooth and do not posses pits or irregularities.  Irregularities produces tags of investment which is prone for fracture by molten alloy leading to casting failure.
  24. 24. TYPES OF ATTACHMENT D i r e c t I n d i r e c t
  25. 25.  In indirect spruing - a connector or reservoir bar is positioned between the pattern and crucible former.  It is common to use indirect spruing for multiple single units and fixed partial dentures.
  26. 26.  The sprue former should be straight to reduce chances of creating turbulence in molten metal entering the mold.  High turbulence of alloy cause porosity
  27. 27. Usually a single sprue is used for small castings.  When two thick sections of a pattern are connected by thin part of wax, 2 separate sprues should be attached to each thick portion.
  28. 28.  The double sprue design is more effective than the single sprue design in decreasing the internal porosity (jpd vol 78 no 4 oct 1997)
  29. 29.  Reservoir is a small amount of additional wax which is added to the sprue former near the junction of wax pattern  It prevents localized shrinkage porosity as the alloy in this part solidifies last after the solidification of metal in mold
  30. 30.  It is used in direct spruing.  The horizontal running bar of indirect spacing provides the same function; they are used when the distance between the crucible and pattern is high.  The reservoir is present in prefabricated plastic sprues also.
  31. 31.  Small auxiliary sprue/vents are applied to thin wax pattern to improve the quality of casting. Usually 18- gauges sprues are used. It is indicated with extremely thin/thick casting to produce nonporous castings.
  32. 32. They help in escape of gases during casting and ensure beginning of solidification in critical areas by acting as a heat sink. It is attached to the wax pattern directly opposite to larger sprue former.
  33. 33.  The sprue is attached to crucible former which constitutes the base of casting relation with casting ring during investing.  It also helps by holding sprue in desired ring.
  34. 34.  Crucible formers are basically of 2 types--- a)Steep-sided cone: used with metal when casted using centrifugal casting force. b)Shallow cone: used to cast metal using stream/air pressure
  35. 35.  They are available as----  Rubber crucible former  Metallic Crucible former  Plastic crucible former
  36. 36.  They form a conical depression in investment, which guides flow of molten metal.  It should be clean and petroleum is applied to prevent formation of rough investment tag.  Then the end of sprue former is passed into the hole and held in position till the molten wax sets.
  37. 37.  Casting rings are used to confine the fluid investment around the wax pattern while the investment sets.  It also allow the hardened investment to be safely handled during burnout and casting
  38. 38.  They are available as--- 1) Shapes - Round - Oval 2) Complete rings I) - Rigid - Metal (stainless steel) - Plastic II) Flexible - Rubber 3) Split rings I) metal II) plastic
  39. 39.  Considerations in selection of castings rings 1) The internal diameter of casting ring should be 5-10mm greater than the widest measurement of the pattern and about 6 mm higher. 2) For single crown/inlay - small rings as used. Diameter - 32 mm 3) For large fixed partial denture – 63mm round/oval shaped casting ring are used
  40. 40. Plastic ring with rubber crucible formers are used. The ring is conical in shape with tapering walls. As the investment sets the investment is tapped out of ring. Then burnout is done with out casting ring, this causes greater expansion
  41. 41.  They are commonly used to produce expansion of mold. Various materials used as ring liners ---- 1. Asbestos liner 2.Cellulose (blotting paper) liner 3.Ceramic ring liner 4.Combination of ceramic and cellulose ring liner
  42. 42. 1. Allow uniform setting expansion of investment by decreasing the confinement of rigid casting ring. 2. In case of wet liner technique ---The absorbed water help in hygroscopic expansion.
  43. 43. 3.Thickness of the liner should be < 1mm 4.The amount of expansion depends on the number of liners used. The expansion seen with 2 liners is greater than one liner
  44. 44. 1. Asbestos liner: Asbestos is refractory to high temperature, they show a sufficient amount of water absorption. There are 3 types of asbestos-  White asbestos (least toxic) – this type is used in dentistry  Blue asbestos (most toxic)  Brown asbestos (Intermediately toxic)
  45. 45.  Asbestos is no longer used in dentistry. As produces 3 types of diseases 1) Asbestosis 2) Bronchogenic lung cancer 3) Mesothelioma – fatal tumour
  46. 46.  This material shows adequate water absorption.  It is burnt during burnout procedure. So to keep the investment in contact with ring after burnout,the liner is kept 3mm short of ring ends.  This also restricts the longitudinal setting and hygroscopic expansion.
  47. 47.  A long cellulose liner is carefully adapted on the walls of casting ring and is tucked in position with sticky wax.  If wet liner technique is used, the lined ring is immersed in water for some time. Then excess water is shaken away. Squeezing of liner should be avoided. The liner should end 3mm short of the casting ring end.
  48. 48. They are basically alumino-silicate fibrous material.  They do not absorb water to large extent, but its network of fibres can retain small amount of water on its surface.
  49. 49.  They are refractory to high temperature.  The binders used in ceramic liner (Ex – neoprene-latex) can contribute to toxicity (stimulate fibrosis/ act as adsorbent surface for carcinogenesis).  They show potential for development of Mesothelioma .They posses fibers of --- Length 5.3-17.8 mm. Diameter 0.2- 0.97 mm
  50. 50.  The wax pattern should be cleaned of any debris, grease or oils.  For this we can use either:- - A commercial wax pattern cleaner, or, - A diluted synthetic detergent. The pattern is left to air dry while the investment is being prepared.
  51. 51.  T h i n f i l m o f c l e a n e r o n p a t t e r n a )r e d u c e s s u r f a c e t e n s i o n o f w a x b )b e t t e r “w e t t i n g ” o f w a x p a t t e r n b y t h e i n v e s t m e n t  The wax pattern should not stand for more than 20-30 min before being
  52. 52. Mixing of investment may be done either by ---- i) Vacuum mixing ii) Hand mixing The incidence of bubble free casting with different investing technique – Open investing - 17% – Vacuum investing - 95%
  53. 53.  The incidence of nodules on casting is more in hand mixing then vacuum mixing. Application of surface tension reducing agent decreased the nodules (Johnston, IJP, 1992, 5; 424-433).  The best method is vacuum mix and vacuum pour technique. But most popular method vacuum mix and open pour.
  54. 54. Advantages of vacuum mixing----- 1) Remove air bubbles 2) Produce smooth castings 3) Increase tensile strength of investment 4) 95% of castings free of nodules. 5) Removes all the gaseous by products of chemical reaction of investment material
  55. 55.  Hand mix for 15 seconds  Vacuum mix for 60 seconds  Working time: 2-3 minutes
  56. 56. Mixing Ratios General --- • •More investment liquid, less water =more expansion • Less investment liquid, more water =less expansion • Begin with a dry bowl Use a maximum of 27ml of liquid Using more liquid results in a weak mold • For 100gms of investment:- Crowns/veneers: 22ml liquid, 5ml distilled water
  57. 57.  • Inlays/Onlays: 16ml liquid, 11ml distilled water  • Follow instructions on investment packet
  58. 58. Second part
  59. 59.  Require very specific W:P ratio’s .  A variation of only 1ml of H2O can significantly alter the setting expansion & the character of the casting surface.  Increasing W:P ratio makes investing process easier but investment will lose strength, cause cracks to occur during heating surface of casting inferiors. After the casting ring has been filled with investment material, any excess should be removed
  60. 60.  The filled ring is now set aside to allow the investment material to complete its setting reaction & the accompanying setting expansion.  Setting is complete in 30-40min.  Hygroscopic technique is used. - Freshly filled investment ring is immediately placed into water bath for 30min. & kept at 100ºF(38ºC).
  61. 61. Expansion of the mold cavity can be increased by-- 1) increasing the no. of layers of asbestos or fibrous ceramic lining the casting ring. 2) increasing the special liquid : water ratio. 3) increasing the total L:P ratio.
  62. 62. 4) Placing the investment in contact with water during setting. 5) Burning out the mold at a higher temp. 3mm on each end is left as it serves to lock the investment within the ring & equalize radial & axial expansion.
  63. 63.  Residual, hardened investment in an unclean mixing bowl will greatly accelerate the set of newly mixed investment  Phosphate investment should not be mixed in an apparatus that has been used for gypsum investment. Residual gypsum will also accelerate the set & will break down at temp. above 2400ºF(1300ºC) liberating sulfurous gases that can be detrimental to the casting
  64. 64.  Ammonia gas is given off during mixing, & it is important to hold the mixed investment under the vacuum after mixing ceases to dissipate some of this gas & thereby reduce the incidence of bubbles adhering to the wax pattern ( this additional holding time will vary from 15-45sec).
  65. 65.  Initial set of the phosphate bonded investment is generally rapid with the liberation of heat.  If burnout is not carried within 1-2hrs, the ring should be stored in a humidor at 100% humidity, not soaked in water since excessive hygroscopic expansion may result
  66. 66.  Carefully grinding or scraping the shiny “skin” off the end of investment just prior to burnout is advisable. This removes a relatively impervious layer, opening the pores of the investment & facilitating gas release as the alloy is cast into the mold.
  67. 67.  Once the investment has set for an appropriate period 45min. it is ready for burnout.  A crucible former is then carefully removed.  It is advisable to begin the burnout procedure while the mold is still wet, because water trapped in the pores of investment reduces the absorption of wax & as water vaporizes, it flushes wax from mold.
  68. 68.  This burnout after 45min determines with a gradual increase in temp. with wax elimination & phenomena of crystalline inversion that accounts for volume increase on thermal expansion.  when the alpha form is converted into beta form at inversion temp. density decreases volume increases, k/a volumetric expansion
  69. 69.  For expansion phenomena to take place in the best possible conditions, it is necessary that internal temp. of casting ring gradually reach prescribed level.  The interval between successive temp. level is such that it permits the external heat to reach the internal areas of casting ring
  70. 70.  Final burnout temp. of casting ring must satisfy fundamental principles:- 1) Give a degree of expansion that is in harmony with the shrinkage of alloy. 2) Maintain the viscosity of alloy at a level necessary for complete filling of thinnest area in mold. 3) Permit controlled cooling.
  71. 71.  These investments are relatively fragile & require the use of metal ring for protection during heating.  So, the mould are usually placed in a furnace at room temp. & slowly heated to 650ºC-700ºC for 60min. & held for 15-30min. at the upper temp.
  72. 72.  At 468⁰C for hygroscopic technique the investment obtains its compensation expansion from 3 sources:- 1) 37ºC water bath expands the wax pattern 2) Warm water entering the investment mould from top adds some hygroscopic expansion. 3) Thermal expansion at this temp. provides the needed expansion.
  73. 73. Advantages ----- 1) Less mold degradation. 2) Cooler surface for smoother castings 3) Convenience of placing molds directly at 468ºC  Rapid heating can generate steam that can cause flaking of the mould walls.
  74. 74.  Too rapid heating may also cause cracking of the investment. In such case, outside layer of the investment becomes heated before the centre sections.  Outside layer starts to expand thermally, resulting in compressive stress in the outside layer that counteracts the tensile stresses in the middle regions of the mold.
  75. 75. Decomposition & alloy contamination is related to a chemical reaction between residual carbon & CaSO4 binder.  CaSO4 + 4C CaS + 4CO 3 CaSO4 + CaS 4CaO + 4SO2  This reaction takes place whenever gypsum investments are heated above 700ºC in the presence of carbon.  Sulfur dioxide as a product of this reaction contaminates gold castings & makes them extremely brittle.
  76. 76.  Methods for rapid burnout procedure are ----- - Placing the mold in a furnace at 315ºC for 30min. & then rapid heating. Or - Directly place into a furnace at the final burnout temp. held for 30min. & cast.
  77. 77. PBI require:- 1) Higher burnout temp. for total elimination of wax patterns. 2) Completion of chemical & physical changes. 3) Prevention of premature solidification of higher melting alloys. Usual burnout temp. range from 750⁰C-1030⁰C. .
  78. 78.  PBI obtain their EXPANSION by 1)EXPANSION OF THE WAX PATTERN- this is considerable because the setting reaction raises the mold temp substantially. 2)SETTING EXPANSION- this is usually greater than gypsum, especially because special liquids are used to enhance such expansion. 3)THERMAL EXPANSION- this is greater when taken to temp higher than those used for gypsum investment
  79. 79.  Heating rate is usually slow to 315ºC & is quite rapid thereafter, reaching completion after a hold at upper temp. for 30min.
  80. 80.  Casting of an alloy into the mold space uses 2 basic requirements. A) Heat source – to melt the alloy B) Casting force – to force molten alloy into mold
  81. 81. 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
  82. 82.  Melting temp of pure gold –1063⁰c  Melting temp of gold alloy-924-960⁰c  Melting temp of base metal alloy-1155- 1304⁰c
  83. 83.  A) Heat Source: Different types of materials and method are used as heat source to melt alloy. Two basic modes are by using 1) Torch flame-- Gas air Gas oxygen Air acetylene Oxygen acetylene. hydrogen oxygen generator 2) Electricity --
  84. 84. Two type of torch tips: 1. Multi-orifice 2. Single-orifice Zones of the blow torch flame:  Zone 1 - colorless zone  Zone 2 – Combustion zone  Zone 3 - Reducing zone  Zone 4 - oxidizing zone
  85. 85.  Gas air torch: -Gas-air torch is used to melt conventional noble metal alloys (used for inlays, crown and bridge) whose melting points less than 1000⁰c
  86. 86.  Used to melt metal ceramic alloys of higher temperature up to 1200⁰c  The tip of torch is available as single orifice/multiorifice. the oxygen pressure is adjusted to 10-15 psi
  87. 87.  The flame is directed onto metal with the nozzle of the torch about 1.5 cm away from the metal.  Complete fluid should be obtained within 30 second at which point the metal is poured into the mould.
  88. 88. The actual production of flame can be done by adjusting the pressure and flow of individual gases . commonly advised pressure for acetylene nozzle is 3.5 N/cm2 and oxygen nozzle 7- 10 N/cm2  one part of acetylene + 2 and half part of oxygen
  89. 89.  The best results are obtained when flame is used with a distance of 10cm between the face of blow torch nozzle and the base of crucible.  If distance is reduced to – - 7.5 mm -slight porosity - 5 mm -increased porosity due to occluded H2 gas
  90. 90.  When the reducing zone is in contact, the surface of the gold alloy is bright and mirror like.  When the oxidizing portion of the flame is in contact with alloy there is a dull film developed over the surface
  91. 91.  The Melting of alloy requires a crucible to act as a platform on which the heat can be applied to the metal. There are three types of casting crucibles available--- Clay Carbon Quartz Zirconia-alumina
  92. 92.  Clay crucibles are used with high noble and noble metal alloys  used for crown and bridges.
  93. 93.  Quartz crucibles are recommended for high-fusing alloys of any type of base metal alloys and palladium alloys
  94. 94.  Carbon crucibles – for high noble crown and bridge and also for higher fusing gold-based metal ceramic alloys.
  95. 95.  Carbon crucibles should not be used in melting of high palladium, palladium silver alloys (to be melted above 1504⁰c) and also with nickel- chromium/cobalt chromium base metal alloys  The crucibles used with noble metal alloys should not be used for melting base metal alloy
  96. 96.  Copper –containing gold alloys and non- copper gold alloys for use with porcelain should not be melted in the same crucible  Crucible should be discarded if it contains large amount of oxides and contaminants from previous metals
  97. 97.  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
  98. 98. Sufficient mass of alloy must be present to sustain adequate casting pressure---  6g m i s t y p i c a l l y a d e q u a t e f o r p r e m o l a r a n d a n t e r i o r c a s t i n g  10g m i s a d e q u a t e f o r m o l a r c a s t i n g  12 g m i s a d e q u a t e f o r
  99. 99. A) Electrical resistance-  It is used to melt ceramic alloys. Here the alloy is automatically melted in graphite crucible.  Provides best means of temperature control. It is quite convenient as compared to blow torch.
  100. 100.  B) Electrical arc melting:  is used to melt higher fusing alloys.  It used to create a electrical arc at the end of two electrodes  The apparatus requires a high electrical input (30A)
  101. 101.  Device for forcing the molten alloy into the mould under pressure after wax has been eliminated
  102. 102.  Alloy is melted in situ in crucible, followed by applied air pressure.  Pressure of 10-15 psi
  103. 103.  Alloy is melted in a crucible, and forced in to mold by centrifugal force.
  104. 104.  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. 3)Electrical resistance - heated casting machine
  105. 105.  Direct current is produce between two electrodes: the alloys and the water cooled tungsten electrode.  Temp between the arc rapidly increases to 4000°C – alloy melts very quickly.  High risk of over heating of the alloy.  Damage may occur even after few seconds of over heating. Direct-current arc melting machine:-
  106. 106.  Metal is melted by an induction field that developed with in the crucible surrounded by water- cooled metal tubing. 4. Induction melting machine:
  107. 107.  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
  108. 108.  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.
  109. 109. A) Trimming is done from the button end of the ring. B) Investment is being pushed out of the casting ring
  110. 110. C) The mold is broken open. D) Investment is removed from the casting. Care must be taken to avoid damaging the margin
  111. 111.  The casting is held in a sandblasting machine to clean the remaining investment from its surface.
  112. 112.  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.  50% Hydrochloric acid  Sulfuric acid  Ultrasonic devices  Gold and palladium based metal ceramic alloys and base metals, these alloys are not generally pickled.
  113. 113.  Pickling solution should be renewed frequently, since it is likely to become contaminated  Precious alloys(Gold-Platinum-Palladium) can be soaked with hydroflouric acid  Nickel Chromium should never be placed in acid because of high reactivity
  114. 114.  The casting is trimmed , shaped and smoothen with suitable burs or stones.  The sprue is sectioned off with a cutting disc.
  115. 115.  Minimum polishing is required if all the procedures from the wax pattern to casting are followed meticulously.  White stone ,rubber wheels, rubber disks, and fine grit are included in the finishing and polishing agents
  116. 116. Third part
  117. 117. Error in the procedure often results in defective casting, these defects are known as casting defects.
  118. 118. According to philips  Distortion  Surface roughness and irregularities  Porosity  Incomplete or missing details Based on location  Internal  external
  119. 119. According to Rosensteil  Roughness  Nodules  Fins  Incompleteness  Voids or porosity  Marginal discrepancy  Dimensional inaccuracies
  120. 120.  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.  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
  121. 121. 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 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
  122. 122.  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. Air bubbles:
  123. 123. Water films:  Wax is repellent to water, & 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 agent 2.Correct L/P ratio (Too high L/P ratio may produce these irregularities)
  124. 124. Rapid Heating Rates It produces  Fins or spines on the casting Cause- because of Flaking of the investment Prevented by:  Heat gradually at least 60min from room temperature to 700 c.  Greater the bulk – more slowly heated.
  125. 125. Under heating  Incomplete elimination of wax residues may occur, if the heating time is too short.  This factor is mainly important for low heat technique. Prolonged heating  During high heat technique, decomposition or disintegration of the investment occurs & the walls of the mold are roughened.  Product of decomposition are sulphorous compounds, which contaminates the casting, this is the reason why the surface of the casting does not respond to pickling sometimes.
  126. 126.  Prevented by- when thermal expansion technique is used, the mold should be heated to the casting temperature & NEVER HIGHER. Liquid/Powder Ratio  The amount of water and powder measure should be accurate.  Too little water- investment too thick & cannot be applied to the wax pattern  Too much water- making investment easier but reproduces poor casting. 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 and - 3 to 4 turns of the spring in centrifugal casting machine.
  127. 127. 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 plane  Space between the pattern is atleast 3mm
  128. 128. 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. Prevented by:  This type of surface roughness or irregularities can be avoided by proper spruing.  Placement of sprue at 45 degree 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.
  129. 129. 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
  130. 130. Localized shrinkage porosity It is caused by premature termination of the molten metal during solidification. It mainly occurs at sprue-casting junction. Cause:  Diameter is too narrow  Length of the sprue is too long  Absence of reservoir  Direction of sprue at 90 degree
  131. 131. Prevented by-  Using sprue of correct thickness  Attach sprue to the thickest portion of the wax pattern  Flaring the sprue at the point of attachment  Placing reservoir close to the attachment
  132. 132. A hot spot is created by the hot metal impinging on the mold wall near the sprue. This hot spot causes this region to FREEZE LAST Since the sprue is already solidified, NO MORE MOLTEN MATERIAL IS AVAILABLE, resulting in shrinkage k/a SUCK BACK POROSITY Suck back porosity
  133. 133. It often occurs at OCCLUSOAXIAL OR INCISOAXIAL LINE ANGLE PREVENTED BY-  Flaring the point of sprue attachment  Reducing the temperature between the mold & molten alloy
  134. 134. 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..  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
  135. 135. 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.
  136. 136. Back pressure porosity Some times referred to as entrapped-air porosity.  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.
  137. 137. Causes  Inability of the air in the mold to escape through the pores in the investment Prevented by:  Proper burnout  Sufficiently high casting pressure  Adequate L/P ratio  Thickness of investment between tip of pattern and end of ring is not greater than 6mm.
  138. 138.  Factors that inhibit the mold filling is: 1. In sufficient venting 2. In sufficient casting pressure, pressure should be applied atleast for 4 sec 3. Incomplete elimination of wax 4. Lower L/p ratio 5. Viscosity of the fused metal
  139. 139. THANK YOU