Spruing,casting and investing


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Spruing,casting and investing

  1. 1. Spruing , Investing and casting DR. MAHMOUD N. ALMUGHANY
  2. 2. Curriculum :  Wax pattern construction .  Sprue ( Def, Role, Requirements and technique )  Casting ring and liner ( types, mold expansion )  Investment material (requirement, composition and types )  Investing ( mixing and pouring )  Burn out  Casting ( Requirement, alloy melting, casting )  Quenching  Pickling  Divesting and finishing  Polishing
  3. 3.  The process consists of surrounding the wax pattern with a mold made of heatresistant investment material. eliminating the wax by heating and then introducing molten metal into the mold through a channel called the sprue.
  4. 4. Wax Pattern Construction
  5. 5. Spruing
  6. 6. Sprue  Definition: It’s a channel through which molten alloy can reach the mold in an invested ring after the wax has been eliminated .
  7. 7. Role of a sprue :  Holds the wax pattern to avoid its distortion. Creates a channel to allow the molten wax to escape from the mold.  Enables the molten alloy to flow into the mold which was previously occupied by the wax pattern.  Sprue can made from : Metal , plastic and wax
  8. 8. Requirement of sprue : Diameter In general, a relatively large diameter sprue is recommended because this improves the flow of molten metal into the mold. Molar around 2.5 mm ( 10 gauge), premolar 2 mm (12 gauge)
  9. 9.  Reservoir : Is a small amount of additional wax added to sprue former 1 mm below wax pattern . Function : to compensate for the shrinkage occurs during solidification of the casting. Reservoir
  10. 10. Location The sprue should be attached to the bulkiest noncritical part of the pattern. away from margins and occlusal contacts . Normally, the largest nonfunctional cusp is used. Fuctional cusps : Buccal cusps of the mandibular posteroir teeth & Lingual cusps of the Maxillary posterior teeth .
  11. 11. Attachment Should be attached at angle to allow the incoming molten metal to flow freely to all portions of the mold, (About 45 degree ).
  12. 12. Length The length of sprue former should keeps the wax pattern 6 mm from the casting ring length to Provides adequate bulk of investment to withstand force. Allows gases to escape from end of mold. The pattern should be placed as possible to the center of ring
  13. 13. Venting Small auxiliary sprues or vents have been recommended to improve casting of thin patterns and may helps in : gases escape during casting. compensate for the shrinkage during solidification
  14. 14. Crucible Former usually made of rubber, which serves as a base for the casting ring during investing and the sprue is attached to it . The pattern is painted with surface tension reducer and then carefully coated with vacuum-mixed investment
  15. 15. Spruing technique Direct The flow of molten metal is straight ( direct from casting crucible to the pattern. In Direct Using aconnector or (runner bar) which the wax pattern sprue is attached .
  16. 16. Number of Sprues Single Double Multiple
  17. 17. Casting Ring and Liner
  18. 18. Casting Ring  The casting ring serves as a container for the investment while it sets and restricts the setting expansion of the mold.
  19. 19. Types of ring according to shape • Round • Oval Complete ring Split ring •Rigid : Metal , plastic •Metal •Flexible: Rubber •plastic
  20. 20. Ring Liner :  Creates a space to allow for investment expansion  Allow setting of investment material under water, to give hygroscopic expansion  Regulate heat transfer through the investment material.  Facilitate removal of investment from the casting ring after casting.  Thickness of liner should be around 1 mm not less.  One or two layers.  3.0-mm short of both ends of ring will allow supporting contact of investment with ring after liner has burned out.
  21. 21.  Material used as liner :  Asbestos liner  Cellulose liner ( paper )  Ceramic liner ( aluminum silicate )  Combination of ceramic and cellulose liner.
  22. 22.  Four mechanisms to produce mold expansion: Setting Expansion of Investment Hygroscopic Expansion Wax Pattern Expansion Thermal Expansion  Setting Expansion Occurs as a result of normal gypsum crystal growth in air. About 0.4% but partly restricted by metal investment ring.
  23. 23.  Hygroscopic Expansion Max expansion: immerse investment-filled ring in water bath at 38°C. Water in bath replaces water used by hydration process space between growing crystals is maintained crystals grow longer  outward expansion of mold. About 1.2 to 2.2% max with expandable ring.  Wax Pattern Expansion While investment is still fluid, expansion occurs when wax is warmed above Temp at which it was formed.  Thermal Expansion when investment is heated in burnout oven. Two purposes :
  24. 24. Investment material
  25. 25. Requirements:  Precise reproduction of wax pattern.  Sufficient strength to withstand burnout and casting.  Sufficiently porous to allow escape of gases.  Expand enough to compensate solidification shrinkage of alloy.
  26. 26. Composition of Investment  Refractory material or filler Material that resist high temperature Expand upon heating Its problem have no cohesion to maintain the shape of investment when dried  Binder Mixed with filler to give some strength  Chemical modifier
  27. 27. Types: Gypsum bonded Phosphate bonded Silica bonded
  28. 28. Gypsum Bonded Investments use with alloys that melt below 1,000°C.  Used with Type I, II, III gold alloys  Composition: Phosphate Bonded Investments Much stronger and can withstand much higher burnout temps (800°C +) For alloys with casting temp > 1150°C Silicate Bonded Investments base metal alloys such as chrom-cobalt and steel The reason is that these alloys melt at temperatures ranging from 1250 - 1400 A silica-bonded investment material can be heated up to 1200C
  29. 29. Investing
  30. 30. Technique Hand mixing Vacuum mixing
  31. 31. Vacuum mixing  Advantages : Reduce the amount of porosity in the investment . Texture of the surface casting is smoother with better detail reproduction . Compressive strength of the investment is increased .
  32. 32. Investment material pouring 1- casting ring is filled up to it’s rim . 2- allow investment to set for about 1 hour . 3- crucible former is carefully removed .
  33. 33. Burn out
  34. 34. Wax Elimination  Wax elimination or burnout consists of heating the investment in a thermostatically controlled furnace until all traces of the wax are vaporized.  Once the investment is heated during the waxelimination procedure, heating must be continued, and casting must be completed. Bring the furnace to 200°C (400°F), and hold this temperature for 30 minutes. Most of the wax is by then eliminated.
  35. 35.  It’s advisable to begin the burn out procedure while the mould is still wet because water trapped in the pores of the investment reduces the absorption of wax .
  36. 36. Burnout ovens manual semiautomatic fully programmable controls
  37. 37. Casting
  38. 38. Melting and casting technique    Requires : heat source : to melt the alloy Casting force : to drive the alloy into the mould
  39. 39. Molten the alloy Using Torch Using electric furnace
  40. 40. Types of torch flame :  Gas air torch  Gas oxygen torch  Oxy- acetylene torch  Hydrogen oxygen generator Types of torch tips :  Multi orifice  Single orifice
  41. 41. Flame zones : (OXY- ACETYLENE flame) 1- Neutral Flame (Acetylene oxygen in equal proportions) 2. Oxidizing Flame (Excess of oxygen) 3. Reducing Flame (Excess of acetylene)
  42. 42.  Air pressure casting machine : Alloy is melted in situ in crucible hollow of the ring , followed by applied air pressure on the melt  Centrifugal casting machine: Alloy is melted in crucible , then forced in the mold by centrifugal force .
  43. 43. Quenching  After the red glow has disappeared from the button, the casting ring is plunged under running cold water into a large rubber mixing bowl
  44. 44. Pickling : Surface of the casting appears dark with oxides and tarnish. This surface film removed by these process ( pickling ) . By place the casting in a dish and pour acid over it . Heat the acid and don’t boil it .  Type of acids : Hydrochloric acid Sulfuric acid Ultrasonic devices
  45. 45. Divesting and Finishing
  46. 46. Polishing
  47. 47. Thank you