Casting Technique and Defects PDF

Casting Technique and Defects Casting Technique and Defects 1  Casting Technique and Defects Introduction The desired accuracy of the casting is about 0.1%; therefore, the lost wax procedure requires specially developed materials tha...

Casting Technique and Defects Casting Technique and Defects 1  Casting Technique and Defects Introduction The desired accuracy of the casting is about 0.1%; therefore, the lost wax procedure requires specially developed materials that compensate for the dimensional changes indicated by the following equation: Shrinkage (wax + alloy) = Investment expansion Spruing: Before investing the wax pattern, it is attached to a wax sprue with its sprue crucible former base. Types of sprues Metallic or wax or plastic. Solid or hollow A. The purpose of spruing the wax pattern: 1. To fix the wax pattern in space so a mold can be made 2. To create a channel for elimination of wax during burnout 3. To form a channel for the entrance of molten alloy during casting. 4. To compensate for alloy shrinkage during solidification B. Sprue size and design: The sprue diameter must be large enough so that it remains open until the casting solidifies and short enough to allow rapid filling of the mold cavity. For large castings, two or more sprues may be necessary to allow molten alloy to reach all parts of the mold cavity before solidifying Point of attachment Sprue attachment must always be made at the bulkiest portion of the pattern. 2  Casting Technique and Defects C. Sprue selection: Sprues may be constructed from wax, metals or plastics. The wax sprue is the most commonly used. Plastic sprues are Not Recommended because: 1. Their higher flow temperatures and thermal expansion characteristics make it difficult to eliminate the sprue. 2. Since the wax melts at a much lower temperature than the plastic sprue and the thermal expansion of the wax is 5 times that of plastic, excessive wax pressure may build up in the mold during burn-out before the plastic sprue softens. D. Orientation in mold: It is essential that, when the investment ring is placed over the pattern-sprue crucible-former assembly, the pattern be 6 mm from the end of the ring. If the pattern is less than 6 mm from the end, there is not enough thickness of gypsum bonded investment to keep the molten gold from breaking through. If there is more than 6 mm of space, the alloy will solidify before the entrapped air can escape; resulting in rounded margins, incomplete casting, or mold fracture N.B: This distance is reduced to 4 mm in case of phosphate bonded investment. Why??. 3  Casting Technique and Defects Lining the ring with a liner: A liner is placed inside the ring to allow lateral expansion of the investment ❖ The functions of the casting ring liner: 1) It forms flexible lining to the inner surface of the rigid metal casting ring, to allow mold expansion. 2) Its thermal insulating properties ensure that the investment mold does not cool rapidly and contract after removal from the ‘burn out’ oven. 3) Permits easy separation of the investment from the ring after the casting is completed. 4) Wet liner allows the investment to set in presence of water providing hygroscopic expansion. ❖ The casting ring liner types: a) Ceramic aluminous silicate. b) Cellulose paper. c) Ceramic cellulose combination. d) Asbestos sheet (no longer used as discovered to be carcinogenic). Mixing the investment: The accurate water-powder ratio is mixed under vacuum. A brush is then used to paint the wax pattern with a mix. 1. Brush The casting ring is applied over the crucible former, and the ring is filled under vibration Technique until it is completely filled. The ring is a container that holds the investment in place during setting and restricts mold expansion. The mix is first hand spatulated. then with the crucible former and pattern in place, the 2. Vacuum ring is attached to the mixing bowl. technique The vacuum hose is then attached to the assembly. The bowel is inverted and the ring is filled under vacuum and vibration 4  Casting Technique and Defects Advantage of vacuum mixing: ✓ Reduce air bubbles and porosity in the investment. ✓ Increase the strength of investment. ✓ Give a smoother cast surface free from nodules. ✓ Better casting fits in the prepared cavity. Filling the ring: ✓ A vibratoris often used to aid the flow of the investment material. ✓ The investment is added until the ring is full. Wax Burn Out: The sprue base and former are removed and the wax pattern ‘burnt out’ to leave the completed mold cavity. The choice of Investment Material Depends On The Type Of Alloy which is to be cast. The temperature to which the investment mold is heated during burn out controls the thermal expansion of the investment: ✓ For gold alloys, 450-700º C is a commonly used burn-out temperature with gypsum bonded investments. ✓ For Ni/Cr alloys a temperature in the range 700-900º C. ✓ For Co/Cr alloys a burnout temperature of 1000º C. 5  Casting Technique and Defects Alloy And Mold Temperature: Heating rate of the investment should enable gases liberation without cracking the mold. The Balance Between The Molten Alloy Temperature And Mold Temperature is important in terms of producing a complete and accurate casting with a fine grain structure. The alloy temperature should be hot enough to ensure fully molten during casting into the mold…should not be so hot Not to oxidize or causes damage of the mold walls. The mold temperature should be hot enough to ensure complete expansion of the mold……………but should not be so hot causing delayed crystallization & coarse grain structure. Casting Machines The aim of casting machines is to cause molten alloy to completely fill the investment mold cavity. The alloy can be melted either by: 1. Gas-air torch 2. Oxyacetylene torch 3. Electrical induction heating 4. Electrical resistance melting The molten alloy can be driven into the mold by: 1. Gravity 2. Air pressure 3. Steam pressure 4. Centrifugal force 6  Casting Technique and Defects Finishing and polishing For Noble Alloys: After the casting has been completed; the ring is removed It is quenched in water to get TWO ADVANTAGES: 1. The noble alloy is left in a soft condition for burnishing and polishing. 2. When the water contacts the hot investment, an aggressive reaction occurs in the investment that becomes friable and soft and the casting is more easily cleaned. The surface of the casting appears dark with oxides and tarnish, which can be removed by pickling Pickling: Is heating the discolored casting in an acid such as 50% HCl or sulfuric acid solution. Base metal Alloys: They are bench cooled to room before the casting is removed from the investment Casting from these alloys are generally Not Pickled. Faults In Castings (Casting Defects) 1. Finning and bubbling. 2. Incomplete casting. 3. Porosity in casting. 4. Oversized or undersized casting. 1) Finning and bubbling: ► Finning: Occurs when the investment is heated up too rapidly in the furnace. This causes the investment to crack. Molten alloy flows into the cracks forming thin ‘fins’ on the casting in regions of the cracks. 7  Casting Technique and Defects ► Bubbling: Appears as spheres of excess material attached to the surface of the casting. These reflect the presence of surface porosities in the investment, a problem which can be overcome by vacuum investing. If defects occur in critical areas can result in a need to re-cast. 2) Incomplete Castings: ► Causes: If the alloy is not properly melted, or if the mold temperature is too low, solidification occurs before the mold can be properly filled. If there is insufficient push force created during casting; the alloy may not flow to all parts of the mold cavity. The problem is therefore more significant for base metal alloys which have lower density 8  Casting Technique and Defects 3) Porosity: ✓ Porosity may be seen as surface pitting on or within the casting. Broken pieces of investment may become embedded in the casting and produce pitting of the surface. is produced by gases which become dissolved in the molten alloy. This type of porosity may affect all parts of the casting. A. Gaseous porosity Its effects can be reduced by: 1. Avoiding overheating of the alloy 2. Casting in an inert gas atmosphere or vacuum. Are caused by an inability of air or other gases within the mold to escape. B. Back pressure A casting subjected to back pressure is rounded at the edges and lacking in effects details ✓ To assist the escape of gases: I. The investment material between the casting and the end of the ring should be thin and yet strong. II. When using silica-bonded; a vent, 0.5 mm in diameter, should be provided to allow escape of gases towards the crucible end of the mold. III. The dense layer of investment material at the base of the ring should be scraped away to facilitate the escape of gases. 4) Undersized Casting: The casting shrinkage of the alloy should be compensated by the setting expansion, thermal expansion of the investment. May Be Due To Faults In Technique as not heating the investment mold enough, producing insufficient compensation for casting shrinkage. 9  Casting Technique and Defects ‫زيادات في الكتاب‬ ✓ The invested ring is allowed to set in air in case of thermal technique or immersed in water bath in case of hygroscopic technique. ❖ The ring is lined with a resilient liner to 1) Provides a cushioning effect and allows for the expansion of the investment against the walls of the ring. 2) Provides excess water to the investment during setting (hygroscopic expansion). The liners used now may be cellulose or ceramic liners instead of asbestos liner used before which is carcinogenic. Wax elimination or burnout consists of heating the investment in a thermostatically controlled furnace until all traces of the wax are vaporized in order to obtain an empty mold ready to receive the molten alloy during the procedure. The ring is placed in the furnace with the sprue hole facing down to allow for the escape of the molten wax out freely by the effect of gravity. Then, a casting machine is used to melt the alloy to completely fill the investment mold cavity. Centrifugal casting machine Finally, the ring is removed and the completed casting is removed from investment and allowed to be finished and polished to be ready for use. 10