In the casting of models for the production of jewellery, a major problem being faced by producers often has to do with the occurrence of various forms of defects. These defects take place at difference stages of the production process and a source of revenue loss. Experienced players in the jewellery production industry are always seeking new and innovative ways to prevent and deal with the occurrence of casting defects.
Casting defects can be classified as:
b) Surface roughness & irregularities
d) Incomplete casting or missing details
e) Surface Discoloration
Distortion of the casting is due to a distortion of the wax pattern. Some distortion of the wax pattern some wax distortion occurs as the investment hardens during manipulation, around it or due to hygroscopic because of release of & setting expansion. stresses caused by: Contraction on cooling Occluded air Moulding & carving Removal Time & temperature of storage It does not cause serious problem, but accounts for some unexplained inaccuracies. Minimized or prevented by proper manipulation of the wax and handling of the pattern
Distortion can be minimized by:
1. Manipulation of wax at high temperature.
2. Investing pattern within one hour after finishing.
3. If storage is necessary, store in a refrigerator.
2. SURFACE ROUGHNESS & IRREGULARITIES
Definition of Surface roughness It is defined as relatively finely spaced surface imperfections whose height, width & direction establish the predominant surface pattern.
Definition of Surface irregularities Surface irregularities are isolated imperfections, such as nodules, that are not characteristic of the entire surface area.
Even under optimal conditions, the surface roughness of the casting is invariably somewhat greater than that of the wax pattern from which it is made.
Excessive roughness or irregularities on the outer surface of the casting requires additional finishing & polishing whereas irregularities on the cavity surface (inner surface) prevents proper seating of the casting.
CAUSES OF SURFACE ROUGHNESS & IRREGULARITIES
1.AIR BUBBLES: Air bubbles on wax pattern causes nodules on the casting. These air bubbles may occur on the outside or inside of the casting.
If the voids occur on the outside, they are theoretically removable, but requires great expense of time and money. If they occur on margins or internal surfaces, successful removal is extremely difficult & often the restoration will have to be recast.
2.WATER FILMS: If the investment becomes separated from the wax pattern in some manner, a water film may form irregularly over the pattern surface as wax is repellent to water. This is manifested as minute ridges or veins on the surface of casting.
3.RAPID HEATING RATES: rapid a heating rate causes cracking of the investment. These cracks produce a casting with fins or spines. This condition is especially seen with cristobalite investment.
4.OVERHEATING: Overheating of investment above 700ºC, disintegrates the investment, liberating Sulphur or Sulphur compounds. These combine in metal in the gold alloy & form sulphide film. This gives a dark casting (Black casting) which cannot be cleaned by pickling.
5.PROLONGED HEATING: Same effect as that of overheating i.e disintegration of investment / mould cavity.
6.UNDER HEATING- Incomplete elimination of wax residues may occur if the heating time is too short or heating temperature is too low.
7.PREVENTION: Heating the investment ring, for adequate time & temperature so as to remove carbon residues. The burn out should be done with sprue hole facing downwards (for the wax to run down). This facilitates its complete removal.
8.COMPOSITION OF THE INVESTMENT- He ratio of the binder to the quartz influences the surface texture of the casting. A coarse silica causes surface roughness.
9.CASTING PRESSURE- Too high casting pressure during casting can produce a rough surface on the casting. A gauge pressure of 0.10 to 0.14 MPa in an air pressure casting machine OR 15 Ibs/sq inch of air pressure. Three to four turns of centrifugal casting machine is sufficient for small castings.
10. IMPACT OF MOLTEN ALLOY: The direction of the sprue former should be such that the molten alloy does not hit a weak portion of the mould surface. Occasionally, the molten alloy may fracture the mould surface on impact, regardless of its bulk. Such a depression / concave area in the mould is reflected as a raised area on the casting, and prevents the complete seating of the casting.
Porosity may occur both within the interior region of a casting and on the external surface. External porosity may be a manifestation of internal porosity. Whenever internal porosity extends to the surface, it can cause for discoloration. The external porosity results in surface roughness, whereas internal porosity weakens the casting. Severe porosity can cause plaque accumulation at the tooth-restoration interface, and secondary caries may result. Although the porosity in a casting cannot be prevented entirely, it can be minimized by use of proper techniques.
Distortion can be minimized by:
1. Use sprue of correct thickness.
2. Decreasing the length of sprue.
3. Increasing the melt temperature.
4. Increasing the mould temperature.
5. Attach the sprue to thickest portion of wax pattern.
6. Flare the sprue at the point of attachment.
7. Place a reservoir close to the wax pattern.
4. Incomplete casting or missing details
Occasionally, only partially complete casting or perhaps no casting at all is found. Cause: Molten alloy has been prevented for completely filling the mould.
Factors that might inhibit the ingress of the liquefied metal are:
1.Insufficient alloy used.
2.Alloy not hot enough. (High viscosity of the fused metal) & so alloy is not able to enter thin parts of mould.
3.Premature solidification of alloy.
4.Low casting pressure. Improper burnout. (Incomplete wax elimination & Insufficient venting of the mould).
5.Pattern too far from end of the ring. Sprue blocked with foreign body.
5. Surface Discoloration
Surface discoloration may be due to:
2. Incomplete elimination of wax.
3. High sulphur content of the torch flame.
4. Absence of reducing agents in the investment.
A source of discoloration often overlooked is the surface contamination of a gold alloy restoration by mercury. Mercury penetrates rapidly into the alloy and causes a marked loss in ductility and a greater susceptibility to corrosion. Thus, it is not a good practice to place a new amalgam restoration adjacent to a high noble alloy restoration.
Causes of Surface Discoloration
1. Failure to use flux.
2. Due to oxidation, when molten alloy is overheated.
3. Use of oxidizing zone of flame.
4. Formation of S compounds (Black casting).