Technical Advise

Appearance:

• Numerous of small and spherical pores resembling pinholes randomly scattered on the surface. These pinholes are usually found abundant on most of the polished surface of heavy items with large flat surface. Extensive polishing usually cannot remove the pores successfully.

Caused by:

• Too high the flask and/or casting temperature rendering the decomposition of the investment powder. The gaseous sulfur dioxide will penetrate into the molten metal.
• High ratio of recycled material or use of heavily contaminated scrap.

Solved by:

• Avoid to use too high the casting and/or flask temperature. The sprue design should be reviewed. The larger sprue in right position can help to reduce the risk of getting gas porosity.
• Avoid to use heavily contaminated scrap for recycling. Prior to recycling, the scrap must be cleaned thoroughly to remove all the residual investment powder and oxide by tumbling and pickling in dilute sulfuric acid.

Appearance:

• Some tiny, almost invisible pores were observed on surface of the casting items.
• Upon higher magnification, a shiny and dentritic structure can usually be seen.
• Fracture may sometimes occur during resizing or applying a little force.

Caused by:

• Improper sprueing position and/or improper sprue size.
• Too low the casting and/or flask temperature.

Solved by:

• The sprue must be placed at the thickest part of the cast. If there are more than one heavy part in the cast connected by other filigree part, the other heavy parts should also be connected with additional sprue. For heavy parts, the cross section of the sprue should ranged from 2.5-3.5 mm.
• For filigree items, the sprue should be as thick as the cross section of the cast. Whilst positioning the sprue, no sharp corners or turning should be present to avoid turbulence. Fluted sprue is always preferred.

Appearance:

• The pores, which appear predominantly after polishing have a typical form of such “crows feet” like appearance and its appearance is sometimes very similar to the shrinkage porosity. The mechanical strength of the cast can be affected and embrittlement usually occurs.

Caused by:

• Oxidation of the alloys which contain high zinc content.
• Over-recycling of the scrap and/or use of the heavily contaminated scrap.

Solved by:

• The melt must be protected from oxygen by covering with protective gas (e.g. nitrogen or argon).
• The recycled material must be thoroughly cleaned to remove the oxide by pickling in dilute sulfuric acid prior to recycling.
• The ratio of recycled materials which contain high zinc content must be kept as low as possible during recycling. It can avoid the presence of excessive zinc oxide.
• Casts with serious oxide inclusion porosity should be subjected to refining instead of further recycling.

Appearance:

• The surface is randomly littered with numerous irregular pores. Under magnification, the inner wall of the pores does not show any signs of dendritic structure. Fracture can sometimes happen with excessive porosity in smaller cross-sectioned regions.

Caused by:

• Over-use of the flux for cleaning- Weakening of the investment.
• Recycling of used wax.

Solved by:

• Limited the use of flux. The excessive flux can enter the flask together with melt. The flux may stick on the inner wall of the flask and renders this porosity problem.
• If this porosity is frequently observed and the cast is often associated with some fins, the quality of the investment should be checked.
• Avoid recycling the used wax since the wax may contain some residual investment fragments which cannot be burnt off and thus caused this inclusion porosity problems.

Appearance:

• A single hard spot or a group of hard spots were observed whilst polishing the item. The hard spots are usually localized on one side of the cast only.

Caused by:

• Oxidation of the Ni-white gold during melting.
• Over-recycling the scrap .
• The alloy contains too much silicon.

Solved by:

• Avoid the oxidation by covering the melt with protective gas such as nitrogen or argon. The presence of oxide is believed to favor the formation of nickel silicide, which is the origin of the hard spot. Since it is lighter than the bulk metal, the hard spot usually localized only on one side.
• Avoid recycling too much the scrap and it must be cleaned thoroughly prior to recycling.
• Use of Si-free alloy or alloys with higher zinc content.

Appearance:

• A single hard spot or a cluster of hard spots were detected whilst polishing the cast. The hard spots usually distribute all over the cast.

Caused by:

• Use of alloys containing too much grain refiners.
• Over-use of the recycled scrap.

Solved by:

• Use of alloys containing less grain refiners.
• Reduce the ratio of the recycled scrap in the cast.

Appearance:

• Single or a cluster of hard spots were detected whilst polishing the cast. The hard spots usually distribute only on one side of the cast. The hard spots are very hard.

Caused by:

• Use of gold with contaminations by PGMs or tungsten.

Solved by:

• Use of gold with higher fineness and reliable sources.
• Refine the gold prior to casting.

Appearance:

• The surface bears a typical dendritic or crystalline structure.
• The Phenomenon is more obvious in the case of heavy cross-sectioned cast.

Caused by:

• Too high casting and/or flask temperature.
• Improper use of forming gas for investment casting.

Solved by:

• Avoid to use too high casting and/or flask temperature since the high operational temperature can favor the decomposition of the investment. The calcium sulfate in the investment powder will decompose to liberate sulfur dioxide and oxygen which can form a gaseous layer between the metal and the investment stopping the molten metal to fill up the space which is generated during the crystallization process.
• Avoid to use forming gas for investment casting since the presence of hydrogen in the forming gas can enhance the decomposition of the investment.
• Re-design the sprue to allow a lower operational temperature.

Appearance:

• The surface of the cast appears rough and sandy just like after sand blasting.
• Usually the cast may also bear some small fins at the edge especially when it located at the top of the casting tree.

Caused by:

• The defect is mainly due to the weakness of the investment flask but it is more commonly found when we do the casting with heavier alloys such as 18K Pd-white gold or even platinum.

Solved by:

• To check the quality of investment. If there is necessity, change the investment powder with better quality.
• Check the water/investment ratio during the flask setting.

Appearance:

• Some visible raised traces were found particularly on the flat surfaces.
• The orientation of the traces on the item runs approximately from the top to bottom of the tree.

Caused by:

• The cause is more related to the investing procedure. The investment slurry contains an excess of free water. The water in the slurry started to separate out upon remaining motionless for minutes.

Solved by:

• The investment should be kept stirring before the setting starts.

Appearance:

• The cast fractured during de-investing without being subjected to any significant strain or deformation. The edge of the fracture usually does not bear any signs of plastic deformation and it is mostly appeared near the sprue.

Caused by:

• Use of alloys with too much silicon.

Solved by:

• Change the alloys to another one containing less or even no silicon since the silicon can weaken the metallic strength once it segregates out from the alloy.
• Quench the flask faster. This can helps to stop the segregation of the silicon and the metallic strength of the alloys is not affected.

Appearance:

• 18K red gold or 18K gold with high copper content becomes very brittle whilst soldering or just de-investing. The same effect happened during other heat treatment process.

Caused by:

• Quenched the alloys slowly in water. This can favors the formation of the intermetallic compounds between copper and gold.

Solved by:

• Fast quenching the alloy in water whenever the alloys was still hot or over 410°C. This can help to freeze the metal alloys in its solid solution state since the atoms will undergo an ordering process to form intermetallic compound when it reached 410°C.
• Use the alloy with less copper content to 18K red gold.

Appearance:

• The fracture is usually found on the shank near the sprue or in the small prongs upon applying a low force. The fracture occurred without any deformation but the rest of the cast can be still very ductile. Sometimes, the fracture surface is found very dirty and full of colored contaminants.

Caused by:

• Oxidation of the metals during melting.
• Too much recycling of the scrap.

Solved by:

• The melt must be protected from oxygen by covering with protective gas (e.g. nitrogen or argon).
• The recycled material must be thoroughly cleaned to remove the oxide by pickling in dilute sulfuric acid prior to recycling.
• The ratio of recycled materials which contain high zinc content must be kept as low as possible during recycling. It can avoid the presence of excessive zinc oxide.
• Casts with serious cracking problems should be subjected to refining instead of further recycling.

Appearance:

• The alloy is found completely brittle and fractured. The alloy cracked without any noticeable deformation and it is nearly not able to withstand any strain. Usually, numerous intergranular fracture can be observed.

Caused by:

• Lead, Bismuth or Antimony was found in the karat gold alloys.

Solved by:

• To use only lead free materials for the casting. Some brass, tin-lead solder may contain substantial amount of lead. Upon casting or soldering, the lead can be introduced to the gold alloy rendering such kinds of cracking problem.
• Avoid to use any tools or equipment containing lead, antimony or bismuth.

Appearance:

• This defect is much pronounced in sterling silver casting. Some red to dark patches were found irregularly on the surface of the casts. The patches are usually not completely removed even with extensive polishing.

Caused by:

• Oxidation of the alloys was occurred during casting. The copper in the alloy is oxidized to cuprous oxide or cupric oxide, which is respectively red and dark in color.
• Too much recycling of the scrap.
• Use of oxygen-containing silver.

Solved by:

• The casting must be covered by protective gas (nitrogen or argon).
• Limited the use of recycled materials and the recycled materials must be cleaned thoroughly by tumbling and pickling in dilute sulfuric acid.
• Avoid to use the silver with unknown quality for the silver casting since the silver may contain a large amount of oxygen gas.

Appearance:

• The color of the jewelry turns yellow or brown after exposing in open atmosphere for certain period of time. It frequently happens to sterling silver and low karat gold jewelry.

Caused by:

• The discoloration is due to the formation of silver or copper sulfide.
• Some parts of the jewelry may be under high stress (such as sand blasting) which is probably more sensitive towards tarnishing.

Solved by:

• To coat the jewelry with anti-tarnishing coating.
• To use the materials which contains anti-tarnishing ingredients.