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Cracks on Jewellery


#1

Hello,

What could be the reason behind cracks seen on the casted jewellery? We cast gold jewellery in 99.5 purity. We are aware that if the gold contain other impurities then this incident is likely. Thank you.


#2

Hi there Binod,

From past experience I found this issue to be related to the main factor of the Heat/Time relationship.

If upon opening up these cracks you see a very grainy or lattice-like structure, then that would confirm a condition of shrinkage porosity.

  1. This style of head requires appropriate molten time to fill.

  2. If the sprue position/size is inadequate the metal will begin to cool at the head prior to the rest of the item.

  3. Restricted flow of molten metal often causes this delay.

  4. Cooling at the back of the narrow shank draws the heat back towards the central sprue area.

  5. Rapid cooling of the head also draws heat from the shoulder areas of the shank.

  6. This in turn develops a significant heat transfer away from the shoulder position producing a very loose grain structure.

Adjusting flask and melt temps may not be the answer as most of these issues are resolved by an improved sprue arrangement.

Slightly higher flask temps may assist in some cases.

Hope this helps.

Regards,

Phil.


#3

Hello Phil,

Thank you very much your insightful input. We have discussed all the possible causes pointed by you with our casting team. They have been advised to follow those points seriously.

Best regards,
Binod.