Day 25 Casting Defects Pits and Porosity

If you haven’t casted your own jewelry, or sent waxes (or CAD Files) off to someone to cast them for you, here is a tip and a quick check to save you cleanup time in the metal. There is nothing more frustration than finishing a casted piece and finding pits and porosity that you have to either solder to fill pits, or have them laser-filled. There are many types of porosity, from bad sprue/gate design or placement, shrinkage porosity, gas porosity, etc. but the easiest to avoid we will cover today. (A sprue or gate is the way the metal gets into the piece of jewelry during casting.)

The easiest way to avoid casting defects is to pay attention to the sprueing or gating of your piece in wax or the model. Sprueing/Gating can be a complicated subject, and can take a long time to get a grasp on the technical end of it, but I will give you a couple of quick tips to get you understanding why it is important.

First, if you are adding a sprue to a piece that you are molding, or a wax you are casting, add the sprue to the thickest part of the piece and use a sprue that, as a rule of thumb, is 1.5 times thicker the area you are adding the sprue to. (For example, if you are adding a sprue to a ring, and the ring has a shank that is 2mm thick and 4mm wide, use a sprue that is approx. 3mm thick (2mm x 1.5)

Now for the easiest way to avoid a defect in the casting. Pay special attention to the joint, or union of the sprue, and the piece of jewelry. If there is not a good complete union, like a pit or a sharp edge, in the solder or wax joint, this will cause pits in your castings. Let me show you how:

In the casting process, the wax is surrounded by investment, similar to Plaster of Paris, for the “burnout” or elimination of the wax. If there is a divot, pit, or sharp edge in either the union of either the sprue-to-jewelry, or the piece to the casting base, (tree) then during the casting process, when there is molten metal flowing through the casting flask, the metal flowing at a high speed and pressure will hit the little investment defect and break it off, and the piece of investment will float around in the metal and end up as a void or defect in the metal, resulting in a pit.

A little care and attention to the union of the wax to the tree, or sprue to the wax will prevent these kind of defects in your castings. I know this is kind of complicated, but I hope I explained it in a way that makes it understandable. I also hope the images help to explain. If it is still unclear, go ahead and post a comment and I will try to answer your questions.

Also if you have a subject you would like me to cover, comment here or on Facebook (JewelryMonk).

Take Care and have a great day! Now go make something shiny!

Doug