Problems with porosity?
Castings with porosity in them can cause all kinds of problems and lead to more work, wasted time and wasted money. Complicating things further is the fact that the visible porosity can be just a part of the problem. The same forces behind the porosity can cause warped bridges, inadequate fit and problems with porcelain.
Porosity can come and go like the wind, appearing one day and vanishing the next. So where does porosity come from, and better yet how do you get rid of it? Overheated alloy, casting rings that are too hot and contaminated metal are some of the causes that are easier to fix. If you are experiencing problems with porosity then this is a place to start troubleshooting.
But a majority of the time porosity can be traced to one cause; improper spruing methods. But expensive sprues, complicated casting rings and castings without a button are not the way to solve the problem. In fact, it is much easier, and if only a couple simple rules are followed high quality results can be repeatedly achieved.
- Never cast with sharp angles
We always want to avoid sharp angles when we sprue as it lessens the possibility of creating turbulence when we cast. Whether spruing a single coping, or a seven unit bridge the path of the alloy should always "flow". To the eye it should look as if the metal will have a clear path from the button to the restoration. If your spruing method has sharp angle, bends and turns then it will be an invitation for problems.
- Always cast from thick to thin
This is usually the most difficult thing to follow and can be the biggest cause of problems. When approaching this we have to consider practicality and common sense, as a wax-up can go from thick to thin and then back to thick again many times over. When we say thick to thin we mean that the casting should go from a greater volume to a smaller volume. Picture it as a funnel, larger at one end and gradually getting smaller at the other end.
In the end porosity can't be eliminated, but it can be controlled. To control the porosity we have to make sure that the metal cools in such a way that the porosity ends up in the middle of the sprue and not the restoration. When metal is heated for the casting process as it melts it will expand, and after the casting takes place as the metal cools it shrinks. But as it shrinks it creates a vacuum that draws metal from the sprue to saturate the restoration. But if the sprue cools before the restoration then it will have nowhere to draw molten metal from, thus resulting in porosity.
This is why for proper casting we must have a button and we must place the waxed restoration in the proper position outside the heat zone of the ring. This allows the metal to cool from both ends, meeting in the center of the sprue and that is where the porosity will be found.
We all want to achieve quality results, and we also want to do it in the most efficient manner. Excessive grinding, polishing and adjusting can be costly. That is why it is important to follow a few simple rules of metallurgy, avoiding pitfalls and achieving the absolute best results.