Dental Metalcasting


By Daniel, the_vizionary@yahoo.com

Introduction

Okay, before we get started, I have been a dental technician for only 2 years, and I am by far not an authoritative expert on this subject. However, I do have enough experience and understanding to impart the neccessary information for an experienced metal caster to get a good idea of how we do it. I have worked in 2 different dental labs, and the casting techniques were basically the same for both. Though all dental labs have their nuances, and many people have unothodox techniques that work for them, the system I am describing is represenative of dental casting in general.

Please keep in mind I am side-stepping a few aspects of this process, as a complete description would either cause confusion or require an understanding of other areas of dental laboratory work, making this article far too long and unfocused. The method described here may be of some use to other systems, it may not. Maybe it will inspire new thoughts, or hybrid techniques. If nothing else, in the words of Tom, "It's always cool to have information from people who do the same thing as you, but in a completely different way."

The Setup

For the level of detail and precision necessary for dental metalwork, we use the lost wax method. Once a crown pattern has been waxed, we sprue it with a length of wax cut from a spool. The attachment is done by carefully heating the sprue and sticking it to the pattern. Obviously, insufficient heat will produce a weak bond, and excessive heat will warp the wax pattern. Often, the pattern end of the sprue is pinched down to a slightly smaller diameter. This tapering increases the velocity of the metal flowing into the pattern. I cannot explain the physics of this, or tell you why the increased velocity helps the final product; I only know that it does. The other end of the sprue is attached to the base of the ring mold. In contrast, this attachment should be tapered out slightly to allow for a smooth entrance for the metal. If venting is necessary, a length of spruing wax is attached at the relevant location on the pattern, and ran back to the base former end. With a centrifugal casting machine and dental investment, venting is almost never necessary (the stuff is expensive for a reason). For large crowns or bridges, a reservoir (a plastic or wax ball "interrupting" the sprue, and placed close to the pattern) is sometimes added to eliminate shrinkage and porosity. This is usually not needed, as the base former creates a large cavity in the investment ring, and thus a natural reservoir. Incidentally, the metal left in this cavity is called a button.

Diagram for setup

The Molding Process

The mold material we use is called investment. I tried to find some information on the chemical make up of dental investment, but I was unable to find anything. The investment we use comes from Microstar Corporation. As I have already stated, its not cheap, but it is excellent material if precision and fine detail are high priorities. I am not plugging for Microstar, but I have no other examples of this material. So just for reference: http://www.microstarcorp.com/invests.htm

The investment is mixed with water and a special liquid that is sold separately. The ratio of water and this special liquid depends on how "tight" or "lose" you want your investment. This sacred knowledge belongs to the waxers, which I am not, so if you want to uncover those secrets, I'm afraid you must either experiment yourself, or find a waxer in a dental lab and appease their wishes for them to part with the ways of their ancient art. Once mixed, the investment is vaccuum mixed to remove air bubbles. While the investment is mixing, we spray a small amount of debubblizer on the patterns, and blow off the excess. If too much debubblizer is left on the pattern, the investment will be diluted in that area, and create a weak spot. Debubblizer is a liquid that reduces the angle of contact when flowing a thick liquid into a mold, or onto a pattern. This makes the investment slide across the pattern, instead of rolling across it and trapping air pockets between the pattern and the investment material. I do not know the chemical content of debubblizer, but it feels and acts like slightly soapy water. You then carefully pour the investment into the former. Patience, practice, and keeping your eyes on what you are doing help to get a good pour.

Burnout of the Wax

Now we set a timer to tell us when the investment is near the hottest stage of its setup process. This information is different with each type of investment, and is given on the package. Basically, this is after the investment is hard, but well before being fully cured. When this point is reached, the ring is removed from the former. The non-gated end is scraped well, as although the investment is designed to be finely porous, any surfaces that have been formed against a smooth surface will also be smooth, and thus nearly air-tight. Scraping removes this smooth surface layer, allowing gases to escape freely once we are casting. The ring is then placed in an oven and left until the wax has burned completely out, and the ring has reached the proper temperature for the type of metal that will be cast. It is important to act quickly, for the ring should still be as hot as possible when placed in the oven to avoid temperature shock. This is only necessary when speed is a factor, and as in all places of work, time is money. If speed is not a factor, you can place a cold ring in a cold oven and heat it slowly.

The device we use for casting is called a broken arm centifugal casting machine. Please excuse the over simplified imagary, but I couldn't get my hands on a camera, and I was already taking too long getting this article together, so I slapped these basic diagrams together:

Picture of casting machine

I'm not even sure what brand our casting machine is, so again just for reference, here are a couple of links to manufacturers of these machines:
http://www.bartcotools.com/P77.html
http://www.handlermfg.com/pages/dental/castmacc.html

Casting the Crown

Once the burnout is complete, and the ring is properly heated, we wind the casting machine and set the retaining pin. We then "break" the caster's arm. The caster arm has a pivoting joint just before the crucible and cradle. Rotating the "wrist" as I call it, in the opposite direction from how the arm will spin, adds extra force to the initial swing, and this makes all the difference in the world. The ingot(s) are then placed in the crucible ready to melt. We use a different crucible for each type of metal, so that left over traces of metal left in the crucible from previous castings do not contaminate our end product. We do keep our crucibles clean, but having a crucible for every type of metal is prudent, and prudence throughout our process is what keeps our dentists happy, and our lab in business!

We put the torch to the metal, careful to get an even melt. Most metals will begin to pool naturally, but some non-precious metals we use actually maintain their shape when heated to their melting temperature. For these metals, we tap the crucible occasionally, watching for the "shell" to break, and the metal to pool. Once the metal pools, we back the torch off a little to avoid overheating, as we do not wish to evaporate important elements from the metal. The ring is then taken out of the oven and placed in the cradle. The crucible is slid forward to meet the ring, the pin is dropped, the torch is taken away, and... woosh! Now for the most part, all the metal flows into the ring. But sometimes, a little metal does not make it into the ring because of the "jerk" of the arm straitening out. This is not a big problem, but when casting gold we follow the arm with our hand for a split second before complete release in order to dampen the sudden jerk. Over time, a drop of gold here and there add up, and since gold casts very easily, we lose nothing by slowing the very beginning of the acceleration.

Conclusion

I do not know how well this system will work with other forms of mold material, nor do I have any idea if a centrifugal casting machine is useful for larger projects. In the dental field, metal casting is on a relatively small scale, so the light load of metal needs pressure to inject it into small areas. With projects using a couple of pounds of metal or more, the sheer weight of the metal alone provides alot of pressure. Searching for centrifugal casting machines on the internet has lead me to sites where multi-ton machines spin large loads of metal, so the pressure of centrifugal injection is apparently of use on any scale, but for me, it remains to be seen if it is a worthwhile system for the backyard caster.

Collected and Styled by Tom.