The process of choosing investments and gypsums for the dental laboratory is a complicated process with many variables. When choosing an investment, the most important factor in putting together your system is the ‘fit’ of the crown or bridge. Some dentists prefer tight fits, while others like the fit to be looser. Each of the materials you can choose from to make the crown or bridge will have different expansion characteristics.
In addition, there are many outside factors which can have an (unwanted) effect on expansion, so technique and control of environment and following mixing and handling instructions are important. Before choosing an investment, the one question you need to ask yourself is:
‘What material will I be putting in the investment; pressable ceramic, ceramic metal or metal & pressable?’
The material you will be putting into the investment will have a direct impact on your desired results. Once you choose your material, there are additional considerations that you need to take into account. I have outlined common questions that should be considered in your investment choice to ensure you get your desired results.
With PRESSABLE CERAMICS, you must think temperature, temperature, temperature. The investment you choose must withstand the top temperature of pressing and the duration of the hold and press times. This is why we do not recommend gypsum investments.
CERMAIC METALS or METAL & PRESSABLE materials you want to ask yourself which materials will be cast and which burnout method will be used. This is important because the temperature at which you burnout and the expansion will effect the results of your cast.
1. Which burnout procedure will I use? When considering
- Slow burnout, Conventional burnout – although it takes more time it is more kind to the investment
- Rapid burnout, Fast burnout – allows for faster production but has limitations to patterns
2. Which ceramic metals will I use?
- Non-precious for ceramic – chrome-cobalt or nickel-chrome have higher temperatures
- Semi-precious for ceramic made of sliver palladium or low gold under 10% will cause changes in expansion
- Precious for ceramic (ceramic alloys) – containing over 40% gold, will have lower temps and lower expansion
Always remember, no matter what investment you are using, expansion is a function of the following factors and may work in combination to have an adverse effect on the desired result:
- The wax used (combining of waxes change the way they move during expansion)
- Wax Expansion during bench-set as investment heats (most technicians don’t think of this at all!)
- Investment Expansion during bench-set
- Thermal Expansion during burnout
- Ambient Temperature in the laboratory
- The Temperature of the investment powder
- The Temperature of the liquid used
Changes to any of these will cause changes in expansion. One way to minimize unwanted results will be to keep the investment powder and the liquid at a constant temperature such as in a refrigerator, set on the warmest of the cool settings. Keeping the investment powder and liquid in a refrigerator until use will minimize these two potential areas of problems.