Bubbles are wonderful in a sparkling wine, but bubbles in a study cast are definitely not something to celebrate! Incorporating air into gypsum/water mixture will have a negative impact on the physical properties of our casts.
Excess air will result casts that have reduced compressive strength and reduced resistance to abrasion. In addition, as the surface of the cast becomes more porous it becomes more difficult to use for diagnostic waxing and other procedures like duplication and creation of putty matrices.
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The best way to avoid incorporating air into a mix is to mix under a vaccuum. Using a vacuum mixer provides a timer as well as a vacuum. The Whip Mix vacuum mixer we use at Spear Education is pictured below. This mixer is user-programmable, allowing up to 20 different mixing times, which is helpful when using a variety of different materials.
In order to demonstrate the effect of vaccuum mixing, I created a small comparison project. For this project, I fabricated three sample “casts” following the manufacturer recommendations for mixing times and powder-to-liquid ratios. The samples were mixed, poured into disposable wax paper cups, allowed to set and then trimmed using a lab model grinder. Lets look at the results:
For the first sample, I tried to do the best I could without a vacuum mixer. I carefully measured 50 grams of powder and the corresponding volume of water. I added the powder to the water in a flexible green mixing bowl and then carefully spatulated by hand. I did my best to remove any bubbles with the assistance of a lab vibration table. The resulting mix was finally poured into a disposable cup marked HM (hand-mixed) and allowed to set.
In the sectioned sample, the bubbles are obvious. The bubbles varied in size, but were fairly evenly distributed.
For the second sample I used the same 50 grams of powder, but this time I used a vacuum mixer. After sectioning the set stone, I found one bubble near the center of the mass. This is a pretty big improvement!
Not satisfied, I mixed one more sample cast. For the final sample, I vacuum mixed the stone but this time I doubled the volume of the mix (100 grams powder). The volume of water was increased to manufacturer recommendations. The results of this sample were interesting.
The third mix really had no bubbles and I liked that, but the sample cast also appeared more homogenous. Variations in the samples are visible in the image below. Note the striations in sample two (50 g Vacuum). With our vacuum mixer and the mixing bowls we use, 100 grams of powder is really the lower threshold necessary to get a good mix. Too little stone and the mixing blades essentially missed the mix.
Maximizing the physical properties of dental materials is something all clinicians should be interested in. Does using a vacuum mixer make a difference? In this case, we can see significant improvement in the sample “casts” when adding the vacuum mixing step and again when the volume of the mix was increased.
(Click this link for four steps to fabricating a soft tissue cast.)
Understanding the value of a vacuum mixer is key. If casts are being fabricated in the dental office, those casts will be improved when a vacuum mixer is used.
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Darin Dichter, D.M.D., Spear Faculty and Contributing Author