Thermal expansion of dental casting alloys and phosphate-bonded investments.
For a dental investment to render accurate castings, it is essential 1) that the setting expansion is low enough not to cause any distortion of the wax pattern, and 2) that the thermal expansion is high enough to compensate fully for the solid thermal contraction of the casting alloy used1-8). However, all phosphate-bonded investments known today when used according to the manufacturers' instructions, are characterized by a relatively high setting expansion, and by a thermal expansion insufficient to compensate adequately for the solid thermal contraction of conventional casting alloys. Recent studies9) have shown that the setting expansion of representative European brands of phosphate-bonded investments may be reduced to an acceptable low value by modifying the mixing liquid. It was the purpose of the present work to study the thermal expansion of thus modified products, and for comparison also the thermal expansion of several dental casting alloys.
Corrosion properties of soldered joints. Part I: Electrochemical action of dental solder and dental nickel-chromium alloy.
The corrosion properties of contact between six Ni-Cr alloys and two dental solders were evaluated. Ni-Cr alloys with low Cr content coupled with silver solder induced continuous galvanic current, whereas Ni-Cr alloys with high Cr and some Mo content coupled with silver or gold solder induced galvanic current for a short time. Six Ni-Cr alloys were divided into two groups according to the dissolution of Ni, one with low resistance and the other with high resistance. The initial equilibrium was induced between high resistance Ni-Cr alloys and both solders by less current density, which implies a smaller degree of corrosive current than that in low-resistance alloys. Corrosion was observed in the silver solder coupled with high-resistance Ni-Cr alloy, but was not found in the gold solder coupled with both alloys. In soldering Ni-Cr alloy, it is better to use a gold solder and a high-resistance Ni-Cr alloy for corrosion resistance.
Setting Changes of Hydrocal in Free State and in Various Hydrocolloid Impressions
A vertical measuring apparatus was devised by Fusayama and used for measuring the free setting expansion of hydrocal. This apparatus is very easy to use and is accurate enough.Perfect curves of dimension, temperature and hardness of hydrocal during setting were made and compared in fig. 2. Plastone and Duroc were used as hydrocal material.Their setting expansions occured at the time of the initial setting. The maximum expansion of Plastone was +0.54% 1 day after pouring, and that of Duroc was +0.05% 6 hours after pouring.The maximum temperature rise from room temperature in the center of a 23×23×50 mm3 block of Plastone was 7.5°C 30 minutes after pouring. That of Duroc was 16°C 13 minutes after pouring. The times requested to reach to these peaks substantially corresponded with those of the highest piches of their setting expansions.At first the hardness increased generally parallel to the curves of their setting expan-sions, and then continued to increase during their drying shrinkage also. They reached B. H. N 7 after 50 minutes in the case of Plastone and B. H. N 8.5 after 30 minutes in the case of Duroc. These results were about 60% of their final hardness measured after 1 week.Elastic hydrocolloid impressions restricted the setting expansion of hydrocal poured in them to some extent. Harder impression colloid supported closer by tray walls restricted it more.The Plastone models poured in various hydrocolloid impressions expanded after one day. This is illustrated in the following data :Technicol-Plastone model +0.26 (±0.06) %Algix-Plastone model +0.51 (±0.10) %Deelastic-Plastone model +0.27 (±0.07) %D. P. Im. Col.-Plastone model +0.35 (±0.09) %