| Porcelain-fused-to metal restorations (PFMRs) is the major type of restoration used to replace missing teeth or missing portion of a tooth. It is widely accepted owing to many advantages, such as high strength, high chemical stability and good appearance. Its quality has been improved constantly in the last four decades, and it has been used more and more commonly in clinic.There are two main kinds of materials to fabricate the metal substructure of PFMRs: precious alloy and non-precious alloy. The former has good biocompatibility, strong corrosion resistance, high precision for casting and good color match. It is the optimal alloy for PFMRs. However, its high cost has limited its use in domestic market. Therefore non-precious alloy is much more commonly used in clinic at present.The metal substructure of PFMRs is fabricated by precision casting technique. To guarantee the casting, more alloy heavier than the casting must be used to fill the mold. In this way, there will be many sprue pins and bases (wasted material / old alloy) left each time. According to the instructions of manufacturer, the wasted material of precious alloy can be reused by adding more than 1/3 ofnew alloy. This method is widely acceptable. As for non-precious alloy, in order to save cost, many domestic laboratories choose the same method mentioned above to reuse the material. But there is still lack of experimental findings to support it. The purpose of this study is to examine if the wasted material of non-precious alloy can be reused, therefore provide some references for clinical applications.In 1999, the committee of International Organization for Standardization established a standard method (ISO9693) to measure mechanical property, metal-ceramic bond strength, thermal expansion coefficient of dental casting alloy. Specimens were prepared and tested just according to this standard.The specimens were divided into four groups. Group 1, set as control, was Ni-Cr alloy, commonly used to fabricate PFMRs in clinic. Group 2, Group 3, Group 4 were the used alloy remained by the previous group. They were reused after sandblasting and cleaning by ultrasonic cleaning machine for 30 minutes. The main findings of this study were as follows:1. The physical properties of Ni-Cr ceramic alloys recasted one up to four successive remelts of the same alloys were compared included tensile strength; yield strength, percentage of elongation. The tensile strength values of 4 groups were 521.85±29.06 MPa , 533.07±20.43 MPa, 503.22±28.95 MPa, 515.85±38.76 MPa; The yield strength values of 4 groups were 281.38±10.93 MPa, 291.60±15.11 MPa, 287.22±14.31 MPa, 303.29±19.06 MPa; There is no significant differences between the four generations of the recast alloys for tensile strength, yield strength; The percentage of elongation values of 4 groups is 17.37±2.36, 20.60±2.80, 22.03±1.39, 22.93±1.61. The percentage of elongation and the microstrures of the recast alloys have some differences .Themicrostructure of the recast alloys were observed with the reversed type of metallosocope and scanning electron microscope (SEM). A significant increase in the amount of interfacial voids is observed when recast alloy is used. Compared with original metal,SEM observation on fracture surface reveals that porosities and defects increase.The elemental composition of alloy remains stable during melting procedures,however,the results of this study indicate that mixing new and previously cast metal can influence the compositional stability of alloy, The physical properties of the repeated castings for the first,second,third,and fourth generations,as compared with the minimun ISO9693 specifecations for alloy.The percentage of elongation is above the minimum ISO9693 specifications of 3% for all the generations. It indicates that the Ni-Cr ceramic alloys can be reused for four generations.2. 32 wax sheets were divided into 4 groups, each group included 8 sheets, and the samples were casted in accord to the groups and fused with porcelain, metal-ceramic bond strength was evaluated, the value of the 4 groups in bound strength were 54.65±2.38MPa,45.96±2.37 MPa,38.46±3.95 MPa,31.43±2.69 MPa. The bond strength values for the specimens 100%new metal are significantly higher than those for the specimens once-cast alloy. There were significant different (PO.05). It can improve the metal-ceramic bond strength by adding new alloys to the reuse surplus alloy.3.Samples of 4 groups were heated from room temperature to 100°C,200oC,300°C,400oC,500°C,600oC respectively in order to measure the values of thermal expansion coefficient(TEC).The TEC values of 4 groups are 13.57 xl(T6/°C, 13.53 xl(T6/oC, 13.61 xl(T6/°C,13.39xl(r6/0C,while the TEC value of Vita 95 body porcelain are 12.7xl(T6/oC.The differences of alloy1 TEC with...
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