Requirements for metal at the bottom of the porcelain
1. porcelain porcelain powder melting point must be higher than the melting point of the alloy (170-270), to ensure the metal coping Crown fused porcelain Grill without metal melting or deformation. Chinese porcelain high NI-CR metal-melting alloys, the melting point 1320 c, ceramic powder using low melting porcelain powders, and its melting point is 871-1065.
2. alloy has high flexural strength and modulus of elasticity of elastic modulus, higher, thicker metal can be done at the bottom, loading without deformation; no flexibility requires the underlying metal hardness, deformation of porcelain. Under normal circumstances, the thickness of the metal base can not be too thin, because the coating can cause shrinkage stress deformation of metal substrate expansion, metal base layer is thinner, the greater the deformation, resulting in PFM Crown does not fit in the neck, but also reduces the thermal stability of metal-ceramic interface, cooled, and prone to fracture. PFM produced should be increased when metal thickness control of PFM Crown after the cooling rate, prevent cleft in the chill.
3. metal structure of fine particle size on mechanical attachment edge makes a good seal, and increased corrosion resistance and hardness.
4. alloy easy casting, cast to fit closely with the prepared tooth, and can compensate for mechanical and physical properties change.
5. easier wetting of the alloy, and weld have a good combination of porcelain.
6. casting alloy shall have the right of casting temperature and shrinkage, and porcelain has a good combination of strength, coefficient of thermal expansion of porcelain and alloy should match.