Properties of Clasps
Properties of clasps should include the following :
Strength. Should be strong enough in thin cross-section to withstand oral forces. Adding molybdenum in small amounts to cobalt chromium alloy increases its strength; however, the addition of nickel decreases its strength while increasing its ductility.
Ductility. In cobalt chromium the grains tend to be large and, therefore, there are only two to three grains across the thickness of a clasp. This reduces its ductility and it is easily broken or distorted.
Malleability. A malleable material can be worked into thinner sections; this property is of importance in wrought clasps.
Proportional limit. The limit beyond which a clasp will permanently deform or fracture.
Torsional elasticity. The rigid position of a clasp arm should be above the survey line. The torsional elasticity of the metal in the more rigid upright part provides flexibility to the horizontal arm and a more effective distribution of stress throughout the structure.
Modulus of elasticity (resilience). The higher the modulus the shallower the undercuts that can be engaged.
Appearance. A tooth-coloured clasp may be more aesthetically pleasing but may not provide other optimal properties. Often gold alloys are more aesthetically acceptable intra-orally than ‘silver’ coloured alloys.