Page 11 - ADITI 2018 Newsletter - Volume 2
P. 11
then fired at a temperature over 1100°C to remove transformed into ''brown parts,'' preserving the same
the remaining wax and to strengthen the ceramic size with a quite porous structure. The final stage of
mold. Before the metal is poured into the ceramic the MIM process is sintering, which is performed in a
mold or “shell”, the mold is preheated to a specific high-temperature furnace under vacuum or a
temperature to prevent the molten alloy from controlled atmosphere. In this stage the residual
solidifying or “freezing off” before the entire mold is binder is removed, and at the end of the process the
filled. Then the molten metal is poured at a parts have shrunk by 17–22%, reaching the precise
temperature of over 1600°C into the ceramic mold. desired dimensions. In certain cases, thermal or
The next step is the cooling process. The natural surface treatments are also required.
cooling process is carried out at temperature of
25°C, after which the ceramic mold is broken away. Fig. 3 various steps in bracket manufacturing using MIM technique
Ceramic mold removal can also be accomplished
chemically, using a heated caustic solution of either After the sintered bracket is obtained mesh pad is
potassium hydroxide or sodium hydroxide, but this added to the base of bracket. Mesh pad are made of
approach is being phased out due to environmental laminating stainless steel wires of different diameter
and health concerns. Then, the orthodontic bracket and configuration. The size of the wire mesh used in
is cut from the tree form. A chemical cleaning manufacturing of various single mesh type bases are
process removes the remaining ceramic slurry, using 40, 60, 80 and 100 meshes. The finest mesh that can
a HF Chemical solution, followed by an ultrasonic be used on metal bracket is 100 gauge, which can
cleaning process to achieve the final smooth casting accommodate up to 155-micron particle size of filler
result. The last stage is the geometric and surface present in the orthodontic adhesive. Spot welding and
roughness analysis, conducted by using a digital brazing are the methods by which mesh can be
microscope. attached to the bracket base.
In spot welding the strands within the mesh are
Fig 2 Various Steps in investment casting welded to each other and to the base of the bracket.
Spot welding process damage the mesh base by
Metal injection molding (MIM) creating flat areas where the welds occurred. These
In this process, metal powders with particle sizes of a flat areas create voids in the base adhesive interface
few microns are mixed with organic binders that are potential areas where brackets can detach.
(typically, wax, thermoplastic resins, and other Additionally, the spot-welded areas create metal
materials), lubricants until a homogeneous mixture is spurs that prevent complete seating of the bracket. It
obtained. Injection of the feedstock is done using an also creates an area of stress concentration by
injection molding machine similar to those used in decreasing the area available for retention which may
the plastics industry. The injected parts, called initiate fracture of the adhesive at adhesive-bracket
''green parts,'' are formed into the desired geometry base interface. These shortcomings are overcome
but at 17–22% oversize to compensate shrinkage when mesh base is united with brackets via brazing
after sintering. The next procedure is the instead of spot welding.
''debinding,'' which is used to remove at least 90% of Brazing: Brazing is a process where metal parts are
the organic binder from green parts by heat, solvent, joined together by melting a filler metal between them
or both (Fig. 3). The green parts have now been at a temperature below the solidus temperature of the
metal being joined and the melting point of the filler is
above 840 degree F. Brazing of the mesh strands
instead of spot welding does not flatten the wires. The
brazing layer contains a combination of silver, gold or
