Transformation of orthodontics bracket geometry in metal injection molding process

Abstract

Orthodontic Bracket is a complex component; Therefore, it requires manufacturing process of a high accuracy like metal injection molding. Metal injection molding consists of three main steps. At first, the feedstock which is a mixture of steel powders and polymer is injected into a mold to form the desired design. Then the molded part is unbound to remove the polymer from the part, this process results in an absence of polymer that generates porosity. Furthermore, the unbound part is sintered to densify steel particles through capillarity force, this process is also the cause of part shrinkage that leads to a geometrical change. The bracket consists of three sections namely the geometrical functions that are affected by the sintering process, they are: wing, trunk, and base. The wing section works as a hook to hold the orthodontic ring, the trunk section supports all the bracket sections, while the base section provides surface contact with orthodontic adhesive. Each section of the bracket has its specific function namely to attach object with different geometries and to withstand a certain load. The functions will not work properly if during the sintering process the geometrical changes caused by shrinkage occurs. This paper presents transformation analysis of orthodontic bracket geometric transformation in the MIM process. In the case of wing geometry, the shrinkage of the outer part of the arch is greater than that of the inner part resulting the reduction on its slope from 35,01° to 32,11°. In the case of square geometry, the corners shrink greater than the edge side so that the square getting rounded, especially the bottom surface of the bracket that the radius of convexity reduced from 43.51 mm to 19.23 mm. The effects of shrinkage due to sintering have less impact on the trunk in the middle of the bracket that is equal to 7.23% on x-axis and 4.994% on y-axis while the wings section shrink by 10.98% on x-axis and -6.637% (elongated) on y-axis. In conclusion, the changes of size and shape are different from each section, depend on the section’s geometry, the sintering tends to effect more on the outer side of geometry. The disproportion of shrinkage will result in overall shape change in the manufacturing outcome. On the wing of the bracket, there will be a thinning and a slope reducing that will increase the risk of fracture and can not refute the orthodontic rubber well. On the base, occurs the rounding that will reduce the surface contact on the adhesive that attaches the bracket to the teeth. To adapt to the geometrical change, the first design must be larger, and the part’s faces must be designed to be more concave.