Interference fit of material extrusion parts

Material extrusion is an Additive Manufacturing process able to fabricate a physical object directly from a virtual model using layer by layer deposition of a thermoplastic filament extruded by a nozzle. The fabrication of functional components implies the need for the assembly with other parts with different properties in terms of material and surface quality. One of the most used assembly method involving plastic materials is the interference fit. It consists of fastening elements in which the two parts are pushed together, by means of a fit force, and no other fastener is necessary. It requires the accurate design of the interference, typically carried out by the designers through diagrams and theoretical formulations supplied by the material manufacturers. At present no theory has been provided for material extrusion parts due to the anisotropic behavior: the mesostructure, the surface roughness and the dimensional deviations mainly depend upon the build orientation. In this work the effects of the surface morphology and the interference grade on the assembly and disassembly forces in an interference fit joint are investigated. For the purpose, a design of experiment with a factorial plan has been carried out. The coupling behavior and the maximum forces are discussed. A new variable namely the real interference has been introduced and a relationship between this variable and the assembly force has been found. Through this model it is possible to know in advance the force necessary to assemble a material extrusion part with an assigned interference grade.