Fabrication and Analysis of Plastic Hypodermic Needles by Micro Injection Molding
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This thesis explores the analysis and fabrication of plastic hypodermic needles. The hypotheses for this work are that replacing metal hypodermic needles with plastic ones will reduce or eliminate the possibility of the second-hand infections from needle sticks and unsterlized reuse and will be more cost and time efficient to recycle. The most critical structural failure mode for plastic needles is buckling due to their shape (thin walled hollow column). The consideration of buckling is critical to avoid structural failure and to ensure reliability for medical applications. The buckling strength of a cannula is analyzed by analytic (Euler buckling theory) and finite element analysis (FEA) methods. A 22 gage needle model (OD 0.7mm, ID 0.4mm, Length 12.7mm) was analyzed. Euler buckling theory was used to calculate the critical buckling load. Numerical approaches using finite element analyses showed very similar results with analytic results. A skin model was introduced to simulate boundary conditions in the numerical approaches. To verify the results of the analyses, cannulas with the same cross-sectional dimensions were fabricated using a micro injection molding technique. To make the parts hollow, a core assembly of straightened wire was used. Using the tip of a 22 gage needle, cannulas with the inverse shape of an actual hypodermic needle were made. The structural (buckling) characteristics of cannulas were measured by a force-displacement testing machine. When buckling occurred, an arch shape was visible and there was an abrupt change in the load plot. Test results showed the relation between the needles length and the buckling load, which was similar to that predicted by Euler buckling theory. However, test values were 60% of the theoretical or analytical results. Several reasons to explain these discrepancies can be found. The first is that an unexpected bending moment resulted from an eccentric loading due to installation off-center to the center of the testing machine or to the oblique insertion. A cannula that was initially bent during ejection from the mold can add an unexpected bending moment. The quality control of cannulas can be another reason. Bent or misaligned core wires produce eccentric cannulas, and the thinner wall section can buckle or initiate fracture more easily. The last reason may be that Euler buckling theory is not fully valid in short cannula, because the axial stress reaches yield stress before buckling occurs. Inelastic deformation occurs (i.e., the modulus is reduced) during compression in short cannula. The Johnson column formula is introduced to explain this situation. Especially for the nylon nanocomposite material tested, a loss in modulus due to moisture absorption may be another explanation for the discrepancies.