| Micro-extrusion technique of plastic melt belongs to micro-extrusion technique and is extensive used in Fused Deposition Modeling (FDM). Rheological behavior of plastic melt in micro-extrusion based on screw spray head of FDM will ultimately affect the quality and precision of products. Flow rules of melt in the spray head and evolution of melt leaving the spray head will be mastered and theoretical support for optimized design of screw and spray head will be provided through the research of micro extrusion spray head.Existing micro-extrusion technology and its spray head structures are discussed firstly, from which a concept of plastic micro-extrusion is proposed. Secondly, combining with experiment, theoretical inference and simulation analysis by PLOYFLOW, a deep research is made on the extrusion process of one particular screw spray head of FDM. In order to find the factors of impacting extrusion capability and stability, we analysis the distribution of melt velocity and pressure fields in the screw spray head, evolution of melt leaving the spray head. To search for the key parameters of impacting extrusion velocity and to achieve optimal design of micro-screw structure, we analysis several geometry parameters of micro-screw, which include helix angle, the effective length of the screw, screw groove depth, screw flight width and clearance between screw flight and machine barrel.After the experimental research, theoretical analysis and simulation analysis by PLOYFLOW, main conclusions in the thesis are as follows:(1) Study on melt rheological activities in original laboratory equipment shows that: Though melt flow rate increases to a certain degree as speed grows, it has basically no relationship with temperature. Pressure difference decreases as temperature increasing. The increasing of speed will also lead to the growth of pressure difference. The flow that near the barrel wall flows slowly in the nozzle, also there are molten material resort areas, nozzle pressure lose the most in the die, which occupies more than half of the whole nozzle pressure loss.(2) Research by orthogonal simulation indicates that: Among several geometric parameters that impacting on extrusion capability, extreme difference descending order is D>C>B>E>A (A—Helix angle, B—Screw effective length, C—Screw groove depth, D—screw groove width, E—Clearance between screw flight and machine barrel), which means fractional change of screw groove depth and screw groove width would have greater influence on extrusion capability. After further analysis on helix angle and screw groove width, a conclusion appears: while helix angle maintain at 10.8°to 14.3°, extrusion capability decrease with its increase. Increase with the growth of screw groove width. Finally the best solutions that are Helix angle at 10.8°, screw effective length at 60mm, screw groove width at 2.8mm and screw groove depth at 1mm is achieved.(3) Different nozzle types and melt flow conditions in different die length are studied by simulation analysis, which indicates that: Among all of the nozzle pressure loss, the pressure loss of die part is more obvious, and the longer of a die, the greater of the loss. Refers to melt in a nozzle, the greater convergence of a nozzle, the greater maximum speed of the melt in the nozzle, and the much more likely to cause instability when it pass through a die.(4) Based on existing equipment, new laboratory equipment is optimal designed by simulation results. Furthermore, the test results are compared with results from simulation results, it turns out that shows some deviations. The causes of deviations are analyzed finally.
|
PDF Full Text Request