| With the great development of micro-processing technology, many products become more and more light, small, tiny and exact. Simultaneously, traditional polymer extrusion molding is turning to micromation, and the polymer micro-extrusion molding appeared. Some tiny and slim tubes such as interventional medical catheters, automobile oil and gas tubes, new type of plastic optical fibers are used widely in the field of medical treatment, automobile, and communication. And the interventional medical catheters with high technical content, much additional value have enormous room for medical treatment. The characters of the interventional medical catheters are tiny size, complicated section and high geometry accuracy, so the design, manufacture, and extrusion molding process of a micro-extrusion die are very complicated, while the traditional polymer extrusion molding technology is inapplicable. Just because of the limit factors like micro-extrusion dies and matched manufacture process, the interventional medical catheters and micro-extrusion dies in our nation mainly depend on importation at present, and the price is quite expensive. This situation makes against the development of our interventional medicine. So this paper is focused on the micro-extrusion molding of polypropylene micro-tubes, and the key technology which is about the rheological design, manufactural method and micro-estrusion molding process of polymer micro-estrusion dies in the condition of micro-scale effect is also studied. The primary items in this paper are as follows:Firstly, based on the lumen-sectional shape of micro-tubes and the lumen distribution characteristics on the cross-section of the product, the micro-tubes were divided into three types, such as centrosymmetric, axisymmetric and uniformly distributed. And from simple to complex in turn, single-lumen micro-tubes, double-lumen micro-tubes with different shape lumens and five-lumen micro-tubes with different diameter lumens were listed and researched as the most representative products of these three types in the paper. Considering the land length of the polymer micro-extrusion die was the key to the quality of products and the productive efficiency, single-lumen, double-lumen and five-lumen micro-tubes were taken as objects. The parameter relationships between the land length and the flow channel structure, the rheological characters of polymer melts were deduced by a theory and the land lengths of the three-type micro-extrusion dies were also fixed. Then, for the extrusion dies of multi-lumen micro-tubes, combined the flow analysis of polymer melts with the rheological theory and the optimum design method of polymer melts, a new design method for the right-angle non-symmetry flow channel structure of the micro-extrusion die was put forward. In this method, the flow balance of polymer melts in the non-symmetry flow channel of the right-angle micro-extrusion die was seen as the optimization objective; the steering angle in the extrusion die was taken as a design variable; and the objective function of optimizing the flow channel structure was established. Through the optimization of asymmetric flow channel in the extrusion dies for double-lumen and five-lumen micro-tubes, the optimization design method of the multi-lumen extrusion die was gradually improved.Secondly, because the lands and air injection holes of the mandrel were very tiny, easily broken and the cross-section structures of the lands were very complex especially for multi-lumen micro-extrusion dies, the precision machining together with micro EDM technology were adopted to process the lands of the mandrel and the die. The cross-section of the mandrel land was processed by once corroding technology of the molding electrode end side. At the same time, the vent with big deep diameter ratio in the mandrel was processed by multi-step punching which combined metal drilling with micro EDM molding and punching. In the course of the assembly of the micro-extrusion die for polymer melts, the assembly error easily leads to asymmetry of the assembly interspace between the lands of the mandrel and the die or leads to difficulty of the extrusive molding. The assembly technics of the mandrel and the die and of the vent pipe and micro adjustable system was studied in this paper. The micro-extrusion die after assembly could do a fine adjustment between the mandrel and made the die not bend or break the land of the mandrel. And also, the vent pipe could be airproofed with high temperature.Finally, the single factor micro-extrusion molding experiments of single-lumen, double-lumen, and five-lumen micro-tubes were studied and analyzed theoretically because the tubes with fine diameter, complex section, thin wall thickness were difficult to be extruded. Through extrusion experiments, the effect order of main process parameters on the size and shape accuracy of micro-tubes and the optimum combination of process parameters were obtained. Experimental results showed that the air injection velocity effected fiercely and decided directly on the size and shape accuracy of micro-tubes. At the same time, the right-angle extrusion die with the asymmetrical channel for the multi-lumen micro tubes was designed by the method. According to the experiments for the single-lumen micro-tubes, the combination of process parameters was determined including an air injection velocity of6ml/min, a screw speed of4r/min, a die temperature of202℃and a pulling speed of28m/min, and a single-lumen micro-tube of the smallest diameter with0.5±0.02mm was fabricated. On the basis of the the experiments for the double-lumen micro-tubes, the optimal combination of process parameters was determined including an air injection velocity of6ml/min, a screw speed of12r/min, a die temperature of218"C, a vacuum degree of the water tank of20%and a pulling speed of12m/min, and the better double-lumen micro-tube of the smallest diameter with1.2±0.02mm was obtained. And the ovalities of outside profile, crescent lumen and circular lumen in the tube section were achieved by0.8%,1.4%and3.0%respectively under the optimal combination of process parameters. According to the experiments for the five-lumen micro-tubes, the optimal combination of process parameters was determined including an air injection velocity of6ml/min, a screw speed of16r/min, a die temperature of202℃, a vacuum degree of the water tank of0and a pulling speed of10m/min, and the rational five-lumen micro-tube of the smallest diameter with1.6±0.02mm was machined. The ovalities of outside profile, main lumen and secondary lumen in the tube section were reached by2.88%,2.09%and3.9%respectively under the optimal combination of process parameters. Through the extrusion experiments, the correctness of the die design method was verified. |
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