Study On Principle Of The Bag Expansion Method And Equipment For Manufacturing Biaxially Oriented Pvc Pipes

Mechanical properties of a polymer may be considerably enhanced by stretching the polymer in the elastomeric state and thereby achieving orientation effect of the molecular chains. The tensile strength and elastic modulus of a self-reinforced pipe with a expansion rate of1.26are higher than regular pipe by30%and50-100%, as a result, about20%material can be saved. And more than30%material can be saved when the expansion ratio increases.The principle of the bag expansion of PVC-U pipes with cloth tube filled with compressed air was studied in this paper. The main parameters considered here are expansion ratio, the temperature, heating time, expanding pressure and Pressure maitaining time, and so on. Numerical calculation and a large number of experiments indicated that the proper technological parameters for continuous expansion were obtained as follows:heating temperature140℃, heating time5min, pressure0.65MPa, pressure maintaining time5s. Theoretically, the material would be heated to elastomeric state, and lower temperature, higher pressure, larger expansion ratio and quicker cooling will be benefit to gain highly oriented pipes with high performance.A continuously bag expansion equipment was innovated according to the principle of inner-pressure expansion to control the main technological parameters. Experiments of both batch inner-pressing expansion and continuously bag expansion of PVC-U pipes with cloth tube filled with compressed air were done to get series of oriented pipes. The performance testing shows that the oriented pipes have uniform appearance with larger diameter, enhanced tensile strength and elastic modulus with a little bit lower elongation ratio at break.As we know that the enhancement of the tensile strength and elastic modulus of a material is usually determined by the orientation ratio which is determined by the expansion ratio. In this paper a new method for evaluating the orientation ratio of the oriented PVC pipes by measuring the transmission speed of an ultrasonic wave through the oriented PVC pipes was introduced. Testing data shows a reseanable tendency with the speed increases as the expansion ratio enlarges.