Study On Forming Theory And Practice Of Continuously Extruding Pure UHMWPE Pipes With The Method Of Forming In Machine Barrel On Single Screw Extruders

Ultra high molecular weight polyethylene(UHMWPE)is a kind of thermoplastic engineering plastic,which has a linear structure,with outstanding comprehensive properties.Because of its super high molecular weight,it takes on a series of outstanding properties,which no other types of plastic can be compared with,it has been called“amazing plastic" at home and abroad.On many occasions,it can substitute such materials as carbon steel,stainless steel,and bronze,it can be used in many fields such as machine,transportation,weaving,paper making,agriculture,foodstuff packing,coal mining,porcelain industry,chemical industry,etc,so it can have a very expansive application foreground.But compared with other types of thermoplastic engineering plastic,a main deficiency of UHMWPE is that it is unable to be formed by general processing equipments for plastics.For the reason that under the condition of being melted this type of ultra high molecular weight polymer only behaves a high elasticity state,whereas behaves hardly any viscous flow state which common polymers always take on,it can not be thus processed with traditional forming methods,such as extruding method and so on,which are developed for the common polymers with the viscous flow state.This is a main technical obstacle to direct wide application of pure UHMWPE products.In order to overcome the difficulty in forming UHMWPE,a common method at present is to modify pure UHMWPE raw material with such low molecular polymers as general polyethylene,and therefore to obtain lower viscidity and much better fluidity of the molten modified UHMWPE,which can be extruded in modified traditional equipments.But this measure is acquired at the cost of sacrificing some certain excellent properties which are unique characteristics of UHMWPE thanks to its unique ultra high molecular weight,and of increasing cost of production.As a matter of fact,this measure with having no choice but to do that isn't an advance in technology.It can be said that at present,the technology of extruding pure UHMWPE on single screw extruders still needs a further breakthrough.In order to conquer the cosmopolitan difficult problem of forming pure UHMWPE,this dissertation tried to find some other way that is on the base of UHMWPE being provided with extrudability in its high elasticity state to imagine a possibility of forming pure UHMWPE pipes in machine barrel on a single screw extruder.It constructed an original conception of a new method of forming in machine barrel and expatiated on technological characteristics of the method.Based on the conception of the method of forming in machine barrel,this dissertation considered that in order to ensure success in the method,one of the keys is to ensure that the extruding system can produce sufficient extrusion pressure.On the basis of characteristic of UHMWPE which under the condition of being melted only behaves a high elasticity state,whereas behaves hardly any viscous flow state which common polymers always take on,this dissertation broke through traditional modes of theoretical analyses for forming process of common polymers with the viscous flow state.According to the classical solid transportation theory,on the basis of static equilibrium of solid friction,it relatively completely constructed a theory of pressure formation during pure UHMWPE raw material was being extruded in machine barrel on a single screw extruder.As the mainbody of the pressure formation theory,a mathematical model of pressure formation on each segment of the extruder was derived in detail,so that a total extrusion pressure formula was obtained to describe the pressure formation when pure UHMWPE raw material got to the screw's head of the machine.At the same time,from the angle of forming process,it was put forward firstly that the direct reason,why pure UHMWPE raw material is unable to be extruded normally on traditional single extruders,is that the extrusion pressure is not high enough.Another key to ensure success in the method of forming in machine barrel on single screw extruders is to ensure that the melting process is enough and reasonable.According to the forming rule of the method of forming in machine barrel,this dissertation analyzed in detail the melting characteristics of pure UHMWPE raw material during it was being extruded in machine barrel on a single screw extruder.On the basis of heat transfer principle,a physical model of the melting process of pure UHMWPE raw material with the method of forming in machine barrel on single screw extruders was constructed.And further,by making use of finite difference method,a corresponding mathematical model,criterions and standards for judging and analyzing calculated data were built also.They formed together a melting theory of extruding pure UHMWPE in machine barrel on a single screw extruder.Based on the above-mentioned theory of extrusion pressure formation and melting theory,this dissertation has successfully developed a set of special purpose CAD software to carry out the computer aided design for the single screw extruder with the method of forming in machine barrel.Under the condition that the performance parameters of pure UHMWPE raw material and the structure parameters of the specific extruder were given,on the one hand,this software could calculate extrusion pressure in each segment of the extruder,display every tendency charts of the extrusion pressure and a tendency chart of total extrusion pressure during the process of pure UHMWPE raw material was being extruded.in the extrude.These charts were used for judgment upon whether the extrusion pressure was appropriate.Thus,the CAD software provided means of computer aided design for optimization of structure parameters of the extruder based on extrusion pressure.On the other hand,this software could simulate temperature variation rule while pure UHMWPE raw material was being extruded in the extruder.The simulation results were used for analyses and judgment upon whether the melting process was enough and reasonable.So that the CAD software provided means of computer aided design for optimization of structure parameters of the extruder based on the melting state.In order to ensure the above-mentioned two factors,which are that extrusion pressure is high enough and that pure UHMWPE raw material is melted enough when it gets to the screw's head of the machine,to be realized,this dissertation paid special attention to the pressurization inner lining inlaid in the combined barrel of a single screw extruder with the method of forming in machine barrel.According to a relative sufficient theoretical analysis,an innovating view for the pressurization inner lining was put forward.It considered that for the pressurization inner lining if instead of opening grooves on its inner wall along the direction of the axis of the screw,several spiral grooves are opened on its inner wall,the heat of internal friction from pure UHMWPE raw material could be reduced efficiently.Traditional modus operandi is to open grooves on an inner lining's inner wall along the direction of the axis of the screw.For the condition of the least effective difference of velocity to be realized,on the basis of particular theoretical analyses of forces acting on an elementary solid plug of the pure UHMWPE and of corresponding velocities,a mathematical model of the optimized spiral angles of the pressurization inner lining was given.Thus,a rationale of reasonable design for the pressurization inner lining was provided.Based on the theory of extrusion pressure formation and the melting theory,this dissertation used the special purpose CAD software above-mentioned to design and develop a special single screw extruding mechanism,which has a special combined barrel with spiral grooves and a multiplex screw with equal screw pitch and changed height of spiral edge.Through the mechanism,a general ST45-25 single screw extruder has been successfully reconstructed into a special single screw extruder with the method of forming in machine barrel.The advanced extruder was used for continuously extruding samples of pure UHMWPE pipes which raw material was pure UHMWPE powder.Consequently,from the experimental angle,the feasibility of the method of forming in machine barrel as well as the accuracy of the correlative theories has been demonstrated.Further,this dissertation took tow kinds of specimen from respectively the pure UHMWPE pipes prepared by the method of forming in machine barrel and molding pressed pure UHMWPE blocks to carry out sliding wear tests for comparison of friction and wear properties between the tow ones made by two different techniques respectively.Test results showed that under the condition of dry friction and paired against steel,friction coefficient of the specimens from pure UHMWPE pipes prepared by the method of forming in machine barrel not only lay in the normal range entirely,but also less than that of the specimens from molding pressed pure UHMWPE blocks.SEM analyses demonstrated that the worn surface of the specimens from pure UHMWPE pipes prepared by the method of forming in machine barrel appeared lighter than that of the specimens from molding pressed pure UHMWPE blocks.The reason why tribological performance of pure UHMWPE pipes extruded by the method of forming in machine barrel is better than that of molding pressed pure UHMWPE blocks should be found in the unique forming process of the method of forming in machine barrel,which does give an excellent tribological characteristic to pure UHMWPE products.The tribological test results proved the advanced technical nature of the method of forming in machine barrel.The method of forming in machine barrel differs from methods of traditional single screw extruding in not only forming technique,but also forming theory.It might be one of the optional ways to solve the cosmospolitan difficult problem of forming pure UHMWPE pipes.The new theory and method could be further expanded to meet the requirement of extruding pure UHMWPE sheet and bar on single screw extruders.