Research On Rotor Structure Design And UHMWPE Processing Characteristics Of Continuous Mixing Extrusion System

Ultra High Molecular Weight Polyethylene(UHMWPE)has many excellent properties such as high impact resistance,self-lubrication and wear resistance,and is widely used in defense,like transportation,sports,medical and other fields.However,due to its extremely high molecular weight and extremely long molecular chain,molecular chain entanglement easily occurs inside,forming a physical entanglement network.Therefore,the melt viscosity is extremely high,the fluidity is extremely poor,and the critical shear rate is extremely low,which makes the Molding process of UHMWPE extremely difficult and has become a worldwide problem.Equipment is fundamental to the Molding process of materials and should conform to the processing characteristics of the materials.UHMWPE processed by conventional equipment still has problems such as low efficiency,large material damage,low material stability and so on.Therefore,new processing equipment design and development is urgent and it can essentially seek breakthroughs for the difficult processing of UHMWPE.Firstly,the continuous hardening extrusion equipment CM-50D with shear and stretching mixed flow field is used to process UHMWPE with molecular weight of 3.0×10~6 g/mol.According to its special melt plasticization law,it can be divided into mixed transport zone.Four characteristic areas of the compression bonding zone is the melt plasticizing zone and the melt conveying zone.The CM-50D can achieve high efficiency and continuous processing of UHMWPE,opening up a new way for its processing.However,there are still insufficient shearing force,local friction and overheating,high processing temperature,difficulty in feeding and clogging of the barrel,resulting in oxidative degradation of the material,and the molecular weight is reduced to 2.1×10~5g/mol.Compared to the molded sample,the storage modulu is reduced by an order of magnitude and the tensile strength is reduced by 34%.Therefore,according to the plasticizing characteristics of UHMWPE,it is necessary to adjust the proportion of the plasticizing function of the continuous mixing and extrusion equipment and redesign and modify the rotor structure.Secondly,according to the in-depth exploration of UHMWPE melt plasticization and the finite element simulation analysis of the mixing rotor,it is shown that reducing the proportion of mixed transport zone and compression bonding zone can significantly shorten the rotor length and reduce the thermal processing history of the material;The wedging angle of the rib and the increase of the gap between the rotor and the barrel can effectively reduce the average shearing force of the mixing rotor,reduce the phenomenon of molecular chain rupture and molecular weight reduction caused by excessive shearing of the material;According to the force analysis,change the positive-to-peer connection mode of the reverse-thread of the mixing rotor to the design of the reverse-thread rotate circumferentially by 30°into a staggered disconnection can eliminate the high-pressure region of the plasticizing space and avoid local occurrence.High shear and local friction overheating;Changing the 90°vertical at the beginning of the staggered reverse screw to a 45°tilt allows the high viscosity UHMWPE melt to be transported forward smoothly,thereby avoiding friction during material accumulation.Excessive heat generation and excessive processing temperature cause the material to undergo thermal oxidative degradation due to excessive heat history.The pre-experiment and optimization of the processing technology show that the processing temperature is reduced by about 50℃,the processing capacity is increased by 60%,and the molecular weight can reach 2.2×10~6g/mol.Finally,UHMWPE was processed and sampled by redesigned and developed special-purpose triangular low-temperature K-type double-rotor continuous-mixing extrusion equipment CM-35S-K.The test analysis showed that the processing temperature of UHMWPE was reduced by about 50℃,And the molecular weight retention rate is increased by 10 times,The entanglement molecular weight Me is reduced by 79%,the entanglement point density is increased by 52 times,the tensile strength is up to 45 MPa,and the elongation at break is up to 760%.After the design and transformation,the tensile strength of the processed samples increased by 61%and the elongation at break increased by 287%,indicating that the design and modification of the closed rotor was very successful,the processing performance was improved significantly,and the molecular weight and ultra-high performance of the material remained very good.Compared with traditional molding and torque mixing,the mechanical properties is better,the processing temperature is lower,the processing continuity and stability is better,and the processing efficiency is greatly improved.