Study On Cold And Hot Upsetting Behavior Of Polymer

The solid-state forming of polymers was developed, in comparison with press working and powder forming of metals, as a special method focusing on the forming of polymer products. It followed the fast development of plastic industries during the 1950s. But it is seen that there isn't successful and practical forging method that can be utilized to make the polymer mechanical parts with self reinforcement, high modulus and high-strength. So during the several decades, researches in this field are experimental, which can't show the practicability in the field of manufacturing polymer parts.In this paper, upsetting, the most basic workstage and deformation form in forging polymer,was picked out; choosing five polymers, using loading compression ratios (deformation degree), deformation temperatures and speeds as parameters, designing and making the temperature control box and punches for these experiments. Through test detecting and data analysis, cold and hot upsetting behaviors of polymer were revealed, focusing on the spring back after removing load in upsetting; The data basics was made up in order to recognize and understand the most difficult problem—"shape fixability" in forging polymer.The results were obtained and problems, which needed to be further discussed, were discovered as follows:(1) Ductility isn't the essential property of polymer, it has relations not only with heating temperatures, but also with stress-strain state and other exterior technological factors.(2) The stress state can be described by tensor. The spheric stess tensor(hydrostatic pressure) is able to compress the free-volume inner polymers without destroying the strength of major chemical bonds inner big molecules. And the macroscopic shape change of polymer is produced by the shear effects resulting from deviatoric stress tensor showing as responds of strain. So under complex stress state, excepting for tensile testing, especially those which compressive stress plays the leading role, the ability of big ductile deformation can be found in polymers. Thus, the difference of "Thoughness" and "Brittleness" during tensile testing can not be found. But now it is in lack of research for the deformation model and mechanism of big molecules under complex stress state.(3) The "phase structure-condensed state" of solid-state polymers is important to analyze ductile deformation mechanism and springback, and it's physical basics for designing the forging temperatures of polymers. Thus, the deformation significance relating to "phase structure-condensed state" is described under different temperatures based on five polymers.(4) The develop of tensile oriented structure is geometrically free of space restraint. But the oriented structures appearing during cold and hot upsetting are restrainted within finite dimension extent, which is the result of shearing strain depending on deviatoric tensor. And the develop mechanisms of these structures and their effects to polymer properties is needed to be studied.(5) The stress-strain curve of cold upsetting might be changed before the disappearing of elastic modulus during hot upsetting. For example, the strain hardening phenomenon of PMMA isn't observed in cold upsetting, but during the hot upsetting with high elastic state under low temperature. For the above phenomenon this paper could not explain yet.(6) There are four plasticity-like behaviours such as "the freeze-strain after forced high elastic deformation", "the residual strain disappearing with ageing after load removal during high elastic deformation", "crystal structure melting after its deformation". "residual strain after leather state(crystal solid phase and non-crystal liquid phase) deformation", during solid state deformations of polymer. After the load is removed, the recoveries without these plasticity-like behaviours is called "essential springback". More widely, the "essential springback" is the sums of instantaneous and ageing recoveries after load removals when the stress exceed the potential hills of polymer chains causing conformation to change and the chains to move, belong to big molecules chains'softness and natural relaxation characteristic, i.e. the sums of recoveries of forced high elastic deformation, high elastic deformation, crystal structural deformation, leather state deformation under complex "phase structure-condensed state" of big molecules after load removals and with stress relaxation, the value of which has great effects on shape fixability of polymers.(7) The relaxation time when stress and strain is balanced in cold upsetting is longer than hot upsetting. This paper could only explain it with hot compressing temperature, which perhaps promotes the balance of stress and strain.(8) Because there are big springbackes in cold upsetting, it is recognized that the shape fixability of cold open-die forging is very poor for polymers, which means it is hard to control the recovery to the required precision. Thus there is a lack of practical values for this method. The suggestion is needed to be severely discussed, with which cold embossing is applied to micro-forming for polymers.(9) The recovery ratio of upsetting increases as the deformation upgrades, because the number and structural units of polymer joining the deformation, accumulated pressure energy, systemtic entropy, the trend of natural relaxation of big molecules all increase.(10) The springback would be effectively decreased using hot upsetting compared with cold upsetting, which is the result from "temperature soften effects" based on mechanics principles and the activation of big molecules' motion based on thermodynamics principles in the procedures of heating. Besides according to the significance of "phase structure-condensed state", it is the result from the change of non-crystal structures from "solid fluents" to "true liquids" during higher than T11 (secondary transform temperature), and with the change of crystal structures from "crystal solid state" to "half melt solid state" during higher than Tac (pre-melting temperature).(11) The deformation springback can be reduced by improving deformation speed, whether in hot or cold upsetting. For cold upsetting, this effect mainly results from three factors as follows:increase of the yield stress by increasing speed, the segments' motion or big molecules' slips motivated by deformation speed, and the compression of polymer's free volume by stress spheric tensor. But for hot upsetting deformation, fast distortion composed with temperature effect will produce the "speed glutinate effect", which not only reduce the deformation springback, but also may transform the deformation springback into the "glutinosity deformation contraction".(12) The shape fixability of solid-state polymers'forging relates to deformation springback, but scale and precision can not be guaranteed without designs in advance. So whether the shape stability is good or not depends on the springback range and control of it. From this paper's testing and analysis, it can be seen that:the recovery ratio of non-crystal polymer PMMA and ABS are smaller than 3% and 5% each, the magnitude of the absolute springback value is 10-1 mm, which is easy for designing to be made up of, thus having good shape fixability; crystal polymer PP, PA6, and PTFE, the average springback ratio is bigger than 10%, the absolute springback value reacher mm leval(less than 10mm), which is hard for designing to be made up of, thus having bad shape fixability, and the reasons are to be further explored.