Melting process and mechanism for vibration induced single‐screw extruder

The effect of a vibration force field on the melting process of an extruder is studied. It is shown that the mechanism for melting differs from conventional theory. Experimental studies of melting of low‐density polyethylene (LDPE) pellets in a vibration‐induced single‐screw (VISS) extruder show that melting is initiated on the inside of the barrel and the surface of screw. Models were developed that explain the melting mechanism in those regions. The melting at the surface of the screw is mainly initiated by frictional work on the pellets by the vibration and rotation of the screw. The melting action at the barrel is induced by a barrel temperature higher than the melting point and propagated by viscous dissipation heating of the melt film produced. The theory is supplemented by a calculation sample, which shows good agreement with experimental data obtained on a transparent barrel VISS (T‐VISS) extruder and a half‐open barrel VISS (H‐VISS) extruder with LDPE. The results of the experiment and calculation sample indicate that the introduction of vibration‐induced field can improve the melting capacity of extruder to a great extent. The present model enables the prediction of processing parameters for VISS extruders, from which the optimum operating conditions can be obtained. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 2504–2514, 2007     

Melting mechanism of a starved‐fed single‐screw extruder for calcium carbonate filled polyethylene

A study of starved‐fed single screw extrusion was initiated to understand the relation between its distinctive melting mechanism and the improved mixing capabilities attained during compounding of a calcium carbonate filler into HDPE. Experiments were carried out in a 63.5 mm single screw extruder, examining the effect of degree of starvation on a conventional and barrier feed screw. Interest was focused on the mixing/melting mechanism of starved‐fed solids‐conveying as it affects the size and number of filler agglomerates observed in the extrudate. The melting performance of both feed screws was examined using pressure and temperature measurements down the screw length as well as direct inspection of the polymer in the screw channel via rapid screw cooling. Both screws showed improved mixing quality with increased starvation.     

Performance study of barrier screws in the transition zone

This paper defines through a mathematical model the advantages and disadvantages of barrier screws as far as their melting and mixing performances in the transition zone are concerned. The melting analysis is based on the Tadmor's original model, and the flow in the melt channel is considered to be non-Newtonian and nonisothermal. The performance of these barrier screws is investigated for the solids channel in terms of melting rate/interface a I contact area; melting efficiency; melting length; solid bed velocity profile; and power consumption in the melt film at barrel surface. For the melt channel, their performance is investigated in terms of pressure buildup; average bulk temperature; power consumption in the melt channel and in the main flight clearance at barrel surface; and average bulk mixing. The present study confirms that the increased-pitch multichannel screw (Ingen Housz screw) outperforms clearly the other barrier screws investigated, since it gives the highest melting rate with reasonable pressure buildup in the melt channel. When compared with conventional screws, all the barrier screws examined give better melting performance.     

啮合同向双螺杆挤出过程聚合物熔融机理研究——螺纹元件组合中聚合物的熔融

借助于装有玻璃视窗的可视化啮合同向双螺杆挤出机 ,对高密度聚乙烯 (HDPE)颗粒在螺纹元件组合中的熔融过程进行了研究。结果发现 ,聚合物颗粒在螺纹元件中熔融时颗粒的变形小。在减导程螺纹元件组合中 ,聚合物颗粒熔融的主要能量来源是机筒的热传导 ,故熔融效率较低 ;在反向螺纹元件组合中 ,熔体的粘性耗散热是聚合物熔融的主要热源     

Contribution of the melting stage to the evolution of the morphology and chemical conversion of immiscible polyamide/polyethylene blends in twin‐screw extruders

This study investigates the effects of processing conditions (the screw speed, throughput, barrel temperature, and screw configuration) on the chemical conversion and morphology evolution of polyamide/polyolefin blends along a twin‐screw extruder. Polymer samples were collected rapidly at specific barrel locations with a special sampling device for subsequent chemical and morphological characterization. Increasing the screw speed or using more restrictive screw modules at the beginning of the melting zone promoted a faster reaction and better dispersion along the extruder. Increasing the throughput or decreasing the barrel temperature slowed the evolution of the morphology and chemical conversion along the extruder because of the lower melting rate. As soon as melting started, the chemical reaction took place. However, high chemical conversion rates required extensive melting, that is, significant interface generation. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Foam extrusion of LDPE with solid phase CO2 as blowing agent

Physical foaming usually requires special metering equipment to inject the blowing agent at high pressures. In this study, low density polyethylene (LDPE) was foamed in the extrusion process using dry ice as physical blowing agent. The blowing agent was metered in form of pellets over the hopper. To determine relevant process parameters and improve the understanding of the process a melting model is applied. The influence of the formation of a melt film and melt pool on the dissolution of the blowing agent is discussed. The theoretical considerations are validated in a design of experiment study. As a result, the efficiency of the process is especially affected by the throughput rate, granules temperature, and blowing agent concentration. Further varied parameters are the barrel temperature in the melting zone and diameter of the dry ice pellets. Fast melting and pressure build‐up were shown to reduce blowing agent losses. POLYM. ENG. SCI., 55:2821–2828, 2015. © 2015 Society of Plastics Engineers     

Fabrication of the self‐reinforced composites using co‐extrusion technique

The results of this work relate to the use of co‐extrusion technology in the preparation of monocomposite pellets. The low‐melting polypropylene copolymer was used as a matrix material. The high strength polypropylene fibers were used as a fibrous reinforcement. Research confirms the possibility to produce the pellets with fibrous structure. The prepared composite material in the form of pellets was processed and shaped using the injection molding technology. Obtained samples were subjected to mechanical testing in the static tensile test and dynamic mechanical analysis. Research complements microscopic observation of scanning electron microscopy. The measurement results confirm the reinforcing effect of the fibers. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 41180.     

3‐D numerical analysis on the mixing performance for assemblies with filled zone of right‐handed and left‐handed double‐flighted screws and kneading blocks in twin‐screw extruders

We have calculated and visualized numerically the mixing performances of four kinds of assemblies in twin‐screw extruders that were composed of right‐handed or left‐handed double‐flighted full flight screws and neutral or left‐handed stagger angle kneading blocks, taking into consideration industrial usage. We have found that the mixing performance of a kneading block interposed between full flighted screws is strongly influenced by the flow of full flighted screws, in particular that of a neutral kneading block. Furthermore, we have proposed the simple mixing indices, which could describe both uniform mixing and heterogeneous mixing, and also applied them to our marker tracking results of four kinds of assemblies. We found that more uniform mixing is obtained for the assembly that has a neutral kneading block before the right‐handed full flight screw, and a more enhanced heterogeneous mixing is obtained for the assembly that has a left‐handed kneading block before the left‐handed full flight screw. The reason for the latter mixing ability will relate to the quasi‐channels of the left‐handed kneading block, through which marker clusters flow while elongating.     

A two‐zone melting model for polymer blends in a batch mixer

The rate of the morphology development of immiscible polymer blends is closely related to the rates of melting of the polymer constituents. A two‐zone model was developed to describe the rate of melting of the polymer pellets of mixtures of two semicrystalline polymers in an internal batch mixer. The mixing chamber was divided into two zones: high temperature and high shear zone, and low temperature and low shear zone. Melting was assumed to occur mainly in the first zone. Moreover, pellets were assumed to be spherical, and melting proceeded from the outersurface to the core in a gradual manner. For a given pellet, the rate of melting depended, among other things, upon the number of passes through the high shear and high temperature zone and the time it experiences therein. The validity of the model was confirmed experimentally in terms of the number and weight fraction of solid pellets as a function of mixing time.     

A Miniature Device for Shock Initiation of Hexanitrostilbene by High‐Speed Flyer

A miniature device for shock initiation of the hexanitrostilbene (HNS) through micro‐charge detonation‐driven flyer was fabricated. This device consisted of the substrate, micro‐charge, flyer, and barrel. Four types of flyer (titanium of 28 μm, aluminum of 22 μm, copper of 22 μm and polyimide (PI) of 55 μm in thickness) were studied and the effect of micro‐charge thickness, diameter, and barrel length were investigated by measuring the average flyer velocities using polyvinylidene fluoride (PVDF) films. The results show that the titanium flyer is more proper for such initiation device compared to aluminum, copper, and polyimide flyer. The average velocity of the flyer increased with the thickness of micro‐charge and the increment was larger when the thickness increases from 0.3 mm to 0.4 mm than when the thickness increases from 0.4 mm to 0.6 mm. The flyer velocity significantly increased with the increase in the diameter of micro‐charge until a plateau appeared at 0.8 mm. The flyer velocity increased first and then decreased sharply with the increase in barrel length. The average velocity for a 28 μm thick titanium flyer was measured to be as high as 2468 m s⁻¹ when the thickness, micro‐charge diameter and the length of barrel were 0.6 mm, 0.8 mm and 659 μm, respectively. The HNS‐IV explosive with density 1.57 g cm⁻³ was initiated by this miniature device.     

Chain extension behavior and thermo‐mechanical properties of polyamide 6 chemically modified with 1,1′‐carbonyl‐bis‐caprolactam

A chain‐extender based on 1,1′‐Carbonyl‐Bis‐Caprolactam (CBC) was melt compounded with a commercial Polyamide 6 (PA6) by using a twin screw extruder. Rheological, thermal, and mechanical tests were performed on the resulting materials to evaluate the chain extension capability of CBC. Rheological tests on the compounded pellets and relative viscosity measurements on solubilized samples evidenced an increase of the viscosity values with the chain‐extender amount, while the opposite trend was determined increasing the high temperature residence time. Terminal group analysis confirmed the increase of the molecular weight with the CBC content and highlighted a preferential reactivity of the chain‐extender with aminic end groups. Differential scanning calorimetry (DSC) tests showed how both melting and relative crystallinity of the up‐graded samples decreased with the CBC amount. Elastic and yield properties of chain‐extended PA6 were similar to those of neat PA6 grades at different molecular weight, while crystallinity drop due to chain extension was responsible of an increase of the strain at break values. POLYM. ENG. SCI., 54:158–165, 2014. © 2013 Society of Plastics Engineers     

Morphology of polymer blends in the melting section of co‐rotating twin screw extruders

The properties of polymer blends are largely determined by the morphological structure of the polymer combinations that are involved. In terms of extruder design, this means it is necessary to have models available for estimating the development of the morphology over the length of the screws. Since significant morphological changes are observed in the melting section, in particular, is it necessary to analyze not only the plasticizing process for binary material combinations but also the initial formation and further development of the morphology in this section of the extruder. In the framework of this study, experimental investigations were conducted into polypropylene/polyamide 6 (PP/PA6) blends with small components (by weight) of the disperse PA phase. Apart from varying the process conditions of screw speed and throughput, the viscosity ratio was also varied through the use of two different PP grades. The degree of melting and the development of the morphology over the length of the screws were determined for the individual tests. The study of blend morphology in the melting section reveals key findings that must be taken into account for modeling the initial formation and further development of the morphology. It is very clear that, on the second component, which melts at higher temperatures, a kind of melt film removal occurs at the surface of the granules as they melt. The drops of second component in the melting section, which are directly adjacent to components that have not yet fully melted in some cases, have already assumed dimensions (in the μm range) similar to those that are seen at the end of the extrusion process. This means that, in the melting section of the twin‐screw extruder, no volumes become detached from or are worn off the already‐molten granule surfaces. An evaluation of scanning electron micrographs also shows that, in the melting section of co‐rotating twin‐screw extruders, virtually all the degradation mechanisms that can essentially be distinguished, such as quasi‐steady drop breakup, folding, end pinching and decomposition through capillary instabilities, take place in parallel.     

X‐ray tomography of feed‐to‐glass transition of simulated borosilicate waste glasses

High‐level waste feed composition affects the overall melting rate by influencing the chemical, thermophysical, and morphological properties of a cold cap layer that floats on the molten glass where most feed‐to‐glass reactions occur. Data from X‐ray computed tomography imaging of melting pellets comprised of a simulated high‐aluminum feed reveal the morphology of bubbles, known as the primary foam, for various feed compositions at temperatures between 600°C and 1040°C. These feeds were formulated to make glasses with viscosities ranging from 0.5 to 9.5 Pa s at 1150°C, which was accomplished by changing the SiO₂/(B₂O₃+Na₂O+Li₂O) ratio in the final glass. Pellet dimensions and profile area, average and maximum bubble areas, bubble diameter, and void fraction were evaluated. The feed viscosity strongly affects the onset of the primary foaming and the foam collapse temperature. Despite the decreasing amount of gas‐evolving components (Li₂CO₃, H₃BO₃, and Na₂CO₃), as the feed viscosity increases, the measured foam expansion rate does not decrease. This suggests that the primary foaming is not only affected by changes in the primary melt viscosity but also by the compositional reaction kinetic effects. The temperature‐dependent foam morphological data will be used to inform cold cap model development for a high‐level radioactive waste glass melter.     

Visual analysis of plastication of long‐glass fiber‐reinforced resins using a glass‐insert heating cylinder

The fiber‐reinforced plastication processes during the injection molding of reinforced resins with long fibers are difficult to observe through visualization cylinders because of possible obstacles, such as (1) opacification of melts, (2) abrasion on the inner surface of glass windows, and (3) breakage of glass windows under high pressures. In this study, we visualized the plastication process of long‐glass fiber‐reinforced resins containing 50 wt% fibers. In plastication experiments using three full‐flight screws with different compression ratios, we observed the instability of the melting process, the generation of solid beds and melt pools, and the breakup of melted segments. We demonstrated that the plastication process depends considerably on the compression ratio and clarified the characteristic molding phenomena related to fiber breakup. Based on our observations, we proposed a model that explains the different fiber breakage distributions at different compression ratios during the melting process of long‐glass fiber‐reinforced resins. POLYM. ENG. SCI., 59:1300–1309 2019. © 2019 Society of Plastics Engineers     

Experimental study for starve‐fed single screw extrusion of thermoplastics

An experimental study of the polymer behavior in a starve‐fed single screw extrusion is presented. Various polymeric materials, semicrystalline low density polyethylene (LDPE), polypropylene (PP), and (LDPE/PS) polyblends were investigated at various operating conditions. A “screw pulling‐out” technique was used to study polymer behavior along the screw. The solid conveying, melting position, the extent of starvation, and the fully filled regions were observed. Polymer samples were stripped off from the screw which was removed from the machine to investigate melting mechanism. It was seen that filling of the screw channel increases with the flow rate at a fixed screw speed, and decreases with the screw speed at a fixed flow rate. Contiguous solids melting mechanism was observed for flood fed extrusion, but it was not observed for starve‐fed extrusion. A new two‐stage physical model of polymer melting has been proposed with conductive mechanism for melting in the starve‐fed region and dispersed melting mechanism in the fully filled region. Melting action seems to be faster for starve feeding than for flood feeding, since the pellets are not compacted into a dense solid bed. It was observed that the pressure and power consumption considerably decrease with starvation. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers     

ϵ‐caprolactone grafting on a poly(vinyl alcohol‐co‐vinyl acetate) in the melt without added initiator

Polycaprolactone (PCL) grafting on vinyl alcohol‐co‐vinyl acetate) (PVA‐Ac), was investigated in the melt at high temperature (170°C), below the PVA‐Ac melting point, by ring opening polymerization of ?‐caprolactone initiated by metal alkoxyde sites present in PVA‐Ac: no additional initiator was used. The obtained average structures were determined by ¹H NMR. As expected, small grafts, with low average polymerization degree (DP), were obtained, between 4 and 12 h of reaction. These DP are due to exchange reactions between hydroxyl groups and PCL growing chains. The PVA‐Ac was shown to be partially substituted by short PCL grafts. The DP linearly increased with the initial Lactone/PVA‐Ac ratio, and the substituted alcohol sites rate were limited to 63%.It was shown that the used reactive system is characterized by a quazi‐living polymerization mechanism. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Effect of screw axial vibration on polymer melting process in single‐screw extruders

A model for investigating the melting process of polymer in a vibration‐induced single‐screw (VISS) extruder is presented. The key feature of this model is as follows: vibration force field is introduced into the overall course of extrusion by the axial vibration of the screw, and the velocity distribution in the polymer melt behaves strongly nonlinear and time‐dependent. To analyze this model, half‐open barrel visible experimental method and low‐density polyethylene material are adopted to investigate the effect of the vibration parameters on the melting process, which goes into further details of study and research on the melting mechanism, and thus, a novel physical melting model is derived. Combining the conservation equations of mass, movement, energy, and constitutive, analytical expressions of the melting rate, the energy consumption, the length of melting section, and the distribution of solid bed are obtained. This model enables the prediction of the processing and design parameters in the VISS extruders from which the optimum conditions for designing VISS extruder and polymer processing are obtained. The theory is supplemented by a calculation sample and experiment, which shows that the introduction of vibration force field can improve the melting capacity and decrease the power consumption of extruder greatly. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 3860–3876, 2006     

Compacted Modified Propellant Blocks as Traveling Charge in the Hybrid Shot Scheme

The burning of compacted modified propellant charges applied as a traveling charge in the hybrid shot scheme was studied. The block charges were manufactured by pressing fine propellant grains coated by a thin film of polyvinyl butyral. A stick from several pressed pellets was insulated over its lateral surface by a thin layer of silicon paste, glued to the back of the projectile and inserted into the barrel of the 23‐mm smooth‐bore laboratory gun. The loose‐packed accelerator charge was placed in the breech. Combustion was initiated by an igniter plug placed between the traveling and breech charges. A set of piezo‐quartz gauges placed in the breech and along the barrel, as well as a frame‐target device were used for recording characteristics of the firings. It is shown that blocks of this type, applied as the traveling charge, provide a stable burning process resulting in high ballistic performance. The block traveling charge preserves its integrity in the course of its motion along the barrel, and burning envelopes its total mass when pressure in the breach passes the maximum value. The descending portion of the pressure diagram demonstrates appreciable transformation, with convex or secondary hump sections. The shape of spatial pressure profiles behind the moving projectile is also transformed, and the pressure at the projectile butt end may be higher than the pressure in the breech. Compared to the conventional charges at the same maximum pressures the muzzle velocity increment attains 340 m s⁻¹ (or 23 %) for a light 35‐g projectile and 200 m s⁻¹ (or 19 %) for a heavy 104‐g projectile.     

Computation of starch cationization performances by twin‐screw extrusion

A theoretical model for the cationization of plasticized wheat starch in a modular seIf‐wiping co‐rotating twin screw extruder was developed. Our objective was to build a model which would be able to predict the evolution of the cationization reaction along the screws, in relation with the processing conditions and the geometry of the twin screw extruder. Based on previous studies on reactive extrusion modeling, the present model takes into account the interactions occurring between the flow conditions encountered in the extruder and the kinetics of the reaction. It allows one to predict the influence of operating parameters such as reagent concentrations, feed rate, screw speed, and barrel temperature on the reaction extent. Depending on conditions, degrees of substitution in the range 0.01–0.05 are obtained, with efficiencies between 30 and 90%. A good agreement is found between theoretical results and experimental measurements, allowing future use of the model for optimization and scale‐up purposes. POLYM. ENG. SCI., 47:112–119, 2007. © 2007 Society of Plastics Engineers