A mathematical model for predicting dynamic behavior of a plasticating extruder

A mathematical model was developed to predict the dynamic behavior of flowrate and melt temperature in a plasticating extruder caused by changes in operating variables such as screw speed, back pressure and barrel temperature. The model has application for on-line computer control of an extrusion process or for simulation purposes off-line. Experimental data for developing the model was obtained from a 2½ in. diameter plasticating extruder producing high impact polystyrene sheet.     

Extrusion Characteristics of Round-Section Dies with VFF

A vibration force field was applied to the whole process of polymer plasticating extrusion by periodical vibration of the screw. It observably affected both the extruder screw extrusion characteristics and the round-section die extrusion characteristics. It also affected the polymer plasticating extrusion process and the quality of the extrudates. An analytical model of typical dynamic extrusion of round-section dies was created. The results showed that a vibration force field can improve extrusion output.     

Analysis on Pulsating Flow of Melt for Reciprocating Extrusion in the Screw Channel

In the reciprocating extrusion, the vibration force field (VFF) is applied to the entire plasticating process by the axial vibration of screw and the novel concept on the polymer dynamic plasticating process being strengthened has been brought forward in the paper. The mathematical model is established that describes the plasticating process under the VFF and the approximative analytical solutions of the velocity distribution, pressure gradient in the screw channel and the plasticating capability are obtained. The theoretical results show that the axial vibration of screw can accelerate the blend capability of polymer and make melt temperature more uniform. With increase of the vibration intensity, the effect of blend and plasticating is enhanced further. The average pressure gradient drops down the channel under the VFF because of lower virtual viscosity of polymer melt and smaller flow resistance in the channel. The comparison between the theoretical analysis and the experimental results shows the plasticating capability of melt is improved with increase of vibration intensity.     

Residence time distribution in a real single screw extruder

The residence time distribution in an industrial single screw extruder was investigated experimentally in the case of melt and plasticating extrusion. The investigations performed proved that the extrusion parameters influence strongly the residence time distribution in the extruder. It was found that the resistance to flow through the die-head of the extruder is very important from this point of view, as well as other parameters like rotational speed of the screw and the screw channel depth. Variation of these parameters can change the residence time distribution over a broad range between the extreme idealized cases of plug flow and flow with perfect mixing. In order to obtain quantitative dependences three moduli were used and a correlation equation was obtained. This equation enables an estimation of residence time distribution on the basis of experimental characteristics of the extruder and the actual extrusion parameters.     

Measurements of residence time distribution for the peroxide degradation of polypropylene in a single-screw plasticating extruder

Measurements of the residence time distribution (RTD) in a single-screw plasticating extruder were carried out during experimental studies of the peroxide-initiated controlled chemical degradation of polypropylene (PP). A radioactive tracer method was employed, and the effect of screw speed, temperature, and reaction on the RTD was examined. An increase of the peroxide concentration resulted in a broader distribution whereas an increase of the extrusion temperature was found to result in a narrower distribution. Use of low screw speeds simply increased the time delay through the extruder without affecting considerably the breadth of distribution. Results obtained from the present experiments were compared with several theoretical models.     

Effect of vibration extrusion on the structure and properties of high‐density polyethylene pipes

BACKGROUND: The axial strength of a plastic pipe is much higher than its circumferential strength due to the macromolecular orientation during extrusion. In this work, a custom‐made electromagnetic dynamic plasticating extruder was adopted to extrude high‐density polyethylene (HDPE) pipes. A vibration force field was introduced into the whole plasticating and extrusion process by axial vibration of the screw. The aim of superimposing a vibration force field was to change the crystalline structure of HDPE and improve the molecular orientation in the circumferential direction to obtain high‐circumferential‐strength pipes. RESULTS: Through vibration extrusion, the circumferential strength of HDPE pipes increased significantly, and biaxial self‐reinforcement pipes could be obtained. The maximum increase of bursting pressure and tensile yield strength was 34.2 and 5.3%, respectively. According to differential scanning calorimetry and wide‐angle X‐ray diffraction measurements, the HDPE pipes prepared by vibration extrusion had higher crystallinity, higher melting temperature, larger crystal sizes and more perfect crystals. CONCLUSION: Vibration extrusion can effectively enhance the mechanical properties of HDPE pipes, especially the circumferential strength. The improvement of mechanical properties of HDPE pipes obtained by vibration extrusion can be attributed to the higher degree of crystallinity and the improvement of the molecular orientation and of the crystalline morphology. Copyright © 2008 Society of Chemical Industry     

Pressure profiles in plasticating extruders

The pressure distribution through the melting and melt zones of a plasticating extruder is discussed, and an analysis is described for predicting the pressure profile. In the stable melting zone, the pressure profile is calculated based on flow in the melt pool, and the pressure is strongly influenced by the flow of the solid bed of plastic. The solid bed flow is primarily determined by the polymer rigidity in the screw compression section. If the size (through a melting analysis) and the velocity (through a solid bed acceleration parameter) of the solid bed along the screw channel are reasonably approximated, the pressure profile is reasonably approximated by this analysis. Inaccurate representations of the size or velocity of the solid bed can yield inaccurate pressure profile prediction. In the unstable melting region, the assumption of a complete melt yields reasonable pressure predictions. The introduction of these concepts into an extrusion model permits a more accurate prediction of the operating RPM of a given screw design in a given machine.     

Maximum pressure developed by solid conveying force in screw extruders

There are two distinct solid conveying theories that can be applied to plasticating screw extruders. One is Darnell and Mol's theory based on a solid-to-solid friction model and the other is Chung's theory based on a viscous shearing model. The two theories predict very different solid conveying performances for a same set of conditions. In this paper, the maximum pressures that can be developed inside plasticating screw extruders by the solid conveying force are calculated using each of the two theories. Comparison of the results may shed some light on the applicability of each theory for a particular extrusion operation.     

On the scale-up of plasticating extruder screws

In the most commonly used scale-up method of plasticating extruder screws, the screw channel depth is increased by the square root of the diameter ratio while the screw RPM is decreased by the square root of the diameter ratio such that the output rate increases proportionally to the square of the diameter ratio. This scale-up method, largely based on the pumping function of the screw, often leads to a higher melt temperature, a higher screw horsepower consumption per unit output rate and an inferior melt quality from the larger diameter screw. Analysis of the common scale-up method reveals that, although the shear rate in the melt is kept constant, the average residence time and the peripheral screw speed are increased for the larger diameter screw. Our recent study on the melting mechanism also reveals that the melting capacity increases less than the pumping capacity. A detailed examination of the common scale-up method in this paper shows that the pumping capacity and the solid conveying capacity increase more than necessary while the melting capacity increases insufficiently.     

可控流变共聚PP的制备工艺及力学性能

用可控流变法降解共聚聚丙烯(PP)获得可控流变共聚PP。考察了基础树脂、过氧化物加入量、工艺参数对最终产品力学性能的影响。研究发现:在QP73N加入少量过氧化物便能显著改善流动性;根据基础树脂和最终产品的熔体流动指数可确定过氧化物的加入量;各种工艺参数对流变性能的影响由大到小依次为:温度、喂料转速、主机转速;喂料转速与主机转速之比越小,熔体流动指数越高;对降解产品力学性能的各项指标影响最大的因素都是挤出温度,其次是喂料转速和主机转速。     

聚合物挤出成型的新方法与新设备

本文论述一种聚合物塑化挤出成型的新方法与新设备,着重分析聚合物塑化挤出新概念和动态塑化挤出过程,并介绍塑料电磁动态塑化挤出设备的原理、结构、性能及特点。     

A die rotating system for moderations of extrusion load and pressure drop profiles for molten PP and wood/polypropylene composites in extrusion processes

A die‐rotating system was proposed in this work for moderations of extrusion forces and entrance pressure drop for molten polypropylene (PP) and wood/polypropylene (WPP) composites in a capillary rheometer and a single screw extruder. The effects of processing conditions and wood loading in PP were of our interests. The extrusion force and entrance pressure drop with and without the die rotating system were monitored in real‐time. This was the first time that the die‐rotating system was used for processing of highly viscous wood/polymer composite materials. It was found that the flow properties of the molten PP and WPP composites obeyed pseudoplastic non‐Newtonian behavior. The behavior was more obvious at wood contents of above 6 wt % and in the capillary rheometer. The rotation of the die could moderate the extrusion load by 60% and entrance pressure drop by 20% in the capillary rheometer, and the entrance pressure drop by 30% in the single screw extruder, especially at the conditions where the viscosities of the WPP and the extrusion rate were high. Greater fluctuations in entrance pressure drop caused by die rotation were observed in the single screw extruder. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci 120:1006–1016, 2011     

Fundamentals of plasticating extrusion. II. Experiments

Experiments were performed to develop quantitative information for designing plasticating extruders for low density polyethylene. Screw design variables explored included feed section length, compression section taper, and minimum channel depth. Operating variables included were screw speed, barrel temperature, and back pressure. A moving picture film illustrates temperature action and cross-channel temperature distribution for some typical experiments using a new type of extruder screw for 2.5 inch and 8 inch diameter extrusion. The information gathered was used to obtain relations between performance and screw dimensions and revealed an optimum combination of feed section length and compression taper.     

A melting model for reciprocating screw injection-molding machines

A theoretical model for melting in reciprocating screw injection molding machines is proposed. The model permits the calculation of the solid bed profile as a function of time during the injection cycle. It consists of a dynamic extrusion melting model for the rotation period, a transient heat conduction model with a phase transition for the screw rest period, and a proposed model for the drifting of the beginning of melting during the injection cycle.     

Screw design in injection molding

The performance of screws of advanced design in injection molding has been investigated with respect to four different objectives: (1) improvement of distributive mixing; (2) improvement of dispersive mixing; (3) increase of plasticating capacity; and (4) reduction of inhomogeneity of melt temperature. The screws used are three zone screws with different compression ratios, screws with pineapple or Maddock/Egan mixing elements, with one or two channel barrier sections, with static mixers mounted in the valve or in the nozzle, or with combinations of these different elements. The best mixing quality is obtained with multi-channel Maddock sections. The highest plasticating capacity and, consequently, the shortest cycle times are achieved with the barrier screws. Temperature measurements show that these screws improve melt homogeneity considerably with a relatively small loss of plasticating time. In all cases, increasing the back pressure gives inferior results compared with improvement of the screw design.     

Developments in the analysis of steady screw extrusion of polymers

A review is undertaken of theoretical and experimental work which has advanced the understanding of the single screw extrusion process for polymers. Detailed consideration is given to the many published theoretical models of solids conveying, melting and melt flow in the channel of a plasticating extruder. Development of such models is traced in terms of the gradual relaxation of simplifying assumptions to provide methods of analysis which give realistic predictions of machine performance. Comparisons between the results of such analyses and observed machine behaviour are also discussed.     

Dynamic behavior of a single screw plasticating extruder part I: Experimental study

The dynamic responses of a 2–1/2 inch single screw plasticating extruder and extrusion line were investigated. Step changes in screw speed, take-up speed, back pressure, and processing materials were used to determine the transient responses of barrel pressures, die pressure, melt temperature, and extrudate thickness. Dynamic responses of the entire extrusion line can be explained by the flow mechanism of the extruder and the logical properties of the polymer used. A capillary rheometer was also used to determine if it could simulate pressure responses in the extruder for screw speed changes. Results showed that capillary rheometer was helpful in estimating the short term pressure responses in the die.     

电磁动态塑化挤出机性能的研究

本文结合理论分析，着重从实验方面分析研究电磁动态塑化挤出机的挤出特性和混合特性，证明了这种新型挤出机具有挤出产量高、挤出稳定、挤出温度低、能耗低和混合效果好等优点。     

Bulk density of solid polymer resins as a function of temperature and pressure

In a plasticating extruder, solid polymers are heated and are subjected to high pressures before they are melted and delivered to a die. In both the solids conveying and melting sections, these temperature and pressure increases will compact the unmelted polymer bed as it moves down the screw channel. Performance of the extruder depends in part on how well the screw design matches the compaction behavior of the resin for a given set of process conditions. The design of these screw sections, however, is often done based on past experience and with little knowledge of the resin compaction behavior. A much improved design would include screw performance prediction using variable bulk density and computer simulations. Computer simulations, however, are often performed using constant solid bulk density because of the lack of reliable density data as a function of both pressure and temperature. An instrument was developed for studying the compaction behavior of pellet and powder resins. Bulk densities and storage friction coefficients are reported for several important thermoplastic resins as a function of temperature and pressure. The bulk density data were fitted to a semi-empirical model.