Surging in screw extruders

Output uniformity is one of the main factors limiting the maximum output obtainable from a single screw plasticating extruder, and is adversely affected by surging. Several causes of surging have been identified, perhaps the most important being instabilities in the melting process. These are caused by periodic break-up of the bed of compacted solid polymer formed in the screw channel. Solid bed break-up is shown, both experimentally and theoretically, to be associated with rapid acceleration of the bed in the downstream direction parallel to the screw flight. A novel method of measuring solid bed velocity and hence acceleration is described. The theoretical model of the melting process is shown to be capable of predicting this acceleration reliably, and therefore the tendency for a particular combination of screw design, material and operating conditions to cause surging.     

Fundamentals of plasticating extrusion. I. A theoretical model for melting

A steady state model for melting in a screw extruder has been developed based on the assumption that a stable solid bed of polymer granules is melted by heat that is conducted from a hot barrel and heat that is generated by viscous dissipation in the film that separates the solid bed and the surface of the barrel. The solid bed gradually decreases in width, as it proceeds in the channel, until it disappears at which point the melting is terminated. The model predicts the solid bed width profile and the required length of melting in terms of physical properties, operating conditions, and geometry of the screw. The model has been tested based on experimental data.     

螺杆冷却时的单螺杆挤出熔融理论研究

利用可视化挤出实验对螺杆冷却情况下的单螺杆挤出熔融机理进行了研究。实验表明，挤出过程中固体床始终保持连续而不会出现固体床破碎现象，螺槽表面会出现聚合物的亚稳态相转变行为。通过建立螺杆冷却时熔融理论的数学模型，用数值方法获得了熔融段聚合物流场的数值解，结果表明，理论预测的固体床宽度和机筒压力与实验结果基本吻合。     

Melting performance of single screw extruders with a grooved melting zone

A novel melting model for single screw extruders with a grooved melting zone was established. The whole solid plug, which came from the grooved feed zone, was ruptured and melted mainly by continuously changing the volume of the barrel grooves and the screw channel in the grooved melting zone. A new single screw extruder platform with hydraulic clamshell barrels was constructed to investigate the melting of solid polymer with different combinations of barrels and screws. The melting model was verified by experiments. The results showed that the melting started earlier and finished in a shorter length for single screw extruders with a grooved melting zone than that for conventional single screw extruders and the melting efficiency was improved by introducing a grooved melting zone to a single screw extruder. The theoretical values are consistent with experimental results. The novel single screw extruder with grooved melting zone can dramatically increase the plasticizing efficiency and the throughput.     

Melting performance in the single screw extrusion of thermoplastics

Melting performance experiments involving three different thermoplastics and three different screw designs have been carried out on a well-instrumented single screw extruder equipped for cold screw extractions. In the case of the particular polystyrene used it was possible to deduce, from measurements made on the extracted screw, the velocity, and hence acceleration, of the solid bed of compacted polymer at points along the screw channel. The experimental results are successfully compared with the performance predicted by a previously established model, the most important feature of which is the ability to allow the solid bed to deform freely and hence to accelerate. The results show that the bed does indeed suffer significant and non-uniform acceleration and that the model can predict both this acceleration and the resulting bed break-up which leads to surging.     

A theoretical melting model for plasticating extruders

A theoretical model for melting in plasticating extruders is described. Compared to previous models, this model introduces more accurate and less restrictive assumptions, adds a mass balance on the entire channel, and replaces certain approximate solutions by exact solutions. Flow of the solid bed is represented by a solid bed acceleration parameter, SBAP, which permits solid bed acceleration in a screw compression section. New experimental melting data for a variety of screw designs, polymers, and extruder sizes are presented and compared to the theoretical predictions. With the optimum SBAP, reasonably accurate model prediction of the melting profiles is observed for a wide variety of cases.     

Experimental study of melting of LDPE/PS polyblend in an intermeshing counter‐rotating twin screw extruder

An experimental study is presented of the melting mechanism in a starve‐fed closely intermeshing counter‐rotating twin screw extruder of a modular Leistritz design. Various polymeric materials, semicrystalline low density polyethylene (LDPE), amorphous polystyrene (PS), and (LDPE/PS) polyblend were investigated at various operating conditions. A “screw pulling‐out” technique was used to investigate polymer behavior along the screw axis. In particular, the solid conveying, melting positions, the extent of starved character along the screw, and the fully filled regions were observed. Polymer samples were stripped off from each screw which was removed from the machine to investigate melting mechanism. Generally, it has been concluded that the melting mechanism revealed by White and Wilczyński for polyolefines has been proved for other polymeric materials under study. This mechanism consists of pellets being dragged into the calendering gap where they are melted due to calendering action. The molten polymer is expelled from the gap and pushes against the pellet bed which is continuously dragged into the gap. The composite modeling of an intermeshing counter‐rotating twin screw extrusion of polyblends has also been discussed. POLYM. ENG. SCI., 52:449–458, 2012. © 2011 Society of Plastics Engineers     

Experimental study of melting of polymer blends in a starve fed single screw extruder

Experimental study is presented of the melting mechanism of polymer blends in a starve fed single screw extruder. Various polyblends were investigated such as PP/PS, HDPE/PS, and PP/PMMA. “Screw pulling‐out” technique was used to study the solid conveying, melting mechanism, and extent of starvation. It has been concluded that melting mechanism consists of two stages: conductive melting in the starve fed region and dispersed melting in the fully filled region. Substantial differences between melting of neat polymers and polymer blends were observed. In the case of polyblends, in the starved region a mixture of two solid polymers is melted by conduction, and in the fully filled region a dispersion of one or two solid polymers in a molten matrix is observed. Although contiguous solids melting mechanism was not seen for starve fed extrusion of polyblends, it was clearly observed for flood fed extrusion. Melting action seems to be faster for starve fed extruders than for flood fed machines, since the polyblend granules are not compacted into a dense solid bed. It was observed the pressure considerably decreases with level of starvation. Filling of the screw increases with an increase of the feed rate of polyblend, and decreases with an increase of the screw speed. Global modeling of the starve fed single screw extrusion of polyblends has been discussed. POLYM. ENG. SCI., 56:1349–1356, 2016. © 2016 Society of Plastics Engineers     

A plasticating model for single-screw extruders

By measuring the solid-bed transfer velocity, width and thickness under various conditions, die following results are obtained. As the result of melting, the solid bed decreases in width and thickness almost with the same rate, and the solid-bed transfer velocity is constant, while a melt layer exists between the solid bed and the screw root; also, when the phenomenon of dam-up occurs, caused by the combined effect of decreasing depth of the screw channel with tin insufficient decrease of solid-bed thickness, the transfer velocity increases proportional to the rate of decrease of channel depth. Consequently, the solid bed is considered to behave us loosely packed particles. A new plasticating model is developed by making the above results an assumption and adopting finite differential calculus with the Newton-Raphson method to obtain accurately the melting velocity, melt profile, and solid-bed temperature. Calculated values are in remarkably good agreement with the experimental values Solid-bed softening point, pressure, and screw torque are also successfully estimated.     

Melting model for modular self wiping co-rotating twin screw extruders

A model for the melting process in a self wiping co-rotating twin screw extruder is described. Self-wiping co-rotating twin screw extruders are modular and starve fed. This leads to melting mechanisms that are different from single screw extruders. The melting process in the modular screw configurations generally occurs in specialized sections such as kneading disk blocks. The model, based on our previous experimental observations, considers the formation of two stratified layers of melt in contact with the hot barrel and solid pellets in contact with the relatively colder screw. In the kneading disk blocks, a part of the solid bed is blocked because of the relative stagger between successive disks. The model predicts both the location of melting and melting lengths in a screw configuration. Calculations for individual screw elements and kneading disc elements are presented first. Melting in a modular configuration of these elements is then considered. The effect of operating variables such as mass flow rate and screw speed on melting is then studied. The model is put in a dimensionless form and the effect of various dimensionless groups is discussed. We make a comparison to the experiment and agreement is good.     

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.     

Studies on the theory of single screw plasticating extrusion. Part I: A new experimental method for extrusion

An extruder specially designed for the study of the single-screw plasticating extrusion process was constructed. Its barrel is equipped with glass windows which are located on both sides of it so that the full process of plasticating extrusion, solid conveying, melting, and melt conveying, can be clearly observed and recorded with photos and video recordings through the transparent windows. The solid profile X/W, the velocity of the solid bed V_sz, the forming positions of the upper melt film and the melt pool, and the position at which the break-up of solid bed occurs, was easily determined with good accuracy. Many dynamic characters of the plasticating extrusion, such as the non-plug solid conveying, the process of the break-up of the solid bed and the disappearance of the, broken pieces of solid, were also observed in the experiments.     

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.     

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.     

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     

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     

Analytical melting model for polymer pellets

The plasticating or melting behavior of polymer pellets sliding on a hot metal surface was investigated. An analytical model was developed for predicting the melting rate and the shear stress of an incompletely compacted solid bed of pellets in terms of the rheological and thermodynamic properties of the polymer, the pellet size, the degree of compaction, and the operating conditions. This investigation extends the results of the previous investigators obtained for a fully compacted solid bed of polymers to an incompletely compacted solid bed of polymers, closely representing the initial stage of the plasticating mechanism inside a screw extruder. Experimental verification of the analytical model developed here was made by testing six different commercial polymers.     

同向双螺杆挤出过程中聚合物颗粒熔融问题探讨Ⅰ—相变分数法与实验验证

在双螺杆挤出过程中，聚合物颗粒挤出熔融过程中较为常见的一种固、液两相共存的形式可以用“海－岛”模型来描述。提出了“海－岛”模型的一种简化模型－中间模型，通过引入“相变分数”对粘性耗散热进行分配，得出了固相分数及熔体平均温度沿螺槽方向的变化规律。通过实验对相关结果进行了验证，对比表明该方法基本能反映聚合物颗粒熔融的真实过程。     

Melting in single screw extruders

A model for the melting of granules in a single screw extruder is presented in Part I. It is consistent with observations of earlier workers and retains some of the ideas introduced by Tadmor in his model; however, it assumes that the solid bed of granules cannot stand large differences of principal stresses and so account has to be taken explicitly of the downstream force balance on the solid bed and in the melt pool. Detailed quasi-analytic results are given for a Newtonian (constant viscosity) fluid in Part II. These illustrate the model for a particularly simple case and have relevance for some materials. A more elaborate numerical scheme is described in Part III for a non-Newtonian model and results are presented for comparison with the predictions of other theories and with experiments.     

Melting in single screw extruders

A model for the melting of granules in a single screw extruder is presented in Part I. It is consistent with observations of earlier workers and retains some of the ideas introduced by Tadmor in his model; however, it assumes that the solid bed of granules cannot stand large differences of principal stresses and so account has to be taken explicitly of the downstream force balance on the solid bed and in the melt pool. Detailed quasi-analytic results are given for a Newtonian (constant viscosity) fluid in Part II. These illustrate the model for a particularly simple case and have relevance for some materials. A more elaborate numerical scheme is described in Part III for a non-Newtonian model and results are presented for comparison with the predictions of other theories and with experiments.