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.     

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.     

Melting of thermoplastics in single screw extruders

Some experiments on the melting of thermoplastic polymeric materials in single screw extruders are described. Although these were of the now familiar screw extraction type, special care was taken to distinguish between material melted by screw rotation and that melted during the subsequent cooling operation. A barrel which could be split longitudinally was also used, thus avoiding some of the disadvantages of axial extraction. A theoretical model is proposed which, unlike previous models, allows the solid bed of material in the screw channel to accelerate naturally, and also allows for the presence of a film of molten material between the bed and the screw. This model gives satisfactory predictions of melting performance. Comparison with experimental results shows that break-up of the solid bed occurs when the model predicts rapid acceleration of the bed. Bed break-up and the resulting surging may be reduced or prevented by the use of screw cooling which has the effect of inhibiting the formation of a melt film at the screw surface.     

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.     

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.     

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.     

单螺杆挤出过程中固体粒子的变形、取向与熔融Ⅲ固体粒子熔融过程的理论与计算

采用带有在线显微观察和摄影系统的全程透明视窗单螺杆挤出机为实验装置 ,在对LDPE物料固体粒子的熔融过程进行实验观察和分析的基础上 ,提出了固体粒子熔融的物理模型和数学模型 ,并对有关模型进行了数学计算。研究结果表明 ,螺槽表层粒子熔融过程不发生熔体迁移的假设与实际情况差异较大 ,而存在熔体迁移的模型与实验现象较为接近 ;而螺槽心层粒子的封闭球壳结构模型与实际情况也存在一定的差异。研究结果进一步指出 ,螺槽心层粒子的熔融前期可以认为不发生熔体迁移 ,但在熔融后期将存在较明显的熔体迁移现象 ;对于固体粒子熔融过程的精确计算既要考虑不同模型的差别 ,还要考虑到粒子形状、粒子运动与变形等因素的影响。     

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 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.     

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.     

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.     

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。     

Analytical melting model for extrusion: Melting rate of fully compacted solid polymers

The melting mechanism inside screw extruders is presently analyzed using numerical, iterative methods that are too complex to be used widely by practicing engineers. Our theoretical and experimental investigations of the melting mechanism have produced simple, analytical equations for predicting the melting rate of fully compacted solid polymers without iterative calculations. The accuracy of these equations was found to be satisfactory. Consideration of the temperature and shear dependencies of the melt viscosity was found to be essential for the accurate prediction of the melting rate.     

Mathematical modeling of melting of polymers in a single-screw extruder

This paper addresses the apparent controversy surrounding the role of the solid bed mechanics in the Maddock melting mechanism. It is demonstrated that the inability of the melting models based on the freely deformable solid bed concept to predict accurately the pressure gradients in the melting zone is not exclusively due to the highly simplified isothermal Newtonian treatment of the melt pool as presumed previously. This study has shown that when using a non isothermal non-Newtonian flow model for the melt pool, the freely deformable solid bed concept still results in unrealistically low pressure gradients while it may give good predictions of the melting rates. To the contrary, when a rigid solid bed is assumed, the pressure predictions tend to represent the experimental data more closely, whereas the theoretical melting rates seems to become less realistic. In view of the fact that both the freely deformable and the rigid solid bed concepts show such inconsistencies, it has been concluded that the mechanics governing the solids and melt transport in the melting zone require some additional examination, most notably, the influence of the constitutive behavior of the solid bed and of the cross-channel melt circulation around the solid bed, and possibly of the melting kinetics for semicrystalline polymers.     

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     

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.     

A theoretical melting model for a reciprocating-screw injection molding machine

A theoretical model for the transient melting behavior in a reciprocating-screw injection molding machine is proposed. The model is based on a steady state extrusion model, Neumann's melting problem, and a heuristic postulate for the transient behavior. The model predictions are compared to experimental melting data for low density polyethylene, acrylonitrile-butadiene-styrene polymers and polyvinyl chloride for a variety of operating conditions and two screw designs. A useful degree of correlation is demonstrated for all experimental cases.     

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     

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.