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Fundamentals of Foam Sheet Extrusion Using a Tandem Extrusion Line
Mark A. Spalding, John L. Sugden, Gary C. Welsh, March 2023

Tandem foam sheet extrusion is a complex process that requires optimization to produce quality sheet at high rates. The goal of this paper is to describe the process, show how rates can be increased, provide a guideline for sheet quality, and provide case studies.

A New Approach for the Local Determination of Energy Input in CO-Rotating Twin Screw Extruders Based on Screw Torsion
Laura Austermeier, March 2023

A targeted energy input in the melting zone of co-rotating twin screw extruders, which is sufficient to plasticize the material without damaging it, is one of the main goals of compounding. High temperatures damage the material just as much as extreme shear, caused by high rotational speeds, does. Fast melting sometimes leads to overstressing the material, which is why a gentle melting zone is preferred. But gentle melting means a longer melting zone and therefore longer extruders, which need more space and energy. This conflict leads to the need of a model to predict the exact energy input for one single element in the current process to find a compromise between both factors. The specific energy input can be described as power per mass flow, which in turn can be simplified to torque and speed of the extruder. At present, no models are available to describe the torque along the extruder with sufficient accuracy. In the present paper a new approach will be carried out, to calculate the determination of energy input by calculating the resistance of the plastic against the screws, causing a torsion of the screws. This can be repatriated to the torque and therefor the energy input. Different zones along the axis of the extruder are distinguished and described with different models. This results in a global model for the breakdown of the torque along the extruder. Basically, the flow of the individual zones is considered and the force of the melt flow on the screw surface is determined.

Compensation of Viscosity Fluctuations Through Local Temperature Adjustment in the Extrusion Die
Prof. Dr.-Ing. Christian Hopmann, Lisa Leuchtenberger, M.Sc., Malte Schön, M.Sc., March 2023

In recycling processes, different material streams with varying degrees of contamination and with different material properties are compounded into new granules. The varying material composition as well as the material degradation impact the melt viscosity from batch to batch. Different viscosities cause local different shear heating in extrusion dies such as binary pre-distributors. The resulting thermal inhomogeneities then lead to film thickness variations, which impair the film quality. In order to adapt the process to changing viscosities, e.g. when processing recyclates, one possible influencing variable is the die temperature in the vicinity of the flow channels. Current research of Hopmann et al. already addresses local cooling through the use of heat pipes in a binary pre-distributor for a low-density Polyethylene (PE-LD) [4]. The adaption of local heat pipe cooling to other materials with different rheological and thermal material properties is focused in this paper. In order to evaluate the applicability of the already developed system for other materials, computational fluid dynamics simulations (CFD-simulations) are carried out. The occurring inhomogeneities in temperature and velocity distribution with and without local heat pipe cooling are compared for PE-LD and a high-density Polyethylene (PE-HD). Finally, it is derermined whether the heat transfer capacities of the heat pipes specified by Hopmann et al. can be universally applied in addition to the existing heat pipe positioning.

New Polymer Processing Additives for Extrusion
Julia Resch, March 2023

Injection molding and extrusion are the two basic processing methods in plastics technology. The most likely applications, especially in case of extrusion, are in the packaging sector. Here, polyolefins represent the most important type of plastic in terms of volume. Worldwide, the total consumption of polyolefins is estimated at over 150 million tons per year. However, during the extrusion of polyolefins such as polyethylene or polypropylene, flow instabilities can occur as soon as the mass flow exceeds a critical value. These instabilities mainly show up in three different forms. They are referred to as the "sharkskin effect", the "stick-slip effect" and the "gross melt fracture". The effects mentioned appear with increasing shear rate. In the case of the sharkskin effect, periodic tearing of the product surface result, producing a sharkskin-like structure. The occurrence of the stick-slip effect manifests itself in products (for example, strands or films) with alternately smooth and jagged structures. If even higher shear rates during the process lead to melt fracture, in which the product undergoes extremely severe deformation. The flow instabilities can limit the production rate and thus also affect cost efficiency. To counteract them, processing aids such as fluoropolymers are added in practice. Fluoropolymers are considered to be extremely environmentally critical. Due to the strong C-F bond, these polymers can stay in the environment for an extremely long time and are therefore considered persistent. Due to this persistence, the introduction of the substances into the environment is so to say irreversible and poses a serious risk to humans and the environment. For this reason, their ban is often called for. In the past, various alternatives have been developed as polymer processing aids. Most of them are based on siloxanes. However, siloxanes also have high stabilities and durabilities, which is why they might be harmful to the environment, as well. The aim of this study therefore was to develop a novel and highly effective processing aid master batch that does neither pose a risk to health nor the environment. The new developed master batch should suppress flow instabilities and thus shift the process towards higher mass throughputs. In order to investigate the effectiveness of the processing aid master batches developed, they were tested with a high-pressure capillary rheometer. The capillary rheometer offers to detect the resulting pressure fluctuations, which are caused by the flow instabilities. For this purpose, a novel and high-resolution measuring nozzle was developed. This nozzle allows complete characterization and high-precision testing of the efficiency of the developed master batches. Various additives were tested. It was shown that the measuring method is suitable for detecting and accurately analyzing flow instabilities in the nozzle. It was also found that polysorbate is suitable as a flow aid for polyolefin extrusion. By adding polysorbate, the pressure instabilities could be suppressed so that surface effects were no longer visible.

The Benefits of Screw Cooling for Improved Solids Conveying for Smooth-Bore, Single-Screw Extruders
Timothy W. Womer, Mark A. Spalding, March 2023

All sections of a single-screw extruder must be operating well to maintain the maximum profitability of the line. The solids conveying section must be able to operate at a rate high enough to keep the metering section full of resin and pressurized. Optimal solids conveying depends on the forwarding and retarding forces on the solid bed, and these forces depend on the barrel and screw temperatures. Usually, a considerable level of care is given to setting the barrel and feed casing temperatures. The temperature of the screw, however, is typically not controlled. Instead, the screw temperature is unknown and often hotter than optimal. Screw cooling can improve solids conveying for many processes. This paper discusses the fundamentals and operational practices for using screw cooling.

Development of a New Granulate Feeding System for Gas Pressurized Extrusion Processes
Vitus Zenz, Dirk Muscat, Nicole Strubbe, March 2023

One major problem of a continuous process like plastic extrusion is their incapability to deal with non-local gas pressure. This is an inherent problem because a continuous process has an open end in the feeding port where pressure can escape. In this study a novel feeding system was developed to enable granulate feeding into gas pressurized processes inside a single- or twin-screw extruder. With this apparatus, gas pressure can be applied inside the extrusion process. The apparatus separates the pressurized extruder from the main feeder of the extruder. It keeps the pressure inside the system while continuously feeding new material into the process. The invention was patented under EP4130532 on 8th of February 2023 [1]. The apparatus was able to handle small amounts of granulates of around 100 - 200 g/h. An applied gas pressure of 0,8 MPa was achieved, with future optimizations aiming towards 1,5 - 2,0 MPa.

Twin Screw Melting Model Prediction Of Experimental Study Of LDPE Melting In A Twin Screw Extruder
Gregory A. Campbell, Mark D. Wetzel, Paul Andersen, Joseph Golba, June 2022

The melting of polymers in a twin-screw (T/S) extruder is an important operation in many industrial processes. Research by Shih, Gogos, Geng and others has identified the physical phenomena that take place during the melting phase transition. This paper describes a new approach for modeling the melting in a twin-screw extruder and the model predictions are compared with an experimental study of Low-Density Polyethylene (LDPE) melting in a co-rotating, intermeshing T/S extruder using on-line visualization and axial scanning of pressure and temperature techniques. This paper focuses on the physics and engineering concepts that are inherent in the melting mechanism in the extruder, and viscous energy dissipation in the melt with un-melted solids. The effects of throughput, Q, and at a constant rotation speed, N, is examined. Low and high Q/N ratios have significantly different axial pressure profiles.

Multilayer Extrusion Of Higher And Lower Modulus Alternating Polymer System
Joel Thambi, Vamsy Godthi, Marco Nefs, Nadia Grossiord, Jongwoo Lee, June 2022

Multi-Layer extrusion (MLE) is an advanced co-extrusion processing technology, which enables two polymer systems to be melt extruded, combined in an alternating format to very small total thickness <100μm and arranged in higher number of layer typically ranging from 8 to 1024. The focus of this paper is to investigate polymeric materials which are high modulus (e.g. LNP™ EXL PC copolymer or polymethyl methacrylate PMMA) and relatively low modulus (e.g. TPU) in nature as an alternating material combination for MLE. By combining different modulus of polymeric materials in MLE films, it is possible to achieve desired balance of different properties like mechanical, thermal, optical, dielectric etc., by synergistically combining the properties of the individual resins. In this paper flexural test is shown as an example to discuss the mechanical performance of MLE films. One of the major challenges of the MLE process is the down-selection of materials that are thermoplastics and have “matching” viscoelasticity at the processing temperature, as assessed by viscosity measurement at lower shear rates. Additionally, in order to ensure inter-layer adhesion, solubility parameters and processing windows of the two resins must be considered. In this study differences in adhesion were noted between PC/PU and PMMA/TPU MLE system. In PMMA/TPU MLE modification of processing temperature resulted in an improved interfacial stability and interlayer adhesion.

A Recycle Stream In The Reactive Extrusion Process For Preparing Lignocellulosic Thermoplastic
Jinlei Li, David J. W. Lawton, Guerino G. Sacripante, Michael R. Thompson, June 2022

Processes needing to extrude biopolymers can be challenged by the poor flow properties often exhibited by this class of materials. Lignocellulose is one such material that is very attractive to the future polymer industry as a potential engineered biopolymer suitable for structural applications. To convert the poorly processable lignocellulose pulp into a flowable thermoplastic, the chemistry of both cellulose and lignin need to be modified, and to do so economically, attention is turned towards reactive extrusion. A reactive solution is required for the modification but also, to simply allow the lignocellulose to flow through the extruder. This study examines the novel idea of a recycle stream in reactive extrusion to reduce the normally high concentration of reactive solution needed. The goal behind the recycle stream was to produce an exiting product requiring minimal recovery of the unreacted solution without the introduction of a contaminant into the process to aid lignocellulose flow. The results showed that a comparable thermoplastic product could be produced with ~50% less reactive solution by recycling 25% of the exit stream back into the process, The recycled polymer was an effective plasticizer for the lignocellulose pulp, lowering the reliance on the reactive solution to offer this function in addition to acting as the modifier.

Relative Humidity Effects on Accelerated Aging of Medical-Grade Polymers: Should ASTM F1980 be Revised to Recommend Constant RH Instead of Constant Moisture Content?
Aniruddha Palsule, Rachel Jacobs, Martin Gibler, Robert J. Klein, June 2022

ASTM F1980 provides a methodology for accelerated aging of sterile barrier systems for medical devices, and is also widely used as the definitive guide for accelerated aging of medical devices and pharmaceutical packaging. ASTM F1980-16, as well as previous versions going back to 2007, emphasize that when increasing temperature to accelerate aging, it is preferable to decrease relative humidity so as to maintain an approximately constant moisture content. However, there is a revision under consideration by the ASTM F02.50 committee that would dramatically change this guidance to indicate a preference (although allowing for other options) to keep relative humidity approximately constant. This change is based on somewhat limited test data and literature review published recently by Thor et al. In this paper, we perform a study looking at eight resins (PP, COC, ABS, PC/PET, Copolyester, PBT, PA66gf, PUR) that have been aged at 60C and three different RH levels to evaluate the impact on aging. Our findings to date indicate that: (i) yes, it is likely that RH should be held constant when increasing temperature in order to keep moisture constant in the resins at a similar level; and (ii) for the medical-grade resins evaluated here, RH level does not significantly impact the physical aging mechanism. We also recommend that further accelerated aging studies are performed to more thoroughly evaluate the impact of moisture content on Q10 factors, corrosion rates, and other endpoints before this dramatic change is made to the ASTM F1980 standard.

Automated Optimization Of A Block-Head-Mixer With An Innovative Algorithm
Felix Vorjohann, Lucas Schulz, Mirco Janßen, Reinhard Schiffers, June 2022

CFD-Simulations are a common tool to design and optimize mixing elements. The manual evaluation and experience-based derivation of an optimized geometry is still an iterative process which is time consuming. In this paper an automated algorithm is developed and tested for a mainly distributive Block-Head-Mixer. To automatically evaluate the flow field of each geometry variant, quality criteria are introduced which enable the assessment of the mixing capability. The investigation showed that the quality criteria are suitable to evaluate the flow field and an optimized candidate compared to a starting geometry could be found automatically.

Investigation of Influences on The Melting of Polyethylene and Polystyrene in a Co-Kneader
Johannes Rudloff | Marieluise Lang | Thomas Hochrein | Martin Bastian, May 2021

The co-kneader is well known for its superior mixing performance and its exact temperature control capabilities. Therefore, it is widely used in the polymer industry for the compounding of shear- and temperature-sensitive materials. In contrast to the considerable amount of scientific work that deals with investigation, modeling or simulation of the process behavior of single and twin screw extruders there are only few studies about the co-kneader. Due to increased quality requirements and the trend for cost reduction by process optimization, this is increasingly becoming a problem for plant construction and processing companies. To address this problem, experimental investigations of the melting behavior of polymer materials in the co-kneader were conducted. In order to determine the melting degree along the extruder length a special barrel was used which can be opened in axial direction. Based on the experimental results, a theoretical consideration for co-kneaders that are operated as plastification extruders is proposed. Therefore a disperse solid melting model is used. A comparison between simulated results and experimental data shows a descent agreement, when the point of melting initiation is estimated accurately.

Reproducibility Analysis of Fiber Length Measur ements During Processing With Twin Screw Extruders
Hatice Malatyali | Volker Schöppner | Seyit Ali Akar | Felix Hanselle | Laura Austermeier, May 2021

The twin screw extruder is used for processing of plastics. One of the most important processing tasks is the preparation of plastics with fillers and reinforcing materials. For the processing of fibers various recommendations can be found. But in an industrial production, with the same specification and process parameters, the resulting fiber length may differ. These variations must be clearly defined and determined. While process deviations occur during compounding, measurement deviations can be detected in the fiber end length measurements. In order to evaluate the experimental investigations and to use it for model validations, the corresponding deviations must be known beforehand. Within this paper, a reproducibility study will be carried out to ensure the reliability of the experimental investigations. The aim of the work is to determine the fiber length degradation along the screw and their deviations. The investigations in this paper are showing that a producible fiber length reduction is possible.

Influence of Twin Screw Configuration and Processing on Ketoprofen Dissolution in Polymer Blends
Laura Restrepo-Uribe | Nicolas Ioannidis, May 2021

The solubility and dissolution enhancement of the poorly soluble drug ketoprofen (KTO) in polymer blends prepared by hot melt extrusion was studied using two different twin screw configurations while changing extrusion processing parameters. Soluplus and Kollidon SR blends were used as solid dispersion excipients. A design of experiments with three melt temperatures, three screw rotation speeds, and three fill factors was performed. Different characterization techniques such as differential scanning calorimetry (DSC), optical and polarized light microscopy, X-ray diffraction (XRD), solid-state nuclear magnetic resonance (ss-NMR), and dissolution testing were used. The results from DSC and XRD showed an amorphous solid solution. An optimal processing condition by twin screw extrusion was found for each screw configuration achieving more than 80% drug release in 8 hours.

Modeling Fully Intermeshing Co-Rotating Twin-Screw Extruder Kneading-Blocks: Part A Conveying Characteristics
Ursula Stritzinger | Wolfgang Roland | Hanny Albrecht | Georg Steinbichler, May 2021

Twin-screw extrusion modeling is in most cases based on analytical approaches that are build on considerable geometric simplifications. These approaches give only rough estimations of the processing behavior. More accurate predictions generally require numerical methods with less drastic simplifications. In this work, we analyzed the pressure-throughput behavior in fully-intermeshing double-flighted kneading blocks. First, we conducted a dimensional analysis based on the Buckingham Π-theorem. Second, for each staggering angle, we determined the characteristic angular position that describes the mean throughput. Based on this position, a parametric design study was carried out by varying the identified dimensionless parameters. To solve the complex flow patterns, 3D CFD simulations were conducted. For each design point we evaluated the dimensionless drag flow-rate and the dimensionless dam-up pressure. As an addition to the two established dimensionless conveying parameters A1 and A2, we propose a novel conveying parameter A3. This new parameter simultaneously enables the description of conveying and non-conveying kneading discs. Our results offer considerably deeper insight into the conveying characteristics of kneading blocks. In addition, they can serve as foundation for screw design and process modeling. For a better understanding of the process, we additionally investigated the power consumption and viscous dissipation in Part B of this publication.

Modeling Fully Intermeshing Co-Rotating Twin-Screw Extruder Kneading-Blocks: Part B Power Consumption
Wolfgang Roland | Ursula Stritzinger | Christian Marschik | Georg Steinbichler, May 2021

Modeling twin-screw extrusion is commonly based on significant geometric simplifications such as the representation of the flow domain as flat channels. Furthermore, the prediction of the conveying characteristics and power demand of kneading blocks is typically based on their approximation as conveying elements. Considering the accurate flow geometry of fully intermeshing co-rotating twin-screw extrusion kneading blocks we analyzed the power characteristics by means of three-dimensional numerical simulations for Newtonian flow. Therefor we first conducted a dimensional analysis to identify the dimensionless characteristic influencing parameters. Next, we derived novel dimensionless power parameters and then conducted a parametric design study. Our proposed power parameters are capable to simultaneously cover conveying and non-conveying screw elements. The results provide new insights in the power characteristics of kneading blocks and are fundamental for screw design, screw simulation, and scale-up. In Part A. [1] of this work we focused on the conveying parameters.

New Concept for Melting in Single Screw Extruders
Robert A. Barr | Jeff A. Myers | Aaron F. Spalding, May 2021

In the late 1950’s Union Carbide’s research engineer, Bruce Maddock, ran several single screw extruder experiments. He established a method to reveal the melting profile in the screw by stopping the screw at speed and quickly cooling it to freeze the polymer. Then he reheated it to create a melt film on the barrel surface so he could pull the screw out. The condition of the polymer in the screw flights was studied. This revealed what Bruce called the solid bed melting mechanism. He showed that at the end of the feed section there was a tightly packed mass of solids in the screw channel. Melting occurred at the barrel surface as the conventional transition section decreased in depth. The melt was scrapped off the screw barrel surface by the rotating screw flight which deposited it into the rear of the screw channel. Thus, the solid bed melting mechanism was discovered. This mechanism has been the basis of all screw designs since. This paper will disclose an alternate melting mechanism which does not use the solid bed theory.

Development of an Analytical Mathematical Modelling Approach for a More Precise Description of Disperse Melting
Marius Dörner | Volker Schöppner, May 2021

The melting behaviour in single-screw extruders is of significant importance. For a high-quality extrusion product, a completely molten and thermal homogeneous, in case of a compound or the use of fillers also uniform concentrated, polymer melt is necessary. Due to the striving for the highest possible economic efficiency of the process, screws which can achieve a higher throughput at the same extruder size through higher screw speeds are often used. In these, however, melting no longer takes place in the classical way with a subdivision into melt eddy and solid bed, as was researched in the 1950s and 1960s by MADDOCK and TADMOR and successively extended by many more. Much more the solid bed breaks up into individual solid particles due to high forces or special screw geometries introducing disperse melting. The mathematical description of this process is not as mature as that of classical melting and is therefore the subject of this paper. An analytical mathematical model is presented which allows the calculation of the temperature development in the particle and the variable melting rate in addition to the actual melting process of the disperse melting. The temperature input by dissipation as well as by barrel temperature is considered. By means of an iterative procedure, complete screw geometries can be checked for the melting behaviour. Furthermore, a statistical experimental design based on the model is used to show which factors favor or impair disperse melting.

Evaluation of the Distributive Mixing Quality Based on Particle Tracking and Delaunay Triangulation
Maximilian Frank | Volker Schöppner, May 2021

In addition to conveying and melting, one of the core tasks of a single screw extruder is the homogenization of the material. Since conventional conveying screws in single screw extruders usually have an inadequate mixing effect, mixing elements at the end of the screw are commonly used to increase the homogenization performance. The melt homogeneity at the end of the screw is very important because it correlates strongly with the product quality and is therefore also directly related to reject rates in the production. However, characterization of the mixing quality is often very difficult because there are many degrees of freedom. In this paper, a new method for characterization of the distributive mixing quality on the basis of 3D CFD simulations is presented. In order to be able to assess the mixing quality, the particle trajectory of an initially defined particle distribution at the inlet of the flow must first be calculated with a particle tracking method. Subsequently, the homogeneity can be characterized by the change in the particle distribution at the end of the flow area. Bin counting is often used for this purpose. However, this method has considerable weaknesses, which will be shown. Consequently, a new characterization method based on the Delaunay triangulation has been developed and implemented in MATLAB. The new method will be demonstrated using selected fictitiously generated as well as simulated particle distributions of some different screw geometries.

Analysis of Leakage Flow in Pressure-Generating Melt-Conveying Zones
Christian Marschik | Wolfgang Roland | Alexander Hammer | Georg Steinbichler, May 2021

In many extrusion analyses, the pumping capability of the extruder screw is overestimated. This is usually due to the effect of the flight clearance being omitted in the mathematical model. The clearance between flight land and barrel surface enables the polymer melt to leak across the flights, thereby reducing the effectiveness with which the screw can pump the polymer melt forward. A few studies have proposed modifications to the widely known pumping model to account for the effect of leakage flow. While most of these consider Newtonian fluids, less attention has been directed towards shear-thinning polymer melts. We propose approximate equations to predict the flow of power-law fluids through the flight clearance of pressure-generating melt-conveying zones. Rather than correct the net material throughput of the single-screw extruder, we locally describe the two-dimensional flow between the flight tip and the barrel surface. Our novel models, which predict the flow rate and viscous dissipation, increase the understanding of the flow of shear-thinning polymer melts across the flights. Implemented in our screw calculation routine (introduced in [1-4]), they also serve as the basic equations for the network elements positioned over the screw flights.

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