Barrier Screw Research Articles

Optimization of an Industrial Recycling Line: The Effect of Processing Parameters on Mechanical Properties of Recycled Polyethylene (PE) Blends

This study concerns the optimization of an industrial recycling line; in other terms, this paper aims to find the optimal processing parameters that allow for a decrease in the loss of stress crack resistance (SCR) using a notched crack ligament stress (NCLS) test and an increase in the gain of the elongation at break, flexural modulus, and Izod impact strength of a polyethylene (PE) blend before and after recycling. The recycling line is composed mainly of a mono- and twin-screw extruder and a filtration system. Hence, the research question is as follows: How can we optimize the recycling process, without compromising the mechanical properties of recycled polyethylene (PE) blends? To answer the research question, Taguchi’s design of experiment and grey relational analysis (GRA) for multiobjective optimization was applied. Experiments were performed according to L16 standard orthogonal array based on five process parameters: mono-screw design, screw speed of the mono- and twin-screw extruder, melt pump pressure, and filter mesh size. Based on grey relational analysis (GRA), the optimal setting of process parameters was identified, and a barrier screw and a higher screw speed for both extruders were allowed to have optimal mechanical properties. Furthermore, the analysis of variance (ANOVA) indicated that the mono-screw design and screw speed of the mono- and twin-screw extruder significantly impact the mechanical properties of recycled polyethylene (PE) blends.

Evolutionary Multi-Objective Optimization of Extrusion Barrier Screws: Data Mining and Decision Making.

Polymer single-screw extrusion is a major industrial processing technique used to obtain plastic products. To assure high outputs, tight dimensional tolerances, and excellent product performance, extruder screws may show different design characteristics. Barrier screws, which contain a second flight in the compression zone, have become quite popular as they promote and stabilize polymer melting. Therefore, it is important to design efficient extruder screws and decide whether a conventional screw will perform the job efficiently, or a barrier screw should be considered instead. This work uses multi-objective evolutionary algorithms to design conventional and barrier screws (Maillefer screws will be studied) with optimized geometry. The processing of two polymers, low-density polyethylene and polypropylene, is analyzed. A methodology based on the use of artificial intelligence (AI) techniques, namely, data mining, decision making, and evolutionary algorithms, is presented and utilized to obtain results with practical significance, based on relevant performance measures (objectives) used in the optimization. For the various case studies selected, Maillefer screws were generally advantageous for processing LDPE, while for PP, the use of both types of screws would be feasible.

Statistical research on the mixing properties of wave based screws by numerical simulations

Abstract Research based on numerical simulation with the CFD software ANSYS POLYFLOW is conducted on the mixing properties for the wave screw elements, as well as barrier screw elements, by using statistical tools. Then, the investigation is conducted in detail on the relationship between pressure, maximum shear rate, mixing index and other flow field characteristics of the two above screws under the same simulation conditions. It is found that polymer melt flow in the wave screw possesses various advantages compared with the normal barrier screw, such as acquiring larger pressure, stronger shearing and stretching action, better mixing and efficiency, which mainly result from the periodic depth change design in the screw groove. On the other hand, the increased wave bulge in the wave screw lessens the space for the polymer melt to be conveyed forward, which can greatly reduce the original function of the secondary flight. Also, the convergent and divergent zones in the wave screw groove produce a much stronger stress favoring the shear and elongation rates, and also lead to a sharp increase of the axial force load on the wave screw. Consequently, for the optimization of the wave screw configuration it is suggested to reduce the axial force without decreasing the effect of the excellent distributive and dispersive mixing.

Design and Evaluation of General Purpose, Barrier, and Multichannel Plasticating Extrusion Screws

Extrusion screw designs and validation are presented for three multiple channel, fractal screws for comparison with common general purpose, and barrier screws using an instrumented single screw extruder with high impact polystyrene (HIPS) and low density polyethylene (LDPE) at varying screw speeds. The fractal screws are designed with multiple channels and pressure–volume–temperature relations to control shear heating with cooling by adiabatic decompression. The general‐purpose design had the highest throughput but did not provide sufficient mixing and so resulted in excessive variation in the melt temperature and pressure at screw speeds above 40 RPM. The barrier screw was a capable design with good performance for LDPE and HIPS with screw speeds from 20 to 60 RPM. However, it tended to provide excessive shear heating at higher screw speeds due to the large surface area of the barrier and mixing sections. The first fractal screw design was a multichannel variant of the general‐purpose design and exhibited good consistency but excessive heating due to the large bearing area between the flights and barrel. The second fractal screw design provided decompression in the feed zone and metering zone to improve throughput but was limited by a poor transition section design. The third fractal screw design remedied these deficiencies with an improved transition section and intermittent clearances for dispersive mixing. Its performance rivaled that of the barrier screw with respect to volumetric output and energy efficiency but provided better melt pressure consistency. Cold screw freezing experiments were performed for all five screws with 5% black, blue, and violet colorants serially added to neat HIPS. The cold screw pulls showed that the general purpose and barrier screws exhibited significant racing of the materials within their screw channels and, thus, broad residence time distributions. Examination of the material cross sections indicated persistent coiled sheet morphologies, which were best dispersed with the third fractal screw. POLYM. ENG. SCI., 60:752–764, 2020. © 2020 The Authors. Polymer Engineering & Science published by Wiley Periodicals, Inc. on behalf of Society of Plastics Engineers.

Analysis of a Single-Screw Extruder with a Grooved Plasticizing Barrel – Experimental Investigation

Abstract In the field of extrusion, a very promising high performance extruder was introduced to the European plastics industry in 1999. This new type of single-screw extruder combines a grooved plasticizing barrel with a barrier screw. Compared to conventional extrusion systems, these extruders have by far a higher output and product quality among other advantages such as lower processing costs and lower energy consumption. In the past, both Newtonian and non-Newtonian models were developed to calculate and predict the melting rate along the flow channel. The aim of this paper is to present experimental investigations on the processes within the melting zone of a single-screw extruder with a grooved plasticizing barrel. To visualize the melting mechanisms, screw extraction experiments as well as dynamic pressure measurements were performed. By means of screw extraction experiments a solid bed was clearly identified only in the solid channel and a melt pool only in the melt channel. Analysis of the dynamic pressure signals also showed the same behavior and therefore allowed in-line characterization of the melting process. Thus, theoretical concepts about the melting mechanisms could be confirmed experimentally.

Vortical Fountain Flows in Plasticating Screws.

Variances in polymers processed by single-screw extrusion are investigated. While vortical flows are well known in the fluids community and fountain flows are well known to be caused by the frozen layers in injection molding, our empirical evidence and process modeling suggests the presence of vortical fountain flows in the melt channels of plasticating screws adjacent to a slower-moving solids bed. The empirical evidence includes screw freezing experiments with cross-sections of processed high-impact polystyrene (HIPS) blended with varying colorants. Non-isothermal, non-Newtonian process simulations indicate that the underlying causality is increased flow conductance in the melt pool caused by higher temperatures and shear rates in the recirculating melt pool. The results indicate the development of persistent, coiled sheet morphologies in both general purpose and barrier screw designs. The behavior differs significantly from prior melting and plastication models with the net effect of broader residence time distributions. The process models guide potential strategies for the remediation of the processing variances as well as potential opportunities to achieve improved dispersion as well as complex micro and nanostructures in polymer processing.

Simulation of Non-Isothermal Non-Newtonian Flow Behavior of PP for Various Injection Molding Screws and Comparison with Experimental Results

Reducing injection molding cycle time is very important for improving productivity. In the cooling phase of injection molding, resin plasticization must be completed in the screw. This study examined the plasticization and flow behavior of polypropylene in injection molding screws using non-isothermal simulation. A standard screw and two types of barrier screws, the open-type, in which the barrier flight is open, and the closed-type, in which it is closed, were used. The geometry used in this simulation was wound using a channel model for a full three-dimensional flow simulation. Through non-isothermal simulation, the pressure distribution, melt flow, and temperature distribution were compared in terms of screw design. The experimental study revealed that in the open-type of the barrier screw, the solid bed and melt pool were well separated by the barrier flight, allowing for better melting than in the standard screw and the closed-type barrier screw. Simulation verified that resin melted better in the open-type barrier screw due to its easy inflow in the barrier flight and high temperature generation in the screw channel. The simulation results proved that the study of simulated flow and plasticization of resin in a screw is very useful for estimating screw performance.

Effect of Barrier Screw Design in Injection Molding Machines on the Plasticization Efficiency of Polypropylene Resin

사출성형의 사이클 타임에서 냉각시간은 가장 긴 시간을 차지하므로 냉각시간을 줄이기 위한 연구가 활발히 진행 중이다. 이에 따라 짧아진 냉각시간 내에 수지를 가소화시킬 수 있는 스크류 디자인에 관한 연구가 필요하다. 본 연구에서는 사출성형기용 barrier screw 디자인에 따른 PP 수지의 가소화 실험을 진행하였다. 실험에 사용한 barrier screw는 barrier flight의 도입부가 열린 타입(open type)과 닫힌 타입(closed type)으로 두 가지이다. 스크류를 계량한 후 정상상태에서 RPM에 따른 토출량, 배압, 토크, 수지온도를 측정하였다. 그리고 스크류를 배럴에서 빼내어 스크류 채널에 분포되어있는 수지의 단면을 관찰하여 PP 수지의 용융현상을 관찰하였다. 그리고 용융 시작점과 용융 완료점을 관찰하여 용융길이의 비교를 통해 스크류 형상에 따른 가소화 능력을 비교하였다.

Transport Behavior of PP in a Plastication Screw of an Injection Molding Machine : Comparison of Standard Screw and Barrier Screw

사출성형은 충진, 보압, 냉각 그리고 취출의 단계를 거치는데 수지의 가소화는 냉각단계에서 이루어진다. 냉각시간 이내에서 사출장치에서는 스크류 회전에 의한 계량이 완료되어야 하므로 빠른 시간 내에 수지를 녹일 수 있는 가소화의 연구가 필요하다. 본 연구에서는 사출성형기용 스크류에서 PP 수지의 가소화에 관하여 실험을 수행하였다. 실험에 사용한 스크류는 스탠다드 스크류와 배리어 스크류이다. 각각의 스크류에 대하여 RPM에 따른 실험결과를 비교하였다. 계량 후 정상적인 가소화 상태에 도달한 후 토출량, 배압 그리고 토크를 측정하였다. 또한 배럴에서 스크류를 빼내어 스크류 채널에 붙어 있는 수지의 단면을 관찰하여 용융 현상을 파악하였다. 그리고 용융 시작점과 용융 완료점을 관찰하여 용융길이의 비교를 통해 두 종류의 스크류에서 가소화 능력을 분석하였다.

Enhancing the electrical conductivity of carbon black/graphite nanoplatelets: Poly(ethylene‐butyl acrylate) composites by melt extrusion

ABSTRACTThe influence of processing parameters such as screw geometry, temperature profile, and screw speed on the electrical properties of hybrid composites consisting of graphite nanoplatelets and carbon black in ethyl butyl acrylate was studied. Two different screws were used to compound the hybrid composites at two different temperatures and two different screw speeds. A beneficial effect was noted with regard to the electrical properties when adding nanoplatelets to the filler system. The cause could be a synergistic effect due to the difference in particle shape of the two fillers. Lower percolation thresholds were obtained with the conventional screw due to less breakage of the graphite nanoplatelets compared to the barrier screw. No significant changes of the electrical properties were observed when changing the temperature profiles or the screw speeds. Furthermore, the melt viscosity of the compounds was not appreciably affected at the rather low filler contents used here. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 42897.

The Plasticating Sequence in Barrier Extrusion Screws Part II: Experimental Assessment

A modular laboratorial single screw extruder was used to perform screw pulling experiments and generate data on the development of plasticating of two polymers along the axis of a Maillefer barrier screw. The effects of the presence of the barrier (by comparison with an equivalent conventional 3-zone screw), of its geometrical parameters and of operating conditions were investigated and compared to the predictions provided by a previously developed computational model. Although the small size of the extruder influenced solids conveying and induced premature and quick melting, the two sets of data were generally in satisfactory agreement.

The plasticating sequence in barrier extrusion screws part I: Modeling

The plasticating sequence in barrier screws is modeled from hopper to die. The aim is to couple this routine to an optimization algorithm and use the software to set optimal operating conditions and screw design. The procedure developed to describe the geometry of the barrier section enables tackling most of the existing solutions. Calculations encompass gravity solids conveying in hopper, drag solids conveying, delay in melting, melting, and melt conveying. The onset and length of melting are decoupled from the beginning and profile of the barrier, providing a more realistic description of the effect of operating conditions and screw geometry. Predictions of melting and pressure axial profiles, output, distributive mixing, melt exit temperature, viscous dissipation, and mechanical power consumption are presented, and the effect of screw speed is discussed for a variety of barrier screw designs. POLYM. ENG. SCI., 54:1791–1803, 2014. © 2013 Society of Plastics Engineers

Barrier Screw Compounding and Mechanical Properties of EAA Copolymer and Cellulose Fiber Composite

Abstract The difficulty of feeding cellulose fibers and thermoplastics into the extruder or injection molding machine is addressed, this being a serious problem in the production of cellulose fiber composites for industrial applications. Agglomerates consisting in cellulose fibers and ethylene-acrylic acid copolymer (EAA) with different cellulose contents and different fiber lengths were processed with two different screws in order to better understand how the dispersion of the fibers can be improved by melt extrusion. A conventional screw with a compression ratio of 4:1 and a screw with barrier flights were used at different screw rotation speeds. The fiber length and fiber content were measured and microscopic analyses were performed in order to estimate the number and size of the cellulose fiber aggregates in the final composites. It was concluded that the barrier screw was more effective than the conventional screw in breaking up the fiber aggregates and dispersing the fibers. More but smaller cellulose aggregates were observed when the barrier screw was used, and the reduction of length was significantly greater for long than for short fibers. In contrast to that was expected, the samples containing the shorter fibers had better mechanical properties, probably due to a better dispersion of the fibers.

Composites of Recycled PET Reinforced with Short Glass Fiber

Composites of recycled poly(ethylene terephthalate) (PET) and short glass fibers (GFs) in different compositions (0, 20, 30, and 40 wt% of GFs) with optimized microstructures and high mechanical performance were obtained through melt processing. Composites showed appropriate dispersion and distribution and suitable bonding of the GF throughout the PET matrix, using either recycled bottle-grade PET in its degraded form or in a solid-state polymerized (SSP) form and GFs treated with either aminosilane or epoxysilane coupling agents. The high level of reinforcement of these PET/GF composites was confirmed by comparison of the experimental elastic modulus values of PET/GF composites with theoretical ones obtained using the Halpin–Tsai model. An important aspect highlighted by this study is that although it has been stated in the literature that this is only possible with the use of twin-screw extruders, these PET/GF composites with optimized microstructure and with high mechanical performance were compounded using a single-screw extruder with a double-flight barrier screw. In general, slightly better mechanical strengths were achieved for the composites based on the SSP recycled PET, which may be associated with its highly entangled amorphous phase arising from its higher molecular weight.

Quantification of dynamic mixing performance of single screws of different configurations by visualization and image analysis

AbstractThe visualization and image analysis techniques developed by the authors for the study of quantifying the dynamic quality of mixing of a single‐screw extruder were employed to investigate the mixing performance of screws of different configurations. The mixing quality was quantified by calculating the variances of the light intensity of the bitmap files cropped from the films taken at the screw length of 15D and 17D of a 45‐mm screw diameter single‐screw extruder. Temperature and screw speed were the two variables studied. While temperature and back pressure were found to have little effect on the mixing quality, screw speed was demonstrated to be a major factor responsible for the mixing fluctuation observed during extrusion. In fact, screw speed and mixing fluctuation appeared to have a relationship that the higher the screw speed, the bigger the fluctuation (i.e., poorer quality of mixing). The experimental results also revealed that the secondary flight of a barrier screw might be essential to improve the quality of mixing. © 2009 Wiley Periodicals, Inc. Adv Polym Techn 28:1–15, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/adv.20142

Relationship between processing method and microstructural and mechanical properties of poly(ethylene terephthalate)/short glass fiber composites

AbstractComposites of recycled poly(ethylene terephthalate) (PET) reinforced with short glass fiber (GF) (0, 20, 30, and 40 wt %) were compounded in a single‐screw extruder (SSE) and in a intermeshing corotating twin‐screw extruder (TSE). An SSE fitted with a barrier double‐flight screw melting section in between two single‐flight sections and a TSE with a typical screw configuration for this purpose were used. The composites were subsequently injection molded at two different mold temperatures (10 and 120°C), with all other operative molding parameters kept constant. The effects of processing conditions on composite microstructure, PET degree of crystallinity, and composite mechanical properties were evaluated. Appropriate dispersive and distributive mixing of the glass fiber throughout the PET matrix as well as fine composite mechanical and thermal‐mechanical properties were achieved regardless of whether the composites were prepared in the SSE or TSE. The performance of the SSE was attributed to the efficiency of the barrier screw melting section in composite mixing. The mold temperature influenced the mechanical properties of the composites, by controlling of the degree of crystallinity of the PET in the composites. For a good balance of mechanical and thermal‐mechanical properties, high mold temperatures are desirable, typically, 120°C for a mold cooling time of 45 s. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci 2008

Quantitative Analysis for Polymer Degradation in the Extrusion Process

Abstract Polymer degradation in the extrusion process decreases quality and productivity. For this reason, it is necessary to prevent polymer degradation. In the extrusion process, polymer degradation is caused by oxidation. It depends on the processing temperature and the amount of dissolved oxygen in the molten polymer. Therefore, a quantitative analysis of these factors is required. As for the degradation characteristics of the material used in this study, temperature and oxygen concentration dependency of the oxidation rate could be quantitatively characterized with an apparatus to evaluate polymer degradation which utilized chemiluminescence generated by an oxidation reaction. Moreover, an online measuring apparatus to analyze dissolved gas in the extruded molten polymer was developed. With this apparatus, the volume ratio of dissolved gases (N2, O2 etc.) to the extruded molten polymer could be quantitatively analyzed and the quality of the extruded molten polymer evaluated. With this apparatus, dissolved nitrogen (an index of entrained air) was analyzed with a full-flight screw and a barrier screw. Furthermore, observation of cross sectional views in the screw channel obtained from the cooling experiment under the operating conditions was carried out for the full-flight screw and the barrier screw. With the full-flight screw, break up phenomenon (collapse of solid polymer) occurred in the screw channel and the amount of nitrogen increased. With the barrier screw, the amount of nitrogen decreased because of prevention of the break up phenomenon in the screw channel. Consequently, it is shown that the use of the barrier screw is suitable for oxygen reduction in the molten polymer, which is a factor in causing polymer degradation.

Recent advances in barrier screw design

Barrier screws have been ubiquitous in the moulding and extrusion industry for almost half a century. However, conventional barrier screws have some drawbacks that limit their applicability. One of the most serious drawbacks of barrier screws is that they are more susceptible to solid bed plugging; severe plugging results in process instabilities (surging). Chris Rauwendaal describes how the optimum design of barrier screws can largely eliminate the chance of plugging while improving the melting and mixing performance.

Rheology, phase morphology, mechanical, impact and thermal properties of polypropylene/metallocene catalysed ethylene 1-octene copolymer blends

Blends of a polypropylene (PP) and a metallocene catalysed ethylene–octene copolymer (EOC) were prepared using a single screw extruder fitted with a barrier screw design. The EOC used had 25wt% 1-octene content and the weight fraction of EOC in the blends covered the range 1–30 wt.% Viscosity values for the blends determined experimentally from dual capillary rheological studies were similar to those calculated theoretically using the log additivity principle described by Ferry. This result together with scanning electron microscopy (SEM) observations and evidence from tan δ curves from dynamic mechanical thermal analysis showed PP and EOC to be partially miscible for blends having 10wt% EOC or less. The tensile modulus, break strength and flexural modulus of the blends decreased with respect to virgin PP as the weight fraction of EOC was increased to 30 wt.% The diminution in mechanical properties was concomitant with an initial increase in elongation at break from 40% for neat PP to 140% for the blend with 15wt% EOC before decreasing to 65% when 30wt% EOC was blended. The optimum impact modification of the PP used in this study, in the temperature range −40 to 23°C, was achieved by blending with between 20 and 30wt% EOC.

Dynamic Performance of Single-Screws of Different Configurations

Abstract A 45 mm diameter single-screw “transparent” extruder was employed for an investigation on the dynamic performance of screws of different configurations. The extruder had eight glass windows installed along the entire metal barrel which enabled visual observation to be easily made. It was demonstrated, with this extruder, that pressure profile appeared to be an important factor for screw performance such as throughput rate, residence time, mixing characteristics, break-up of solid-bed, etc. The full-flight screw had the lowest pressure profile among all the screws tested, and its performance was found to be generally inferior to that of the mixing screws. The Darnell and Mol solid plug flow theory was shown to be applicable only to the mixing screws for the solid flow behaviour in the entire solid conveying section. The secondary flight of the barrier screw was found to effectively suppress solid bed break-up, but only at low screw speed. A screw with a Maddock mixing device was found to have the highest throughput rate. Other findings were believed to be of practical significance in the screw design.