Three-dimensional flow simulation of a film-casting process

Three-dimensional flow simulation of a film-casting process was performed using a finite element method assuming an isothermal and steady Newtonian flow. The simulation was carried out under industrial operation conditions. The neck-in and the edge bead phenomena could be simulated. The effects of draw ratio, air gap length, and die width on these phenomena are discussed. The neck-in and the edge bead phenomena were affected by the draw ratio and air gap length and not by the die width. The neck-in value, which was defined as the difference between the die width and film width at the chill roll, increased with the draw ratio and air gap length.     

Measurement and modelling of the film casting process: 2. Temperature distribution along draw direction

In the case film process a polymer melt is extruded through a slit die, stretched in air and cooled on a chill roll. During the path in air the melt cools and a reduction of both thickness and width takes place; obviously, temperature distribution, thickness and width reductions are function of draw ratio and stretching distance.Temperature distribution along the draw direction was measured as function of flow rate during film casting experiments performed with an iPP resin. A non-contacting method of measurement, based on a narrow-band IR pyrometer, was adopted.A good qualitative agreement is shown between experimental temperature data and predictions of a model accounting of radiation emissivity dependence upon film thickness. Differences are consistent with discrepancies of film thickness evolution along draw direction, indeed the model slightly over predicts both film thickness reduction and, parallel, temperature decrease along the draw direction.     

Experimental studies on the development of a cast film

The cast film process is of significant industrial interest. This work presents quantitative experimental results for the characterization of a film casting operation in the region between the die exit and the chill roll. The polymer melt employed is a commercial PET sample that has been characterized in both shear and extension‐dominated flows. Pointwise measurements of both the film temperature and the local velocity are made over the entire region between the die and the chill roll. The velocity increases in the gap, although the strain rate decreases as the polymer moves from the die to the chill roll. For a particular axial position, the velocity decreases significantly near the edges of the film. There are modest (～20 °C) decreases in the temperature between the die and the chill roll, with the greatest variations again occurring near the edges of the film. The experimental results are explained by the coupling between the temperature, velocity fields, and geometry of the experimental apparatus. POLYM. ENG. SCI. 45:443–450, 2005. © 2005 Society of Plastics Engineers.     

The processing of polypropylene cast films. I. Impact of material properties and processing conditions on film formation

Material properties have a significant impact on the response of polymers to industrial flow processes such as film casting. During film casting, molten polymer is extruded through an (approximately) rectangular die and then cooled rapidly. Subsequent biaxial stretching can be employed to manipulate the film properties. In this work, we investigate the effect of polymer viscosity (molecular weight), draw ratio, and die temperature on polypropylene film formation. We measure the width, two components of the velocity vector, and the temperature profiles throughout the film web. The polymer rheology and the die temperature affect the film geometry and temperature profiles in the air‐gap. The magnitude of the machine direction velocity component depends on both the machine and transverse position in the film. The velocity shows a maximum at the centerline and decreases toward the film edge. The measured transverse velocity component, a consequence of the film neck‐in, is seen to decrease from the film edges to the centerline. POLYM. ENG. SCI., 2010. © 2009 Society of Plastics Engineers     

Causes of edge beads in cast films

Cast polymer films are extruded through a uniform thin slit onto a quench roll. Between the die and the quench roll, thick edges called edge beads form, which have to be trimmed from the film, and are often scrapped. The relation between edge beads and neck-in is shown. Three causes of edge beads are discussed: surface tension, die swell, and an edge stress effect. Surface tension and die swell effects can be important in low viscosity and elastic materials respectively. The predominant cause of edge beads is an edge stress effect which occurs because the film is stretched between the die and the roll. The edge elongates in uniaxial stress while the center material elongates in plane strain.     

Dynamics and criteria for bubble instabilities in a single layer film blowing extrusion

In this paper, the performance of a new in‐line scanning camera system for the study of various bubble instabilities in film blowing extrusion is critically evaluated. Three commercial film‐grade polyethylenes, LmPE, LLDPE and LDPE, were used to generate the bubble instabilities. Reliable and objective criteria for differentiating various bubble instabilities such as draw resonance, helicoidal instability, and frost line height instability are proposed by using the new device. Detailed dynamics of each bubble instability was carefully investigated as a function of time in a broad range of the take‐up ratio (TUR), blow‐up ratio (BUR) and frost line height (FLH). In addition, effects of melt temperature and mass flow rates on dynamics of the bubble instabilities are discussed. It was found that the new system could capture the main characteristics of all bubble instabilities quantitatively. It was also found that magnitude and periodicity of radius variation during draw resonance of LmPE decreased as TUR increased at constant FLH and BUR. This implies that the origin of draw resonance in film blowing seems to be different from that observed in fiber spinning. In the case of helicoidal instability, eccentricity, which defines the deviation of the bubble center from the die center, decreased as TUR increased. However, the bubble could not be stabilized as expected. A graphical quantification approach to determine the stable zone in the bubble stability map is also discussed.     

薄膜流延过程中拉伸共振现象的研究进展

介绍了熔体流延过程中的拉伸共振现象、拉伸共振形成的机理以及在牛顿流体和粘弹性流体中拉伸共振现象的研究进展 ,讨论了加工参数 (拉伸比、纵横比和空气间隙等 )和聚合物弹性对拉伸共振的影响。     

Mathematical Modeling of Air-Drag Spinning for Nonwoven Fabrics

Air-drag spinning is one of the oldest fabrication processes for making fibrous materials. The entire process is similar to the melt spinning, except that: (1) high-velocity air jets are used instead of a take-up roll to pull and attenuate the extruded melt into fine filaments; (2) process temperature is about 100°C, higher than that used in the melt-spinning process for a given polymer. Figure 1 shows two types of air-drag spinning (spray spinning) used in industry [1–9]. Type 1 has a built-in air jet in the melt extrusion die, whereas the jets are located closely but differently in Type 2. In Type 1, the angle between the fiber and air-flow direction is about 30 to 60°, whereas it is about 1 to 7° in Type-2 design. A steep angle between fiber and air-flow direction results in breaking fiber, and the final nonwoven web consists of a lot of short fiber elements. A shallow angle tends to draw the fiber tangentially so that the web is formed from random, but substantially continuous, fibers.     

Numerical simulation of the cast film process

The cast film process is studied and a numerical simulation is performed using a finite element method. Two dimensional equations involving the thickness and a mean velocity are used by considering the film as a thin layer. The process is assumed to be isothermal between the die exit and the quick cooling on the chill roll. The width of the film is computed iteratively as a free surface problem by means of a finite element method. The thickness distribution of the film is obtained by a finite volume method. This approach allows modeling of the well known “dog-bone” or “edge bead” defect with a reasonable computation time.     

Identifying breach mechanism during air-gap spinning of lignin–cellulose ionic-liquid solutions

To be able to produce highly oriented and strong fibers from polymer solutions, a high elongational rate during the fiber-forming process is necessary. In the air-gap spinning process, a high elongational rate is realized by employing a high draw ratio, the ratio between take-up and extrusion velocity. Air-gap spinning of lignin–cellulose ionic-liquid solutions renders fibers that are promising to use as carbon fiber precursors. To further improve their mechanical properties, the polymer orientation should be maximized. However, achieving high draw ratios is limited by spinning instabilities that occur at high elongational rates. The aim of this experimental study is to understand the link between solution properties and the critical draw ratio during air-gap spinning. A maximum critical draw ratio with respect to temperature is found. Two mechanisms that limit the critical draw ratio are proposed, cohesive breach and draw resonance, the latter identified from high-speed videos. The two mechanisms clearly correlate with different temperature regions. The results from this work are not only of value for future work within the studied system but also for the design of air-gap spinning processes in general. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47800.     

Orientation and crystallinity in film casting of polypropylene

In the cast film process a polymer melt is extruded through a slit die, stretched in air, and cooled on a chill roll. During the path in air the melt cools while being stretched. Film casting experiments were carried out with an isotactic polypropylene resin. The temperature and width distributions were measured along the draw direction. Further, the crystallinity and Hermans orientation factor were measured on the final film. The process was described by a simple thermomechanical model derived elsewhere. The evolution of the molecular orientation parameters was calculated on the basis of a dumbbell model coupled with velocity and temperature distributions provided by the thermomechanical model. The experimental crystalline orientations of the final films collapsed into a single step‐shaped curve (from low to high orientation) if plotted versus the stress calculated by the model at the frozen line. The experimental values of the crystallinity and Hermans orientation factors are discussed on the basis of predictions of the dumbbell model for melt orientation at the frozen line and the crystallinity data obtained in quiescent conditions under the same cooling rate. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 1981–1992, 2002; DOI 10.1002/app.10422     

高速纺细旦聚乳酸拉伸丝的研制

选用国产聚乳酸切片为原料,利用自行设计的中试设备,通过预取向-拉伸两步法制得聚乳酸细旦拉伸丝,研究了切片干燥条件、纺丝温度、纺丝速度、拉伸温度、拉伸倍数对聚乳酸细旦拉伸丝的影响。研究表明:采用转鼓干燥及短甬道熔纺工艺,纺丝速度3200 m/min,经3.0倍拉伸后,制得单丝纤度小于1.1 dtex的聚乳酸细旦拉伸丝,纤维性能良好。     

Effect of processing on the crystalline orientation, morphology, and mechanical properties of polypropylene cast films and microporous membrane formation

Cast films of a high molecular weight linear polypropylene (L-PP) were prepared by extrusion followed by stretching using a chill roll. An air knife was employed to supply air to the film surface right at the exit of the die. The effects of air cooling conditions, chill roll temperature, and draw ratio on the crystalline orientation, morphology, mechanical and tear properties of the PP cast films were investigated. The crystallinity and crystal size distribution of the films were studied using differential scanning calorimetry (DSC). It was found that air blowing on the films contributed significantly to the uniformity of the lamellar structure. The orientation of crystalline and amorphous phases was measured using wide angle X-ray diffraction (WAXD) and Fourier transform infrared (FTIR). The amount of lamellae formation and long period spacing were obtained via small angle X-ray scattering (SAXS). The results showed that air cooling and the cast roll temperature have a crucial role on the orientation and amount of lamellae formation of the cast films, which was also confirmed from scanning electron microscopy (SEM) images of the films. Tensile properties and tear resistance of the cast films in machine and transverse directions (MD and TD, respectively) were evaluated. Significant increases of the Young modulus, yield stress, tensile strength, and tensile toughness along MD and drastic decreases of elongation at break along TD were observed for films subjected to air blowing. Morphological pictograms are proposed to represent the molecular structure of the films obtained without and upon applying air cooling for different chill roll temperatures. Finally, microporous membranes were prepared from annealed and stretched films to illustrate the effect of the PP cast film microstructure on the morphology and permeability of membranes. The observations of SEM surface images and water vapor transmission rate of the membranes showed higher pore density, uniform pore size, and superior permeability for the ones obtained from the precursor films prepared under controlled air cooling.     

铸片系统对BOPP薄膜的影响研究

介绍了双向拉伸聚丙烯（BOPP）薄膜铸片系统的设备结构和工作原理，详细地论述了铸片系统中模头、气刀、压边嘴、激冷辊以及水槽对BOPP薄膜的生产过程和性能的影响，并提出了一些注意事项和解决方法，对生产实践有相当的参考价值。     

Thermomechanical analysis and modeling of the extrusion coating process

During the extrusion coating process, a polymer film is extruded through a flat die, stretched in air, and then coated on a substrate (steel sheet in our case) in a laminator consisting of a chill roll and a flexible pressure roll. The nip, i.e. the area formed by the contact between the pressure and the chill rolls, constitutes the heart of the extrusion coating process. Indeed, in this region, some of the most critical properties, such as adhesion, barrier properties, optical properties, are achieved or lost. In this article, we first present an experimental investigation of the coating step, which enables to characterize the leading thermomechanical phenomena. It is shown that there is no polymer macroscopic flow in the nip, but a local flow within the asperities of the steel substrate surface. This microscopic flow, at the interface between the film and the substrate, is slowed by strong cooling conditions in the nip. Several models are then proposed, giving access to the temperature profile through polymer thickness and substrate, the pressure distribution in the nip as well as the behavior of the polymer melt in the nip at the interface with the substrate. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

Elongational flow and melt-spinning instability of concentrated suspensions of small particles in polymer melts

It is suggested that the existence of yield values in the elongational viscosity of concentrated suspensions of small particles in polymer melts leads to enhanced instability of uniaxial stretching and melt-spinning behavior. This is supported by analyses of filament stability. Severe instabilities are found in experiments on both simple stretching and melt spinning of filaments of suspensions of carbon black, titanium dioxide, and calcium carbonate in polystyrene. Necking and low elongations to break are observed in the former case. The melt-spinning results show “draw resonance” occurring at low drawdown ratios and high amplitudes of disturbances in the unstable region.     

聚甲醛纤维的一步法制备及其性能研究

采用自制的熔融纺丝一体化设备,使用不同熔融指数的聚甲醛的共混料为原料,通过DSC、TGA等测试以及纺丝工艺过程的分析,得出了最佳的原料配比和熔融纺丝工艺。研究了牵伸倍数对聚甲醛纤维拉伸强度的影响,结果表明:在10倍牵伸下,可以制备出拉伸强度为6.2 cN/dtex的聚甲醛纤维。     

Studies on blown film extrusion. III. Bubble instability

An experimental study has been carried out to investigate flow instabilities in blown film extrusion. Two types of flow instabilities were observed, depending on whether a bubble was under uniaxial or biaxial stretching. Under biaxial stretching, the phenomenon of a surface wave-type instability was observed, yielding wavy bubble shapes which very much resembled water waves at the free surface. Under uniaxial stretching, another type of instability, frequently referred to as draw resonance, was observed. It was also observed that, once draw resonance occurs, the amplitude and frequency of bubble diameter pulsing increased with stretch ratio. Quantitative information was obtained from a series of motion pictures taken of bubble diameter in both types of flow instability. It was observed further that an increase in extrusion melt temperature enhanced the severity of bubble instability.     

Studies on the mechanism of the instability of polymers in tensile testing

This study deals with the mechanism of the instability of tensile force occurring in tensile testing for polymer materials. It was found that through a proper transformation of inertia coordinates, a tensile test can be described in the same governing differential equations with the same boundary conditions as those employed in melt spinning. This fact suggests that the nature of the instability is simply due to draw resonance rather than any other cause. The effects of extension rate in tensile testing and the specimen dependence on the instability of tensile force were experimentally investigated by using specimens with various molecular orientations. The results are interpreted by applying the principles of draw resonance in melt spinning.