Development of an extrusion die with rollers for foaming polymers

Ideas concerning the use of a flat slot die with rollers in the technology of extrusion foaming of polymers are developed. Based on the experimental study of low‐density polyethylene foaming with isobutane, dependences of the cell density and foam density on the polymer solution viscosity, gap width between rollers, die temperature, and talc concentration have been analyzed. A model is formulated for estimating the intensity of coalescence of supercritical bubbles in an inhomogeneous field of flow velocities. The number density of supercritical nuclei formed in a polymer–gas solution in the limiting case of high pressure drop rate is estimated. The obtained simple estimations allow the results of experiments to be justified on a qualitative level. POLYM. ENG. SCI., 55:2256–2269, 2015. © 2015 Society of Plastics Engineers     

Carbon dioxide foaming of glassy polymers

The mechanism of foaming a glassy polymer using sorbed carbon dioxide is studied in detail. A glassy polymer supersaturated with nitrogen forms a microcellular foam, if the polymer is quickly heated above its glass transition temperature. A glassy polymer supersaturated with CO₂ forms this foam-like structure at much lower temperatures which indicates the T_g-depressing effect of CO₂. Having this interpretation in mind, the overall sample morphology, i.e., a porous foam enclosed by dense outer skins, can be completely explained. The dense skins, however, are not homogeneous but show a nodular structure when analyzed by SEM and AFM. Foaming experiments with samples having a different thermal history suggest that the nucleation mechanism underlying the foaming process is heterogeneous in nature. © 1994 John Wiley & Sons, Inc.     

Reactive modification of PBT with applications in low density extrusion foaming

By contrast to polyethylene terephthalate (PET), extrusion foaming of polybutylene terephthalate (PBT) to medium–low densities has been seldom reported in the literature. In this study, a commercial linear PBT resin was reactively modified in a batch mixer to a branched structure with a higher molecular weight (MW) and a broader molecular weight distribution (MWD) as evidenced by rheological analysis. Chain branching was also accomplished by single screw extrusion, where the competing degradation reaction needed to be taken into account. Optimization of the extrusion operational conditions, which also involved the use of suitable consititutive equations, led to the production of a branched resin with viscoelastic characteristics suitable for low density extrusion foaming by injection of physical blowing agents (PBA). The branched product made under the optimized conditions showed good foamability. The effect of pressure drop rate on cell nucleation rate and the effect of the crystallization rate on cell density and cell morphology are examined. POLYM. ENG. SCI., 47:244–253, 2007. © 2007 Society of Plastics Engineers.     

Melt viscoelasticity of polyethylene terephthalate resins for low density extrusion foaming

The rheological properties of conventional polyethylene terephthalate (PET) resins are not particularly suitable for low density extrusion foaming with physical blowing agents; as a result, chemically modified resins through chain extension/branching reactions are often used. Such resins have overall higher melt viscosity and higher melt strength/melt “elasticity” than unmodified materials. In this work, following a review of the prior art on PET chemical modification, an unmodified and a chemically modified resin were selected and characterized for their melt viscoelastic properties including shear and dynamic complex viscosity over a broad shear rate/frequency range, storage and loss modulus, and die swell. Certain rheological models were found to provide better fits of the entire viscosity curve for the unmodified vs. the modified resin. Foamed extrudates having variable densities (from about 1.2 to 0.2 g/cc), were prepared by carbon dioxide injection in monolayer flat sheet extrusion equipment. Foams with increasingly lower density, below 0.5 g/cc, were obtained by increasing gas pressure only in the case of the chemically modified resin. The effects of variables such as concentration of the physical blowing agent, resin rheology, resin thermal properties and choice of process conditions are related to product characteristics including density, cell size and crystallinity.     

高适应性挤出模具机头结构设计

探讨了挤出模具机头结构对模具适应性的影响,提出了对配方、工艺条件有高适应性模具结构的设计方法。     

Bubble coalescence in foaming process of polymers

This paper investigates the effect of rheological behavior on dynamics of bubble coalescence. In situ observations on polymers with different rheological behaviors were conducted to study coalescence dynamics. Four patterns of interface deformation were observed. Deformation during coalescence was analyzed by applying image‐processing schemes and rheological models to the in situ observations. Polymer with Newtonian behavior showed nucleation and coalescence of spherical bubbles that created a planar interface. The planar interface was deformed by a combination of biaxial and planar elongational flows. Coalescence in polymers with non‐Newtonian behavior is characterized by penetration with a parabolic shaped interface. The deformation of parabolic interface was due to biaxial elongational flow. However, the interface deformation of the polymer with cross‐linked structure was due to an integration of biaxial and elongational flow. Further, cross‐linking was also observed to have an effect on coalescence time. POLYM. ENG. SCI. 46:680–690, 2006. © 2006 Society of Plastics Engineers     

Conjugate thermal simulation for sheet extrusion die

The thermal effects on manifold temperature uniformity and output flow uniformity are important for polymer extrusion die design. Lin and Jaluria (Lin and Jaluria, Polym. Eng. Sci., 37, 1582 (1997)) has carried out a numerical study on conjugate heat transfer for extrusion polymer flow under the assumption that the die body surface is in uniform temperature or heat transfer coefficient. In this study, we have solved the non‐uniform body surface temperature as part of the simulation solutions based on heat flux boundary conditions (including radiation and convection heat transfer). The body temperature is computed in conjugated with the melt polymer flow with non‐linear viscous shear heating effect. The relative tough thermal conditions are set to test uniformity of the temperature distribution on the manifold wall. We also give the results of the heat transfer effect on the flow velocity distribution. POLYM. ENG. SCI., 54:682–694, 2014. © 2013 Society of Plastics Engineers     

单甘脂对聚苯乙烯挤出发泡性能的影响

利用超临界二氧化碳挤出发泡法,研究了单硬脂酸甘油脂（GMS）母粒的添加量对聚苯乙烯（PS）发泡性能的影响。采用毛细管流变仪研究了GMS添加量对PS/GMS体系的流变特性的影响,观察并测试了发泡材料的微观泡孔结构。研究结果表明,GMS的添加会降低树脂的黏度。母粒中含有的多组分GMS和少组分乙烯-醋酸乙烯（EVA）共同影响制品的表观密度、平均泡孔直径和泡孔密度等参数。在GMS添加量为1.05%,EVA添加量为0.45%时,制品的平均泡孔直径最小,泡孔密度最大。     

Numerical simulation of polymer foaming process in extrusion flow

Computer simulations of polymer foaming processes in extrusion flow have been carried out in order to improve current understanding of viscoelasticity and bubble growth effects on die-swelling in the production of polymer foam. The linear and non-linear viscoelastic materials functions of a commercial low density polyethylene (LDPE) melt have been extracted by fitting its rheometric data with constitutive models including a simple viscoelastic model (SVM), the exponential Phan-Thien–Tanner (EPTT) model and the double convected pom–pom (DCPP) model. Simulations of LDPE melt under extrusion flow without foaming show that the predictions of the die-swell by the SVM are in reasonably good agreement with the results obtained from the EPTT and DCPP models. By comparison of the simulation results of LDPE foaming in extrusion flow using the Bird–Carreau model and the SVM, a cooperative effect of polymer viscoelasticity and bubble growth on the die-swell has been quantified. The numerical results also show that the density of polymeric foam decreases significantly with the increasing concentration of foaming agent, and that the combination of the SVM and bubble growth model can account for some essential physics of polymer foaming process in extrusion flow.     

Correlation between die opening gauge and die swell for steel-rubber composite extrusion

Steel-belted tires are usually made by extruding rubber onto a bank of wires. The resulting steel-polymer composite must meet a gauge specification. However, the composite swells extensively after leaving the die throat, taking on a more or less corrugated appearance. This paper presents an experimental study to understand the influences different parameters have on die swell for this sort of extrusion. Basically, the experiment is conducted to correlate die opening gauge to die swell. Parameters that affect die swell extrudate expansion are identified and investigated. Die opening is varied throughout the experimental process, and the die opening and belt gauge are measured to calculate die swell. To achieve a more uniform gauge across the width of the belt, the die throat opening is divided into five zones. Each zone of each die throat is considered. Experimental parameters held constant include wire parameters, polymer parameters, and setup parameters. The experimental data result in an overall plot of die swell percent vs. die throat opening gauge. This plot can be used as a guideline for predicting required gauge opening in a steel belt gauge development. The study shows that die swell percents for extruded belts is more predictable than previously thought, and debunks the notion that die swell percent is constant. With regard to the time and effort for optimizing future die throat development, some concluding remarks and recommendations are made based on the analysis of the experimental results obtained in this study.     

Strain hardening in polypropylenes and its role in extrusion foaming

Extensional viscosity of several polypropylene polymers and their blends was measured and the foam processing of these blends using carbon dioxide blowing agent was studied. Foaming was carried out on a co‐rotating twin‐screw extrusion line, with a gear pump to build pressure. A linear isotactic polypropylene and two branched polypropylenes were considered. The uniaxial extensional viscosity was quantified and the foam characterized based on bulk density, cell size, and cell concentration. The linear polymer exhibits no strain hardening, while both branched polymers show pronounced strain hardening. Blends of low concentrations of branched polymer in the linear polypropylene show significant strain hardening down to 10‐wt% branched polypropylene. Strain hardening is expected to prevent cell coalescence and lead to higher cell concentrations. The branched polymers were found to have a lower cell concentration than the linear polymer. Yet blends of linear and branched polypropylenes attained a cell concentration higher than either of the neat polymers. This suggests that even small amounts of branched polypropylene blended in linear polypropylene can improve the foaming process. Polym. Eng. Sci. 44:2090–2100, 2004. © 2004 Society of Plastics Engineers.     

Solid state extrusion of semicrystalline polymers

The dynamics of solid state extrusion of polypropylene, high density polyethylene, and poly(tetrafluorethylene) have been investigated at various extrusion temperatures and piston velocities. A model of solid state extrusion of semicrystalline polymers is proposed. The formulae are obtained relating the value of the extrusion pressure with the parameters of the process and polymer properties. It has been found that for certain values of the parameters, defects of two main types emerge. A mechanism of defect formation is suggested.     

Continuous extrusion of high-modulus semicrystalline polymers

A process has been developed by which very high-modulus semicrystalline polymer films can be extruded continuously from a melt. This is accomplished by controlled cooling of the melt in a two-stage flow channel. A temperature gradient along the flow channel quenches the melt prior to an area reduction in which the polymer undergoes solid-state orientation. Analysis of high-density polyethylene tapes extruded by this process shows that they have properties similar to samples hydrostatically extruded at 120°C. Infrared analysis was used to determine both the degree of crystallinity and degree of orientation in these tapes as well as previously prepared hydrostatically extruded samples.     

An innovative method of die design and evaluation of flow balance for thermoplastics extrusion profiles

In this article, a computational and experimental method for flow balancing of a U‐shaped die profile with nonuniform thicknesses is presented. The approach was to implement a flow restricting mechanism along the melt flow path. A parametric study based on the restrictor dimensions was carried out to attain a preliminary optimal design. Simulations were performed using Fluent software to analyze the flow velocity at the die exit. Experimental study was then carried out at various restrictor positions for the purpose of attaining a desirable flow balance. The velocity at various segments of the die exit was measured utilizing an innovative procedure by implementing the “separating blades.” Experimental findings were compared with those of simulations which showed an acceptable agreement. The results suggest that a flexible die can be designed to achieve a flow balance under various processing conditions. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

聚丙烯片材挤出发泡工艺及发泡体系研究

采用自行研制的高熔体强度聚丙烯(PP),通过挤出片材发泡实验,研究了口模温度、挤出温度、螺杆转速等工艺条件以及PP熔体强度和发泡成核剂对片材发泡效果的影响。PP发泡片材最佳挤出工艺条件为:挤出温度210℃,口模温度160℃,螺杆转速40 r/min。PP熔体强度为13 cN,发泡成核剂用量为6 phr时,发泡片材密度最低(0.450g/cm~3),片材表面光滑平整,挤出发泡效果最好。     

Effect of viscoelastic properties of the base polymer on extrusion foaming with thermally expandable microcapsules

Thermally expandable microcapsules (MCs) are applicable to the foam processing of polypropylene (PP). The purpose of this study is to clarify the influence of processing conditions, such as temperature, shear rate and resin selection, on the behavior of thermally expandable MCs in PP. In this study, MC was added to several types of PP and extrusion foaming was performed. The average diameter of the MCs after processing was calculated to characterize the expansion of PP including MCs. Two types of molecular architecture, homo- and impact-PP, were examined in this study. The expansion behavior could be summarized by the shear viscosity at the extrusion die. Irrespective of the molecular architecture, data were summarized on one curve on a plane that expressed the relation between the shear viscosity and average MC diameter. Shear viscosity is thus an effective parameter to examine base resin selection for MC foaming. POLYM. ENG. SCI., 60:558–562, 2020. © 2020 Society of Plastics Engineers     

如何解决挤出模头渗料难题

模头咬模现象也称为口模泄料、模头渗料、积垢,是挤出加工过程中一种常见的故障问题。这种故障会引起多种产品缺陷,如可见挤出产品表面缺料、加工流程中断和产品完全开     

Design of an extrusion die with a variable choker bar

A novel approach is proposed to adjust flow nonuniformities caused by production variations from an extrusion die. A mathematical model that is based on the lubrication approximation was developed, and the effect of four types of production variations was examined separately. To each production variation, a specially designed choker bar was constructed and inserted into the extrusion die to correct flow nonuniformities. © 1994 John Wiley & Sons, Inc.     

On‐line NIR sensing of CO2 concentration for polymer extrusion foaming processes

An on‐line sensor using near infrared (NIR) spectroscopy is developed for monitoring CO₂ concentration in polymeric extrusion foaming processes. NIR absorption spectra are acquired by a probe installed at the foaming extruder die. The calibration curve relating the absorbance spectrum at 2019 nm to the dissolved gas concentration is derived so as to infer dissolved CO₂ gas concentration on‐line from measured NIR spectra. Experimental results show the developed on‐line NIR sensor can successfully estimate dissolved CO₂ concentration in the molten polymer and illustrate that the developed NIR sensing technique is among the more promising methods for quality control of polymeric extrusion foaming processes.