- November 01, 2007, David J. Bentley, Jr., Editor
Troubleshooting Extrusion Processes With Hands-On Computational Procedures
by Natti S. Rao, Plastics Solutions International Consulting
APPLICATION: Most machine and process related problems in extrusion processes are solvable on the shop floor where they occur.
The important components of extrusion coating equipment are the extrusion screw that creates the melt from the solid resin and the film die that distributes the melt stream over the substrate film. These machine elements influence the quality of the product significantly. They can be designed by means of the state-of-the-art numerical software with good results. In daily practice where quick estimates of the parameters involved are necessary, the application of this kind of sophisticated software is time consuming and expensive. This situation has led to the development of practical design formulas that are easily applicable while considering resin rheology.
Troubleshooting machine and process related problems in extrusion processes such as film manufacturing and extrusion coating is gaining increasing importance as the drive for reducing production costs intensifies. Experience indicates that most problems are solvable on the shop floor where they occur by applying simple calculation procedures of plastics engineering to analyze the process and the converting machine design. Starting from basic polymer rheology, this paper provides methodology on the basis of easily applicable formulas to answer questions concerning why one needs viscosity data, how to compare the behavior of different resins, and why does screw output decrease. The answers to such questions are illustrated by means of interactive examples prompting one to solve the exercises presented with the help of the formulas given with each example.
The paper presents practical design formulas for dimensioning extrusion coating screws. It shows that the resin rheology can be reliably considered by means of the power law exponent that can be conveniently obtained from melt flow curves. To calculate the melt flow distribution in a film die, one must be able to predict the pressure drop in the die. The paper therefore presents a practical relationship for this quantity.
Troubleshooting Extrusion Problems In Coextrusion Film Applications
by Karen Xiao, Brampson Engineering, Inc.
APPLICATION: The paper presents suggested solutions to problems encountered in the coextrusion process and offers an actual case study to help with the illustrations.
A big challenge in actual extrusion operations is to identify extrusion problems in the most efficient way to reduce scrap and downtime. With the emergence of coextrusion film applications, the tendency now is to have seven or more layers to achieve the desirable final film properties in the most economical way possible. It is therefore increasingly difficult to pinpoint the problematic layer or layers. In this paper, problems often encountered in coextrusion will be identified, and solutions will be suggested. Finally, the paper will present a real-world case study to help with the illustrations. The goal of the presentation is to try to summarize the common issues that are frequently seen in coextrusion and present a systematic way of tackling these issues.
Many issues often occur in coextrusion. One involves haze lines. This phenomenon typically exhibits lines in the film in the machine-direction (MD) where these lines are typically hazier than rest of the film. Melt stability is a second issue. As the name implies, the bubble is unstable and therefore can result in reduced line output and in rolls of film that are sub-standard. Gels present a third problem. This is probably one of the most common issues, but it is also least understood area. There are many types of gels that can coexist in the same process. As a first step, one must therefore identify the types of gels. Interfacial instability is another issue. There is a potential for interfacial instability to occur as long as there are two or more materials flowing next to each other. The literature documents two main types of interfacial instability as the paper discusses. Poor gauge is also a problem. The paper discusses each issue mentioned here and others in greater detail.
New Metering Pump Technology For Laminating Adhesives
by Bruce Menk
APPLICATION: A linear displacement pump can process one, two, and three component solvent borne, aqueous, and solventless adhesives.
A new metering pump technology will handle one, two, and three component adhesives that are available in solvent or water or solventless. The technology uses a linear displacement pump (LDP). This is a double-acting, positive displacement, continuous flow, rod displacement pump driven by PLC controlled DC servo motors. The systems have wide ranging dispensing parameters. This paper discusses the comparative advantages of an LDP design, control features, dispensing parameters, and economic efficiencies of operation versus batch processing.
Typical plural component dispensing systems in use today incorporate fluid metering devices such as piston pumps, gear pumps, flow meters, or progressive cavity pumps. The metering accuracy of these pumps can be affected by wear, viscosity variations, and phasing issues. A “clean slate” approach to fluid metering with a new pump design and control system overcomes limitations of other fluid metering pumps.
A linear displacement pump is a positive displacement pump that uses linear motion as opposed to rotary motion to move fluid. A LDP implementation features a volumetrically accurate double-acting continuous flow rod pump. This has two sets of fluid cylinders where a center carriage plate reciprocates between the top and bottom plates driven by a ball screw. One set of cylinders is mounted on the top of the carriage plate, and one set of pump rods is mounted on the bottom of the carriage plate. As the carriage traverses down, the bottom set of cylinders are dispensing while the top set of cylinders are filling. When the pump reciprocates to traverse in the up direction, the top set of cylinders are dispensing while the bottom set of cylinders are filling. A linear transducer provides pump position feedback allowing an automation control device to reverse the pump at the end of each stroke.
Integral to the operation of this LDP is a crossover valve or 4-way switching valve. A crossover valve directs fluid to and from the cylinder sets. In this LDP implementation, each set of cylinders is filled and dispensed from a single port. This design eliminates the use of check valves generally found in pumps featuring separate fill and dispense ports. When the LDP changes direction, the pneumatically actuated crossover valve switches the fill/dispense function of each cylinder set.
Polyethylene Co-extruded Blown Film: Layer Arrangements For Optimum Optics And Stiffness
by Lou Mele and Wen Lin, NOVA Chemicals
APPLICATION: Layer arrangements influence the optical and tensile properties of three- layer polyethylene film structures.
The blown film coextrusion process provides the opportunity to create film structures with optical and physical properties that typically could not be achieved in monolayer structures alone. A study evaluated the effect of layer arrangements on the optical and tensile properties of three-layer polyethylene film structures. Raman spectroscopy and atomic force microscopy (AFM) were used in an attempt to evaluate the hypotheses generated in a previous study and to establish a better understanding of the mechanism that contributed to the combination of good optical and stiffness properties. Good correlation between surface roughness and surface haze was observed, and the mechanism of the improvement on the optical properties was confirmed. An optimal layer arrangement was found and applied in the co-extruded blown films with LLDPE, LDPE, and HDPE blends. A recommendation was presented on materials selection and structure design that would give optimum optics and stiffness for the multilayer co-extruded films.
The exceptional growth in the flexible packaging industry is fueled by form as much as function. Considerations regarding package appearance and texture are as important as product protection in garnering consumer attention today. In addition to high quality print and graphics, packages are also required to sparkle, shine, and stand up.
The versatility of polyethylene is well suited to today's complex packaging structures. It performs especially well in co-extrusions where available grades can provide the necessary toughness, heat-seal, stiffness, and optical properties. Previous studies have shown that the layer arrangement has a significant effect on the optical properties of co-extruded film, and the mechanism was speculated upon two hypotheses. Special crystalline structures (i.e. transcrystalline structure) are formed on the interfaces between each layer within the co-extruded film which have an impact on the overall optical properties. The roughness of the outside surface layer is solely responsible for the optical properties. The velocity profile at the die exit of the outside layer varies as layer distribution changes and consequently changes the surface roughness of the co-extruded film.
In the first part of this paper, experiments were undertaken on a microstructure level aimied at better understanding the mechanisms and verifying the hypotheses. Based on the results, a variety of three-layer co-extruded films were made using linear low density polyethylene (LLDPE), high density polyethylene (HDPE), and low density polyethylene (LDPE) in an attempt to obtain optimal performance with respect to both optical and mechanical properties. This study of the impact of layer arrangement on the optical properties of polyethylene three-layer film has confirmed that the roughness of the outside surface layer is primarily responsible for the optical properties of co-extruded multilayer films.
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