PFFC - November 2007 - (Page 53) David J. Bentley, Jr., Editor TAPPI, 15 Technology Parkway South, Norcross, GA 30092; 770-446-1400; 800-332-8686 (US); 800-446-9431 (Canada); FAX 770-446-6947 E X E C U T I V E S U M M A RY 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. E X E C U T I V E S U M M A RY 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. To obtain the complete version of these papers, go to www.tappi.org, and select “the PLACE” under the Publications/Bookstore heading. WWW.PFFC-ONLINE.COM Submit manuscripts for publication to dbentley@tappi.org. Obtain information about the PLACE Division from www.tappi.org NOVEMBER 2007 | 53 http://www.tappi.org http://www.tappi.org http://www.tappi.org http://WWW.PFFC-ONLINE.COM
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