DIE DESIGN

Accurate description of flow of the polymer melt through the die requires knowledge of the viscoelastic behavior of the polymer melt. The polymer melt can no longer be considered a purely viscous fluid because elastic effects in the die region can be significant. Unfortunately, there are no simple constitutive equations that adequately describe the flow behavior of polymer melt over a wide range of flow conditions. Thus, a simple die flow analysis is generally very approximate, while more accurate die flow analyses tend to be quite complicated. 

Finite element analysis (FEM) has become a popular method for numerical simulation of flow through dies. One of the benefits of EEM is that it can handle non-linear fluids well. A newer numerical technique gaining popularity is boundary element analysis (BEM) Three-dimensional flow analysis with BEM can handle complex flow geometries well however, BEM at this point is not as good as FEM in handling nonlinear fluids. Less detailed analyses often use control volume analysis to reduce the computational effort. The different numerical techniques will be discussed in more detail in Chapter 12. 

Michaeli s book on extrusion dies [1] contains some information on the use of FEM in die design. The book by Crochet, Davies, and Walters [30] contains general information on the use of FEM in the analysis of non-Newtonian flow. More recent books on computer simulation in polymer processing are the books by Tucker [38] and O Brien [39]. The last book deals specifically with modeling for extrusion. 

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L. E. Sartor, "Slot Coating Fluid Mechanics and Die Design," Ph.D. dissertation. University of Minnesota, 1990. 

Labelling die variables as T, S and M, widi reference symbols, a, b, c respectively in trials involving a low (1) and high (h) levels of each variable, each trial having die result jc, die factorial procedure would produce die design code as follows ... 

To extract die EPBC liquids, die plant employs a cryogenic temperature process in combination with a refluxed demedianizer process in two trains. The two turboexpander trains are die heart of die process and bodi have operated at efficiencies higher dian originally expected at die design stage. 

With most homopolymers and copolymers the apparent viscosity is less dependent on temperature and shear stress (up to 10 dyn/cm ) than that of the polyolefins, thus simplifying die design. On the other hand the melt has a low elasticity and strength and this requires that extruded sections be... 

MW is die molecular weight K is die design factor HS is die hygiene standard. 

The design and operation of a proeess plant form an integral part of safety and systematie proeedures and should be employed to identify hazards and operability and, where neeessary, should be quantified. During die design of a new plant, die hazard identifieation proeedure is repeated at intervals. This is first performed on the pilot plant before the full-seale version as the design progresses. Potential hazards whose signifieanee ean be assessed with the help of experiments are often revealed by this study. 

When a molten plastic is forced through a die it is found that under certain conditions there will be defects in the extrudate. In the worst case this will take the form of gross distortion of the extrudate but it can be as slight as a dullness of the surface. In most cases flow defects are to be avoided since they affect the quality of the output and the efficiency of the processing operation. However, in some cases if the flow anomaly can be controlled and reproduced, it can be used to advantage - for example, in the production of sheets with matt surface finish. Flow defects result from a combination of melt flow properties, die design and processing conditions but the exact causes and mechanisms are not completely understood. The two most common defects are... 

Can all affected people hear die alann or see die designated signal ... 

To properly apply the system safety techniques to die design and operation of potentially hazardous teclmologies, die design engineer must have a clear miderstanding of die system and be able to prepare a written response to questions such as ... 

To identify areas of die design drat may possess a significant hazard potential... 

H. R. Kavianian, J. Rao, and G. V. Brown, Application of Hazard Evaluation Teclmiques to die Design of Potentially Hazardous Industrial Chemical Processes , NIOSH, Cincinnati, OH, 1992. 

Wolfenden, R., and Frick, L., 1987. Transition state affinity and die design of enzyme inhibitors. Chapter 7 in Enzyme Mechanisms, edited by M. I. Page and A. Williams. London, England Royal Society of London. 

Although there is no limit theoretically to the shapes that can be created, practical considerations must be met. These relate not only to product design but also to mold or die design, since these must be considered one entity in the total creation of a usable, economically feasible product. In the sections that follow, various phases considered important in the creation of such products are examined for their contribution to and effect on design and function. 

The mold or die should also be recognized as one of the most important pieces of production equipment in the plant. These controllable, complex devices must be an efficient heat exchanger and provide the product s shape. The mold or die designer thus has to have the experience or training and knowledge of how to produce the tooling needed for the product and to meet required tolerances with the plastic to be processed. 

Tolerance The penalty for having an unbalanced wall is the reduction of tolerance control. Tolerance limits are usually at least doubled. Also, with certain plastics it is more difficult to process them, such as those with low melt strength. Although the balanced wall is the ideal, having it is not always possible. Recognize that the unbalanced wall can be extruded with proper die design and control of the extruder line from upstream to downstream equipment. 

Brookfield, CT 06804-0403 Series of SPE sponsored seminars and workshops are held in various locations throughout the year. Subjects range from Die Design Principles to Plastic Part Design for Economical Injection Molding. A full range of seminars are also conducted in conjunction with SPE s annual technical conference (ANTEC). http //www.4spe.org... 

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