Polymer Processing

Principles and Design

Donald G. Baird, Dimitris I. Collias

• Publisher: Wiley
• Year: 2014
• Print ISBN: 9780470930588
• eBook ISBN: 9781118354728
• Edition: 2
• Topic: Technology & Engineering
• Subtopic: Physics
• Index: Physics

Fundamental concepts coupled with practical, step-by-step guidance

With its emphasis on core principles, this text equips readers with the skills and knowledge to design the many processes needed to safely and successfully manufacture thermoplastic parts. The first half of the text sets forth the general theory and concepts underlying polymer processing, such as the viscoelastic response of polymeric fluids and diffusion and mass transfer. Next, the text explores specific practical aspects of polymer processing, including mixing, extrusion dies, and post-die processing. By addressing a broad range of design issues and methods, the authors demonstrate how to solve most common processing problems.

This Second Edition of the highly acclaimed Polymer Processing has been thoroughly updated to reflect current polymer processing issues and practices. New areas of coverage include:

• Micro-injection molding to produce objects weighing a fraction of a gram, such as miniature gears and biomedical devices
• New chapter dedicated to the recycling of thermoplastics and the processing of renewable polymers
• Life-cycle assessment, a systematic method for determining whether recycling is appropriate and which form of recycling is optimal
• Rheology of polymers containing fibers

Chapters feature problem sets, enabling readers to assess and reinforce their knowledge as they progress through the text. There are also special design problems throughout the text that reflect real-world polymer processing issues. A companion website features numerical subroutines as well as guidance for using MATLAB^®, IMSL^®, and Excel to solve the sample problems from the text. By providing both underlying theory and practical step-by-step guidance, Polymer Processing is recommended for students in chemical, mechanical, materials, and polymer engineering.

1
IMPORTANCE OF PROCESS DESIGN

The intention of this chapter is not merely to present the technology of polymer processing but to initiate the concepts required in the design of polymer processes. A knowledge of the types of polymers available today and the methods by which they are processed is certainly needed, but this is available in several sources such as Modern Plastics Encyclopedia (Green, 1992) and the Plastics Engineering Handbook (Frados, 1976). In this chapter we present primarily an overview of the major processes used in the processing of thermoplastics. In Section 1.1 we begin by classifying the various processes and point out where design is important. In Section 1.2 we present a case study concerned with film blowing to illustrate how the final physical properties are related all the way back to the melt flow of a polymer through the die. Finally, in Section 1.3 we summarize the principles on which polymer process design and analysis are based.

1.1 CLASSIFICATION OF POLYMER PROCESSES

The major processes for thermoplastics can be categorized as follows: extrusion, postdie processing, forming, and injection molding. We describe specific examples of some of the more common of these processes here.

The largest volume of thermoplastics is probably processed by means of extrusion. The extruder is the main device used to melt and pump thermoplastics through the shaping device called a die. There are basically two types of extruders: single and twin screws. The single-screw extruder is shown in Figure 1.1. The single-screw extruder basically consists of a screw (Fig. 1.2) that rotates within a metallic barrel. The length to diameter ratio (L/D) usually falls in the range of 20 to 24 with diameters falling in the range of 1.25 to 50 cm. The primary design factors are the screw pitch (or helix angle, θ) and the channel depth profile. The main function of the plasticating extruder is to melt solid polymer and to deliver a homogeneous melt to the die at the end of the extruder. The extruder can also be used as a mixing device, a reactor, and a devolatilization tool (see Chapter 8).

FIGURE 1.1 Typical single-screw extruder. (Reprinted by permission of the author from Middleman, 1977.)

FIGURE 1.2 Two different extruder screw geometries along with the various geometric factors that describe the characteristics of the screw. (Reprinted by permission of the publisher from Middleman, 1977.)

There are an equal number of twin-screw extruders in use as single-screw extruders today. There are many different configurations available including corotating and counterrotating screws (see Fig. 1.3) and intermeshing and nonintermeshing screws. These extruders are primarily adapted to handling difficult to process materials and are used for compounding and mixing operations. The analysis and design of these devices is quite complicated and somewhat out of the range of the material level in this text. However, some of the basic design elements are discussed in Chapter 8.

FIGURE 1.3 Cross-sectional view of corotating and counterrotating twin-screw extruders.

The extruder feeds a shaping device called a die. The performance of the single-screw and corotating twin-screw extruders is affected by resistance to flow offered by the die. Hence, we cannot separate extruder design from the die design. Problems in die design include distributing the melt flow uniformly over the width of a die, obtaining a uniform thermal history, predicting the die dimensions that lead to the desired final shape, and the production of a smooth extrudate free of surface irregularities. Some of these design problems are accessible at this level of material while others are still research problems (see Chapter 6).

There are many types of extrusion die geometries including those for producing sheet and film, pipe and tubing, rods and fiber, irregular cross sections (profiles), and coating wire. As an example, a wire coating die is shown in Figure 1.4. Here metal wire is pulled through the center of the die with melt being pumped through the opening to encapsulate the wire. The design problems encountered here are concerned with providing melt flowing under laminar flow conditions at the highest extrusion rate possible and to give a coating of polymer of specified thickness and uniformity. At some critical condition polymers undergo a low Reynolds number flow instability, which is called melt fracture and which leads to a nonuniform coating. Furthermore, the melt expands on leaving the die leading to a coating that can be several times thicker than the die gap itself. (This is associated with the phenomenon of die swell.) The problems are quite similar for other types of extrusion processes even though the die geometry is different. The details associated with die design are presented in Chapter 7.

FIGURE 1.4 Cross-head wire coating die. (Reprinted by permission of the publisher from Tadmor and Gogos, 1979.)

We next turn to postdie processing operations. Examples of these processes include fiber spinning (Fig. 1.5), film blowing (Fig. 1.6), and sheet forming (Fig. 1.7). These processes have a number of similarities. In particular, they are free surface processes in which the shape and thickness or diameter of the extrudate are determined by the rheological (flow) properties of the melt, the die dimensions, cooling conditions, and take-up speed relative to the extrusion rate. The physical and, in the case of film blowing and sheet forming, the optical properties are determined by both the conditions of flow in the die as well as cooling rates and stretching conditions of the melt during the cooling process. Furthermore, slight changes in the rheological properties of th...

Table of contents

• Cover Page
• Title Page
• Copyright Page
• Contents
• Preface
• Preface to the First Edition
• Acknowledgments
• 1 Importance of Process Design
• 2 Isothermal Flow of Purely Viscous Non-Newtonian Fluids
• 3 Viscoelastic Response of Polymeric Fluids and Fiber Suspensions
• 4 Diffusion and Mass Transfer
• 5 Nonisothermal Aspects of Polymer Processing
• 6 Mixing
• 7 Extrusion Dies
• 8 Extruders
• 9 Postdie Processing
• 10 Molding and Forming
• 11 Process Engineering for Recycled and Renewable Polymers
• Nomenclature
• Appendix A Rheological Data for Several Polymer Melts
• Appendix B Physical Properties and Friction Coefficients for Some Common Polymers in the Bulk State
• Appendix C Thermal Properties of Materials
• Appendix D Conversion Table
• Index