Plastic design

Degradable plastic is a plastic designed to undergo a significant change in its chemical stmcture under specific environmental conditions, resulting in a loss of some properties that may vary as measured by standard test methods appropriate to the plastic and the appHcation in a particular period of time that determines its classification. 

Case study 2 plastic design materials for a pressure vessel... 

At low strains there is an elastic region whereas at high strains there is a nonlinear relationship between stress and strain and there is a permanent element to the strain. In the absence of any specific information for a particular plastic, design strains should normally be limited to 1%. Lower values ( 0.5%) are recommended for the more brittle thermoplastics such as acrylic, polystyrene and values of 0.2-0.3% should be used for thermosets. 

This book will not provide extensive engineering equations since they are readily available from industry that are reviewed in Appendix A PLASTICS DESIGN TOOLBOX and references (3, 6,10,14, 20, 29, 31, 36, 37, 39, 43 to 125). Equations will be reviewed throughout this book where they are required to understand the behavior of plastics in order to meet different load requirements (static to dynamic). What this book provides is information on the behavior of... 

As these problems were encountered in the past, it became evident that we did not have at hand the physical or mathematical description of the behavior of materials necessary to produce realistic solutions. Thus, during the past half century, there has been considerable effort expended toward the generation of both experimental data on the static and dynamic mechanical response of materials (steel, plastic, etc.) as well as the formulation of realistic constitutive theories (Appendix A PLASTICS DESIGN TOOLBOX). 

This is the one serious limitation in plastic design problems. Even if the designer did wait for data on one material, chances are the final design might be switched to another plastic or formulation. Thus, as a compromise, data from relatively short-term tests are extrapolated by means of theory to long-term problems. However, when this is done, the limitations inherent in the procedure should be kept in mind. 

The first step in applying FEA is the construction of a model that breaks a component into simple standardized shapes or (usual term) elements located in space by a common coordinate grid system. The coordinate points of the element corners, or nodes, are the locations in the model where output data are provided. In some cases, special elements can also be used that provide additional nodes along their length or sides. Nodal stiffness properties are identified, arranged into matrices, and loaded into a computer where they are processed with certain applied loads and boundary conditions to calculate displacements and strains imposed by the loads (Appendix A PLASTICS DESIGN TOOLBOX). 

Uncertainty about a material s properties, along with a questionable applicability of the simple analysis techniques generally used, provides justification for extensive end use testing of plastic products before approving them in a particular application. As the use of more FEA methods becomes common in plastic design, the ability of FEAs will be simplified in understanding the behavior and the nature of plastics. 

A common pressure vessel application for pipe is with internal pressure. In selecting the wall thickness of the tube, it is convenient to use the usual engineered thin-wall-tube hoop-stress equation (top view of Fig. 4-1). It is useful in determining an approximate wall thickness, even when condition (t < d/10) is not met. After the thin-wall stress equation is applied, the thick-wall stress equation given in Fig. 4-1 (bottom view) can be used to verify the design (Appendix A PLASTICS DESIGN TOOLBOX). 

The early development of modern plastic materials (over a century) can be related to the electrical industry. The electronic and electrical industry continues to be not only one of the major areas for plastic applications, they are a necessity in many applications worldwide (2,190). The main reasons is that plastic designed products are generally basically inexpensive, easily shaped, fast production dielectric materials with variable but controllable electrical properties, and jn most cases the plastics are used because they are good insulators (Chapter 5, ELECTRICAL PROPERTY). 

Designing has never been easy in any material, particularly plastics, because there are so many. Plastics practically provide more types with the many variations that are available than any other material. Of the more than 35,000 different plastics worldwide, only a few hundred are used in large quantities. Unfortunately, some designers view plastics as a single material because they are not aware of all the types available (Appendix A, PLASTICS DESIGN TOOLBOX). 

The problem of acquiring complete knowledge of candidate material grades should be resolved in cooperation with the raw material suppliers. It should be recognized that selection of the favorable materials is one of the basic elements in a successful product-configuration design, material selection, and conversion into a finished product (Appendix A PLASTICS DESIGN TOOLBOX). 

Here are examples in the selection of the many resources available to the plastics designer and other plastics users also review references 3, 6, 10,14, 20, 29, 31, 36, 37, 39, 43 to 125. 

Published by the Plastics Design Library, PDLCOM is an exhaustive reference source of how exposure environments influence the physical characteristics of plastics. Data include resistance to thousands of chemicals, weathering and UV exposure (i.e. color change after accelerated weathering or outdoor exposure) sterilization (radiation, ethylene oxide, steam, etc.) thermal air and water aging environmental stress cracking and much more. 

---