LITMIR.BIZ

Скачать книгу

developed for testing metals and other similar materials. The Izod impact test, for example, was derived from the manual for testing metals. Because of the drastically different nature of plastic materials, the test methods often had to be modified. As a result, a large number of nonstandard tests were written by various parties. As many as eight to ten distinct and separate test methods were written to determine the same property. Such a practice created chaos among developers of the raw materials, suppliers, design engineers, and ultimate end‐users. It became increasingly difficult to keep up with various test methods or to comprehend the real meaning of reported test values. The standardization of test methods acceptable to everyone solved the problem of communication between developers, designers, and end‐users, allowing them to speak a common language when comparing the test data and results. As plastic materials are introduced for use in more rigorous applications, such as structural components in automobiles in the form of high strength composites, the need for test methods that are applicable to these new applications will be important and lead to further development.

In spite of the standardization of various test methods, we still face the problem of comprehension and interpretation of test data by an average person in the plastics industry. This is due to the complex nature of the test procedures and the number of tests and testing organizations. The key to overcoming this problem is to develop a thorough understanding of what the various tests mean and the significance of the result to the application being considered (1). Unfortunately, the plastics industry has placed more emphasis on how and not enough on why, which is more important from the standpoint of comprehension of the test results and understanding the true meaning of the values. The lack of understanding of the real meaning of heat deflection temperature, which is often interpreted as the temperature at which plastic material will sustain static or dynamic load for a long period, is one such classic example of misinterpretation. In the chapters to follow, we concentrate on the significance, interpretation, and limitations of physical property data and test procedures. Finally, a word of caution: it is extremely important to understand that the majority of physical property tests are subject to rather large errors. As a general rule, the error of testing should be considered ±5 percent. Some tests are more precise than others. Such testing errors occur from three major areas: (1) material variation, (2) the basic test itself, (3) the operators conducting the tests, and (4) variations in the test specimens. While evaluating the test data and making decisions based on test data, one must consider the error factor to make certain that a valid difference in the test data exists (2).

1.2. SPECIFICATION AND STANDARDS

      A specification is a detailed description of requirements, dimensions, materials, and so on. A standard is something established for use as a rule or a basis of comparison in measuring or judging capacity, quantity, content, extent, value, and quality.

      A specification for a plastic material involves defining particular requirements in terms of density, tensile strength, thermal conductivity, and other related properties. The specification also relates standard test methods to be used to determine such properties. Thus, standard methods of test and evaluation commonly provide the bases of measurement required in the specification for needed or desired properties (3).

      As discussed earlier, the ultimate purpose of a standard is to develop a common language, so that there can be no confusion or communication problems among developers, designers, fabricators, end‐users, and other concerned parties. The benefits of standards are innumerable. Standardization has provided the industry with such benefits as improved efficiency, mass production, superior quality goods through uniformity, and new challenges. Standardization has opened the door to international trade, technical exchanges, and the establishment of common markets. One can only imagine the confusion the industry would suffer without the specific definition of fundamental units of distance, mass, and time and without the standards of weights and measures fixed by the government (4).

      Standards originate from a variety of sources. The majority of standards originate from the industry. The industry standards are generally established by voluntary organizations that make every effort to see that the standards are freely adopted and represent a general agreement. Some of the most common voluntary standards organizations are the ASTM International, the National Sanitation Foundation, the Underwriters Laboratories, the National Electrical Manufacturers Association, and the Society of Automotive Engineers. Quite often, the industry standards do not provide adequate information or are not suitable for certain applications, in which case private companies are forced to develop their standards. These company standards are generally adapted from modified industry standards.

      The federal government is yet another major source of standardization activities. The standards and specifications related to plastics are developed by the U.S. Department of Defense and the General Services Administration under the common heading of Military Standards and Federal Standards, respectively.

      After World War II, there was a tremendous increase in international trade. The International Standards Organization (ISO) was established for the sole purpose of international standardization. ISO consists of the national standards bodies of over 145 countries from around the world. The standardization work of ISO is conducted by technical committees established by the agreement of five or more countries. ISO’s Technical Committee 61 on plastics is among the most productive of all ISO committees.

1.3. PURPOSE OF SPECIFICATIONS

      There are many reasons for writing specifications, but the major reason is to help the purchasing department purchase equipment, materials, and products on an equal basis. The specifications, generally written by the engineering department, allow the purchasing agent to meet requirements and ensure that the material received at different times is within the specified limits (5). The specifications are intended to ensure batch‐to‐batch uniformity, as well as remove confusion between the purchaser and supplier—we all know that more often than not, what is provided by the supplier is not what is expected by the purchaser.

1.4. BASIC SPECIFICATION FORMAT

      Many guidelines and directives have been formulated for writing specifications. The specifications for materials should include the following:

      1 A descriptive title and designation

      2 A brief but all‐inclusive statement of scope

      3 Applicable documents

      4 A classification system

      5 Definitions of related terms

      6 Materials and manufacturing requirements

      7 Physical (property) requirements

      8 A sampling procedure

      9 Specimen preparation and conditioning requirements

      10 Reference to, or descriptions of, acceptable test methods for determining conformance

      11 Inspection requirements

      12 Instructions for retest and rejection

      13 Packaging and marking requirements

      1.4.1. Classification System

Since plastic materials are seldom supplied without the addition of certain additives and fillers, a classification system must be used to avoid confusion. For example, the specification for acetal materials covers three main types of acetal resins: homopolymer, copolymer, and terpolymer. The resin types are subdivided into classes according to the grade descriptions. The group 1, class 1 represents general‐purpose homopolymer acetal resin and group 2, class 3 represents impact modified copolymer acetal resin. Table 1‐1 lists the detailed requirements for acetal materials.

      1.4.2. Requirements

      Physical requirements should

Скачать книгу