Plastics selection procedures

However, in general these fabrication and performance advantages are common to all plastics and so a decision has to be made in regard to which plastic would be best for a particular application. Rather than compare the basic raw material costs it is better to use a cost index on the basis of the cost to achieve a certain performance. Consider again the material selection procedures illustrated in Section 1.4.1 in relation to strength and stiffness. 

Plastics are families of materials each with their own special advantages. The major consideration for a designer is to analyze what is required as regards to performances and develop a logical selection procedure from what is available. 

Plastics are families of materials each with their own special advantages. An example is polyethylene (PE) with its many types include low density PE (LDPE), high density PE (HDPE), High molecular weight PE (HMWPE), etc. The major consideration for a designer and/or fabricator is to analyze what is required as regards to product performances and develop a logical selection procedure from what is available. 

Chapter 3 describes the various components of plastics and how they are transformed into three-dimensional functional solids. The chapter is not intended as a technical manual from which polymers can be synthesized and plastics prepared but as an overview of selected procedures and materials relevant to museum professionals concerned with plastics. Knowledge of plastics technology helps to date plastics objects, to interpret chemical analysis of plastics and to understand the causes of deterioration. Polymerization, additives and shaping processes contribute to the physical and chemical properties, useful lifetime and degradation pathways of plastics. The optical, chemical and physical properties of plastics are detailed in Chapter 4. 

Advances in the use of TP and TS plastic adhesives have made possible the adhesive bonding of RP structural and nonstructural parts in appliances, automobile, aircraft, medical devices, and so on. Adhesives with strengths higher than some metals are used (epoxy, etc.). The wealth of adhesive technologies that are available could make adhesive selection a task if one does use the proper approach such as determining specifically what performance requirements are needed (as with any selection procedure). The best adhesive for an application will depend on processing considerations and meeting the performance requirements. Tables 5.23 and 5.24 provide information on types and use of adhesives. 

As emphasized throughout this book, many different types of auxiliary equipment and secondary operations can be used to maximize overall processing plant productivity and efficiency. Their proper selection, use, and maintenance are as important as the selection of the processing machines (injection molder, extruder, etc.). The processor must determine what is needed, from upstream to downstream, based on what the equipment has to accomplish, what controls are required, ease of operation and maintenance, safety devices, energy requirements, compatibility with existing equipment, and so on. This chapter provides examples of this selection procedure and its importance in evaluating all the equipment required in a processing line. Details on all the equipment that is available can be obtained from plastics industry trade publications, usually compiled in an annual issue. These and other pertinent publications are included in the reference section (1-4, 33, 271-289). 

Some plastics can be used with many different processes, but certain others require a specific process. Generally, process selection takes place before material selection, as a range of materials may be available for different processes or only one method of processing may be available. The one process situation could be very unprofitable or restrictive to product performance. Figures 11-2 through 11-7 provide summary guides to selection procedures for processes, products, and materials (3). It is important to realize that the fabrication process can markedly influence product performance. 

The four major thermoplastics—polyethylene, polypropylene, PVC, and polystyrene—together represent over 85% by volume of world plastics consumption. Because of their lower prices, these commodity materials dominate the market, and in any materials selection procedure there are good economic reasons for considering them first before turning to the more expensive engineering plastics. 

If it is necessary to maximise performance, the engaged area should be as large as allowed by the design and the plastic s surface should be chemically treated or roughened if at all possible. Finally, a high-performance adhesive of low modulus - preferably toughened - should be used. The selection procedure of Section 5.2 helps locate a suitable material. Note in the presence of a steel part, most high quality anaerobic adhesives will set satisfactorily - even if one surface is non-metallic - but two non-metallic surfaces will invariably require a primer if an anaerobic adhesive is to cure. 

This chapter explores the influences leading to the use of particular plastics in particular applications. The first section identifies the characteristics which are decisive in motor industry applications, the second summarizes the strengths and weaknesses of the different polymer groups, the third offers suggestions for a materials selection procedure, and the fourth looks at the requirements of the different application areas. This is intended to provide a logical background to the detailed application surveys in Chapters 4, 5, 6 and 7. 

Experiments with unproven materials are discouraged in the motor industry, and until quite recently the quality of suppliers data did not encourage a detailed selection exercise. Since the late 1970s two factors have combined to help make sophisticated selection procedures a reality. One is the generation of meaningful multi-point data by the material suppliers the fruits of this were discussed in Designing with plastics in Chapter 2. The other is the development of computerized databases. 

The use of SPME for CE has not (yet) been studied widely. Li and Weber (170) reported an off-line SPME-CE approach for the determination of barbiturates in urine and serum, utilizing a sorbent of plasticized PVC coated around a stainless steel rod. Eor extraction, the coated rod was inserted for 4 min in a Teflon tube containing 50 p.1 of sample, and next the rod was repeatedly desorbed in another Teflon tube which each time contained 5 p.1 of desorption solution. This solution was transferred to an injection vial and an aliquot was injected into the CE system (Eigure 11.19). The extraction procedure appeared to be selective and effectively allowed the handling of very small samples. 

The selection of the evaluation method(s) depends on various factors such as the specific type of plastic, the type of flaw to be detected, the environment of the evaluation, the effectiveness of the evaluation method, the size of the structure, and the economic consequences of structural failure. Conventional evaluation methods are often adequate for baseline and acceptance inspections. However, there are increasing demands for more accurate characterization of the size and shape of defects that may require advanced techniques and procedures and involve the use of several methods. 

The technology of manufacturing the same basic type or grade of plastics (as with steel and other materials) by different suppliers may not provide the same results. In fact a supplier furnishing their material under an initial batch number could differ when the next batch is delivered and in turn could effect the performance of your product. Taking into account manufacturing tolerances of the plastic, plus variables of equipment and procedure, it becomes apparent that checking several types of materials from the same or from different sources is an important part of material selection and in turn their use. 

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