Material Selection Based on Service Life

In view of the complex failure and aging mechanisms of plastics, materiai seiection is of particuiar significance. To make a selection from the available types and grades of materials is not a task for a materials specialist alone, but a joint task to be performed together with design and production. Selection and adaptation of material groups and material types as well as meeting specifications requires an involved and iterative process. 

Beginning with the available total material palette, selection goes through a three-stage process of elimination in order to prepare the decision for the optimum compromise candidates from a wide variety of commercial types or lay the groundwork for further material development or modification. 

Service life is one of the evaluation criteria in each of the following three steps toward materials selection  

If the decision to use a thermoset material was made during Step 1, the rules for 

Assuming specific medium influence, its concentration, and operating temperature, media class I to X and the applicable resin groups are determined (example DIBt media lists [7]). This will result in the suggestion of several resin groups with key numbers, see also Appendix A. 17. 

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Information on the service life pattern of selective materials is based on the following data ... 

Select materials based on their functional suitability to the service environment. Materials selected must be capable of maintaining their function safely and for the expected life of the equipment, and at reasonable cost. 

Most screening machines are designed and fabricated such that their useful service lives are measured in decades rather than years in most applications. However, some applications can dramatically shorten service Ufe. Abrasive materials can cause premature wear, particularly when process rates are high. Some materials attack the screener s metal components. Salts, such as sodium chloride, calcium chloride, and ammonium sulfate, are particularly aggressive. In such cases, selection of the materials of construction is critical to maximizing screener service life. In this matter, it is best to work closely with the manufacturer, sharing all pertinent process parameters so recommendations can be made based on their experience with similar systems. 

T q)ically, cost analysis should be done on the basis of installed cost plus operating cost considering the useful service life of the equipment. It is also possible to base the selection solely on the initial cost of the equipment which consists of equipment and installation costs. A more accurate basis would consider the cost of the equipment over its useful life. This calculation is based on some discounted cash flow considerations and depreciation of the cost overthe life ofthe equipment. Tables 12.1-12.3 compare the costs of piping, vessels, and lining systems for different material selections. The reader should consult these tables to compare the initial equipment cost. 

The mechanical properties of a conductive material play an important role when selection is performed for any particular textile application. Metallic filaments usually compromise good electroconductive characteristics with poorer mechanical properties. A very high stiffness and lower stretchability of metallic fibers not only makes the woven or knitting process difficult but also reduces their service life. On the other hand, polymeric fibers or yams exhibit good elongation and recovery properties. The combination of nonstretchable metallic fibers with stretchable polymeric yams creates a new class of metal-based electroconductive fibers, which is known as co-spun polymeric-metal yams (Fig. 28.3). 

Selecting an elastomer for an application requires consideration (like for plastics and foams) of many factors, including the mechanical and physical service requirements, the product s life cycle, the material s processability, and its cost (see Figs. 6-24 and 6-25 and Tables 6-12 and 6-13). A wide range of properties is available, based on the many different compounds that can be produced. 

A material and component life assessment concept, that is traditionally based on the theory of creep only, has to be established for the new fuel types (bio-based wood chips, straw, peat, recycled waste, etc.). The OPTICORR approach aims at development of modelling tools for high-temperature active oxidation and corrosion to predict metal loss as a function of the service conditions and gas compositions. The development is based on selected cases and corresponding experimental studies. 

Fill Materials. Since the fill material is an additional fabrication material that becomes a part of the design construction, procurement documentation is required to specify a fill material type and thereby implement the via fill process. The selection and documentation of the fill material require the same consideration as the base laminate preference. This is especially critical when targeting a lead-free-compatible process. Currently, an industry-based material specification for via fill material does not exist.Therefore, specific fill-material brands may be named on the drawing, or some other form of user/supplier agreement must be established.The fabricator has preferences for the type of material used for via fill. Just as suppliers often have preferences for a specific solder mask brand, they also often prefer to use of a specific via fill material around which they have developed their principal processes. Supplier preferences can be driven by specific via fill material characteristics, such as accessibility, equipment compatibility, process supportability, plateability, and/or shelf/pot life. This may complicate source selection, or it might influence the use of a dedicated service center for the hole-fill process. The fabricator may not always know the reliability of its preferred material for a given via structure or end-use environment. 

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