Material Choice

        This text considers chiefly those materials that are used for shaped products. The first consideration in material choice is whether plastics or some other solid material such as metal, ceramic, or a natural polymer such as wood will be used. All these materials have advantages in various applications. Table 6.3 shows the areas of advantage for plastics, which are linked to plastic's unique properties. 

Table 6.3 Advantages and Disadvantages of Plastic Materials Versus Other Common Materials Such as Metals, Ceramics, and Wood.

        Even though all plastics share some similar characteristics, each one has particular characteristics that distinguish it from all others. These characteristics are best understood within the context of the molecular and micro structure of the particular plastic and therefore a discussion of typical characteristics is an important part of each section of this text where that material is considered in detail. The purpose of this chapter is to provide an overview of the types of considerations that should be given to all plastic materials.

6.5.1. Databases

        After the decision has been made to use a plastic material, a specific plastic must be chosen. The wide variety of plastics and grades within each plastic type make this choice interesting and challenging. Several databases of plastics properties are available to assist in comparing the properties of each plastic type. These databases include online computer services (such as the GE database), encyclopedia sections (such as the Modem Plastics Encyclopedia or the Plastics Technology Manufacturing Handbook and Buyer's Guide), tables and charts published in plastics textbooks, and computer databases available on disk and CDs (such as from IDES). A searchable database is especially valuable because it allows rapid access to data for many products and sorting of plastics based on multiple selection criteria. 

        But although they are valuable, databases must be considered only as general guides. Several factors contribute to the need for caution in using the data contained within the databases. The following is a description of some of these factors. 

        Multiple Grades. Each plastic is offered for sale in a number of grades. These grades, which are largely functions of density (crystallinity), melt index (molecular weight), and molecular weight distribution, can be very significant in determining the properties of the plastic. The values in the databases often are averages for the several grades. 

        Multiple Additives. Some resin manufacturers offer plastic resins that are formulated for special properties. These resins can be considered different grades of a particular resin type. For instance, fire-retardant grades, impact grades (to which tougheners may have been added), filled grades (with inorganic fillers), reinforced grades (with chopped fiberglass), and many others can be purchased in most resin types. 

        Different Blends. The ability to produce resins with a wide range of properties by mixing two or more polymers has led to a wide variety of new resin types. These blends may be standard products for a resin manufacturer, but materials made on demand by compounders also are available. Even within a well-known resin type, such as ABS, the ability to copolymerize, graft polymers together, and blend and mix polymers leads to innumerable possible combinations of the basic components and, therefore, many different property combinations. 

        Improvements. New polymers are constantly being developed, and existing resins are constantly being improved. These improvements, either for cost or property enhancement, inevitably change the properties. Because improvements are made on a continuous basis, databases are always outdated, no matter how often they are reprinted or changed. 

        Additives. The database data generally omit the effects of additives. In many cases, the presence of major additives, such as an inorganic filler, can significantly change the properties.

6.5.2. Material Selection Methodology

        The wide variety of plastics and the wide range of properties available among plastic materials suggest that some organized method be used to help in the process of selecting a particular resin for a particular application. The following multistep process is one selection methodology that can be used. 

        Step 1: Define the Criteria for Material Selection. The overall product-realization process provides the framework that will serve as a basis for making the material selection. This framework includes the functional specification (which defines what the part must do and the environment in which it must operate), the appearance characteristics, the constraints on the part, the consequences of part failure, the codes and standards under which it might be required to operate, and the costs. The criteria should be well defined before the material selection process is begun. Although no list can cover all the properties that must be considered for all applications, the list in Table 6.1 covers constraints that should be considered in almost all applications. 

         Step 2: Determine the Advantages and Disadvantages of Each Polymer Type. This step is a preliminary screening of all polymers to determine which of them can logically satisfy the need. If the properties of certain plastics being considered are unknown, then those plastics should be retained for consideration at this phase of the selection process. Generally, however, the properties of plastic types are understood sufficiently to eliminate some and thus simplify the selection process. Some key properties or constraints of the particular application are usually sufficient to conduct this preliminary screening. For example, if the application is a structural one, low-density polyethylene can usually be excluded. Likewise, if the application requires high elongation, acrylic and polystyrene can be excluded. The exclusion of polymers that do not meet performance specifications will result in a shortened list of candidate polymers. 

         Step 3: Select Polymers for Special Properties. This step involves considering polymers in terms of special properties, especially those associated with the actual conditions under which the polymer must operate. This step eliminates many polymers, further shortening the list of acceptable ones. These special properties may be somewhat more subtle than those properties considered in previous steps, thus requiring a simultaneous evaluation of the importance of the special property in the actual use of the material along with the relative excellence of the performance of the plastic. For example, an application for a toy may be chiefly indoors, thus limiting the amount of UV exposure that will be encountered. Under this condition, a material that is UV-sensitive could be acceptable, although a UV-tolerant polymer could be rated slightly higher because of its additional versatility; that is, the part could be used outdoors without difficulty. Those polymers that are acceptable in a particular property might have different degrees of acceptability and therefore might be ranked according to their performance in the various environments, although some may simply be "acceptable" with no ranking possible. For instance, nylon is acceptable for most moderately high-temperature environments, but it is not as good as phenolic. The following are some possible use conditions that might be considered:

• Water solubility.
• Oil solubility.
• Solvent resistance (to a particular solventor group of solvents).
• Flame retardance.
• Corrosion resistance.
• Oxidation resistance.
• UV resistance.
• Heat resistance.
• Electrical resistance or some other electrical property.
• Permeation resistance (barrier properties).

        Other performance criteria should be considered at this stage as well. For instance, the ability of the material to be painted is important in some applications, such as automotive exterior panels. The ability of the material to be bonded is important if the part is a component in an assembly that must be bonded together. The functional specifications should be carefully considered for any special performance criteria that are not among the more common use considerations just mentioned. 

        Step 4: Rank the Resins and Make a Preliminary Selection. The short list of acceptable resins produced by following the previous steps can now be ranked according to the full range of performance criteria. Some criteria and constraints, such as size, shape, and mechanical properties, still have not been applied. These criteria are interrelated and so must be considered concurrently. For instance, if the constraint on size is quite broad, even materials normally considered to be flexible or weak can be strengthened by making them thicker or by making them in some structural form, such as an I-beam. Hence, the application of a constraint and the consideration of mechanical or other properties should be done iteratively. 

         Ranking the resins is done by considering the most important property and then giving a numerical rank based on that property. (The highest rank number should be given to the resin with the highest performance. In other words, if four resins are being ranked, the highest-ranked resin would be a 4.) As an example, suppose that stiffness is the most important property. A database of plastic properties would be consulted for all of the resins on the short list, and the stiffest resin would receive the highest ranking on that property, with others ranked in descending order based on their stiffnesses. A multiplier, or weighting factor (WF), is then assigned to the property to reflect the importance of this property relative to the other properties. For instance, stiffness might be given a weighting factor of 10, which would be the highest weighting factor because stiffness is the most important property. All of the stiffness ranks given to all potential resins would then be multiplied by this weighting factor to obtain a stiffness score for each resin for the stiffness property. All the candidate resins would then be ranked according to the next most important property, which might be solvent resistance. These ranks would be multiplied by a weighting factor (7, perhaps) that represents the importance of solvent resistance to the final performance of the part. Scores for solvent resistance would thus equal the ranking multiplied by 7. 

        This procedure would be followed for all of the properties having significance. When the resins have been ranked and scored for all significant properties, the scores are totaled for each resin. The resin with the highest total score would then be the first choice. This process is illustrated in Table 6.4. A blank table of this type is given in Appendix 3. 

Table 6.4 Methodology for Ranking and Scoring Resins According to the Importance of the Properties.

        Considerable ambiguity is likely at this stage of the material-selection process, especially as the various specific grades of resin are considered. Still, by considering the ranking that was obtained in the previous steps and the general capability of each resin and grade in meeting all of the performance criteria, an overall ranking of resins can be achieved. The highest-scoring resin is, then, the first choice for making the prototype or working model. 

6.5.3. Computer-aided Selection

        An alternative to the selection methodology just suggested is a computer-aided selection process. These semiautomated systems usually employ some formalized decision-making process that asks the designer to answer a series of questions and thereby narrow the selection process down to a few material candidates. 

        All these computer-aided selection systems require that the preliminary steps in the design process be completed. The functional specification must be prepared, a sketch of the part made, the constraints identified, the processing rules established, mechanical considerations given to the part size and shape, and cost considerations applied. These functional specifications and constraints are input to the computer by establishing the primary and secondary functions. For instance, the primary function of a case for a hand calculator might be to "enclose components." Secondary functions might be to "resist impact" or to "mask scratches." Using this approach, the designer is required only to specify the functions to be performed and the conditions under which a part is expected to operate. In satisfying the functional requirements of the part, the designer may modify the basic geometry or some special environmental need that will modify the material choice. All the standard choices are already embedded in the software so that the input for a particular part is simply a process of answering questions. The answers lead through a tree structure that eventually ends up with a complete or nearly complete function description of the part within the computer memory. 

        The material selection is done by the computer based on the major and minor functional specifications. The computer database contains properties for all of the major plastic materials. As the functional specifications are input, the computer eliminates the inappropriate materials from candidacy. This elimination process sometimes requires decisions in addition to those for the functional specification. For instance, the computer may ask what manufacturing process is to be used. Eventually, the computer narrows down the choices to a single material.