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Polymers are used extensively in electrical/electronic devices. Because the commercial life cycles of these devices are very short, it is important for manufacturers to determine the most suitable polymer as quickly as possible in the product development process. Choosing the right polymer also helps ensure that the device meets its marketing, performance, and manufacturing-cost goals. Because there are many polymers from which to choose, the selection of the most favorable one is a difficult process. Lists published by producers or compounders of engineering polymers contain hundreds or even thousands of formulations. There are several approaches followed by designers and engineers when selecting a material for a particular application: * a commonly used material is "force-fit" into the application; * the suggestions of experienced colleagues are followed, although the colleagues' knowledge may be limited to those polymers with which they are most familiar; * choice is made based upon guesswork or speculation; * nylon is the only material considered, even though a more suitable or cost-effective material may be available; * material suppliers are contacted for suggestions. While the preferred method of material selection is to consult the material supplier, who is familiar not only with a wide range of polymer families, but also with specialty grades and materials that have been successfully used in similar applications, some designers and engineers choose to select materials based upon their own experience and knowledge. For those who elect to proceed without the assistance of material suppliers, a simplified five-step approach to selecting materials for E/E applications is as follows: 1. Define all requirements at the outset of the product development process. 2. Begin with a short list of polymer families that have previously been used successfully in E/E applications. 3. Determine which polymers are most likely to be capable of fulfilling key E/E requirements of the current application. 4. Select the best standard grade of material. 5. If a standard grade is not appropriate, seek assistance before changing the design. Defining All Requirements The emphasis is on the word all. A common cause of delays and problems with polymers in E/E applications is that a key requirement has been overlooked. Although most prerequisites are obvious, difficulty often arises when less obvious ones go unnoticed. Following are some of the requirements to be considered for narrowing polymer selection to the family level: * What is the function of the part? * What are the service conditions, from assembly to shipping and end use, that make up the part's environment? * What are the key electrical requirements? * How does the part interface with other parts of the assembly? * What secondary operations will the part undergo? * What long-term dimensional tolerances are required? * What are the impact loading conditions? * What mechanical properties are required? * What agency approvals are needed for each country in which the part will be distributed? * Will the part be black, natural, or colored? The Short List The ideal polymer exists for almost any application; the challenge is to find it. If too many materials are considered, time and money are lost. If too few are considered, options are limited or unnecessary compromises are made to the design. Table I lists polymer families that have been used most often in E/E applications worldwide. All of the polymers in these families solidify into either an amorphous or semicrystalline structure. These chemical structures can help to identify polymers that would be appropriate for a particular application, or at least help rule out some polymer families as being unsuitable. Amorphous materials exhibit less shrinkage and warpage than semicrystalline materials. However, impact strength and elongation are typically better with amorphous materials. On the other hand, semicrystalline materials offer better chemical resistance, especially to organic solvents, than amorphous types, and also offer better melt flow characteristics for easier filling of thin-walled parts. Polymer Cost A comparison of the cost of resins should be based on cost per volume, not cost per pound. Although polymer families can vary widely in cost per pound, they can also vary widely in density. A stronger, lighter, but more expensive polymer often proves to be the most economical. For example, when the maker of an electrical fuse box sought a lower-cost material to replace a 45% glass and mineral- reinforced flame-retardant polybutylene terephthalate (PBT), Laestra syndiotactic polystyrene (SPS) was suggested, because it would be warpage-free and more dimensionally stable than PBT. Trial runs showed that Laestra G/30 VOCT1, an SPS with only 30% glass and no minerals, would meet requirements and be more economical. Though the SPS polymer cost more per pound than the original PBT, it actually cost the user 10% less per part, because the lower density of the SPS more than offset its slightly higher cost per pound. Though polymers are sold by weight, evaluation by volume is a more direct measure of actual part cost. If one polymer is 20% lighter than another, 20% more parts per pound can be produced from an equal volume of material, which is illustrated by the example of the fuse box. While the flame-retardant Laestra compound with 30% glass reinforcement costs 12% more per pound than the more highly filled PBT, it weighs 20% less per pound, saving almost 10% per part. It also has better melt flow, allowing for easier molding and faster cycle times, thereby lowering molding costs. In addition, the lower warpage of the Laestra SPS reduced scrap loss that resulted from warpage. Key E/E Requirements That Identify Material Finalists Generally, there are one or two key property requirements that cannot be met by some polymer families, thus eliminating them from consideration for a particular application. It is also important to note that a key property identified by a material supplier may not have been recognized as a key property by the manufacturer. For example, consider an application that has several requirements, including a CTI requirement of 450 volts or greater. Because many flame-retardant materials have CTI values below 250 volts, the requirement of 450 volts becomes a key property in eliminating many polymer families from consideration. The following key properties may simplify material selection in E/E applications: * flammability rating; Polymer Families Used Most Frequently in E/E Applications. * comparative tracking index; * temperature (RTI) resistance; * processability (mold shrinkage, warpage, flow and fill); * chemical resistance; * snap-fit characteristics; * laser markability. Grade Selection Grade selection is important but tedious given the number of polymer families and grades available. And it usually involves the consideration of additives, principally flame retardants and reinforcing materials. Following are some key points to be considered when determining the appropriate additives for use in E/E applications: * Many additives, such as flame retardants, UV stabilizers, and colorants, tend to reduce impact strength and elongation; * Flame-retardant compounds do not resist UV/weathering as well as those that do not contain flame retardants; * Glass and carbon reinforcing fibers develop higher strength than mineral reinforcements; * Mineral reinforcements control warpage better than glass or carbon fiber; * Glass beads reduce warpage and improve surface appearance, but do not significantly improve mechanical properties. It is preferable to find a standard material that satisfies requirements without compromising the design. However, material suppliers are happy to develop special or modified materials for a specific application. These specialty materials are often necessary to meet UL approvals (a process that can cause project delays), and they can fulfill other essential requirements. In fact, many commonly used materials were initially developed as specialty materials for a specific application. Developing a specialty material is almost always a better alternative than compromising a design. When the requirements have been identified and the polymer choice has been narrowed to the family level, attention to several key requirements will aid in timely selection of the best grades. These requirements include mechanical properties (strength, stiffness, and impact), UL (or other agency) requirements, and color and special requirements (e.g., flame retardancy, lubricity, and laser markability). (See Table 2.) Getting Help It is important to note that a polymer scientist or materials engineer is a specialist with extensive knowledge in the attributes and capabilities of many polymer materials. Another significant fact is that many material databases contain information only on standard materials, not modified or special grades. Therefore, the most effective and efficient product-development strategy encompasses two courses of action. The first is to focus on defining the requirements of the application. Subsequently, a pa\rtnership should be established very early in the project with an experienced, full-line supplier of polymers for E/E applications, who will recommend the best material and grade selection for the application. Jim Johnson LATI USA Mount Pleasant, South Carolina Reference Society of Plastics
Engineers
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