Synthetic polymeric materials form a key branch of production in the chemical industry. In the period from 1950 to 1973, these materials, which are usually also referred to as plastics*, underwent an upswing which could scarcely be matched by any other sector of the chemical industry. Some of the reasons for this were the drop in the prices of raw materials during this period, the lowering of production costs due to increased plant capacity, and the development of new, more efficient technologies for producing and processing these materials and, not least, the properties of the materials. However, the growth of the polymeric materials industry is due in large part to the versatility of these materials (i.e., their wide range of properties).
Polymeric materials are now almost exclusively manufactured synthetically from organic compounds (monomers) by various processes, such as polymerization, polycondensation, or polyaddition. The monomers are linked together to form macromolecules which are the characteristic feature of the microstructure of polymeric materials (HERMANN STAUDINGER, 1881–1965). Chemical composition, structure, and morphology are parameters which have a close relationship with the macroscopic properties of these materials. Therefore, it will be helpful to have a fundamental understanding of the chemistry of these materials to briefly illuminate some of the most important structure-property relationships. For an in-depth study, the reader is referred to [1.1] and to other review works [2.1 to 2.5].
Thermoplastics are polymeric materials made up of linear or branched macromolecules. Macromolecules having a regular structure are capable of the order required to form crystals, while an irregular arrangement of polymer chains gives rise to an amorphous structure. Polymeric materials are said to be semi-crystalline (partially crystalline) when they contain both domains of crystalline order as well as amorphous phases.
Macromolecules attach themselves to one another by means of physical binding forces (e.g., van der Waals forces and hydrogen bonding). These binding forces can be overcome when sufficient thermal energy is supplied, i.e., these polymeric materials can make the transition to the molten state. In doing so, a crystalline structure is converted into an amorphous phase. The processing of thermoplastic polymeric materials from the powdery or granular form of the raw materials into finished or semi-finished parts is most commonly accomplished by heating the polymer to the molten state in one of many different processes (see also Figure 7.1).
The mechanical properties of polymeric materials, especially those of thermoplastics, depend to a much greater extent on temperature, time, and on the magnitude and nature of an applied load than those of metals. In addition, many environmental effects, such as UV radiation or exposure to certain chemicals, play a significant role in aging and related changes in properties. This can be difficult to quantify in strength-related calculations. The conditions used in processing (e.g., injection molding process) can have an effect on the properties of the finished product.
There are a variety of technical tools available to designers, including computer-aided design tools (CAD and CAE) to relieve the burden of routine tasks and perform complex analyses. There is also design methodology that organizes and classifies design principles in systematic manner, thereby making it easier to find optimum solutions [6.1–6.3]. Nevertheless, a design emerges not only from theoretical technical knowledge, but also from an individual’s thought processes that relate to both form and function, as well as manufacturability [6.4, 6.5].
Polymeric materials can be processed from raw materials to finished parts by a wide variety of manufacturing processes and technologies. An overview of the most important manufacturing processes and manufacturing resources is given in Figure 7.1. The reader may consult the basic literature, e.g., [7.1] to [7.11] for more detailed descriptions of these processes.
Particular attention will be paid to designing for injection molding, because of the great importance of injection molding for processing thermoplastic polymeric materials.
Structural elements that are required to have high deformability should be designed so that they are capable of withstanding the flexural or torsional loads associated with the application (see also Section 6.1). Two examples of such designs common in parts made from polymeric materials are snap-fit or interlocking joint elements and elastic elements. Another common feature in parts designed for high deformability is their relatively thin wall thickness. For example, integral hinges are structural elements having extremely low wall thicknesses.
The classical method of mechanical fastening using a metal bolt and a metal nut is comparatively unimportant when joining polymeric materials. Specific advantages, such as the ability to achieve a high and constant force cannot be exploited and such joints require many individual parts making the assembly operation relatively labor-intensive.
Creep or loss of prestress is a common problem when metal fasteners are used to assemble plastic parts. In order to keep the loss of prestress as low as possible, the following guidelines should be adhered to.
Ribs are an efficient design feature for increasing the rigidity and load-bearing capacity of structural parts subjected to flexural or torsional stresses. They provide an alternative to the use of thicker walls. As a result, consumption of material is reduced and production costs in injection molding are lowered. On the other hand, considerations favoring ease of production for ribbed structures are often diametrically opposed to those favoring a good stress distribution.
Gear wheels are designed to transmit movements and forces or moments without slippage. In the field of precision engineering, the transmission of movement (at relatively low torques) is frequently the more important task of toothed drives. Accordingly, special gear tooth designs that are particularly free of play, have low friction, and are quiet in operation find application in a variety of products [11.1–11.4]. The tooth depth of such gear wheels is usually less than 1 mm, which leads to problems in moldmaking and in dimensionally accurate production*. These machining problems are not addressed to any significant extent here. Probably the smallest gearwheel** produced from polymeric material (POM) by conventional means today is used for driving the second hand of a wrist watch [4.18]. It weighs 0.00056 g, has eight teeth and an outer diameter of 1.32 ± 0.01 mm. The mold cavity was produced by spark erosion and the electrodes by disk shape milling cutters [11.35].
The generic term “bearing” encompasses the mounting of moving machine parts. A friction bearing is a special type of bearing in which this movement takes the form of sliding, thus causing sliding friction. Accordingly, the contact surfaces move relative to one another.
Polymeric materials are suitable for making wheels and rollers subjected to relatively low loads. Due to their unique properties, plastics extend the range of performance possible with more traditional materials for wheels and rollers, particularly when the following properties are demanded:
