Plastics Engineering

Summary

Plastic engineering functions on two principal fronts. The first is the application of the material properties of plastics to the solution of specific engineering problems. Examples include the application of Bakelite (a phenol formaldehyde resin) to the production of electric insulators and epoxy-based fiber-reinforced polymers (FRPs) to the remediation of infrastructure. The second principal function of plastics engineering is the design and application of methods to produce the material objects desired. Typical examples of this aspect of plastics engineering include designing and manufacturing molding devices and other machines to produce plastic materials and objects. These two fronts can be thought of as engineering with plastics and engineering of plastics.

Definition and Basic Principles

The term plastics engineering can mean engineering with plastics or engineering of plastics. In the first sense, plastics engineering refers to using plastics in engineering applications. Second, plastics engineering is the development of plastic materials and applications or methods for their formation and manipulation.

Plastics in Engineering Applications. Traditional engineering materials such as metals and wood often provide structural strengths and weights that are excessive or harm the product. They also pose problems regarding production, energy consumption, and the degree to which they can be recycled. In many cases, traditional materials are replaced by plastics because of their strength and lower manufacturing costs. Replacing a metal gear with a corresponding plastic gear is a prime example. Metal gears require individual machining processes at every stage of production. Some of these processes include producing a die, boring center holes and setscrew holes, milling in keyways, and shaping each gear tooth. In many applications, only metal gears can provide the necessary physical strength the product needs. In many other applications, however, metal gears, which are more expensive and labor-intensive to create, can be substituted with gears made from plastic. A plastic gear made from high-density nylon can be injection molded by the thousands from a single die at a fraction of the cost and the time required to produce a single machined metal gear. The plastic gear is strong enough to withstand the strain placed on it in use, is much less massive—which means it requires fewer support mechanisms than metal—and is usually recyclable.

Plastic materials, especially thermoplastics, are also easily machined using standard woodworking and metalworking tools. They do not require the high-strength materials needed for machining metals and can be machined to close tolerances. Thermoplastics have the property of softening when heated. The heat generated by the cutting tool at the point of contact weakens their physical structure and makes them easier to machine. They are also much softer materials than the cutting tools, so they can be cut easily in machining operations. Thermosetting plastics, however, are harder to work with as they do not soften with heating but become brittle and prone to fracture. The localized heating at the cutting edges in machining operations tends to make the surface of the material more brittle, requiring other steps such as sanding and polishing to produce a finished surface. Thermosets also tend to be considerably harder materials than thermoplastics, compounding the problems encountered in machining operations carried out on the materials. This is because thermosetting polymers tend to have highly cross-linked, three-dimensional structures at the molecular level. For that reason, they are used primarily as matrix materials in fiber-reinforced polymers and composite structures.

Using fiber-reinforced polymers allows the manufacture of structures that could not otherwise be made. A fiber-reinforced polymer consists of a fibrous material, such as woven glass or carbon fibers, consolidated and bound into a solid polymer matrix. Fiber-reinforced polymer structures are typically laid up by hand, consolidated (compressed together) with the polymer resin matrix material, and cured with heat and pressure. This method can produce uniquely shaped, strong, and lightweight structural pieces. Fiber-reinforced polymers can also be used in mass-production methods. Thermoplastic materials can be employed to create many relatively simple shapes that do not call for high strength. Variations of these methods, such as extrusion and pultrusion, represent combinations of these methodologies.

Development of Plastic Materials. Plastics engineering also relates to the development of the plastics themselves for specific properties and to the industrial methods of preparing, handling, and manipulating the materials. Plastics begin as monomers—compounds whose molecules can add to themselves repeatedly in a chain reaction to form very large polymer molecules. Each polymer molecule may comprise several hundreds or thousands of monomer units. Thermoplastic polymers are almost always linear polymers, in which the monomers form a single head-to-tail chain from beginning to end. Thermosetting polymers usually comprise branched polymer molecules, in which monomer molecules have a structure that allows them to participate in more than one polymerization chain reaction. This results in a highly interconnected three-dimensional molecular structure. Plastics engineering at this level involves the molecular design and synthesis of the monomeric material and subsequent chemical and physical tests of the polymeric material. After the design of the molecule has been engineered, the method of producing the material in quantity must be devised. This involves the design, construction, and operation of processes from which the monomeric material is obtained in a stable state of sufficient purity so it will not prematurely undergo polymerization.

To get the material from the point of manufacture to the point of use, appropriate packaging and handling methods must be determined or devised. Once in use, the materials must be manipulated to turn them into the desired products. At this point, plastics engineering deals with designing and preparing the machinery and processes required for product formation. This includes any machining processes and an assortment of molding methods.

Background and History

The history of plastics engineering extends back several centuries. European explorers of the New World and Asia found the indigenous peoples using natural latexes and resins for waterproofing clothing, bouncing balls, and other everyday objects. From the late eighteenth to the mid-nineteenth centuries, many efforts were made to adapt those materials to the purposes of European industry. This meant that methods to manipulate the materials had to be developed. At the time, chemistry did not include atomic theory, and practicing chemists were restricted to experimenting through trial and error. The first significant advance in manipulating natural plastics was the masticator, a device that could cut and recut a raw rubber mass. The process permitted the rubber to be calendered, or rolled out in thin sheets, without crumbling. Calendering could also bind fabric and plastic together to form a waterproof material.

Industrial applications of plastic materials were limited to using natural resins until the accidental discovery of nitrocellulose. A semisynthetic resin formed by the nitration of cellulose (from cotton), nitrocellulose exhibited many of the properties of thermoplastic polymers and eventually became the basis of the celluloid industry. In 1884, George Eastman, founder of the Eastman Kodak Company, began to mechanically produce celluloid in thin, flexible sheets through a process invented by Belgian chemist Leo Hendrik Baekeland. This celluloid film became the basis for the photography industry.

The realization of a workable, predictive atomic theory greatly assisted in the further development of plastics. Rapidly developing understanding of molecular interactions led Baekeland to create the first fully synthetic polymer, Bakelite, a formulation of phenol and formaldehyde, in 1907. Bakelite was developed to fill a specific need—a material for electric insulators. It proved to be much more than that, as its properties made it an excellent replacement for many natural materials that were in ever-decreasing supply. One of its earliest applications was as a substitute for elephant ivory, used for making billiard balls. Bakelite was the first engineered thermosetting plastic and was developed simultaneously with the production method. This method involved heating a mass of the component mixture, or prepolymer, to drive the polymerization reaction to completion while confining it under pressure within a shaped die or mold.

The many ways of modifying nitrocellulose, combined with an advancing comprehension of the polymerization process within the context of atomic theory, resulted in the development of numerous new materials. For example, between 1930 and 1939, polymer research produced an average of one new polymeric material per day. This necessitated the creation of new methods for turning those materials into products and applying their properties to engineering problems. These factors were crucial to the outcome of World War II, and research and development in these areas have continued unabated.

The same properties that make plastics useful also make them environmentally problematic, especially because the primary source of raw materials for plastics production has shifted from coal tar residues to petroleum. Many recyclable thermoplastics have been discarded in landfills and garbage heaps. By the early twenty-first century, plastics engineering had developed a more circumspect view of plastic applications, focusing on developing methods of recycling discarded plastic goods, including thermoset plastics, which historically had not been recyclable.

How It Works

Polymerization. To understand the principles of plastics engineering, one should appreciate the basic principles of polymerization. Polymers begin with specific molecules that are referred to as monomers. These contain a functional structure that allows them to form chemical bonds successively between individual molecules in a chain reaction. In a chain reaction, a molecule forms a bond to a second molecule, enabling the second to bond to a third, the third to a fourth, and so on. Successive bond-forming reactions typically occur several thousand times before some condition is encountered that terminates the progress of the reaction chain. In linear polymers, this reaction process forms molecules that have a structure essentially as simple as that of their monomers but are much larger.

If the monomer molecules are selected in such a way that at least some of the molecules in the prepolymer can take part in more than one chain reaction at the same time, the reaction process produces a three-dimensional polymeric structure. In this structure, the linear molecular chains become bonded to each other, or cross-linked, as the monomer molecules become bonded to each other in the chain reactions. This results in a structure in which the polymers are intimately intertwined in a three-dimensional network that spans the entire mass. In principle, this can turn an entire multiton mass of resin into a single, very large molecule. However, in practice, this never happens because of the various ways polymerization chain reactions are terminated during the process.

Linear and Three-Dimensional Polymers. Linear polymers are generally thermoplastic, meaning they become pliable when heated and eventually melt. Three-dimensional polymers, on the other hand, are generally thermosetting, becoming brittle, breaking down, and decomposing when heated instead of becoming pliable and melting.

Thermosetting Materials. Thermosetting plastics, also known as thermosets, cannot be reformed once polymerized except through mechanical machining methods. As such, thermosetting materials are typically used in on-the-spot production methods such as injection molding and extrusion molding. Thermosetting plastics are stronger than thermoplastics because they form from rigid three-dimensional bonds.

Thermoplastic Materials. Thermoplastic materials can be reformed (and therefore recycled) when heated to a pliable state. They can quickly convert from raw materials to finished products and from waste materials to new finished products. Plastics engineering, at this stage, begins with research and development of the molecular structure of the monomeric material to assess and characterize the properties of the polymeric material that it will produce. Such activity often includes chemically modifying and functionalizing the monomer molecules in specific ways to obtain desired properties in the resulting polymer. Once the molecular structure has been established, the engineering focus shifts from the production of the material in the laboratory to the production of the material in bulk. As plastics are typically produced on a scale of many millions of tons annually, continuous-flow methods of production are preferred, although some specialty plastics are best prepared by batch processes. The plastics engineer has the task of identifying and adapting or developing the most suitable process for producing the material and optimizing the process in practice. The processes used typically produce thermosetting resins for transshipment and to use in polymerization reactions at other locations. Thermoplastics are produced by methods that make them easily transportable in solid form. Various process types are used in the subsequent processing and production of plastic objects.

Applications and Products

Fiber-Reinforced Polymers. Plastics engineering involving fiber-reinforced polymers, particularly their use in advanced composite materials, begins with engineering the design of the desired finished component. This includes the nature and form of the reinforcing materials, often polymeric materials, employed in the composite structure and their distribution pattern within the finished structure. Because the shape and form of a thermosetting plastic cannot be altered after polymerization is complete, any object made from such materials must be prepared in its final shape. The plastics engineer uses molds and forms made specifically for that purpose. The active surfaces of the molds and forms correspond precisely in shape to the desired final shape of the object being made. The reinforcing material, impregnated with the thermosetting resin, is laid into the mold or form according to the design specifications, and the resulting stack is enclosed in special containment fabrics. Next, the contents are subjected to reduced pressure, usually by use of a vacuum pump. This permits an even distribution of atmospheric pressure that will compress and consolidate the stack and remove any extraneous gases that could interfere with the product's structural integrity. Applying heat at this point completes the polymerization reaction while assisting with eliminating gaseous wastes. Once cooled, the resulting composite structure is removed from the mold. If the removed part is of suitable quality, it is sent to the next stage in the manufacturing process.

Pultrusion. Pultrusion is another method of producing thermoplastic fiber-reinforced polymers. In pultrusion, an appropriately designed bundle of continuous fiber strands is drawn through a die along with a molten thermoplastic matrix material. The die consolidates the material combination, and as the matrix material solidifies upon exiting the die, a continuous fiber-reinforced structure is produced. Examples of pultruded products include fiberglass rods and reinforced water hoses.

Injection Molding. Both thermosetting and thermoplastic polymers can be used in the injection molding process, though the practical requirements are very different for the two types of polymers. In injection molding, a cavity in the shape of the desired object is filled with molten thermoplastic or a thermosetting prepolymer resin. After the solid polymer forms, the object is removed from the mold, and the process is repeated. When a thermoplastic material is being used, the plastic material is fed into the mold under pressure from a heated hopper. If a thermosetting resin is being used, precautions must be taken to ensure that the polymerization reaction only occurs within the mold cavity and after the cavity has been filled. The process can rapidly produce a large number of intricately designed objects that would require many independent machining operations if the objects were made from traditional materials such as wood or metal. Successive molding steps enable the production of multicolored objects.

Thermoforming. The thermoforming process applies only to thermoplastic materials. In this process, a sheet of solid thermoplastic material, such as polystyrene or Poly(methyl methacrylate), commonly called Plexiglas, is placed in a mold or form and heated to a temperature at which the material becomes highly pliable but does not melt. This allows the sheet to deform and adopt the shape of the mold, assisted by changing the pressure on one side of the sheet of material. The method is most suitable for relatively flat or low-profile designs.

Blow Molding. The process of blow molding also applies only to thermoplastics. It is similar to thermoforming in that it uses a difference in pressure to force softened thermoplastic material to adopt the shape of a mold. Blow molding uses three-dimensional molds, however, and is typically used to produce open structures such as plastic bags, bottles, and other similar objects. In blow molding, a metered quantity of plastic material is set into a mold, where it is then blown outward by a sudden blast of heated air to the shape of the mold. The method is most suitable for open, three-dimensional shapes having little or no surface detail.

Extrusion. The process of extrusion is closely related to pultrusion. In extrusion, the plastic material is pushed through a die under pressure rather than drawn through with continuous fiber-reinforcement materials. Random-oriented fiber-reinforcement materials can be used in the extrusion process if they are blended with molten plastic before entering the die. Extrusion can be used only to produce structures that have a constant cross-sectional profile along their entire length, as determined by the die profile. Quite complex cross-sectional designs can be produced in this way, but they are essentially only two-dimensional.

Compression and Transfer Molding. Both compression and transfer molding are used primarily for thermosetting resins and rely on heating a filled mold to drive the cross-linking reactions in the polymer body to completion. The processes use either a powdered resin or a preformed plug that will be set into a final desired form. Compression molding is the process that was developed by Baekeland for the production of Bakelite objects, and it uses heat and pressure to restrict the material to the final form of the product throughout the polymerization process. In transfer molding, the resinous material is first preheated and liquefied in a separate chamber before being transferred to the object mold and undergoing polymerization.

Rotational Molding. The process of rotational molding uses centrifugal force to distribute the plastic material within a spinning or rotating mold. The material may be injected as a liquid or added as a finely ground powder that is fused within the mold. The process is used exclusively for the production of hollow objects.

Foams. Various methods and treatments of resins, typically thermosetting resins, result in the formation of foams. The forceful addition or release of gas within the bulk polymer structure expands the material by pushing the component molecules apart. The gases may be produced as a by-product of the polymerization reaction or added as an extraneous component. Urea-formaldehyde foams are typically produced by the first method, as the polymerization reaction produces quantities of water vapor. Expanded foams such as Styrofoam are generally produced by adding a low-boiling liquid such as pentane to be absorbed by the solid plastic material. Subsequent rapid heating quickly vaporizes the liquid, and the resulting pressure of the vapor acts to expand the solid material. In other applications, combinations of materials such as urea and formaldehyde solutions react quickly to produce a solidified foam structure. Such materials are generally applied as sprays to provide an insulating and sealing layer on flat surfaces.

Casting and Encapsulation. These are basically simple molding procedures in which a liquefied plastic material is introduced into a mold without the application of increased or decreased pressure, forming a specific shape as the material solidifies.

Calendering. Thermoplastic materials can be formed into thin sheets by calendering, a process by which the material is pressed between heated rollers. The process can be used to laminate a cloth insert layer with plastic material and impart various surface textures to the resulting sheet.

Mechanical Machining Methods. The inherent pliability and shear properties of plastics, particularly of thermoplastics, make them highly amenable to shaping with traditional tools such as saws, drills, planes, shapers, lathes, sanders, and millers. The materials are easily cut by steel tools, which are harder than any plastic. For this reason, mock-up designs are often initially constructed as plastic models rather than as metal constructs, allowing design engineers to test various physical properties, such as aerodynamic stability and wind resistance, that are directly related only to the shape of the structure and not to the materials used.

Careers and Course Work

A career in plastics engineering requires a solid foundation in general and applied mathematics, chemistry, and physics, beginning in high school and continuing into postsecondary education. In college, students interested in a plastics engineering career should study organic chemistry, analytical chemistry, physical chemistry, physics, and applied mathematics and take specialized courses in polymer chemistry, reaction kinetics, and industrial chemistry. This will guide the student into postgraduate academic programs and positions in industry. Those interested in careers related to the engineering of plastic materials for industrial applications should study chemistry, physics, applied mathematics, and engineering principles. Specialized coursework will be determined by the chosen branch of engineering: mechanical, chemical, civil, industrial, or polymer.

The many applications of engineered plastics guarantee that many careers will involve training in the uses and applications of plastics. Civil engineering technologists and technicians can expect to encounter applications of fiber-reinforced polymers, forms, and molded plastics. Aircraft maintenance engineers will be required to undertake training in using and repairing fiber-reinforced polymers and advanced composite structures as a regulated licensing requirement. Automotive technicians can specialize in repair techniques using engineered plastic components.

Social Context and Future Prospects

Plastics have become so entrenched in the workings of the modern world that careers involving plastics engineering seem destined to be as persistent as the materials themselves. The economic impact of the plastics industry in the United States is hundreds of billions of dollars every year, and the industry provides millions of jobs. Plastics' great strength and chemical stability are a great strength and failure. Plastic goods were not designed considering resource conservation but as disposable items. The negative impact of this practice was realized only considerably after plastics' use became widespread, making the management of used plastics problematic. Because of the increasing price of plastics due to high petroleum costs and a broader understanding of the issues plastic poses, landfills were mined for plastics, mainly nylon. Scientists have also turned to developing methods of reusing and recycling plastics.

The plastics industry continually researches new materials and products to maintain a constant supply of cheap, convenient, and ultimately disposable plastic products for consumer demand. At the same time, consumers increasingly demand that the plastics industry be more circumspect with and accountable for its use of plastics as a commodity. The Great Pacific Garbage Patch—a Texas-sized swirl of plastic garbage floating in the Pacific Ocean—brought public awareness to issues associated with plastic use and encouraged research of alternatives. Human health concerns about the plastic additive bisphenol A (BPA) also brought further attention. Governments also began taking action in the twenty-first century, banning plastic bags at retail and grocery stores in cities across America and several countries. 

The long-term effects of the plastics industry are not fully understood. Most scientists agree that plastic often contains highly toxic, possibly carcinogenic chemicals that can disrupt the human endocrine system. These chemicals accumulate in the environment, including in drinking water, and are likely to pose negative repercussions for decades. However, because plastics hold a critical place in everyday life, creating cleaner, safer, and earth-friendly alternatives is at the forefront of plastics engineering. Success has been observed in alternatives made of vegetable fats, corn, woodchips, and recycled food.

The future of plastics engineering is developing along three distinct fronts, each offering broad opportunities as career choices. One is the traditional production and support base of the plastics industry. The second, potentially the most significant, is the growing industry of plastics abatement, which involves recovering used plastics and mitigating plastic residues in the environment. The third front is the field of bioplastics, or plastics produced by living organisms through biological processes instead of chemical alteration of petroleum sources. All three require the services of plastics engineers.

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