Rubber and Plastic Products Industry
This group deals with products made from plastic and rubber resins, including the actual production of the material. It contains establishments engaged in manufacturing these products. Its industries include:
-Tires and Inner Tubes
-Rubber and Plastics Footwear
-Gaskets, Packing, Sealing Devices, and Rubber and Plastic Hoses and Belts
-Fabricated Rubber Products (not elsewhere classified)
-Miscellaneous Plastic Products
Material Production: Establishments in this major group often start from "scratch" in producing their own material with which to make products.
POLYMERIZATION is the formation of plastics by either addition of monomers to form one large molecule, or by condensation of water from reactive molecules. Thermoplastics are the result of addition processes and are generally the weaker, more flexible types of plastics. Thermosets are permanent, high strength, and brittle plastics resulting from condensation processes. Plastic products are numerous and popular for being relatively inexpensive to produce. Polymerization is often used to create synthetic rubber, and is generally a very inexpensive process.
COMPOUNDING is the mixing of elements and fillers with natural or synthetic rubber to form specific elastomers. Many elastomers are used for their resistance to electrical current and their flexibility. They do not, however, respond well to heat.
VULCANIZATION is the chemical reaction caused by heating a mixture of rubber (natural or synthetic) and sulfur. It is to numerous forms of elastomers because it increases strength drastically. The most common application of Vulcanization is in the tire industry where high strength and resistance to wear is needed.
COMPOSITES are heterogeneous solids consisting of two or more components that are mechanically or metallurgically bonded. There are three basic types of composites; laminar, particulate, and fiber reinforced. Common methods of producing composites include; compression molding, filament winding, pultrusion, autoclave curing. Quite often, there are no names for composite construction that simply puts two separate materials together in one shape or form. It is advantageous to add strengthening materials to a material that is often too weak when left pure.
Forming: Rubber formation processes are limited to casting and extrusion, and do not include any type of mechanical working unless above some melt temperature. Plastic formation processes are quite extensive and cover a broad range.
CASTING is one of the oldest and most common methods of forming. It requires the melting of a solid, heating it to the proper temperature, treating it to produce a desired chemical composition, and then pouring it into a cavity or mold for solidification. A wide range of sizes and shapes are possible with casting including large and bulky to small and detailed products. Some common casting processes include:
Some machining is required in casting processes to remove cores, gates, fins, left by air holes in the molds. Casted parts offer the best resistance to working stresses because they are melted for forming and not mechanically worked. Rubber is formed into useable parts through die casting and injection molding.
Hot working forming processes require that the material be mechanically worked or deformed when heated. Hot working is attractive because it is very easy to shape materials at higher temperatures. Massive deformation is available without sacrificing much strength. Some disadvantages include the need for machining after the process and the presence of poor dimensional accuracy.
EXTRUSION requires that the material be compressively forced to through a suitable shape (called a die) to form a product with reduced cross section. The extrusion process is like squeezing toothpaste out of a tube. The two types of extrusion used in hot working are direct and indirect. In direct extrusion, a ram pushes the material through the die. In indirect extrusion, the ram pushes the die back around the material. A typical application of extrusion for rubber or plastic is the formation of simple garden hose.
DRAWING is a plastic forming process in which a flat sheet or plate of material is formed into a recessed, three-dimensional part with depth several times the thickness of the material. Hot drawing is used, primarily, for forming thick-walled parts of simple geometry. Plastic bowls and cups are often formed with this process.
Other hot working processes include HOT SPINNING and PIERCING. Hot spinning is the plastic forming of parts from a flat rotating disk of material. Piercing is the production of thin walled, seamless tubing by center punching solid, circular stock with a bullet like tool.
Cold working forces require the material to be mechanically worked or deformed cold. Such processes are advantageous because they provide good dimensional accuracy and good surface finish. Greater forces are required to cold work, and there are also losses in strength. However, machining can sometimes be eliminated with proper working. Cold working is limited to a few plastics in this major group.
SQUEEZING is a forming process where the material is forced to compact. It is used to produce features such as sudden tapers, rounded ends, and thinner sections. Common squeezing techniques include:
BENDING requires deformation of the workpiece about a linear axis. Plastic bending is done, but it is rare. Bending is used to form metal to numerous geometric shapes such as right angles, rounded corners, and tight seams. Common bending applications include:
DRAWING is a forming process where plastic (permanent) flow occurs over a curved axis. A good way to envision drawing is taking a flat sheet of material and using a punch to push a cup-like shape from the sheet. The materials used for furniture and fixtures include metal and plastic. Some common drawing techniques include:
Machining: Little machining is required in the rubber and plastics major group. Some for plastics, but almost none for rubber. Most products are complete after the formation stage. For example, only minor machining such as removing sprues and imperfections is required of casted parts.
SAWING produces chips by a succession of small cutting edges or teeth arranged in a narrow line on a saw blade. Sawing processes cutting off sections of rubber to notching plastic. Saws can be classified into three distinct types; reciprocating, band, and circular.
WATER JET machining removes material through erosion effects of a high velocity, small diameter jet of water. It is commonly used for cutting and slitting of wire, and for rubber cable stripping. Abrasives can be added to the water to handle stronger materials.
TURNING is a high speed process where the workpiece rotates and a longitudinally fed, single point tool does the cutting. Variations of turning include tapering, facing, and boring. Turning requires the use of a lathe except in some boring operations. Multiple diameter poles and rods are easily produced in a turning process.
DRILLING uses a rotating tool with two edges that is pressed into the workpiece to produce a hole or void. Precision holes are required for the ultimate assembly of some plastic products. Types of drilling tools include; straight and taper shank, deep hole, combination, subland, and spade. Feed rates depend on the type and thickness of the material. Drilling is often followed by counterboring or countersinking to accommodate fastening devices.
FILING requires cutting teeth arranged in succession along the same plane on the surface of a file to remove material chips. Types of files include; single-cut, double-cut, vixen-cut, and rasp-cut. Files are also classified by coarseness and construction. Filing is important to plastic industries in easily removing sharp edges that could cause injury in finished products.
ABRASIVE machining uses very small cutting edges that are integral parts of abrasive particles to produce chips from the material. A good example is using sandpaper to smooth a rough surface. Types of abrasives include; quartz, aluminum oxide, silicon carbide, and diamond. Some common types of abrasive processes include grinding and lapping. Grinding uses a wheel of some shape on which abrasives are bonded. Lapping requires that fine particles be embedded into a soft material (like paper) which serves to finish the part. Abrasive machining is limited to small amounts of material removal. However, it can provide extremely smooth surfaces.
ABRASIVE JET machining removes material through the action of a focused stream of abrasive laden gas. Common abrasives include; aluminum oxide, silicon carbide, sodium bicarbonate, and glass beads. AJM is used to debur plastic parts, deflash small castings, and touch up molds. AJM is very cheap and good for difficult to reach areas. It can machine very hard materials, but it has a slow material -removal rate. Stray cutting may occur, along with the embedding of particles in the workpiece.
CHEMICAL machining is classified as non-traditional machining in that it doesn't produce chips. It employs some type of chemical reaction (sometimes enhanced by electrical or thermal energy) to remove material. Chemical machining often take the place of processes such as deburring, grinding, and polishing. It can also be used to etch decorative surfaces on rubber products.
ULTRASONIC machining is a material removal process used to erode holes and cavities in hard workpieces. It uses shaped tools and high frequency mechanical motion. Another form of ultrasonic material removal is the stripping of finishes via underwater ultrasonic wave emission.
Assembly: The actual putting together, joining, and additional finishing of components constitutes assembly. Not many processes define a fashion in which plastic and rubber products are put together, it's just done in (hopefully) the fastest, most efficient manner. Once together, there has to be a means of joining the components. Most rubber and plastic parts don't require assembly and are complete after the machining stage.
ADHESIVE BONDING uses composite systems with several compounds to join two surfaces together. The bonding material may exist as liquids, pastes, solids, tapes, or films. Common types of adhesives include:
The surfaces of all components to be joined must be clean and dry to insure proper bonding.
MECHANICAL FASTENING requires that a joint be formed by mechanically interlocking or interfering with no fusion or bonding present. Fastener types include integral and discrete. Integral fasteners are machined areas of wood such as tabs and seams that fit into other parts. Discrete fasteners are separate pieces such as nails, bolts, screws, rivets, and wires.
COATING is performed to impart certain properties to surfaces such as resistance to weather and wear, different colors, and smoother finishes. Coatings can be classified into organic and inorganic types. Common coatings include paint, enamel, anodization, and plastic laminations.
Inspection and Testing: Most of the processes already described in the Rubber and Plastics occur at only one point in the overall manufacturing path. Inspecting and testing, however, are performed numerously throughout all processes. It can consist of a simple check to see if a piece is present, or a minute measurement of the surface of a finished part. Inspection and testing tell whether or not a particular process was performed adequately.
DESTRUCTIVE TESTING does not allow for use of the tested part after inspection because it imparts some type of mechanical deformation. An example is fatigue testing of rubber materials notorious for their poor resistance to repeated mechanical workings. This type of testing is done on a sampling basis. Other types of destructive testing include impact, creep, torsion, and shearing.
NON-DESTRUCTIVE TESTING allows for use of the part after inspection. It includes a wide range of optical and mechanical techniques. Mechanical techniques using instruments such as calipers, micrometers, and scales may contact the part, but they leave no significant mechanical deformation. Optical techniques include structured light, triangulation, holography, and interferometry. An example of a NDT application is the use of a projected grid pattern to detect a drilled hole.