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Plastic: Manufacturing Basics

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Plastic: Any of various complex organic compounds produced by polymerization, capable of being molded, extruded, cast into various shapes and films, or drawn into filaments used as textile fibers.
-- Webster's Dictionary

The Basics of Manufacturing

The term "plastics" encompasses organic materials, such as the elements carbon (C), hydrogen (H), nitrogen (N), chlorine (Cl) and sulfur (S), which have properties similar to those naturally grown in organic materials such as wood, horn and rosin. Organic materials are based on polymers, which are produced by the conversion of natural products or by synthesis from primary chemicals coming from oil, natural gas or coal.

The plastic production process begins by heating the hydrocarbons in a "cracking process." Here, in the presence of a catalyst, larger molecules are broken down into smaller ones such as ethylene (ethene) C2H4, propylene (propene) C3H6, and butene C4H8 and other hydrocarbons. The yield of ethylene is controlled by the cracking temperature and is more than 30% at 850ĀčC and such products as styrene and vinyl chloride can be produced in subsequent reactions. These are then the starting materials for several other types of plastics. Therefore, this process results in the conversion of the natural gas or crude oil components into monomers such as ethylene, propylene, butene and styrene.

These monomers are then chemically bonded into chains called polymers. Different combinations of monomers yield plastic resins with different properties and characteristics. Each monomer yields a plastic resin with different properties and characteristics. Combinations of monomers produce copolymers with further property variations.
The resulting resins may be molded or formed to produce several different kinds of plastic products with application in many major markets. The variability of resin permits a compound to be tailored to a specific design or performance requirement. This is why certain plastics are best suited for some applications while others are best suited for entirely different applications. For instance, impact strength measures the ability of a material to withstand shock loading. Heat resistance protects the resin from exposure to excessive temperatures. Chemical resistance protects the resin from breakdown due to exposure to environmental chemicals.

Some examples of material properties in plastic product applications are:

Hot-filled packaging used for products such as ketchup
Chemical-resistant packaging used for products such as bleach
Impact strength of car bumpers

The Structure of Polymers

Polymers are created by the chemical bonding of many identical or related basic units and those produced from a single monomer type are called homopolymers. These polymers are specifically made of small units bonded into long chains. Carbon makes up the backbone of the molecule and hydrogen atoms are bonded along the carbon backbone.

Polymers that contain primarily carbon and hydrogen are classified as organic polymers. Polypropylene, polybutylene, polystyrene and polymethylpentene are examples of these.

Even though the basic makeup of many polymers is carbon and hydrogen, other elements can also be involved. Oxygen, chlorine, fluorine, nitrogen, silicon, phosphorous and sulfur are other elements that are found in the molecular makeup of polymers. Polyvinyl chloride (PVC) contains chlorine. Nylon contains nitrogen. Teflon contains fluorine. Polyester and polycarbonates contain oxygen. There are also some polymers that, instead of having a carbon backbone, have a silicon or phosphorous backbone and these are considered inorganic polymers.

The Additives

When plastics emerge from reactors, they do not have the desired properties that make it a material of choice, that is, it is considered a raw material. In order to achieve a commercial product, the plastic is subject to further treatment and the inclusion of additives which are selected to give it specified properties. Most polymers are blended with additives during raw material processing into their finished parts. Additives are incorporated into polymers to alter and improve their basic mechanical, physical or chemical properties. Additives are also used to protect the polymer from the degrading effects of light, heat, or bacteria; to change such polymer properties as flow; to provide product color; and to provide special characteristics such as improved surface appearance or reduced friction.

Types of Additives:

antioxidants: for outside application
colorants: for colored plastic parts
foaming agents: for Styrofoam cups
plasticizers: used in toys and food processing equipment

Two Characterizations Of Plastic

A Thermoset is a polymer that solidifies or "sets" irreversibly when heated. Similar to the relationship between a raw and a cooked egg, once heated, a thermoset polymer can't be softened again and once cooked, the egg cannot revert back to its original form.

Thermosets are valued for their durability and strength and are used primarily in automobiles and construction, although applications such as adhesives, inks and coatings are also significant. Other examples of thermoset plastics and their product applications are:

Polyurethanes:

mattresses
cushions
nsulation
ski boots
toys

Unsaturated Polyesters:

lacquers
varnishes
boat hulls
furniture

Epoxies:

lglues
coating for electrical circuits
helicopter blades

A Thermoplastic is a polymer in which the molecules are held together by weak secondary bonding forces that soften when exposed to heat and return to its original condition when cooled back down to room temperature. When a thermoplastic is softened by heat, it can then be shaped by extrusion, molding or pressing. Ice cubes are a common household item which exemplify the thermoplastic principle. Ice will melt when heated but readily solidifies when cooled. Like a polymer, this process may be repeated numerous times.

Thermoplastics offer versatility and a wide range of applications. They make up the greatest share of plastics used in food packaging because they can be rapidly and economically formed into any shape needed to fulfill the packaging function. Examples include milk jugs and soda bottles. Other examples of thermoplastics are:

Polyethylene:

packaging
electrical insulation
milk and water bottles
packaging film
house wrap
agricultural film

Polypropylene:

carpet fibers
automotive bumpers
microwave containers
external prosthesesyes

Polyvinyl chloride (PVC):

sheathing for electrical cables
floor and wall coverings
siding
credit cards
automobile instrument panels

Processing Methods

There are a variety of different processing methods used to convert resins into finished products. Some include:

Extrusion - This continuous process is used for the production of semi-finished goods such as films, sheet profiles, tubs and pipes. They are termed "semi-finished" because they must be further processed before they become useful articles. Plastic material is first loaded into a hopper and then fed into a long heated chamber through which it is moved by the action of a continuously revolving screw. At the end of the heated chamber, the molten plastic is forced out through a small opening called a die that is cast in the shape of the finished product. As the plastic extrusion comes from the die, it is fed onto a conveyor belt where it is cooled by blowers or by immersion in water. The operation's principle is the same as that of a meat mincer but with added heaters in the wall of the extruder. Examples of products include lawn edging, pipe, film and window trim.

Injection molding - Since this process can produce moldings of high quality and with great accuracy, it is very widespread. It is predominately used for thermoplastics but smaller amounts of thermosets and elastomers are also processed this way. In injection molding, plastic material is also put into a hopper, which feeds into a heating chamber. A plunger pushes the plastic through the heating chamber where the material is then softened into a fluid state. At the end of this chamber, the resin is forced into a closed mold. Once the plastic cools to a solid state, the mold opens and the finished product is ejected. This process is used to make such items as butter tubs, yogurt containers, closures, fittings and razors.

Blow molding - Blow molding is a process used in conjunction with extrusion. The die forms a molten tube of thermoplastic material. Using compressed air, the tube is then blown to conform to the interior of a chilled mold which clamps around the tube. Overall, the goal is to produce a uniform melt, form it into a tube with the desired cross section and blow it into the exact shape of the product. This process is intended for use in manufacturing hollow plastic products and its principal advantage is its ability to produce hollow shapes without having to join two or more separately molded parts. This method is used to make items such as commercial drums and bottles.

Rotational Molding - This process is relatively simple in concept since heat is used to melt and fuse a plastic resin inside a closed mold without using pressure. Rotational molding consists of a mold mounted on a machine capable of rotating on two axes simultaneously. Solid or liquid resin is then placed within the mold and heat is then applied. Rotation distributes the plastic into a uniform coating on the inside of the mold until the plastic part cools and sets. This process is used to make hollow configurations. Common rotationally molded products include shipping drums, storage tanks and some consumer furniture and toys.


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