Our everyday world is full of plastic - its in our car, power tools, television, computer, in practically everything we use to do things. Plastic is cheap, lightweight, strong, often attractive, and can be combined with a wide range of materials. Despite this, most of us know very little about plastic and how it is made. When we think about plastic, most of us think it is something that easily changes shape and is easy to melt. We'd know better if we taken the time to learn a little about the process and the machinery used to create plastic. While much has been written about plastics and the processes of making plastic, very little has been said about the evolution of plastic machinery - which for most of us is unlike anything we have ever seen before.
What is Plastic?
Plastic is a polymer that is basically a long chain of repeating carbon-based molecules or monomers. Derived from the Greek word plastikos, plastic meant to form or able to be formed into different shapes. For centuries people have been using natural organic polymers in the form of waxes and shellacs, as well as fabrics and ropes, which are all based on a plant polymer known as cellulose. By the early 19th century, the rubber taken from a rubber tree was the material of choice. Today plastics refers to a range of mostly synthetic or semi-synthetic materials that can be molded or extruded into objects, films or filaments that are often rigid and hard.
Prior to the appearance of the fully synthetic plastic, most plasticswere found in nature, such as silk, cellulose, natural rubber, amber, horn, wax, bitumen, shellac, and gutta percha.The plastics we use today are made from a wide variety of chemical elements including carbon, silicon, hydrogen, nitrogen, oxygen and chloride. These modern forms of plastic are synthesized from oil or natural gas by a process of polymerization whereby short chains of chemicals (monomers) are joined in the presence of a catalyst to make long chains of new polymers. This complex process was initiated or sustained by heat, pressure, or one or more catalysts.
Plastics can be divided into two main groups:
- Thermoplastics - are soft and can be reheated and remolded many times over.
- Thermosetting - are hard and unchangeable once they have been cooled down and set.
The Plastic Time Line
Before the invention of the first synthetic plastic, Bakelite by Leo Hendrik Baekelund in 1909, people used anything with a natural resin to make all sorts of things from spoons, buttons and combs. Horns, tusks, turtle shells and natural resins from plants and trees were popular examples. \ Since the ancient Egyptians,methods of using these natural polymers often involved primitive applications of heat, mixing and molding techniques. In the case of some of the earliest semi-synthetic plastics like vulcanite, casein and celluloid, methods consisted of experimenting with various elements like sulfur, cellulose and nitric acid, applying heat and some basic molding techniques.
As the industrial revolution unfolded, the process of making plastic became more scientific and increasingly mechanical. New mechanical devices gave inventors more control over the application of heat and chemicals, and made it easier for them to alter the properties of natural polymers like rubber and cellulose. In 1834 this led to the idea that it might be possible to alter the properties of a natural polymer by heating it and mixing it with another element, i.e., sulphur. When Friedrich Ludersdorf and Nathaniel Hayward found that rubber didn't become sticky in warm weather if it was mixed with sulphur, they started a revolution.
Called vulcanization, the process of applying heat to a polymer and mixing it with sulphur was developed and perfected by Thomas Hancock and Charles Goodyear. Cooking rubber in sulphur joined the separate isoprene polymers in rubber and improved its structural integrity as well as other properties. This paved the way for a completely new kind of product one of which turned out to be the modernTyre. (Vulcanization is named after the god, Vulcan, who used both sulphur and heat).
Bewley applies an existing extrusion process to the production of gutta percha tubes for the first submarine telegraph cable.
Samuel Peck patents the method of mixing shellac with wood to make the predecessor of today's framed family photographs.
Alexander Parkes develops a synthetic ivory named pyroxlin, which he calls Parkesine. Parkesine is made from cellulose treated with nitric acid and a solvent. The result is a hard dough, ivory-like material that could be molded when heated.
John Hyatt discovers that camphor works better as a solvent and proceeds to design much of the basic industrial machinery needed to produce plastic materials. Since cellulose was the main element used in the synthesis of his new material, he calls it celluloid. Hyatt's engineer, Charles Burroughs, designs specialized tools and machinery for celluloid production. His stuffing machine is patented in 1872 and is considered the predecessor of the modern injection moulding machine.
Charles Cross, Edward Bevan, and Clayton Beadle patent a new artificial silk called rayon and sell the rights for the new fabric to the French Government. Cheap and smooth on the skin, rayon is weak when wet and is easily creased. Also produced in a transparent sheet called cellophane.
John Hyatt uses celluloid to fabricate a material in a strip
format for movie film.
Leo Hendrik Baekelund finds that mixtures of phenol and formaldehyde produce an extremely hard material when heated, mixed and allowed to cool. Known as phenolic or phenol-formaldehyde he calls the new material bakelite and is the first synthetic thermosetting resin.
Improvements in chemical technology lead to an explosion in the development of new plastic resins. Among the earliest examples are polystyrene (PS) and polyvinyl chloride (PVC), developed by the I.G. Farben company of Germany. Polystyrene is a rigid, brittle plastic that is the basis for one of the most popular plastics known as styrofoam.
Wallace Carothers develops the first molecular design of materials. His work leads to the discovery of a synthetic nylon fiber. The first applications are the bristles for toothbrushes and a replacement for silk, namely nylon.
Polyamide - or nylon. the first purely synthetic fiber, is introduced by the Du Pont Corporation at the 1939 World's Fair in New York City.
Reginald Gibson and Eric Fawcett discover polyethylene (PE) at the British industrial giant Imperial Chemical Industries (ICI). Polyethylene evolves into low density polyethylene (LDPE) and high density polyethylene (HDPE). Polyethylene is cheap, flexible, durable, and chemically resistant. LDPE is used to make films and packaging materials, including plastic bags, while HDPE is used more often to make pipe, containers, plumbing, and automotive fittings.
German chemists synthesize the first of a series of synthetic rubbers that eventually become the basis for synthetic rubber production during World War II. By 1942 rubber production is mostly under Japanese control.
American, British, and German companies produce polymethyl methacrylate (PMMA), better known as acrylic. As hard and more transparent than glass, acrylic is sold as a replacement for glass under the name of plexiglas.
Roy Plunkett discovers teflon by accident in 1938. Used to refine uranium for the atomic bomb. By the 1960s, Teflon "non-stick" frying pans and dental floss are all the rage. Is used to synthesize the miracle fabric GoreTex which can be used to make rain coats that breathe keeping you moisture free.
I.G. Farben Industries of Germany file a patent for polyepoxide or epoxy. As a thermoset plastic that cures when a "hardener" is added, epoxies come into wide use as coatings and super glues.
Rex Whinfield and James Dickson develope polyethylene terephthalate or PET. Used for synthetic fibers in the postwar era under names like polyester, PET is more impermeable and abrasion resistant than other low-cost plastics. Is best known for its use in making bottles for Coke and popular soft drinks.
A total of 50 factories in the United States are manufacturing rubber, producing twice as much as the world's natural rubber production before the beginning of the war.
When exposed to fluorine gas it is discovered that polyethylene becomes a much more robust polyfluoroethylene. Experiments with polyethylene lead to another material known as polypropylene (PP). It is used in everything from plastic bottles to carpets to plastic furniture, and is very heavily used in automobiles. Earl Tupper's tupperware features a complete line of sealable polyethylene food containers that Tupper cleverly promoted through a network of housewives who sold Tupperware as a means of bringing in some money.
1950 - 60
As plastics continue to improve and so much of the environment is moulded, fabricated and constructed of plastics that we hardly notice its existence. The Synthetic Age is upon us.
The Development of Plastic
The Plastic Process
There are many processes for plastics. Selection of a process depends on many factors including:
1. Quantity and production rate
2. Dimensional accuracy and surface finish
3. Form and detail of the product
4. Nature of material
5. Size of final product
The plastics process has three phases:
Heating - to soften or melt the plastic
Shaping/Forming - under constraint of some kind
Cooling - so that it retains its shape.
While thermoplastics start as regular pellets or granules and can be remelted, thermosetting plastics start as liquids/syrups, often called resins, as powders or partially cured products which need heat for the shaping phase. The shaping is accompanied by a chemical reaction, which means that the material does not soften on reheating. The reaction may be exothermic (giving heat out), in which case cooling is required.
There are roughly seven processing methods for making plastic objects:
is the principal method of forming thermoplastic materials. Modifications of the injection process are sometimes used for thermosetting plastics. In injection molding, plastic material is put into a hopper which feeds into a heated injection unit. A reciprocating screw pushes the plastic through this long heating chamber, where the material is softened to a fluid state. At the end of this chamber there is a nozzle which abuts firmly against an opening into a cool, closed mold. The fluid plastic is forced at high pressure through this nozzle into the cold mold. A system of clamps hold the mold halves shut. As soon as the plastic cools to a solid state, the mold opens and the finished plastic is ejected from the press.
is the method employed to form thermoplastic materials into continuous sheeting, film, tubes, rods, profile shapes, and filaments, and to coat wire, cable and cord. In extrusion, dry plastic material is first loaded into a hopper, then fed into a long heating chamber through which it is moved by the action of a continuously revolving screw. At the end of the heating chamber the molten plastic is forced out through a small opening or die with the shape desired in the finished product. As the plastic extrusion comes from the die, it is fed onto a conveyor belt where it is cooled, most frequently by blowers or by immersion in water.
is a method of forming hollow articles out of thermoplastic materials. In blow molding, a plastic material is formed into a molten tube of thermoplastic material using compressed air to blow up the tube to conform to the interior of a chilled blow mold.
consists of heating thermoplastic sheet to a formable plastic state and then applying air and/or mechanical assists to shape it to the contours of a mold. Air pressure may range from almost zero to several hundred psi.
is like compression molding in that the plastic is cured into an infusible state in a mold under heat and pressure. It differs from compression molding in that the plastic is heated to a point of plasticity before it reaches the mold and is forced into a closed mold by means of a hydraulically operated plunger.
Reaction Injection Molding
In reaction injection molding, two liquid components, polyols and isocyanates, are mixed in a chamber at relatively low temperatures (75° - 140° F) before being injected into a closed mold. An exothermic reaction occurs, and consequently RIM requires far less energy usage than any other injection molding system.
is the most common method of forming thermosetting materials. Compression molding is simply the squeezing of a material into a desired shape by application of heat and pressure to the material in a mold. Plastic molding powder, mixed with such materials or fillers as woodflour and cellulose to strengthen or give other added qualities to the finished product, is put directly into the open mold cavity. The mold is then closed, pressing down on the plastic and causing it to flow throughout the mold. It is while the heated mold is closed that the thermosetting material undergoes a chemical change which permanently hardens it into the shape of the mold.