The History and Future of Plastics (3)-The Development of New Plastics

In 1907 Leo Baekeland invented Bakelite, the first fully synthetic plastic, meaning it contained no molecules found in nature. Baekeland had been searching for a synthetic substitute for shellac, a natural electrical insulator, to meet the needs of the rapidly electrifying United States. Bakelite was not only a good insulator; it was also durable, heat resistant, and, unlike celluloid, ideally suited for mechanical mass production. Marketed as “the material of a thousand uses,” Bakelite could be shaped or molded into almost anything, providing endless possibilities.

Hyatt’s and Baekeland’s successes led major chemical companies to invest in the research and development of new polymers, and new plastics soon joined celluloid and Bakelite. While Hyatt and Baekeland had been searching for materials with specific properties, the new research programs sought new plastics for their own sake and worried about finding uses for them later.

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The History and Future of Plastics (2)-The First Synthetic Plastic

The first synthetic polymer was invented in 1869 by John Wesley Hyatt, who was inspired by a New York firm’s offer of $10,000 for anyone who could provide a substitute for ivory. The growing popularity of billiards had put a strain on the supply of natural ivory, obtained through the slaughter of wild elephants. By treating cellulose, derived from cotton fiber, with camphor, Hyatt discovered a plastic that could be crafted into a variety of shapes and made to imitate natural substances like tortoiseshell, horn, linen, and ivory.

This discovery was revolutionary. For the first time human manufacturing was not constrained by the limits of nature. Nature only supplied so much wood, metal, stone, bone, tusk, and horn. But now humans could create new materials. This development helped not only people but also the environment. Advertisements praised celluloid as the savior of the elephant and the tortoise. Plastics could protect the natural world from the destructive forces of human need.

The creation of new materials also helped free people from the social and economic constraints imposed by the scarcity of natural resources. Inexpensive celluloid made material wealth more widespread and obtainable. And the plastics revolution was only getting started.

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The History and Future of Plastics (1)-What Are Plastics, and Where Do They Come From?

Plastic is a word that originally meant “pliable and easily shaped.” It only recently became a name for a category of materials called polymers. The word polymer means “of many parts,” and polymers are made of long chains of molecules. Polymers abound in nature. Cellulose, the material that makes up the cell walls of plants, is a very common natural polymer.

Over the last century and a half humans have learned how to make synthetic polymers, sometimes using natural substances like cellulose, but more often using the plentiful carbon atoms provided by petroleum and other fossil fuels. Synthetic polymers are made up of long chains of atoms, arranged in repeating units, often much longer than those found in nature. It is the length of these chains, and the patterns in which they are arrayed, that make polymers strong, lightweight, and flexible. In other words, it’s what makes them so plastic.

These properties make synthetic polymers exceptionally useful, and since we learned how to create and manipulate them, polymers have become an essential part of our lives. Especially over the last 50 years plastics have saturated our world and changed the way that we live.

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How Plastics Are Made(5)-Thermoplastic and Thermoset Processing Methods

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

Extrusion – This continuous process is used to produce films, sheet, profiles, tubes, and pipes. Plastic material as granules, pellets, or powder, 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. The chamber is a cylinder and is referred to as an extruder. Extruders can have one or two revolving screws. The plastic is melted by the mechanical work of the screw and the heat from the extruder wall. At the end of the heated chamber, the molten plastic is forced out through a small opening called a die to form the shape of the finished product. As the plastic is extruded from the die, it is fed onto a conveyor belt for cooling or onto rollers for cooling or by immersion in water for cooling. The operation’s principle is the same as that of a meat mincer but with added heaters in the wall of the extruder and cooling of the product. Examples of extruded products include lawn edging, pipe, film, coated paper, insulation on electrical wires, gutter and down spouting, plastic lumber, and window trim. Thermoplastics are processed by continuous extrusion. Thermoset elastomer can be extruded into weatherstripping by adding catalysts to the rubber material as it is fed into the extruder.

Calendering – This continuous process is an extension of film extrusion. The still warm extrudate is chilled on polished, cold rolls to create sheet from 0.005 inches thick to 0.500 inches thick. The thickness is well maintained and surface made smooth by the polished rollers.  Calendering is used for high output and the ability to deal with low melt strength. Heavy polyethylene films used for construction vapor and liquid barriers are calendered. High volume PVC films are typically made using calendars.

Film Blowing – This process continuously extrudes vertically a ring of semi-molten polymer in an upward direction, like a fountain. A bubble of air is maintained that stretches the plastic axially and radially into a tube many times the diameter of the ring. The diameter of the tube depends on the plastic being processed and the processing conditions. The tube is cooled by air and is nipped and wound continuously as a flattened tube. The tube can be processed to form saleable bags or slit to form rolls of film with thicknesses of 0.0003 to 0.005 inches thick.  Multiple layers of different resins can be used to make the tube.

Injection Molding – This process can produce intricate three-dimensional parts of high quality and great reproducibility. It is predominately used for thermoplastics but some thermosets and elastomers are also processed by injection molding. In injection molding plastic material is fed into a hopper, which feeds into an extruder. An extruder screw pushes the plastic through the heating chamber in which the material is then melted. At the end of the extruder the molten plastic is forced at high pressure into a closed cold mold. The high pressure is needed to be sure the mold is completely filled. Once the plastic cools to a solid, the mold opens and the finished product is ejected. This process is used to make such items as butter tubs, yogurt containers, bottle caps, toys, fittings, and lawn chairs.  Special catalysts can be added to create the thermoset plastic products during the processing, such as cured silicone rubber parts. Injection molding is a discontinuous process as the parts are formed in molds and must be cooled or cured before being removed. The economics are determined by how many parts can be made per cycle and how short the cycles can be.

Blow Molding – Blow molding is a process used in conjunction with extrusion or injection molding.  In one form, extrusion blow molding, the die forms a continuous semi-molten tube of thermoplastic material. A chilled mold is clamped around the tube and compressed air is then blown into the tube to conform the tube to the interior of the mold and to solidify the stretched 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 used to manufacture hollow plastic products and its principal advantage is its ability to produce hollow shapes without having to join two or more separately injection molded parts. This method is used to make items such as commercial drums and milk bottles. Another blow molding technique is to injection mold an intermediate shape called a preform and then to heat the preform and blow the heat-softened plastic into the final shape in a chilled mold. This is the process to make carbonated soft drink bottles.

Expanded Bead Blowing – This process begins with a measured volume of beads of plastic being placed into a mold. The beads contain a blowing agent or gas, usually pentane, dissolved in the plastic. The closed mold is heated to soften the plastic and the gas expands or blowing agent generates gas. The result is fused closed cell structure of foamed plastic that conforms to a shape, such as expanded polystyrene cups.  Styrofoam™ expanded polystyrene thermal insulation board is made in a continuous extrusion process using expanded bead blowing.

Rotational Molding – Rotational molding consists of a mold mounted on a machine capable of rotating on two axes simultaneously. Solid or liquid resin is placed within the mold and heat is applied. Rotation distributes the plastic into a uniform coating on the inside of the mold then the mold is cooled until the plastic part cools and hardens. This process is used to make hollow configurations. Common rotationally molded products include shipping drums, storage tanks and some consumer furniture and toys.

Compression Molding – This process has a prepared volume of plastic placed into a mold cavity and then a second mold or plug is applied to squeeze the plastic into the desired shape. The plastic can be a semi-cured thermoset, such as an automobile tire, or a thermoplastic or a mat of thermoset resin and long glass fibers, such as for a boat hull. Compression molding can be automated or require considerable hand labor. Transfer molding is a refinement of compression molding. Transfer molding is used to encapsulate parts, such as for semi-conductor manufacturing

The formation of plywood or oriented strand board using thermoset adhesives is a variant of compression molding. The wood veneer or strands are coated with catalyzed thermoset phenol formaldehyde resin and compressed and heated to cause the thermoset plastic to form into a rigid, non-melting adhesive.

Casting – This process is the low pressure, often just pouring, addition of liquid resins to a mold. Catalyzed thermoset plastics can be formed into intricate shapes by casting. Molten polymethyl methacrylate thermoplastic can be cast into slabs to form windows for commercial aquariums. Casting can make thick sheet, 0.500 inches to many inches thick.

Thermoforming – Films of thermoplastic are heated to soften the film, and then the soft film is pulled by vacuum or pushed by pressure to conform to a mold or pressed with a plug into a mold. Parts are thermoformed either from cut pieces for thick sheet, over 0.100 inches, or from rolls of thin sheet. The finished parts are cut from the sheet and the scrap sheet material recycled for manufacture of new sheet. The process can be automated for high volume production of clamshell food containers or can be a simple hand labor process to make individual craft items.

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How Plastics Are Made(4)-The Two Plastic Types, Based on Processing

A Thermoset is a polymer that solidifies or “sets” irreversibly when heated or cured. Similar to the relationship between a raw and a cooked egg, a cooked egg cannot revert back to its original form once heated, and a thermoset polymer can’t be softened once “set”. Thermosets are valued for their durability and strength and are used extensively in automobiles and construction including applications such as adhesives, inks, and coatings. The most common thermoset is the rubber truck and automobile tire.  Some examples of thermoset plastics and their product applications are:

•  Mattresses
•  Cushions
•  Insulation

Unsaturated Polyesters:
•  Boat hulls
•  Bath tubs and shower stalls
•  Furniture

•  Adhesive glues
•  Coating for electrical devices
•  Helicopter and jet engine blades

Phenol Formaldehyde:
• Oriented strand board
• Plywood
• Electrical appliances
• Electrical circuit boards and switches

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 common household items 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 are commonly 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 carbonated soft drink bottles. Other examples of thermoplastics are:

•  Packaging
•  Electrical insulation
•  Milk and water bottles
•  Packaging film
•  House wrap
•  Agricultural film

•  Carpet fibers
•  Automotive bumpers
•  Microwave containers
•  External prostheses

Polyvinyl Chloride (PVC):
•  Sheathing for electrical cables
•  Floor and wall coverings
•  Siding
•  Automobile instrument panels

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How Plastics Are Made(3)-Additives

When plastics emerge from reactors, they may have the desired properties for a commercial product or not. The inclusion of additives may impart to plastics specific properties. Some polymers incorporate additive during manufacture. Other polymers include additives during processing into their finished parts. Additives are incorporated into polymers to alter and improve 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 processing properties such as melt flow; to provide product color; and to provide special characteristics such as improved surface appearance, reduced friction, and flame retardancy.

Types of Additives:

  • Antioxidants: for plastic processing and outside application where weathering resistance is needed
  • Colorants: for colored plastic parts
  • Foaming agents: for expanded polystyrene cups and building board and for polyurethane carpet underlayment
  • Plasticizers: used in wire insulation, flooring, gutters, and some films
  • Lubricants: used for making fibers
  • Anti-stats: to reduce dust collection by static electricity attraction
  • Antimicrobials: used for shower curtains and wall coverings
  • Flame retardants: to improve the safety of wire and cable coverings and cultured marble
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How Plastics Are Made(2)-The Structure of Polymers

As we have discussed, polymers can be homopolymers or copolymers. If the long chains show a continuous link of carbon-to-carbon atoms, the structure is called homogeneous. The long chain is called the backbone. Polypropylene, polybutylene, polystyrene and polymethylpentene are examples of polymers with homogeneous carbon structure in the backbone. If the chains of carbon atoms are intermittently interrupted by oxygen or nitrogen, the structure is called heterogeneous. Polyesters, nylons, and  polycarbonates are examples of polymers with heterogeneous structure. Heterogeneous polymers as a class tend to be less chemically durable than homogeneous polymers although examples to the contrary are numerous.

Different elements can be attached to the carbon-to-carbon backbone.  Polyvinyl chloride (PVC) contains attached chlorine atoms. Teflon contains attached fluorine atoms.

How the links in thermoplastics are arranged can also change the structure and properties of plastics. Some plastics are assembled from monomers such that there is intentional randomness in the occurrence of attached elements and chemical groups. Others have the attached groups occur in very predictable order. Plastics will, if the structure allows, form crystals. Some plastics easily and rapidly form crystals, such as HDPE—high density polyethylene. HDPE can appear hazy from the crystals and exhibits stiffness and strength. Other plastics are constructed such that they cannot fit together to form crystals, such as low density polyethylene, LDPE. An amorphous plastic typically is clear in appearance. By adjusting the spatial arrangement of atoms on the backbone chains, the plastics manufacturer can change the performance properties of the plastic.

The chemical structure of the backbone, the use of copolymers, and the chemical binding of different elements and compounds to a backbone, and the use of crystallizability can change the processing, aesthetic, and performance properties of plastics. The plastics can also be altered by the inclusion of additives.

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How Plastics Are Made(1)-The Basics of Plastic Manufacturing

The Basics of Plastic Manufacturing

The term “plastics” includes materials composed of various elements such as carbon, hydrogen, oxygen, nitrogen, chlorine, and sulfur. Plastics typically have high molecular weight, meaning each molecule can have thousands of atoms bound together. Naturally occurring materials, such as wood, horn and rosin, are also composed of molecules of high molecular weight. The manufactured or synthetic plastics are often designed to mimic the properties of natural materials. Plastics, also called polymers, are produced by the conversion of natural products or by the synthesis from primary chemicals generally coming from oil, natural gas, or coal.

Most plastics are based on the carbon atom. Silicones, which are based on the silicon atom, are an exception. The carbon atom can link to other atoms with up to four chemical bonds. When all of the bonds are to other carbon atoms, diamonds or graphite or carbon black soot may result. For plastics the carbon atoms are also connected to the aforementioned hydrogen, oxygen, nitrogen, chlorine, or sulfur. When the connections of atoms result in long chains, like pearls on a string of pearls, the polymer is called a thermoplastic. Thermoplastics are characterized by being meltable. The thermoplastics all have repeat units, the smallest section of the chain that is identical. We call these repeat units unit cells. The vast majority of plastics, about 92%, are thermoplastics1.

The groups of atoms that are used to make unit cells are called monomers. For some plastics, such as polyethylene, the repeat unit can be just one carbon atom and two hydrogen atoms. For other plastics, such as nylons, the repeat unit can involve 38 or more atoms. When we combine monomers, we generate polymers or plastics. Raw materials form monomers that can be or are used to form unit cells. Monomers are used form polymers or plastics

When the connection of the carbon atoms forms two and three-dimensional networks instead of one-dimension chains, the polymer will be a thermoset plastic. Thermoset plastics are characterized by not being meltable. Thermoset plastics, such as epoxy adhesives or unsaturated polyester boat hulls and bathtubs or the phenolic adhesives used to make plywood, are created by the user mixing two chemicals and immediately using the mixture before the plastic “sets up” or cures.

The formation of the repeat units for thermoplastics usually begins with the formation of small carbon-based molecules that can be combined to form monomers. The monomers, in turn, are joined together by chemical polymerization mechanisms to form polymers. The raw material formation may begin by separating the hydrocarbon chemicals from natural gas, petroleum, or coal into pure streams of chemicals. Some are then processed in a “cracking process.” Here, in the presence of a catalyst, raw materials molecules are converted into monomers such as ethylene (ethene) C2H4, propylene (propene) C3H6, and butene C4H8 and others. All of these monomers contain double bonds between carbon atoms such that the carbon atoms can subsequently react to form polymers.

Other raw material chemicals are isolated from petroleum, such as benzene and xylenes. These chemicals are reacted with others to form the monomers for polystyrene, nylons, and polyesters. The raw materials have been changed into monomers and no longer contain the petroleum fractions. Still other raw materials can be obtained from renewable resources, such as cellulose from wood to make cellulose butyrate. For the polymerization step to work efficiently, the monomers must be very pure. All manufacturers purify raw materials and monomers, capturing unused raw materials for reuse and byproducts for proper disposition.

Monomers are then chemically bonded into chains called polymers.There are two basic mechanisms for polymerization: addition reactions and condensation reactions. For addition reactions a special catalyst is added, frequently a peroxide, that causes one monomer to link to the next and that to the next and so on. Catalysts do not cause reactions to occur, but cause the reactions to happen more rapidly. Addition polymerization, used for polyethylene and polystyrene and polyvinyl chloride among others, creates no byproducts. The reactions can be done in the gaseous phase dispersed in liquids. The second polymerization mechanism, condensation polymerization, uses catalysts to have all monomers react with any adjacent monomer. The reaction results in two monomers forming dimers (two unit cells) plus a byproduct. Dimers can combine to form tetramers (four unit cells) and so on.  For condensation polymerization the byproducts must be removed for the chemical reaction to produce useful products. Some byproducts are water, which is treated and disposed. Other byproducts are raw materials and recycled for reuse within the process.  The removal of byproducts is conducted so that valuable recycled raw materials are not lost to the environment or exposed to populations. Condensation reactions are typically done in a mass of molten polymer. Polyesters and nylons are made by condensation polymerization.

Different combinations of monomers can yield plastic resins with different properties and characteristics. When all monomers are the same, the polymer is called a homopolymer. When more than one monomer is used, the polymer is called a copolymer. Plastic milk jugs are an example of homopolymer HDPE. Milk is satisfactorily packaged in the less expensive homopolymer HDPE. Laundry detergent bottles are an example of copolymer HDPE. The aggressive nature of the detergent makes a copolymer the right choice for best service function.  Each monomer yields a plastic resin with specific properties and characteristics. Combinations of monomers produce copolymers with further property variations. So, within each polymer type, such as nylons, polyesters, polyethylenes, etc, manufacturers can custom make plastics that have specific features. Polyethylenes can be made to be rigid or flexible. Polyesters can be made to be low temperature melting adhesives or high temperature resistant automobile parts. The resulting thermoplastic polymers may be melted to form many different kinds of plastic products with application in many major markets.The variability of the plastic either within plastic family types or among family types permits a plastic to be tailored to a specific design and performance requirements. This is why certain plastics are best suited for some applications while others are best suited for entirely different applications. No one plastic is best for all needs.

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
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What Is the Injection Molding Process

The injection molding process is a technique used to create a wide range of objects such as plumbing parts, plastic toiletry items, toothbrush handles, toys, and even the dashboards in cars. Materials used in the injection molding process vary, but they frequently include plastics or synthetic polymers that can be easily molded. Low marginal cost per molded item, variability of material, general efficiency, and nearly complete products are all advantages of the injection molding process. The initial investment in the machinery can be expensive, however, and the cost of powering the machines can be high as well, presenting a few disadvantages to injection molding.

To begin the injection molding process, plastic granules are dispensed from a hopper into a heated container with a plunger. The plunger is usually moved by a hydraulic pump that presses the heated plastic horizontally towards the mold. As the plastic moves through the chamber it passes a series of heaters that eventually melt it, and then it reaches a nozzle that directs it past a gate and into the mold. Once inside the mold, the plastic cools and hardens. The mold itself is kept at a cool temperature to facilitate efficient cooling of the plastic, which begins almost immediately.

The mold must maintain a set amount of pressure to make up for the natural shrinkage of the plastic material as it cools. Often times, clamping units are included in the system to create this pressure. Once the plastic resin has cooled sufficiently, the mold is opened and the plastic part is removed. The empty mold is then ready to close and receive the next shot, or dose, of melted plastic for the next part.

While the injection molding process is quite streamlined, there are several problems that can potentially occur if the equipment malfunctions. Many of these problems occur in the beginning of the process, and can be fixed by adjusting the machinery. Common complications include burning and general imperfections in the plastic parts, warping of the parts, and incomplete filling of the mold.

Burning and general imperfections are most frequently caused by overheating. If the plastic is left in the heating chamber too long, or if the heaters are powered too high, the plastic could burn or develop gas bubbles that create an uneven surface. Warping is also a result of a temperature error, but it usually involves an uneven mold surface temperature. This causes the plastic inside the mold to cool at different rates, shrinking unevenly and warping the plastic part. Finally, if the nozzle malfunctions, then the mold might not be fully filled, causing another error in the shape of the plastic parts.

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How To Get Injection Molding Training

An injection molding system is the mainstay of production lines that feature a lot of prefabricated parts that are eventually put together. Having a machine that allows you to create a great amount of parts for a particular product is great if you want to really pick up your production in the manufacturing aspect side of things. While it may be a good idea to adapt to an injection molding technology setup for your factory, it isn’t exactly a walk in the part.

The machine used for the injection molding training is quite a complicated one. It’s not something that you could probably understand after seeing it. It has thousands of moving parts that must move in a precise and organized manner to make sure that the desired products turn up as good as they could possibly be. While reading the articles on the website of the manufacturer as well as reading the official manual may add to your knowledge, you would probably feel that such a familiarity with the machine would not come after reading a book and a website. To be really familiar with the injection molding system, you should try getting some training. Here’s how you do it:

There are government run agencies that conduct training for technical courses. Injection molding training can be one of them if a considerable amount of people would sign up. Finishing a training program led by the government would probably allow you to get a certificate that you could hold on to in case somebody asks you for proof for training.

There are also injection molding systems manufacturers that are willing to offer training. In fact, many would probably offer training to those who are going to buy their products. Training would be very important for the end user and it’s going to be to the buyer and seller’s advantage if the person who bought the product could go in training. This would be great in observing the actual operation of the machine as well as the maintenance, upkeep, customization and other details related to the injection molding system.

Take special classes from specialists that would offer this type of seminar. Information learned from the experts who have real life and actual plant operations would really enrich your insights on how to work with injection molding systems. The training in the company and the manuals can build your theoretical base but taking notes from the grizzled veterans can really make you a better technician.

Always regularly attend more training to update what you know regarding the technology. The wave of research and development is never static so it’s very important that you’re up to date with the news and buzz in the industry. Other people may be developing new and better ways to do products at the end of the day so you should be open to always learn and improve the techniques that you already know.

It’s a very tricky subject so you should be ready to put in some time to learn the ropes of injection molding system.

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