3 Basic Steps of the Injection Molding Process

Injection molding is a popular manufacturing method for many reasons. It has proven especially valuable to those in the consumer product development sector, since plastics are a primary component of many consumer products, and injection molding is one of the best ways to manufacture plastics. Let’s take a quick look at the three major phases of the injection molding process, and then discuss the advantages and disadvantages of the process.

Injection Molding Process, Basic Step 1: Product Design

Design is one of the most important facets of the production process because it’s the earliest opportunity to prevent expensive mistakes later on. (Of course, determining whether you have a good idea in the first place is also important, but more on that here.) There are many objectives to design for:function, aesthetics, manufacturability, assembly, etc. The right design is one that accomplishes the required objectives to a satisfactory level, but it may take a lot of creativity to get there. Product design is most often accomplished with computer aided design (CAD) software, like SolidWorks. (Click herefor nine pro tips on how to best use SolidWorks in design and engineering.) Proficiency with CAD software is vital because it allows for quicker iterations and more accurate prototyping if necessary.


Some specific ways to avoid costly mistakes during the product design process are to plan for uniform wall thickness whenever possible, and to gradually transition from one thickness to another when changes in thickness are not avoidable. It is also important to avoid building stress into the design, such as corners that are 90 degrees or less. (Read more about Injection Molding Defects here.)

A skilled team of design engineers will be able to brainstorm, design, and improve upon a variety of solutions to meet the particular complexities of a specific project. The design team at Creative Mechanisms has combined decades of experience creating elegant solutions to complex problems. Meet some of our team here, here, or here, or visit our Customer Testimonials page to see what previous and current clients have to say about our product design capabilities. We think you’ll be impressed.

Injection Molding Process, Basic Step 2: Mold Design

After a looks-like, feels-like design has been tested and slated for further production, the mold (or die) needs to be designed for injection mold manufacturing. Molds are commonly made from these types of metals:

  • Hardened steel: Typically the most expensive material to use for a mold, and generally the longest-lasting (which can drive down price per unit). This makes hardened steel a good material choice for products where multiple hundreds of thousands are to be produced.
  • Prehardened steel: Does not last as many cycles as hardened steel, and is less expensive to create.
  • Aluminum: Most commonly used for single cavity “Prototype Tooling” when a relatively low number of parts are needed for testing. Once the injection molded parts from this tool are tested and approved, then a multi cavity steel production tool is produced. It is possible to get many thousands of parts from an aluminum tool but typically it is used for lower quantities.
  • Beryllium-Copper alloy: Typically used in areas of the mold that need fast heat removal or where shear heat is concentrated.

Just as with overall product design, mold design is another opportunity to prevent defects during the injection molding process. We have previously written blogs on the Top 10 Injection Molding Defectsand Avoiding Mistakes in Injection Molding, but here are some examples of how poor mold design can be a costly mistake:

  • Not designing the proper draft: This refers to the angle at which the finished product is ejected from the mold. An insufficient draft can lead to ejection problems, costing significant time and money.
  • Improperly placed or sized gates: Gates are the openings in a mold through which thermoset or thermoplastic material is injected. Each will leave a vestige (scar), which can create aesthetic or functional problems if not properly placed.

The number of parts (cycles) required, as well as the material they will be made of will help drive decision-making as to how and with what materials to create the mold.

Injection Molding Process, Basic Step 3: The Manufacturing Process

When a product has been properly designed, approved, and die cast, it’s time to start the actual manufacturing! Here are the basics of the injection molding process…

Thermoset or thermoplastic material in granular form is fed through a hopper into a heating barrel. (Learn more about the differences between plastics in our PLASTICS course.) The plastic is heated to a predetermined temperature and driven by a large screw through the gate(s) and into the mold. Once the mold is filled, the screw will remain in place to apply appropriate pressure for the duration of a predetermined cooling time. Upon reaching this point, the screw is withdrawn, the mold opened, and the part ejected. Gates will either shear off automatically or be manually removed. This cycle will repeat over and over, and can be used to create hundreds of thousands of parts in a relatively short amount of time.

injection molding process.png

photo courtesy of substech.com

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Mold Flow Analysis is Critical to the Consumer Product Development Process

old flow analysis is an often overlooked but important step in the injection molding process, and it’s absolutely critical to do whenever a large number of parts are going to be produced. Let’s learn more about this process, and how it can improve Return on Investment (ROI) for engineering companies, and simplify the consumer product development process. But first…

The basics of injection molding and why it’s a great manufacturing process:

Injection molding is a manufacturing process for producing parts in large volume. It is most typically used in mass-production processes where the same part is being created thousands or even millions of times in succession. Once the initial costs for design and tooling have been paid, the price per unit during injection molded manufacturing is extremely low. The price also tends to drop drastically as more parts are produced. Injection molding is most often used for parts made from various types of plastics. Visit our Plastics page, or take our PLASTICS course which are two of the most comprehensive resources available to learn about plastic characteristics, uses, and more.

Thermoset or thermoplastic material in granular form is fed through a hopper into a heating barrel. The plastic is heated to a predetermined temperature and driven by a large screw through the gate(s) and into the custom designed mold. Once the mold is filled, the screw will remain in place to apply appropriate pressure for the duration of a predetermined cooling time. When cooling is complete, the mold is opened, and the part ejected. Gates will either shear off automatically or be manually removed. This cycle will repeat over and over, and can be used to create hundreds of thousands of parts in a relatively short amount of time. Read our blog Avoiding Design and Engineering Mistakes in Injection Molding.

What is mold flow analysis?

Mold flow analysis is the process of simulating an injection molding cycle with a particular plastic and analyzing the results. Mold flow analysis should occur before the injection molding process ever begins, through the use of specialized software that simulates the design of the part to be manufactured. Since the flow of the liquid material in the mold makes a massive difference to the behavior of the product, this step can save a great deal of effort down the road. This software creates color maps of different properties of the design as they would be reflected in the actual mold flow. These may include heating/cooling, fill pattern, injection pressure, potential air traps, shear stress, fiber orientation, and many more properties. Mold flow analysis is a careful, hands-on-process meant for experts.

How does mold flow analysis improve the injection molding and manufacturing process?

The consumer product development process is much more complex than just creating a design and having a machine spit out material in a desired shape; plastics are not as inert as they seem and do not operate like static building blocks. The color maps created by mold flow analysis assist in adaptive changes of the design in order to create a quality product before the molding process actually occurs. This ensures that when a prototype or product goes into production, it is going to perform and behave optimally. Despite the overhead that it adds to the process, mold flow analysis more than makes up for this in terms of final quality. In fact, it has been considered a factor in keeping North American manufacturing competitive in the face of cheaper product development processes elsewhere. Furthermore, it demonstrates a commitment to quality, and is a good indicator of design and engineering firms who are competent, reliable, and produce quality solutions.

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Next Level Product Development

Companies that develop product on a continual basis are well served by doing a regular review of their product development process. Try to identify areas where there is a gap between where you are now and where you would like to be. Each companies process is different but some of the general segments to evaluate are:

Concept Development
Mechanical problems
Production Costs
Speed of Development
General Innovation
Design for Manufacturability
Collaboration and Communication
What segments of the process have the largest gap? Which segment are the easiest to work on first? Typically, you want to see improvements in the quality of the output, the speed in which that quality work happens and the efficiency of that segment.

Looking at Concept Development for example you will want to see a greater variety of innovative concepts produced in a shorter time. With Mechanical development you will want to quickly develop mechanisms that are reliable, use the fewest parts and deliver the desired result efficiently.

Improvements in the process will come from identifying the area’s most in need. Evaluating each step of the current process and being open to doing things differently is the required mindset for improvement. The worse thing someone can say during this examination is “that’s the way we have always done it.” There is always room for improvement. There is also always a cost for those improvements, usually in time, money and human resources. The trick is to work on the areas where you’re going to get the biggest return on your investment.

Evaluating each step and identifying ways to consolidate them while increasing the quality of the output is not easy. Some of the ways to accomplish this are eliminating nonproductive steps, upgrading the tools used, and of course bringing more collaboration and teamwork to solve problems. The process of self-evaluation is certainly challenging but when you are talking about a long product development cycle small increases in efficiency can reap large rewards.

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Prototype Iterations in product development

People experienced in developing new mechanical product know that it is an iterative process. You concept, develop and then iterate a product. The prototype iteration process starts with the development of a Solidworks File that defines all parts of the assembly. A model is made of the assembly from 3D prints or CNC cut parts of each piece of the assembly. That prototype is evaluated and tested to find every improvement possible at the stage. These changes are then incorporated back into the Solidworks file and new model is built. The process is repeated until the product is ready for manufacturing.

The variety of acceptance of this process is interesting to me. There are those who don’t iterate and those who iterate too much. The more inexperienced developers get frustrated by the fact that problems even exist on a first prototype and focus on blame rather than on discovery. The other end of the spectrum is a very experienced person that can’t stop the process even as the changes proposed have diminishing returns.

The inexperienced developer that expects everything to be perfect after the first round of prototyping is usually reacting to the cost of prototyping and views having to spend money on a second round of prototyping as some sort of failure that should have been prevented. We in the service industry have the responsibility of educating our clients that iteration is expected. It is not a failure. You can’t foresee every aspect of how something is going to work by looking at the model on a screen. The transition from screen to reality brings unexpected problems and challenges. That needs to be clear from the onset. The value of making a better product by making changes in the early stage is clear. It is far more expensive to make changes after the product has been launched. Sometimes it is even too late. In today’s instant social media age, a bad reputation happens so quickly. It is much better to spend the time and money up front than to move forward with a product that you know has problems, but you don’t want to take the time and money now to fix.

On the other end of the spectrum is someone who can’t stop the process. Granted there are always going to be improvements that can be made. Losing site of the big picture though can sink a product for no good reason. You must ask yourself “is moving that button over .100 really going to sell any more product”? Will any body but you notice that change when using the product. Are you making this change to improve the usability, aesthetic or functionality of the product? Will it lower product or distribution cost, ease assembly or improve the return on investment in any way? If the answer is no, then why are you spending time and money on it? Analysis to paralysis is very real for some people, teams and companies. Nothing is perfect but there is appoint of diminishing returns.

Take your iteration process to the next level. Develop a product you are proud of. Create something that people will use and enjoy using. Invest in a product that will bring you a return on that investment. If you have produced a product that does these things then perhaps it will have a long enough life that you will have an opportunity to make a second generation incorporating all the learning that you have gathered along the way.

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Manufacturing Problems and Solutions

Product development concerns problem solving. Most products are conceived by identifying a problem and envisioning a product as the solution. The development of that product from the initial vision to the retail shelf proceeds through a series of problems and solutions. Experience solves some problems and trial and error solves others. Problems that arise during manufacturing present some of the most frustrating areas in the product development process. This is especially true for injection molded products.

The development process for injection molded products is usually long and arduous. By the end of that process, you’ve spent a lot of time and money on development and tooling. Now you face a production deadline that looks a bit scary, especially when you finally get the parts from the molder and they just aren’t right. Stay calm! A little bit of advanced planning, some knowledge of often underutilized technologies, and a good part review and troubleshooting process can alleviate some of that stress.

One of the biggest mistakes people make is failing to schedule first, second, and third shot reviews of injection molded parts. These reviews should be an expected part of the process. In today’s fast-paced, low-cost environment, optimism leads to skipping this necessary process.

First shots are the first complete parts off the injection molding machine. The designer reviews these parts and checks their dimensions against the files that were provided to the toolmaker. Expect to make adjustments from this review. Second shots are the parts made after those adjustments. With luck, everything has been addressed and your parts are good at this point; however, it is possible—and expected from a scheduling standpoint—that another round of adjustment is necessary.

Another issue is that all today’s available tools and information are not utilized. I have seen molds made improperly because the manufacturer did not read the data sheet for the material to be molded and the length of the runner was too long. Material suppliers possess a great deal of knowledge and material-specific literature for mold design and processing. Use it. Mold flow analysis software is also widely available now. It should be used during the part design and the mold design processes. This software simulates how the plastic will flow through the mold. It will even show the optimal location where the plastic should be injected (gate location). The cost of molds today should mandate this step in every mold build. Another fairly new technology, “scientific molding,” involves the separate control of the molding parameters that yields very consistent parts. Discuss this technology with the molder to see if the parts being molded would benefit from its use.

The part review and troubleshooting process directly affects the bottom line. The parts need to be right. Parts rarely come out completely as expected. It doesn’t much matter why the parts are not the way you expected. With time as your enemy, all that matters is fixing it. This can mean adjusting the mold only a few thousands of an inch to make a snap work better or a latch engage properly. In a best-case scenario, the adjustments are “steel safe,” meaning that you add plastic to the part by removing material from the mold. If you must reduce material from the part, then you must weld material onto the mold and machine it down—a much lengthier process.

Product development entails a series of problems and their solutions. The stress at the end of the process sometimes makes the last steps the most frustrating. Planning for that, doing everything you can to prevent the problems before they happen, and having a good process in place for part review and troubleshooting make it all a bit less painful.

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What is Engineering For Manufacturing

Engineering for manufacturing is the process of providing the information the manufacturer needs to produce the product. Typically, that information takes the form of 3-D CAD files. These files define every part with all the detail necessary for the manufacturing process. The files also display the assembly of those parts to show the complete product, list a bill of materials (BOM), and show an exploded view. The manufacturing process to be used to make each part requires that each part be design by an engineer who has a thorough understanding of that manufacturing process. In addition to designing a part that can be produced, good engineering for manufacturing focuses on cost reduction and reliability. Reducing the parts count and simplifying the assembly process play important parts of engineering for manufacturing.

Plastic injection molding factories used to require 2-D control drawings, but most now only require 3-D CAD files to make their molds today. The molds are CNC-cut directly from the CAD files. Some industries do require full sets of 2-D drawings, which can be expensive to produce. Many sheet metal vendors specify 2-D drawings with tolerances specified for all parts and also require that the Bill of Materials and the exploded view accompany the drawings.

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Hot & Cold Runners For Injection Molding

“Hot runner” is a term used in injection molding that refers to the system of parts that are physically heated such that they can be more effectively used to transfer molten plastic from a machine’s nozzle into the various mold tool cavities that combine to form the shell of your part. Sometimes they are called “hot sprues.” You can contrast the term “hot runner” with its opposite, and the historically more common “cold runner.” Cold runners are simply an unheated, physical channel that is used to direct molten plastic into a mold tool cavity after it leaves the nozzle. The primary difference is that hot runners are heated while cold runners are not.

Injection Molding Hot Runner Assembly

While hot runners are not required for injection molding processes, they can be useful to ensure a higher quality part. They are particularly beneficial with challenging part geometries that require lower margin of error in the flow properties of the molten plastic (i.e. where inopportune cooling or temperature deltas might result in uneven flow). Further, hot runners can be beneficial in reducing wasted plastic during high volume shoots. Because cold runners are unheated, the channel needs to be larger and thus more plastic needs to be shot during each cycle. If you are shooting a large number of parts while iterating to get the design correct you could easily run up the cost of plastic above the cost of a hot runner assembly. The downside to hot runner technology, absent the aforementioned example, is that it is more expensive by default than a cold runner setup.

The advantage of hot runners is that, if designed properly, the plastic will flow from the machine’s nozzle more uniformly into the gate locations. A gate location is the point at which molten plastic enters the injection mold tool cavity. Gate location, plastic temperature, the design of internal mold cavities, and the material properties of the plastic itself as well as that of the mold tool all have an important impact on the success or failure of the injection molding process. Hot runner technology has been around since the middle of the 20th century but it was used only sporadically until technological improvements and market forces surrounding the price of material inputs made it more viable in the 1990s.

Hot runners are designed to maximize manufacturing productivity by reducing cycle time. One of the reasons they didn’t take hold when they first came out is that they needed to maintain the molten plastic at a uniform temperature while the injection mold cavity is simultaneously being cooled. This requires a fair level of complexity. The initial (now obsolete) designs implemented internal heating with isolated heaters inside channel cavities. Internally heated hot runner designs resulted in solidified plastic on the internal boundaries of the channel with molten plastic much more localized to the specific heater location. By contrast, externally heated runners utilize heated nozzles and a heated manifold and based on the high thermal conductivity of metal they are able to maintain much more even flow properties for the internal plastic.

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Plastics: An American Success Story

The United States is one of the most successful industrial powers in history, and plastics have been critical to that success. Plastics manufacturing is America’s third-largest industry. One million Americans work directly in this industry, producing materials vital to other industries. The plastics industry benefits the American people as well, providing a plethora of goods that are durable, lightweight, inexpensive, and environmentally beneficial.

The plastics industry grew with American industry and helped propel the development of the United States into an industrial giant. At the end of the 19th century industry in the United States saw explosive growth, ignited by improvements like railroad expansion, the rise of electrical power, and the use of scientific investigation to address industrial problems. It was during this period that John Wesley Hyatt invented celluloid, the first man-made polymer.

Developed as a substitute for increasingly rare ivory, celluloid provided a cheaper alternative for expensive items like tortoiseshell brushes, billiard balls, combs, and even linen shirt collars. This revolutionary new product freed manufacturing from the limits of natural resources, which was good for people and good for the environment. An 1878 ad for celluloid proclaimed, “Celluloid [has] given the elephant, the tortoise, and the coral insect respite in their native haunts, and it will no longer be necessary to ransack the earth in pursuit of substances which are constantly growing scarcer.”[1]

In 1907 Leo Baekeland invented the first fully synthetic polymer, Bakelite, as a substitute for shellac, another natural material used for electrical insulators. Bakelite also proved extremely versatile and ideal for mass production. Baekeland marketed it as “the material of a thousand uses,” and it soon became popular in commercial products like telephones, radios, and jewelry as well as in the electrical machinery that was powering America’s industrial growth.

The success of Bakelite led large chemical companies to invest in polymer development. One of these research programs ended a long-running quest for a synthetic substitute for silk. DuPont scientist Wallace Carothers made the breakthrough in 1936 with a polymer that could be spun into fabric: nylon. The public embraced nylon stockings so enthusiastically that scammers in the 1940s tried to pass off once-coveted silk stockings as the real (nylon) thing.

During World War II the plastics industry proved its value to American industry. Fought on battlefields around the globe, the war was also a contest of manufacturing capacity. To win the war American industries searched for materials that could substitute for and improve the performance of scarce natural resources like rubber, metal, wool, wood, and cotton. In all cases plastic provided the answer. Plastics went into war materials of all sorts, from helmet liners, to bomber windshields, to components of the first atomic bomb.

World War II proved a boon for the plastics industry. Plastic production tripled between the outbreak of the war and its end in 1945, and growth continued after the war as plastic consumer products flooded the market. The postwar boom of plastic accompanied a postwar surge in the population. This new generation of Americans, the Baby Boomers, grew up in a world of previously unknown material abundance, thanks in large part to inexpensive plastic.

Plastics also provided for many technological advances. Inventions like padded foam dashboards and plastic bicycle helmets improved safety. Lighter cars boosted fuel efficiency. Medicine benefited tremendously from the use of plastic, and plastics made technologies like cell phones and high-powered computers a possibility.

Americans have not always embraced plastics despite their many benefits. Since the days of celluloid, which acquired a reputation of being dangerously flammable, people have looked at unfamiliar materials with suspicion. Public anxieties continue to shape perceptions of plastics. There is widespread concern that polyvinyl chloride, a common plastic, leaches toxins. Fears abound over bisphenol A (BPA), an additive in some plastics, even though no harmful effects have been proven and the Food and Drug Administration classifies BPA as safe.

The plastics industry is also blamed for waste and litter problems. Anti-waste activists attack plastic packaging without considering the important role it plays in protecting food. Without plastic packaging food prices would increase, and there would be less food available to feed the world’s population. Furthermore, activists who target plastic bags often overlook the environmental liabilities of paper bags: the manufacturing and transportation of paper bags consume more energy and produce more air pollution than that of plastic bags.

The environmental benefits of plastics are often ignored by those seeking to limit plastic use. Most of the fossil fuels used to make plastics are by-products of refining that would otherwise be discarded as waste. Because of their light weight compared to other materials like glass or metal, plastics require less fuel to transport. Plastics make vehicles themselves lighter as well, adding to fuel efficiency. Plastics are also excellent insulators. Therefore, they conserve nonrenewable fossil fuels and help reduce emissions of greenhouse gases like carbon dioxide.

Furthermore, the plastics industry has been promoting recycling since the Society for the Plastics Industry proposed the numbered codes for recycling plastics in 1988. Most municipal recycling systems were created as a result of the industry’s recommendations.

For the past century plastics have improved the lives of billions through advancements in safety, medicine, transportation, energy efficiency, and technology. Plastics have increased the standard of living and created widespread material prosperity. It would be impossible to imagine our world without plastic, a substance that allows us to “devise materials to meet precise specifications. This is a great step toward the practical mastery of the world we live in.”[2]

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Science of Plastics


Plastics are a group of materials, either synthetic or naturally occurring, that may be shaped when soft and then hardened to retain the given shape. Plastics are polymers. A polymer is a substance made of many repeating units. The word polymer comes from two Greek words: poly, meaning many, and meros, meaning parts or units. A polymer can be thought of as a chain in which each link is the “mer,” or monomer (single unit). The chain is made by joining, or polymerizing, at least 1,000 links together. Polymerization can be demonstrated by making a chain using paper clips or by linking many strips of paper together to form a paper garland.


Naturally occurring polymers include tar, shellac, tortoiseshell, animal horn, cellulose, amber, and latex from tree sap. Synthetic polymers include polyethylene (used in plastic bags); polystyrene (used to make Styrofoam cups); polypropylene (used for fibers and bottles); polyvinyl chloride (used for food wrap, bottles, and drain pipe); and polytetrafluoroethylene, or Teflon (used for nonstick surfaces). Although many polymers are hydrocarbons that contain only carbon and hydrogen, other polymers may also contain oxygen, chlorine, fluorine, nitrogen, silicon, phosphorus, and sulfur.

Natural polymers, such as cellulose and latex, were first chemically modified in the 19th century to form celluloid and vulcanized rubber. The first totally synthetic polymer, Bakelite, was produced in 1907. The first semisynthetic fiber, rayon, was developed from cellulose in 1911. However, it was not until the global disruption caused by World War II, when natural sources of latex, wool, silk, and other materials became difficult to obtain, that synthetics were mass produced. Synthetic rubber was needed for tires, and nylon was needed as a replacement for silk for parachutes. Today synthetic polymers in the form of plastics are in wide use, and the plastics industry is one of the fastest growing in the United States and around the world. The industry produces approximately 150 kilograms of polymers per person annually in the United States.


Monomers can be chemically joined together in two ways: addition polymerization or condensation polymerization. Addition polymerization has three basic steps: initiation, propagation, and termination. In this type of polymerization the monomers join by adding on to the end of the last “mer” in the chain, just like making a chain of paper clips. Polyethylene, polystyrene, and acrylic are examples of plastics formed by addition polymerization. These polymers are often thermoplastic in nature: they can be heated and made soft and then hardened when cooled. They are easily processed, reprocessed, or recycled. See the attached tables, Some Addition Polymers and Some Condensation Polymers, for examples of each type.

During condensation polymerization a small molecule is eliminated as the monomers join together. Nylons, some polyesters, and urethanes are examples of condensation polymers. These polymers can be thermoplastic or thermosetting. Although all plastics are in a liquid state at some point in processing and are solid in the finished state, once a thermoset polymer is formed, it cannot be melted and reformed.

The monomers in a polymer may be arranged in a variety of ways. For example, the monomers may have a linear arrangement like a long chain of paper clips, although the tetrahedral carbon bonds actually give the chain a zigzag configuration. Polyethylene is the simplest example of a linear polymer.

Polyethylene Zigzag Structure


If the monomers not only form straight chains but also form long side chains off the main backbone, the resulting polymer is described as branched and may look like a tree branch or the stems of a bunch of grapes. Another arrangement occurs when the long chains are chemically linked together, forming a mesh-like structure known as a crosslinked configuration. Vulcanized rubber, which is formed by reacting natural rubber (isoprene) with sulfur, is an example of a crosslinked polymer.

The polyvinyl alcohol is cross-linked using borax, Na2B4O7x10H2O (sodium tetraborate).


The polymer molecules can also have different arrangements. If the arrangement has no particular order or form, like the arrangement of spaghetti on a plate, the polymer is said to be amorphous (having no shape). Amorphous polymers are often transparent and, therefore, are used as food wrap, headlights, and contact lenses. These materials also tend to have lower melting points. If the arrangement is in a distinct pattern, the polymer is said to be crystalline. The higher the degree of crystallinity, the less light passes through. Such materials are either translucent or opaque. This quality depends on the degree of crystallization and the presence of additives. Crystalline polymers have greater strength and tend to have higher melting points.

Characteristics of Polymers

Polymers seem to have a limitless range of characteristics along with properties that allow them to be dyed in an endless array of colors. Their properties can be enhanced by additives. Being able to design or engineer polymers for specific applications makes plastics unique materials. Although each polymer has unique characteristics, most polymers have some general properties:

  1. They are resistant to chemicals.
  2. They are insulators of heat and electricity.
  3. They are light in mass and have varying degrees of strength.
  4. They can be processed in various ways to produce fibers, sheets, foams, or intricate molded parts.

The raw material for manufacturing plastic products is called a resin. Some of the most common resins are polyethylene (PE), polyethylene terephthalate (PET), polypropylene (PP), polyvinyl chloride (PVC), and polystyrene (PS). These resins are often used in packaging. The Recycling Code chart in the linked PDF shows the recycling code for these resins.

Some Additional Polymers

Polymer name Monomer(s) Polymer Use









Most common polymer. Used in bags, wire insulation, and squeeze bottles


Polypropylene CH2=CH








Fibers, indoor-outdoor carpets, bottles

















Styrofoam, molded objects such as tableware (forks, knives, and spoons), trays, videocassette cases
Polyvinyl chloride





(vinyl chloride)





Clear food wrap, bottles, floor covering, synthetic leather, water and drain pipe





-CF2-CF2 Nonstick surfaces, plumbing tape, chemical-resistant containers and films


Polymethyl methacrylate

(Lucite, Plexiglas)






(methyl methacrylate)








Glass replacement, paints, and household products

(Acrilan, Orlon, Creslan)









Fibers used in knit shirts, sweaters, blankets, and carpets



Polyvinyl acetate





(vinyl acetate)







Adhesives (Elmer’s glue), paints, textile coatings, and chewing gum
Natural rubber  CH3








Rubber bands, gloves, tires, conveyor belts, and household materials

(neoprene rubber)











Oil- and gasoline-resistant rubber

Styrene butadiene rubber1.png

Styrene butadiene rubber1.png

Styrene butadiene rubber













Non-bounce rubber used in tires

Some Condensation Polymers

Polymer name Monomers Polymer Use








Fibers, molded objects

(Dacron, Mylar, Fortrel)




Linear polyesters, fibers, recording tape

(Glyptal resin)




Cross-linked polyester, paints

(Casting resin)




Cross-linked with styrene and benzoyl peroxide, fiberglass boat resin, casting resin







Mixed with fillers, molded electrical cases, adhesives, laminates, varnishes
Cellulose acetate

(cellulose is a polymer of






Photographic film




Water-repellent coatings, temperature-resistant fluids and rubber






Foams, rigid and flexible, fibers

Recycling Codes for Plastic Resins

Recycling code Polymer and structure Uses



Poly(ethylene terephthlate) (PET)

Bottles for soft drinks and other beverages



High-density polyethylene

Containers for milk and other beverages, squeeze bottles



Vinyl/polyvinyl chloride

Bottles for cleaning materials, some shampoo bottles



Low-density polyethylene

May have some branches

Plastic bags, some plastic wraps




Heavy-duty microwavable containers




Beverage/foam cups, toys, window in envelopes


All other resins, layered multimaterials, some containers Some ketchup bottles, snack packs, mixture where top differs from bottom



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The History and Future of Plastics (7)-The Future of Plastics

Despite growing mistrust, plastics are critical to modern life. Plastics made possible the development of computers, cell phones, and most of the lifesaving advances of modern medicine. Lightweight and good for insulation, plastics help save fossil fuels used in heating and in transportation. Perhaps most important, inexpensive plastics raised the standard of living and made material abundance more readily available. Without plastics many possessions that we take for granted might be out of reach for all but the richest Americans. Replacing natural materials with plastic has made many of our possessions cheaper, lighter, safer, and stronger.

Since it’s clear that plastics have a valuable place in our lives, some scientists are attempting to make plastics safer and more sustainable. Some innovators are developing bioplastics, which are made from plant crops instead of fossil fuels, to create substances that are more environmentally friendly than conventional plastics. Others are working to make plastics that are truly biodegradable. Some innovators are searching for ways to make recycling more efficient, and they even hope to perfect a process that converts plastics back into the fossil fuels from which they were derived. All of these innovators recognize that plastics are not perfect but that they are an important and necessary part of our future.

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