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The Pluses of Plastics

Versatility and durability make them ideal for many uses.

By RICK HUDSON

Though we at RL Hudson provide a large number of rubber products, we are also involved in numerous custom-molded plastic projects. Plastics really are amazing materials, and I am always impressed by the innovative ways in which our in-house engineers utilize them.

Since I began contributing Tech Session columns back in 1995, I’ve written about everything from polyurethane (my first column) to shaft seal function to, more recently, the testing equipment we use here at RL Hudson. For those of you who are interested, my past columns are all archived in the Knowledge Base section of our web site (www.rlhudson.com). Looking over the list of topics the other day, I was shocked to realize that I have never devoted a column to plastics. It’s high time I rectify that oversight, so here’s a look at what defines plastic materials, how they are classified, some examples of how they can be used, and a brief explanation of how we use sophisticated software when it comes time to think about molding.

ELASTOMER OR PLASTIC? Let’s start with a little basic terminology. Many of the seals and custom-molded products that we at RL Hudson supply are molded from rubber compounds with specific resilience properties suitable for a given application. These compounds take as their primary ingredient specific elastomers, such as nitrile and silicone. These elastomers are really just polymers, long chains of repeating molecules composed of atoms like carbon and hydrogen. Elastomers, then, are known as polymeric materials.

So are plastics. The term “plastics” is a general one and refers to a wide range of mainly synthetic (or semi-synthetic) materials that can be formed into useful products. As a matter of fact, the word “plastics” derives from a Greek root (plastikos) meaning “to form.” This ability to be shaped into a variety of objects (everything from intricate components for larger assemblies to filaments to films) is referred to as plasticity. Like elastomers, plastics are composed of long chains of repeating atoms.

The chief difference between plastics and elastomers is how they respond to heat. Heated elastomers typically undergo a chemical reaction (known as vulcanization, or cure). This reaction results in the formation of permanent connections (cross-links) between the long molecular chains. These cross-links (think of them as chemical bridges) give cured elastomers three-dimensional structures and allow them to be formed into useful shapes (such as shaft seal lips and O-rings). Because they are shaped using heat (a thermal increase) and cannot later be remelted (i.e. they are permanently “set”), many elastomers are good examples of thermoset materials.

On the other hand, plastics undergo a physical change (softening) when heated, but these changes are reversible, and no permanent chemical curing occurs. Plastics often begin in pellet form, then become softer and more fluid as heat increases. This fluidity allows them to be shaped in a variety of ways, including through extrusion, calendaring, injection molding, and blow molding. As they cool, plastics harden (as with curing), but no chemical cross-linking occurs. The changes are purely physical and, with the reapplication of heat, reversible. Because they retain the ability to be reshaped using heat (i.e. they are not permanently “set”), many plastics are also sometimes referred to as thermoplastic materials.

A quick word of caution: This distinction between elastomers and plastics (again, based on their response to heat) is generally true and therefore a useful thing to remember. However, be aware that there are such things as thermosetting plastics and, conversely, thermoplastic elastomers (TPEs), so nothing, as they say, is absolute.

CLASSIFICATION SYSTEMS From a design perspective, plastics offer a seemingly endless array of opportunities. There are currently over four dozen basic types of plastic, which, when compounded and otherwise manipulated, can result in literally tens of thousands of distinct and very useful materials. These plastic materials are sometimes classified according to the ways in which their constituent molecules are arranged. Plastics containing very well organized, dense arrangements of molecules are often referred to as crystalline. Plastics containing some organized molecule alignments as well as some random arrangements are known as semi-crystalline. And plastics in which the molecular arrangement is entirely random are called amorphous.

A given plastic’s degree of crystallization can have a big impact on its physical properties (with more crystallization typically adding rigidity). Degree of crystallization can also affect a plastic’s visual aspect (i.e. how it looks and reacts to light), and plastics can be classified according to their light transmitting properties. A transparent plastic is one through which you can see. You cannot see through a translucent plastic, though light will still pass through the material. An opaque plastic allows for the passage of neither sight nor light.

From a chemical point of view, I find molecular-based classifications interesting, but I’m always more interested in how a given material will perform. That’s what matters most, right? Fortunately, plastic materials can also be classified according to their mechanical, chemical, thermal, and electrical properties. Three general classifications are in use today: commodity plastics, engineering plastics, and high-performance plastics.

Commodity plastics are the low-end types and are typically used for high-volume disposable products where low cost is a priority. Engineering plastics are the next step up and are used for parts requiring a better range of properties, including greater durability. And, as the name implies, high-performance plastics are the high-end types. High-performance plastics are used for applications requiring excellent mechanical, chemical, thermal, and/or electrical attributes. Here at RL Hudson, our in-house engineers are very experienced with a wide range of plastics, especially engineering and high-performance plastics.

MATERIAL PROFILES Nylon is a great example of an engineering plastic. As I mentioned earlier, many polymers are composed of repeating carbon and hydrogen atoms; nylon also contains nitrogen and oxygen within its chemical structure. Often designated by the letters PA (for polyamide fiber), nylon was introduced back in the late 1930s and has since found wide acceptance due to its great strength and durability. Known by a number of trade names (including Zytel® from DuPont), nylon can be blended with other materials (such as minerals or glass) to further augment strength, dimensional stability, and heat resistance. Our engineers recently used a glass-filled nylon compound to replace what had been a metal part for an engine application, and this resulted in a significant cost savings for the customer.

Polytetrafluoroethylene often serves as the basis for extremely useful high-performance plastics. Sometimes called PTFE but known most widely by the DuPont trade name Teflon®, polytetrafluoroethylene features fluorine atoms within its chemical structure. These fluorine atoms are strongly bonded to carbon atoms, making the material extremely resistant to chemical breakdown. PTFE also functions well across a wide temperature range. As a result, PTFE parts are often used in aggressive chemical and thermal environments that would break down lesser materials.

These are just a couple of the plastics we use regularly. We also have wide experience designing custom components made from acetal, acrylonitrile-butadiene-styrene (ABS), polycarbonate (PC), polypropylene (PP), polyetherether ketone (PEEK), high-density polyethylene (HDPE), and many other plastics.

MOLDING ADVICE Of course, a good plastic material must still be properly molded in order to be truly useful. Many of the plastic parts our engineers here at RL Hudson design are injection molded. Injection molding is so-named because plastic pellets are melted and injected under pressure from a heating chamber through a sprue, runner, and gate system into closed mold cavities. Once filled and cooled, the mold is opened and the part removed.

Once our engineers have designed a part and selected a material, they utilize a mold flow analysis (MFA) program to spotlight any potential molding glitches that might arise. The MFA program they use is Pro/ENGINEER Plastic Advisor™, which works within Pro/E to predict how a material will flow into a mold cavity under variable conditions. These variables include temperature, pressure, and gate and runner size and location. This ability to anticipate molding performance (and make any changes that might be needed) saves our customers time that might otherwise be lost awaiting factory evaluation of a design.

So, suffice to say, plastics are very important to industry, and to us here at RL Hudson. If you have an application that might be well suited for a custom-molded plastic part, do not hesitate to call us at
1-800-722-6766. We’ll be happy to help!