Choosing the Right Plastic

There are tens of thousands of different injection molded thermoplastics with widely varying properties. Many of these are simply modifications of various plastic resins obtained by the addition of other materials such as glass fiber, mineral fillers, lubricants or carbon fiber. Many of the basic plastics are familiar to everyone in terms of their usage, although we may not be familiar with their technical names.

For example:

  • Polyethylene - refrigerator containers with flexible tops.
  • Cellulose acetate - screwdriver handles.
  • ABS - computer case, keyboards.
  • TPR - the soft portion of handles, toothbrush handles, etc.
  • Nylon - combs.
  • Polypropylene - plastic buckets, garbage cans.
  • Acrylic - window glazing, fighter canopies.
  • Styrene - disposable picnic ware.
  • Polycarbonate - eyeglass lenses, "bulletproof" glass, police shields.

If you want more information on the usage of plastics in all types of products, click this link to the American Plastics Council. You may return via the back button on your browser. Click here.

It is obvious that there is a wide variety of properties that can be considered by the designer or inventor of plastic articles. Firedrake can consult and advise on the plastic that would be best for a given application, but, if there is a question of what the best material might be, a variety of materials might be run from a given mold, so that the customer can compare the properties in the actual application. This can be a several step process. For example, in a recent automotive application, after a "best" material had been selected, a series of experiments with additives showed that the addition of a rubber modifier to the material improved performance by more than 75% and reduced the cost by 5% by allowing a faster production rate.

When the designer or inventor wishes to select a material for the product, twelve properties should be considered:

  • 1) Cost. Some raw materials cost as little as 75 cents a pound, while some specialty materials may cost as much as 50 dollars a pound.
  • 2) Hardness. Some materials are quite hard, rivaling the softer metals in surface hardness. These are generally filled materials such as glass filled nylon, while others may be as soft and rubbery as a gum eraser.
  • 3) Clarity. Plastic materials can be clear and clear tinted with almost any color. There is a wide variety of clear materials available to choose from. Some, such as acrylic can be used for lenses and sight glasses, while others may be more cloudy than clear, such as translucent refrigerator containers.
  • 4) Flexural strength. The resistance to breakage when bent may be an important factor in material selection. Some materials, such as rubbers, will not break, while others will break with only a slight amount of bending.
  • 5) Stiffness. The resistance to bending can be an important property, especially where the application is a spring, or requires a resistance to flexing during use. This can be important in snap fit and a variety of closures that require resistance to sustained loads.
  • 6 Tensile strength. The resistance to stretching or breaking when pulled is important in many applications, such as handles, or where a part must hold up under repeated stresses.
  • 7) Creep. All materials relax or distort under pressure, and plastics are no exception. Constantly applied pressure can cause a permanent deformation in a part. This can be detrimental, as when a fastener may become loose over a period of time, or may be applied usefully as when a plastic insert in a nut or bolt flows into the threads, locking it against vibration.
  • 8)Temperature resistance. Applications requiring exposure to heat, as in coffee brewers, hair dryers, etc. may demand high temperature resistance, while other applications may require flexibility at freezing temperatures.
  • 9) Elongation. It may be important to consider how much a material will stretch before it breaks. This is generally most critical in parts made of rubber materials such as shock absorbers.
  • 10) FDA acceptance. Materials in contact with drugs, food and beverages, as well as materials intended for interior body contact such as medical implants require FDA compliance to assure that they are biologically safe. Unfortunately, the FDA is slow to test new materials and even slower to accept them, making many modern materials unavailable for current applications.
  • 11) Chemical resistance. Many plastics, like polyethylene and nylon are immune to a wide variety of chemicals, while others are easily attacked by a number of common substances. These characteristics may be employed usefully, as when common disposable items are made of styrene which is easily broken down in the environment by common solvents, sunlight and bacterial action, or where long life is needed, as in plumbing fittings or medical implants.
  • 12) Color. Most plastics can be easily colored, but some, particularly those with fillers or modifiers, may be difficult to color satisfactorily. Colorability is most important in visible consumer applications. One of the foremost suppliers of color additives is Bayer, where you can order a color chart, as well as learning about the coloring of plastics. You can return with the back button on your browser.

Aesthetic appeal may or may not be a factor in the design of a particular part. Nobody is too much worried about the finish, color,etc. in a gear, but items that come into customer contact or are a part of the visual environment should be pleasing to the eye and touch. In the past, most plastic products were made with a high gloss finish, but the more modern approach is to employ a matte or textured finish that does not show fingerprints and scratches the way a gloss finish does. There are exceptions, of course, where optical transparency or ease of cleaning is important. Many materials, such as those with a rubber fill or glass filled materials, are naturally matte finished, and cannot be made glossy, other than painting them. In a like manner, external contours of consumer products should be pleasant as well as utilitarian, without sharp corners or edges to catch dust.

If you would like more information on types of plastics, their properties and, really, more than you ever wanted to know about plastic, visit this wonderful website by the University of Southern Mississippi: The Galleria.