WHAT KIND OF PLASTIC?
There are tens of thousands of different injection molded thermoplastics with widely varying properties. Many of these are simply modifications of basic plastic resins obtained by the addition of other materials such as fillers, lubricants, glass fibers, carbon fiber, minerals or abrasive materials. Many of the basic plastics are familiar to everyone in terms of their usage, although we may not be familiar their technical names. For example:
Polyethylene -------------refrigerator containers with flexible tops, "Tupperware"(tm)
Cellulose acetate---------Screwdriver handles
ABS-------------------------Computer cases, keyboards
TPR-------------------------"Softgrip" tool handles, toothbrush handles
Polypropylene------------Ballpoint pen bodies
Acrylic---------------------Window glazing, fighter canopies, dice
Styrene---------------------Swizzle sticks, disposable forks and spoons
Polycarbonate------------Eyeglass lenses, "bulletproof" window glazing
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 for more information on plastics usage.
It is obvious that there is a wide variety of properties that can be considered by the designer or inventor of plastic articles. Dragonjewel 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 may be run from a given mold, so that the customer can compare 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 addition of a rubber modifier to the material improved performance by more than 75% - - -and reduced the cost by 5% by allowing a faster cycle time.
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 90 cents a pound, while 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 as a gum rubber eraser.
3) Clarity. Plastic materials can be clear and clear tinted with a 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.
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 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 fits and a variety of closures.
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 its shape under repeated stresses or continuing loads.
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 flows into the threads, locking the nut 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 rubber materials.
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.
11) Chemical resistance. Many plastics, like polyethylene and nylon, are immune to a wide variety of chemicals, while others are easily attacked by a wide range 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 a 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 for plastics is Bayer. They have an informative website, where you can order a color chart showing many standard colors for plastics, as well as learning about the process of coloring molded items. You can return via the back button on your browser. Click here to go to the Bayer site on coloring of plastics.
Aesthetic appeal may or may not be a factor in the design of a particular part. Nobody is too much worried about finish, color, etc. in a gear, but items that come into customer contact or are a portion of the visual environment should be pleasant to the eye and to 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 scratches and fingerprints the way that a gloss finish does. There are exceptions, of course, as when optical transparency is needed, or where ease of cleaning is required. Many plastics, such as glass filled materials or those with impact modifiers are naturally matte finished, and cannot be made shiny. Likewise, external contours of consumer products should be pleasant as well as utilitarian, and should avoid sharp edges or other scratch or cutting hazards.
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. You can return via the back button on your browser. Information on many plastics properties.