OPTICAL/COLORABILITY PROPERTIES OF PLASTICS

  Many plastic materials are transparent and used in optical applications. Some of these materials are acrylics, styrene, PVC, polycarbonate, ABS, and Epoxy. The properties measured and presented in the material suppliers literature are concerned with items, such as the % Haze (cloudiness) in a material, the transmittance capability (how much light gets through the material), yellowness index (appearance), and the index of refraction (how much light is bent as it goes into and out of the material)

  Transparent colored materials transmit that portion of the visible spectrum that allows the eye to see the desired color. Most plastic materials are not transparent and the color of the base material may limit the selection of colors available.

WEAR CHARACTERISTICS OF PLASTICS
  Wear characteristics of a material are very difficult to define. It can mean being resistant to scratching when the part is cleaned. It might mean being resistant to abrasion when the wind blows sand against it. It might mean running another part against it. It might mean being able to maintain its appearance after considerable handling.

  A material like glass may be very resistant to scratching yet can be readily abraded by sand blasting, as evidenced by the pits in a windshield. Conversely, another material like acrylic is easily scratched when wiped and yet is much more resistant than glass to abrasion from sand blasting. It is usually best to devise a test that will duplicate actual use conditions to accurately determine a material's suitability for an application.

  Many plastics are specifically formulated for running against surfaces. The base polymer may exhibit self-lubricating properties. Additives such as TFE, silicone oil, molybdenumdisulfide, and carbon are used to further enhance the bearing capabilities of some materials. Materials have their bearing properties even further enhanced by the addition of additives, such as TFE.

MACHINABILITY
  Plastic stock shapes may be easily machined; however, the tool geometry and speed must be adjusted for optimum performance with a specific material. The tolerances for machining plastics usually should be larger than applied to metals. The tolerances must be larger because of thermal expansion and the shape changing from the relaxation of internal stresses within the material. In critical applications, it may be necessary to premachine the part slightly oversize and STRESS RELIEVE or ANNEAL the part before taking the final cuts.

  Annealing is the baking of a material, without melting or distorting the part, for a time to relax the internal stresses. The internal stresses are usually caused by uneven cooling, that is the outside of the part cools much faster than the inside when the blank is made. This uneven cooling can also cause variations in the properties from the outside to the inside.

  The poor thermal conductivity of plastics requires that care is taken to prevent the area being machined from getting too hot. The type of tool, depth of cut, rate of feed, and coolant flow may have to be adjusted. If a coolant is used, MAKE SURE IT DOES NOT CHEMICALLY ATTACK THE PLASTIC BLANK.

Check the supplier literature for specific recommendations on the types of tools, speeds, etc., to be used with a particular material.

TOLERANCES
  Many designers will ARBITRARILY put a +/-.005 tolerance on a part if it is to be machined. Quiz the designer if the tolerances can't be increased. Remember that a piece of paper is about .003 inch thick, +/- .03 is equal to 1/16 of an inch, and +/- .06 equals 1/8 of an inch. Look at a ruler to visualize the size of the tolerance and think about the tools available to make the cut. Work with the designer to specify the tolerances really needed to make his part work and that can really be produced with the equipment available.