Gemini Plastics Inc. Engineering Basics Tensile Modulus thermal conductivity shear strength tensile coefficient expansion deflection elongation yield point

Graphic-epoxy composites	40,000,000
Steel	30,000,000
Aluminum, 1000 series	10,000,000
Epoxy-glass laminates	5,800,000
Polyester-glass reinforced	2,000,000
Nylons, 30% glass reinforced	1,400,000
Acrylics	500,000
Cast epoxy	450,000
Polycarbonate	450,000
Acetal, copolymer	410,000
Polyethylene; high molecular weight	100,000

Stress, strain and modulus are related to each other by the following equation. The modulus or stiffness of a material can be determined when the material is loaded in different ways, such as tension, compression, shear, flexural (bending) or torsion (twisting). They will be called tensile modulus, also known as plain modulus, flexural modulus or torsional modulus. MODULUS = STRESS/STRAIN

Many plastics are good thermal insulators; that is, heat does not travel through them easily. We experience this every time we pick up a hot pan by its plastic handle. The conductivity of plastics is 300 to 2500 times less than most metals. This property shows why it takes a long time for a casting or other molded parts to cool down in the middle. Internal stress can be set up in a material because of the differences in the cooling rates between the outside of a part and the core.

Shear strength is the strength of a material when the material is loaded. The surfaces of the material are being pulled in opposite directions. Some examples of items that experience shear loading are the nail holding a picture on the wall, the cleats of athletic shoes, and tire tread as a car speeds up or slows down.

The maximum strength of a material without breaking when the load is trying to pull it apart is Tensile Strength. This is the system used by the suppliers to report tensile properties in their literature, such as strength and elongation.

The units are usually given in inches per inch per degree Fahrenheit. It is the change in length (inches) of one inch of a part caused by changing the temperature one degree.

Polyethylene	0.000140
Acrylics	0.000060
Acetal, copolymer	0.000047
Polycarbonate	0.000037
Aluminum, 1000 series	0.000013
Polycarbonate, 30% glass reinforced	0.000009
Steels	0.000008
Glass	0.000004

Example: Assuming an acrylic material, how much will a 10" dimension change if the temperature changes 40¼F? Change in length = original length x the coefficient of expansion x the change in temperature = 10 x 0.00006 x 40 = 0.024"

In addition to changing size, the strength and modulus of elasticity of plastic materials tend to decrease as the ambient temperature increases. The standard test for determining the deflection temperature under load (DTUL) at 66 and 264 psi provides information on the ability of a material to carry a load at high temperatures. The 66 psi means a light load and the 264 psi means a heavy load on a beam. The temperature of the loaded beam is raised until a certain amount of deflection is observed. The temperature when that deflection is reached is called the DTUL. Plastics usually have a higher DTUL at 66 psi than 264 psi because of the lower load.

Elongation is always associated with tensile strength because it is the increase in the original length at fracture and expressed as a percentage. An example would be to pull on a 1" wide piece of paper that is 4" long. It tears with no visible elongation or nearly 0% elongation. Now do the same thing to a 1" x 4" piece of taffy. It will stretch several times its original 4" length before it fractures. Assume that it is stretched to a 12" length, then (12" /4")(100) = 300% elongation.

The yield point is that point when a material subjected to a load, tensile or compressive gives and will no longer return to its original length or shape when the load is removed. Some materials break before reaching a yield point, for example, some glass-filled nylons or die cast aluminum.