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POLYMER PROPERTIES AND SELECTION

Selecting the right polymer is critical to your successful design of a new plastic
product. Design engineer is faced with a myriad of design considerations, coupled
with an ever growing number of available resins and specialty polymers and should
be aware of the operating environment to which the final product will be subjected.
Like many other materials, plastic has a host of different properties under the
categories of physical, mechanical, thermal and electrical.
PHYSICAL PROPERTIES
1. Density is defined as the measure of mass per unit volume of any material,
expressed either as pounds per cubic inch (lb/in.3) or grams per cubic centimeter
(g/cm3). Density is used to calculate the relationship between weight and volume of
plastic in a specific part design. The density value of a polymer material tells what
each cubic inch (or centimeter) of the part weighs.
2. Specific Gravity is a ration that compares the density of any material divided by
the density of water, both measured at 73° F (23° C).
3. Shrinkage defines the expected reduction of the plastic part dimension as the
part cools in the mould, compared to the original mould dimension, is usually given
as inch per inch (in./in.), or centimeter per centimeter (cm/cm), and the range is
between zero for low shrinkage materials, and approximately 0.050 in./in. (0.127
cm/cm) for very high shrinkage materials. Actual shrinkage values depend on many
variables such as temperature, pressure, injection time, and wall thickness of the
part. In general, amorphorus materials have lower shrinkage than crystalline
materials, and glass-reinforced or filled materials have less shrinkage than unfilled,
or neat resins.
4. Water Absorption is expressed as the percentage increase in weight of a
material due to absorption of water. This property determines the effects on
mechanical and electrical properties of a specific plastic. Plastics with low
absorption rates tend to be more dimensionally stable.
5. Transparency (Opacity) are properties that determine the light transmission
abilities of a given plastic and are usually measured as haze and luminous
transmittance.
6. Modulus (Toughness) is a measure of flexibility and refers to the material's
ability to absorb mechanical energy without fracturing, usually measured by the area
under the stress/strain curve of a plastic text specimen. In general, high impact
unfilled plastic materials have excellent toughness values.
7. Brittleness is a lack of toughness, and is exhibited by lower impact
strength and higher stiffness properties.
8. Elasticity is the ability of a material to return to its original size and shape after
being deformed. Most plastic materials have very limited elasticity.
9. Plasticity is the inverse of elasticity in so far as it is a value of the ability of a
polymer to remain deformed. Plastic materials exhibit plasticity on being stressed

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