Property Theory




Specific Gravity / Density Specific Gravity

The specific gravity is the ratio of the density of a material to the density of water at 4℃.
The specific gravity is very important factor in determining cost of material per unit volume or 

determining material costs of parts manufactured.


Density

Density is defined as the mass per a unit volume of the material at a specific temperature (23℃ in case of plastics), 

expressed in g/cm3.


Specific Gravity vs. Density

Though often understood as there is no difference, as the density of water at 23℃ is a little less than 1, 

the following formula is used to convert the specific gravity into the density.

Density = specific gravity×0.99756


Melt Flow Index(MI) Melt Flow Rate(MFR)

Melt flow index value is the rate of extrusion of molten resins through a die of a specified length and diameter under prescribed conditions of temperature, load, and piston position in the barrel as the time measurement is being made (ASTM D1238-88). As the load (constant shear stress) is placed on the piston, molten polymer is extruded through a die (constant shear rate). Then, MFI is calculated as the weight of polymer extruded after 10 minutes. A material with a high MI value has a good melt flow which enables sophisticated injection molding and reduction of cycle time. However, in selecting MI value, one should consider its effects of other factors such as heat resistance level, impact strength and so on. The below table illustrates the relative importance of increasing and decreasing the melt index value in general.


Flexural Properties

Flexural properties are measured most often in plastics by using a three-point bending jig attached to a “universal” type tensometer. The test consists of measuring the stress developed at the surface of a prescribed test piece supported near each endand loaded at the center.


Flexural Strength

Is equal to the maximum stress in the outer fibers at the moment of break. Most thermoplastics do not get ruptured even after the specimen is significantly elongated, therefore a flexural modulus at the point of 5% elongation is accepted instead. This value represents resistance to the strain when deflected. specimen. Greater the flexural modulus represents greater stiffness, and lesser value of it means greater flexibility.

         Flexural Strength.jpg




Tensile Test

In order to measure the mechanical properties of polymer resin, observing material's behavior against the applied load is fundamental. This relationship is measured on stress-strain curve by observing the level of strain (ability to withstand a load) as the specimen made with material is stressed (application of load).

            Tensile Test1.jpg      


    StressStrainCurve.gif


Tensile Stress

The tensile load per unit area of minimum original cross-section, within thegage boundaries, carried by the test specimen at any given moment. It is expressed in force per unit area, usually ㎏/㎠.


Tensile Elongation

The increase in length produced in the gage length of the test specimen by atensile load. It is expressed in units of length, usually millimeters.Further, this can be expressed in percentage elongation.


Principle

Rate of deformation is of vital importance in determining the behavioral nature of a polymer. High rates of deformation lead to a brittle behavior while low rates of deformation often allow ductile yielding. In many applications, plastics may be subject to sudden shock loading, and some estimation of their effect must be made.
Thus, a various impact tests have been derived, each of which studies material behavior at one point on the general curve of strength properties as a function of speed of testing.
Methods standardized for materials testing usually have the advantage of ready measurement of the energy required to break an arbitrary shaped test piece, information not easily calculated from tensile and flexural tests. This situation does not necessarily hold true in the light of recent developments, but the widespread acceptance of impact test such as Izod and Charpy pendulums should ensure their use in material comparisons.
Test Methods In test of Izod type, the specimen is held as a vertical cantilever beam and is broken by a single swing of the pendulum with the line of initial contact at a fixed distance from the specimen clamp and from the centerline of the notch and on the same face as the notch.
(ASTM, Designation: D-256-88) In the test of Carpy type, the specimen is supported as a horizontal simple beam and is broken by a single swing of the pendulum with the impact line midway between the supports and directly opposite the notch.


Test Methods(Izod)

In test of Izod type, the specimen is held as a vertical cantilever beam and is broken by a single swing of the pendulum with the line of initial contact at a fixed distance from the specimen clamp and from the centerline of the notch and on the same face as the notch.
(ASTM, Designation: D-256-88) In the test of Carpy type, the specimen is supported as a horizontal simple beam and is broken by a single swing of the pendulum with the impact line midway between the supports and directly opposite the notch.

Impact Strength.jpg


Heat Deflection Temperature

Principle Heat deflection temperature is the temperature at which a simple beam with the load applied at itscenter deflects 0.01inch (0.254㎜).


Test Methods

A test method is classified into the following two, according to the load.

Method A : 264 psi (18.6kgf/㎠)

Method B : 66 psi (4.6kgf/㎠)

The specimen is loaded with a determined load, and immersed in the oil.
The oil is heated at the speed of 120℃/hr.
after a pre heating for 3-5 minutes.
Then, the specimen starts getting deflected as the temperature of the oil rises, and the temperature, at which the specimen gets defected by 0.01 inch, is the heat deflection temperature of the material.






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