Tensile Strength
The amount of force required to elongate
the plastic by a defined amount. The higher the value, the
stronger the material. Or the maximum stress sustained by a
material before failure in tension. When the maximum stress
occurs at the yield point, it is called tensile strength at
yield. When maximum stress at break: tensile strength at
break.
Thermal Conductivity
The measure of the ability of a
material to conduct heat along its length or through its
thickness. Or the rate of heat flow under steady conditions
through unit area, per unit temperature gradient in the
direction perpendicular to the area. The higher the value, the
more conductive: a material with a low value for thermal
conductivity acts like an insulator
Thermoelasticity
Rubber-like elasticity exhibited by a
rigid plastic resulting from an increase in temperature.
Thermoforming
The process of forming a thermoplastic sheet
into a three-dimensional shape by clamping the sheet in a
frame, heating it to tender it soft and flowable. Then
applying differential pressure to make the sheet conform to
the shape of a mold or die positioned below the frame.
Thermoplastics
Materials that become soft when heated and
solid when cooled to room temperature. This softening and
setting may be repeated many times.
Thermoplastic Isomers
The family of polymers that resemble elastomers in that they can be repeatedly stretched without
distortion of the unstressed part shape, but are true
thermoplastics and thus do not require curing.
Thermoplastics vs. Thermosets
A thermoplastic is a polymeric
material or plastic which becomes soft and formable when
heated and rigid when cooled. This process may be repeated a
number of times without chemically altering the material. A
thermoset is a polymeric material which undergoes irreversible
chemical changes when cured through heat, catalysts or
ultraviolet light: cross-linking prevents movement of
molecular chains after curing. Once cured, the structure
cannot be changed.
Thermosets
Materials that may not be reheated and softened
again. Once the structural framework is set, these plastics
cannot be reformed.
Thickness
Depth of the material and contributory to pressure
requirements; thickening reduces the pressure required to fill
the part.
Titanium dioxide
A white pigment available in two crystalline forms, rutile and
anatase, the former being the most widely used white and
opacifying pigment in thermoplastics, printing inks and paints
Undercut
A protuberance or indentation that impedes
withdrawal from a mold.
Underflow
The dominant flow of two confronting flows, over the
other. The lesser flow reverses direction giving poor surface
appearance and structural strength. Underflow should be
avoided by positioning gates so that the flow fronts meet at
the end of filling.
Uniform Cooling Time
Cooling time the same throughout the
part to avoid warping.
UV absorber
An additive which protects materials by absorbing UV
radiation.
UV stabiliser
Additive which stabilises organic materials against UV
radiation.
Vacuum Forming
A method of forming plastic sheets or films
into three-dimensional shapes, in which the plastic sheet is
clamped in a frame suspended above a mold, heated until it
becomes softened, drawn down into contact with the mold by
means of a vacuum, and cooled while in contact with the mold.
Often used interchangeably with thermoforming.
Vicat Softening Temperature Undercut
Vicat Softening
Temperature, is a measure of the temperature at which a
plastic starts to soften at specified test conditions
according to ISO 306. It is determined with a standard
indenter (a flat-ended needle of 1 mm 2 circular cross
section) penetrating into the surface of a test specimen under
a predefined load. The temperature at 1 mm penetration is
quoted as the VST in Co. VST gives an indication of a
material’s ability to withstand limited short-term contact
with a heated object. For material comparisons only.
Virgin Material
Any plastic compound or resin that has not
been subjected to use or processing other than that required
for its original manufacture.
Viscosity, MFR, MVI, and
MV
Viscosity is the resistance to
steady flow shown within the body of a material. It is
internal friction or the measure of a polymer melt’s
resistance to flow. In testing: the ratio of the shearing
stress to the rate of shear of a fluid. Which ‘Newtonian
viscosity’, the ratio of shearing stress to rate of shearing
strain is constant. In non-Newtonian behavior – which is the
usual case with plastics – the ratio is not constant but
varies with the shearing stress. Such a ratio is often called
the apparent viscosity at the corresponding shearing stress.
It represents one point on the flow curve. MFR, or melt flow
rate, is the mass of thermoplastic material extruded in a
given time through a defined orifice under specified
conditions. Also called "flow rate." The expression MVI, or
melt volume index, equals MVR. MV, or melt viscosity, is a
measure of a polymer at a given temperature at which the
molecular chains can move relative to each other. It is
expressed as the quotient of the real shear stress t and the
real rate of shear y. Melt viscosity is considerably dependent
on the molecular weight: the higher the molecular weight the
greater the entanglements and the greater the melt viscosity.
Warp
To twist or deform from a desired shape. Often caused
by molded in stress or shrinkage.
Warpage
Dimensional distortion of a plastic part due to strain
resulting from injection molding or other conversion methods.
Zones
Sections of the barrel which are controlled
individually by temperature controllers.
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