|
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.
Valence Number of electrons gained, lost, or shared by an atom in
bonding to one or more other atoms.
Valve gating 1) A type of gate in which a pin is held in the
gate or channel by spring tension. As the plunger comes for-ward, it
compresses the plastic in the runner. When this pressure build–up is
sufficient to overcome the spring tension, the pin is then pushed back
(pulled) and the fast decompression of the plastic fills the cavity at
extremely high speed. 2) A type of hot runner mold gating that uses
hydraulic or pneumatic pressure to open and close the gate opening to
the part.
Variation Variation is the difference in things that are
supposed to be the same.
VARTM (Vacuum-Assisted Resin Transfer Molding) Is a low-cost
alternative to resin transfer molding (RTM). It features lower tooling
costs, room temperature processing and ease of scalability.
Velocity of Approach Average velocity of fluid in a duct head
of the point considered.
Vent Shallow channel in a mold which allows air, gas, or
volatile to exit as the melt enters the cavity. Tiny channels at the end
of the flow path that allow air to escape the cavity.
Vented barrel Port in a barrel through which volatile’s
moisture can be removed.
Vented screw Two-stage screw with vent in second stage to
remove volatile or water from the plastic.
Vertical flash ring The clearance between the force plug and
the vertical wall of the cavity in a positive or semi-positive mold;
also the ring of excess material that escapes from the cavity into this
clearance space.
Vibration welding Method of fusing two plastic parts by
vibrating (rubbing) the mating surfaces together at relatively low
frequencies — 90 to 120 Hz.
Vicat softening point The temperature at which a flattened
needle of 1 mm2 circular cross section will penetrate a
thermoplastic specimen to a depth of 1 mm under a specified load using a
selected uniform rate of temperature rise. ASTM D1525.
Vinyl Class of resins containing materials such as polyvinyl
chloride, polyvinyl acetate, etc.
Virgin Material Any plastic compound or resin that has not
been subjected to use or processing other than that required for its
original manufacture.
Viscoelasticity This property, possessed by all plastics to
some degree, dictates that while plastics have solid-like
characteristics such as elasticity, strength and form-stability, they
also have liquid-like characteristics such as flow depending on time,
temperature, rate and amount of loading.
Viscometer (Viscosimeter) Instrument for measuring the
viscosity of a fluid. Types of viscometers are: 1) Poiseuille, 2)
Stokes, 3) Couette, 4) Newton, 5) Coulomb, according to the effect used
to measure the viscosity.
Viscosity Breaking (Visbreaking) Thermal treatment (cracking)
of petroleum fractions, with or without catalyst.
Viscosity Gravity Constant Correlation of specific gravity and
viscosity: low for paraffinic, high for napththenic oils.
Viscosity Index Relationship between viscosity change of an
oil with change in temperature; an empirical measure of the temperature
coefficient of viscosity.
Viscosity Range Ratio of viscosity of a fluid at its lowest
temperature to that at its highest temperature, or lowest to highest
shear.
Viscosity The measure of the resistance of a fluid to flow
(either through a specific orifice or in a rotational viscometer). The
absolute unit of viscosity measurement is the poise (or centipoise).
Kinematic viscosity is expressed in strokes. A material's resistance to
flow. High substances are thick and slow-flowing, like honey. Low
viscosity substances are thinner and runnier, like water.
Viscous Fluid Fluid having appreciable fluid friction.
Viscous Forces Tangential frictional forces. The action of
such internal shear forces results in a conversion of mechanical energy
into heat or unavailable thermal energy.
Voids Pockets of unfilled space or vacuum in a molded part
generally caused by shrinkage during cooling. Bubbles of air or other
gases trapped in the part.
Volatile loss Weight loss by vaporization.
Volatile Materials which are capable of being driven off as a
vapor during molding.
Volatility Relative tendency of a substance to evaporate.
Controlled by vapor pressure for components in a homogeneous liquid
obeying Raoult’s Law.
Vortex Swirl Rotation of a liquid as if it were a solid body;
there is no interchange of position between particles. |
- The guide to polymer processing technology,
materials, additives & equipment -
Life insurance