A round, hollow tube of molten plastic that is
extruded from the head of the blow molding machine.
The tendency for a parison to lose its
tubular shape because of conditions in the extrusion process.
Generally, the bottom of the parison tends to drape
irregularly, causing the entire parison to lose its original
shape, creating difficulties in molding the part.
Parison Melt Strength
Parison melt strength depends directly
on the melt characteristics of the resin being extruded. The
ability to extrude a parison of sufficient dimensions to
produce the desired part depends on the melt strength of the
parison. The larger and heavier the parison, the greater its
melt strength requirements.
Parison Pinch Bars
Various methods are used to close or seal
the bottom of the parison before mold closing or part
inflation. The devices are called pinch bars and are usually
spring loaded, or hydraulically or pneumatically actuated.
They may be attached to the bottom of the mold or positioned
directly under it.
A pinch-off is needed when the parison falls
outside the cavity of the mold. It is the protruding edge
separating the cavity from the flash pocket, and it compresses
the flash to the point of severance. Inserted beryllium copper
is preferred because the alloy has thermal conductivity equal
to that of the aluminum alloy used in the mold. Steel pinches
are used when pinch wear is critical - for example, when
molding materials such as polycarbonates are used.
Introducing air pressure into the parison
before closing the mold halves. This provides better
distribution of wall thickness and prevents the parison wall
from coming in contact before the inflation of the part.
Varying the wall thicknesses in a parison
to conform to the wall thickness requirements of a given part.
The bottom portion of a parison that is severed
by the lower pinch-offs and falls outside the mold.
The ability of a material to withstand
continuous and permanent deformation by stresses exceeding the
yield value of the material without rupture.
To render a material softer, more flexible and/or
more moldable by the addition of a plasticizer.
A substance or material incorporated in a material
(usually a plastic or an elastomer) to increase its
flexibility, workability or extensibility. Some plasticizers
have been known to have detrimental effects on certain types
of plastic, end use testing is recommended.
An objectionable coating gradually formed on metal
surfaces of molds during processing of plastics due to
extraction and deposition of some ingredient such as pigment,
lubricant, stabilizer or plasticizer.
The large metal plates the mold attaches to on a
plastic molding machine.
PLC (Performance Level Categories)
As defined by UL: "In order
to avoid an excessive level of implied precision and bias,
material performances for several tests are recorded as PLC,
based on the mean test results (rather than recording the
exact numerical results)". PLC levels are assigned to electric
properties, tested according to UL 746A
Polycarbonate Resin (PC)
A family of special types of
polyesters in which groups of dihydric phenols are linked
through carbonate linkages.
The product of a polymerization reaction.
The product of polymerization of one monomer is called a homopolymer, monopolymer or simply a polymer. When two
different monomers are polymerized simultaneously the product
is called a copolymer. The term terpolymer is sometimes used
for polymerization products of three monomers.
A linear polymer is a polymer in which the
monomers are bound to each other in a straight chain without
any branches. Branched polymers have branched connections of molecules.Copolymers are polymers with repeating molecular
units from at least two different monomers. Two kinds of
arrangements are possible: random and alternating, resulting
in random copolymers and alternating copolymers.Such polymers
are called block-copolymers, characterized by both monomers A
and B forming the backbone chain of the polymer. They have
repeating monomers in linearly connected blocks. Another
possibility is the formation of a graft-copolymer, which is
essentially a branched-chain structure. It has side chains
composed of one type of monomer unit attached to the backbone
or main chain from another monomer unit.
The process of converting a monomer or a
mixture of monomers into a polymer. Addition polymerization is
the stepwise addition of a simple repeated unit. Or, the
reaction that yields a product that is an exact multiple of
the original monomeric molecule. Condensation polymerization
is the combination of functional molecules, leading to the
formation of a polymer with the liberation of simple
by-products, usually water.
A general term referring to the relative
positions, arrangement in space, and freedom of motion of
atoms in a polymer molecule.
The class of polymers made by polymerising relatively simple
Time, usually in hours, during which a two or three
component product can be used after it is mixed. Sometimes
measured in terms of time to gel and/or double in viscosity.