Injection molding is the principal method of forming
thermoplastic materials. Modifications of the injection process
are sometimes used for thermosetting plastics.
In injection molding, plastic material is put into a hopper which
feeds into a heated injection unit. A reciprocating screw pushes
the plastic through this long heating chamber, where the material
is softened to a fluid state. At the end of this chamber there is
a nozzle which abuts firmly against an opening into a cool,
closed mold. The fluid plastic is forced at high pressure through
this nozzle into the cold mold. A system of clamps hold the mold
halves shut. As soon as the plastic cools to a solid state, the
mold opens and the finished plastic is ejected from the press.
The problem with injection molding of thermosetting materials is
that, under heat, these plastics will first soften, then harden
to an infusible state. Thus it is essential that no softened
thermosetting material in the heating chamber be allowed to
remain there long enough to set. Jet molding, offset molding and
molding using a screw-type machine overcome this problem by
liquefying the thermosetting plastic material just as it goes
through the injection nozzle into the mold, but not before.
Blow molding is a method of forming hollow articles out of
Blow molding is a process of forming a molten tube of
thermoplastic material, then with the use of compressed air,
blowing up the tube to conform to the interior of a chilled blow
mold. The most common methods are extrusion, injection, and
injection-stretch blow molding.
The continuous-extrusion method uses a continuously running
extruder with a tuned die head that forms the molten plastic
tube. The tube is then pinched between two mold halves. A blow
pin or needle is inserted into the tube and compressed air is
used to blow up the part to conform to the chilled mold interior.
Accumulator-extrusion is similar, however, the molten plastic
material is accumulated in a chamber prior to being forced
through a die to form the tube.
Injection blow molding is a process of injection molding a
preform (similar to a test tube), then taking the tempered
preform to a blow mold to be filled with compressed air to
conform to the interior of the blow mold. Injection-stretch blow
molding can be a single-stage process similar to standard
injection blow molding, by adding the element of stretch prior to
blow forming. Also, a two-step process is possible, where a
preform is made in an injection molding machine, then taken to a
reheat-stretch blow molding machine for preform reheating and
final blow forming in a blow mold.
Thermoforming of plastic sheet has developed rapidly
in recent years. This process consists of heating thermoplastic
sheet to a formable plastic state and then applying air and/or
mechanical assists to shape it to the contours of a mold.
Air pressure may range from almost zero to several hundred psi.
Up to approximately 14 psi (atmospheric pressure), the pressure
is obtained by evacuating the space between the sheet and the
mold in order to utilize this atmospheric pressure. This range,
known as vacuum forming, will give satisfactory reproduction of
the mold configuration in the majority of forming applications.
Transfer molding is most generally used for thermosetting
plastics. This method is like compression molding in that the
plastic is cured into an infusible state in a mold under heat and
pressure. It differs from compression molding in that the plastic
is heated to a point of plasticity before it reaches the mold and
is forced into a closed mold by means of a hydraulically operated
Transfer molding was developed to facilitate the molding of
intricate products with small deep holes or numerous metal
inserts. The dry mold compound used in compression molding
sometimes disturbs the position of the metal inserts and the pins
which form the holes. The liquefied plastic material in transfer
molding flows around these metal parts without causing them to
Reaction Injection Molding
Reaction injection molding (RIM) is a relatively new processing
technique that has rapidly taken its place alongside more
traditional methods. Unlike liquid casting, the two liquid
components, polyols and isocyanates, are mixed in a chamber at
relatively low temperatures (75� - 140� F) before being injected
into a closed mold. An exothermic reaction occurs, and
consequently RIM requires far less energy usage than any other
injection molding system.
The three major types of polyurethane RIM systems are rigid
structural foam, low-modulus elastomers, and high-modulus
Reinforced RIM (R-RIM) consists of the addition of such materials
as chopped or milled glass fiber to the polyurethane to enhance
stiffness and to increase modulus, thus expanding the range of
Compression molding is the most common method of forming
thermosetting materials. It is not generally used for
Compression molding is simply the squeezing of a material into a
desired shape by application of heat and pressure to the material
in a mold.
Plastic molding powder, mixed with such materials or fillers as
woodflour and cellulose to strengthen or give other added
qualities to the finished product, is put directly into the open
mold cavity. The mold is then closed, pressing down on the
plastic and causing it to flow throughout the mold. It is while
the heated mold is closed that the thermosetting material
undergoes a chemical change which permanently hardens it into the
shape of the mold. The three compression molding factors --
pressure, temperature and time the mold is closed -- vary with
the design of the finished article and the material being molded.
Extrusion molding is the method employed to form thermoplastic
materials into continuous sheeting, film, tubes, rods, profile
shapes, and filaments, and to coat wire, cable and cord.
In extrusion, dry plastic material is first loaded into a hopper,
then fed into a long heating chamber through which it is moved by
the action of a continuously revolving screw. At the end of the
heating chamber the molten plastic is forced out through a small
opening or die with the shape desired in the finished product. As
the plastic extrusion comes from the die, it is fed onto a
conveyor belt where it is cooled, most frequently by blowers or
by immersion in water.
In the case of wire and cable coating, the thermoplastic is
extruded around a continuing length of wire or cable which, like
the plastic, passes through the extruder die. The coated wire is
wound on drums after cooling.
In the production of wide film or sheeting, the plastic is
extruded in the form of a tube. This tube may be split as it
comes from the die and then stretched and thinned to the
dimensions desired in the finished film.
In a different process, the extruded tubing is inflated as it
comes from the die, the degree of inflation of the tubing
regulating the thickness of the final film.
Method of Plastic
Gas Assisted Injection Molding (GAIN)
Reaction Injection Molding (RIM/SRIM)
Extrusion Blow Molding
Injection Blow Molding
Injection Stretch Blow Molding
Rod, Pipe, Sheet, Profile Extrusion
Extrusion Coating and Laminating
Melt Blown Nonwoven Fiber