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Injection molding machine
Components
For thermoplastics, the
injection molding machine converts granular or
pelleted raw plastic into final molded parts
via a melt, inject, pack, and cool cycle. A
typical injection molding machine consists of
the following major components, as illustrated
in Figure 1 below.
FIGURE 1.
A single screw injection molding machine for
thermoplastics
 Machine
specification
Clamping
tonnage and shot size are commonly used to
quickly identify the size of the injection
molding machine for thermoplastics. Other
parameters include injection rate, injection
pressure, screw design, mold thickness, and the
distance between tie bars.
Machine
function
Injection
molding machines can be generally classified
into three categories, based on machine
function:
Auxiliary
equipment
The major
equipment auxiliary to an injection molding
machine includes resin dryers,
materials-handling equipment, granulators,
mold-temperature controllers and chillers,
part-removal robots, and part-handling
equipment.
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The injection
system consists of a hopper, a reciprocating screw
and barrel assembly, and an injection nozzle, as
shown in Figure 1. This
system confines and transports the plastic as it
progresses through the feeding, compressing,
degassing, melting, injection, and packing stages.
FIGURE 1. A
single screw injection molding machine for
thermoplastics, showing the plasticizing screw, a
barrel, band heaters to heat the barrel, a
stationary platen, and a movable platen.
The hopper
Thermoplastic
material is supplied to molders in the form of small
pellets. The hopper on the injection molding machine
holds these pellets. The pellets are gravity-fed
from the hopper through the hopper throat into the
barrel and screw assembly.
The barrel
As shown in
Figure 1, the barrel of
the injection molding machine supports the
reciprocating plasticizing screw. It is heated by
the electric heater bands.
The
reciprocating screw
The
reciprocating screw is used to compress, melt, and
convey the material. The reciprocating screw
consists of three zones (illustrated below):
While the outside
diameter of the screw remains constant, the depth of
the flights on the reciprocating screw decreases
from the feed zone to the beginning of the metering
zone. These flights compress the material against
the inside diameter of the barrel, which creates
viscous (shear) heat. This shear heat is mainly
responsible for melting the material. The heater
bands outside the barrel help maintain the material
in the molten state. Typically, a molding machine
can have three or more heater bands or zones with
different temperature settings.
FIGURE 2. A
reciprocating screw, showing the feeding zone,
compressing (or transition) zone, and metering zone.
The nozzle
The nozzle
connects the barrel to the sprue bushing of the mold
and forms a seal between the barrel and the mold.
The temperature of the nozzle should be set to the
material's melt temperature or just below it,
depending on the recommendation of the material
supplier. When the barrel is in its full forward
processing position, the radius of the nozzle should
nest and seal in the concave radius in the sprue
bushing with a locating ring. During purging of the
barrel, the barrel backs out from the sprue, so the
purging compound can free fall from the nozzle.
These two barrel positions are illustrated below.
FIGURE 3. (a)
Nozzle with barrel in processing position. (b)
Nozzle with barrel backed out for purging.
The mold
system consists of tie bars, stationary and moving
platens, as well as molding plates (bases) that
house the cavity, sprue and runner systems, ejector
pins, and cooling channels, as shown in
Figure 4. The mold is essentially a
heat exchanger in which the molten thermoplastic
solidifies to the desired shape and dimensional
details defined by the cavity.
FIGURE 4. A
typical (three-plate) molding system.
An mold system is
an assembly of platens and molding plates typically
made of tool steel. The mold system shapes the
plastics inside the mold cavity (or matrix of
cavities) and ejects the molded part(s). The
stationary platen is attached to the barrel side of
the machine and is connected to the moving platen by
the tie bars. The cavity plate is generally mounted
on the stationary platen and houses the injection
nozzle. The core plate moves with the moving platen
guided by the tie bars. Occasionally, the cavity
plate is mounted to the moving platen and the core
plate and a hydraulic knock-out (ejector) system is
mounted to the stationary platen.
Two-plate
mold
The vast
majority of molds consist essentially of two halves,
as shown below. This kind of mold is used for parts
that are typically gated on or around their edge,
with the runner in the same mold plate as the
cavity.
Three-plate
mold
The
three-plate mold is typically used for parts that
are gated away from their edge. The runner is in two
plates, separate from the cavity and core, as shown
in Figure 5 below.
FIGURE 5.
(Left) A two-plate mold. (Right) A three-plate mold.
Cooling
channels (circuits)
Cooling channels are passageways located within the
body of a mold, through which a cooling medium
(typically water, steam, or oil) circulates. Their
function is the regulation of temperature on the
mold surface. Cooling channels can also be combined
with other temperature control devices, like
bafflers, bubblers, and thermal pins or heat pipes.
The hydraulic
system on the injection molding machine provides the
power to open and close the mold, build and hold the
clamping tonnage, turn the reciprocating screw,
drive the reciprocating screw, and energize ejector
pins and moving mold cores. A number of hydraulic
components are required to provide this power, which
include pumps, valves, hydraulic motors, hydraulic
fittings, hydraulic tubing, and hydraulic
reservoirs.
The control
system provides consistency and repeatability in
machine operation. It monitors and controls the
processing parameters, including the temperature,
pressure, injection speed, screw speed and position,
and hydraulic position. The process control has a
direct impact on the final part quality and the
economics of the process. Process control systems
can range from a simple relay on/off control to an
extremely sophisticated microprocessor-based,
closed-loop control.
The clamping
system opens and closes the mold, supports and
carries the constituent parts of the mold, and
generates sufficient force to prevent the mold from
opening. Clamping force can be generated by a
mechanical (toggle) lock, hydraulic lock, or a
combination of the two basic types.
A typical molded system consists of
the delivery system and the molded part(s), as shown
in Figure 6.
FIGURE 6. The
molded system includes a delivery system and molded
parts.
The delivery
system, which provides passage for the molten
plastic from the machine nozzle to the part cavity,
generally includes:
The delivery
system design has a great influence on the filling
pattern and thus the quality of the molded part.
Cold runners
After molding, the cold-runner
delivery system is trimmed off and recycled.
Therefore, the delivery system is normally designed
to consume minimum material, while maintaining the
function of delivering molten plastic to the cavity
in a desirable pattern.
Hot runners
The hot-runner (or runnerless)
molding process keeps the runners hot in order to
maintain the plastic in a molten state at all times.
Since the hot-runner system is not removed from the
mold with the molded part, it saves material and
eliminates the secondary trimming process.
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