Injection mold China is the process in which melted plastic is
forced into a mold where it is shaped and cooled. In packaging, it is the usual
method for manufacturing closures of various types, Injection molding is also
used for producing margarine tubs, 5-gal pails, and other containers and
package components, A very important application is production of performs for
injection blow molded bottles.
Injection Molding Machines
The injection molding process begins with delivery of the
plastic, usually in the form of pellets, to the injection molding machine. This
is in essence an extruder much like those used for producing cast or blown
film, as described in Chapter 7, with one addition. The screw has the
capability to move backwards and forwards in the barrel; this is known as a
reciprocating screw. As the screw turns, the melted plastic is contained within
the barrel and accumulates ahead of the screw, and behind a valve ay the end of
the barrel, The pressure created by the melt forces the screw backwards in the
barrel. When the desired volume of melt is accumulated, the screw stops
rotating. It is then driven forward, mechanically or hydraulically. This forces
the melted plastic through a nozzle into the mold. Typically the pressure is
raised to a high level and held there after the mold is filled, This is called
packing the mold, After a period of time (determined by the processing
necessary for the particular parts being made), the screw is retracted, screw
rotation begins, and the process starts over again.
Injection molding
machine (reprinted with permission from)
Some injection molding machines are designed with separate
plasticizing and injection functions, particularly those designed for
production of large parts or those used for processing of materials that are
temperature sensitive. In one such design, a non-reciprocating extruder feeds
the plastic into a separate chamber, known as the shooting pat. When the
desired amount of melt is accumulated in the shooting pot, an injection piston
forces the melt into the mold. This design allows the extruder screw to rotate
continuously, and also provides greater accuracy in shot volume. A third design
involves a plunger, instead of a screw, The plunger forces the plastic over a
heated torpedo, melting it, and then the plastic is forced into the mold. The
major disadvantage of this process is poor homogeneity of the melted plastic
due to lack of mixing. Consequently, the torpedo design has been almost totally
abandoned.
The size of
injection molding machine is usually specified in terms of injection capacity
and clamp force. These factors are not directly related. Other important
variables are L:D ratio, barrel size, plasticizing rate, injection rate, and
injection pressure. Major components of injection molding machines include the
injection unit, the clamp unit, and the machine base which contains the power
and control units.
Injection Mold Units
Injection molds have two main parts, or platens. Typically
the half containing the hollow cavity side is attached to the
injection mold China machine, and is referred to as the fixed platen. Because the melted plastic
enters through this side, it is also called the hot side, or the hot half.
The section of
the mold containing the concave core is usually the piece that moves back and
forth to open and close the mold, and is referred to as the moving platen, In
some design, especially in stack molds, the mold cavity is mounted on an
intermediate section called the floating platen, rather than directly on the
injection molding machine.
Injection mold, with
mold cavities and (reprinted with permission from)
The clamp unit platens hold the two halves of the mold
together, providing the force needed to keep the mold closed when the plastic
is injected, This clamping action is provided using hydraulics or by mechanical
toggles, Because of the very high pressures that are common in injection
molding, the clamping units must be able to apply high counter-pressures to
keep the molds from being forced open. Injection pressures of up to 200 Mpa are
not unusual. The clamping pressure required is determined by multiplying the
pressure in the mold by the projected area of the parts.
To achieve
accurate dimensions in the finished part, the mold cores must be aligned
accurately with the cavities. Mold halves have large guide pins in the corners
to assure alignment.
The mold is
cooled, usually by water, to solidify the plastic. When the plastic object has
been cooled sufficiently to maintain its shape, the mold is opened, along the
mold parting line, and the object is ejected. The cycle time in injection
molding depends on the size of the molded part, as well as on the molding
conditions and on the thermal and mechanical characteristics of the plastic
being molded. In some case, the cycle time can be as shorts as two seconds.
injection mold China are usually made from high quality tool-grade steel.
They may include beryllium copper inserts to improve heat transfer rates for
more rapid cooling. Molds are quite expensive, and often contain wear plates
which can easily be replaced, so that the molds can be more easily refurbished
and are a longer life.
