Injection molding machines
differ in both injection unit and clamping unit. The name of the injection molding machine is generally based on the type of injection unit used.
Two types of injection units are widely used today. The reciprocating-screw machine
is the most common. This design uses the same barrel for melting and injection of plastic. The alternative
unit involves the use of separate barrels for plasticizing and injecting the polymer. This type is called a screw-preplasticizer machine
or two-stage machine
. Plastic pellets are fed
from a hopper into the first stage, which uses a screw to drive the polymer forward and melt it. This barrel feeds a second barrel, which uses a plunger to inject the melt into the mold. Older machines used one plunger-driven barrel to melt and inject the plastic. These machines are referred to as plunger-type injection
injection systems to the reciprocating screw: (a) screw preplasticizer, and (b) plunger type
Clamping designs are of three types: toggle, hydraulic, and hydromechanical. Toggle clamps
include various designs. An actuator moves the crosshead forward, extending the toggle links to push the moving platen toward a closed position. At the beginning of the movement, mechanical advantage is low and speed is high; but near the end of the stroke, the reverse is true. Thus, toggle clamps provide both high speed and high force at different points in the cycle when they are desirable. They are actuated either by hydraulic cylinders or ball screws driven by electric motors. Toggle-clamp
units seem most suited to relatively low-tonnage machines.
Two clamping designs: (a) one possible toggle clamp
design (1) open and (2) closed; and (b) hydraulic clamping (1) open and (2) closed. Tie rods used to guide movuing platens not shown.
are used on higher-tonnage injection-molding machines, typically in the range 1300 to 8900 kN (150 to 1000 tons). These units are also more flexible than toggle clamps in terms of setting the tonnage at given positions during the stroke. Hydromechanical clamps
are designed for large tonnages, usually above 8900 kN (1000 tons); they operate by (1) using hydraulic cylinders to rapidly move the mold toward closing position, (2) locking the position by mechanical means, and (3) using high pressure
hydraulic cylinders to finally close the mold and build tonnage.