Injection Moulding machines are broadly classified into two types,
1) Plunger type Injection Moulding Machine.
2) Screw type Injection Moulding Machine.
1) Plunger type Injection Moulding Machine
The earliest moulding machines were of plunger type as illustrated in fig.and still many of these types of machines are used. In this type of injection moulding machine the resin is fed from hopper into the barrel and heated through the input of thermal energy from heaters around the barrel. The molten resin collects in a pool in the barrel called the injection chamber.
The molten resin is then pushed forward by action of a plunger driven by a hydraulic system at the head of the machine. To facilitate the melting of any residual solid material and to give better mixing of the melt, the molten resin is pushed past a torpedo or spreader that, along with a back pressure plate, imparts shear to the melt. Then the resin flows through a nozzle into the mould.
2) Screw type Injection Moulding Machine
Figure shows a schematic drawing of the injection end of a reciprocating screw machine. The extruder screw, which is contained in the barrel, is turned most often by a hydraulic motor (as contrasted to an electric motor attached to a gear system) point, at which time the screw stops. The rotary shutoff valve is rotated so that when the injection plunger advances, the material is injected into the mold. The main advantages of a two-stage screw are that the material passes over the whole
As the screw turns, it picks up material from the hopper. As it progresses down the screw, the resin is compacted, degassed, melted, and pumped past the nonreturn flow valve assembly at the injection side of the screw. This, in essence, is a check valve, which allows material to flow only from the back of the screw forward.
As the material is pumped in front of the screw, it forces back the screw, hydraulic motor, and screw drive system. In so doing, it also moves the piston and rod of the hydraulic cylinder(s) used for injection. Oil from behind the piston(s) goes into a tank through a variable resistance valve, called the back pressure valve. Increasing this resistance requires higher pressures from the pumping section of the screw, and results in better mixing, a slower cycle, and greater energy consumption.
The screw will continue to turn, forcing the carriage back until a predetermined location is reached. Then the rotation is stopped, and an exact amount of melted material is in front of the screw and will be injected into the mold at the appropriate time in the cycle. This is accomplished by using the hydraulic injection cylinder(s).
