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.
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.
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.
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.
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.
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.
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 delivery system
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.