Plastic Injection Molding Basics
- Plastic injection molding uses thermoplastic materials (plastics that soften and fluidize when heated.) The plastics used in the injection molding process are liquefied at temperatures between 220° C and 270° C.
- After liquefaction, the molten plastic is pushed into a closed die, where it hardens into the shape configured by the die cavity.
The Plastic Injection Molding System
Here is simple representation of a plastic injection molding system. See below for explanation.
Hopper
The thermoplastic material used in the plastic injection molding process is supplied in the form of small pellets. These pellets are loaded into the hopper, then gravity-fed into the barrel and the screw assembly.
Barrel
This a chamber in which the reciprocating screw is located. The barrel is heated by electric heater bands.
Reciprocating Screw
The reciprocating screw compresses and melts the plastic material. It also moves the material toward the die.
The reciprocating screw is divided into three zones: the feeding zone, the transition zone, and the metering zone. The outside diameter of the screw remains constant throughout these zones; but the depth of the screws' flights decreases from the feeding zone to the metering zone.
The flights of the screw compress the material against the inside of the barrel. This motion creates viscous (shear) heat. Although there are heater bands on the barrel, this shear heat is the primary heat that melts the plastic. (The main function of the heater bands is to maintain the temperature of the molten material.)
Nozzle
The nozzle forms a sealed connection between the barrel and the sprue bushing of the mold. The temperature of the nozzle is usually set near the plastic’s melt temperature.
The mold’s sprue bushing has a concave radius. The nozzle fits into this radius with the aid of a locating ring.
