Plastic Injection Molding

[attachment=18229]

Plastic Injection Molding

It is a manufacturing technique for making parts from thermoplastic and thermoset materials
In contrast to the extrusion (which makes continuous parts of constant cross section), injection molding make discrete parts (with complex and variable cross section)
Molten plastic is injected at high pressure into a mold, which is the inverse of the desired shape.
The mold is made from metal, usually either steel or aluminium
Widely used for manufacturing a variety of parts, from the smallest component to entire body panels of cars

Injection Unit

From hopper – hole (feed throat)
Barrel made of heavy steel cylinder to withstand the pressure and temperature involved in melting the resin
2 types of system used in injection molding;
Reciprocating screw- similar to extruder screw but with unique reciprocating action
Ram injector

Reciprocating Screw Injection Molding Machine


The entire screw move forward and pushes the molten resin out through the end of barrel
To ensure the resin does not flow backward, a check valve or nonreturn valve is attached to the end of screw
Normally the screw will stay in the forward position, until resin began to harden in the mold

Ram Injection- Injection Molding Machine


In this type of injection molding, the resin is fed from a hopper into the barrel, and heated through thermal energy from the heaters
The molten resin is collect in a pool in a barrel celled injection chamber
The molten resin is then push forward by the action of plunger (ram or piston)
To five better mixing, the molten resin is pushed past a torpedo/spreader, impart shear to the melt

Lubrication and Cooling

Obviously, the mold must be cooled in order for the production to take place. Because of the heat capacity, inexpensiveness, and availability of water, water is used as the primary cooling agent. To cool the mold, water can be channeled through the mold to account for quick cooling times. Usually a colder mold is more efficient because this allows for faster cycle times. However, this is not always true because crystalline materials require the opposite: a warmer mold and lengthier cycle time.