The function of Injection Molding Process is to convert Polymer into the desired shape of Components. The polymer is heated in a cylinder under Pressure in a Machine called Injection Molding Machine. The desired shape is provided by a tool Known as Injection Mold.
One of the earliest forms of Plastic Moulding was Compression Moulding. Here, a fixed amount of plastic is placed in the lower half of a mould and heated before the upper half of the mould is closed over the top of it. The mould remains closed while the part cools and when it is taken off the ‘Flash’ (excess material that seeps between the two halves of the mould) is removed .Transfer Moulding introduces a plunger, or ram, that pushes the plastic through a barrel and into the mould cavity, which is already closed. Transfer Moulding reduces the amount of waste and removes the need for de-flashing. Some waste material is still produced though, in the barrel and interconnecting parts of the mould(depending on its shape).Plunger Moulding has the plunger mounted horizontally and the plastic fed into the barrel from a hopper mounted on top.
The last major innovation, which led to the Injection Moulding Machine, was the extruding, Archimedean or plasticizing screw. The Screw is a tapered bar with a spiral of flights along its length and was first used, not surprisingly, for extruding .The height and pitch of the flights varies according to which of the three zones of the screw they occupy and hence, which job they are designed to do as the screw turns: Feeding, Compressing or Metering.
1. The Feed zone flights are long and designed to move the material along the barrel as quickly as possible. As they move along, the granules are heated by friction (from the movement of the screw inside the barrel and from the movement of the granules themselves) and by the inside of the barrel against which the granules are forced by movement of the flight.
2. In the Compression zone the flights become shorter and the plastic granules are further heated and compressed, removing any air pockets. The plastic is melted by now and becomes thoroughly mixed or homogenized by the continuing movement of the screw.
3. The Metering zone contains the shortest flights, which are designed to pump the plastic or melt, through the extrusion die. Various methods of incorporating
Archimedean screw extruders into injection molding machines were developed until the most efficient was found; the Reciprocating Screw. Basically the screw is capable of moving back and fore as well as turning so that it can act as a plunger and an extruder. As melt builds up at the end of the screw, the screw moves backwards screw, has traveled the screw is driven forward, pushing the melt through the nozzle, along the sprue and into the mould cavity.
The injection molding process occurs cyclically. Typical cycle times range from 10 to100 seconds and are controlled by the cooling time of the thermo plastic or the currying time of the thermosetting plastic.
The plastic resin in the form of pellets or powder is fed from the hopper and melted. In a reciprocating screw type injection molding machine, the screw rotates forward and fills the mold with melt, holds the melt under high pressure, and adds more melt to compensate for the contraction due to cooling and solidification of the polymer. This is called the hold time.
Eventually the gate freezes, isolating the mold from the injection unit, the melt cools and solidifies. Next the screw begins to rotate and more melt is generated for the next shot. In the soak time the screw is stationary and the polymer melts by heat conduction from the barrel to the polymer. The solidified part is then ejected and the mold closes for the next shot.
There is a good demand for plastic Injection Molded Articles in Automobile, Electronic, Electrical and Mechanical Industry. Injection Molded Articles such as bearing hose, Gears, Covers etc, are finding increase in use and replacing conventional metallic components. The process has been modified and developed in numerous ways and now there are many different types of Injection Moulding, such as:
- Conventional Injection Molding,
- Twin /Triple Injection Moulding,
- Multi-component injection molding,
- Multi-station injection molding,
- Reaction injection molding,
- Gas injection molding.
Thermoplastic Materials like HDPE, PP, LDPE ,LLDPE,GPPS, PCetc can be injected in Mold & molded into variety of products starting from consumer goods i.e. mugs, caps closures etc to industrial goods such as Coil ,Fork, Transformer, housing etc.
In addition to the Injection Molding Machine and the Molds, the Auxiliary Equipments required as Hot Air Oven, Weighing Balance, Cranes, Scrap Grinder, Cooling Tower, Maintenance Equipments like Lathe ,Drilling Machine, Grinder, Electrical Testing Equipments etc. and miscellaneous items like Brass /Copper Rods ,Nylon Head Hammer, Vice , Mould Release Spray, etc.