Injection molding is a method, a polymer is heated to a extreamly plastic state and forced to flow under high pressure into a mold cavity wherever it solidifies.
The molded part known as molding is then removed from the cavity. It produces discrete elements that are almost always net shape.
The production cycle time is usually in the range 10 to 30 seconds. The mold may contain more than one cavity so that multiple moldings are produced each cycle.
Complex shapes are possible with injection molding. Injection molding is economical only for large production quantities.
Injection molding is the most widely used molding process for thermoplastics.
Some thermosets and elastomers are injection molded, with modificatiaons in equipment and operating parameters to allow for cross-linking of these materials.
Part size will vary from about 50 g upto about 25 kg. The upper limit represented by elements such as refrigerator doors and automobile bumpers.
The mold determines the part shape and size and is the special tooling in injection molding.
For large complex parts, the mold can cost hundreds of thousands of dollars. For small parts, the mold can be built to contain multiple cavities, also making the mold expensive.
The functions of the injection unit are to melt and homogenize the polymer and then inject it into the mold cavity.
The clamping unit is concerned with the operation of the mold. Its functions are
hold the two halves of the mold in proper alignment with each other
keep the mold closed during injection by applying a clamping force sufficient to resist the injection force
open and close the mold at the appropriate times in the molding cycle.
The clamping unit consists of two platens a fixed platen and a movable platen. The mechanism is basically a power press that is operated by hydraulic piston.
The cycle for injection molding of a thermoplastic polymer are
Mold is closed and clamped.
A shot of melt, which has been brought to the right temperature and viscosity by heating and by the mechanical working of the screw, is injected under high pressure into the mold cavity. The plastic cools and begins to solidify when it encounters the cold surface of the mold. Ram pressure is maintained to pack additional melt into the cavity to compensate for contraction during cooling.
The screw is rotated and retracted with the non-return valve open to permit fresh polymer to flow into the forward portion of the barrel. Meanwhile, the polymer in the mold has completely solidified.
The mold is opened, and the part is ejected and removed.
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