Injection molding is a process in which a polymer is heated to a highly plastic state and forced to flow under high pressure into a mold cavity, where it solidifies. The molded component, called a molding, is then taken out from the cavity. The process produces separate components that are almost always net shape. The production cycle time is normally in the range of 10 to 30 sec, although cycles of 1 min or longer are not uncommon for large parts. Also, the mold may contain more than one cavity, so that several moldings are produced each cycle. Complex and intricate shapes are possible with injection molding. The challenge in these cases is to fabricate a mold whose cavity is the same geometry as the part and that also allows for part removal.
Part dimension can range from about 50 g (2 oz) up to about 25 kg (more than 50 lb), the higher limit represented by components like refrigerator doors and automobile bumpers. The mold determines the component shape and size and is the special tooling in injection molding. For huge, complex parts, the mold can cost hundreds of thousands of dollars. For tiny parts, the mold can be built to contain various cavities, also making the mold expensive. Thus, injection molding is inexpensive only for large production quantities. Injection molding is the most extensively used molding process for thermoplastics. A number of thermosets and elastomers are injection molded, with modifications in equipment as well as operating parameters to permit for cross-linking of these materials.
PROCESS AND EQUIPMENT:
Fig.1 Diagram of an injection molding machine, reciprocating screw type.
Equipment for injection molding evolved from metal die casting. A large injection molding machine is shown in Figure2. As illustrated in the schematic in Figure 1, injection molding machines consist of two principal components: (1) the plastic injection unit and (2) the mold clamping unit. The injection unit is like an extruder. It is having a barrel with that is fed from one end by a hopper containing a supply of plastic pellets. In the interior of the barrel is a screw whose operation surpasses that of an extruder screw in the following respect: in addition to turning for mixing and heating the polymer, it also acts as a ram that rapidly moves forward to inject molten plastic into the mold. A non return valve mounted near the tip of the screw prevents the melt from flowing backward along the screw threads. Afterward in the molding cycle the ram retracts to its former position. Because of its double action, it is called a reciprocating screw, a name that also identifies the machine type.
Fig.2 Injection Molding Machine:
Fig.3 Construction of Injection Molding Machine:
Elder injection molding machines used a simple ram (without screw flights), but the advantage of the reciprocating screw design has led to its widespread adoption in today’s molding plants.
Fig.4 Injection Molding Process:
To summarize, the functions of the injection unit are to melt and homogenize the polymer, and after that inject it into the mold cavity. The clamp unit is concerned with the operation of the mold. Its functions are to (1) hold the two halves of the mold in proper alignment with each other; (2) keep the mold closed during injection by applying a clamping force sufficient to resist the injection force; and (3) open and close the mold at the appropriate times in the molding cycle. The clamp unit consists of two platens, a fixed platen and a moveable platen, and a mechanism for translating the latter. The mechanism is basically a power press that is operated by hydraulic piston or mechanical toggle devices of various types. Clamp forces of several thousand tons are available on large machines.
Fig.5.1 Typical molding cycle: Step (1) mold is closed
Fig.5.2 Typical molding cycle: Step (2) melt is injected into cavity.
Fig.5.3 Typical molding cycle: Step (3) screw is retracted.
Fig.5.4 Typical molding cycle: Step (4) mold opens, and part is ejected.
The cycle for injection molding of a thermoplastic polymer proceed in the following sequence, illustrated in Figure 5.1 to 5.4.
(1) The mold is closed and clamped.
(2) An injection of melt, which has been brought to the precise temperature and viscosity by heating and the mechanical working of the screw, is inserted under high pressure into the mold cavity. The plastic cools and begin to solidify when it encounters the cold surface of the mold. Ram pressure is maintained to set additional melt into the cavity to compensate for contraction during cooling.
(3) 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. In the meantime, the polymer in the mold has completely solidified.
(4) The mold is opened, and the component is ejected and removed.