Injection Molding, Closures, Rotational Molding, Compression Molding, and Tubes

Injection Molding

Injection molding is the process in which melted plastic is forced into a mold where it is shaped and cooled. In packaging, it is the usual method for manufacturing closures of various types, Injection molding is also used for producing margarine tubs, 5-gal pails, and other containers and package components, A very important application is production of performs for injection blow molded bottles.

Injection Molding Machines

The injection molding process begins with delivery of the plastic, usually in the form of pellets, to the injection molding machine. This is in essence an extruder much like those used for producing cast or blown film, as described in Chapter 7, with one addition. The screw has the capability to move backwards and forwards in the barrel; this is known as a reciprocating screw. As the screw turns, the melted plastic is contained within the barrel and accumulates ahead of the screw, and behind a valve ay the end of the barrel, The pressure created by the melt forces the screw backwards in the barrel. When the desired volume of melt is accumulated, the screw stops rotating. It is then driven forward, mechanically or hydraulically. This forces the melted plastic through a nozzle into the mold. Typically the pressure is raised to a high level and held there after the mold is filled, This is called packing the mold, After a period of time (determined by the processing necessary for the particular parts being made), the screw is retracted, screw rotation begins, and the process starts over again.

 Injection molding machine (reprinted with permission from)

Some injection molding machines are designed with separate plasticizing and injection functions, particularly those designed for production of large parts or those used for processing of materials that are temperature sensitive. In one such design, a non-reciprocating extruder feeds the plastic into a separate chamber, known as the shooting pat. When the desired amount of melt is accumulated in the shooting pot, an injection piston forces the melt into the mold. This design allows the extruder screw to rotate continuously, and also provides greater accuracy in shot volume. A third design involves a plunger, instead of a screw, The plunger forces the plastic over a heated torpedo, melting it, and then the plastic is forced into the mold. The major disadvantage of this process is poor homogeneity of the melted plastic due to lack of mixing. Consequently, the torpedo design has been almost totally abandoned.

      The size of injection molding machine is usually specified in terms of injection capacity and clamp force. These factors are not directly related. Other important variables are L:D ratio, barrel size, plasticizing rate, injection rate, and injection pressure. Major components of injection molding machines include the injection unit, the clamp unit, and the machine base which contains the power and control units.

Injection Mold Units

Injection molds have two main parts, or platens. Typically the half containing the hollow cavity side is attached to the injection molding machine, and is referred to as the fixed platen. Because the melted plastic enters through this side, it is also called the hot side, or the hot half.

      The section of the mold containing the concave core is usually the piece that moves back and forth to open and close the mold, and is referred to as the moving platen, In some design, especially in stack molds, the mold cavity is mounted on an intermediate section called the floating platen, rather than directly on the injection molding machine.

 Injection mold, with mold cavities and (reprinted with permission from)

The clamp unit platens hold the two halves of the mold together, providing the force needed to keep the mold closed when the plastic is injected, This clamping action is provided using hydraulics or by mechanical toggles, Because of the very high pressures that are common in injection molding, the clamping units must be able to apply high counter-pressures to keep the molds from being forced open. Injection pressures of up to 200 Mpa are not unusual. The clamping pressure required is determined by multiplying the pressure in the mold by the projected area of the parts.

      To achieve accurate dimensions in the finished part, the mold cores must be aligned accurately with the cavities. Mold halves have large guide pins in the corners to assure alignment.

      The mold is cooled, usually by water, to solidify the plastic. When the plastic object has been cooled sufficiently to maintain its shape, the mold is opened, along the mold parting line, and the object is ejected. The cycle time in injection molding depends on the size of the molded part, as well as on the molding conditions and on the thermal and mechanical characteristics of the plastic being molded. In some case, the cycle time can be as shorts as two seconds.

      Injection molds are usually made from high quality tool-grade steel. They may include beryllium copper inserts to improve heat transfer rates for more rapid cooling. Molds are quite expensive, and often contain wear plates which can easily be replaced, so that the molds can be more easily refurbished and are a longer life.