How does the injection molding machine work?

The greatest quantities of plastic parts are made by injection molding. The process consisits of feeding a plastic compound in powdered or granular from from a hopper through metering and melting stages and then injecting it into a mold. After a brief cooling period, the mold is opened and the solidified part ejected. In most cases, it is ready for immediate use.

Several methods are used to force or inject the melted plastic into the mold. The most commonly used system in the larger machines is the in-line reciprocating screw, as shown in below.

The screw acts as a combination injection and plasticizing unit. As the plastic is fed to the rotating screw, it passes through three zones as shown: feed, compression, and metering. After the feed zone, the screw-flight depth is gradually reduced, forcing the plastic to compress. The work is converted to heat by shearing the plastic, making it a semifluid mass. In the metering zone, additional heat is applied by conduction from the barrel surface. As the chamber in front of the screw becomes filled, it forces the screw back, tripping a limit switch that actives a hydraulic cylinder that forces the screw forward and injects the fluid plastic into the closed mold. An antiflowback valve prevents plastic under pressure from escaping back into the screw flights.

The clamping force that a machine is capable of exerting is part of the size designation and is measured in tons. A rule-of-thumb can be used to determine the tonnage required for a particular job. It is based on two tons of clamp force per square inch of projected area. If the flow pattern is difficult and the parts are thin, this may have to go three or four tons.

Many reciprocating –screw machines are capable of handling thermosetting plastic materials. Previously these materials were handled by compression or transfer molding. Thermosetting materials cure or polymerize in the mold and are ejected hot in the range of 35-410℉(190-210℃). Thermoplastic parts must be allowed to cool in the mold in order to remove them without distortion. Thus thermosetting cycles can be faster. Of course the mold must be heated rather than chilled, as with thermoplastics.

Ways of injection molding plastic material are sketched below. The oldest is the single-stage plunger method. When the plunger is drawn back, raw material falls from the hopper into the chamber. The plunger is driven forward to force the material through the heating cylinder where it is softened and squirted under pressure into the mold. The single-stage reciprocating screw system has become more popular because it prepares the material more thoroughly for the mold and is generally faster. As the screw turns, it is pushed backward and crams the charge from the hopper into the heating cylinder. When enough material has been prepared, the screw stops turning and is driven forward as a plunger to ram the charge into the mold. In a two-stage system, the material is plasticized in one cylinder, and a definite amount transferred by a plunger or screw into a shot chamber from which a plunger injects it into the mold.

An injection molding machine heats to soften, molds, and cools to harden a thermoplastic material. Operating-temperature is generally between 150℃ and 380℃(300℉ and 700℉ ) with full pressure usually over 35 and up to 350 MPa ( 5000 to 50000 psi ). The mold is water cooled. The molded piece and sprue are withdrawn from the injection side and ejected from the other side when mold is opened. The mold is then closed and clamped to start another cycle. Thermosetting plastic can be injection molded but have to be polymerized and molded before they set in the machine. This may be done in a reciprocating screw machine here one charge at a time is brought to curing temperature. By another method, sometimes called jet molding, performs are charged one at a time into a single-stage plunger machine.

Machines are available for molding sandwich parts. One cylinder and plunger injects a measured amount of skin material into the mold, and then a second cylinder squirts the filler inside the mass Finally, a final spurt from the first cylinder clears the core material from the sprue. The aim is to produce composites with optimum properties. Either case or core may be foamed.