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1、 Mold Construction1. The injection molding and machine1.1 The injection moldingInjection molding is principally used for the production of the thermoplastic parts, although some progress has been made in developing a method for injection molding some thermosetting materials. The problem of injecting
2、 a melted plastic into a mold cavity from a reservoir of melted material has been extremely difficult to solve for thermosetting plastics which cure and harden under such conditions within a few minutes. The principle of injection molding is quite similar to that of die-casting. The process consists
3、 of feeding a plastic compound in powdered or granular form 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.Injection-molding machines can be arranged for manual operation, automatic
4、 single-cycle operation, and full automatic operation. The advantage of injection molding are: (i) a high molding speed adapted for mass production is possible; (ii) there is a wide choice of materials providing a variety of useful properties; (iii) it is possible to mold threads, undercuts, side ho
5、les, and large thin sections.1.2 The injection-molding machineSeveral 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 . The screw acts as a combination injection and plasticizing unit. As th
6、e plastic is fed to the rotating screw, it passes through three zones: 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 meterin
7、g 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 activates a hydraulic cylinder that forces the screw forward and injects the fluid plastic into the closed mold. An an
8、tiflowback 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 .The injection-molding machine ,the reciprocating-screw injection system and is measured in tons. A rule-of-thumb can be
9、 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 to three or four tons.Many reciprocating-screw machines are capable of handing thermose
10、tting 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 375410. Thermoplastic parts must be allowed to cool in the mold in order to remove them without distortion.
11、Thus thermosetting cycles can be faster. Of course the mold must be heated rather than chilled, as with thermoplastics.Machines are available for molding sandwich parts. One cylinder and plunger injects measured amount of skin material into the die, and then a second cylinder squirts the filler insi
12、de 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.2.Feed systemIt is necessary to provide a flow-way in the injection mould to connect the nozzle (of the inj
13、ection machine) to each impression. This flow-way is termed the feed system. Normally the feed system comprises a sprue, runner and gate. These terms apply equally to the flow-way itself, and to the molded material which is removed from the flow-way itself in the process of extracting the molding.A
14、typical feed system for a four-impression, two plate-type mould. It is seen that the material passes through the sprue, main runner, branch runners and gate before entering the impression. As the temperature of molten plastic is lowered while going through the sprue and runner, the viscosity will ri
15、se; therefore, the viscosity is lowered by shear heat generated when going through the gate to fill the cavity. It is desirable to keep the distance that the material has to travel down to a minimum to reduce pressure and heat losses. It is for this reason that careful consideration must be given to
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