【正文】 ocess of forming thermoplastic materials. It is also one of the oldest. Currently injection molding accounts for 30% of all plastics resin consumption. Since raw material can be converted by a single procedure, injection molding is suitable for mass production of plastics articles and automated onestep production of plex geometries. In most cases, finishing is not necessary. Typical products include toys, automotive parts, household articles, and 文獻(xiàn) consumer electronics goods, Since injection molding has a number of interdependent variables, it is a process of considerable plexity. The success of the injection molding operation is dependent not only in the proper setup of the machine variables, but also on eliminating shottoshot variations that are caused by the machine hydraulics, barrel temperature variations, and changes in material viscosity. Increasing shottoshot repeatability of machine variables helps produce parts with tighter tolerance, lowers the level of rejects, and increases product quality ( ., appearance and serviceability). The principal objective of any molding operation is the manufacture of products: to a specific quality level, in the shortest time, and using a repeatable and fully automatic cycle. Molders strive to reduce or eliminate rejected parts, or parts with a high added value such as appliance cases, the payoff of reduced rejects is high. A typical injection molding cycle or sequence consists of five phases: 1 Injection or mold filling 2 Packing or pression 3 Holding 4 Cooling 5 Part ejection Injection Molding Overview Process Injection molding is a cyclic process of forming plastic into a desired shape by forcing the material under pressure into a cavity. The shaping is achieved by cooling (thermoplastics) or by a chemical reaction (thermosets). It is one of the most mon and versatile operations for mass production of plex plastics parts with excellent dimensional tolerance. It requires minimal or no finishing or assembly operations. In addition to thermoplastics and thermosets, the process is being extended to such materials as fibers, ceramics, and powdered metals, with polymers as binders. Applications Approximately 32 percent by weight of all plastics processed go through injection molding machines. Historically, the major milestones of injection molding include the invention of the reciprocating screw machine and various new alternative processes, and the application of putersimulation to the design and manufacture of plastics parts. Development of the injection molding machine Since its introduction in the early 1870s, the injection molding machine has 文獻(xiàn) undergone significant modifications and improvements. In particular, the invention of the reciprocating screw machine hasrevolutionized the versatility and productivity of the thermoplastic injection molding process. Benefits of the reciprocating screw Apart from obvious improvements in machine control and machine functions, the major development for the injection molding machine is the change from a plunger mechanism to a reciprocating screw. Although the plungertype machine is inherently simple, its popularity was limited due to the slow heating rate through pure conduction only. The reciprocating screw can plasticize the material more quickly and uniformly with its rotating motion, as shown in Figure 1. Inaddition, it is able to inject the molten polymer in a forward direction, as a plunger. Development of the injection molding process The injection molding process was first used only with thermoplastic polymers. Advances in the understanding of materials, improvements in molding equipment, and the needs of specific industrysegments have expanded the use of the process to areas beyond its original scope. Alternative injection molding processes During the past two decades, numerous attempts have been made to develop injection molding processes to produce parts with special design features and properties. Alternative processes derivedfrom conventional injection molding have created a new era for additional applications, more designfreedom, and special structural features. These efforts have resulted in a number of processes,including: Coinjection (sandwich) molding 文獻(xiàn) Fusible core injection molding) Gasassisted injection molding Injectionpression molding Lamellar (microlayer) injection moldin Livefeed injection molding Lowpressure injection molding Pushpull injection molding Reactive molding Structural foam injection molding Thinwall molding Computer simulation of injection molding processes Because of these extensions and their promising future, puter simulation of the process has alsoexpanded beyond the early layflat, empirical cavityfilling estimates. Now, plex programs simulate postfilling behavior, reaction kiics, and the use of two materials with different properties, or two distinct phases, during the process. The Simulation section provides information on using CMOLD the Design topicsare several examples that illustrate how you can use CAE tools to improve your part and molddesign and optimize processing conditions. Coinjection (sandwich) molding Overview Coinjection molding involves sequential or concurrent injection of two different but patible polymer melts into a cavity. The materials laminate and solidify. This process produces parts that have a laminated structure, with the core material embedded between the layers of the skin material. This innovative process offers the inherent flexibility of using the optimal properties of each material or modifying the properties of the molded part. 文獻(xiàn) FIGURE 1. Four stages of coinjection molding. (a) Short shot of skin polymer melt (shown in dark gr