【正文】 cations 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 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 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 FIGURE 1. Four stages of coinjection molding. (a) Short shot of skin polymer melt (shown in dark green)is injected into the mold. (b) Injection of core polymer melt until cavity is nearly filled, as shown in (c). (d)Skin polymer is injected again, to purge the core polymer away from the sprue. Fusible core injection molding Overview The fusible (lost, soluble) core injection molding process illustrated below produces singlepiece, hollow parts with plex internal geometry. This process molds a core inside the plastic part. After the molding, the core will be physically melted or chemically dissolved, leaving its outer geometry as the internal shape of the plastic part. 文獻 FIGURE 1. Fusible (lost, soluble) core injection molding Gasassisted injection molding Gasassisted process The gasassisted injection molding process begins with a partial or full injection of polymer melt into the mold cavity. Compressed gas is then injected into the core of the polymer melt to help fill and pack the mold. This process is illustrated below. FIGURE 1. Gasassisted injection molding: (a) the electrical system, (b) the hydraulic system, (c) the control panel, and (d) the gas cylinder. Injectionpression molding Overview The injectionpression molding process is an extension of conventional injection 文獻 molding. After a preset amount of polymer melt is fed into an open cavity, it is pressed, as shown below. The pression can also take place when the polymer is to be injected. The primary advantage of this process is the ability to produce dimensionally stable, relatively stressfree parts, at a low clamp tonnage (typically 20 to 50 percent lower). Lamellar (microlayer) injection molding Overview This process uses a feedblock and layer multipliers to bine melt streams from dual injection cylinders. It produces parts from multiple resins in distinct microlayers, as shown in Figure 1 below. Combining different resins in a layered structure enhances a number of properties, such as the gas barrier property, dimensional stability, heat resistance, and optical clarity. Livefeed injection molding Overview The livefeed injection molding process applies oscillating pressure at multiple polymer entrances to cause the melt to oscil