Unit 6  Injection Molding
Injection molding (Fig 6.1) is the predominant process for fabrication of thermoplastics into finished forms, and is increasingly being used for thermosetting plastics, fiber-filled composites, and elastomers.weigh翻译
It is the process of choice for tremendous variety of parts ranging in weight from 5g to 85g. It is estimated that 25% of all thermoplastics molded. If newer modification, such as reaction injection molding, and the greatly increased rate of adoption of plastics as substitutes for metals are considered, it is likely that the worldwide industrial importance of injection molding will continue to increase. Currently, probably close to half of all major processing units is injection molding machines. In 1988, a dollar sale of new injection molding machinery in the U.S. was approximately 65% of total major polymer machinery sales volume; this included 4,600 injection molding units. The machines and their products are ubiquitous and are synonymous with plastics for many people.
A reciprocating screw injection molding machine combines the functions of extruder and a co
mpressive molding press. It takes solid granules of thermoplastic resin, melts and pressurizes them in the extruder section, forces the melt at high velocity and pressure through carefully designed flow channels a cooled mold, then ejects the finished part(s), and automatically recycles. This machine is a descendant of the plunger type “stuffing machine” patented by the Hyatt brothers in 1872 to mold celluloid. In 1878, the Hyatts developed the first multicavity mold, but it was not until 1938 that Quillery (France) patented a machine incorporating a screw to plasticize the elastomer being molded. In 1956, Ankerwerk Nuremberg commercialized the modem reciprocating screw injection molding machine for thermoplastics. Today, over 50 machine manufacturers are listed in Modern Plastics Encyclopedia, offering machines to the U.S. market ranging from 2 to 6,000 tons clamping capacity. (A machine with a 10,000-ton capacity has been built to mold 264-gallon HDPE trash containers.) A host of suppliers of auxiliary equipment, molds, instruments, and controls service this major segment of the polymer industry.
Injection molding is particularly worthy of intensive study because it combines many areas of interest extrusion, mold design, rheology, sophisticated hydraulic and electronic controls,
robotic accessories, design of complex products, and, of course, the integration of materials science and process engineering. The objectives of injection molding engineers are simple enough: to obtain minimum cycle time with minimum scrap, to attain specified product performance with assurance, to minimize production costs due to downtime or any other reasons, and to steadily increase in expertise and competitiveness. Profit margins for custom injection molders are said to be generally skimpy; an established way to improve profits is to be selected for more demanding, higher margin jobs demand the highest level of efficiency and competence.
This text will concentrate on the reciprocating screw machine thermoplastics, which has largely replaced the older reciprocating plunger types except for very small-capacity machines.
Injection Molding Materials
It is not possible to injection-mold all polymers. Some polymers like PTFE (Poly-tetre-fluoro-ethylene), cannot be made to flow freely enough to make them suitable for injection
molding. Other polymers, such as a mixture to resin and glass fiber in woven or mat form, are unsuitable by their physical nature for use in the process. In general, polymers which are capable of being brought to a state of fluidity can be injection-molded.
The vast majority of injection molding is applied to thermoplastic polymers. This class of materials consists of polymers which always remain capable of being softened by heat and of hardening on cooling, even after repeated cycling. This is because the long-chain molecules of the material always remain as separate entities and do not from chemical bonds to one another. An analogy car, be made to a block of ice that can be softened (i.e. turned back to liquid), poured into any shape cavity, and then cooled to become a solid again. This property differentiates thermoplastic materials from thermosetting ones. In the latter type of polymer, chemical bonds are formed between the separate molecule chains during processing. In this case the chemical bonding referred to as cross linking is the hardening mechanism.
In general, most of the thermoplastic materials offer high impact strength, corrosion resista
nce, and easy processing with good flow characteristics for molding complex designs. Thermoplastic are generally divided into two classes: namely crystalline and amorphous. Crystalline polymers have an ordered molecular arrangement, with a sharp melting point. Due to the ordered arrangement at molecules, the crystalline polymers reflect most incidents light and generally appear opaque. They also undergo a high shrinkage or reduction in volume during solidification. Crystalline polymers usually are more resistant to organic solvents and have good fatigue and wear-resistant properties. Crystalline polymers also generally are denser and have better mechanical properties than amorphous polymers. The main exception to this ruler is polycarbonate, which is the amorphous polymer of choice for high-quality transparent molding, and has excellent mechanical properties.

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