Shaped polymer fabrication injection molding

Injection molding is one of the most widespread methods used to make polymer products. A cursory look around our homes, schools, work places or cars suffices to find an abundance of injection molded products. One of the reasons that this conversion method is so popular is that it enables us to create complex shapes in a single fabrication process. 

An important step in the manufacture of any plastic product is the fabrication or the shaping of the article. Most polymers used as plastics when manufactured are prepared in pellet form as they are expelled from the reactor. These are small pieces of material a couple of millimeters in size. This resin can then be heated and shaped by one of several methods. Thermoset materials are usually compression molded, cast, or laminated. Thermoplastic resins can be injection molded, extruded, or blow molded most commonly, with vacuum forming and calendering also used but to a lesser extent. 

Structural Foams. Structural foams are usually produced as fabricated articles in injection molding or extrusion processes. The optimum product and process match differs for each fabricated article, so there are no standard commercial products for one to characterize. Rather there are a number of foams with varying properties. The properties of typical structural foams of different compositions are reported in Table 4. The most important structural variables are again polymer composition, density, and cell size and shape. 

Matrix-type implants are fabricated by physically mixing the drag with a polymer powder and shaping the mixture into various geometries (e.g. rod, cylinder, or film) by solvent casting, compiession/injection molding or screw extrusion. 

Polyethylene is the least costly of the major synthetic polymers. It has excellent chemical resistance and can be processed in a variety of ways (blown film, pipe extrusion, blow molding, injection molding, etc.) into myriad shapes and devices. Fabrication methods will be briefly discussed in Chapter 8. 

The successful utilization of Reaction Injection Molding (RIM) to fabricate complex polyurethane shapes In a single step from relatively low viscosity streams has led to a search for other chemical systems which can be fabricated by the RIM process. The rapid polymerization of molten caprolactam by anionic catalysis has been utilized to develop attractive nylon RIM systems. The incorporation of a rubber segment In the polymer chain allows the fabrication of high Impact or even elastomeric nylon parts. The combination of a rubber phase with the high melting (215°C) crystalline nylon phase provides useful properties at low temperatures as well as at elevated temperatures. 

Another area in which preceramic polymers can be utilized effectively is as binders for ceramic powders in near net shaping fabrication processes, such as compression or injection molding with subsequent sintering. Alternatively, an active filler and a polymer [67,68], as reported by Greil and Seibold, can be used in such fabrication. Other potential applications of preceramic polymers is in the general area of coatings, especially for carbon-carbon composites [69], and in the synthesis of nanostructured ceramic particles and composites [70-73]. 

Development work has been going on for several years at The Carborundum Company on p-oxybenzoyl polymer systems. 3 The early work mainly focused on the homopolymer (EKONOLjI This polymer has excellent thermal stability and also very good friction and wear properties and has found use recently as an additive to PTPB for molded shapes and coatings. The homopolymer however is very difficult to fabricate by itself and this has led to the development of copolymer systems which retain the excellent thermal stability of the homopolymer but have sufficient flow for compression and injection molding. 

Injection molding, an economical mass production technique, has also been used to fabricate microcantilevers out of thermoplastic polymers (McFarland et al. 2004 McFarland and Colton 2005a, b). In this process, a molten polymer such as polypropylene is forced under pressure into a steel cavity (mold) the shape of the cavity defines the dimensions of both the base and the cantilever(s). Injection-molded microcantilevers have been shown to be of equal caUber to commercial silicon miCTOcantile-vers. McFarland et al. (2004) and McFarland and Colton (2005a, b) specified in detail the fabrication of injection-molded microcantilevers. Despite their advantages over silicon-based cantilever arrays, polymeric cantilever arrays are not commercially available. 

Processibility still remains a concern, since processibility appears to be much easier to obtain for undoped conducting polymers than for the same polymers in doped forms. If the polymer must be doped after fabrication of a shaped article, the problem of introducing the dopant uniformly is serieous, especially when it is required that the same dopant which easily diffuses into the article must eventually be thermally stable. What is needed here appears to be new methods for processing conducting polymers. One attractive possibility would be a process analogous to conventional RIM (reaction injection molding) in which polymer articles are directly fabricated from a mixture of monomer and dopant. 

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