Molded Parts and Extrudates

Bowman [192] conducted a systematic study of the structure-property relations of injection molded polyacetals (polyoxymethylene or POM) and observed correlations between process conditions, structures and mechanical properties. Barrel temperature effects were studied as they are known to influence both microstructure and mechanical properties [193]. Increased barrel temperature was shown to reduce the outer skin layer while increasing the extent of the equiaxed, unoriented core, resulting in a decreased tensile yield strength parallel to the injection direction. 

An example of the multilayered structures common in polyacetals is shown in the polarized light micrographs (Fig. 5.39). They depict a uniformly nucleated crystalline structure formed 

In contrast to polyacetal, PE and PP, multilayered textures have not been observed for poly uty-lene terephthalate) [201] or nylon. Nylon and polyCbutylene terephthalate) moldings both exhibit a low crystallinity or amorphous surface layer, with little or no orientation, and a crystalline core which depends on mold conditions [192]. This amorphous skin is due to the rapid quenching of the polymer at the surface and the high glass transition temperature of these materials, not to the flow of the polymer. Thus, a simple skin-core texture rather than a multilayered texture has been observed for these polymers. 

Polarized light micrographs show details of the spherulitic structure in molded nylon (Fig. 5.42A) which is similar to PBT [201]. The nonspherulitic skin, a transition zone and a spherulitic core region are observed. This is as expected, as the quench rate declines away from the surface, e.g. in the transition zone, and thus there is some nucleation of spherulites. A view of the transition zone (Fig. 5.42B) shows round spherulites in a fine textured matrix. The 

A cross section of a PC disc would appear as a regular series of grooves and lands (raised, flat regions) with some periodicity. Depths can be on the order of 60 nm with periodicities around 1.5/xm and groove land ratio of 1 3. The substrates [202] were injection molded commer- 

Bowman [157] conducted a systematic study of the structure-property relations of injection molded polyacetals (polyoxymethylene or POM) and observed correlations between process conditions, structures and mechanical properties. 

34 SEI of a fractured, molded POM test bar, containing a high void level, shows a skin-core morphology. Elongation of the voids at the skin surface is due to high orientation whereas the more rounded voids and the semicircular flow front in the core results from less orientation in that region of the mold. 

An example of the multilayered structures common in polyacetals is shown in the polarized light micrographs (Fig. 5.35). They depict a uniformly nucleated crystalline structure formed due to mold filling and variations in the rate of cooling of the melt. The skin surface in the microtomed section (top in Fig. 5.35A) is birefringent, non-spherulitic and highly oriented. The molecular chains are oriented parallel to the injection direction. The central portion of the bar consists of a core (bottom Fig. 5.35A) with randomly oriented spherulites (Fig. 5.35C). It has no preferred molecular or lamellae orientation. There are usually one or more layers between the skin and core that are transitional shear zones with intermediate 

Structure. In polyacetals this has been termed transcrystallinity [159,146]. Transcrystalline growth is controlled by the heat flow to the mold wall and is initiated by the cold mold wall. The melt at the wall cools rapidly, and dense spheru-lite nucleation takes place adjacent to the wall. Spherulites nucleated close to the wall, in the thermal gradient, have parabolic interfaces 

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Irradiation of polyolefins, particularly the family of polyethylenes, represents an important segment of the radiation processing. Polyolefins can be irradiated in many forms, such as pellets and powders, films, extruded and molded parts or as wire and cable insulation. 

It has been reported that three kinds of distinct fibrils could be observed in oriented LCP fibers, extrudates, and mold parts they are 50-nm microfibrils, 500-nm fibrils and 5 /xm macrofibrils (88). For highly oriented fine fibers, the LC domains are elongated along the fiber direction with a size of about 500 nm... 

Thermotropic polymers are melt processable and thick extrudates and molded parts are formed with high strength characteristics as in fiber reinforced thermoplastics. 

Liquid crystalline polymers [326,329-336] have found application as high modulus fibers and films with unique properties due to the formation of ordered lyotropic solutions or thermotropic melts which transform easily into highly oriented, extended chain structures in the soUd state. Thermotropic polymers are melt process-able and thick extrudates and molded parts are formed with high strength characteristics as in fiber reinforced thermoplastics. 

Features For use in most polyolefins primarily suited forwh. and bik. pigments in high impact PS, improves appearance and color development of extruded and molded parts... 

Fluorosilicone compounds can be processed by the same methods used for silicone elastomers based on PDMS. They can be milled, calendered, extruded, and molded. A large proportion of fluorosilicone compounds is used in compression molding. Molded parts produced in large series are made by injection molding, and parts with complex shapes are produced by transfer molding. Calendering is used to produce thin sheets and for coating of textiles and other substrates. 

The twin-screw injection molding extruder is an injection molding machine that is capable of both blending/compounding and extrusion in one step. Because it is a one step process, the fibers never go through the entire extrusion process as well as the pelletization that limits the fiber size, but are blended into the molten plastic before injection. The screw part of this machine is based on a non-intermeshing, counterrotating twin-screw extruder (Chapter 5). One of the screws in this machine is capable of axial movement and has a non-return valve on the end. This action enables the screw to inject and mold parts. 

After the extruded or molded part is cooled, it becomes hard and rigid as the temperature falls below the softening point of the binder. It may then be moved to the baking furnace. 

They may be extruded or blow molded. Parts may be thermally formed and heat or solvent welded. Some applications are blow-molded containers, pipe, ventilating ducts, and molded parts. 

NitrinexEPDM. [A.Schulman] EPDM for wire and cable insulation, automotive parts, hoses, seals, extruded and mold goods, weatherstripping. 

Typical polymer materials studied in optical thin section include extrudates or molded parts, such as semicrystalline polyoxymethylene multiphase polymers, such as rubber toughened nylon filled polymers, such as carbon black filled nylon fibers, such as polyester and rayon and films which are too thick to transmit light. Two examples are of nylon imaged in polarized... 

Polyethylene terephthalate is most often extruded into films or fibers, or blow molded into bottles. Polybutylene terephthalate is primarily found in injection molded parts. Such parts are highly crystalline, which makes them opaque. Polybutylene terephthalate is often modified with glass fibers or impact modifiers. Table 24.1 contains applications by processing method and resin. 

ABS (30% acrylonitrile, 20% butadiene, and 50% styrene) is a tough plastic with outstanding mechanical properties ABS is one of the few plastics that combines both toughness and hardness. So the applications include ballpoint pen shells, fishing boxes, extruded pipes, and space vehicle mechanical parts. There s more than 20 pounds of ABS molded parts in an automobile. 

Resin. Different meanings in different parts of the petrochemicals industry (1) any thermoset polymer (2) polymers of any kind, including plastics ready for fabrication (3) plastics that have been molded or extruded and the now obsolete (4) natural polymers such as shellac, rosin,... 

As shown by Fig. 3.11 for an applied force, the creep strain is increasing at a decreasing rate with time because the elongation of the spring is approaching the force produced by the stress. The shape of the curve up to the maximum strain is due to the interaction of the viscosity and modulus. When the stress is removed at the maximum strain, the strain decreases exponentially until at an infinite time it will again be zero. The second half of this process is often modeled as creep recovery in extruded or injection-molded parts after they cool. The creep recovery usually results in undesirable dimensional changes observed in the cooled solid with time. 

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