Amorphous polymers molding

In conventional amorphous polymers, molded part shrinkage can usually be predicted using a single measured coefficient of thermal expansion since the CTE values for the flow, transverse, and thickness directions are very similar. Knowledge of CTE or actual shrinkage behavior in the flow and width directions for a... 

This paper attempts to treat LCP molded part shrinkage first from a theoretical viewpoint related to directional and volumetric CTE s, and then from a more empirical perspective, where both actual shrinkage and CTE data are examined. Differences between LCP and conventional amorphous polymer molded part shrinkages are highlighted. A method is suggested for predicting potential shrinkage in a molded LCP article. 

Warpage Resistance 5 to 10% glass fibers 5 to 10% carbon Cost Cost Amorphous polymers are inherently nonwarping molding resins. Only occasionally... 

Anisotropic materials have different properties in different directions (1-7). 1-Aamples include fibers, wood, oriented amorphous polymers, injection-molded specimens, fiber-filled composites, single crystals, and crystalline polymers in which the crystalline phase is not randomly oriented. Thus anisotropic materials are really much more common than isotropic ones. But if the anisotropy is small, it is often neglected with possible serious consequences. Anisoiropic materials have far more than two independent clastic moduli— generally, a minimum of five or six. The exact number of independent moduli depends on the symmetry in the system (1-7). Anisotropic materials will also have different contractions in different directions and hence a set of Poisson s ratios rather than one. 

Parison cooling significantly impacts the cycle time only when the final parison thickness is large. In thin blown articles the mold is opened when the pinched-off parts have solidified so that they can be easily stripped off thus they are the rate-controlling element in the cooling process. For fast blow molding of even very thin articles, the crystallization rate must be fast. For this reason, HDPF, which crystallizes rapidly, is ideally suited for blow molding, as are amorphous polymers that do not crystallize at all. 

They are atactic amorphous polymers which have good light transparency (92%) and yield transparent moldings and films. As was noted for polyalkyl acrylates, the solubility parameters decrease as the size of the alkyl groups increases. The flexibility also increases as one goes from polymethyl methacrylate (PMMA) to polyaryl methacrylate and then decreases as the size of the alkyl group is further increased. 

Kim (82) estimated PED from compressive experiments on molded disks of a number of materials, as shown in Fig. 5.17. High modulus, yielding, amorphous polymers such as PS dissipate a large amount of mechanical energy, compared to lower modulus, polycrystalline polymers, as shown again in Fig. 5.17. Iso-PED and corresponding iso-ATadiab contours can be obtained from a number of cylindrical specimens compressed to various strains at various initial temperatures, as shown in Fig. 5.18(a) and 5.18(b). From such plots, the expected ATadiab from one or more successive E deformations can be obtained, as shown in Fig. 5.19, for PS compressed to successive eo = 1 deformations. 

In amorphous state, solid polymers retain the disorder characteristic for liquids, except that the molecular movement in amorphous solid state is restrained. The movement of one molecule versus the other is absent, and some typical liquid properties such as flow are absent. At low stress, polymers display elastic properties, reverting to a certain extent to the initial shape in a relaxation process. However, they can be irreversibly deformed upon application of appropriate force. The deformation and flow of polymers is very important for practical purposes and is studied by a branch of science known as rheology (see e.g. [1]). The combination of mechanical force and increased temperature are commonly applied for polymer molding for their practical applications. The polymers that can be made to soften and take a desired shape by the application of heat and pressure are known as thermoplasts, and most linear polymers have thermoplastic properties. 

The optical properties of semicrystalline polymers are often anisotropic. On the other hand, amorphous polymers are normally isotropic unless directional stresses are frozen in a glassy specimen during fabrication by a process such as injection molding. Anisotropy can often be induced in an amorphous polymer by imposing an electric field (Kerr effect), a magnetic field (Cotton-Mouton effect), or a mechanical deformation. Such external perturbations can also increase the anisotropy of a polymer that is anisotropic even in the absence of the perturbation. 

Poly(ester-imide)s useful as molding plastics are made from terephthalic acid, trimellitic anhydride, 4-(aminomethyl)cyclohexanemethanol, and 1,4-cy-clohexanedimethanol [244]. Blends of this poly(ester-imide) with polycarbonate are also patent protected [245]. An amorphous polymer containing units derived from N-3-hydroxyphenyl trimellitimide and units derived from p-hy-droxybenzoic acid [246] and another having units derived from 9,9-bis(4-ami-nophenyl)fluorene [247] have similar claims. 

Molten polyesters show low viscosity and small extmdate swell. For these reasons, they have been blended with amorphous polymers to improve the latter s processability and chemical resistance. Elastomers have also been added to polyesters to improve impact resistance. Blends of polyester (either PET or PBT) with polycarbonate, PC, are the most popular (viz., Bay/oF, BCT4201, Calibre , Dialoy P, Ektar MB (with PCTG), Idemitsu SC, MakroblencT, MB4300, NovadoF, Pocan , R2-9000, Sabre , SC 600, Stapron E, Ultra-blencF KR, Valox , Xenoy 1000, etc.). Presence of PC in PET/PC increases crystallization rate of PET, which translates into faster injection molding cycle and lower part distortion upon demolding than those observed for neat PET. 

The primary motivations for blending the thermoplastic polyesters with other polymers are (a) to improve the solvent resistance and process-ability of amorphous polymers such as PC, styren-ics, PPE, etc., (b) to reduce the mold shrinkage of polyesters associated with their crystallization,... 

Polymer Specimens. The materials used in this work were polyimide (PI),polyamide-imide (PAl), polyether-ether-ketone (PEEK), polyphenylene sulfide (PPS) and polyether sulphone (PES). The chemical formulas and physical properties of the specimen polymers are summarized in Table I. The specimen polymers, except PPS, were unfilled while the PPS specimen was filled with glass fiber of Uo wt. %. PAI and PES are amorphous polymer with considerably high glass temperature. The polymers, except PI, can flow at hi temperatures and allow the use of injection molding. 

Shrinkage during solidification always tends to reduce the dimensions to values lower than those of the mold. The magnitude of this shrinkage is shown in Figure 23.12. It appears that for amorphous polymers a reduction in volume of circa 10% occurs when the polymer is cooled from the processing temperature to the ambient for crystalline polymers, this reduction may amount to 20-25% (Fig. 23.12). 

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