Volume 3, Issue 4

The present paper is concerned with a crystallization process which occurs in the interior of a polymer melt, when its boundary (mold wall etc.) is suddenly quenched. As polymer melts are prone to supercooling, dispersed nucleation can take place at a distance from the boundary, if the local temperature has become low enough. This results in (delayed) formation of a crystallization zone moving into the bulk of the material. An approximate methematical treatment of this phenomenon enables an easy (semi-quantitative) interpretation of experimental results which were obtained with the aid of a technique of visual inspection on a high density polyethylene (HDPE) sample.

The interaction between flow, heat transfer and thermodynamic behavior of amorphous polymers during packing-cooling stages in injection molding of thermoplastics has been studied numerically. A finite-volume method with staggered meshes has been used to solve simultaneously the continuity, momentum and energy equations coupled to an equation of state. The Generalized Newtonian (GN) fluid model has been chosen, including pressure effects during packing. The simulation, applied to a polystyrene melt injected into a rectangular shaped cavity, allows the calculation of the pressure, velocity and temperature distributions during the packing and cooling phases. The final shrinkage of the resulting molded part is also calculated.

By injection molding two kinds of frozen-in orientation will arise: During the filling phase the outer layers of the molding will be sheared and simultaneously cooled. This results in highly oriented layers at the outside of the molding (orientation by filling). During the packing phase the melt, forced into the cavity, gives rise to low rates of deformation in the molding, but since the viscosity has become high by cooling, high stresses and frozen-in orientation will also result (orientation by packing). This last kind of orientation can be avoided by high pressures in the filling phase, followed by immediate mechanical sealing of the cavity (injection molding without packing), and hereby a better quality of the moldings can be achieved. Some results with moldings of PS and PP are given.

The theoretical foundation of system analysis is given after historical development of the problem. The key concepts and the technical systems of things are discussed. Based on investigated procedures of injection molding of polymers and other matters a general definition of injection molding is given. This enables us to work out the general model of injection molding of matter, as well as the system analysis of molds for injection molding. From this analysis, it follows that the mold has different functions: primary shaping of the product as well as structuring on the molecular and supramolecular level. The mold is at the same time a very complicated and complex technical system of things. The morphological classification of different systems gives the basis for better process control.

Polybutadiene-based polyurethane/poly (methyl methacrylateco-glycol dimethacrylate) interpenetrating polymer networks (PU (HTPB)/P(MMA-co-EGDMA)-IPN) was synthesized by simultaneous polymerization. FTIR and dynamic mechanical spectra (DMS) show no apparent chemical bonds between the two component polymers. The results of DMS also demonstrated that the interpenetration of networks exists mainly between P(MMA-co-EGDMA) and the hard segments of PU(HTPB). The results of transmitting electron microscopy (TEM) indicate that when the amounts of P(MMA-co-EGD-MA) are from 20 to 80 wt%, the domain sizes increase from 100 to 200 Å to several microns, and the morphology features vary from the interpenetration of microphase separation structure to the boundary interpenetration of cellular structure. Besides, the IPN has dual phase continuity. With increasing amounts of P(MMA-co-EGDMA), the stress-strain curves indicate that the corresponding IPN materials transform from reinforced rubbers to toughened plastics. The mechanical properties of the full-IPN are far superior to those of the corresponding linear blends, and also higher than those of the corresponding semi-IPN.

Granular high density polyethylene has been irradiated to relatively low doses prior to processing. The molecular weight was, at the lower doses, increased and the molecular weight distribution broadened with increasing dose. At the two highest doses an insoluble gel was introduced. These variously irradiated granules were processed by two routes, compression and injection molding, and selected structural features and mechanical properties ascertained. For all granule doses compression molded samples could be produced, with the plaque density decreasing with increasing granule dose. While the ductility of the moldings improved with increasing granule dose, the modulus and yield stress both declined as the density of the moldings decreased. Injection molded samples could only be prepared from granules free of gel. The density of the moldings depended on both granule dose and barrel temperature: in general, the density first increased then declined with increasing granule dose, an effect associated with preferred molecular orientation. The yield stress and modulus both appeared to correlate better with melt flow index rather than density, to infer that preferred molecular orientation is significant in injection moldings and can override the influence of density.

The dynamic experiments were made on the solutions of poly (l,4-phenylene-2,6-benzobisthioazole) (PBT) in poly(phosphoric acid) at frequencies from 10 −2 to 10 2 radian· sec −1 . For the liquid-crystalline solutions, the logarithmic plots of G′ and G″ against frequency show abnormally small slopes and those of η′ exhibit large slopes at low frequencies in contrast to the plots for the optically isotropic solutions. However, the viscoelastic behavior of liquid-crystalline solutions is regarded as linear. The temperature of anisotropic-isotropic phase transition of solution is higher for high-molecular-weight PBT than for lowmolecular-weight PBT. The dilution of solution lowers the transition temperature. The transition goes through various stages of mixed phases. The method of reduced variables is applicable to the isotropic solutions, and the resulted composite curves were discussed. The method, however, is inapplicable to the anisotropic solutions, suggesting that the configuration, interaction and aggregation state of molecules in liquid-crystalline solutions may change sensitively by temperature. The slow relaxation mechanisms are involved in the mesophase systems, but the quick shrinkage of highly oriented nematic mesophase at the onset of relaxation produces the zigzag-fibrillar structure of fibers and films made by stretch spinning or casting.

Four kinds of inorganic fibres: carbon, silica glass, brass and aluminium, were filled in two kinds of low density polyethylenes (PE, Melt Flow Rates (MFR) at 190°C, 2.16 kg; 4 and 8 g/10 min) by extrusion molding. The filling ratio of fibres was varied from 0 to about 20% in volume. The melting and crystallization of the above systems were measured by differential scanning calorimetry. The fibre-filled PE was crystallized at a temperature higher than that of the original PE. This phenomenon was observed through a wide range of cooling rate. The temperature of the main melting peak moved slightly to the low temperature side and a broad shoulder peak appeared, as the filling ratio of fibre increased. It was also recognized that the maximum temperature of the molten state affected the crystallization behavior of the system. This was observed for the fibre-filled PE having low melt flow rate. The variation of fibre species scarcely affected the melting and crystallization behavior. The results suggested that the filled fibre accelerated the nucleation process and retarded the crystal growth of PE.