Volume 32, Issue 5

Liquid Composite Molding (LCM) processes are cost efficient manufacturing alternatives to traditional autoclave technology for producing near-net shape structural composite parts. However, process induced defects often limit wider usage of LCM in structural applications. Thorough knowledge of these defects, as well as their formation mechanisms and prevention techniques, is essential in developing improved LCM processes. In this article, process induced defects in liquid molding processes of composites, categorized into preform, flow induced and cure induced defects, are reviewed. Preform defects are further presented as fiber misalignment and fiber undulation (waviness and wrinkling). The respective causes, detrimental effects, and possible prevention methods of these defects are presented. Thereafter, flow induced defects are classified as voids and dry spots. Dry spot formation mechanisms in LCM processes and available prevention techniques are summarized. In addition, void formation mechanisms, adverse effects on composite properties, and removal techniques are presented. Cure induced defects include microcracks, void growth and geometrical distortions (warpage and spring-in). Each of these defects are discussed along with their underlying causes as well as their control and reduction schemes.

In polymer processing, crystallization generally occurs in complex, inhomogeneous and coupled mechanical (flow, pressure), thermal (cooling rate, temperature gradient) and geometrical (surface of processing tools) conditions. A first route to understand crystallization in processing conditions is to design model experiments to isolate the specific influence of a given parameter. The emphasis will be laid here on the influence of: (i) shear flow through rheo-optical measurements using the commercial RheoScope module, (ii) high cooling rates obtained with the modified hot stage Cristaspeed (up to 2 000 °C min −1 ) and (iii) high pressures in the original Cristapress cell (up to 200 MPa). Numerical simulation is also a useful tool to understand and predict the coupled phenomena involved in crystallization. Based on Avrami's ideas and equations, a general differential formulation of overall crystallization kinetics has been proposed by Haudin and Chenot (2004). It is able to treat both isothermal and non-isothermal cases, and has been extended to crystallization in a limited volume without and with surface nucleation inducing transcrystallinity.

The knife-over-roll coating process of plastisol formulations for flooring applications has been investigated. The coexistence of smooth spherical PVC particles and rough calcium carbonate aggregates in a Newtonian plasticizer makes this suspension a more complex system than those encountered in the literature. The shear viscosity has been measured with Couette and capillary devices in the wide range of shear rates encountered in the process, resulting in a non-monotonous viscosity curve. A numerical model based on the lubrication approximation theory allows accounting for this non-monotonous behavior. It predicts the deposited thickness, the pressure field between roll and knife and the separating force as a function of the plastisol rheology and the coating parameters.

Achieving an appreciable weight reduction in PP based composite materials, particularly the ones reinforced by glass fibers, is quite challenging while enhancing their mechanical properties and fullfilling other enviromental concerns. To address this issue, low density composites of Poly(propylene) (PP) and waste cellulose fibers (WCF) were produced by high-shear thermo-kinetic mixer. This technique facilitates the ease of processing for the mass production of such composite materials due to the availability of high shear rates and relatively short processing times during manufacturing. The structure-property behavior of the molded samples was investigated as a function of WCF content. Briefly, one-fold increase in elastic modulus, 18 % increase in tensile strength, 87 % increase in flexural modulus and 27 % increase in flexural strength of PP were achieved by the addition of 30 wt.% WCF. The significant enhancements in mechanical properties were mainly attributed to the homogeneous dispersion of intrinsically stiff WCF filler in the PP matrix as a direct result of the high-shear mixing. These results mainly suggest that waste cellulose fibers can be used as an effective reinforcing agent in PP matrix instead of highly dense, non-renewable and non-biodegradable fibers, such as glass fibers, that prevents further stresses on the environment. Along with this, the reuse of waste cellulose fiber in PP matrix, particularly at high concentrations like 30 wt.%, evenly corresponds to the reduction of total PP consumption for PP based composite production. The main conclusion of the study is that the extensive blending technology gives us the ability to produce high performance thermoplastic based composite materials as well as addressing the world-wide waste disposal problem by reusing of natural wastes, which is a great opportunity to ensure sustainability and reduce enviromental and economical costs for many industries.

We fabricated polymer blends of recycled-PC (PC) and recycled-PET (PET) using high-shear processing technology. We also assessed its structure, morphology, and physical properties. Results of DSC measurements show that T g of PC and T c of PET shifted to higher temperatures by kneading under high-shear conditions. Results show further that the T m of PET shifted to a lower temperature. Moreover, compatibilization progressed. TEM observations show that the PET domain was not confirmed completely under the kneading condition of 1 000 min −1 /10 s or more. Compatibilization progressed. Tensile test results show that the PET ratio of 30 wt% (1 000 min −1 /10 s) has higher breaking strain than neat PC does. Chemical resistance test results obtained by good solvent of PC demonstrated that blending of PET makes PC elution difficult.

A novel slit die with an adjustable gap is designed to perform transient swell measurements while changing the die gap dynamically. A generalized control model is developed to predict time-dependent swell using characteristic relaxation time and corresponding steady state swell as model parameters. Further, a series of slit dies are designed and used to measure steady state thickness swell of a high-molecular-weight blow-molding grade high density polyethylene resin under various operating and geometrical conditions. A generalized expression for steady state thickness swell is obtained by applying multi-variable nonlinear regression on thus obtained steady state thickness swell data, and is used in the empirical control model to predict transient extrudate swell.

This paper addresses the role of solvent volatility on diameter selection of bicomponent polymer nanofibers produced via gas jet fibers (GJF) process whereby an axisymmetric turbulent gas jet is used for liquid jet initiation, liquid jet stretching, and solvent evaporation. Several morphological forms, such as interpenetrating network (IPN), bi-lobal, and core-shell are obtained by spinning homogeneous solutions of two immiscible polymers in two mutually miscible solvents. The diameter selection of fibers of the same morphology, e. g., bi-lobal, is achieved by spinning polymer solutions of two sets of solvents of different volatility. The results show that fiber diameter is strongly dependent on the value of vapor pressure of the solvents while the morphology is strongly dependent on the vapor pressure difference of the two respective solvents. The diameter selection by GJF process is most prominent for IPN nanofibers, moderately prominent for core-shell fibers, and almost indifferent for bi-lobal nanofibers. The relative rates of solvent evaporation are useful in interpreting the experimental trend.

The micro injection molding process is a rapidly growing area in plastics processing technology. In this process, the polymer is exposed to both high shear rates and large thermal gradients. In view of the versatility of the process, both commodity and engineering polymers have been used in micro injection molded products. In the present work, poly(oxymethylene) (POM), a partially crystalline engineering polymer, was employed to evaluate the relationships between processing conditions, on one hand, and the morphology and properties of the final part, on the other hand. An unsymmetrical mold cavity to make parts in the form of stepped plaques was used in the study. This resulted in substantial differences in morphology, crystallinity and shrinkage of the zones of different constant thicknesses in the micro parts. Depending on the molding conditions and the location on the micro-part, the microstructure can display up to five crystalline layers. Of particular interest, shish-kebab crystalline structures were observed within the skin of the step with the smallest thickness. Differential scanning calorimetry (DSC) tests are used to distinguish between the melting points of the shish and kebab components of this particular structure. The degree of crystallinity as determined by wide angle X-ray diffraction (WAXD) and shrinkage across the thickness were also found to be highest in the step with the smallest thickness.

In this study, we address heat-seal properties of poly (lactic acid) (PLA), blended with Poly (butylene adipate-co-terephthalate) (PBAT). The objective is to correlate blends crystalline structure and morphology to corresponding heat-seal of blends films. The SEM micrographs show a two-phase elongated morphology where stretched ellipsoids developed through elongational flow during the cast film process. To distinguish the effect of crystallization, we also prepared amorphous and crystalline PBAT films and then compared them to blends with PLA. Heat-sealed areas were created by putting film surfaces in intimate contact for 1 s at the pressure of 0.5 N/mm 2 or Pa and in the temperature range of 70 to 140 °C. Thermal analysis shows that the crystalline structure of PBAT has a significant effect on shifting its heat-seal initiation temperature (T si ) up to 20 °C. Regarding the blends, incorporation of PBAT as a dispersed phase lowers T si of blend samples. Here, gradual decrease in PBAT crystallinity caused by the hindering effect of PLA rigid molecules correlates with the shift in heat-seal initiation temperature. As mentioned above, elongated disperse morphology with higher aspect ratio of the dispersed phase compared to spherical dispersed domain, is formed through film cast process. This enhances the adhesion process by providing higher contact area. The blends also show higher toughness and better puncture resistance, which is an asset for flexible packaging applications and would enhance the mechanical performance of the seal layer.

Carbon-based nanoparticles have unique electrical, thermal, barrier and mechanical properties. When incorporated into polymer matrices, the resulting nanocomposites are potentially suitable for a wide scope of advanced applications. In practice, the properties of the nanocomposites are strongly determined by the level of dispersion achieved and by the degree of polymer/particle interfacial bonding. Production and processing of nanocomposites are often carried out in successive thermo-mechanical cycles. These may change the state of nanoparticle dispersion. This work analyzes the evolution of the dispersion of graphite nanoplates (GnP) in a polypropylene matrix during compounding in a co-rotating twin screw extruder and subsequent processing in a single screw extruder, aiming at a better understanding of the kinetics and stability of dispersion. Dispersion was evaluated along the compounding and processing stages and correlated with the composite electrical conductivity, an important engineering property. Two commercial GnP were used as received and chemically modified to graft PP-g-MA (fGnP-PP). Compositions with 2 or 10 wt.% of GnP and fGnP-PP were studied.

Polyethylene chains having functional maleic anhydride groups were grafted on several graphene derivatives. These chain grafted graphene derivatives were added to polyethylene and the properties of the obtained nano-composites were investigated. Modified Hummers' method was employed to produce graphite oxide (GO). Then amino-functionalized GO (AFGO) was prepared through the functionalization of GO by ethylenediamine. Thermally reduced GO (TRGO) was prepared by the heating of the GO in the presence of nitrogen. TRGO was amino-functionalized to obtain amino functionalized nano-graphite (AFNG). Low density polyethylene (PE) and polyethylene grafted maleic anhydride (PEgMA) nano-composites, containing 1 to 3 wt% of the obtained fillers, were produced by solution mixing. With the incorporation of amino-functionalized graphene into PEgMA, the amine groups on the graphene surface and the maleic anhydride in PE chains covalently bonded and improved the mechanical properties of the nano-composites; by comparing PEgMA nano-composite with 1 wt% AFGO and 1 wt% GO, a 155 percent enhancement in the elongation at break was observed. The modulus and tensile strength of these nano-composites increased over the pure matrix. In addition, the effect of PEgMA content in the 1 wt% AFGO nano-composites was studied and the optimum ratio of PEgMA to PE was found to be 0.40 to 0.60. At this ratio, the best mechanical properties were achieved. Also, at 2 wt% nano-filler the elongation at break of the AFNG nano-composite was higher than that of the TRGO nano-composite. AFNG created more chain grafting but AFNG exfoliates more. The electrical conductivity of TRGO powders by amino-functionalization decreased about 20 times. Therefore, the electrical conductivity of the graphene nano-composites was higher than amino-functionalized graphene nano-composites.

Recently, several companies have started to use the foaming technology in blow molding processes, primarily in extrusion blow molding. Despite the design complexity involved in the preform blow molding method, substantial advantages result when microcellular foaming and blow molding are combined. In preform and extrusion blow molding, the preform (i. e., the parison) undergoes significant biaxial stress during the inflation stage. Since either extensional or shear stress can dramatically improve cell nucleation, an externally applied stress can cause small-scale, local pressure variations throughout the sample, thus reducing the energy barrier for cell nucleation. So, unlike the current low-pressure foam blow molding technology, where cell nucleation occurs before inflating the preform/parison, we used a high-pressure system to prevent premature foaming in the shaping stage. Consequently, cell nucleation was induced after biaxial stresses were created to induce a higher cell density.

In large thick plastic pipe extrusion, the residence time in the cooling chamber is long, and the melt inside the pipe sags under its own weight, causing the product to thicken on the bottom (and to thin on the top). To compensate for sag, engineers normally shift the die centerpiece downward. This paper focuses on how this decentering triggers unintended consequences for elastic polymer melts. We employ eccentric cylindrical coordinates, to capture exactly the geometry of our problem, the flow between eccentric cylinders. Specifically, we arrive at an exact analytical expression for the axial and lateral forces on the die barrel using the polymer process partitioning approach, designed for elastic liquids. We choose the Oldroyd 8-constant framework due to its rich diversity of constitutive special cases. Since our main results are in a form of simple algebraic expression along with two sets of curves, they can thus be used not only by engineers, but any practitioner. We close our paper with detailed dimensional worked examples to help practitioners with their pipe die designs.

The rheological characterization of Sheet Molding Compound (SMC) and its modelling is crucial for reliable process simulations. In the past, characterization and material modelling were mainly focusing on SMC with low glass fiber content and a high filler fraction. Due to new application areas, SMC without fillers and with high glass fiber contents, and SMC with carbon fibers become more important. Therefore, these three types of SMCs are characterized in this work, using an inline rheological tool. Differences regarding their compressibility and their flow dependency are identified and considered in an analytical shell modelling. The comparison of the different materials leads to a better understanding of the phenomenological parameters related to the viscosity and friction in the models. Furthermore, the importance to properly consider all relevant material-specific effects becomes evident.