Volume 35, Issue 2

Selective laser sintering (SLS) of thermoplastic powders allows for the creation of parts with complex geometries and functionally robust designs. Based on the principle of selectively fusing regions in a layer-by-layer process, SLS has gained increasing interest as an economical way to supplement small batch production for traditional manufacturing techniques. To introduce SLS parts into functionally demanding applications, it is imperative that they exhibit good surface quality to prevent wear caused by surface friction. Parts produced with SLS, however, typically have a higher surface roughness than those produced through traditional manufacturing techniques. Understanding the relationship between processing parameters and surface quality will allow one to better predict and control surfaces for end-use parts. A design of experiments was formulated to investigate the effects of four process parameters on parts produced across the powder bed: powder type, roller speed, laser power, and scan spacing. To analyze the surface roughness and area ratio of the parts, Focus Variation (FV) technology was implemented. Statistical models were developed to estimate the surface roughness and area ratio of manufactured specimens.

In this study, a organosilicon modified waterborne polyurethane (WPU) is synthesized with polyethylene glycol 1,4-butanediol adipate ester diol (PBA) to form the soft segment, dimethylolpropionic acid (DMPA) as the hydrophilic chain extender, and isophorone diisocyanate as the hard segment to synthesize the WPU prepolymer, and aminoethyl aminopropyl dimethicone (AEAPS) as the graft chain extender. The properties of the formed WPU films are then characterized by using Fourier transform infrared spectrometry, thermogravimetric analysis, X-ray diffraction, and dynamic mechanical analysis. It is found that when the amount of AEAPS in the WPU is increased from 0 to 30 wt%, the particle size of the AEAPS modified WPU emulsion is increased from 84.8 nm to 271.9 nm and maintained high centrifugal stability. Moreover, the water absorption of the WPU film is reduced from 43.4% to 24.1%, and the hardness is enhanced from 3H to 5H, while the glass-transition temperature (Tg) of the soft segment of the modified WPU shifts from -37.4 °C to -44.3 °C, and the Tg of the hard segment shifts from 73.6 °C to 118.1 °C. Therefore, the overall performance of AEAPS modified WPU is improved.

It is well-known that strength and stiffness are commonly inversely related with toughness and ductility for organic filler filled elastomer nanocomposites. These performances are governed by the dispersion of organic fillers and interface of elastomer nanocomposites. Herein, the designed physical and chemical hybrid-network based on tannic acid (TA) as interface regulator and cross-link agent can endow graphene/elastomer nanocomposites with reinforcement as well as toughness simultaneously. The results indicate the formation of a strong and stable network structure composed of elastomer chains and graphene, contrary to traditional graphene/elastomer nanocomposites. The present composites with a physical and chemical hybrid-network effectively improve the load transfer and show excellent mechanical properties.

A plastic micro-tube is extremely small, and the melt is still in a molten state when the melt exits the die. The inner layer gas still flows in the micro-cavity after leaving the die, therefore, the diameter of the micro-tube increases gradually. When the outer layer gas leaves the die, it blows directly to the outer wall of the micro-tube. The flow of the double layer gas has a great influence on the extrusion of the micro-tube. To this end, a double layer gas-assisted extrusion model based on gas/melt two-phase flow is established. It focuses on the overlay effect of the double layer gas flow on the forming micro-tube. Through a finite element numerical simulation of the micro-tube extrusion process, forming the double gas layer inside and outside the tube wall, respectively, we obtain the shape, velocity, pressure drop and first normal stress difference of the micro-tube. The analysis shows that the double gas layer inside and outside the tube wall have asymmetrical effect on the melt, and they must be analyzed at the same time; the first normal stress difference is generated at the entrance of the die, exit of the die and downstream of the die exit; it reflects the extrusion deformation of the gas to the micro-tube, the degree of extrusion on the micro-tube wall, the distribution of the velocities X and Y, and the distribution of the pressure drop, to some extent. Compared with the micro-tube gas-assisted extrusion experiment, when the gas pressure is large, the result is consistent with the phenomenon of irregular corrugation on the wall surface of the micro-tube, the wall at the exit of the die is rapidly thinned and the wall downstream of the die exit gradually thins.

Polyphenylene ether (PPE) is a polymer with very high glass transition temperature, superior thermal stability but poor melt processability. Mainly to improve its processability and chemical resistance, it is blended with Polystyrene (PS) and Nylon-6, respectively. In this research, ternary blends of PPE/PS/Nylon-6 were prepared using co-rotating twin screw extrusion and test specimens were injection molded. The influence of Maleic anhydride and screw speed on the mechanical and thermal properties of the ternary blends was investigated. The 40/40/20 blends, without Maleic anhydride, processed at screw speed of 600 min –1 showed highest tensile, flexural and impact strengths of 65.74 MPa, 82.22 MPa and 8.88 kJ/m 2 , respectively. Complete miscibility of the three polymers in the ternary blend was evidenced by scanning electron micrographs. Glass transition temperature of the ternary blend was 127 °C. High thermal and dimensional stability of the blend was evidenced by lowest volatile mass and highest residual mass at 800 °C along with decomposition temperature of 458.6 °C. Maleic anhydride was not effective as compatibilizer in improving the mechanical properties of the ternary blend.

While a lot of research can be found in the field of bulk resistance of carbon filled polymers, comparatively few papers focus on contact resistance between compound and metal contacts. Due to that small number of researches that deal with contact resistances, studies of the influence of injection molding conditions and parameters on the contact resistance are also very rare. In contradiction to that, these influences on bulk resistance have been studied. The objective of this work was to investigate the electrical contact resistance of overmolded tinplated copper contacts after modifying flow situations and molding conditions by procedural and constructional methods in contact areas. Metal pins were overmolded with a polypropylene compound containing 45 vol.% graphite, utilising an insert injection molding process. To affect the flow situation at the contacts, several processing parameters, such as mold temperature and injection speed, were modified. In addition, the contact alignment related to melt flow direction was varied. Electrical properties were studied and related to macroscopic and microscopic connection properties and flow situations in contact regions. It was found that the contact resistance is a significant factor while examining electrical resistances of overmolded samples. Furthermore, it was shown that the various flow situations had an essential impact on contact resistances. Weld lines at the position of the contact caused a decreased contact resistance. The correlation of the weld line effect, the filler orientation and contact resistance were successfully investigated by μ-CT. Regarding processing parameters, it was observed that a high mold temperature of 120 °C increased not only bulk conductivity, but also had a positive impact on contact conductivity. Macroscopic and microscopic connection mechanisms of contact surfaces were interpreted and connected to the experimental observations.

This paper reports the low velocity impact behavior of preloaded E-glass/polypropylene sandwich composite plates. In particular, the effects of the type of preload and pre-strain amount on the impact behavior of composite plates are reported. Low velocity impact tests of specimens subjected to biaxial tension, compression and tension-compression (shear) were carried out using a drop-weight impact machine under a hemispherical impactor. Deformations ranging from 250 to 500 microstrains were imposed by a special fixture fabricated for this purpose. Single impact loadings were applied to the composite sandwich structures at different impact energies which were varied from rebounding case (10 J) to the perforation case (40 J). Impact results were explained in terms of typical contact force – deformation (F-D) curves and energy – time diagrams. The maximum contact force, deformation and absorbed energy of the specimens were compared to investigate the influence of pre-strain amount. In addition to the single impact tests, repeated impact behavior of composite sandwich structures subjected to different preload types were obtained with the same impact energy levels. The experimental results showed that the maximum contact force and maximum absorbed energy were considerably different in these situations. However, the repetition number of the specimens at the higher impact energies subjected to shear preloads was largely unaffected.

Enhancing mechanical properties and suppressing vibration are important requisites in developing composite materials for dynamic structural applications such as automotive and aircraft. In this work, mechanical and vibrational properties of twill woven CFRP composites fabricated by compression molding, are studied by varying tow sizes (3k and 6k). Density, void, tensile, bending, impact strength, hardness, and free vibration tests were conducted as per ASTM standards to study their properties and investigate the impact of tow sizes on composites. In experimental modal analysis, Frequency Response Function (FRF) plots were also recorded. Scanning Electron Microscope (SEM) was used to analyse the fracture behavior of the composites. A comparison between the observed results of 3k and 6k twills are made and discussed in detail. From the experimental investigation, it is found that higher tow size decreases the tensile, flexural and hardness strength but increases the natural frequency and damping coefficient. Interfacial bonding between fiber and matrix was affected by the presence of voids in the composites as evident from SEM micrographs.

In the present investigation, a novel chemical treatment was introduced for the extraction of nanocellulose fibers from sugarcane bagasse and applied as reinforcement material to enhance the physical properties and thermal stability of epoxy nanocomposites. Epoxy nanocomposites with different weight fractions were fabricated using a wet layup process followed by furnace heating to remove the residual moisture content. The influence of surface modified sugarcane nanocellulose fiber loading on morphological (transmission electron microscope) properties of epoxy nanocomposites was investigated. The porosity and water absorption increase with the increment in fiber weight fraction for both treated and untreated nanocellulose fiber-epoxy composites. Among the various treatment processes, the alkali-treated fibers reinforced epoxy composites showed better thermal stability and water absorption resistance under 10 wt.% of nanocellulose fiber reinforcement.

Insoluble sulfur (IS) is prepared at low temperature with graphene oxide (GO) as stabilizer and ammonium persulfate as initiator. The FTIR and Raman patterns show that the non-conjugate double bond of GO can couple to the long chain free radicals of IS. The TGA and EDS results show that the mass fraction of IS is 79.75 wt% in particles by refluxing method, much higher than that in particles by hydrothermal method, when the mass ratio in reactants of GO to sulfur was 3: 1. Under constant content of sulfur, the tensile stress and elongation at break of NBR/GO-S(h) and NBR/GO-S(r) composites are higher than those of NBR/Reduced GO composites with the addition of 3 phr GO-S(h), GO-S(r) and Reduced GO particles into NBR. It can provide a new technology for the preparation of IS with lower temperature, little energy consumption and less environmental pollution. It is also a new method for the modification and application of GO in rubber.

Plastic pyrolysis has been studied over the years and has been proven to be a viable method of converting waste materials into useful products. Low-density polyethylene (LDPE) is of particular interest as it makes up a significant part of solid waste stream. Aspen HYSYS was used to model the steady-state pyrolysis of waste low-density polyethylene (LDPE) based on a combination of kinetic and thermodynamic approaches. The results of the numerical optimisation (at a basis of 10 kg/h feed) showed that 100% conversion and a maximum of 91.6% oil yield can be achieved at a temperature of 449 °C and a purge gas flowrate of 1.21 kg/h. Reaction temperature was found to be a more important process factor than purge gas flowrate. Correlations were also developed for the prediction of oil yield and reaction conversion based on the process temperature and purge gas flowrate. The analysis of variance (ANOVA) showed that the correlations for both reaction conversion and oil yield were statistically significant.