Injection Molding of Low-Density Polyethylene (LDPE) as a Model Polymer: Effect of Molding Parameters on the Microstructure and Crystallinity

Due to its relatively simple structure, low-density polyethylene (LDPE) can be considered as a model polymer for the study of its properties. Herein, the effect of processing variables on the microstructure and crystallinity of injection-molded LDPE specimens was quantitatively determined. The polymer was injected at different temperature conditions in the barrel and the mold. The specimens were characterized by scanning electron microscopy and X-ray diffraction. With the data obtained, an analysis of variance (ANOVA) was carried out, and response surface graphs (SRP) were constructed to quantify and to observe the behavior of the processing variables, respectively. Different models were obtained to predict the effect of the experimental factors on the response variables. The results showed that the interaction of the two temperatures has the greatest effect on the size of the spherulite, while the temperature of the mold affects the crystallinity. The SRP showed different behaviors: for the spherulite, the size increases with the mold temperature, while for the crystallinity, higher values were observed at an intermediate mold temperature and a low melt temperature. The results presented herein are valuable for setting empirical relations between the microstructure, crystallinity, and the molding conditions of LDPE.

Morphological, mechanical, and thermal properties of injection molded polylactic acid foams/composites based on wood flour

In this work, injection molding was used to produce polylactic acid foams using azodicarbonamide as a chemical foaming agent and to study the effect of wood flour concentration (15, 25, and 40% wt.) on morphology (scanning electron microscopy), density (gas pycnometry), as well as mechanical (tensile, flexural, and impact) and thermal (differential scanning calorimetry) properties. In particular, density reduction was controlled by the amount of material injected (shot size). The results showed that polylactic acid properties increased with wood content, but decreased with density reduction. Nevertheless, specific flexural modulus (per unit weight) always increased with foaming. Foaming was also shown to significantly increase polylactic acid crystallinity.

An investigation into reinforced and functionally graded lattice structures

Lattice structures are regarded as excellent candidates for use in lightweight energy-absorbing applications, such as crash protection. In this paper we investigate the crushing behaviour, mechanical properties and energy absorption of lattices made by an additive manufacturing process. Two types of lattice were examined: body-centred-cubic (BCC) and a reinforced variant called BCC z. The lattices were subject to compressive loads in two orthogonal directions, allowing an assessment of their mechanical anisotropy to be made. We also examined functionally graded versions of these lattices, which featured a density gradient along one direction. The graded structures exhibited distinct crushing behaviour, with a sequential collapse of cellular layers preceding full densification. For the BCC z lattice, the graded structures were able to absorb around 114% more energy per unit volume than their non-graded counterparts before full densification, 1371 ± 9 kJ/m3 versus 640 ± 10 kJ/m3. This highlights the strong potential for functionally graded lattices to be used in energy-absorbing applications. Finally, we determined several of the Gibson–Ashby coefficients relating the mechanical properties of lattice structures to their density; these are crucial in establishing the constitutive models required for effective lattice design. These results improve the current understanding of additively manufactured lattices and will enable the design of sophisticated, functional, lightweight components in the future.

Morphological, physical and mechanical properties of nanocrystalline cellulose filled Nylon 6 foams

Nanocomposite foams based on Nylon 6 and nanocrystalline cellulose were prepared via extrusion and injection molding to study the effect of nanocrystalline cellulose concentration (0 to 5%), chemical foaming agent content (0, 1%, and 2%), and mold temperature (30℃ and 80℃) on the morphological, physical, and mechanical properties of the samples. Nanocrystalline cellulose content, especially between 1 and 3 wt%, was very effective in reducing the cell size and increasing the cell density of the foam structure. Nanocrystalline cellulose addition (0–5%) was found to increase density (4% for composites and 20% for foams), tensile strength (10% for composite and 13% for foams), tensile modulus (20% for composites and 34% for foams), and flexural modulus (37% for composites and 29% for foams), but decreased the impact strength (35–40% for composites and 20–40% for foams). Foaming agent addition (1%) was able to improve the specific tensile (10%) and flexural (12%) moduli, tensile strength (14%), elongation at break (6%), and impact strength (27%). Finally, higher mold temperature decreased skin thickness and, consequently, decreased the mechanical properties, mostly tensile strength of the foam samples (1% for composites and 18% for foams).

Effect of the skin layer on the overall behavior of closed-cell polyethylene foam sheets

This article presents an experimental investigation of the effect of the skin layer on the mechanical behavior of a closed-cell polymeric foam material. The skin layer is a thin layer with increased density due to the manufacturing processes, which results in inhomogeneity. The skin-layer effect is investigated by comparing the stretch-stress characteristics of specimens with and without this skin layer on a particular closed-cell polyethylene foam sheet. The characteristics were recorded via uniaxial tension and compression tests, respectively, in all principal manufacturing directions. Thus, the skin-layer effect can be obtained by analyzing the recorded data points, which are also presented. The skin-layer-free specimens are prepared with a particularly designed assembly in accordance with the ASTM standards.

Asymmetric microcellular composites: Mechanical properties and modulus prediction

In the first part of this study, asymmetric microcellular composites were prepared by injection molding to study their morphological properties as a function of temperature gradient inside the mold (0–60℃), as well as foaming agent (0–1%) and natural fiber (0–30%) contents. High-density polyethylene, flax fiber, and azodicarbonamide were used for the matrix, reinforcement, and chemical blowing agent, respectively. From the samples produced, mechanical properties (tensile, flexion, torsion, impact) are analyzed in this second part. Mechanical properties were found to be strongly influenced by density reduction and natural fiber content. It was also found that fiber addition provides higher reinforcement in flexion than torsion and tension. Also, flexural modulus and impact strength were relatively unaffected by foaming agent content for the range of parameters studied. From the experimental data obtained, a simple mechanical model based on density profile is presented to predict the elastic moduli of asymmetric structural composite foams.