The effect of density profile on the flexural properties of structural foams

An Approach to the Impact Simulation on Foamed Injection Molded Polypropylene Parts: An Example of Application in the Automotive Industry

An approach to the simulation of foamed injection molded Polypropylene parts subjected to impact loading is presented in this paper. The proposed method, which considers strain-rate-dependent material properties and the possible occurrence of fracture, is, in particular, suitable for parts manufactured with core-back technology. The method was developed to be used within the functionality of a commercial Finite Element solver using a shell-type element mesh. The material model is based on a three-layer structure, with two compact skin layers and a foamed core layer made of expanded material. The properties of the foamed material are assumed as those of the compact grade scaled by a suitable factor, which is identified via inverse engineering on a set of bending tests executed on specimens having different foam densities. The fracture of the material is then predicted using a damage model which considers the effects of triaxiality. The approach is then validated on industrial parts from the automotive sector, subjected to impact in a component test. Despite the simplicity of the presented approach, which makes this method suitable for industrial applications and especially for early-stage design, the validation shows a sufficiently accurate simulation of part behavior under the impact, with a reasonable prediction of damage and fracture.

Backpressure Optimization in Foam Injection Molding: Method and Assessment of Sustainability

Inspired by the Industry 4.0 trend towards greater user-friendliness and self-optimization of machines, we present a novel approach to reducing backpressure in foam injection molding. Our method builds on the compressibility of polymer-gas mixtures to detect undissolved gas phases during processing at insufficient backpressures. Identification of a characteristic behavior of the bulk modulus upon transition from homogeneous to heterogeneous polymer-gas mixtures facilitated the determination of the minimum pressure required during production to be determined, as verified by ultrasound measurements. Optimization of the pressure conditions inside the barrel by means of our approach saves resources, making the process more sustainable. Our method yielded a 45% increase in plasticizing capacity, reduced the torque needed by 24%, and required 46% less plasticizing work and lower pressures in the gas supply chain. The components produced exhibited both improved mechanical bending properties and lower densities. From an economic point of view, the main advantages of optimized backpressures are reduced wear and lower energy consumption. The methodology presented in this study has considerable potential in terms of sustainable production and offers the prospect of fully autonomous process optimization.

Polypropylene structural foams: Measurements of the core, skin, and overall mechanical properties with evaluation of predictive models

Relationships for the prediction of various linear mechanical properties of polymeric sandwich foams obtained in injection processes were studied in comparison with shear, tensile, and flexural tests. The samples were obtained by a core-back foam injection molding process that enables one to obtain sandwich materials with dense skins and a foamed core as revealed by the morphological analysis. Tensile, shear, and flexural moduli were investigated for the skin, the core, and the overall foamed structure. In addition, the Poisson’s ratio of the skin was also determined. The core properties were specifically analyzed by machining the samples and removing the skins. Tensile and shear properties of the core can be well described by the Moore equation. The tensile modulus can be calculated by a linear mixing rule with the moduli of the skin and of the core in relation to the thickness of the layers. Shear and flexural moduli are described by a linear mixing rule on the rigidity in agreement with the mechanics of beams. Tensile modulus, out-of-plane shear modulus, and flexural modulus can finally be predicted by the knowledge of only very few data, namely the tensile modulus and Poisson’s ratio of the matrix, the void fraction, and thickness of the core. The equations were proved to be physically meaningful and consistent with each other.

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.

Theoretical evaluation of tensile and flexural moduli of structural polymer foams

In this paper, the tensile and flexural behaviors of structural polymer foams with integrated skin layer are studied. Several models are reviewed to show the dependence of Young’s modulus of cellular solids on void fraction. Effective flexural modulus of structural foam is computed based on the Euler–Bernoulli beam theory and results are compared with experimental data available for uniform density core. The effects of skin thickness, core relative density, and density transition profile on the elastic modulus are investigated for symmetrical structural foams. Lower and upper bounds of effective moduli are obtained to evaluate the role of structural foams in increasing specific mechanical strength of polymeric structures. The optimum thickness of solid skin is then estimated in terms of relative density of cellular core to get maximum specific tensile or flexural modulus. Density profile of structural foams with graded density core is also optimized to find the most possible value of specific flexural modulus. The present study is expected to provide some useful information for evaluation and designing of structural foams with the aim of material saving and improvement in mechanical properties.

Bulk Density Reduction of Injection Molded Thermoplastic Foams via a Mold Design Approach

This study investigates the inter-relationships amongst processing parameters and mold design and their effects on relative density of injection molded foams. The limitation in lowering the overall bulk density of polymeric foams is one of the main drawbacks in injection foaming process. A sheet mold with a rectangular cavity was designed and manufactured which, includes an overflow channel connected with the main cavity via a secondary gate, the size of which was varied in this research. The effects of three parameters including chemical blowing agent weight percentage, gate width, and part thickness on reduction of relative density were investigated. Full factorial test experiments were applied in this research work. The results were used to determine the optimum conditions in terms of low foam bulk density and high cell density. The results showed that enhanced cell structure can be achieved at a larger part thickness and adjusting the secondary gate to an optimum thickness. The conclusions revealed that part thickness and secondary gate width were the most influential factors on reduction of relative density and part weight.

Mechanical properties of polypropylene structural foams with fiber-reinforced skins

This article reports on the effect of skin thickness, core density, and skin composition on the mechanical properties of sandwich structural foams. In this study, the skin layers are polypropylene reinforced by different amounts of hemp fibers while the core is the same polypropylene foamed using different amounts of chemical blowing agent. The sandwich structures are characterized mechanically in terms of tensile and flexural properties. The results show that fiber introduction in the skins not only improved the mechanical properties, but also produced finer cell structures in the core. Finally, a mechanical model including fiber concentration, skin thickness, and density profile in the core is proposed to predict with high accuracy the elastic modulus of the sandwich foams.

Prediction of the Shear Modulus of Polymer Structural Foams

In this work, a model is proposed to predict the shear modulus of structural polymer foams. The calculations include morphological parameters such as skin thickness, core density, and a transition zone of variable density between the skin and core sections. A comparison between several models is made with experimental data taken on polyethylene structural foams.

Structural Characterization and Mechanical Behaviour of LDPE Structural Foams. A Comparison with Conventional Foams

Structural foams are composed of two solid layers enclosing a foamed core. The application of sandwich structural foams has rapidly increased in the last decade. Injection moulding is currently used to produce these foams, being not common to produce conventional foams of similar densities and chemical compositions in a similar process. In this paper an alternative route to produce structural foams has been used. This method allows fabricating conventional foams with the same chemical composition and density than the structural foams, so comparisons between properties of both kinds of materials can be made in a proper way, i.e. avoiding effects of different chemical compositions and/or different densities. The structural and mechanical properties in tension, compression and bending have been characterized both for structural and conventional foams based on a low density polyethylene. The results have showed that the sandwich structure of structural foams improves a 50% the mechanical behaviour in bending, however no improvements in compression or tension have been found.

Tensile Modulus Prediction of Structural Foams using Density Profiles

In this study, the tensile moduli of LDPE structural foams are related to their density profiles measured by X-ray densitometry. A review of the most important mechanical models for cellular materials are presented and adapted to include the complete density profile. Using experimental data, predicted values with this new approach are much closer (4% deviation) when compared with other models for the range of parameters studied.

Compression Moulding of Polypropylene Foams and Their Properties

Compression moulding was used to produce polypropylene foams using azodicarbonamide as a chemical blowing agent. The morphology of the foams is reported in terms of cell size, cell density, and skin thickness using 1.5, 2, 2.5 and 3% of blowing agent. The resulting foams were characterized mechanically in terms of tensile and flexural moduli and comparison with mechanical models was made to predict foam performance. In general, mechanical properties, cell density, skin thickness and foam density decreased, while cell size increased with increasing amount of blowing agent. It was also found that simple mechanical models based solely on foam density and skin thickness were not able to correctly predict all the experimental data. For better prediction, foam density profile must be used.

Effect of Mold Temperature on Morphology and Mechanical Properties of Injection Molded HDPE Structural Foams

In this study, HDPE structural foams are produced by injection molding under different mold temperatures to study the effect of this variable on average cell dimension, cell density, and skin thickness ratio. Samples are also produced by setting independently the temperature of the fixed and moving plate of the mold to detect the sensitivity of foam structure to a temperature gradient in processing. The resulting foams are also characterized in terms of mechanical properties including impact and flexural tests. It has been found that for homogeneous mold temperatures, symmetrical skin thicknesses are obtained, which increase with decreasing mold temperature. On the other hand, by keeping one mold face at a constant temperature and varying the second one, asymmetric skin thicknesses are obtained. The degree of asymmetry is found to increase as the temperature difference between both molds increased. Furthermore, decreasing mold temperature produces a small increase in average cell sizes and reduced cell density. In general, both impact strength and flexural moduli of the structural foams increase with increasing skin thickness. For the particular case of asymmetric foams, the flexural moduli are slightly higher when the load is applied on the thicker skin; while much higher impact strength is obtained when the falling weight strikes the samples on the face having the smaller skin thickness.