Modelling of anisotropic suede-like material during the thermoforming process Giovanni Lelli*1, Massimo Pinsaglia2, Ernesto di Maio2 1Alcantara S.p.A. (Application Development Center), 2University of Naples "Federico II" (Department of Materials and Production Engineering) *Corresponding author: Strada di Vagno, 13, I-05035 Nera Montoro, TR, ITALY, [email protected] Abstract: Physical and mechanical studies of The appearance and tactile feel of the Alcantara® have shown very pronounced material is similar to that of suede, making it anisotropic nonlinear features. Using suitable for a large number of luxury constitutive equations borrowed from the applications, including furniture, fashion & modelling of biological tissues like tendons accessories, contract and automotive. In and/or arteries under the form of hyperelastic particular, the most important market sector is free-energy functions, a good representation of currently represented by the premium such mechanical features can be obtained. In automotive segment, where Alcantara® particular, a combination between the represents a new category of which it is the optimization module and a modified unique product, though many imitations have hyperelastic model in COMSOL been attempted. Indeed, its characteristics Multiphysics® can be used to determine the (mechanical resistance, breathing ability, macroscopic mechanical parameters and to enduring life, easy care, colour fastness, apply them to the simulation of the behaviour resistance to wrinkling) make Alcantara® the of Alcantara® when it undergoes high- best alternative to leather for applications like temperature processes like thermoforming. seating, dash trimming and headliners for many premium OEM automotive suppliers. Keywords: Thermoforming, Hyperelastic, Although many manufacturing processes Anisotropic of automotive components are still based on traditional techniques, some large-scale and 1. Introduction high-rate productions require a more industrial approach. An example is represented by Alcantara® is a trade name given to a thermoforming of car headliners. composite material used to cover surfaces and Thermoforming is a manufacturing process forms in a variety of applications. The material where a thermoplastic sheet is heated to a was developed in the early 1970s by Miyoshi malleable forming temperature, formed to a Okamoto, a scientist working for the Japanese specific shape in a mould, trimmed to create a chemical company Toray Industries. In 1972, a usable product and finally cooled. The driving joint-venture between the ENI Italian chemical force to stretch the sheet into or onto a mould group and Toray Industries gave rise to is provided either by vacuum or by a Alcantara S.p.A. countermould. Alcantara® is created via the combination Alcantara® is not able to hold the shape by of an advanced spinning process (producing itself after a thermoforming cycle, so it must very low denier bi-component "islands-in-the- be always combined with a thermoplastic sea" fiber) and chemical and textile production backing (usually, a felt made by glass and processes (needle punching, impregnation, thermoplastic polymer fibres is used for the extraction, splitting, buffing, dyeing, finishing manufacturing of car headliners). This can be etc.) which interact with each other. Alcantara achieved by sticking the product on the S.p.A. has the only European integrated backing using either a resin or a thermoplastic manufacturing cycle of ultra-micro-fibrous adhesive. non-woven textiles: 3 production units (from Depending on the complexity of the final raw materials to the final products), each shape, the moulding process can be carried out supplying the following, are present in the either in one step (i.e., the covering and the plant. backing are shaped together) or in two steps From a technical standpoint, the resulting (i.e., the backing is pre-formed and placed in product can be defined as a composite material the mould, then the covering is heated and in which the reinforcement is a non-woven glued on it, so to avoid wrinkles near small-ray structure of polyester ultra-microfibers in a curvatures). In any case, if the covering is porous polyurethane matrix. stretched far beyond its elastic limits (usually next to small hollows or reliefs) the material Moreover, to check the predicting ability can either break or tend to a release of residual of the model, a reference mould was built, in stresses which manifest itself in the form of a the shape of a paraboloid with an undulate detaching from the backing. For this reason, a basis (Figure 2). Standard Alcantara® with a trial-and-error approach is usually applied, 10×10 mm grid printed on its surface, using either “torture moulds” (to check if the combined with an Acryl-Butadiene-Styrene material is able to withstand the most critical 2mm-thick sheet by means of a thermoplastic deformations expected for the final shape of adhesive polyurethane-based film, was used to the manufactured part) or at worst smoothing get a thermoformed sample for the direct the sharpest edges of the final mould if some measurement of local deformations. The unexpected problem occurs. Obviously, this moulding was performed through a one-step approach is very expensive and time- process: firstly, the materials (held by a consuming, so a software tool able to highlight 650×340 mm frame throughout the entire the critical points and to predict whether the process) are heated from the covering side by covering is able to withstand high local IR lamps, then both sides of the mould move deformations or not can be very useful and towards one another to give the structure its cost-effective. final shape. After cooling and ejecting the finished part from the mould, local 2. Experimental deformations can eventually be measured by comparing the final dimensions of each As past experiences proved that this quadrangle on the surface with the initial material is characterized by different square (Figure 3). mechanical performances in warp and weft directions, it is worth pointing out that, although the microstructure of Alcantara® was represented by a three-dimensional entanglement of PET ultra-microfibers surrounded by a PU porous matrix, a simplification has been made, assuming that the overall mechanical behaviour could be described by an orthotropic model. Hence, tensile uniaxial tests based on the ASTM D638–I standard were performed on samples cut along 0° (warp, longitudinal), 90° (weft, transverse) and 45° (diagonal) directions, using an INSTRON 5565 Figure 2. Reference lab-scale mould (paraboloid) dynamometer equipped with a climate for one-step thermoforming chamber. For the sake of simplicity, as thermoforming of Alcantara® car parts is carried out at temperatures around 90°C, this value has been chosen for tests (Figure 1). 80 70 60 50 40 Load (N) Load 30 L T 20 D 10 0 Figure 3. Example of thermoformed part for the 0 10 20 30 40 50 60 70 80 90 100 110 120 130 measurement of local deformations. Backing: ABS Displacement (mm) (2 mm thick). Adhesive: Thermoplastic Figure 1. Load (N) / displacement (mm) curves at polyurethane-based film 90°C for the three directions analyzed: warp (blue), weft (red), diagonal (green). 3. Theory and Governing Equations Ψ is depending upon both the first invariant I1 and a constant commonly identified as μ: As it was said before, the microstructure of Alcantara® is characterized by a preferential 1 I 3 orientation of the fibres due to the process 2 1 sequence needed to get the final non-woven product. In particular, fibres seem randomly A fundamental aspect to be considered for distributed at a micro-scale level, but at the any material described by a hyperelastic law is same time they are clearly characterized by a the degree of volumetric compressibility. preferential orientation in the longitudinal Hyperelastic materials can be reasonably direction at a macroscopic level. Moreover, at considered either weakly compressible or even deformations lower than 10% the material uncompressible, with an equivalent Poisson shows a clearly nonlinear behaviour, which coefficient near to the upper limit 0.5. In this can be probably ascribed to the rearrangement case, the deformation energy density can be of fiber distribution during the first load stages. decoupled as a sum between a purely isochoric This implies the need to introduce nonlinear contribution and a purely volumetric anisotropic constitutive equations. contribution U: A similar behaviour has been already observed for “soft composites” like fiber- p reinforced rubber composites (formed by cords 1 U I1 3 p J el 1 with high tensile strength reinforcing an 2 2 elastomer, as for example tires and conveyer belts) (Tuan, et al., 2007), as well as for where: biological tissues like arteries and tendons, which often exhibit anisotropic properties F X χ X X , deformation gradient (Gasser, et al., 2006). Therefore, a particular T CFF , right Cauchy Green tensor Helmholtz free-energy function was used, which allows to model a composite in which a J el det(F), spherical (dilatatio nal) elastic matrix material is reinforced by families of volume variation fibres and the mechanical properties of this 1 3 kind of composites depend on preferred fiber FF J el , isochoric (distortional) directions. The description of the constitutive deformation, det(F ) 1 model is given with respect to the reference T 2 3 undeformed configuration, under the CFFC J el , modified right hypotheses that the deformation