Thermoforming gets a stamping lesson from metal
Original article date: February 1998
A French metal stamping simulation tool that uses finite element analysis has been extended to cover thermoforming. Alan Quinn explains the significance.
Press forming of continuous fibre reinforced thermoplastic (FRTP) sheets, known as thermoforming, offers a promising fabrication option for lightweight structural composite components. Thermoplastic polymers have several improved mechanical and physical properties compared to thermoset polymers and, perhaps most important for industry, they offer the possibility for rapid part production using the press forming process.
Today, thermoforming relies heavily on ‘trial and error’ practices which are costly, inefficient and
provide little scope for a detailed understanding and optimisation of the process. Now, an explicit finite element code has been developed to simulate the thermoforming process. Detailed temperature dependent rheological models are used which account for intraply shearing, squeeze flow, fibre reorientation and fibre buckling. Advanced friction laws – temperature and pressure dependent – are used to account for the interply sliding mechanism between plies.
The forming of complex thin sheet metal shapes such as automotive body parts is usually performed using metal stamping techniques in which an initially flat sheet, or blank, is pressed by a punch into a die cavity having the required shape. For the stamping of simple shapes, experience is usually sufficient to design the required tooling so that a satisfactory part may be formed without tearing, wrinkling or excessive elastic springback occurring. In the case of more complex shapes the stamping process may become very involved, possibly requiring multiple stamping operations and the use of draw beads to encourage plastic stretching and avoid wrinkling, or local lubrication to allow easier sliding of the blank between the tools and reduce excessive local thinning. In these cases previous knowledge is of limited use and invariably an expensive and time consuming trial-and-error design, manufacture and try-out process is normally unavoidable.
Within the past five years, numerical simulation techniques based on the finite element method have made tremendous advances and today the stamping of very complex parts may be computer designed. The try-out process is now able to be performed on the computer leading to substantial savings in tooling costs and the time to production. The ability to experiment ‘numerically’ gives the potential for a much better understanding of the forming of a part and, consequently, the opportunity to produce better quality parts. Contours of thickness distribution and plastic strain distribution give vital information to the tool designer before any final decisions on tooling and the forming process must be made.
The process has been further developed to permit the analysis of certain classes of fibre reinforced thermoplastic materials (FRTP). The FRTP materials being considered at present are pre-consolidated, stacked, continuous fibre reinforced thermoplastic materials having either unidirectional or woven fabrics. Typical examples of commercially available materials being investigated include unidirectional carbon fibre reinforced APC2-AS4, woven carbon fibre reinforced PEI-Cetex, and woven glass fabric reinforced GF-PA12. Through careful control of the forming temperature and pressure cycles it is possible to produce high quality, light weight, complex-shaped structural parts within a short cycle time competitive with conventional metal stamping.
Additional complexities for the simulation of FRTP stamping include modelling of the stacked plies, characterisation of the thermo-viscoelastic material and analysis of the transient heat transfer. In addition to laminate thickness and stress and strain distributions during forming, the designer is also interested in temperature, cooling rate and pressure distributions, all of which influence the final part quality. The simulation should identify regions of possible material failure such as delamination, fibre breakage or wrinkling. Finally, prediction of post forming residual deformations due to thermally induced stresses should be possible.
A commercial metal stamping simulation tool – Pam-Stamp, from PSI Group, headquartered in Rungis, France – that uses finite element analysis has been extended to cover thermoforming. In metal stamping analysis, the blank deformations may be easily described using a conventional elasto-plastic material law. FRTP materials, however, form under a combination of various ply (intraply shearing and in plane stretching) and interply slip (shearing and rotation) mechanisms. Squeeze flow and resin percolation due to the high pressure and low matrix viscosity may also occur.
The approach is to model each ply independently using shell finite elements and to impose an interface viscous friction law to govern the interply sliding between the individual plies. This approach may correctly account for all of the important forming mechanisms except percolation, which is considered unlikely in rapid press forming of high quality structural parts.
A temperature-dependent viscoelastic material law has been developed to characterise the in-plane deformations of each ply within the laminate. This law separates the viscous matrix and elastic fibre contributions. The matrix viscosity terms are temperature dependent. The fibre directions and modulus, for either unidirectional or woven fabrics, must be specified depending on the fabric type.
Various failure modes are possible during thermoforming, examples include fibre fracture, excessive delamination between plies and morphological defects such as the creation of voids. The latter are usually caused by inadequate pressure application or too fast a cooling rate during the forming and post-cooling phases. Investigations are underway to identify suitable process windows for the materials. The software will then be able to identify critical regions by monitoring the current state of applied pressure and cooling rates with respect to the limits of the process window.
A further common problem in the forming of complex shaped FRTP parts is the formation and control of fibre or ply buckling. This is particularly severe in the case of woven fabrics which undergo large shear strains. The dynamic explicit finite element formulation and the shell elements being used to model the plies, together with an appropriate material law, is able to directly capture the ply buckling phenomena.
An analysis must correctly predict the time dependent (transient) heat transfer process between the tools and the laminate and the temperature distributions throughout the laminate during forming. The transient temperature distribution at any position within each ply, or interply are then used as input to the coupled thermomechanical material properties and the temperature dependent interply viscous friction laws.
The key features of the governing heat transfer equations are as follows:
* An anisotropic two dimensional model is used for the in-plane heat transfer distributions. Coefficients of heat conduction are required in both the fibre and transverse to the fibre directions.
* The through-the-thickness heat transfer between neighbouring plies and the tool-to-ply contact is treated using a one dimensional model.
This theory has been implemented and has been validated against both classical and simple composite laminate test cases. A full three dimensional heat transfer example has been demonstrated with 20 plies of APC2-AS4.
Future work will further investigate full scale industrial problems.
February 1998