Shrinkage and warpage of in-mold-laminated parts

Calculation and experimental verification of shrinkage, warpage and residual stresses of in-mold laminated parts During the manufacture of decorated parts, such as those used, for example, in the automotive the widely used film laminating and painting are more and more replaced with the process of in-mold lamination. In this process the ready cut and usually thermoformed film is placed into the open mold and a thermoplastic melt is injected and a laminated part is produced. The dimensioning of the injection molds and thermoform molds is problematic. The starting point is the part with its desired form and dimensions. The questions which arises is: how must the appropriate thermoform mold or injection mold be designed so that the part can be produced with the required exactness in dimensions and form? At present the designing is done using empirical methods. It would be desirable to support the designer through the use of simulation programs. In the framework of this project, the shrinkage, warpage and residual stresses of parts produced by in-mold film lamination will be calculated as well as the pre-shaping of the films in thermoforming operations. The calculations will be done with the commercial FEM-Programs ABAQUS and MOLDFLOW. The results of the calculations will be compared to experimental results. Three simple geometries-plate part, angled part and box shaped part - will be used. Furthermore, the influence of the process parameters and geometry on shrinkage, warpage and residual stresses will be examined.

The in-mould lamination technology allows the manufacturing of a decorated composite part. The decorating film is placed into the open mold, a polymer melt is injected and thus a laminated part is produced. The decorating films are mostly made of printed multi-layer plastic films. Their advantages are less environmentally detrimental painting and the various properties that can be achieved through the composite films such as scratch resistant surfaces etc. If the part is not flat, the film is first thermoformed and then inserted in the injection mould. The thermoforming and the in-mould lamination of the film with an appropriate plastic material both cause shrinkage and warpage, which lead to problems getting the desired shape and dimensions of the part. The aim of this project was to examine the use of simulation programs to calculate the ideal construction of the part, the thermoform mould and the injection mould. This can save time and money for the construction of the part to manufacture and the reconditioning of the moulds can be avoided, too. First the heating of the film was simulated using Abaqus, a FEM-program. It was investigated how to homogenize the temperature distribution in the film with various heating devices. After that the process of thermoforming was calculated and hence the shell thickness of the film, too. Using two self-programmed interfaces the results from the simulation of the process of thermoforming were transmitted to Moldflow, an injection moulding simulation program. The calculation of the in-mould lamination process was done using the program Moldflow. The results from the injection moulding simulation were transmitted with another interface to the program Abaqus. Using Abaqus shrinkage and warpage, which result from residual stresses in in-mould laminated parts, were calculated. Shrinkage and warpage are mainly the cause for problems to get the appropriate exactness in dimensions and shapes of the part. The developed and elaborated way makes the combination of the processes of thermoforming and injection moulding possible. The influence of the process parameters on shrinkage and warpage can be investigated in various calculations. Within the acceptance of the project the experimental verification was cancelled for reasons of economy, therefore only studies of the process parameters could be done within the research work. The simulation of the thermoforming process was verified by some experiments which were carried out using the thermoforming machine of the department of plastics processing.