Improvements of thermoforming of thermoplastic composites using a collection of rubber particles as a soft mould half

Abstract

Compression moulding is the ideal candidate for large series production of thermoplastic composite parts. Improvements in this production technique will make it more appealing for those markets that are reluctant to use composites because of their development costs. Unlike other composites processing systems, the compression moulding press is capable of producing fibre-reinforced plastic parts in significant volumes, with the accuracy, repeatability and speed to which, for example, the automotive industry has been accustomed in the stamping of metal parts. This thesis aims at a better understanding of the behaviour of the rubber mould during compression moulding of thermoplastics and consequently at the reduction of the development costs and improving the design of the rubber mould. The classical problems that need to be addressed when designing a rubber mould are the correct dimensions to accommodate the laminate and the positions of the details. The standard process, though, does not take into account the temperature changes in the mould during production and in particular the effect of the coefficient of thermal expansion of the rubber. In this thesis, an envisioned method to reduce this problem is to add a certain amount of aramide in the rubber mould, in order to restrict the expansion due to increased temperature. The second issue that has to be considered is the friction between the melted thermoplastic and the rubber mould. The use of lubricant is extremely effective, but can be used only in a prototyping phase, as the lubricant affects the mechanical properties of the thermoplastic composite. Proper modelling of the rubber forming process, considering the correct rubber parameters, allows identification of the problems that might occur during manufacturing. The way to eliminate those problems numerically, though, is computationally challenging as well as uncertain and time consuming. With those results in mind, an improved method was developed which substitutes the flexible rubber mould with a collection of rubber particles. The collection of rubber particles acts in a way similar to that of a fluid and has the advantage of filling the mould completely so that there is always contact between the rigid and the flexible mould. The new method allows the manufacturing of a wider range of products and allows the reduction of development costs related to the definition of the proper rubber mould shape. To be able to describe the collection of rubber particles as a homogeneus material, a series of tests has been designed for the determination of some of their physical parameters. The obtained material has a very variable stiffness, from a very low modulus when the particles are not compressed, to two orders of magnitude higher values when compaction is almost complete. Bulk and shear modulus are related to the Poisson’s ratio that does not vary much during the entire process, having a value always slightly below 0.5. This value is consistent with the fluid-like behavior in the beginning of the process and with the, almost incompressible, solid rubber block at the end of the process. Finally, the parameters found have been used to model the compression moulding process with a collection of rubber particles. Modeling is not strictly necessary because most of the existing problems in the conventional production method have been eliminated. However it might become useful when the limitations of the new production technique will be explored and in particular for those geometries that are not possible with the conventional method.