Thermoplastic Foam Injection Moulding of Ultralight Structural Components With Highly Reinforced Surface Layer

The trend toward lightweight construction continues unabated across all industries. Due to their low density; plastics play an important role here and are increasingly replacing metals. Possibilities for implementing a lightweight construction strategy range from the use of continuous fibre-reinforced thermosets to short fibre-reinforced thermoplastic components. By combining thermoplastic foam moulding (TFM) and back injection of continuous fibre-reinforced tapes; such lightweight structural components can be realized economically in high volumes. Structural lightweight design with plastics will continue to gain in importance as weight is to be saved in many industries; but especially in all areas of transportation. Lighter vehicles not only consume less fuel; but they also emit less CO2. Of course; the weight savings also mean lower consumption of raw materials. Plastic components in themselves already bring considerable weight reductions. The use of foamed components can make an additional contribution here. The aim is to produce integral skin foams from thermoplastics which; thanks to their sandwich-like structure; offer very high lightweight potential combined with very high flexural rigidity. Integral foams are foam structures with a compact outer skin and a porous core. In this process; the density decreases continuously toward the centre of the component. An important process to produce so-called integral skin foams is thermoplastic foam moulding (TFM). The foam structure can be achieved by chemical and physical blowing agents. A combination of the two process variants is also possible; whereby particularly high weight reductions can be achieved. The advantages of these injection moulded components are their low density combined with good mechanical properties; the functional and process integration possibilities; and the design freedom they offer. Preliminary study In a preliminary study; the influences of process parameters and foaming technologies on the weight and flexural modulus were investigated on a Wittmann-Battenfeld Smartpower 60/130 using a simple test component (plate: 150 x 40 x 5 mm3). The polyamide Grilon TS from EMS-Grivory as a technical material was used. In various experimental test series; integral foams were produced both by physical foaming or by adding chemical blowing agents in the injection moulding process. The respective weight savings; microstructures and flexural elastic moduli were determined and compared. The flexural modulus of elasticity was determined in a 3-point flexural test according to DIN EN ISO 178. As expected; the flexural modulus decreases with increasing weight savings. Thus; at the maximum weight savings of 43%; a reduction in flexural modulus of just under 32% is produced. In a subsequent series of tests; trials were carried out with different melt metering volumes since the weight reduction is directly influenced by the metering volume. Thus; the optimum ratio between weight and bending modulus can be determined. As the metering volume increases; the mould is filled less and can thus foam more. For an endothermic blowing agent; the best combination between weight and flexural modulus would be a weight saving of 34%; i.e. a metering volume of 25 cm3. In subsequent trials with physical blowing agents; greater weight savings were achieved; but with larger cell sizes; which had a negative effect on the flexural modulus. Further trials with nucleating agents will follow here to achieve better foam distribution. Public founded project with industry Based on the preliminary study various process variants were being used to produce integral foams to analyse the potential of TFM. Process variants that were investigated in detail include standard TFM with one material (like in the preliminary study); sandwich moulding with two different materials and back-foaming of continuous fibre-reinforced tapes. The back-foaming of continuous fibre-reinforced tapes allows particularly rigid components to be realized and therefore offers great potential. This research work was carried out in a project funded by Innosuisse; Swiss Innovation Agency; and two industry partners. The potential of TFM for various materials and blowing agents (chemical as well as physical) was being investigated. One possible cause of failure of such sandwich structures is a lack of bond adhesion between the face sheets and the foamed core; i.e. delamination. To prevent such a failure; the bonding should be optimized as far as possible. The bond strength can be quantitatively determined by peel tests on the tensile testing machine. A component specially designed for this test serves as the test specimen. In the injection moulding process; a continuous fibre-reinforced tape is back-moulded from two sides so that the tape remains exposed in the centre of the component. The exposed tape allows clean peeling. Investigations at IWK showed that the following variables have a positive influence on the bond strength: €¢A high pre-heating temperature of the tape semi-finished products (> melting temperature of the tape matrix) and a high melt temperature favour the adhesion properties; primarily due to the higher mobility of the molecular chains. €¢Due to a low mould temperature; the components exhibit a compact boundary layer. This has a positive effect on the boundary layer and the bond adhesion. €¢For low-filled materials; a low injection speed is recommended. For highly filled materials; the injection speed has no significant influence. Project example surf fin One potential application of the back-foaming of continuous fibre-reinforced tapes is a Surf fin. In surfing; the characteristics of the fin are of immense importance; its behaviour in the water influences the overall handling of the surfboard. The most important characteristic value is the so-called flex (bending stiffness) of the surfing fin. Also very important is the total weight of the surfboard and therefore of the individual components. In order to meet these two requirements for a surf fin; conventional surf fins are produced using infusion or RTM processes with thermoset resin systems; among other things; but this is correspondingly costly and time-consuming. By integrating unidirectional thermoplastic carbon tapes into the injection moulding process; similar properties can be achieved with a much more efficient manufacturing process. A first high-end surf fin is on the market produced using injection moulding and meets the technical requirements (from KWB; Switzerland). The carbon tapes are inserted into the injection mould and back-injected. Under a bending load; the tapes on the outer sides form a compression and tension band (sandwich component). Thanks to the lower density of the plastic core; the weight of the fin remains low. The next optimization step is to further reduce the weight of the fin. This can be achieved by minimizing the density of the core component using TFM.