Thermoplastic composites with continuous fibre reinforcements, sometimes also referred to as organosheets, come with excellent weight-specific mechanical properties such as strength and stiffness. Moreover, these materials are tough and allow short cycle times in part production. Also fuelled by the rapid development of Hybrid Moulding (forming & over-injection moulding in one operation), these materials are now starting to replace more traditional materials such as aluminium and steel. But there is still room for further improvement: the new group of neolaminates marketed under the brand name W8SVR from Huesker Synthetic are made completely from thermoplastic UD tapes, and have the potential to further improve the weight/performance ratio of thermoplastic composites, increase surface smoothness and reduce waste, according to Huesker Synthetic´s Michel Jansen.
1. History of composites
The modern era of composites began in the beginning of the 20th century, when scientists first developed plastics, like vinyl esters, polystyrene, phenolic resins and polyester. These new synthetic materials outperformed natural resins derived from plants and animals that were so far used in making glues and binders. In 1935, Owens Corning was the first company to launch glass fibre onto the market, hereby laying the foundation for modern fibre composites and fibre-reinforced plastics (FRP).
The composites industry matured in the seventies with the design and manufacture of improved plastic resins and reinforcing fibres including aramid and carbon. With carbon fibre reinforced composites being around ten times stronger than steel at the same weight, composites have now found their way into diverse applications like aerospace, marine engineering, wind energy, building and construction, sporting goods, and transportation.
Initially, only thermoset resins were used as matrix systems to make composites. Thermoset resins most widely used today are polyesters, vinyl esters and epoxy resins, all of which are supplied in a liquid state and require a chemical reaction for curing, which can take considerable time. Also, once cured, the chemical reaction cannot be reversed, which means that remolding, reshaping and recycling of thermoset composites is extremely difficult. In the eighties and nineties of the last century we also saw thermoplastic resin systems being introduced to the composites industry. Compared to thermosets, thermoplastics show improved formability, energy absorption, impact strength, and significantly improved fatigue resistance and recyclability. However, the decisive advantage of thermoplastic composites lies in their suitability for mass production. Unlike thermoset composites, thermoplastic composites do not require chemical curing and are much faster to process. This explains the great interest in thermoplastic composites at that point in time. Even so, mass processing proved more problematic than initially expected as processing costs were too high, and market acceptance did not reach the expected level.
2. Thermoplastic renaissance
According to the Composites market report published by AVKIndustrievereinigung Kunststoffe eV in 2018, the market for thermoplastic composites has grown significantly more than the overall market for composites over the previous five years. The key driver in this development were significant improvements in processing technologies, both in ‘long fibre’ reinforced thermoplastics like LFT/GMT as well as in endless fibre reinforced composite applications. Machine manufacturers such as Engel Austria, the Krauss Maffei Group and Arburg now have ‘tried and proven’ machines that enable mass production of automotive
parts using hybrid forming technology. This new technology combines thermoforming of organosheets and injection moulding in one process with cycle times of only one minute.
3. Next development stage in lightweight construction: Neolaminates made from thermoplastic UD tapes
Stiffness and tensile strength in composites are largely determined by fibre length and fibre orientation. Optimum values can therefore be achieved using continuous fibres which are completely stretched and lie straight in the intended load direction of the component, so that tensile and compression forces can be absorbed as directly as possible by the fibre. Exactly these properties can be found in UD tapes: the stretched continuous fibre (with fibre weight fractions up to 70%) is embedded in a thermoplastic matrix to which it has good adhesion. Depending on the field of application as well as the individual customer
requirements, neolaminates can be produced either by weaving or crossplying of UD tapes. W8SVR neolaminates from Huesker Synthetic can theoretically have ‘any’ fibre/matrix combination, but Glass/PP, Glass/PA6, Glass/PA66, Carbon/PP, Carbon/PA6 or PA66, carbon/PA12, carbon/PC, carbon/PPS are the most commonly used materials.
4. Neolaminates made from woven UD tapes
Woven UD Tape (see Fig. 1) sheets have better draping properties and are therefore easier to form into complex geometries than cross-plied sheets. The mechanical properties nevertheless come very close to the properties of crossplied Neolaminates, because the use of wide and thin UD tapes results in significantly fewer ondulation points and very little ondulation depth (see Fig. 2). Compared to traditional (also woven) organosheets, woven UD tapes achieve higher strength, higher stiffness and greatly improved surface smoothness.
5. Neolaminates made with Automatic Tape
Placement (ATP) Automated tape laying machines (see Fig. 3) allow straight placement of UD tapes (see Fig. 4). The tape laying head has a pressure roller and brings heat into the interface of two tapes so that the resulting laminates can be in-situ consolidated. The fibre orientation can be freely chosen by the customer so that expected forces can be absorbed in a load-compliant manner. This way, maximum
stiffness and strength are achieved at lowest possible weight. At the same time, the ATP process enables the near-net-shape production of laminates (see Fig.5). Compared to conventional organosheets, which are usually produced only in rectangular form, material scrap is reduced by up to 40 percent.
6. Stronger, stiffer, smoother, faster
Compared to older generations, the new generation of neolaminates has significantly improved mechanical properties at the same weight per unit, allowing for additional weight reduction and/or cost benefits from material savings in the future. Due to the lower material input, the thermoplastic matrix can also be heated and cooled even faster, so that even better cycle times can be achieved than with conventional organosheets.