Novel process enables more uniform wall thickness in formed composites

9 February 2016
Christian Gröschel and Dietmar Drummer
A shaping process that works with gas pressure instead of matched dies, and without diaphragms, improves the distribution of wall thickness in formed thermoplastic composite parts.

Continuous-fiber-reinforced plastics (FRPs) are enjoying increasingly widespread use owing to their combination of excellent mechanical properties and light weight. In airplanes, for example, these qualities help to lower fuel consumption over the life cycle of the composite material.1 For optimal performance, the load-carrying fibers (carbon, glass, or aramid) must be aligned correctly with an equal fiber volume content over the part.2 A number of different processes, following the same basic steps, are available to shape parts from thermoplastic FRPs, which are mostly used as sheets (organosheets). Because of its short cycle times, a commonly used process is stamp forming with matched dies made out of metal (see Figure 1).3 This process type is very similar to the deep drawing of metals (e.g., to create seamless pans) except that other forming mechanisms occur. Instead of stretching the sheet to shape the part, the fibers are draped (i.e., they move) within the molten matrix.4, 5 Therefore, a disadvantage of this process is that it may also entail unwanted movement of fibers and matrix.

In stamp forming with matched dies made of metal, nonuniform pressure distribution occurs during shaping and leads to nonuniform wall thickness within the finished part.6, 7 This results in different fiber-volume contents and thus affects the ability to achieve consistent mechanical performance. In particular, the apex points that first come into contact with the stamp tend to thin. This is because of undesired squeeze flow of the matrix and fiber slip in the direction of the pressure gradient.8, 9

In our novel Twin-O-Sheet process, illustrated in Figure 2, gas pressure is used instead of matched dies.10 Only one or two female molds are needed, thus offering increased flexibility for part variations. Our process is similar to diaphragm forming but obviates the use of separate diaphragms. Consequently, in addition to one-sided parts, closed hollow bodies can be blown up into shapes. Following shaping and bonding, injection molding can also be included (not done in this work) if the process is run on an injection molding machine. This makes Twin-O-Sheet a so-called one-shot process.


Schematic of stamp forming with matched dies.


Schematic of the Twin-O-Sheet process without injection molding. p: Gas pressure.


Optical and tactile measurement points, and thickness at apex points. Within the standard deviation (error bars) the thinning can be neglected. fs: Fixed die side. ms: Moving die side.

For the work described here, we maintained constant shaping parameters, i.e., sheet temperature (220°C when shaping starts) and gas pressure (39 bar). Optical and tactile measurements of fiber orientation along the length axis, as well as gas injection (shaping) time showed that uniformly distributed (hydrostatic) gas pressure leads to uniform wall thickness within the parts. This is because the equally distributed pressure reduces matrix squeeze flow and fiber slip. The tactile measurement showed no thinning at the apex points (see Figure 3). The slightly higher thickness in the optical measurement is caused by the preparation and the resulting viewing angle. We used the optical measurement method because it offers the possibility of more measurement points within a single scan (not shown in this article). A downside of the optical method, however, is that the specimen needs to be cut apart to perform the scan of the cross section.

The results of our testing show that, like diaphragm forming, the Twin-O-Sheet process prevents the unwanted movement of fibers and matrix during the forming process. Better quality with regard to the distribution of thickness within parts can thus be achieved. Furthermore, our process offers additional advantages such as the lack of diaphragms, which would remain in the closed hollow body, and thus the possibility of integrated production using injection molding machines.11

High-performance composite parts require constant wall thicknesses for optimal properties. Unlike stamp forming, where thinning occurs, our new Twin-O-Sheet process makes uniform wall thicknesses possible and also enables production of closed hollow bodies in a single step. We are currently developing a process model to describe the influence of the process parameters on the forming behavior.


Authors

Christian Gröschel
Institute of Polymer Technology

Dietmar Drummer
Institute of Polymer Technology


References

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DOI:  10.2417/spepro.005866