Towpregs

Towflex (Hexcel Composites) is based on carbon, glass or aramid fibres impregnated with polypropylene, polyamide, PPS, polyetherimide, PEEK. Product forms include flexible Towpreg, woven fabric, braided sleeving, UD tape, chopped compression moulding compound, moulded plates, thermoformable laminates. 

In addition to the reduced cost, on-line consolidation also offers benefits for design flexibility and performance. With localized heating, this process is inherently suitable for manufacturing parts with large surfaces and moderate curvatures, such as fuselage structures and deep submersibles [5]. Because the towpreg is fully consolidated and locked in the vicinity of the melting zone as it is placed onto the structure, there is no conceptual limitation... 

Models of the intimate contact process that have appeared in the literature are commonly composed of three parts or submodels. The first submodel is used to describe the variation in the tow heights (surface waviness or roughness) across the width of the prepreg or towpreg. The second submodel, which is used to predict the elimination of spatial gaps and the establishment of intimate contact at the ply interfaces, relates the consolidation pressure to the rate of deformation of the resin impregnated fiber tow and resin flow at ply surface. Finally, the third submodel is the constitutive relationship for the resin or resin-saturated tow, which gives the shear viscosity as a function of temperature and shear rate. 

Fig. 18.2 O n-line simulation Correlation between degree of cure and spin-spin relaxation time for both static and moving towpreg. Reprinted with permission from [20].

Applied Composite Technology (ACT), Fayette, Utah, USA—manufacture composite products, a leading producer of prosthetic limbs. Also produce rifle and pistol barrels. AppUed Fiber Systems, Clearwater, Florida, USA— manufacture Towflex, which includes continuous carbon fiber powder coated with a variety of thermoplastics, such as Nylon-6, PP, PET, PES, PEI, PPS and PEEK, avaUable as a flexible towpreg suitable for hot filament winding. Acquired by Hexcel in April 2001. 

Torres L, Nunes J P, Silva J F and Marques A T (2008) New PVC matrix towpregs and composites, Mater Sci Forum 587 241-245. 

Youjiang Wang, A.R. and Muzzy, J. (1996) Braided thermoplastic composites from powder-coated towpregs. Part I towpreg characterization. Polym. Compos., 17, 497—504. 

Production of Thermoplastic Towpregs and Towpreg-Based Composites... 

The powder-coating equipment developed to produce the towpregs is composed of five modules (Figure 1) (i) fiber supplying system, (ii) pneumatic... 

The fibers are pulled from the roving and spread by a pneumatic device to facilitate polymer penetration. Then, in a deposition chamber, a thermoplastic powder is continuously supplied by a variable speed, double-screw miniextruder. By means of an air fan, the powder forms a cloud and adheres to the fibers. Three rolls conveniently placed inside the chamber increase the fibers path and, consequently, the deposition time and amount of adhering thermoplastic powder. Next, the fibers pass through a furnace to soften the polymer particles and promote their adhesion to the surface. Finally, the towpregs are cooled down and woundup on a spool with variable speed. 

Figure 2. Production of pre-consolidated tapes (a) view of the die, (b) GF/PP towpreg and PCT samples.

The influence of the linear fiber pull speed, temperature and moisture content in the coating chamber, as well as the PP powder particle size on the amount of polymer deposited in the towpregs was established (Figure 4a-d). After setting each parameter, approximately 100 m of towpreg were produced, weighed, and the PP mass fraction was determined from the density of the components. 

As a further step towards the optimization of the coating process, the influence of the operational variables on the final composite mechanical properties was studied. Four different parameters (polymer powder size, fiber pull speed, and furnace and PCT die temperatures) were independently varied. Then towpregs were produced under preset conditions, and composite plates were obtained by compression molding and subjected to mechanical testing (three-point bending and tensile tests). The overall results are summarized in Table 3. 

The technique developed by Klett et al was followed a wrapping wire mandrel was used to facilitate the towpreg lay-up and the coated tow is wrapped around an aluminum mandrel by rotating end-over-end. A limitation of this procedure is that the fibers in one layer are misaligned with respect to another layer. 

A theoretical model was developed to simulate the consolidation by compression molding, both in isothermal and non-isothermal conditions. - The model assumes a towpreg lamina as an array of fibers with attached particles (Figure 5a). The stacking of a number of these laminae together with different possible arrangements leads to a preform (Figure 5b). 

Figure 5. Geometry of a towpreg laminate (a) towpreg lamina, (b) arrangements of laminae...

In this equation, C is a constant that depends on the towpreg properties, the press operation, and the Hber/polymer arrangement. The variables hjf and Vy are the final composite thickness and fiber volume fiaction, respectively. For hexagonal and square packing, the values of C are, respectively ... 

For the utilization of the model, it is also necessary to consider the momentary shear rate, y, of the molten polymer through the fiber interstices. As can be seen from Eqs. (1) and (2), the pressure necessary to consolidate a towpreg with a hexagonal fiber/polymer arrangement is approximately 4/3 of that for a square arrangement. At this stage, using the viscosity, it is possible to express the temperature dependence of the pressure by Eq. (2) and to consider an isothermal and a non-isothermal approach. 

Figure 7. Scheme of the towpreg laminate used in the non-isothermal model. 

A pultrusion head was designed, built, and mounted on a conventional 60 kN PULTREX pultrusion line (for thermoset matrix composites), thus adapting it to the production of continuous profiles made from thermoplastic matrix towpregs. The pultrusion head (Figure 11) includes three main parts (i) preheating furnace, (ii) pressurization and consolidation die, and (iii) cooling die. 

The process involves three phases. First, the towpregs are guided into the preheating furnace. Then, they pass through the first part of the pultrusion head, where the consolidation occurs. The consolidated material then enters the cooling die where it cools down to a required temperature. Finally, after leaving the pultrusion head, the profile is cut to specified lengths. 

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