Discontinuous fiber-reinforced thermoplastic composites have earned a permanent place in the designer’s material palette. In particular, emerging options that use flat fibers offer new opportunities in both performance and design. For decades, glass-fiber reinforced polymers have enabled engineers to replace metal in structural housings, brackets, frames and functional components, delivering corrosion resistance, high specific stiffness and strength, and the design freedom of plastic processing methods like injection molding.
As discussed in my October 2024 Plastics Technology article on getting the most from fiber-reinforced thermoplastic composite parts, achieving “metal-like” performance in injection molded components is not guaranteed. It requires thoughtful attention to part design, gate location and material flow in the cavity. When these factors are optimized, discontinuous reinforced composites offer tremendous opportunity. When they are overlooked, familiar pitfalls such as weld lines, variable part performance and excessive warpage can quickly erode the advantages designers seek.
Figure 1: The schematic shows the differences in the cross section of a flat fiber (left) vs. traditional round cross section (right). Top: high-magnification images of a flat (left) and round (right) glass fiber incorporated in a polyamide matrix.
In that article, I also alluded to another new variable that compounders and resin suppliers are utilizing to expand the utility of these materials. That variable is the geometry or cross-section of the glass fibers themselves and whether to use fibers with a traditional round or rectangular “flat” cross section. Often marketed as “low-warpage” grades, these materials are gaining interest among designers and injection molders looking to improve dimensional stability without sacrificing mechanical performance. Understanding where they add value, and where fundamentals still apply, is essential.
Round vs. Flat Fibers — The Claim
Investigation into using different fiber cross sections is nothing new within composite material development. Since the introduction of the traditional cylindrical “round” fiber by Owens Corning in the 1930s, researchers have been experimenting with how different cross sections can yield additional performance benefits to this class of materials. However, until recently, the economics of fabricating these alternative shapes have prevented real commercialization of these substitutions.
Several glass-fiber manufacturers now offer what they call a “flat” fiber technology, with multiple reported benefits including reducing the tendency of part warpage due to fiber orientation, improving material flow and imparting higher mechanical performance over the traditional round glass fibers.
As Figures 1 and 2 highlight, these flat fibers have a rectangular or oval appearance when compared to round fibers. In addition to the thickness and the length of the glass fibers, the glass manufacturers can manipulate the ratio of the width to the fiber thickness, often referred to as the flatness ratio, to achieve performance variations. Typically, that flatness ratio is either 2:1 or 4:1, with the expected improvement in mechanical performance when the thinner fibers are used.
Figure 2: Notched impact strength data of the different resin populations with the different fiber cross sections through two generations of regrind. Source: The Madison Group and RTP Company.
Flat Fiber Designer Benefits
The benefits to a designer or OEM for considering flat fibers include a composite material that exhibits more isotropic shrinkage and greater mechanical properties, particularly at higher glass-fiber loadings. In one study, we examined different recipes compounded with the two glass-fiber cross sections, while also varying loading levels but maintaining the polymer matrix. Those different compounds were then molded into plaques, and tensile specimens were machined out of the plaques at different orientations to see how the properties of the molded part changed as a function of the fiber orientation.
Results of the tensile testing of round vs. flat glass-fiber in a PA 6 matrix.
The results, shown in the table, highlight that while we can get similar strength and modulus values when the fibers are aligned in the direction of loading — 0° — the traditional round glass fibers showed a larger drop-off in both properties, as the loading deviated from the direction of loading. Finally, the weld-line strength of the different cross sections did not substantially change. Therefore, from a design perspective, while we still need to account for fiber orientation, our design may be slightly less sensitive to this variable.
In addition to the improvements in the tensile behavior of the resin, the flat-fiber technology also yields better notched impact strength when compared to the traditional round cross-section fibers (Figure 3). This impact benefit was sustained when the resin was processed through two different passes of regrind as well. While there is inherently significant variability in this test, the trends suggest that the benefits are not short lived and can still yield improvement in applications where regrind is allowed. Again, the benefit is more pronounced as the fiber loading is increased.
Figure 3: Image showing the flatness of the lid geometry with the 50% glass-fiber filled resins, where the flat fiber compound (50F) on the left shows significantly lower warpage to the traditional 50% round glass-fiber compound (50R) on the right.
In addition to the mechanical performance improvements that can be achieved with the flat-fiber technology, the dimensional stability of molded parts can also be improved. In a separate study, we molded lid geometries at different thicknesses and gate configurations to see how the flatness of the parts changed with the different glass-fiber cross sections. This study considered the same four compounds that were used to look at the performance benefits mentioned above.
Figure 4: Graph showing how the pressure to fill the lid geometry in Figure 2 changed with the different cross sections. R- Round Fiber; F- Flat fiber.
The study concluded that the benefits of the flat fibers were most pronounced at thinner wall sections — 1 mm in this case — as compared to a traditional 3-mm wall thickness part. However, there were also benefits if non-ideal gating schemes were used where there were changes in flow direction during the mold-filling process. While the exact mechanism for this benefit could not be precisely identified, the study did confirm that the flat fibers could offer a potential material solution to reduce part warpage without the need to change a part design or gating. This benefit could prove extremely useful to designers, especially when dealing with tight-tolerance parts.
How Flat Fibers Benefit Processors
While it is no new revolution that materials can be developed that help improve the mechanical performance of molded plastic parts, there is usually a trade-off in how easily the material can be processed. Again, the flat-fiber suppliers claim that changing the cross section of the glass fibers can actually help improve moldability by reducing the composite material’s viscosity. The study of our four materials showed that in benchtop and capillary rheometer testing, as well as in the actual molded parts, the viscosity of the material did appear to be either maintained or reduced. This led to reduced pressure to mold our lid geometries and allowed for a wider processing window when compared to the round glass fibers (Figure 4). Therefore, there does appear to be some validity to the claim that these glass fibers may actually make a processor’s job easier.
Additionally, the designer’s benefits of dimensional stability and retention of mechanical properties through regrind cycles also benefit the processor. With these advantages, molders could potentially meet the print specification more easily while also introducing regrind with less concern for its influence on overall part performance. This could help yield a less expensive and, potentially, greener part.
A Useful Option
While changing the glass-fiber cross section may not provide a silver-bullet solution for either designers or processors, it does offer a unique tool in certain applications. The benefit appears to be most pronounced in thin-walled and highly filled resin systems, which can traditionally be difficult to mold and maintain dimensional stability. Therefore, if both the part designer and molder pay attention to the fundamentals of optimizing fiber-reinforced composite parts and consider this new grade of material, they may find that they can more easily manufacture their parts and maintain the original design intent.
This article was originally published in the April 2026 edition of Plastics Technology.
