Novel modeling approach for fiber breakage during molding of long fiber-reinforced thermoplastics

Abrahan Bechara,Angel Yanev,Dave Brands,Sebastian Goris,Tim Osswald

doi:10.1063/5.0058693

Abrahan Bechara, Angel Yanev + Show 3 more

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Long fiber-reinforced thermoplastics (LFTs) are an attractive design option for many engineering applications due to their excellent mechanical properties and processability. When processing these materials, the length of the fibers inevitably decreases, which ultimately affects the mechanical performance of the finished part. Since none of the existing modeling techniques can accurately predict fiber damage of LFTs during injection molding, a new phenomenological approach for modeling fiber attrition is presented. First, multiple controlled studies employing a Couette rheometer are performed to determine correlations between processing conditions, material properties, and fiber length reduction. The results show shear stress and fiber concentration impact fiber damage. Based on these findings, a phenomenological model to predict breaking rate and unbreakable length of a fiber under giving conditions is developed. The model is based on the beam theory with distributed hydrodynamic stresses acting on a fiber. Fiber–fiber interactions are accounted for and correlated with the fiber volume fraction via a fitting parameter. The model tracks both the number-average and weight-average fiber length during processing, which can in turn be used to extract the fiber length distribution.

Highlights

The use of injection-molded long-fiber thermoplastic composites has been rapidly increasing in the past decade, mainly due to the market push for more fuel-efficient vehicles as well as electric vehicles
The results show shear stress and fiber concentration impact fiber damage
The main objective of this study is to develop a new model to predict fiber length during injection molding of long fiber-reinforced thermoplastics

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The use of injection-molded long-fiber thermoplastic composites (or LFTs) has been rapidly increasing in the past decade, mainly due to the market push for more fuel-efficient vehicles as well as electric vehicles. While the transportation industry constitutes 80% of LFTs’ world usage, their use is increasing in applications, such as durable consumer appliances, electronics, and sporting goods.[1,2] Since their advantage is the high aspect ratio of the fibers, a primary concern for manufacturers is preserving the fibers’ length throughout the molding process.[3] Initial fiber length in LFT pellets typically ranges from 10 to 15 mm. Fiber length measurements suggested a remaining average fiber length in the molded part in the 1–3 mm range.[4,5,6,7,8]. Bailey and Kraft found that most damage occurs during plastication, as did Lafranche et al and others.[4,5,6,7] Most notably, they found higher fiber lengths in the core region than the skin region in molded parts, observed in other studies, which suggests that the characteristic flow regime during mold filling causes uneven fiber breakage.[5,9] These findings go hand in hand with the recent findings on the inhomogeneous fiber density distribution in injection molding of LFT.[10]

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