Process Induced Defects in Liquid Molding Processes of Composites

Abstract

Liquid Composite Molding (LCM) processes are cost efficient manufacturing alternatives to traditional autoclave technology for producing near-net shape structural composite parts. However, process induced defects often limit wider usage of LCM in structural applications. Thorough knowledge of these defects, as well as their formation mechanisms and prevention techniques, is essential in developing improved LCM processes. In this article, process induced defects in liquid molding processes of composites, categorized into preform, flow induced and cure induced defects, are reviewed. Preform defects are further presented as fiber misalignment and fiber undulation (waviness and wrinkling). The respective causes, detrimental effects, and possible prevention methods of these defects are presented. Thereafter, flow induced defects are classified as voids and dry spots. Dry spot formation mechanisms in LCM processes and available prevention techniques are summarized. In addition, void formation mechanisms, adverse effects on composite properties, and removal techniques are presented. Cure induced defects include microcracks, void growth and geometrical distortions (warpage and spring-in). Each of these defects are discussed along with their underlying causes as well as their control and reduction schemes.

Dynamic pressure control in VARTM: Rapid fabrication of laminates with high fiber volume fraction and improved dimensional uniformity

“…In conventional VARTM, the limited compaction pressure on the preform (i.e., at most 1 atm) causes lower fiber volume fraction compared with the processes that utilize higher compaction pressure (e.g., RTM and autoclave molding) . In addition, void content is generally higher in VARTM which might significantly degrade the mechanical properties and environmental durability of composites . Applying cyclic debulking to enhance nesting of fibers prior to infusion may help increase the fiber volume fraction by decreasing the layup thickness .…”

Mejora en el comportamiento a la fatiga de compuestos nanoestructurados bidireccionales de fibra de vidrio y poliuretano bajo cargas uniaxiales y altas deformaciones

“…Otro resultado que debe tenerse en cuenta tiene que ver con el hecho de que las placas fabricadas por PUR 2 % NS presentaron un V f 5% superior a las placas fabricadas sin NS. Esta pequeña variación está asociada al efecto que tiene la NS en el tiempo de gel y la etapa de compactación [5] durante el proceso de infusión. Como se ha determinado anteriormente [24], el agregado de NS aumenta el tiempo de gel, indicando que habrá más tiempo para que la etapa de compactación del proceso de infusión permita obtener un espesor menor, aumentando ligeramente el V f [22].…”

“…Sin embargo, este gradiente fomenta también la formación in-situ de burbujas de aire, dado que presiones inferiores a la atmosférica inducen el desgasado de la resina. Cabe destacar que esta formación de burbujas de aire ocurre principalmente en el frente de infusión [5]. Por el bajo espesor del frente de infusión, del orden de los milímetros, el avance del frente permite un desgasado in-situ de la resina, evitando la formación de poros y mejorando la resistencia a la fatiga del material compuesto.…”

Mechanisms of Origin and Classification of Out-of-Plane Fiber Waviness in Composite Materials—A Review

“…Hallander et al [28] stated that materials with lower inter-ply friction are also more sensitive to fiber wash-out. In infiltration processes, especially when using higher pressures such as in resin transfer molding (RTM), the injected resin can force the fibers to be locally "washed" (Figure 24), mainly occurring at the injection port, and also wholescale movements of plies leading to wavy regions and resin rich zones [1,132]. Due to the compaction of the layers in thickness direction by double-sided RTM molds, the layers are more prone to in-plane waviness.…”