Conference Paper

Surface Porosity in Out-of-Autoclave Prepreg Processing: Causes and Reduction Strategies

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Abstract

Composite laminates cured out-of-autoclave (OOA) using vacuum-bag-only (VBO) prepregs can sometimes exhibit significant surface porosity on the tool-side surface when using only a liquid release agent. This porosity may be eliminated by using a release film or a resin-rich surfacing ply between the tool and part during cure, or by post-cure operations such as gel-coating and painting. Because such solutions can add significant time and cost to the overall manufacturing process, new strategies to inhibit surface porosity need to be developed. However, the mechanisms of void formation and tool-part interactions that lead to such surface porosity are not fully understood. The following paper presents a parametric study that clarifies the underlying causes of surface porosity in OOA-VBO carbon fiber-epoxy laminates and determines the relative importance of key manufacturing parameters, including tool plate surface topology, release methodology, prepreg material, and room-temperature vacuum hold time. Laminates are manufactured under various conditions using traditional OOA-VBO protocols. Then, optical microscopy and image analysis are used to characterize the surface porosity in terms of pattern and area. Thus, these parameters are related to expected part quality.

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... Recent studies have suggested that surface porosity results from air that becomes trapped between the prepreg and tool plate during layup [6] [7] [8]. In a previous study, we reported that composite laminates made from woven prepreg exhibit more surface porosity than laminates made from unidirectional (UD) prepreg, an observation attributed to the initial morphology of the prepreg [6]. ...
... Recent studies have suggested that surface porosity results from air that becomes trapped between the prepreg and tool plate during layup [6] [7] [8]. In a previous study, we reported that composite laminates made from woven prepreg exhibit more surface porosity than laminates made from unidirectional (UD) prepreg, an observation attributed to the initial morphology of the prepreg [6]. Air entrapment is less likely with UD prepregs because of the more uniform surface topography. ...
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In this study, we employ a parametric approach coupled with surface analysis to identify the source(s) of surface porosity and to develop effective mitigation strategies. Results confirmed that surface porosity was primarily associated with air that was trapped at the tool-prepreg interface during layup. The magnitude and distribution of surface porosity was affected by multiple parameters, including vacuum hold time, freezer and out time, and material and process modifications that affect air evacuation. The results indicated that prepreg out time (and thus tack) and vacuum quality were the primary drivers of surface porosity; for example, surface porosity decreased by 83% after just four days of out time and by 99% after 14 days of out time. These factors were used to formulate guidelines to mitigate surface porosity by (a) increasing the driving force for air evacuation and/or (b) increasing the permeability of the tool-prepreg interface.
... In contrast, UD prepregs typically exhibit a smooth surface (12.7 μm). This is attributed to different fiber architectures of the woven and UD prepregs [316]. The weaving patterns of woven fabrics are such that there are regions (overlaps and underlaps) for easy air entrapment; this phenomenon has been studied using XCT [240]. ...
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... Furthermore, it has been proposed that air entrapped between the first ply and the tool during layup is a major contributor to the issue of surface porosity in cured laminates. 7 Surface porosity or pitting may not have a large structural effect, but acceptance criteria tend to be strict regarding this issue, as they compromise the aesthetic appearance of components. As a result, rework and finishing operations are often required to improve appearance. ...
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