When specifying a carbon fibre laminate for structural or precision applications, three factors are closely linked and critically important: fibre volume fraction (Vf), wet-out quality, and void content. Together, these determine not only mechanical performance, but also dimensional stability, machining behaviour, and cut-edge quality.
Understanding how these factors interact helps engineers make informed decisions when comparing materials and manufacturing processes — particularly for applications where consistency, precision, and repeatability matter.
What Is Fibre Volume Fraction (Vf)?
Fibre volume fraction (Vf) is the proportion of fibre within a composite laminate, expressed as a percentage of the total laminate volume. In carbon fibre reinforced polymers, the fibres carry most of the load, while the resin binds the fibres together and transfers shear between them.
- Higher Vf means more load-bearing fibre and less excess resin
- This typically results in higher stiffness and strength for a given thickness
- Lower Vf usually indicates excess resin, increasing weight without proportional mechanical benefit
What Does “Wet-Out” Mean?
Wet-out describes how completely resin penetrates and saturates the carbon fibre reinforcement. Proper wet-out means resin has fully impregnated both the fibre bundles and the spaces between them, leaving no dry fibre regions.- Consistent mechanical properties
- Reliable interlaminar shear strength
- Improved fatigue resistance
- Predictable CNC machining behaviour, particularly at edges and thin features
Voids in Composite Laminates — Why They Matter
Voids are trapped air pockets or regions of missing resin within a laminate. Even low levels of void content can have a measurable impact on performance, durability, and consistency.
- Act as stress concentrators
- Reduce compressive and interlaminar strength
- Lower fatigue life
- Increase part-to-part variability
From a manufacturing perspective, voids are often a sign of incomplete impregnation, poor consolidation, or process instability.
Reference: Hsiao, K.T. et al., “Void formation in resin transfer molded composites,” Composites Part A.
Vf, Wet-Out, and Voids Must Be Considered Together
These three factors are tightly linked:
- Increasing Vf without sufficient resin mobility can compromise wet-out
- Improving impregnation without consolidation control can reduce Vf
- Poor air removal leads to voids regardless of nominal Vf
A high-quality structural laminate achieves high and repeatable Vf, complete wet-out, and void-free consolidation simultaneously.
How Common Composite Processes Compare
Hand Layup / Wet Layup
- Simple tooling and flexible geometries
- Highly operator-dependent
- Greater variability in Vf and void content
Resin Infusion / VARTM
- Improved control compared to wet layup
- Wet-out challenges in dense fabrics or thick sections
- Sensitive to flow design and permeability
Prepreg + Autoclave
- Excellent consolidation and mechanical performance
- Higher material and processing costs
- Limited by autoclave size and cycle time
Compression / Closed-Mould Processes
- Good repeatability for suitable formats
- Constraints on geometry and laminate architecture
Reference: Campbell, F.C., Manufacturing Processes for Advanced Composites, Elsevier.
How α-FAB Addresses These Trade-Offs
α-FAB is Juno Composites’ proprietary liquid moulding technology, developed to deliver laminates that are:
- Fully wetted through the fibre architecture
- Void-free
- Manufactured at a high, repeatable fibre volume fraction
By tightly controlling resin delivery, impregnation, and consolidation, α-FAB avoids the traditional trade-offs between wet-out, void control, Vf, and productivity.
Voids, Edge Quality, and Imaging-Critical Applications
In applications where carbon fibre components are cut, machined, or sectioned, void content has a direct impact on edge quality and functional performance.
- Edge chipping or micro-breakout
- Fibre pull-out at laminate boundaries
- Localised resin loss
X-Ray, Imaging, and Inspection Applications
In X-ray imaging fixtures, radiographic inspection tooling, and metrology components, voids and edge defects can:
- Scatter or attenuate imaging signals
- Introduce artefacts or contrast variations
- Reduce image clarity and repeatability
A void-free laminate with complete wet-out supports clean cut edges and uniform material density at the boundary.
Reference: ASM Handbook, Volume 21: Composites, ASM International.
Typical Fibre Volume Fraction in Our Products
For our 3k 2×2 twill orthogonal carbon fibre sheets & plates, laminates are manufactured to a typical fibre volume fraction (Vf) of 56%. Where 12k 2×2 twill orthogonal carbon fibre sheets are 60%. Individual product pages list Vf where applicable.