Manufacturing of Glass & Carbon Fibre Composites: Hand Lay-up, Spray-up & Spray Mechanics

Open moulding processes like hand lay-up and spray-up represent the foundation of the structural composites industry. They are used to fabricate large-scale structures such as boat hulls, wind turbine blade shells, storage tanks, and automotive body panels. Although liquid moulding (RTM) and autoclave prepreg curing offer higher fibre content, open moulding remains highly dominant due to its extremely low tooling costs and design flexibility. Indian manufacturers like Suzlon, Kemrock, and EPP Composites utilize these techniques extensively for industrial components.

• Describe the step-by-step procedure of hand lay-up and spray-up open moulding.
• Compare hand lay-up and spray-up in terms of fibre volume fraction ($V_f$), mechanical properties, and capital cost.
• Identify key defects in open moulding (air voids, resin-rich areas, incomplete wet-out) and their mitigation.
• Calculate the resin-to-fibre weight ratio and composite density for a target ply thickness.
• Reference composite manufacturing standards such as ISO 1268 and ASTM D2584.

3.1 Hand Lay-up Process Mechanics

Hand lay-up is a manual process where reinforcement (chopped strand mat, woven roving, or carbon fabric) is placed in a one-sided open mould and saturated with liquid resin (polyester, vinyl ester, or epoxy) containing catalysts and promoters.

• Steps: Apply release agent $\rightarrow$ apply gel coat (for surface finish) $\rightarrow$ lay reinforcement $\rightarrow$ apply resin $\rightarrow$ roll out with serrated rollers to remove entrapped air and compact the ply.
• Fibre Volume Fraction ($V_f$): Low, typically 20–35% due to manual rolling variation.

3.2 Spray-up Process Mechanics

Spray-up is a semi-automated process where continuous glass roving is fed into a chopper gun mounted on a spray system. The gun chops the fibres (typically 25–50 mm length) and sprays them simultaneously with catalyzed resin onto the mould surface. Manual rolling is still required to consolidate the mixture.

• Advantage: Higher deposition rate; ideal for large, complex-shaped moulds.
• Limitation: Isotropic properties only; lower mechanical properties due to short, discontinuous fibres ($V_f \approx 15-25\%$).

3.3 Comparison of Open Moulding Processes

3.4 Density of Composites by Weight Fractions

Given the weight fractions of fibre ($W_f$) and matrix ($W_m = 1 - W_f$):

Problem: A hand lay-up laminator prepares a glass/polyester composite panel. The reinforcement used is E-glass ($W_f = 35\%$, density $\rho_f = 2.54$ g/cm³) and the matrix is unsaturated polyester ($W_m = 65\%$, density $\rho_m = 1.20$ g/cm³). Calculate:

1. The overall density of the composite ($\rho_c$).
2. The fibre volume fraction ($V_f$).

Solution:

1. Calculate composite density $\rho_c$:

1. Calculate fibre volume fraction $V_f$ using the density relationship:

Interpretation: The resulting composite has a density of 1.472 g/cm³ and a fibre volume fraction of 20.3%. This low $V_f$ is typical for hand lay-up using chopped strand mats and manual rolling. To achieve higher performance, the laminator must increase rolling force or transition to vacuum bagging to squeeze out excess resin.

EPP Composites (Rajkot, Gujarat) is a leading manufacturer of composite chemical storage tanks and ducting. They use hand lay-up and spray-up to build corrosion-resistant linings on moulds before completing structural winding. gel coats are qualified under international corrosion standards.

In Indian boatyards and wind energy facilities, open moulding operations must comply with IS 14887 and state pollution control board norms regarding VOC (styrene) emissions, often requiring high-efficiency ventilation or transitioning to low-styrene resins.

• Gel Coat: A specially formulated pigmented polyester resin applied first to the mould to provide a smooth, glossy protective surface.
• Serrated Roller: Metal tool used in open moulding to compact the laminate and release entrapped air bubbles.
• VOC: Volatile Organic Compounds; in composites, mainly styrene monomer vaporised during open cure.
• Woven Roving: Coarse glass fabric providing high directional strength in hand lay-up laminates.

1. ISO 1268-2 — Fibre-reinforced plastics — Methods of producing test plates — Part 2: Contact moulding (hand lay-up)
2. ASTM D2584 — Standard Test Method for Ignition Loss of Cured Reinforced Resins (used to verify fibre weight fraction)
3. IS 14887 — Bureau of Indian Standards (BIS) code for glass fibre reinforced polyester resin systems
4. ASTM D5687 — Standard Guide for Preparation of Flat Composite Panels

1. Explain the mechanism of styrene crosslinking in unsaturated polyester resins. What role do methyl ethyl ketone peroxide (MEKP) and cobalt naphthenate play?
2. Why do composites manufactured by spray-up display inferior mechanical properties compared to those produced by hand lay-up with woven fabrics?
3. Calculate the weight of epoxy resin required to produce a 5 kg carbon fibre plate with a target fibre volume fraction of 40%. (Densities: carbon = 1.80 g/cm³, epoxy = 1.15 g/cm³).

Q1. What is the purpose of applying a gel coat to the mould surface in hand lay-up?

• C) To provide a durable, weather-resistant, and aesthetically pleasing cosmetic finish

Q2. Compared to spray-up, hand lay-up using woven roving reinforcement produces laminates with:

• A) Higher fibre volume fraction ($V_f$) and better tensile strength

Q3. Which defect is most likely to occur in open moulding if rolling is done insufficiently?

• B) Interlaminar air voids and dry spots

Q4. The typical fiber volume fraction ($V_f$) range for a manual hand lay-up composite is:

• C) 20% to 35%

Q5. Which standard method is used to determine the fiber-to-resin ratio by burning off the polymer matrix?

• B) ASTM D2584 (Ignition Loss)