Resin Feed Network Design and Infusion Front Control

Why Feed-Network Design Matters

Vacuum infusion pulls catalyzed resin through a dry fiber stack by creating a pressure differential between a resin feed line and a vacuum line. The goal is a single, even resin front that advances from the feed side to the vacuum side without leaving dry spots or channeling along unintended paths. Poor network design is the leading cause of infusion defects. Understanding how resin flows through reinforcement lets you arrange your feed and vacuum lines before a drop of resin is committed.

Darcy's Law and Flow-Front Basics

Resin movement through a fiber preform follows Darcy's Law: flow rate is proportional to permeability and pressure gradient, and inversely proportional to resin viscosity. You do not need to solve the equation by hand, but the concept tells you three practical things:

1. Higher permeability = faster flow. Flow media (a coarse mesh layer) has very high in-plane permeability and acts as a distribution highway. Biaxial glass fabric has much lower permeability and controls through-thickness fill.

2. Lower viscosity = faster flow. Infusion-grade vinyl ester is formulated for low viscosity. Mixing temperature, pot temperature, and the time elapsed after catalyzing all affect viscosity. Work within the resin supplier's recommended pot-life window.

3. Pressure gradient drives everything. The vacuum line is your low-pressure sink. Resin enters at atmospheric pressure at the feed port and travels toward vacuum. The front shape reflects the path of least resistance.

Common Feed-Line Patterns

Center-line feed / perimeter vacuum: One feed line runs along the centerline of a rectangular panel; vacuum lines run along both long edges. The front splits left and right symmetrically and travels roughly half the panel width to each vacuum line. Useful for wide, flat panels.

Perimeter feed / center vacuum: Feed line surrounds the part; vacuum line runs down the center. The front converges inward. Good for narrow panels or when a central stiffener is present.

Edge feed / opposite-edge vacuum: The simplest layout for a rectangle. Resin enters one short or long edge and exits the opposite edge. Race-tracking risk is highest at corners because resin reaches the bag edge first and can channel along it before filling the field.

Tapered panels and hull sections: As width changes, so does the distance from feed to vacuum. A single straight feed line on a tapered hull section will deliver resin faster to the wide end. Solutions include angling the feed line to follow the taper, using branch feeds of varying spacing, or segmenting the vacuum line to equalize arrival time.

Joggled steps: A thickness change interrupts permeability. Plan a secondary feed leg that crosses the step, or position a short flow-media patch over the step to bridge the gap.

Race-Tracking and How to Prevent It

Race-tracking occurs when resin finds a low-resistance channel — typically along a bag edge, a wrinkle, or an unsealed spiral-wrap end — and sprints ahead of the main front. The fast channel then blocks vacuum from reaching adjacent dry fiber. Prevention measures:

• Seal spiral-wrap ends with tacky tape so resin exits only at intended ports.
• Keep bag bridges tight against the part edge; bridge folds create channels.
• Extend peel ply to — but not past — the part boundary so no open fiber sits against the bag edge.
• For tapered geometries, stagger feed-line branches to slow resin advance at the wide end.

Vacuum-Line Placement Rules

• The vacuum line is always the last thing resin should reach. Place it at the point geometrically farthest from the feed, accounting for the actual flow path through the reinforcement.
• Use a resin trap (catch pot) between the part and the pump. Without a trap, resin entering the pump causes immediate pump failure and a safety hazard.
• Maintain vacuum at or above the level recommended by the resin supplier — typically 25 in Hg or higher for infusion-grade systems. Test the bag for leaks for a minimum of 15–30 minutes before introducing resin.

Monitoring Front Progression

Once resin enters the preform, watch the translucent wet front advance. Marks on the mold at measured intervals let you confirm the front is on schedule and symmetrical. If one side leads the other by more than your design tolerance, clamp the feed line serving the fast side partially to slow that branch. When resin first appears at the vacuum-line spiral wrap, close the resin feed valve to stop additional resin intake, then maintain vacuum through gelation to compact the laminate.

Safety Requirements for This Lab

PPE is mandatory throughout: nitrile gloves (double-glove for extended resin contact), safety glasses or chemical splash goggles, and an air-purifying respirator with organic-vapor cartridges rated for styrene/vinyl ester vapors. A disposable Tyvek suit protects skin and clothing.

Ventilation: Infusion-grade vinyl ester releases styrene vapor. Work only in the designated ventilated infusion area. Confirm exhaust ventilation is operating before opening any resin container.

MEKP handling: Methyl ethyl ketone peroxide is a strong oxidizer and can cause serious eye and skin injury. Measure it into a separate container before adding to resin — never add resin to the MEKP container. Store MEKP away from heat, open flame, and incompatible materials. Know the location of the eyewash station.

Spill response: Have absorbent material and a labeled waste container at the workstation. Catalyzed resin waste must be spread thin to dissipate exotherm before disposal per your facility's hazardous-waste procedure.

Electrical: The vacuum pump is a running motor. Keep resin and solvent away from the motor housing. If a resin trap overflows, shut off the pump before clearing it — this is a lockout/tagout situation if the pump requires service.