Chopper Spray Gun Product

Overview

The chopped-strand spray gun is a hand-held or robotic composite application tool that integrates fiber chopping, resin spraying, and catalyst application in a single pass. Continuous roving fiber (from a large creel) feeds into the gun, where a high-speed rotating cutter head chops the fiber into 25–50 mm fragments. Simultaneously, catalyzed resin sprays through an atomizing nozzle, coating the chopped fiber. The fiber-resin mixture is deposited on a mold surface, building up layers at rates 2–10 kg/m²/min.

This process is the industry standard for rapid, labor-efficient composite construction of large, relatively simple shapes: boat hulls (marine), storage tanks, pipe, air ducts, bathtubs, and shower enclosures. Unlike filament winding (rotational symmetry) or pultrusion (linear), spray-up accommodates complex, non-symmetric, curved surfaces.

The primary trade-off: Spray-up creates a loose, porous laminate (fiber volume fraction 25–35%, often poor fiber wetting) compared to prepreg (55–65% fiber) or RTM (45–60% fiber). To improve properties, the layup is compacted via roller or vacuum bag post-spray. Environmental concerns (styrene vapor, noise) have driven adoption of robotic (enclosed) spray-up and lower-styrene vinyl-ester resins.

Roving Feed & Chopping

The gun pulls continuous roving from a large Roving Feed Guide creel (often mounted directly on the spray gun or on a separate stand). The roving passes through a Guide Tube (ceramic or PTFE liner, 2–5 mm bore) maintaining controlled tension via a Tension Wheel brake.

At the gun head, the roving enters the Roving Chopper Head, a high-speed rotating cutting mechanism (1000–3000 RPM) consisting of two oppositely-rotating or counter-rotating blade wheels (male and female profile, scissors action). The blades shear the roving into 25–50 mm fragments—length is adjustable by changing blade spacing.

Chopper physics:

• Blade speed relationship: At 3000 RPM and ~100 mm diameter chopper wheel, surface speed ≈ 10 m/s. A roving fiber advancing at 5 m/s through the chopper is sheared at a relative velocity of 5 m/s (reasonably efficient).
• Fiber damage: Dull blades or high-speed mismatch between roving and blade velocity causes fiber shredding (breaking fibers into <10 mm fragments, reducing strength).

The chopped fiber exits via a Fiber Chute chute that accelerates fragments toward the spray zone, mixing them with resin before impact on the mold surface.

Resin Atomization & Spray

The Resin Spray Nozzle is an HVLP (high volume, low pressure) or airless electric spray atomizer delivering catalyzed resin at 0.5–2 L/min:

1. HVLP spray: Air compressor (80–100 PSI) powers a Spray Pump diaphragm pump (1–3 GPM output), pressurizing resin to 2–3 bar. The Nozzle Tip (0.5–2.0 mm orifice, carbide or hardened steel) meters resin flow, and an Air Cap with atomizing air streams (4–6 small vents) breaks the resin into a fine mist (droplet size 100–200 microns).

2. Airless electric pump: Electric Spray Motor (1–2 kW) drives a piston pump pressurizing resin to 20–40 bar, atomizing directly at the nozzle without atomizing air. Airless is more efficient (higher resin transfer, less overspray) but noisier and requires electric power.

A Spray Needle metering needle controls flow rate (0–100% proportional), allowing operators to adjust deposition rate in real-time. The spray pattern (flat fan or round cone) is set by the Air Cap.

Catalyst Application

A separate Catalyst Supply applies liquid catalyst (MEKP for polyester, amine for epoxy) via a fine mist nozzle (0.3 mm orifice), proportional to resin flow. This dual-spray approach allows operators to:

1. Control gelation time: More catalyst = faster gelation (5–10 min), less catalyst = slower (30–60 min).
2. Compensate for temperature: In cold shops, additional catalyst accelerates cure; in warm shops, reduce catalyst to prevent gelation in the gun.

The Catalyst Pump is a small diaphragm pump (0.1–0.3 GPM); the Catalyst Valve proportional solenoid maintains a constant ratio of catalyst to resin (e.g., 2% catalyst by volume).

Air Supply

The Air Supply & Motor provides compressed air at 80–100 PSI for:

1. Chopper motor: Pneumatic turbine (1–2 kW, 1000–3000 RPM) consuming 50–100 CFM at full speed.
2. Spray atomization: Additional 50–100 CFM for air cap atomizing pressure.

Total air demand: 100–200 CFM (2.8–5.7 m³/min) at 5.5–7 bar.

A shop air compressor (3–5 kW rotary screw or reciprocating) supplies the line, with a Air Regulator proportional reducing valve and Moisture Trap removing water (critical to prevent moisture contamination of resin, which causes foaming and voids).

A Silencer muffler attaches to the gun exhaust, reducing noise from 95+ dB (chopper at 3000 RPM) to 85–90 dB (still loud, requiring hearing protection).

Trigger & Control

The operator holds the Trigger Handle (ergonomic aluminum or composite grip, weight 3–5 kg) and activates via a Trigger Lever mechanical trigger. The trigger proportionally activates a solenoid valve that:

1. Turns on the Spray Pump.
2. Engages the chopper motor pneumatic valve.
3. Meters catalyst flow via Catalyst Valve.

The Flow Control (external screw adjuster) limits maximum spray flow (0–100%), allowing operators to preset safe deposition rates. A Spring Balancer optional pneumatic cylinder assists operator arm weight, reducing fatigue during extended spraying sessions.

Spray-Up Process & Technique

Hand-spray workflow (typical boat hull):

1. Mold preparation: Release agent applied, mold surface cleaned.
2. Gel coat (optional): Thin polyester or epoxy layer (0.5–1 mm) sprayed first for finish and UV protection.
3. Structural layers: Operator holds gun 200–300 mm from mold, perpendicular to surface (critical for proper fiber/resin deposition). Trigger activated, moving gun in overlapping passes, building layers 2–5 mm per pass.
4. Curing: Room temperature or oven, typically 2–8 hours to demoldable strength.
5. Compaction (optional): Roller or vacuum bag applied post-cure to consolidate fiber and improve properties (void content 25–35% → 15–20%).

Spray pattern: Typical gun produces 50–200 mm wide pattern at 300 mm standoff distance. A 5 m long boat hull, 1 m wide, requires multiple overlapping passes. Deposition rate 5 kg/m²/min means 5–10 minutes per structural layer (20–50 kg/m² final).

Fiber & Resin Mixing Quality

The quality of fiber-resin mixing directly affects final properties:

Good spray-up (wet fiber, homogeneous):

• Fiber volume 30–35%, resin well-distributed.
• Tensile strength ~50–100 MPa (polyester).

Poor spray-up (dry fiber pockets, resin pools):

• Fiber volume 25–28%, voids 8–12%.
• Tensile strength ~30–60 MPa (20–40% reduction).

Key controls:

• Gun distance: Too close (100 mm) = resin-rich pools and overspray; too far (500 mm) = dry fiber and low deposition rate.
• Spray angle: Perpendicular (90°) is best; angled spray creates uneven mixing.
• Speed: Too fast = dry fiber, high voids; too slow = resin-rich, heavy, costly.
• Roving tex: Fine roving (200–400 tex) requires fast chopper speed (3000 RPM) and slow gun speed to avoid bundling; coarse roving (2400 tex) is more forgiving.

Environmental & Safety Concerns

Styrene emissions: Polyester spray-up releases styrene vapor (10–20% of resin by weight), a volatile organic compound (VOC). Open-air spray-up exposes operators to 50–200 ppm styrene (permissible exposure limit is 50 ppm in many jurisdictions).

Mitigation:

• Local exhaust ventilation: Spray booth with capture hood and exhaust ductwork.
• Lower-styrene resins: Vinyl-ester (5% styrene) or epoxy (0% styrene) reduce emissions by 50–75%.
• Robotic spray: Enclosed system with integrated exhaust containment (capital cost $200k+, but enables large production).

Hearing: Chopper noise 95+ dB requires hearing protection (earplugs or earmuffs, -20 dB attenuation).

Fiber exposure: Loose glass fibers inhaled during spray can cause respiratory irritation. Dust masks (P100 or organic vapor cartridge) recommended.

Spray-Up vs. Other Processes

Advantages:

• Flexibility: Any shape, complex curves, inserts easily integrated.
• Speed: 10–50 parts/day with single operator and gun.
• Labor-efficient: Minimal skill training vs. hand layup.
• Mold cost: Simple open molds, reusable indefinitely.

Disadvantages:

• Quality: Lower fiber content (25–35%), higher void content (8–12%) vs. prepreg or RTM.
• Emissions: Styrene vapor release requires ventilation infrastructure.
• Surface: Only one side molded; reverse side requires post-finishing.
• Waste: 10–20% overspray and trim waste.

Typical economics:

• Material cost: Polyester resin $5–8/kg, glass roving $8–12/kg, net ~$13–20/kg of composite.
• Labor: 1–2 operators/gun, $20–25/hour, ~$5–10/kg sprayed.
• Finished part: Small boat hull or tank, $30–60/kg selling price (material + labor + overhead + margin).

Large marine composites (boat manufacturer Bénéteau, SeaCraft) employ 20–50 spray guns in dedicated facilities, producing hundreds of hulls annually. Automotive (Class B panels, interiors) and industrial (tanks, ducts, pipe) rely on spray-up for economic mid-volume production.