FRP Spray-Up and Chopper Gun Setup: Catalyst Ratio, Glass Content and Compaction

The chopper gun is the workhorse of open-mould FRP production: it turns continuous roving and catalysed resin into a finished laminate in a fraction of the labour time of hand lay-up, which is why bathtubs, tank shells, pool linings and boat hulls still come off spray-up lines by the thousand. It is also the process where quality is easiest to lose, because everything — glass content, catalyst ratio, thickness, voids — depends on equipment calibration and operator technique rather than on a pre-cut material. This guide walks through the setup that a production shop should verify every shift: metering and catalyst ratio, chop length and glass content, pass thickness and exotherm control, roll-out technique, and the QC checks that catch a drifting gun before it ships bad parts.

1. How Spray-Up Works and Where It Wins

In spray-up, a chopper gun simultaneously cuts continuous glass roving into short strands (typically 25–50 mm) and sprays them mixed with catalysed resin onto an open mould. The operator then compacts the deposit with a fin or mohair roller, driving out entrapped air and working resin into the fibre. Deposition rates of up to about 5 kg/min of composite make it dramatically faster than hand lay-up for large, simply-shaped parts, and no reinforcement cutting pattern is needed. The trade-offs are equally clear: random fibre orientation and low glass content cap mechanical properties, overspray wastes 20–30% of material, and consistency lives or dies on operator skill. Spray-up is the right process for large-area parts of moderate strength demand; it is the wrong process when you need the directional strength of fabric, tight tolerances, or high fibre fractions.

2. The Resin System: Promoters, Catalysts and Pot Life

Production spray-up uses a pre-promoted resin drum: accelerator and promoter are mixed into the 55-gallon drum with an air-driven bunghole mixer (collapsing-propeller type), and a minimum of 25 minutes of mixing is needed for homogeneity. The catalyst — almost always MEKP — is metered at the gun. Typical working windows:

Parameter	Typical range	Consequence of being out of range
MEKP catalyst (vs resin weight)	1.0–2.0% (standard polyester and vinyl ester; to 2.5% in cold shops)	Low: tacky, under-cured laminate. High: gel before the pass is rolled; exotherm runaway in thick sections
Cobalt accelerator (in drum)	0.2–0.4% (e.g. 0.3% of 6% cobalt naphthenate for a spray-up vinyl ester)	Shifts gel time; never "fix" an over-catalysed mix with more cobalt
Working pot life after catalysis	10–20 min at shop temperature	Too short = rolling cannot keep up; too long = slow line and resin drain-off
Resin line condition	No moisture or oil in air or catalyst line	Contamination deactivates peroxide or poisons the cure

Temperature moves the whole curve: above ~25 °C reduce the dosage or switch to a lower-reactivity MEKP grade; below ~15 °C use a higher active-oxygen grade or additional promoter. Two-part systems (prepromoted resin in one pot, peroxide-loaded resin in the other) exist for vinyl esters where the drum's shelf life at room temperature is limited to a few weeks — mix only what will be used within about 4 hours.

3. Calibration: Measuring the Ratio Instead of Guessing It

The single most valuable habit in spray-up production is a timed, weighed calibration at the start of every shift. The standard procedure:

• Prime steady-state. Flush and prime the resin lines with the catalysed formulation; discard the first seconds of output — they are not representative.
• Weigh resin output. With the chopper disabled, collect resin for exactly 30 s into a tared container; repeat three times and average.
• Weigh catalyst output over the same period and compute catalyst as a percentage of (resin + catalyst); adjust the metering control if outside the resin supplier's range.
• Weigh glass output. With resin disabled, collect chopped glass for 30 s — weigh immediately, chopped glass picks up ambient moisture fast.
• Compute glass content: glass% = glass ÷ (glass + resin) × 100. Example: 115 g glass and 385 g resin gives 23% — well below a typical 28% target.
• Adjust one variable at a time (resin pump output or roving feed), re-collect, and recheck until within tolerance — typically ±2 percentage points of the laminate schedule target.

Record pump setting, catalyst control position, roving feed, air pressures and chop length on an equipment settings card kept with the gun, dated and re-verified at each calibration. When a laminate dispute arises weeks later, that card is the only defensible record.

4. Glass Content: Target Windows and Their Consequences

Spray-up laminates live in a narrow glass-content band. Structural spray-up specifies 25–35% glass by weight, and the process ceiling is about 35% — above that the chopped strand will not wet out under rolling. Standardized test plates confirm the window: ISO 1268-2 targets ~32 ± 4% glass by mass for chopped-roving spray-up plates.

Glass content (wt%)	Laminate density (g/cm³)	What it means
23	~1.28	Resin-rich; below structural target — flexural strength and stiffness suffer
25	1.303	Minimum for light-duty parts (linings, covers)
30	1.347	Standard structural spray-up target
32 ± 4	1.32–1.37	ISO 1268-2 test-plate window
36	~1.41	Over-saturated risk: dry strand, poor roll-out, voids

Density values above follow from the rule of mixtures with ρ(resin) = 1.12 and ρ(glass) = 2.55 g/cm³ — a handy shop check, because a weighed coupon's density immediately tells you whether glass content has drifted. For confirmation, burn-off to ISO 1172 (or ASTM D2584) on a coupon cut from the trimmed part gives the true glass and filler content.

5. Output Rate, Coverage and Thickness Control

With a typical three-strand gun and medium nozzle, expect resin output around 3.6 kg/min and glass around 1.8 kg/min — 5.4 kg/min of total deposit. Convert that into planning numbers using the laminate density at 30% glass (1.347 g/cm³):

Target wet thickness	Theoretical coverage rate	Practical note
1.5 mm	~160 m²/h	First pass / skin coat territory
2.0 mm	~120 m²/h	Standard per-pass build with roll-out
2.5 mm	~96 m²/h	Upper end for one pass on vertical faces
3.0 mm	~80 m²/h	Only on horizontal, well-drained moulds

These are theoretical ceilings: part shape, gun transit speed and overspray cut them substantially. In practice, operators control thickness by spraying-and-checking — wet-film gauge probes at intervals during the pass — rather than by timing alone, because mould geometry never matches the theoretical spray fan. Budget the waste too: spray-up typically carries a ~25% material waste factor versus ~15% for hand lay-up, so a part needing 100 kg of net laminate consumes about 125 kg of purchased material.

6. Pass Thickness and Exotherm Control

The cure reaction is exothermic, and heat builds with thickness. For parts over ~3 mm, spray in stages: a first pass of 1–2 mm, subsequent passes of 2–3 mm, each pass applied only after the previous one has substantially gelled — this prevents the peak exotherm of a thick, unvented mass from burning (thermal discolouration, micro-cracking) or distorting the mould. Cumulative heat scales directly with laminate mass: at 30% glass, a 1.5 mm first pass stores about 3 kJ/m² per kelvin of adiabatic rise, a two-pass 4 mm build about 9 kJ/m² per kelvin, and a three-pass 7 mm laminate about 15 kJ/m² per kelvin — which is exactly why the schedule matters more than the total thickness alone.

• Never bury a fresh pass inside an already-hot laminate; let the previous pass dissipate first.
• Cold moulds slow cure and invite under-cure at the interface; warm resin with a long pot life is the fix, not more catalyst.
• Watch for the classic error: increasing cobalt to "speed things up" on a thick part — it shortens gel time and raises the peak exotherm simultaneously, the exact combination that burns laminates.

7. Spray Pattern, Fan Overlap and Operator Technique

Good spray-up is a geometrical discipline. The resin fan should be slightly wider than the glass fan so every strand lands wet; nozzle selection sets the resin fan angle (typically 25°–60°, e.g. 30° and 40° for common SS nozzle types), and the glass fan is matched by adjusting the chopper height relative to the nozzles. The operator works top-down and left-right in steady passes, overlapping each pass by about one-third of the previous pass width to give uniform thickness.

Thickness uniformity is a function of gun distance, transit speed and overlap — not of "spraying more where it looks thin". Chasing thin spots after the fact produces resin-rich islands that shrink and print through the gel coat.

• Keep gun-to-mould distance constant (the equipment guide distance the calibration card records); stepping closer floods the fan with resin.
• Roll out every pass immediately, before gel; a pass that gels unrolled is a void-rich scrap risk.
• On vertical and overhead surfaces, reduce per-pass thickness and let tack carry the laminate between passes.
• Chop length check: spray a short burst onto card at working distance and measure several strands — 25–50 mm is the typical specification window.

8. Roll-Out and Void Content: Where Strength Is Actually Made

The sprayed stream is full of entrapped air; compaction is what turns it into a laminate. Serrated fin rollers force resin into the fibre bundles and push air to the surface; a subsequent mohair pass smooths the surface over the gel-coat interface. Unrolled laminates keep void contents that act as stress concentrators, blisters in immersed service, and porosity that wicks moisture. The QC proxy is simple: rolled spray-up at correct glass content reaches the laminate schedule's density; if weighed coupons come in light, voids (or low glass) are the cause.

Where properties matter more than open-mould economics allow, the same deposit can be upgraded: vacuum bagging the sprayed lay-up removes air and excess resin, raises glass content and halves the void problem — at a cost that only selective, high-strength areas usually justify.

9. Health, Safety and Shop Environment

Spray-up is the highest-emission open-mould process: atomized resin plus chopped fibre means overspray, styrene vapour, and airborne dust at trim. Non-negotiables:

• Styrene exposure control: the OSHA permissible exposure limit for styrene is 100 ppm (with a short-term allowance), while ACGIH recommends a far lower 10 ppm TWA — modern shops ventilate to the stricter number. Supplied-air or appropriate respirators per the site's exposure assessment.
• MEKP is a hostile oxidizer: dedicated pump-out lines, no contamination with accelerator (direct mixing of cobalt and peroxide reacts violently), separate spill kit, and eye/face protection at the gun.
• Overspray housekeeping: strip accumulated overspray from the booth regularly — cured overspray is a fire load and a dust source at breakdown.
• Static and storage: organic peroxides need controlled-temperature storage; rotate stock by active-oxygen certificate, not by date of arrival.

10. QC Checklist: Catching a Drifting Gun Before It Ships Parts

Build the shift routine around these checks, in this order:

• Before production: gel-time cup test on a fresh catalysed sample at shop temperature; line pressure and catalyst metering verification; fan-pattern card check for chop length and pattern uniformity.
• Every shift: 30-second weighed calibration of resin, catalyst and glass; record on the settings card; adjust only one variable at a time to return to the ±2-point glass window.
• During production: wet-film thickness checks per pass; visual roll-out verification (no gloss resin pools, no dry strand); pass-schedule adherence for thickness over 3 mm.
• After cure: Barcol hardness (ASTM D2583) against the resin supplier's cured baseline as the through-cure gate; coupon burn-off to ISO 1172 / ASTM D2584 for glass and filler content on a lot basis; density check as the void proxy.
• Records: settings card per gun, gel-time log, calibration weights, and coupon results — traceable to the shift and operator.

Run this routine and the chopper gun stops being the black box it is reputed to be: every laminate property the schedule promises — glass content within ±2 points, thickness within the pass plan, cure within the Barcol window — becomes a measured fact rather than an operator's opinion. For the process choices beyond open moulding, our comparisons of resin selection for FRP equipment and SMC/BMC compression moulding cover when to step up from spray-up to closed or higher-fibre processes.

Related Reading

Pultrusion surface defects: diagnosis and process windows · Filament winding machine commissioning · FRP composite waste and recycling in production