Pultrusion Defects: Diagnosis, Root Causes, and Corrective A

Last updated: September 2026

Pultrusion defects halt production, scrap thousands in material, and compromise structural integrity. Thermosetting resins cure once — irreversibly. We run pultrusion lines every day — supplying our own FRP profiles and GFRP rebar, and commissioning the YLJ machines we build for customers — so every defect in this guide is one our floor teams have diagnosed and corrected under production conditions. What follows is the systematic diagnosis and correction routine we use, covering the 12 most critical pultrusion defects with specific parameters for implementation on the production floor.

Quick Defect Diagnosis Guide

Identify the issue and jump directly to the solution:

Surface Defects:

• Liquid resin beads on surface → Section 3.1: Surface Droplets
• Fine cracks or crazing → Section 3.2: Surface Cracks
• Rounded elevations, hollow pockets → Section 3.3: Blisters/Bubbles
• Fibers protruding from surface → Section 3.4: Fiber Bloom
• Rough, scaly texture; material sticks to die → Section 3.5: Peeling/Scaling
• Uneven color, spots, streaks → Section 3.6: Color Variation

Internal Defects:

• Internal cavities (NDT required) → Section 4.1: Voids/Porosity
• Layer separation (NDT required) → Section 4.2: Delamination
• Soft, low hardness, flexible → Section 4.3: Incomplete Cure
• Wavy fibers, wrinkled mats → Section 4.4: Fiber Misalignment

Dimensional Defects:

• Profile curves or bows → Section 5: Warping/Bowing

Understanding Core Process Relationships

Before diagnosing defects, understand the three critical process relationships that determine quality.

The Pull Speed–Temperature–Cure Triangle

Faster pulling speed = less time in heated die = lower degree of cure

Higher die temperature = faster cure reaction = can support faster pulling

Target: >95% cure conversion at die exit

When one variable changes, another must compensate. Increasing line speed requires either raising temperature or reformulating the resin for faster cure.

Resin Viscosity and Fiber Wet-Out

High viscosity (>800 cP) → resin cannot penetrate fiber bundles → trapped air → voids

Low viscosity (200–400 cP) → complete fiber saturation → fewer voids

Control viscosity through: bath temperature (most immediate), filler content, resin formulation

Pull Force as Real-Time Diagnostic

The pulling force is the process “heartbeat”:

• Steady force = stable equilibrium
• Gradual increase (5–15% over hours) = die fouling, resin building on walls
• Sudden spike = fiber break, material blockage, setup problem
• Baseline increase >25% = chrome plating worn; schedule die maintenance

Monitor pull force continuously. Trend analysis reveals problems before visible defects appear.

Rapid Reference: Defect Diagnostic Matrix

Section 3: Surface Defects — Diagnosis and Repair

3.1 Surface Droplets and Resin Migration

Identification: Liquid or semi-cured resin beads appear on the profile surface, creating glossy droplets or a “sweating” appearance.

Root Cause Analysis: Incomplete cure before die exit. Three mechanisms converge:

1. Insufficient heat input relative to pulling speed — material exits die before reaching 95% cure; unreacted resin remains mobile
2. Low fiber content (<50%) creating resin-rich zones — polyester shrinkage of 5–8% by volume in resin-rich areas; gap width 0.1–0.5 mm forms between product and mold wall
3. Shrinkage gaps form between product and mold wall — uncured resin migrates through gaps and accumulates on surface as droplets

Immediate Corrective Actions:

1. Reduce pull speed by 20–30%
   Example: 2.0 ft/min → 1.5 ft/min
   Increases residence time in heated zone

2. Raise die temperature 5–10°C in curing zones
   Example: Zone 3: 170°C → 178°C
   Critical for thick profiles (>20 mm)

3. Emergency: Stop line 30–60 seconds, restart at reduced speed

Long-Term Prevention:

• Increase fiber volume fraction to 55–60%: add 2–4 rovings if design permits; reduces resin-rich zone volume
• Add low-shrinkage fillers: calcium carbonate 10–15% by weight; alumina trihydrate 15–20% by weight; reduces volumetric shrinkage from 8% to 3–4%
• For thick sections (>20 mm): reduce final die zone temperature 10–15°C to draw exothermic heat from profile core (e.g., Zone 4: 175°C → 162°C)

3.2 Surface Cracks and Crazing

Identification: Fine fissures or deep cracks on profile surface, typically perpendicular to pulling direction.

Root Cause Analysis: Excessive internal stress during cure from two mechanisms.

Thermal Stress:

• Surface temperature: 175–180°C (contact with heated die)
• Core temperature: 150–160°C (heated by exotherm only)
• Temperature differential: 15–25°C
• Differential thermal expansion creates tensile stress at surface
• Mathematical relationship: Stress (σ) ∝ E × α × ΔT

For thick profiles (>25 mm), gradient increases with thickness — surface cures and rigidifies first while the core continues expanding, causing surface cracks to relieve tensile stress.

Cure Shrinkage Stress:

• Polyester resin: 5–8% volumetric shrinkage during cure
• Resin-rich surface layer lacks fiber reinforcement
• Unconstrained shrinkage generates internal tensile stress exceeding material tensile strength

Immediate Corrective Actions:

1. Reduce pull speed by 30–40%
   Allows uniform heat penetration
   Example: 1.5 ft/min → 0.9 ft/min

2. Lower high-temperature initiator by 10–15%
   Reduces exothermic peak temperature
   Typical: t-butyl perbenzoate from 0.8% → 0.7%

3. Paradoxical approach (specific cases only):
   Increase die temperature 5–10°C
   Accelerates surface cure before internal stresses develop
   Use only when surface demonstrably under-cured

Long-Term Prevention:

Optimized multi-zone temperature profile (prevents cracks):

• Zone 1: 130°C (controlled gelation)
• Zone 2: 155°C (progressive cure)
• Zone 3: 175°C (cure completion)
• Zone 4: 160°C (thermal equilibration)

Additional measures:

• Surface veils: C-glass veil 15–30 g/m² reinforces resin-rich surface layer and distributes stress
• Low-shrinkage fillers: 15–25% by weight reduces volumetric contraction
• Post-cure annealing for thick sections: 4–8 hours at 120–130°C relieves residual stresses and completes polymerization to >98%

3.3 Blisters, Bubbles, and Surface Porosity

Identification: Rounded elevations beneath which hollow, gas-filled pockets exist. Diameter: <1 mm to several millimeters.

Root Cause Analysis: Trapped gases expand within curing resin.

Moisture vaporization (primary cause):

• Hygroscopic resins absorb 0.1–0.3% water during storage
• Calcium carbonate filler: 0.1–0.5% moisture; alumina trihydrate: 0.3–1.0% moisture
• Water vaporizes at 100°C; die operates at 130–180°C
• Steam pressure: 1 atm at 100°C → 7 atm at 170°C

Formation sequence: material enters die and begins heating → moisture vaporizes → surface cures first (contact with heated die) → hardened surface traps steam → pressure builds, forms blister cavity.

Immediate Corrective Actions:

1. Reduce pull speed by 25–35%
   Extends time before surface gelation
   Allows volatiles to escape
   Example: 1.8 ft/min → 1.2 ft/min

2. Increase die temperature 5–10°C
   Accelerates surface cure after volatiles escape

3. Increase reinforcement level
   Add 2–4 rovings if design permits

Long-Term Prevention:

• Resin storage: 18–25°C, <50% RH, sealed containers
• Filler pre-drying: 2–4 hours at 110°C; target <0.05% moisture; cool in desiccator before mixing
• Fiber storage: keep sealed until use; discard if exposed to high humidity >24 hours
• Resin degassing: 25–30 inHg vacuum for 15–20 minutes after mixing; visual confirmation of no rising bubbles
• Vacuum-assisted impregnation: 0.2–0.5 bar vacuum at impregnation bath; capital cost $15,000–$50,000; scrap reduction typically justifies investment in 6–12 months
• Surface veil: 25–50 g/m² continuous filament mat as outermost layer

3.4 Fiber Bloom and Exposed Fibers

Identification: Individual fibers or bundles protrude from surface. Rough, “hairy” texture. White sparkling appearance under light.

Root Cause Analysis: Insufficient resin coverage over reinforcement.

• Surface fiber volume >65% leaves insufficient resin to encapsulate outermost fibers
• Resin viscosity >800 cP prevents penetration of 2,000–4,000-filament glass fiber rovings
• Cured resin debris or worn chrome plating scrapes resin off profile as it passes through die

Immediate Corrective Actions:

1. Reduce fiber count if design permits
   Remove 1–2 rovings
   Increases surface resin availability

2. Lower resin viscosity
   Raise bath temperature 5–10°C
   Example: 25°C → 32°C
   Viscosity reduction: ~40% per 10°C

3. Emergency die clearing
   Stop 30–60 seconds, restart slowly
   Dislodges debris

Long-Term Prevention:

• Surface veil: C-glass veil 25–40 g/m², 5–10 mm wider than profile width, positioned as outermost layer
• Resin viscosity target: 300–500 cP at processing temperature; raise bath temperature 10–15°C or reduce filler loading 5–10%
• Die maintenance: weekly acetone/MEK clean; monthly copper sulfate test for chrome thickness; quarterly polishing; re-chrome annually when wear exceeds 0.1 mm (cost $3,000–$15,000; chrome life 50,000–200,000 feet for glass fiber)

3.5 Surface Peeling, Flaking, and Scaling

Identification: Surface material adheres to die walls, tears away producing rough, scaly finish. Sharp pull force increase. Can progress to complete mold blockage.

Root Cause Analysis: Premature resin gelation at die inlet.

• Die inlet temperature >140°C (polyester) initiates cure before material is moving through die
• Partially cured resin is tacky and adhesive
• Pulling force tears surface away, leaving scaled finish
• Release agent failure: insufficient zinc stearate (<1%) or degraded agent

Immediate Corrective Actions:

Counter-intuitive but effective:

1. Increase die inlet temperature 10–15°C
   Example: 135°C → 148°C
   Accelerates cure past tacky stage
   Material becomes hard, releases cleanly

2. Reduce line speed 20–30%
   Controlled cure progression

3. Stop-and-restart
   Stop 30–60 seconds
   Stuck material fully cures
   Often pulls through on restart

4. Increase low-temperature initiator 5–10%
   Promotes earlier, more controlled cure

Long-Term Prevention:

• Internal release agent: zinc stearate 2–3% by weight; thorough dispersion during mixing; consider carnauba wax 0.5–1% as supplement
• Die surface specification: roughness <0.4 μm Ra; chrome thickness minimum 25 μm; no pitting, scratches, or exposed steel
• Die inlet temperature control: install dedicated thermocouple at inlet; target 120–135°C for polyester; monitor actual temperature, not setpoint

3.6 Color Variation and Surface Discoloration

Identification: Non-uniform surface color: spots, streaks, patches, or gradual shifts along length.

Root Cause Analysis:

• “Hot spots” in die create localized overheating and differential cure rates
• Thermal degradation of organic pigments at >150–180°C
• Heavy pigments settle in resin bath (iron oxide density: 5.2 g/cm³ vs. polyester resin: 1.1 g/cm³)

Immediate Corrective Actions:

1. Verify die temperature uniformity
   Check all thermocouple readings
   Test heater element resistance

2. Increase resin bath agitation
   Manual stirring every 15–30 minutes
   Prevents pigment settling

3. Check for contamination
   Inspect bath for foreign material
   Verify no previous color mixing

Long-Term Prevention:

• Temperature control: quarterly thermocouple calibration to ±2°C using NIST-traceable standards; replace if drift >3°C
• Pigment selection: thermally stable inorganics — iron oxides (stable to 250°C), titanium dioxide (stable to 300°C), carbon black (stable to 400°C); avoid organic pigments that degrade at 150–180°C
• High-shear mixing: >3,000 rpm for 10–15 minutes; add pigment slowly; visual verification of no particles or streaks
• Continuous bath agitation: slow-speed paddle at 30–60 rpm; prevents settling without entraining air
• UV stabilizers for outdoor applications: UV absorbers 0.5–1.5% (benzotriazoles), HALS 0.5–1.0%, heat stabilizers 0.3–0.5%

Section 4: Internal Defects — Hidden Structural Threats

4.1 Internal Voids and Porosity

Identification: Small cavities throughout laminate, not visible externally. Detection requires ultrasonic C-scan, X-ray radiography, or microsection analysis (destructive).

Root Cause Analysis: Incomplete fiber wet-out during impregnation.

• Resin viscosity >800 cP cannot penetrate 2,000–4,000-filament glass fiber bundles
• Resin must flow into spaces ~10–20 μm wide; flow rate governed by Q = (ΔP × r⁴) / (8 × μ × L)
• Bath residence time of 10–30 seconds at typical pull speeds may be insufficient for complete wet-out

Performance Impact:

• Interlaminar shear strength: -7% per 1% void content
• Compressive strength: -30% at 5% voids
• Flexural strength: -10% per 1% void content
• Moisture absorption: 2–5× higher with voids; freeze-thaw and chemical attack pathways

Immediate Corrective Actions:

1. Reduce pull speed by 30–40%
   Example: 2.0 ft/min → 1.3 ft/min
   Increases impregnation bath residence time

2. Raise resin bath temperature 10–15°C
   Example: 25°C → 37°C
   Viscosity reduction: ~70% for 15°C increase
   Target viscosity: 300–500 cP

Long-Term Prevention:

• Vacuum-assisted impregnation: 0.2–0.5 bar vacuum; reduces voids from 4–5% to <1%; capital $15,000–$50,000; payback 6–12 months through scrap reduction
• Resin viscosity control: target 300–500 cP; monitor continuously with viscometer
• Preformer optimization: gradual compression over 300–500 mm; avoid sharp transitions; progressive pressure increase
• Quality specification: structural applications <2% void content; high-performance <1%; verify with ultrasonic C-scan or acid digestion (ASTM D2734)

4.2 Delamination (Interlaminar Separation)

Identification: Planar cracks parallel to reinforcement layers. Detection: ultrasonic pulse-echo, coin-tap acoustic testing, active thermography.

Root Cause Analysis:

• Contamination-induced: oil, grease, or moisture on fiber surfaces prevents resin adhesion and bond formation between layers; sources include handling, guide rollers, and airborne contaminants
• Thermal stress-induced: high exothermic peak (>200°C); glass fiber CTE 5×10⁻⁶/°C vs. polyester resin CTE 60×10⁻⁶/°C (12× mismatch) creates interfacial stress exceeding bond strength
• Insufficient resin between layers: dry spots where mats contact without resin; excessive preformer squeeze-out

Performance Impact:

• Compressive strength: -40 to -70%
• Compression-after-impact strength: -50 to -70%
• Eliminates load transfer between layers

Immediate Corrective Actions:

1. Stop production immediately

2. Investigate contamination sources:
   - Inspect fiber materials
   - Check rollers and guides for oil
   - Review handling procedures

3. Replace suspect materials
   - Do not attempt cleaning
   - Discard contaminated reinforcements

4. For thermal stress:
   - Reduce exothermic peak 10–20°C
   - Lower initiator concentration
   - Reduce pull speed for thick sections

Long-Term Prevention:

• Cleanliness protocols: air filtration MERV 8–11 minimum; clean cotton or nitrile gloves; keep materials covered; clean guide rollers weekly
• Complete fiber wet-out verification: periodic cross-section microscopy at 50–100× magnification; document with photographs
• Thermal stress management: gradual temperature profiles; target exotherm <200°C; alumina trihydrate 20–30% as heat sink filler
• Critical applications: 100% ultrasonic inspection; reject if delamination >25 mm diameter

4.3 Incomplete Cure

Identification: Dull, non-glossy surface; low Barcol hardness (<35 for polyester); flexibility when should be rigid; tacky areas; poor heat resistance.

Root Cause Analysis:

• Heat penetrates by conduction at a rate proportional to 1/thickness²; center of a 25 mm profile heats 6× slower than a 10 mm profile
• Polyester requires 150–180°C for practical production rates; cure time doubles for every 10°C reduction
• Insufficient initiator (<0.5% total), wrong initiator for temperature profile, or contamination with inhibitors

Performance Impact:

• Tensile strength: -30 to -40%
• Compressive strength: -40 to -50%
• Flexural modulus: -30 to -35%
• Glass transition temperature: fully cured polyester Tg = 100–120°C; under-cured Tg = 60–80°C

Immediate Corrective Actions:

1. Reduce pull speed by 40–50%
   Most effective immediate intervention
   Example: 1.5 ft/min → 0.8 ft/min

2. Increase die temperature 15–20°C across all zones
   Example: 165°C → 182°C

3. Verify resin formulation
   Check batch records
   Confirm catalyst addition
   Remake batch if suspect

Long-Term Prevention:

• Barcol hardness (field test): hourly sampling; target >40 for polyester, >45 for vinyl ester
• DSC analysis (definitive): daily production samples; degree of cure = (ΔH_total – ΔH_residual) / ΔH_total; target >95% (residual heat <5%); sample from core of thick sections
• Process targets: 95–98% cure at die exit; thick sections accept 90–92% then post-cure; never accept <85%
• Post-cure for critical applications: 4–8 hours at 120–140°C (polyester); achieves >98% cure; increases Tg by 10–20°C; relieves residual stresses

4.4 Fiber Misalignment and Wrinkles

Identification: Fibers deviate from parallel orientation. Mats exhibit folds or wrinkles. Visible in cross-section cuts.

Root Cause Analysis:

• Misaligned creels cause angled fiber feeding; cumulative small errors create significant deviation
• Tension variation: target <5% between rovings; actual often 10–20% without calibration; low-tension rovings sag and meander
• Sharp corners in preforming tools cause fiber bunching; excessive compression creates buckles

Performance Impact: FRP strength proportional to cos²θ (θ = misalignment angle):

• 5° misalignment: -15% tensile, -20 to -30% compressive strength
• 10° misalignment: -30% tensile, -40 to -50% compressive strength
• Wrinkles create 3–5× stress concentrations and preferential delamination sites

Immediate Corrective Actions:

Requires line shutdown:

1. Stop production

2. Systematic alignment check:
   - Verify creel rack alignment
   - Check guide plate positions
   - Inspect preformers for damage
   - Measure die entrance alignment

3. Tension balancing:
   - Measure each roving: 0.5–2.0 kg target
   - Adjust to ±5% variation

4. Pull test piece at reduced speed, cut cross-section, examine fiber placement

Long-Term Prevention:

• Laser alignment: full line alignment creels to puller; tolerance ±1 mm from centerline; quarterly verification
• Creel tensioning: monthly calibration; target 0.5–2.0 kg per roving; ±5% maximum variation
• Preformer design: smooth transitions with 25 mm minimum radius; gradual compression over 300–500 mm; no sharp edges; polished surfaces
• Setup documentation: photographic records, detailed drawings, setup checklist with sign-off, pull test piece before full production

Section 5: Dimensional Defects — Warping and Bowing

Identification: Profile curves, bows, or twists instead of remaining straight, making assembly impossible for many applications.

Root Cause Analysis:

Asymmetric thermal stress (primary cause): Profile exits die at 150–180°C and begins cooling. If one side cools faster, it shrinks faster, creating an internal bending moment. Sources of cooling asymmetry include air drafts, asymmetric fan placement, and non-uniform die exit temperatures.

Unbalanced reinforcement architecture: Glass fiber CTE is 5×10⁻⁶/°C versus polyester resin CTE of 60×10⁻⁶/°C (12× mismatch). During cool-down from 170°C to 25°C (145°C change), glass shrinks 0.07% while resin shrinks 0.87%. If reinforcement is not symmetric about the neutral axis, differential shrinkage creates permanent curvature.

Mechanical misalignment: Puller clamps misaligned with die exit apply a constant bending force to the hot, low-stiffness profile; misalignment as small as 2–3 mm causes visible bow.

Immediate Corrective Actions:

1. Implement controlled cooling
   Water bath: 30–40°C, 1–2 meters long
   OR air cooling ring with uniform 360° flow

2. Verify die temperature uniformity
   Check all zone readings
   Use thermal imaging if available

3. Eliminate air drafts
   Close doors near die exit
   Shield area from HVAC airflow

4. Check puller alignment
   Verify co-linear with die exit

Long-Term Prevention:

• Symmetric reinforcement design: mirror fiber placement about both axes; equal rovings in all quadrants; surface veils applied symmetrically
• For inherently asymmetric profiles (angle sections, C-channels): add reinforcement to resin-rich side; or post-production straightening fixtures
• Laser alignment: comprehensive system alignment, creels to puller; ±1 mm co-linearity; quarterly verification
• Support system: roller supports after puller at 1–2 metre spacing; height-adjustable; free-rolling
• Material selection: vinyl ester (5–6% shrinkage vs. 8% polyester); carbon fiber for precision applications (near-zero CTE)

ASTM D3917 tolerance standards:

• Straightness: ±0.125% of length; twist: ±1° per metre
• Precision applications: ±0.05% straightness; ±0.5° per metre twist

Frequently Asked Questions

How do I determine optimal pull speed for a new profile?

Start at 1 ft/min and increase incrementally. Pull test pieces at 1.0, 1.5, 2.0, and 2.5 ft/min. Test degree of cure at each speed using DSC and Barcol hardness as a quick field check. Test flexural strength at the most promising speeds, then plot cure versus speed and identify the maximum speed maintaining >95% cure. Apply a 10–15% safety factor for production variability. Thick sections (>20 mm) require 30–50% slower speeds than thin sections (<10 mm) due to the inverse-square relationship between section thickness and heat penetration rate.

What is the most effective way to reduce internal voids?

Vacuum-assisted impregnation delivers the biggest impact because it actively removes air from fiber bundles — passive methods rely only on resin flow. Enclosing the impregnation bath in a vacuum chamber at 0.2–0.5 bar can reduce void content from 4–5% to below 1%. Capital cost is $15,000–$50,000, with payback typically within 6–12 months through scrap reduction. If vacuum impregnation is not feasible, reduce resin viscosity to 300–400 cP by raising bath temperature 10–15°C, and reduce pull speed 30–40% to extend fiber residence time in the bath.

How do I extend die life when chrome is wearing faster than expected?

Chrome wear rate depends on fiber type (carbon is 3–5× more abrasive than glass), fiber volume fraction (65% causes approximately 2× wear versus 55%), and pull force level (force more than 15% above baseline indicates a problem generating excess heat and friction). To extend life: maximize internal release agent (zinc stearate 2–3%), maintain optimal cure to minimize die wall adhesion, and control fiber tension. For carbon fiber applications, consider diamond-like carbon (DLC) coating, which costs approximately 2× traditional chrome but delivers 3–5× longer service life. Re-chrome at 25% pull force increase baseline — the copper sulfate test maps wear distribution across the die cavity.

Can surface defects be repaired after production?

Limited cosmetic repair is possible, but structural defects cannot be corrected. Internal voids, delaminations, incomplete cure, fiber misalignment, and dimensional deviations beyond tolerance are permanent once the resin has cured. Minor surface roughness can be addressed with light sanding (320+ grit, removing less than 0.5 mm) with risk of exposing fibers if over-sanded. Surface coating with paint or gel coat hides blemishes but does not restore mechanical properties and adds 20–50% to cost. For structural applications, repair is not acceptable — defective material must be scrapped. The cost of prevention through process control is consistently lower than the combined cost of repair attempts and scrapped material.

How often should thermocouples be calibrated?

Quarterly calibration is the standard for production pultrusion lines. Use NIST-traceable reference standards and check at multiple temperatures — room temperature, 100°C, 150°C, and 180°C. Accept if deviation is within ±2°C; replace if deviation exceeds ±3°C; document all results. More frequent calibration is warranted for lines operating above 180°C, equipment subject to mechanical stress, or lines with a history of temperature-related defects. Calibrate immediately after any heater replacement. The cost of a thermocouple ($20–50) is negligible compared to the cost of producing defective material from inaccurate temperature control.

How do I prevent color inconsistency in architectural profiles?

Control four factors. First, use a high-shear mixer above 3,000 rpm for 10–15 minutes with pigment added slowly to prevent agglomeration. Second, install a slow-speed paddle agitator at 30–60 rpm in the resin bath to prevent settling of heavy pigments (iron oxide at 5.2 g/cm³ settles rapidly in polyester resin at 1.1 g/cm³). Third, calibrate thermocouples quarterly to ±2°C — hot spots cause differential cure rates and color shifts. Fourth, specify thermally stable inorganic pigments: iron oxides (stable to 250°C), titanium dioxide (300°C), carbon black (400°C). Organic pigments degrade at 150–180°C and should not be used in pultrusion applications.

Pull force increases gradually over an 8-hour shift. What is causing this?

Gradual force increase indicates die fouling — resin building on the die wall interior. A 5–10% increase represents minor fouling; above 15% is significant. Root causes are: die inlet temperature too high, causing resin to initiate cure before material is moving through the die (reduce inlet zone 5–10°C); insufficient internal release agent (increase zinc stearate to 2–3%); or worn chrome plating retaining deposits (schedule re-chroming). The immediate correction is a stop-and-restart: stop the line for 60–90 seconds to allow stuck material to fully cure, then restart — this often clears the blockage. If the problem persists, the die must be removed and cleaned, typically requiring 2–4 hours of downtime. Weekly die cleaning and monthly polishing prevent the fouling cycle from developing.

What degree of cure should be targeted at die exit versus after post-cure?

Target 95–98% cure at die exit for thin profiles (under 10 mm). For thick sections above 20 mm, 90–92% at die exit is acceptable provided a post-cure cycle is included — 4–8 hours at 120–140°C for polyester achieves above 98% cure, increases Tg by 10–20°C, and relieves residual stresses. Never accept below 85% at die exit — profiles at this cure level lack dimensional stability and will continue shrinking after production, causing warpage. Barcol hardness provides a rapid field check (target above 40 for polyester), but DSC analysis of core samples is the only method that accurately measures cure of thick sections, where the surface may read acceptable hardness while the core remains significantly under-cured.

How We Control These Defects on Our Own Lines

Because we manufacture both the profiles and the pultrusion machines that produce them, our floor routine for defect control touches tooling, material, and machine settings in one pass — the same discipline we hand over during commissioning of every line we ship:

• Die-zone temperature discipline. Multi-zone die heating is PLC-controlled and logged per zone; when surface defects appear, zone setpoints are the first variable we review — before touching line speed.
• Pull-speed ramping on profile change. New profiles start at reduced line speed and step up in increments while first-article sections are dimensionally checked at the cut-off saw.
• Resin bath monitoring. Bath temperature and viscosity are checked each shift; resin beyond its working life is discarded regardless of remaining volume — the cheapest defect prevention there is.
• Creel and guidance checks. Fiber tension is balanced across the creel before startup; a single over-tensioned package shows up as longitudinal defect patterns within minutes of running.
• Per-shift die inspection. Die lands are inspected and cleaned at shift change; anything scored or plated is pulled from service — a damaged die produces defects no parameter change can fix.

This is the practical difference between a machine builder and a machine builder who runs production lines of their own: when a defect appears, we can tell whether the cause lives in the tooling, the material system, or the machine — because we own all three.