Pultrusion Process Engineering: Fiber Wet-Out, Die Heat & Pull Force Physics

Pultrusion is a continuous manufacturing process used to produce composite profiles (rods, tubes, structural beams, cable trays) with constant cross-sections. Unlike extrusion, where material is pushed through a die, pultrusion pulls continuous fibres through a resin impregnation bath, a preforming guide, and a heated steel die where curing occurs. It yields profiles with very high longitudinal strength ($V_f > 60\%$). Indian manufacturers like Kemrock Industries and EPP Composites utilize pultrusion to manufacture structural beams and electrical ladder networks.

• Describe the process elements of a pultrusion line (creels, bath, preformer, die, puller, cutter).
• Formulate the viscosity-dependent fiber wet-out kinetics in the resin bath.
• Analyze the thermal profile and cure kinetics of thermosets inside the heated pultrusion die.
• Calculate the total pulling force required based on viscous drag and friction parameters.
• Reference pultrusion design and testing standards such as ASTM D3917 and ISO 1268-9.

3.1 Pultrusion Process Line Elements

A pultrusion line operates continuously in the following sequence:

1. Fibre Creel: Stacks of continuous glass/carbon rovings or mats.
2. Resin Bath: Thermoset resin (polyester, vinyl ester, epoxy) containing catalysts, fillers, and internal release agents.
3. Preformer Guides: Steel plates that fold and shape the wet fibres into the target profile shape, squeezing out excess resin.
4. Heated Die: A chrome-plated steel die (typically 0.6–1.2 m long) heated to 130–160°C. The resin matrix cures as it passes through.
5. Pulling System: Reciprocating clamps or caterpillar pullers that pull the cured profile forward.
6. Cut-off Saw: Cuts profiles to target lengths.

3.2 Fiber Wet-Out Kinetics

Impregnation kinetics of the roving bundle by liquid resin is governed by capillary flow (Darcy's Law variant):

Where:

• $\mu$: Resin viscosity (must be kept low, $< 2.0$ Pa·s, to ensure rapid penetration).
• $R_{bundle}$: Radius of the fiber roving bundle.
• $K$: Preform permeability.
• $P_{cap}$: Capillary pressure driving penetration.

3.3 Die Heat and Curing Zones

The temperature profile along the die length ($z$) is divided into three zones:

1. Zone 1: Entry/Heating: Resin viscosity drops as it heats up, improving final wet-out.
2. Zone 2: Gelation: Polymerization reaction begins. Viscosity rises rapidly to the gel point. The reaction is highly exothermic; the local temperature often exceeds the die set temperature.
3. Zone 3: Solidification & Shrinkage: Curing reaches completion. The polymer shrinks slightly ($1-3\%$), separating from the die wall, which reduces sliding friction.

3.4 Pulling Force Physics

The total pulling force ($F_{pull}$) must overcome viscous drag, compaction forces, and sliding friction:

Where:

• $F_{viscous}$: Viscous resistance of uncured resin at the die entrance.
• $F_{compaction}$: Force to compress the fiber volume into the die profile.
• $f_s$: Friction coefficient between cured composite and chrome-plated die.
• $P_N(z)$: Normal contact pressure along die length (drops to near zero after shrinkage).
• $C$: Perimeter of the profile.

Problem: A pultrusion line produces a solid cylindrical composite rod of diameter $D = 20.0$ mm (perimeter $C = 62.8$ mm) at a line speed $v = 0.50$ m/min. The die length is $L = 1.0$ m. The average normal pressure of the composite against the die wall before shrinkage is $P_{avg} = 0.25$ MPa over the first 0.60 m of the die. After gelation and shrinkage, the normal pressure is negligible. The sliding friction coefficient is $f_s = 0.15$. Calculate the pulling force component ($F_{friction}$) required to overcome this sliding friction.

Solution:

1. Identify the contact area ($A_{contact}$) where normal pressure is active:

1. Calculate the total normal force ($F_N$):

1. Calculate the friction pulling force component $F_{friction}$:

Interpretation: The pulling system must exert at least 1,413 N of force just to overcome the sliding friction of the curing composite against the die walls. The motor and puller clamps must be rated higher to account for viscous drag and compaction resistance at the die entry.

Kemrock Industries & Export Limited (Halol, Gujarat) is an established composite manufacturer utilizing multi-cavity pultrusion lines to produce structural grating profiles and electrical cable trays. They use vinyl ester matrices to satisfy corrosion and electrical insulation specs.

Pultruded profiles are qualified under ASTM D3917 guidelines to check for surface defects (sluffing, blistering, scale) and verify fiber-to-resin ratios.

• Creel: Metal frame supporting multiple yarn balls or spools of reinforcement fiber.
• Gelation: Transition of resin from liquid to solid state during polymerization.
• Pull Force: Tension force required to draw the fibers and resin through the pultrusion line.
• Preformer: Die guides that pre-shape the fiber bundle before it enters the heated die.
• Sluffing: Resin buildup and peeling on the pultruded surface; a defect showing under-cure or poor release agent dosage.

1. ASTM D3917 — Standard Specification for Dimensional Tolerance of Thermosetting Glass-Reinforced Pultruded Shapes
2. ISO 1268-9 — Fibre-reinforced plastics — Methods of producing test plates — Part 9: Pultrusion moulding
3. ASTM D4385 — Standard Practice for Classifying Visual Defects in Thermosetting Reinforced Plastic Pultruded Products
4. IS 10192 — Bureau of Indian Standards (BIS) guidelines for fiber reinforced thermoset structures

1. Sketch the temperature profile and curing state of a thermoset resin as a function of distance inside a heated pultrusion die. Explain the role of the exotherm peak.
2. Discuss how resin viscosity behaves inside the pultrusion die. Why does it drop initially before rising exponentially?
3. How do internal release agents (e.g., zinc stearate) function in a pultrusion resin formulation, and how do they impact pull force?

Q1. Pultrusion differs from extrusion because the material is:

• B) Pulled through the die under tension rather than pushed

Q2. What happens to the normal pressure of the composite against the die wall once curing reaches high conversion?

• C) Decreases to near zero due to polymerization shrinkage

Q3. The viscosity of the resin in the impregnation bath must be kept low to ensure:

• A) Rapid and complete fiber wet-out (impregnation)

Q4. Which visual defect represents resin accumulation and peeling on the pultruded surface?

• C) Sluffing

Q5. Which Indian city is home to major pultrusion manufacturing clusters producing structural profiles?

• B) Halol