Application Areas, Common Problems and Solutions of Pultrusion Process

 

The composite material pultrusion process is a process method for producing composite material profiles by impregnating continuous fibers or their fabrics with resin under the traction of traction equipment and heating the molding die to solidify the resin.

 

The pultrusion process has specific and obvious advantages. According to experts, the main advantages are: simple equipment, low cost, high productivity, easy to form an automated production line, and stable product quality; it can give full play to the role of reinforcing materials, high mechanical properties, especially longitudinal strength and modulus; high effective utilization rate of raw materials, basically no corner waste; the longitudinal and transverse strength of the profile can be adjusted to meet different usage requirements; its length can be cut as needed.

 

Pultruded epoxy FRP products are mainly used in the following 7 fields:

 

The electrical field is one of the development focuses and is currently the most widely used field, such as transformer air duct positioning rods, high-voltage insulator core rods, high-voltage cable protection tubes, cable racks, insulating ladders, insulating rods, electric poles, track guards, cable distribution racks, motor parts, etc.;

 

The chemical anti-corrosion field is the fastest growing field in recent years. Typical products include pipe network support structures, sucker rods, downhole pressure pipelines, wastewater treatment equipment, chemical baffles, railings, stairs, platform handrails, grille floors, etc. in chemical, petroleum, papermaking, metallurgical and other factories;

 

The building structure field is mainly used for light structures and upper structures of high-rise structures Structures or special-purpose structures, such as movable house structures, profiles for door and window structures, trusses, light bridges, railings, tent brackets, ceiling structures, large boron structures, etc.;

 

Sports and entertainment fields such as fishing rods, hockey sticks, skis, pole vaulting poles, bows and arrows, etc.;

 

Transportation fields such as car shelves, truck frames, refrigerated compartments, car spring plates, luggage racks, bumpers, decks, electric train track guards, etc.;

 

Energy fields are mainly used for solar collector brackets, wind turbine blades, oil well conduits, etc.;

 

Aerospace fields such as aircraft and spacecraft antenna insulation tubes, spacecraft motor parts, aircraft composite I-beams, trough beams and square beams, aircraft tie rods, connecting rods, etc.

 

 

 

Peeling

When particles of cured resin come out of the mold on the surface of the part, this phenomenon is called peeling or shedding.

 

Corrective Action:

Increase the temperature of the inlet feed end of the mold to cure the resin earlier. Reduce the line speed to allow the resin to cure earlier. Stop the line for cleaning (30 to 60 seconds). Increase the concentration of low temperature initiator.

 

Blistering

When blistering occurs on the surface of the part.

 

Corrective Action:

Increase the temperature of the inlet end mold to cure the resin faster Reduce the line speed, which has the same effect as above Increase the reinforcement level. Blistering is often caused by voids caused by low glass content.

 

Surface Cracks

Surface cracks are caused by excessive shrinkage.

 

Corrective Action:

Increase mold temperature to speed up cure Reduce line speed, which has the same effect as above Increase filler loading or glass content to increase toughness of the resin-rich surface, thereby reducing shrinkage,

 

Stress and Cracks

Increase the amount of low temperature initiator or use an initiator lower than the current temperature. Add surface liners or veils to the part

 

Internal Cracks

Internal cracks are usually associated with too thick a section and can occur in the center of the laminate or on the surface.

 

Corrective Actions:

Increase the temperature at the feed end to cure the resin earlier. Reduce the mold temperature at the end of the mold to act as a heat sink to reduce the peak of the exotherm. If the mold temperature cannot be changed, increase the line speed to reduce the temperature of the outer contour of the part and the peak of the exotherm to reduce any thermal stress. Reduce the initiator level, especially the high temperature initiator. This is the best permanent solution, but some experimentation is required. Replace the high temperature initiator with an initiator that has a lower exotherm but better curing effect.

 

Color Difference

Hot spots can cause uneven shrinkage, which can cause color difference (also known as color shift)

 

Corrective Action:

Check heaters to ensure they are in place so there are no temperature differences across the moldCheck resin mix to ensure fillers and/or pigments are not settling or separating (color difference)

 

Low Barcol Hardness

Low reading on Barcol hardness tester; due to incomplete cure

 

Corrective Action:

Reduce line speed to speed up cure of resinRaise mold temperature to increase cure rate and cure depth in moldCheck mix formulation for overplasticizationCheck for other contaminants such as water or pigments that can affect cure rateNote: Barcol hardness readings can only be used to compare cures using the same resin. They cannot be used to compare cures using different resins, as they are produced using their own specific glycols and have different depths of crosslinking.

 

Shrinkage

Irregular surface shape caused by excessive shrinkage

 

Corrective Action:

Add more glass to reduce shrinkage during cureAdd a shrink reducer (low shrinkage additive) or increase filler loading

 

Mold Blockage

Mold becomes blocked by glass, causing part to break out or the pull head cannot move the part

 

Corrective Action:

Lower mold inlet temperature; pre-cure at the die can cause blockagesAn acceptable level of pre-molding reduction reinforcement can be achieved by evenly passing the roving through each molding guide and without fiber breakage or twisting during molding. Only a limited number of fiber ends can be placed in a given location. If pulling multiple cavities, check to make sure all ends enter the correct mold. Check mold surface. Cuts in the mold can eventually cause part to break out. Such a condition may appear as a blockage, but actually result in part cracking. Excessively rough (worn) mold surfaces can cause the same problem.

 

Bend

Parts bend as they cool out of the mold

 

Corrective Action:

Check that a symmetrical reinforcement pattern is used, especially if a liner is used. Uneven glass distribution. Adjust the reinforcement strands to offset part bending. Check if a symmetrical thermal model is used; unbalanced heating results in uneven curing speeds, which can produce different shrinkage rates. Check part design. Some asymmetrical designs can cause bending. If this is the cause, a cooling device may be necessary.

 

Bubbles or pores

Bubbles or pores will appear on the surface.

 

Corrective Measures:

Check if excess water vapor and solvents are caused during mixing or due to improper heating. Water and solvents will boil and evaporate during the exothermic process, causing bubbles or pores on the surface. Reduce line speed, and/or increase mold temperature to better overcome this problem by increasing the hardness of the surface resin. Use a surface mask or surface felt. This will strengthen the surface resin and help eliminate bubbles or pores.

 

 

Common Defects and Improvement Measures for Pultruded Products

 

Defective Area	Phenomenon	Reason
Bird’s Nest	The reinforcing fibers entangled with each other at the mold inlet, causing the product to be damaged inside the mold.	The possible causes include fiber breakage, the effect of fiber drape, high resin viscosity, excessive resin adhesion to the fibers, excessively high pulling speed, and improper mold inlet design.
Unstable Curing	A sudden increase in adhesion inside the mold can cause the product to be damaged within the mold.	Excessively high pulling speed; sudden backflow of hot resin caused by pre-curing
Rough Appearance	Poor surface finish with scaling	Insufficient glass fiber content; excessively high stress in the curing zone, causing crawling/creep.
Mold Sticking	Some parts/products adhered to the mold, causing tensile failure of the product.	Low fiber volume content; insufficient filler loading; poor performance of the internal mold release agent or too little dosage
Not Fully Cured		Speed too high; temperature too low; mold too short; improper resin system selection.
White Powder	After demolding, a white powdery substance is attached to the surface of the product.	Poor surface finish on the inner surface of the mold; the product sticks to the mold during demolding, causing damage to the product surface.

 

Grooves and Unevenness	The flat part of the product is uneven, with local groove-like marks.	Low fiber content; too few fiber yarns in localized areas; the mold sticks to the product, scratching the product surface.
White Spot	On the surface layer of products containing surface mat and continuous mat, localized whitening or exposed white fibers occur.	The yarn and mat are not fully impregnated with resin; the mat layer is too thick, or the mat itself has poor performance with impurities mixed in, forming air bubbles between mat layers; the resin layer on the product surface is too thin.
Crack	There are microcracks on the product surface.	The cracks are only in the surface layer. Excessive resin layer thickness causes surface cracks; uneven resin curing leads to concentrated thermal stress, resulting in stress cracking.
Surface Fuzzing	Fibers are exposed on the product surface.	Excessive fibers prevent the resin from bonding fully with the fibers.
Surface Peeling and Breakage		Excessive resin layer remaining; insufficient pressure inside the mold; too low fiber content.
The Product is bent and Twisted/Deformed.		Uneven and non-synchronized curing of the product generates curing stress. After demolding, the pressure drops, and the product deforms under the stress. Uneven distribution of materials in the product leads to different degrees of curing shrinkage. The product is not fully cured when demolded, causing deformation under the pulling force.