Pultrusion Process

 

Overview Pultrusion is a process for the continuous production of linear composite products. It uses resin as the matrix material and fiber and fabric as the reinforcing material. Under the traction of external force, it goes through impregnation, preforming, hot mold curing, and finally forms a continuous regular cross-section product.

 

 

According to the equipment, it can be divided into horizontal and vertical types; according to the traction method, it can be divided into crawler traction, reciprocating traction and ring traction; according to the resin matrix, it can be divided into thermosetting and thermoplastic pultrusion processes; according to the new pultrusion, it can be divided into pultrusion processes such as winding, weaving, and injection.

 

Characteristics of Pultrusion Process

Advantages

▶Automated and continuous production process;

▶High production efficiency, multi-mode and multi-piece;

▶The fiber content in pultruded products can be as high as 80%, which can give full play to the mechanical properties of continuous fibers and have high product strength;

▶The longitudinal and transverse strength of the product can be adjusted arbitrarily

▶The product performance is stable and reliable, and the fluctuation range is within ±5%;

▶The raw material utilization rate is above 95%, and the scrap rate is low.

 

Disadvantages

▶Cannot use non-continuous reinforcement materials;

▶The product shape is monotonous, and can only produce line profiles (non-variable cross-section products), and the transverse strength is not high.

 

Raw Materials for Pultrusion Molding-Matrix Resin

 

Raw Materials for Pultrusion – Reinforcement Materials

 

Glass Fiber

▶Ordinary E glass fiber is the main one, S glass fiber is used for high strength, and M glass fiber is used for high stiffness;

▶Cheap price, excellent comprehensive performance, high cost performance;

▶High density and low modulus.

 

 

Aramid Fiber

▶High strength and high modulus, high temperature resistance, acid and alkali resistance,

▶Aging resistance, expensive

▶Good toughness, excellent impact resistance

 

 

Carbon Fiber

▶Low density, significant weight reduction, excellent performance, expensive

▶High-strength T700 is mainly used for load-bearing structures, and T300 cloth is used for exterior parts

 

 

Basalt Fiber

▶High strength and high modulus, high temperature resistance, acid and alkali resistance, aging resistance, expensive

▶Good toughness, excellent impact resistance

 

 

Raw Materials for Pultrusion – Commonly Used Additives

 

Initiators: commonly used are MEKP, TBPB, BPO, TBPO, BPPD.

Flame Retardants: usually compounds of phosphorus, bromine, chlorine, antimony and aluminum, among which aluminum hydroxide is widely used.

Fillers: commonly used are calcium carbonate, kaolin, talc, silica, hydrated alumina.

Mold Release Agents: commonly used internal mold release agents are stearates, lipids, and organic phosphorus compounds.

Low Shrinkage Agents: commonly used are polyvinyl acetate, polymethyl methacrylate, polystyrene, thermoplastic polyurethane and polyester.

Other Additives: including coupling agents, inhibitors, toughening agents, diluents, antioxidants, etc.

 

Analysis of Pultrusion Process Technology

Schematic diagram of conventional pultrusion process

Viscosity changes of resin matrix in hot mold during pultrusion process

 

Effect of pultrusion speed on the exothermic peak position of resin matrix

 

 

Schematic diagram of the process behavior of the molded product in the hot mold during pultrusion

 

 

Assume that the viscous shear stress between the gel area molded product and the inner wall of the mold is Tadh, the minimum thickness of the conical gel area is t, the thermal tensile strength of the gel is Sgel, and the width of the gel area is Y. In order for the pultrusion process to proceed smoothly, the following inequality must be satisfied:

 

 

Combined with the UPR Pultrusion Process, the Following Measures can be Taken to Make the Above Inequality Hold True:

 

■ In order to shorten the gel zone length Y, a fast-curing resin system can be used and (or) the gel zone mold temperature can be increased. The worst option is to reduce the pultrusion speed, otherwise the gel zone will stick to the mold and become blocked (Collar).

■ Use a high-efficiency internal release agent to reduce the viscous shear force Tadh between the molded object in the gel zone and the inner wall of the mold and (or) improve the finish of the working surface of the molding mold, otherwise the gel zone will stick to the mold and become blocked (Collar).

■ Reduce the thickness of the resin-rich layer on the surface of the molded object by increasing the volume content of the reinforcing material to maximize the t value. In addition, a faster curing speed also contributes to the t value. However, a too large t value will cause the product to fall off (Pill generation).

■ In order to improve the thermal strength of the resin gel, a reasonable resin formula and the addition of fillers are also helpful.

 

At present, domestic enterprises lack awareness and ability in the overall application research of pultrusion molding. The scale and level of application, design methods and concepts, material foundation and supporting, manufacturing processes and equipment are seriously backward, which restricts the development of the composite material industry.

 

To this end, DT New Materials focuses on the pain points of the development of the composite materials industry and invites China’s top expert in carbon fiber composites, Professor Zhu Bo from Shandong University, and a team of front-line experts and senior engineers with more than 20 years of experience to share pultrusion technology experience and related cases.