Pultrusion vs. RTM what's the difference?

Robert Glass

Head of Marketing

Reading time: 4 minutes

Pultrusion and Resin Transfer Molding (RTM) are two widely used manufacturing processes for producing composite materials.

While both methods have their merits, pultrusion offers several distinct advantages over RTM. Let’s explore the benefits of pultrusion and compare them to the RTM process.

Pultrusion is a continuous manufacturing process that involves pulling reinforcing fibers, typically glass or carbon fibers, through a resin bath and then into a heated die, where the resin is cured and the composite’s final shape (geometry) is formed. On the other hand, RTM is a closed-mold process in which dry fibers are placed in a mold, and liquid resin is injected under pressure into the mold to impregnate the fibers and cure the composite.

One of the key advantages of pultrusion is its ability to produce continuous, uniform profiles with high fiber volume fractions. The pultrusion process allows for precise control over the fiber alignment and resin distribution, resulting in composites with excellent mechanical properties and consistent dimensions throughout the length of the profile. In contrast, RTM can sometimes result in variations in fiber content and resin distribution, leading to inconsistent mechanical properties and dimensional tolerances.

Pultruded composites also offer consistent strength-to-weight ratios compared to RTM composites

The continuous fiber reinforcement in pultruded profiles provides excellent load-bearing capacity and stiffness while keeping the weight to a minimum. This makes pultruded composites ideal for applications where weight reduction is critical, such as in aerospace, automotive, and sporting goods industries. RTM composites, although capable of producing strong and lightweight parts, may not achieve the same level of efficiency as pultrusion due to the potential for inconsistent fiber content and resin distribution.

Another advantage of pultrusion is its even distribution of additives and reinforcements alignments during the manufacturing process. By introducing fillers, pigments, fire retardants, or additional fibers, pultruded composites can be tailored to meet specific performance requirements, such as enhanced fire resistance. RTM, on the other hand, may face challenges in evenly dispersing additives throughout the composite due to the injection process, potentially resulting in uneven distribution which may compromise performance.

Looking at manufacturing efficiency, pultrusion offers faster production speeds and higher output rates compared to RTM. The continuous nature of the pultrusion process enables the production of long, constant cross-section profiles, reducing the need for secondary operations and minimizing waste. RTM, being a slower batch process, may require more complex mold designs and additional post-processing steps, which can increase production time and cost.

RTM is well suited to complex 3D geometries such as UAV wings or complex shaped automotive geometries which are not possible via pultrusion. Pultrusion is specialized for 2D shapes, like window frame profiles, or bus side panels, telecommunications radomes, and of course composite tubes.

Pultrusion provides several advantages over RTM composite manufacturing

These include consistent mechanical properties, high fiber volume fractions, excellent strength-to-weight ratios, flexibility in incorporating additives, dimensional stability, resistance to environmental factors, and manufacturing efficiency. While RTM has its own strengths and applications, pultrusion offers a compelling solution for industries seeking high-volume, high-performance composites with consistent quality and reliable performance.

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