Pultrusion vs. resin transfer molding (RTM) - What's the difference?

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 when it comes to making straight profiles. Here Kim Sjödahl , our VP of technology and R&D, explores the benefits of pultrusion and compares 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 geometry is formed. 

On the other hand, RTM is a closed-mold process in which dry fibers, often in the form of a preform, are placed in a mold and liquid resin is injected under pressure into the mold to impregnate the fibers. Final curing is achieved either at room temperature or by heating the mold for a faster cure cycle. 

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. RTM can also produce consistent quality but is more dependent on the typically hand-laid preform. 

Strength-to-weight ratios 

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. 

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.  

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, require more complex mold designs and additional post-processing steps, which can increase production time and cost. 

Pultrusion offers faster production speeds and higher output rates compared to RTM

RTM is well suited to complex 3D shapes, such as complex automotive component geometries and non-prismatic shapes that 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. 

Benefits from both

Both pultrusion and RTM feature fully molded surfaces, meaning both upper and lower surfaces can be visual surfaces, as opposed to single side tooling or machining. RTM can, in some circumstances, match the mechanical properties achieved with pultrusion. Therefore, the two processes can be complementary on applications such as bus and train external panels, with pultrusion for the straight sections and corner shapes by RTM. Both are needed to make a complete product. 

Both pultrusion and RTM offer consistent mechanical properties, high fiber volume fractions, excellent strength-to-weight ratios, dimensional stability, resistance to environmental factors, and manufacturing efficiency. While RTM has its own strengths and applications in shape flexibility, pultrusion offers a compelling solution for industries seeking high-volume, high-performance composites with consistent quality and reliable performance. Together, components made by the two processes can produce almost anything. 

To find out how Exel’s expertise in pultrusion could benefit your engineering or product design, continue reading here about composites manufacturing techniques.