Innovative Pultrusion Process! Thermoplastic Overmolding of Thermoset Pultruded Parts

 

 

Functionalized Semi-Finished Pultruded/Pultruded Profiles

 

Searching for a low-cost process to add functionality to its semi-finished composite profiles and tubes and reduce the use of adhesives in its products, Epsilon Composite, together with its injection molding partner Somocap, developed and patented a thermoplastic composite injection overmolding process. Image credit, all images (including those consistent with title): Epsilon Composite

 

Epsilon Composite (Gaillan Médoc, France) specializes in carbon fiber/epoxy pultrusion and pultrusion, which enable the manufacture of medium- to large-scale series of high-performance carbon fiber composite parts at a faster rate and lower cost than many other processes.

Since its founding in the late 1980s, the company has been manufacturing semi-finished pultruded profiles for a variety of applications in aerospace, industrial, and other markets. For many of the end-use parts made from Epsilon Composite’s profiles and tubes—aircraft struts, industrial components, technical rollers, and more—metal or plastic end fittings and inserts are often bonded to them to add functionality or provide connection points for other parts.

 

However, bonding requires several additional steps that are often manual: surface preparation, application of the adhesive, and curing. This makes adhesive bonding too costly and labor-intensive for many high-volume applications that require thousands of parts per year.

 

Mechanical fasteners can also be used in some cases, but this also presents other challenges, such as added weight and assembly steps, as well as the need to machine holes in the pultruded part, which often reduces its mechanical properties.

 

 

In 2012, the company began experimenting with alternative methods for adding end fittings or other features to components made by pultrusion or pultrusion. “Our goal was to find a way to produce a low-cost, high-quality product—a holy grail,” explains Alexandre Lull, deputy CEO of Epsilon Composite.

 

One idea was to use composite injection overmolding as a method for attaching metal end fittings to pultruded profiles. Lull says injection overmolding, which primarily uses glass- or carbon-fiber-reinforced thermoplastics, was chosen because of its potential to complement the low cycle times and high-volume capabilities of the pultrusion or pultrusion processes the company already uses.

The challenge, however, was that while Epsilon’s pultruded profiles are made of epoxy or other thermoset resin matrices, overmolding introduces a thermoplastic matrix, injected at high temperature and pressure, with a different coefficient of thermal expansion.

 

 

Process Development and First Prototypes

 

Over the next few years, Epsilon searched for the best way to realize its idea of ​​overmolding hybrid thermoset composite pultruded parts with thermoplastics. To do this, Epsilon worked closely with its injection molding partner, Somocap (Jatxou, France). “We brought engineering and composite material knowledge, and they brought injection molding machinery and process knowledge,” explains Ambroise Latron, Epsilon’s head of R&D.

 

 

Carbon fiber/epoxy pultrusion and pultrusion specialists. These images show Epsilon’s pultrusion process before (above) and after (below) the profile is pulled through a heated die for curing.

 

According to a U.S. patent application filed in 2021, the process involves several steps:

 

First, a hollow tubular thermoset composite profile is produced by pultrusion or pultrusion. The end of the profile is then machined to allow for the shape of the end fitting to be attached, which provides a rough surface area for the profile to be attached. Next, a tool/stopper is placed inside the profile in an injection machine, and thermoplastic is then injected around the profile and end fitting under specified heat and pressure, bonding them together.

 

Ultimately, this method could be used as a way to attach traditional metal end fittings to pultruded profiles or tubes, or the metal could be replaced entirely, and a thermoplastic composite tip could be added by overmolding. The first demonstration of the technology involved injecting polyetheretherketone (PEEK) on top of a carbon fiber/epoxy pultruded tube.

 

“PEEK is injected at very high temperatures, around 300°C, so we had to make sure the resin matrix of the [thermoset] composite tube could sustain that temperature for a short period of time,” Latron said.

 

Through trial and error, the R&D team figured out the right combination of a fast injection overmolding process and the desired glass transition temperature (Tg) of the thermoset matrix to produce repeatable results without damaging the pultruded tube, despite material variation.

 

The patent states, “This approach is simple because it can require very few steps, but it allows obtaining components that can withstand strong traction forces, compression forces, temperature gradients using materials with different expansion coefficients.”

 

The initial demonstration parts not only proved the feasibility of the process, but also exceeded performance expectations, Lull said. This led the company to patent the technology and begin introducing it to customers for commercial applications.

 

Lull notes that a range of materials have been used, from relatively low-cost polyamide 6 (PA6) filled with glass fibers to higher-performance materials such as PEEK, polyphenylene sulfide (PPS) or polyetherimide (PEI) reinforced with glass or carbon fibers. Unreinforced resin systems can also be used if desired.

 

Benefits of overmolding are said to include reduced cost, reduced weight and improved impact resistance compared to other methods such as bonding, using mechanical fasteners or even wrapping filaments on top of end fittings. Corrosion risks can also be eliminated if metal is replaced with thermoplastics.

 

The solution can also have added sustainability benefits: removing chemical solvents and adhesives from the process, plus using thermoplastics as a joining method, allows the two components to be separated at the end of the part’s life (EOL) by adding heat, increasing the potential for recyclability. “Also, there is no scrap from the injection process. Any scrap that is generated can be melted down and reused in the injection molding process,” Lull adds.

 

Commercial Applications: Industrial, Aerospace, Agriculture

 

After developing and validating a prototype, the first commercial use case for the technology was in 2015 with a US company that manufactured pultruded rods for use in industrial robots.

 

 

First Application: industrial. One of Epsilon’s core end markets, industrial applications such as robotic arms or robotics, was the first application for its injection overmolding joining process.

 

“We initially worked with industrial customers, but our first major business was aerospace,” says Nall. In 2018, the company began working on an R&D project with Airbus (Toulouse, France). This work grew out of a previous R&D collaboration in which Epsilon demonstrated its pultrusion technology for the manufacture of high-performance tubular structural struts with bonded metal end fittings.

 

Epsilon successfully demonstrated the performance of the part and the reliability of the manufacturing process to TRL 6 and Airbus’ internal standards – however, the project was not pursued commercially because structural bonding was considered too risky for critical aerospace structures.

 

To address these risks, Epsilon developed and patented a process specifically designed to ensure bonding according to aerospace standards, but the process was more expensive, and in parallel, Epsilon and Somocap developed a new overmolding process and began commercial production of industrial parts.

 

Therefore, for the next development iteration of these struts with Airbus, Epsilon introduced overmolding of the end fittings as a solution to meet the needs for optimized cost and high reliability of both the part and the process.

 

Lull explains that an intensive development process of more than a year and a half ensued, aimed at finding the right parameters and toolset to optimally overmold the end fittings onto the thermoset composite pultruded tube.

 

Ultimately, the struts proved successful and delivered 50% cost savings over traditional composite struts made from filament winding or prepreg. The struts were eventually commercially qualified by Airbus, and Epsilon continues to supply these parts to the aerospace market