The Perfect Fusion of Thermoplastic and Thermosetting Composite Materials Solves the Problem of the Inability to Connect Traditional Metal Parts/inserts with Pultruded Profiles in Batches

 

01 Introduction to Pultrusion Process

 

Pultrusion is a continuous molding method for making composite materials with constant cross-section, which combines “pulling” and “extrusion”. In contrast to extrusion, which pushes the material, pultrusion pulls the material. The fibers are impregnated, extruded, heated and cured, and cut to length under external traction to form the final product.

 

JH Watson applied for a very early pultrusion patent in 1944. This was followed by a patent applied for by MJ Meek in 1950. Based on a patent applied for by Rodger B. White in 1952, Glastic Company in Cleveland, Ohio provided the first commercial pultrusion product. The patent granted to WB Goldsworthy in 1959 helped initiate the promotion and dissemination of related knowledge within the industry. Therefore, W. Brandt Goldsworthy is also widely regarded as the inventor of the pultrusion process.

 

 

Schematic Diagram of Pultrusion Process

1. Continuous Roll Reinforced Fiber/Woven Fiber Mat
2. Tension roller
3. Resin
4. Resin-impregnated fiber
5. Mold and heat source
6. Pulling mechanism
7. Composite material after curing

 

Currently, the more common ones are thermosetting resin pultrusion such as epoxy resin and polyurethane. The equipment and process are relatively mature and widely used.

 

 

 

 

Typical Pultruded Products

 

Problems

 

Pultruded profiles are often not used alone, and metal parts need to be bonded to the ends to increase functionality or provide connection points for other parts.

 

The current bonding process is basically a manual process (such as grinding the bonding surface, applying structural adhesives, and final curing). For many large-scale applications that require thousands of parts each year, this greatly improves efficiency and costs.

 

Second, although mechanical fasteners can also be used in some cases, this will also increase weight and assembly steps, and holes need to be processed on the pultruded parts, which destroys the continuity of the fibers and reduces the mechanical properties of the product.

 

02 Thermoplastic Injection Molding/Overmolding Perfect Solution

 

Epsilon Composite (France) and its partner Somocap use a solution for thermoplastic overmolding of hybrid thermosetting composite pultruded parts and submit relevant patent protection 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 the shape of the end fitting to be connected, which provides a rough surface area for the profile to be connected; next, the tool/plug is placed inside the profile in the injection machine, and then thermoplastic is injected around the profile and end fitting under specified heat and pressure, bonding them together.

 

 

 

Pultrusion Process and Subsequent Curing by Heating the Mold

 

Process Difficulties

The thermoplastic matrix is ​​injected under high temperature and pressure. Different materials have different thermal expansion coefficients. It is necessary to comprehensively consider the process parameters such as the product’s traction, compression force and temperature gradient. At the same time, the glass transition temperature (Tg) of the thermosetting resin must match the thermoplastic matrix to avoid damage to the thermosetting pultruded profile.

 

03 The First Prototype Sample

The first prototype garbage profile uses a carbon fiber/epoxy resin pultruded tube with polyetheretherketone (PEEK) injected on the top. PEEK is injected at a temperature of about 300°C, and it is necessary to ensure that the resin matrix of the thermosetting composite tube can maintain this temperature for a short time. Through repeated trials, Epsilon found the right combination of rapid injection molding process and the required glass transition temperature (Tg) thermosetting matrix without damaging the pultruded tube to meet the needs of mass production.

 

 

Epsilon Functionalized Semi-Finished Pultrusion/Pultruded Profiles

 

At the same time, Epsilon has validated a range of materials, from relatively low-cost glass-fibered polyamide 6 (PA6) to higher-performance materials such as PEEK, polyphenylene sulfide (PPS) or polyetherimide (PEI) reinforced with glass or carbon fibers. Other resin systems can also be used if required.

 

Advantages

 

The benefits of overmolding include reduced cost, reduced weight and improved impact resistance compared to other methods such as bonding, using mechanical fasteners or winding 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 by high temperature at the end of the part’s life (EOL), increasing the potential for recyclability. In addition, there is no waste from the injection molding process. Any waste generated can be melted down and reused in the injection molding process.

 

04 Commercial Applications

 

Aerospace

 

The first commercial use case for the technology was pultruded rods for industrial robots. But the company’s first major business was in aerospace.

 

In 2018, Epsilon began an R&D project with Airbus. This work stemmed from 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 parts and the reliability of the manufacturing process to TRL 6 and Airbus’ internal standards. However, the structural bonding was too risky for aerospace structures and was not carried out commercially.

 

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. In parallel, Epsilon and Somocap developed a new overmolding process and began commercial production of industrial parts. Therefore, for the next R&D iteration of these struts with Airbus, Epsilon introduced overmolding of the end fittings as a solution to meet the needs of optimized cost and high reliability of both the part and the process.

 

A year and a half long development process ensued, aimed at finding the right parameters and tooling to optimally overmold the end fittings onto the thermoset composite pultruded tube. Ultimately, the struts proved successful and delivered a 50% cost savings over conventional 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.

 

The technology eliminates the risk of failure of adhesive or even mechanical fasteners, and we have proven that the process can consistently produce thousands of reliable, high-quality parts with no scrap. The solution enables reliable and effective evaluation of parts using non-destructive inspection (NDI) methods that cannot be used on bonded parts.

 

 

Figure 2 Industrial applications, such as robotic arms or robotics, are the first applications of its injection overmolding connection process (above), aircraft struts (below)

 

In the agricultural field, Epsilon worked with customers to replace the steel boom arms of industrial tractors with 50-meter-wide pultruded composite booms. Steel booms cannot be 50 meters wide due to weight and rigidity issues.

 

Composites are stiffer, lighter, and eliminate potential corrosion issues, so there are many benefits, but productivity is the biggest advantage. Larger booms can spray farmland in a shorter time, thereby improving efficiency.

 

05 Future Development

 

There are other composite booms on the market, and Epsilon’s is a truss-shaped pultruded structure, which makes it highly optimized for high stiffness at the lowest possible weight and minimum material usage, thereby reducing overall costs.

 

This technology has become Epsilon’s preferred solution for high-volume parts for the end market, which is very cost-competitive. Epsilon’s R&D team is continually optimizing its overmolding process, making the overall process and workflow between pultrusion and injection faster and more efficient, and developing low-cost or modular tooling for low-volume applications.