MM-Welding solution automates, functionalizes composite joining

As manufacturers seek to increase production rates and reduce costs, assembly becomes a potential bottleneck and pain point that demands innovative solutions. While mechanical fasteners and adhesive bonding have been used in composite assemblies for decades, they also typically involve multistep process chains and other issues that increase cost and delivery time.

Bridging the benefits of fastening and bonding, ultrasonic techniques are resolving some of these challenges while offering processing in seconds, as well as improving efficiency and productivity for a range of composite materials including traditionally troublesome constructions like laminated sandwich panels.

Issues with mechanical fasteners, bonding

Mechanical fasteners are commonly used in composites for quick joining of dissimilar materials while enabling future disassembly for service or repair. However, hole drilling has long been an issue, necessitating extra material in the drilled areas to address the cuts in load-bearing fibers. It also creates dust/debris and multistep process chains (logistics, machining, cleaning, inspection, fastener installation and final inspection).

Another issue is that certain types of composite materials — like sandwich structures, particle foams and nonwoven fibrous mats — are not well suited for direct assembly with screws or nut-and-bolt fastening due to issues with local compression and weak bearing strength. Thus, it is often necessary to first create a “hard point” or reinforced area for installing the fastener.

Alternatively, adhesive bonding is excellent for directly joining components together but can make disassembly impossible or very difficult. Another option is bonded fasteners adhered to the surface of composite structures. The challenge here is finding a suitable adhesive for both the fastener and substrate materials as well as achieving acceptable bond performance on both materials.

History and use of MultiMaterial-Welding

Ultrasonic joining in plastics and polymer materials is well-established. Used for decades, it encompasses direct ultrasonic welding of similar thermoplastics (Fig. 1), staking components onto thermoplastics (Fig. 2) and fixing metallic elements onto thermoplastics (Fig. 3), enabling hybrid assemblies that have been growing in automotive and mobility applications. All of these techniques use ultrasonic energy to melt the thermoplastic polymer, resulting in material fusion and/or form-locked connections.

Founded in 2015, MultiMaterial-Welding (MM-Welding AG, Biel, Switzerland), a business that collaborates with fastening technology specialists Bossard Group (Zug, Switzerland), has developed novel MM-Welding technologies that combine ultrasonic energy and the melting and form-locking capabilities of thermoplastics to create secure fixations/fastener installations. The company’s LiteWWeight and InWWerse solutions can be tailored to specific applications and are used by a wide range of companies in automotive, aerospace and railway applications including BMW, VW, Stellantis, Jeep, Swandoo and Storopack.

Fastener installation using MM-Welding can also be qualified using AI-assisted data processing. By combining the process data from the ultrasonic equipment with machine learning and AI algorithms, it is possible to reliably predict application-specific metrics (such as pull-out strength), providing in-line, real-time verification of fastener installation quality and performance (see Fig. 7 in last section).

Case study 1: Composite sandwich floor panel

To better understand the “how it works” explanation below, this case study shows how MM-Welding’s technology was used with sandwich floor panels that needed a connection point for a standard metric screw to enable attachments.

5M (Kunovice, Czech Republic) is a producer of lightweight, high-strength composite panels for railway interiors. It wanted to improve its traditional system for creating fastening points (Fig. 4a), which was slow and costly.

This previous process required removing material and gluing a block of plywood into the sandwich core before completing the panel lamination. Then a threaded insert was machined into the plywood block.

Bossard worked with 5M to use MM-Welding’s LiteWWeight Double Pin fastener insert. This new process comprised drilling a hole directly into the sandwich material and embedding the Double Pin fastener insert into the hole using ultrasound in 1-2 seconds (Fig. 4b). This not only eliminated process steps but ensured right-first-time assembly by automating fastener positioning and installation.

How it works

Similar to ultrasonic welding, MM-Welding uses high-frequency vibration from an ultrasonic stack — which typically comprises a piezoelectric transducer or converter, a booster that modifies the amplitude of the vibrations and a horn or sonotrode that applies the vibrations to the parts — to create local friction. This melts and flows the thermoplastic materials to create a mechanical positive lock and fused connection.

In this way, porous and lightweight materials (for example, cellular or foam cores) — which pose a local crushing challenge for standard screws or rivets — and geometric undercuts actually become a strength for MM-Welding because they provide space to anchor into. MM-Welding offerings currently include LiteWWeight and InWWerse products.

LiteWWeight technologies are designed to functionalize thermoplastic polymer and composite components, reducing part count and simplifying manufacturing processes and logistical requirements. One method achieves fastener fixation through form-lock connection, where the fastener melts and interlocks with the parent material. This includes LiteWWeight Pin and Double Pin technologies, which are used in sandwich constructions.

Another method combines fastener melting with load spreading across a larger footprint versus a pin. This is the principle for LiteWWeight zEPP for expanded polypropylene (EPP) foam materials and also for MM-Welding’s Lotus solutions used for nonwoven materials. The polymeric fastener inserts do not need removal before recycling, which aids circularity.

InWWerse technologies are used to install metallic elements onto thermoplastic materials, and are designed to overcome challenges like low surface energy materials and fastening for aesthetically sensitive components. InWWerse also creates form-locked connections, which address concerns about process reliability and mechanical performance while enabling joining of chemically dissimilar thermoplastic materials. Because the input of energy is localized, the volume of material affected by the fastener and the process is minimized, which reduces marking defects, commonly referred to as “read through.” This is important for Class A surfaces, such as thin automotive trim parts.

Case study 2: EPP fastening

Known for its optimal energy absorption and lightweight properties, EPP is ideal for crashpad elements in automobiles such as those produced by protection specialist Storopack (Metzingen, Germany). However, using direct assembly with screws can cause local crushing where screw heads and washers apply force. This makes it difficult to achieve an adequate assembly preload to securely retain the fastened parts. A load-spreading, surface bonding solution could avoid this, but adhesively bonded fasteners are often metallic, which also presents a material separation challenge at end of life (EOL).

Storopack chose MM-Welding’s LiteWWeight zEPP technology (Fig. 5) to create a connection point into the EPP materials for several reasons. The installation process uses no chemicals and creates a clean, durable connection that is also space efficient. Specifically, the zEPP insert provides adequate mechanical anchoring into the foam without requiring excessive section depth or surface footprint. Also, because the LiteWWeight zEPP fastener inserts are polymer, they can go through the same EOL material recovery and recycling systems as the EPP component — i.e., they don’t need to be removed first. This minimizes process steps, time and waste streams for more efficient recycling.

MM-Welding worked with Storopack to optimize the design and installation of the zEPP fastener insert. Storopack required a passage hole for an M5 bolt, with precise positioning of the surface relative to the back of the part. This insert was designed with a yellow color to provide a strong contrast with the black foam, making quality checks easier. The ultrasonic installation parameters — such as trigger criteria, pressing speed, oscillation amplitude and end conditions — were also optimized to ensure the best mechanical properties and z-positioning for this specific foam density.

MM-Welding then collaborated with Storopack to develop an automated system for installing the LiteWWeight zEPP fasteners within the EPP crashpads. The final solution comprised a turning table that enables installation of the zEPPs on one side of the table while a worker can unload the assembled part and load the next ones on the other side. The ultrasonic stack was mounted onto a CNC portal which also included a pick-and-place feeding system supported by a vacuum channel in the ultrasonic horn. The system can pick up the zEPP and move it to the defined connection location with control in the x-, y- and z-directions. This was important, because Storopack needed a precise contact surface for the screw head to be able to connect to the crash pad body. And the increased speed of this solution also facilitated automotive throughput rates.

 

Increasing applications, recycling and digital twin/thread solutions

MM-Welding is working to increase awareness of this fastening technology and to increase use cases, for example, in aerospace. Sandwich panels are a typical material used for aircraft interiors, and there are already LiteWWeight products for such applications, but more are being developed.

Another key area is recyclability of assemblies, which customers are discussing daily. This includes, for example, assemblies that can be easily disassembled at EOL. MM-Welding can enable this by creating mono-material structures, where the fastening inserts are made from the same material as the structure. The zEPP solutions for EPP are one example, where the base material and the fastener inserts are all made from polypropylene (PP). But MM-Welding is also developing zEPP fasteners made using PP recycled from the EPP foam, which enables a closed loop for materials but also an easier, lower cost solution for recycling at EOL.

There are also many companies seeking to replace Nomex honeycomb sandwich panels with thermoplastic composite panels in aircraft interiors. This is another example of enabling mono-material solutions, as the MM-Welding fastener inserts can be designed out of the same thermoplastic matrix as the panels. Even though the current materials being explored by MM-Welding include polyetherimide  and polyphenylene sulfide, which are lower cost, it is also possible to have a fully PEEK or PAEK (polyetheretherketone or polyaryletherketone) construction with both the panels and fastener inserts out of the same material. Again, this is easier to manage at EOL, and using the higher performance (and -cost) thermoplastic polymers is simply a matter of adapting the MM-Welding design and process parameters to get the desired results. This approach could also significantly speed interiors installation because the MM-Welding process takes only a couple of seconds, compared to the long processes for creating potted connections for fasteners or adhesive bonding.

MM-Welding has also completed significant work with the University of Applied Sciences and Arts Northwestern Switzerland (FHNW) to model the mechanics of its fastener insert processes. This includes finite element modeling and simulation as well as data management — specifically, how to use the insert installation data to predict the properties and performance of the completed fastenings. These SmartSolutions are described briefly above (Fig. 7) and use a data acquisition unit to collect process data plus customized algorithms for analysis. The work with FHNW has built this data acquisition and training pipeline, enabling the prediction of mechanical properties for each fastening point. According to the company, this has huge potential for digital twins of products for customers and digital threads throughout the supply chain — every single connection point could be tracked and labeled with its properties. MM-Welding SmartSolutions are already being offered to customers. However, the use of such systems is still new for many companies and will take time to develop.