- A Thermal Model for Laser Welding of Thermoplastic Polymers
Transmission laser welding began to increase in use for the joining of thermoplastic polymers from around 1996. However, in order to provide more confidence in its use, TWI created this CRP project in 2000 to investigate some fundamental requirements for the transmission laser welding process. This included an understanding of the behaviour and response of materials in a laser beam, including the effects of material properties on the welding process and the effect of different applied welding parameters.
- Thermoplastic Polymers - A Joining Solution
The mechanical properties of welds made between thermoplastic polymers are dependent on several factors, including temperature, time, pressure, surface roughness, processing histories and the distribution of polymer chains in the component itself. Using the optimum welding conditions can deliver desirable results, but this may require an extensive set of welding experiments. To solve this, TWI worked to develop a computer model that could perform much of this work to simulate defined welding conditions for polymer chain movements crossing the interface. The model was then validated against experimental measurement of toughness for the welded polymer.
- Testing Time for Polymer Composite L-joints
Our CRP work also investigated design tools for assessing damage in composite materials and joints. Building on previous work to assess delamination and impact damage in lap shear joints, this 2004 project provided details of mechanical tests and theoretical modelling work on adhesively bonded carbon-epoxy and glass-epoxy joints.
Finite element analysis was used to model the effect of flaws and damage on both the static and fatigue performance of adhesively bonded composite joints, allowing us to develop computer models to assess the structural integrity of composite material joints subjected to fatigue loads.
- Clamping/Joint Gaps for Through-Transmission Laser Welding
This 2010 project aimed to assess clamping pressure distribution and the effect of clamping pressure on the strength of plastic welds achieved via through transmission laser welding with both a fixed and a sliding clamp system. The process uses a laser transmissive and a laser absorbing workpiece with an absorbent layer at the interface. This project also investigated the effect of irregularities in the workpieces on the through-transmission laser welding process.
- Ultrasonic Welding of Glassy Thermoplastic Polymers
Also in 2010, our experts investigated ultrasonic welding for glassy thermoplastic polymers, which have a glass transition temperature above room temperature. This means that the ultrasonic energy can propagate through the material before causing heating. To assess the mechanisms responsible for heat generation in glassy thermoplastic polymers TWI’s experts used temperature measurement, high speed video and dynamic contact. This allowed us to identify the main heating mechanisms present at different stages of the ultrasonic welding process, measure the temperature at the weld interface during an ultrasonic weld of polymethylmethacrylate (PMMA), and identify the main mechanism responsible for heating, particularly during the first 100 milliseconds of the ultrasonic weld in glassy, amorphous thermoplastics.
- Linear Friction Welding of Carbon Fibre Reinforced Plastic
TWI investigated the use of linear friction welding (LFW) to join continuous fibre carbon reinforced PEEK (Polyetheretherketone) thermoplastic composite (CFRTPC). Fibre degradation was mitigated against thorough the addition of an unreinforced PEEK interlayer that was critical in increasing join strength from 17MPa (Figure 1) to match that of an adhesively bonded joint without the need for surface pre-treatment. The research found that the most successful parameters for the CFRTPC joint were 50Hz, ±1.0mm, 40MPa, while the PEEK interlayer showed adequate heat generation with non-degraded polymer forged out of the joint. Visual assessment from PEEK welding suggests that applying LFW to CFRTPCs using current metal welding equipment at TWI had the potential to effectively produce welded joints. The welded coupons achieved an apparent lap shear strength of 25.4MPa without any surface preparation.
- Diffusion Bonding of Polymer Based Micro- and Nano-fluidic Devices
Used in industries such as clinical chemistry, medical, life sciences, biotech and more, microfluidic devices (also known as plastic manifolds, bonded plastic manifolds, plastic pneumatic manifolds, plastic air boards, plastic valve manifolds, plastic medical manifolds and plastic flowcells) are comprised of a series of micro-scale channels. This project was created with the aim of developing process parameters and then to manufacture a defect-free and transparent diffusion bonded micro-channelled thermoplastic demonstrator. Our experts were able to produce demonstrators from both polymethyl methacrylate (PMMA) and polycarbonate (PC), showing that diffusion bonding with laser channelling was a candidate for manufacturing the next generation of microfluidic devices (Figures 2-4).
Dedicated Industrial Member Support and Other Projects
Much of the work conducted at TWI is undertaken in direct support of our Industrial Members, providing impartial, independent and often confidential support to solve specific challenges. This work has helped build up a unique body of knowledge and expertise at TWI that crosses industry sectors and capabilities, allowing us to bring innovations and ideas from one area to another to deliver the best possible solutions for our Members, including in relation to polymer welding. Although much of this work is necessarily confidential, there are some polymer welding-related projects that we can share some details of with you…
- Failure Investigation of Welded Moulded Plastic Components
Our close working relationship with a number of automakers and Tier 1 suppliers of major automotive companies has given us an awareness of the challenges around the welding of moulded plastic components. These components offer a number of advantages to the industry, but the weld quality remains dependent on the material and joint design for the specific welding process, making verification of weld quality a challenging task. To help address this we undertook an assessment of the weld integrity of a moulded plastic component by destructive and non-destructive examination (Figure 5). The component, a vapour separator made from Hostaform® C13031 (POM), was welded using linear vibration welding (LVW) before verification of the weld quality by visual inspection and microtoming and, although there were already tell-tale signs of poor weld (Figures 6a, 6b, 6c), computed tomography (CT) scans, which provided the most revealing flaws that were indicative of poor welding procedures (Figures 7-9). As a further outcome from this work, TWI developed a three-day training course and assessment, including practical demonstrations, to improve understanding of material selection, design and fabrication of moulded plastic components as well as helping to set up and develop a new British Standard, BS 89100; Joining of thermoplastic moulded components.
- Metal to Composite Joining Breaches a New Frontier
Another breakthrough at TWI was the creation of a technique for composite to metal joining that uses metal surface pre-treatment that allows the polyester resin and glass fibre composite to bond to it. Sculpting the surface of the metal into small peaks and troughs creates a prepared area for the polyester resin to run into to create the join, although some applications may require a further adhesive layer. These ‘Comeld’ joints fail at a much higher loads and absorb far more energy before failure than a conventional joint of identical dimensions (Figures 10-11).
- A New Approach to Thermoplastic Composite Induction Welding
TWI invented a new method for induction heating composites containing carbon fibres that generated sufficient heat to be used for adhesive curing or fusion bonding. The technique exploits the electrical conductivity of carbon fibres in the composite parts and is capable of concentrating the heat around the joint interface, rather than close to the top surface of the composite where the proximity of the induction coil is greatest. This prevents the problem of needing to remove excess heat from the surface of the composite to avoid thermal damage and provides greater control over the process. Results of the temperature rise show that, despite being closer to the source of the electromagnetic radiation, the top laminate experienced a much smaller temperature rise than the bottom laminate (Figures 12-13).
- Real-Time Temperature Monitoring for Polymer Welding
Our expert teams also addressed the challenge of using laser welding to join difficult-to-weld polymers through thermal management and the control of interface temperatures. We used a Hamamatsu LD-Heater to monitor the interface temperature while transmission laser welding three materials; polybutylene terephthalate (PBT), polyphenylene sulphide (PPS) and thermoplastic elastomers (TPE). This allowed us to develop a welding procedure specification for transmission laser welding of PBT, PPS and TPE following the production of demonstration coupon samples (Figures 14-16). As well ensuring fine temperature control and the identification of any defects, we were able to deliver demonstrable benefits in terms of productivity, reduction in reject rates and cost/time savings over other joining methods such as ultrasonic, hot plate or adhesives.
These examples comprise just some of the project work undertaken at TWI in relation to polymer welding technologies – to find put more about our polymer welding services, please see here:
https://www.twi-global.com/what-we-do/research-and-technology/technologies/welding-joining-and-cutting/polymer-welding