TLDR
Reducing Bottlenecks and Consumables in High-Performance Thermoset Composite Manufacturing
The manufacturing of high-performance thermoset matrix composite parts typically involves three distinct steps: layer deposition, preform consolidation/wetting, and matrix curing. Automated material deposition technologies, such as Automated Fiber Placement (AFP), utilize rollers to deposit the composite layers. However, the current generation of high-performance thermosetting matrix polymers require curing times on the order of hours to fully cross-link the polymer from a liquid matrix to a glassy solid.
This lengthy curing process often creates a bottleneck in the manufacturing workflow, limiting the overall production rate. Additionally, the multi-step process requires the use of various consumable materials, such as vacuum bagging and release films, which add to the overall cost and material waste.
The return in popularity of snap-cure thermosetting matrix materials presents an opportunity to address these challenges. Snap-cure thermosets offer significantly shorter curing times compared to traditional thermosets, with processing cycles ranging from 18 minutes at 110°C to as low as 1.5 minutes at 160°C. This breakthrough in curing time reduction has the potential to revolutionize thermoset composite manufacturing by enabling in-situ consolidation and curing during the automated deposition process.
By combining the layer deposition, consolidation, and curing steps into a single processing operation, manufacturers can potentially eliminate the bottleneck at the final curing stage and reduce the need for numerous process consumables. This integrated approach not only streamlines the manufacturing workflow but also offers the potential for improved part quality and reduced material waste.
Time and Material Inefficiencies in Traditional Thermoset Composite Manufacturing Processes
Traditional thermoset composite manufacturing processes typically involve three distinct steps that lead to time and material inefficiencies:
Layer deposition - Depositing the composite layers, often using automated technologies with rollers.
Preform consolidation/wetting - Consolidating the layers together and allowing the resin matrix to wet out the reinforcement fibers. This often requires consumables like vacuum bagging materials.
Matrix curing - Curing the thermoset resin matrix, which for high-performance thermoset polymers can require cure times on the order of hours to fully cross-link the polymer from a liquid to a solid.
The long cure times required in the final matrix curing step are called out as a bottleneck that reduces manufacturing efficiency. The traditional processes also require many consumable materials used in the preform consolidation step.
Developing a Layer-by-Layer Curing Approach for Snap-Cure Thermoset Prepregs Using Automated Deposition
To address the challenges associated with traditional thermoset composite manufacturing processes, researchers have proposed a novel layer-by-layer (LbL) curing approach that leverages the unique properties of snap-cure thermoset prepregs. This innovative approach aims to combine the layer deposition, consolidation, and curing steps into a single, efficient processing operation using automated deposition technologies.
The LbL curing approach involves the use of a heated tool and a consolidation roller to deposit and partially cure each layer of snap-cure thermoset prepreg in a sequential manner. The prepreg is heated and compressed by the roller, initiating the curing process. The layer is cured to a degree of cure (α) below the gelation point (αGel = 0.41) before the next layer is deposited. This process is repeated until all layers are deposited and a minimum degree of cure of 0.85 is achieved in each layer.
To investigate the feasibility and performance of the LbL curing approach, researchers conducted a study using a 2x2 test matrix to evaluate the effect of processing parameters on laminate quality. The parameters of interest were tool plate temperature (110°C and 120°C) and consolidation roller contact pressure (1.08 bar and 1.55 bar). Laminates were manufactured using a research-based automated deposition system that mimicked an AFP process.
The LbL curing approach was compared to conventional hot-press (HP) molding, where laminates were manufactured using heated platens under similar temperature and pressure conditions. The degree of cure and temperature of each layer were monitored in real-time using embedded thermocouples and a cure kinetics model.
Optical microscopy was employed to assess the laminate quality, focusing on the porosity distribution. Cross-sections of the cured laminates were polished and analyzed using image processing techniques to quantify the porosity content and distribution.
Achieving High-Quality Laminates with Reduced Porosity Through In-Situ Consolidation and Curing in a Single Processing Operation
The layer-by-layer (LbL) curing approach, utilizing snap-cure thermoset prepregs and automated deposition technology, has demonstrated promising results in achieving high-quality laminates with reduced porosity. By combining in-situ consolidation and curing in a single processing operation, this approach offers a potential solution to the inefficiencies associated with traditional thermoset composite manufacturing processes.
The study conducted by the researchers revealed that the LbL curing approach produced laminates with evenly distributed micro-porosity, with an average porosity content of 2.9%. In contrast, laminates manufactured using the conventional hot-press (HP) molding method exhibited higher porosity levels, averaging 4.7%. Notably, the porosity in the HP laminates was concentrated in the inter-ply regions, where layers were laminated together.
The differences in porosity distribution between the LbL and HP laminates can be attributed to the distinct pressure application techniques employed in each method. The HP method applies uniform pressure across the entire laminate surface, lacking a pressure gradient that would facilitate the migration of trapped air between layers. Conversely, the LbL method utilizes a moving roller nip point, which effectively squeezes out air between layers and, to a lesser extent, within layers.
Although the LbL laminates were approximately 13% thicker than the HP laminates due to reduced time under pressure, the LbL approach demonstrated the potential to produce high-quality laminates with improved porosity distribution. This improvement in laminate quality can lead to enhanced mechanical properties and performance of the final composite parts.
The successful implementation of the LbL curing approach opens up new possibilities for high-rate, automated composite manufacturing using in-situ consolidation and curing of thermoset prepregs. This solution addresses the bottlenecks and inefficiencies associated with traditional multi-step processes, offering the potential for reduced cycle times, decreased material waste, and improved part quality.
However, further work is needed to optimize the process variables for laminate quality, manage roller contamination, and investigate the application of the LbL approach to non-flat geometries. As the technology matures, it has the potential to revolutionize the manufacturing of high-performance thermoset composite parts, enabling faster production rates, lower costs, and improved sustainability.
References
I would like to express my gratitude to Robin Hartley and James Kratz, the authors of the research paper "CFRP layer-by-layer curing using research-based automated deposition system" published in the journal Manufacturing Letters. Robin Hartley and James Kratz are from the Bristol Composites Institute at the University of Bristol.
Their valuable contribution to this blog post is greatly appreciated. Their research on the layer-by-layer curing approach for snap-cure thermoset prepregs has provided the foundation for this informative and engaging content. Robin Hartley and James Kratz's dedication to advancing composites manufacturing technology is commendable, and I thank them for sharing their findings with the scientific community through their published work.
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