Composites 1 - Week 4.pdf

# Composite manufacturing processes and tooling This topic explores the essential elements of composite part manufacturing, emphasizing the crucial role of tooling in achieving desired shape and quality, and detailing the steps from layup to curing [4](#page=4) [5](#page=5). ### 1.1 The role of tooling The tool is paramount as it defines the final shape and surface quality of any composite part. Key requirements for tooling include [4](#page=4) [5](#page=5) [6](#page=6) [7](#page=7): * **Defining Geometry:** The tool dictates the precise dimensions and form of the composite part [10](#page=10) [4](#page=4) [5](#page=5). * **Surface Quality:** It determines the smoothness and finish of the part's surfaces [4](#page=4) [5](#page=5) [6](#page=6) [7](#page=7). * **Leak-Tightness:** Essential for achieving effective compaction of the composite material [10](#page=10) [4](#page=4). * **Process Condition Resistance:** The tool must withstand the temperature and pressure applied during manufacturing, which can be up to 177 degrees Celsius and 5 bar [10](#page=10). * **Durability:** It must be robust enough for the intended production volume, potentially for around 1000 parts [10](#page=10). * **Accuracy:** The required precision of the part's dimensions influences tool design [5](#page=5). * **Configuration:** Tools can be male (single-sided), female (single-sided), or matched-die (double-sided). Male molds are easier to process but risk bridging and result in a rough outer surface, while female molds yield a smooth outer surface. Matched-die molds provide two smooth surfaces [6](#page=6). * **Surface Finish:** Tools need to be machined, sanded, and polished to achieve the desired surface finish on the part [5](#page=5). * **Self-Support and Transport:** The tool's design should consider its stability and ease of movement [5](#page=5). * **Coefficient of Thermal Expansion (CTE) Match:** Ideally, the CTE of the tooling material should match that of the composite part to prevent distortion like springback and warping during temperature changes [5](#page=5) [8](#page=8) [9](#page=9). ### 1.2 Tooling materials Various materials are used for tooling, each with distinct advantages and disadvantages: #### 1.2.1 Steel tooling * **Pros:** High durability, weldability, repairability, and excellent vacuum integrity [11](#page=11). * **Cons:** Difficult to form into complex curves and shapes, high initial cost, significant mass, and high thermal mass which can affect curing cycles [11](#page=11). #### 1.2.2 INVAR tooling INVAR is a steel alloy containing 36-42% nickel [12](#page=12). * **Extra Pro:** CTE close to zero, making it ideal for both thermoset and thermoplastic composites [12](#page=12). * **Pros:** Retains steel's advantages (high durability, weldability, repairability, vacuum integrity) [12](#page=12). * **Cons:** High initial cost, limited availability, difficult to form complex shapes, and significant mass [12](#page=12). #### 1.2.3 Aluminium tooling * **Pros:** Low cost, easily machined into complex shapes, excellent thermal conductivity for efficient heat transfer, lower mass than steel, and easily repaired by welding and grinding [13](#page=13). * **Cons:** High CTE makes it unsuitable for highly compound curved parts. It's also not suitable for curing above 350 degrees Celsius and is less durable than steel [13](#page=13). #### 1.2.4 CFRP tooling Carbon Fiber Reinforced Polymer (CFRP) tooling. * **Pros:** CTE of the tooling can be matched to the product, low density for easy handling, and can be a low-cost tooling method [14](#page=14). * **Cons:** Temperature is restricted by the glass transition temperature (Tg) of the resin, it is less durable, and its production is labor-intensive [14](#page=14). > **Tip:** When selecting tooling material, consider the part's complexity, production volume, required accuracy, and the curing temperature and CTE of the composite material [13](#page=13) [14](#page=14) [5](#page=5). ### 1.3 Composite manufacturing steps The manufacturing process typically involves layup, vacuum bagging, and curing. #### 1.3.1 Layup This is the process of placing composite material plies onto the tool [15](#page=15) [17](#page=17). * **Tool Material:** Often a negative steel mold [15](#page=15). * **Assisted Placement:** Laser Projection Systems (LPS) can assist in accurately placing plies [15](#page=15) [17](#page=17). * **Quality Control (QC):** Crucial at this stage to ensure correct ply orientation and placement [15](#page=15) [17](#page=17). * **Debulking:** Compacting the laminate between plies to remove trapped air and consolidate the material is performed during layup [17](#page=17). > **Example:** In an aerospace application, precise placement of each carbon fiber ply on a steel mold is critical. A laser projection system guides the operator, and debulking steps are performed after every few plies to prevent voids [15](#page=15) [17](#page=17). #### 1.3.2 Vacuum bagging Once all plies are placed, vacuum bagging is applied to achieve compaction and prevent air entrapment [18](#page=18) [19](#page=19). * **Functions:** * **Compaction:** Applies external pressure to consolidate the laminate [18](#page=18). * **Air Prevention:** Seals the layup to prevent atmospheric air from entering the laminate [18](#page=18). * **Main Components:** * **Vacuum Bagging Film:** An airtight plastic sheet forming the outer seal [18](#page=18). * **Sealant Tape:** Creates a leak-proof seal between the vacuum bag and the tool [18](#page=18). * **Breather Layer:** A porous material that allows air to escape from the laminate and reach the vacuum port [18](#page=18). * **Release Film:** Prevents the laminate from bonding to the tool or the breather layer [18](#page=18). * **Peelply:** Acts as a barrier, allowing air to escape and providing a protective surface after cure, as well as an improved surface for subsequent bonding [18](#page=18). The bagging process involves placing all auxiliary layers, closing the vacuum bag securely, and performing a leak test [19](#page=19). > **Tip:** A thorough leak test before curing is essential. Leaks in the vacuum bag can lead to voids and poor part quality [19](#page=19). #### 1.3.3 Curing Curing is the process of consolidating and hardening the composite material, typically through heat and pressure [20](#page=20) [21](#page=21) [22](#page=22). * **Heating Methods:** * **Autoclave:** Uses temperature, vacuum, and external pressure to cure the part. This is a common method for high-performance composites [20](#page=20). * **Oven:** Used for "out-of-autoclave" curing, applying temperature and vacuum only [20](#page=20). * **Heated Mould:** Involves applying temperature, vacuum, and pressure directly to the mould [20](#page=20). * **Autoclave Cure Details:** * **Cure Cycle:** A specific sequence of temperature and pressure ramps and holds is followed [21](#page=21) [22](#page=22). * **Cure Monitoring:** Temperature is monitored using thermocouples [21](#page=21) [22](#page=22). * **Phases of Hardening:** The cure cycle typically involves a ramp-up phase, a hold phase (where hardening occurs), and a cool-down phase, with pressure and vacuum being controlled throughout [22](#page=22). #### 1.3.4 Release and finishing After curing, auxiliary materials are removed [23](#page=23) [24](#page=24). * **Waste Generation:** This stage often generates significant amounts of waste from consumable materials like release films and breather cloths [23](#page=23) [24](#page=24). * **Improvements:** Using reusable bags or double-sided molds can reduce waste [23](#page=23) [24](#page=24). * **Subsequent Steps:** The released part then undergoes machining (milling, drilling, waterjet, laser cutting), inspection, assembly, and final finishing [24](#page=24). > **Example:** After a part is cured in an autoclave, the vacuum bag, breather, release film, and peelply are carefully removed. This creates a significant amount of waste that manufacturers aim to reduce through process optimization or reusable tooling [23](#page=23) [24](#page=24). --- # Introduction to composite materials and their applications This section introduces the fundamental concepts of composite materials, outlining the lecture structure and key references, while briefly exploring design cases and end-of-life considerations. ### 1.1 Lecture overview and structure The lecture series on composites is structured into seven main topics [2](#page=2): 1. Introduction [2](#page=2). 2. Material properties [2](#page=2). 3. Production and tooling (Parts 1 & 2) [2](#page=2). 4. Assembly and joining [2](#page=2). 5. Automation and thermoplastics [2](#page=2). 6. Recycling/End of life [2](#page=2). 7. Degradation [2](#page=2). ### 1.2 Key reference materials Essential readings for this course include: * "Composites I" by Michael C. Niu (ISBN 962-7128-06-6, Edition 3 or higher) [2](#page=2). * Additional information and lecture slides are available on Moodle as PDF documents [2](#page=2). * "Composite materials: introduction" by R.P.L. Nijssen, also accessible on Moodle [2](#page=2). ### 1.3 Design case example A practical design case discussed is the requirement for a G650 overhang panel, illustrating the application of composite materials in specific engineering contexts [3](#page=3). ### 1.4 Importance of material properties and end-of-life considerations Understanding material properties is crucial in the design and application of composites. Furthermore, considerations regarding the end-of-life of these materials, including recycling and degradation, are vital aspects of their lifecycle [2](#page=2) [3](#page=3). > **Tip:** When studying composite materials, always link the theoretical concepts to practical design cases and their environmental implications, such as end-of-life scenarios [2](#page=2) [3](#page=3). --- # Resin infusion and injection processes This section details resin infusion and injection methods in composite manufacturing, contrasting them with prepreg processes and elaborating on Vacuum Assisted Resin Transfer Moulding (VARTM) and Vacuum Infusion, including the principles of resin flow through porous media and Darcy's law [27](#page=27) [28](#page=28). ### 3.1 Comparison with prepreg processes Resin infusion processes differ significantly from prepreg processes in their fundamental approach to composite manufacturing. In resin infusion, dry fabric is initially placed on a mould, followed by the application of a vacuum bag to create a seal. Resin is then introduced and flows through the dry fibers, driven by a pressure difference, before the part is cured. Conversely, prepreg processes involve laminating layers of fibers that are already pre-impregnated with resin onto a mould. A vacuum bag is also applied, and the stacked layers are then cured [28](#page=28). ### 3.2 Types of injection processes Injection processes for composite manufacturing can be broadly categorized into two main types: Resin Transfer Moulding (RTM) and Vacuum Assisted Resin Transfer Moulding (VARTM), also known as vacuum infusion [29](#page=29). #### 3.2.1 Resin Transfer Moulding (RTM) RTM involves the injection of resin into a closed mould cavity under pressure, where it impregnates a pre-placed reinforcement material [29](#page=29). #### 3.2.2 Vacuum Assisted Resin Transfer Moulding (VARTM) / Vacuum Infusion VARTM utilizes a vacuum to draw resin through the reinforcement material, which is typically laid up in an open or semi-closed mould. The process begins with dry fibers being placed on a mould. A vacuum bag is then applied to compact the laminate and create a sealed environment. Resin is subsequently infused through the material by the vacuum pressure. Curing can occur under ambient or elevated conditions [30](#page=30). **Advantages of VARTM:** * It is a closed process, offering better health, safety, and environment (HSE) conditions [30](#page=30). * Tooling expenses are moderate [30](#page=30). * It is suitable for manufacturing large composite parts [30](#page=30). **Disadvantages of VARTM:** * Product quality is considered medium [30](#page=30). * Process control is not optimal [30](#page=30). > **Tip:** VARTM is often favored for its cost-effectiveness and suitability for large structures compared to prepreg methods, though it may compromise on the highest levels of quality and process precision. ### 3.3 Tooling for vacuum infusion Tooling for vacuum infusion processes must meet specific requirements to ensure successful part fabrication. The tooling must be leaktight to maintain the vacuum integrity. It also needs to be temperature resistant to the processing conditions, which are typically lower than those required for prepregs. Curing often occurs post-moulding in an oven. Optionally, heated tooling, which can be oil, electric, or water-heated, can be employed. The mould is often manufactured in two steps: first, a master mould or plug is created, and then the actual mould is produced from this master [31](#page=31) [32](#page=32). ### 3.4 Typical products from VARTM The VARTM process is well-suited for manufacturing large composite components. Examples of typical products include wind turbine blades, bridges, and boat hulls, highlighting the capability to produce very large parts [33](#page=33). ### 3.5 Curing of large parts The curing of large composite parts manufactured via infusion processes can be achieved through several methods. Room temperature cure is possible due to the exothermic reaction of the resin, though this requires careful management. Alternatively, heated tooling can be used or the part can be cured in an oven or a hotbox [35](#page=35). ### 3.6 Comparing vacuum injection versus prepreg production Vacuum injection processes offer several advantages over prepreg production: * There are no time restrictions on the layup process [36](#page=36). * The process pressure of 1 bar simplifies and reduces the cost of tooling [36](#page=36). * Many resins can cure at room or slightly elevated temperatures, simplifying post-cure requirements [36](#page=36). * Material costs are significantly lower [36](#page=36). * Material placement is quicker than with prepregs [36](#page=36). However, vacuum injection also presents certain drawbacks: * There is less control over the production process, leading to a higher risk of errors [36](#page=36). * The process is more sensitive to leaks, which can compromise vacuum integrity [36](#page=36). * Errors in resin mixing can occur [36](#page=36). * Predicting resin flow through the fiber material is challenging [36](#page=36). * There is less control over the final product quality [36](#page=36). * The resulting parts may have higher void content [36](#page=36). * The fiber volume fraction ($V_f$) achieved might be lower [36](#page=36). ### 3.7 Principles of resin flow in porous media The injection of a viscous liquid resin into a porous medium, such as a dry fiber structure, is governed by specific physical principles. The speed of the resin flow is inversely dependent on its viscosity, denoted by $\eta$. The resin will naturally follow the path of least resistance. This flow resistance is related to the open channels within the porous medium, a property quantified by permeability, denoted by $k$. Permeability is inversely related to the fiber volume fraction [38](#page=38). #### 3.7.1 Darcy's Law Darcy's law describes the flow of a fluid through a porous medium. In its differential form, the pressure gradient ($dp/dx$) is proportional to the flow velocity. A key representation of Darcy's law is: $$Q = -\frac{kA}{\eta} \frac{dp}{dx}$$ where: * $Q$ is the volumetric flow rate (m$^3$/s) [39](#page=39). * $k$ is the permeability of the porous medium (m$^2$) [39](#page=39). * $A$ is the cross-sectional area of the porous medium perpendicular to the flow (m$^2$) [39](#page=39). * $\eta$ is the dynamic viscosity of the fluid (Pa·s) [39](#page=39). * $dp/dx$ is the pressure gradient along the flow direction (Pa/m) [39](#page=39). An alternative form of Darcy's law used in the context of filling a mold of length $L$ is: $$Q = -k \cdot A \cdot \frac{\Delta P}{\eta \cdot L}$$ Here, $\Delta P$ represents the total pressure drop across the length $L$ [39](#page=39). > **Tip:** Understanding Darcy's Law is crucial for predicting and controlling how resin will fill complex mold geometries in infusion processes. Factors like viscosity, permeability, and pressure difference are key variables. The figures and illustrate how the pressure drop and, consequently, the flow front speed change as the mold fills. As the resin flows, the pressure drop across the filled portion of the mold decreases. This leads to a reduction in the flow front speed as the mold progressively fills [39](#page=39) [40](#page=40). ### 3.8 Injection pattern and simulation Designing an effective injection pattern is vital for ensuring that the resin completely and uniformly fills the mold without trapping air. This often involves a combination of simulation and testing [41](#page=41). > **Example:** For a complex mold, simulation software can predict the resin flow paths, identify potential dry spots or air traps, and help optimize the placement of resin inlets and vacuum outlets before committing to physical tooling. The document includes visual references to vacuum infusion processes underscoring the practical application of these principles [42](#page=42) [43](#page=43) [44](#page=44). --- # CTE mismatch and its effects CTE mismatch is a significant phenomenon in composite manufacturing that can lead to undesirable outcomes such as springback and warping of composite parts during or after processing. This arises from differences in how constituent materials within a composite expand or contract with temperature changes [8](#page=8) [9](#page=9). ### 4.1 Understanding CTE Mismatch The Coefficient of Thermal Expansion (CTE) quantifies the tendency of a material to change its shape in response to temperature fluctuations. In composite materials, which are typically made of multiple constituents (e.g., fibers and a matrix), these individual materials often possess different CTE values. This difference is known as CTE mismatch [8](#page=8) [9](#page=9). ### 4.2 Mechanisms of Springback and Warping When a composite part is subjected to thermal cycles during manufacturing processes, such as curing or cooling, the constituent materials attempt to expand or contract at different rates due to their differing CTEs [8](#page=8) [9](#page=9). * **Internal Stresses:** This differential expansion or contraction generates internal stresses within the composite material [8](#page=8) [9](#page=9). * **Differential Strain:** Different layers or regions of the composite experience varying amounts of strain [8](#page=8) [9](#page=9). * **Deformation:** These internal stresses and differential strains can cause the material to deform from its intended shape, leading to either springback or warping [8](#page=8) [9](#page=9). * **Springback:** This refers to the elastic recovery of a material after a load or stress has been removed. In the context of CTE mismatch, it can manifest as a tendency for the part to return to a slightly different shape after cooling from processing temperatures [8](#page=8) [9](#page=9). * **Warping:** This is a permanent deformation of the part, causing it to deviate significantly from its planar or intended geometry. Warping is often a consequence of residual stresses built up during processing that are not fully relieved or balanced [8](#page=8) [9](#page=9). > **Tip:** Understanding the individual CTE values of all constituent materials is crucial for predicting and mitigating CTE mismatch effects. ### 4.3 Factors Influencing CTE Mismatch Effects Several factors can influence the severity of CTE mismatch and its resulting effects: * **Material Properties:** The specific CTE values of the fibers, matrix, and any other reinforcement materials are primary determinants [8](#page=8) [9](#page=9). * **Volume Fraction:** The relative proportions of different materials within the composite can significantly impact the overall CTE and the resulting stresses [8](#page=8) [9](#page=9). * **Laminate Stacking Sequence:** For layered composites (laminates), the order and orientation of the plies play a critical role in how thermal stresses are distributed and how the part deforms [8](#page=8) [9](#page=9). * **Processing Conditions:** The temperature profiles, cooling rates, and pressure applied during manufacturing can all influence the development and relaxation of internal stresses [8](#page=8) [9](#page=9). ### 4.4 Mitigation Strategies Addressing CTE mismatch is essential for producing high-quality composite parts. Strategies include: * **Material Selection:** Choosing constituent materials with compatible CTEs can minimize mismatch [8](#page=8) [9](#page=9). * **Design Optimization:** Carefully designing the laminate stacking sequence to balance thermal stresses across the thickness and in-plane of the part [8](#page=8) [9](#page=9). * **Process Control:** Implementing controlled heating and cooling cycles to manage stress development and relief [8](#page=8) [9](#page=9). * **Post-Processing Treatments:** Utilizing annealing or other treatments to reduce residual stresses after initial processing [8](#page=8) [9](#page=9). > **Example:** A composite part made with a high-CTE matrix and a low-CTE fiber, when heated during curing, will experience the matrix trying to expand more than the fibers. This can lead to internal stresses that, upon cooling, result in the part bending or warping. --- ## Common mistakes to avoid - Review all topics thoroughly before exams - Pay attention to formulas and key definitions - Practice with examples provided in each section - Don't memorize without understanding the underlying concepts

| Term | Definition | |------|------------| | Composites | Materials made from two or more constituent materials with significantly different physical or chemical properties which remain separate and distinct at the molecular level throughout the finished structure. | | Tooling | Molds, dies, or fixtures used to shape and solidify composite materials during the manufacturing process, defining the final geometry and surface quality of the part. | | Leak-tight | A condition where a system or component prevents the unintended passage of fluids or gases, crucial for maintaining vacuum and pressure during composite manufacturing. | | Compaction | The process of pressing layers of material together to remove voids and ensure good adhesion, often achieved through vacuum bagging in composite manufacturing. | | Durability | The ability of a tool or composite part to withstand repeated use or environmental conditions without significant degradation in performance or appearance. | | Coefficient of Thermal Expansion (CTE) | A measure of how much a material expands or contracts in response to changes in temperature. Mismatches in CTE between the tool and the composite part can cause defects like warping. | | Springback | The elastic recovery of a material after being deformed. In composite manufacturing, it refers to the tendency of the part to slightly change shape after being released from the mold due to internal stresses. | | Warping | A distortion or twisting of a material from its intended flat or regular shape, often caused by internal stresses or differential thermal expansion. | | Vacuum bagging | A composite manufacturing process where a flexible film is sealed over the part and tool, and then air is evacuated to compact the laminate layers and facilitate curing. | | Autoclave | A pressurized, heated vessel used for curing composite materials under controlled temperature, pressure, and vacuum conditions to achieve high performance. | | Layup | The process of placing layers of reinforcing fabric (plies) onto a mold in a specific orientation and sequence to build up the composite structure. | | Debulking | A process step in composite manufacturing where vacuum is applied to consolidate multiple layers of prepreg or dry fabric to reduce thickness and remove trapped air before final cure. | | Release film | A thin film applied to the mold surface or between layers in composite manufacturing to prevent the part from adhering to the mold or other materials. | | Breather layer | A porous material placed within a vacuum bag that allows air and volatile byproducts to escape from the laminate during cure, facilitating uniform compaction. | | Peelply | A fabric layer placed directly on the laminate surface before vacuum bagging, which can be peeled off after cure to provide a clean, textured surface suitable for secondary bonding and to allow air escape. | | Resin Transfer Moulding (RTM) | A composite manufacturing process where dry reinforcing fibers are placed in a closed mold, and then liquid resin is injected under pressure. | | Vacuum Assisted Resin Transfer Moulding (VARTM) / Vacuum Infusion | A composite manufacturing process where dry fibers are placed on an open mold, a vacuum bag is applied, and resin is infused into the fiber preform by vacuum pressure. | | Permeability (k) | A measure of a porous material's ability to allow fluids to pass through it. In resin infusion, it influences the speed and direction of resin flow through the fiber preform. | | Darcy's Law | A fundamental law in fluid dynamics that describes the flow of a fluid through a porous medium. It relates the volumetric flow rate to the pressure gradient, viscosity, and permeability of the medium. |