Flow and consolidation management - A158
| Flow and consolidation management | |
|---|---|
| Systems knowledge article | |
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| Document Type | Article |
| Document Identifier | 158 |
| Themes | |
| Prerequisites | |
Overview[edit | edit source]
Flow and consolidation management is concerned with knowing, understanding, and managing how the matrix and reinforcement move during/after the materials deposition process steps. The debulking, consolidation and compaction of a laminate is critical because material properties, part dimensions, part weight, and part quality are greatly affected by the fibre volume fraction and the magnitude of porosity and dry fibres inside a laminate.
The combined effect of the material deposition, flow & consolidation, and thermal transformation steps determine local porosity, resin/fibre volume fraction, and fiber misalignment (misorientation, wrinkling and waviness).
Porosity[edit | edit source]
Porosity is the collective term given to regions within composite laminates that are neither fibre, nor matrix. Porosity above a certain amount (typically 2% in most aerospace applications) is considered a defect due to its effect on mechanical properties. Porosity can be caused by voids, or a lack of resin (dry fibres) due to other effects.
Voids are generated by the presence of entrapped air during the layup process, volatiles/off-gassing of entrapped moisture or by-products during thermal transformation, and vacuum bag or tooling leaks. Voids are dissipated by gas transport out of the part, through some type of the breather system, and out through the vacuum lines, or by void shrinkage/collapse due to high local resin pressure. Typical sources and sinks of voids in a laminate are shown in the figure below.
Dry fibre regions are those that have no matrix in them at all because no resin flowed into them during manufacturing. This causes substantial local degradation in mechanical properties. Additionally, areas that have more than the intended matrix content are of concern. These resin rich areas have a surplus of resin and a deficit of fibers, making them structurally brittle, weak, and heavy.
Both infusion and prepreg processes involve the flow of resin relative to the fibers. In the case of infusion, the flow of resin into the empty fiber architecture is clearly the basis of the material deposition process step for the matrix. Flow on both the macro-scale (through the laminate and around each fibre tow), and the micro-scale (in-between the filaments of each tow) are necessary to obtain good quality parts. Improperly developed infusion processes can be particularly prone to dry fibre regions, since the physics of the infusion process will determine if the resin flows into all regions of the part. Well developed (robust) infusion processes can be consistent, reliable and reduce environmental hazards.
The importance of resin flow in prepreg processes may be more subtle but just as important. Resin flow is important in the debulking process (by which excess gaps and air are removed from the part during layup) and the flow/consolidation portion of the thermal transformation step (where gas is removed from the part and where resin may flow from a high pressure zone of the part to a low pressure zone).
During the consolidation step, geometry (part shape) plays an important role. Concave areas are pressure reducers whereas convex areas are pressure intensifiers. This effect is amplified with increasing part thickness. Gaps caused by poor positioning of the layers, improper placement of the vacuum bag, and other deviations from the nominal will exacerbate the situation.
Future content[edit | edit source]
The above is just a summary of some important concepts and considerations to do with flow & consolidation management. Further details on topics such as volume fraction and fibre misalignment are to come.
The content related to flow & consolidation management in the Knowledge in Practice Centre is currently quite limited as we are in the early stages of content creation and have focused on the theme of thermal management. We encourage you to check back in the future for more content in this theme.
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Welcome
Welcome to the CKN Knowledge in Practice Centre (KPC). The KPC is a resource for learning and applying scientific knowledge to the practice of composites manufacturing. As you navigate around the KPC, refer back to the information on this right-hand pane as a resource for understanding the intricacies of composites processing and why the KPC is laid out in the way that it is. The following video explains the KPC approach:
Understanding Composites Processing
The Knowledge in Practice Centre (KPC) is centered around a structured method of thinking about composite material manufacturing. From the top down, the heirarchy consists of:
- The factory
- Factory cells and/or the factory layout
- Process steps (embodied in the factory process flow) consisting of:
The way that the material, shape, tooling & consumables and equipment (abbreviated as MSTE) interact with each other during a process step is critical to the outcome of the manufacturing step, and ultimately critical to the quality of the finished part. The interactions between MSTE during a process step can be numerous and complex, but the Knowledge in Practice Centre aims to make you aware of these interactions, understand how one parameter affects another, and understand how to analyze the problem using a systems based approach. Using this approach, the factory can then be developed with a complete understanding and control of all interactions.
Interrelationship of Function, Shape, Material & Process
Design for manufacturing is critical to ensuring the producibility of a part. Trouble arises when it is considered too late or not at all in the design process. Conversely, process design (controlling the interactions between shape, material, tooling & consumables and equipment to achieve a desired outcome) must always consider the shape and material of the part. Ashby has developed and popularized the approach linking design (function) to the choice of material and shape, which influence the process selected and vice versa, as shown below:
Within the Knowledge in Practice Centre the same methodology is applied but the process is more fully defined by also explicitly calling out the equipment and tooling & consumables. Note that in common usage, a process which consists of many steps can be arbitrarily defined by just one step, e.g. "spray-up". Though convenient, this can be misleading.
Workflows
The KPC's Practice and Case Study volumes consist of three types of workflows:
- Development - Analyzing the interactions between MSTE in the process steps to make decisions on processing parameters and understanding how the process steps and factory cells fit within the factory.
- Troubleshooting - Guiding you to possible causes of processing issues affecting either cost, rate or quality and directing you to the most appropriate development workflow to improve the process
- Optimization - An expansion on the development workflows where a larger number of options are considered to achieve the best mixture of cost, rate & quality for your application.
