Systems approach to composite materials - A230

Systems approach to composite materials
Document Type	Article
Document Identifier	230

Overview

A practitioner is always driven by the performance (function) of the product, but constrained by the manufacturing. In the case of composite materials, manufacturing is often challenging. Because of its relatively new emergence, the maturity of the manufacturing of fibre reinforced polymer composites (FRPs) is not as well established as it is for traditional materials such as metals or ceramics. In order to reduce the risk associated with composites manufacturing, a structured systematic approach should be taken towards the development of best practice.

Manufacturing Relationship to Design (MSTEP Approach)

Link to MSTE Classes within the MSTEP Approach in Systems Knowledge

As popularized by Ashby, engineering design can be described as the interplay of four classes of choices: the intended function/performance, the choice of material, the choice of shape, and the process that can turn the required material into the desired shape of the product . From this thinking comes a structured approach to selecting the best material and shape for the product, whilst taking into account the feasibility of such material/shape combination through consideration of the process. However, and in particular with composites, the definition of process should be expanded to cover two other items, namely tooling and equipment, that then allow for a full definition of the process.

Traditional (Ashby) framework for engineering design and manufacturing.	CKN's expanded MSTEP approach to manufacturing.

MSTE classes

The KPC framework is an expansion of the traditional design and manufacturing approach to more explicitly address the process. Traditionally, design can be thought of in terms of the desired function (F), material (M), and shape (S) of the part. The process step (P) is not only a function of the material (M), shape (S), but also the tooling and consumables (T), and equipment (E) that are used. Particularly for composites, the interaction of material, shape, tooling & consumables, and equipment during a process step (abbreviated as MSTEP) is critical in determining the outcome of that particular manufacturing process step, and ultimately the quality of the finished part.

Note that the MSTEP depiction of a manufacturing process represents only a single process step of the entire manufacturing workflow. The complete manufacturing process is a collection of individual process steps that come together to form the factory manufacturing process.

Learn more about the MSTE classes within the MSTEP approach, and their interactions and impact on processing outcomes here.

Defining a manufacturing process

Discretized, a process is nothing more than a set of equipment and tooling used to perform a specific action (or process step) on the part or material. And a complete manufacturing process is the collection of process steps required to manufacture the finished part from raw material to out the factory door. Defining which steps to perform and which equipment and tooling to use is based on the initial material state and the part geometry to be constructed. In other words, it is the part and material that determine which processing steps must be performed and which equipment/tooling can be used to perform them. The layout of these processing steps are what define the manufacturing workflow.

This is illustrated in the following example, which shows a resin transfer moulding (RTM) process workflow broken down as a collection of individual process steps.

Example workflow for a resin transfer moulding (RTM) manufacturing process.

By convention, the composites industry names and classifies manufacturing processes after one or more critical processing steps in the manufacturing workflow or after the material form that is processed. Indeed, many composite manufacturing processes contain very similar steps, however, they may differ in process classification due to differences in a particular process step or the material forms involved. Across the entire manufacturing factory workflow, classified processes might only differ from one another in the arrangement of process steps, inclusion/exclusion of specific steps, and/or differences in the equipment, tooling, part (shape), and material involved.

For examples of typical composite manufacturing processes with a description of the key process step(s) that influence its naming and classification, see the common composite manufacturing processes page.

The manufacturing factory

Link to Factory Description in Systems Knowledge

Manufacturing process steps must occur within a physical space in the factory. This is what is referred to as a factory cell, wherein specific equipment and tooling are housed. The production part flows through the factory, taking shape with each successive processing step it undergoes. For each process step there may be a different MSTEP combination. Moreover, there may be multiple process steps that occur within the same physical space (cell) in the factory.

Viewed using a systems level approach, the part being produced moves through several distinct cells within the factory during its manufacturing process. Within each cell, the part is subject to various processes resulting in trackable outcomes. A general factory layout depiction organized by various factory cells is shown below. Example factory layout:

Factory overview displaying relations between the equipment, tooling, part (shape), and material as the part flows through the factory.

Learn more about the systematic breakdown of a composite manufacturing factory here.

Manufacturing Themes

Link to Systems Knowledge volume

The interactions that occur within each factory cell, as well as the outcomes, can be classified according to processing themes. In the processing of composite materials, there are five primary processing themes. A given manufacturing process may incorporate multiple themes, however this discretized structure can still be used to systematically approach the problem. In order to achieve intended manufacturing outcomes, one must properly control and track all process state-variables across each of the themes. Any outcomes arising in upstream processes (those within TM for example) will influence downstream outcomes (those within RSDM for example).

These themes are, in order of general manufacturing steps:

1. Thermal and cure/crystallization management (TM)
2. Materials deposition and consolidation management (MDCM)
3. Residual stress and dimensional control management (RSDM)
4. Machining and assembly management (MAM)
5. Quality/inspection management (QIM)

You can learn more about the composite processing themes here.

Manufacturing Outcomes

Link to Manufacturing Outcomes within the System Parameters page in Systems Knowledge

Manufacturing outcomes are the result of the intrinsic material process-structure-relationship associated with composite materials. Outcomes are those parameters that are tracked for evaluation to define quality and producibility. Outcomes that fail to satisfy the set upon manufacturing requirements of the processed material are known as defects.

Manufacturing outcomes may be measured or tracked as:

• Intermediate outcomes – (e.g. temperature seen in a material) changes of a material during a process step.
• Final outcomes – (e.g. final resin degree of cure) usually measured at the end of a process step and independent of any knowledge of process history.

It should be pointed out that manufacturing outcomes are very often dependent on system interactions. For a given manufacturing outcome of interest, upstream dependencies have the potential to be root causes for downstream dependencies as ‘knock-on’ effects.

Click here to learn more about and examples of manufacturing outcomes and their system dependencies, (in the Systems Knowledge volume) where this topic is covered in more detail.

References

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Engineered materials (designed to have specific properties) made from two or more constituent materials with different physical or chemical properties. The constituents remain separate and distinct on a macroscopic level within the finished structure.

With regards to manufacturing, risk is the combination of the probability and consequences of undesirable manufacturing outcomes. Manufacturing risk can lead to technical issues, program/schedule delays, and cost overruns.

Any manufacturing and/or decision making activity that occurs during any stage of the development design cycle (e.g. conceptual design to production).

In the context of Knowledge in Practice, practice refers to the systematic use of science based knowledge to reduce composites manufacturing risk, cost, and development time.

Material and process (M), Shape (S), Tooling and consumables (T) & Equipment (E) - all have an interlinked effect on the Process step (P). (See MSTE factory ontology)

For polymer matrix composites (PMCs), resin refers to the matrix; the continuous material phase that binds the reinforcement together, maintains shape, and transfers load. Resins are divided into two main groups: thermosets and thermoplastics.

Resin transfer moulding (RTM) involves loading a preform into a two (or more) piece, matched tool, closing it, and injecting resin under pressure (~15-100 psi, or ~1-7 bar).

Well suited to small to medium sized parts, limited to large sizes due to injection pressure loads and tool cost.

An individual station within a factory where a given set of tasks are accomplished (also known as a "work cell"). Some cells may directly add value to the product (e.g. deposition), while others may serve support roles that are critical to maintaining part quality (e.g. receiving, storage, inspection & shipping).

Key components of all composite manufacturing processes. Collectively, the four themes represent the time-temperature-pressure-vacuum history, which is traditionally used to define a manufacturing cycle.

The four processing themes are:

• Thermal management
• Material deposition management
• Flow and consolidation management
• Residual stress and dimensional control management

(Same as "Theme")

Outcomes represent the range of response/sensitivity to factory system attributes. Those that fail to satisfy manufacturing requirements are known as defects. Examples of manufacturing outcomes include process parameter outcomes, material structure outcomes, and material performance outcomes.

The capability of the manufacturing process to produce parts of acceptable quality (e.g. meet engineering, manufacturing, regulatory requirements), repeatably and robustly.

Degree of cure (DOC) is an indication of how far the chemical curing reaction (crosslinking process) has advanced in a thermoset resin.

DOC is defined with a number between 0 and 1 (or 0% and 100%) where 100% is a fully cured resin. It does not have to fully reach 100% for the resin to become solid or the part to be used. In some aerospace applications, resins are only cured to about 90%. Higher the degree of cure, higher the mechanical properties.