System parameters - inputs and outcomes - A108

 

System parameters - inputs and outcomes
Systems knowledge article
General system description breakdown
Document Type	Article
Document Identifier	108
Themes	Thermal management Material deposition management Flow and consolidation management Residual stress and dimensional control management
Tags	Consumables Equipment Equipment parameters Material parameters Tooling Tooling parameters Material Shape Shape parameters System control
Prerequisites	Systems Knowledge

Overview

In many applications, the outcomes of a given process are due to a number of interacting parameters. Considering the effect of all interacting parameters together is known as a systems level approach. Such an approach can be applied to composites manufacturing. In composite manufacturing processes, the interacting parameters may be attributed to the part material (M), part shape (S), tooling and consumables (T), or the equipment (E). For example, during thermal transformation, the equipment supplies temperature, pressure, and vacuum, which interacts with the material and geometrical properties of the part and tooling to affect the part quality. This includes outcomes such as final degree of cure (DOC), porosity, wrinkling, warpage, spring-in, etc. In other words, part quality is influenced by the MSTE inputs for each process step. This is a useful distinction, as it allows engineers and practitioners to simulate and tailor the appropriate parameters to pass specifications and achieve acceptable part quality.

Any parameter of a system can be constrained physically, by specifications, or by self-imposed desires of the manufacturer/designer. An example of a physical constraint would be the maximum temperature an unloaded oven can reach. This is dictated by the physical design of the oven and cannot be changed. A specification, on the other hand, can be physically changed but is in place to meet regulations. For example, a company in the supply chain for a large OEM may have process specifications imposed on them by the OEM. An example may include the cure window that the material must adhere to. Finally, a designer or manufacturer may impose their own constraints on their manufacturing processes in order to achieve their cost, rate, and quality metrics.

To understand how system parameters come together to influence the system response in a general sense refer to system interactions. For a detailed description of thermal management related system effects refer to the following pages:

• Effect of material in a thermal management system
• Effect of shape in a thermal management system
• Effect of tooling in a thermal management system
• Effect of equipment in a thermal management system

Level I

Level II

Material parameters

Link to material page in Systems Catalogue

Material parameters include the material properties, material formulation (eg. dry fabric, prepreg, resin etc.), and the associated specifications for the material. The specifications define the processing requirements specific to the material system. The Manufacturer Recommended Cure Cycle (MRCC) for a given resin system is an example. The material parameters along with the shape parameters define the outcome sensitivity for the system. That is, how the part responds to the imposed boundary conditions.

Shape parameters

Link to shape page in Systems Catalogue

The shape parameters include the physical description of the part. That is, its internal and external geometry. Each of these parameters will influence the way the part behaves during processing and must be considered. For example, a thick part will take much longer to reach a uniform internal temperature as compared with a thin part. Additionally, local shape changes may influence both mechanical and thermal properties of the part.

Tooling and consumable parameters

Link to tooling and consumables page in Systems Catalogue

Combined with the equipment parameters, the tooling parameters form the boundary conditions for the part. Tooling material and thickness are important parameters as these will influence the mechanical and thermal properties of the tool, which will in turn affect outcomes such as part temperature and residual stress induced by tool-part interaction. Material properties and thickness are also of importance for the consumables in a vacuum bag setup, as they too will influence heat transfer across the bagside of the part.

Equipment parameters

Link to equipment page in Systems Catalogue

Equipment applies energy (eg. heat) to the tool/part assembly in order to affect it in some way.

The equipment parameters define the environment in which the tooling, part, and/or raw materials sit. Common equipment parameters that can be controlled are temperature, pressure, and vacuum. In a convective heating system, airflow is another important parameter as it will greatly influence the heat transfer coefficient. However, airflow is difficult to control unless specifically designed for.

Outcomes

Outcomes are the parameters of a system that are tracked and evaluated in order to provide metrics for acceptance. The part thermal history is a common outcome. If the intended thermal history of the part falls within the required bounds, then the outcome is deemed acceptable. If all outcomes are deemed acceptable, then part quality is achieved. If an outcome is unacceptable then the part has not achieved the intended quality and troubleshooting must take place. Ensuring that the part achieves its thermal management outcomes is highly important, as not meeting the requirements could cause defects in the final part, which may not appear until the later in the factory workflow or even once the part is in service. For example, thermal outcomes such as part temperature may have a direct or indirect effect on later processing themes, such as residual stress and dimensional control.

Many outcomes may be challenging or detrimental to measure directly. Because of this, proxy outcomes are often used. Degree of cure (DOC) is an example. While the DOC of the part is of significant concern for thermal transformation, it is difficult to measure directly without advanced or destructive measurement techniques. Therefore, part temperature during cure is typically used a proxy.

Material parameters

Link to material page in Systems Catalogue

The material parameters along with the shape parameters define the outcome sensitivity for the system. That is, how the part responds to the imposed boundary conditions.

Typical material parameters

Material type	Material form
Epoxy resin Polyester resin Vinylester resin Carbon fibre Glass fibre Aramid fibre etc.	Dry fabric Prepreg Neat resin Laminate Sandwich panel etc.

The material parameters are the type and form of the material system. Together, these influence the material properties (see the table below). The properties can generally be classified as chemical, physical, or mechanical. In the case of thermal management, important properties are shown in the table below. There are also process requirements associated with the material system, which act as constraints. These define the global specifications of the material system, such as the acceptable fibre volume fraction for example. Furthermore, the process requirements also define the temperature, pressure, and vacuum history required for the material system to achieve its quality metrics. The manufacturer recommended cure cycle (MRCC), for example, represents the baseline process required to achieve the nominal mechanical properties of the material system. Additionally, there may be process requirements unique to the part itself.

Typical material properties of concern for thermal management (TM)

TM	Specific heat capacity	Thermal conductivity	Density	Fibre volume fraction	Degree of cure	Cure rate	Heat of reaction	Gelation time	Glass transition temperature	Degree of crystallization	Crystallization rate	Crystallization enthalpy	Melt temperature
Chemical					Y	Y	Y	Y
Physical	Y	Y	Y	Y					Y	Y	Y	Y	Y
Mechanical

The table can be simplified by characterizing the cure or crystallization kinetics for the material system. The cure kinetics captures the degree of cure, cure rate, heat of reaction, gelation time, and glass transition temperature for thermoset resins. Similarly, crystallization kinetics encompasses the degree of crystallization, crystallization rate, crystallization enthalpy, glass transition temperature and melt temperature for thermoplastic resins. As such, the primary material properties of concern with respect to thermal management are the specific heat capacity, thermal conductivity, density, fibre volume fraction, and the cure or crystallization kinetics. The former three properties and the fibre volume fraction (for composite tools) are also applicable to the tooling and consumables; they define how heat flows through the tool.

Shape parameters

Link to shape page in Systems Catalogue

The shape parameters include the physical description of the part. That is, its internal and external geometry. Each of these parameters will influence the way the part behaves during processing and must be considered. For example, a thick part may take longer to reach uniform internal temperature as compared with a thin part. Additionally, local shape changes may influence both mechanical and thermal properties of the part.

Typical shape parameters

External geometry	Internal geometry
Thickness Surface area Configuration	Volume Local shape changes

Typically, producibility requirements for the part require that the final shape be constrained. For example, in the case of L-shapes or C-channels, spring-in may result in a deviation of the laminate angle to an unacceptable level. As a result, measures to mitigate such shape changes may be taken.

Tooling and consumable parameters

Link to tooling and consumables page in Systems Catalogue

Combined with the equipment parameters, the tooling parameters form the boundary conditions for the part.

Typical tooling parameters

Geometry	Material properties	Layout
Thickness Volume Configuration (shape/substructure) etc.	Thermal conductivity Density Specific heat capacity Fibre content Coefficient of thermal expansion (CTE) Stiffness Hardness etc.	OML IML

Typical consumable parameters

Geometry	Material properties	Location
Thickness	Thermal conductivity Density Specific heat capacity Permeability	Position on part/tool

The above list is not exhaustive but covers many parameters associated with tooling and consumables. Most items can be further expanded to form a complex set of potential parameters. For example, the contour of a tool may be flat, C-shaped, L-shaped, cylindrical, etc. In many cases, the tooling and consumables are not heavily constrained for manufacturers, allowing for a number of variables to be tweaked. Tooling material and thickness are important parameters as these will influence the mechanical and thermal properties of the tool, which will in turn affect outcomes such as part temperature and residual stress induced by tool-part interaction. Material properties and thickness are also of importance for the consumables in a vacuum bag setup, as they too will influence heat transfer across the bagside of the part.

A complex part may require a complex tool, however there are practical limits as to the design of the tool. For example, very sophisticated metallic tools require challenging machining which might not be feasible. Moreover, the tool must generally be rigid enough to not deform during use. Therefore, there is often a minimum thickness requirement for the tool. Finally, the tool must be able to handle the process it is intended for. If the tool is to be involved in 500+ cure cycles, but is not durable enough, it may not be able to handle numerous temperature and pressure cycles; not to mention the use before and after cure. Similarly, the consumables must be able to withstand the processing conditions. For example, high temperature sealant tape and bagging material must be used if the process requires vacuum bagging under high temperature conditions.

Equipment parameters

Link to equipment page in Systems Catalogue

Equipment applies energy (eg. heat) to the tool/part assembly in order to affect it in some way.

In composites manufacturing, equipment apply energy to the material in many different ways. Mechanical work, heat, chemical, photochemical, or electrical energy are common. The mode of energy application depends on the equipment and the intended use. For example, robotic layup of prepreg imparts mechanical energy to deposit the material on the tool. Oven or room temperature curing of a part relies on the transfer of heat, whereas UV curing of a thermoset resin utilizes photochemical energy. Curing is also possible by strictly chemical energy, such as using ammonium hydroxide to cure. Finally, induction welding is an example of imparting electrical energy on a thermoplastic composite.

The equipment parameters define the environment in which the tooling, part, and/or raw materials sit. The parameters vary depending on the equipment and application. The table below represents general parameters for the given process steps. Common equipment parameters that can be controlled are temperature, pressure, and vacuum. In a convective heating system, airflow is another important parameter as it will greatly influence the heat transfer coefficient (HTC). However, airflow may be difficult to control unless specifically designed for.

Typical equipment parameters

Airflow	Applied temperature	Heating/cooling rate	Applied pressure	Applied vacuum	Volume	Position
Air velocity/distribution	Determined by cure cycle	Determined by cure cycle	Determined by pressurization cycle	Determined by vacuum cycle	Internal volume for tool-part loads	In reference to robotic layup

Depending on the controls of the equipment and the process, any one of these parameters could be a constraint. Generally, equipment constraints are defined by the production requirements and the equipment capabilities. The capabilities of the equipment are an obvious constraint. A part requiring a consolidation pressure greater than 1atm cannot be processed in an oven since an oven does not support pressure application beyond vacuum pressure. Production requirements are local specifications to support changes to production. This includes increasing the throughput rate, processing rate (eg. fast cure cycles), and defining the number of parts that may be placed in given piece of equipment for a given process. For example, an autoclave may be used as a bus stop where many different parts are cured at the same time (i.e. a batch cycle), or it may be a dedicated autoclave, designed to cure one specific part at a time.

Outcomes

The outputs of a system could be any number of parameters, useful or not. Outcomes are those parameters that are tracked for evaluation to define quality and producibility. They represent the state of the system and include the relevant output parameters for each MSTE class. For most manufacturing systems, the outcomes are often associated with the shape or material, as it is the final state of the part that is of concern. For example, the part temperature or part thickness.

It is important to recall the process-structure-property relationship, when referring to outcomes. The process influences the material structure, which in turn influences the material properties. For example, if the processing temperature is too low for a given thermoset resin, there will be a lower degree of crosslinking (i.e. lower degree of cure), resulting in lower molecular weight and reduced mechanical properties. Therefore, outcomes in upstream themes, such as thermal management will impact the outcomes in downstream themes, such as residual stress and dimensional control management.

Understanding of the part outcomes and their evolution from a chemical, physical, and mechanical perspective are key in being able to demonstrate material equivalency. This is necessary in demonstrating that lab scale tests are indicative the completed structure.

Common outcomes for the part. Adapted from.

PART	Thermal management	Material deposition management	Flow and consolidation management	Residual stress and dimensional control management
Chemical	Temperature history (min/max temp, time at max temp, etc) Gelation time Degree of cure
Physical	Degree of crystallization Glass transition temperature and time to vitrification Melt temperature Toughening precipitation Crystal morphology and distribution	Fibre volume fraction Fibre alignment/orientation Laps/gaps Resin-starved/resin-rich areas	Fibre volume fraction Wrinkling/waviness (morphology, distribution, etc.) Porosity and void distribution Ply bridging Ply thickness
Mechanical			Core movement/crush	Thermal strains Cure shrinkage Crystallization shrinkage Microcracks, delamination, disbonding, etc. Warpage Spring-in

Work is ongoing on a more expansive outcome matrix, which will be posted here shortly.

Material	Shape	Tool	Equipment

Cell filled	Do we care about this parameter in the factory cell/ Can the factory cell affect or change the parameter value
Cell marked with "X"	Actively measure, checking

Theme	Parameter of any process	Receiving	Testing	Storage	Material deposition	Thermal Transformation	Demoulding	Trimming and machining	Inspection	Assembly	Coating	Packaging and shipping	Service
Thermal and cure/crystallization management	EXTENDED OUTCOME TABLE COMING SOON
Material deposition and consolidation management
Residual stress and dimensional control management
Machine and assembly management
Quality and inspection management

Acceptable and unacceptable outcomes (defects)

An outcome may be deemed acceptable or unacceptable if it meets the producibility/process requirements of the part/material. An acceptable outcome infers part producibility. That is, the quality of the part is acceptable and may continue on to the next stage of the factory workflow. In order for producibility to be achieved, all outcomes must be deemed acceptable. If an outcome is unacceptable (i.e. it is outside the specification window), then it is considered a defect and part producibility has not been achieved; the process must be troubleshooted.

As a baseline, typical acceptance values follow those of the processing specs from the material supplier data sheets. For thermal management, this means following the manufacturer recommended cure cycle (MRCC) and tracking the necessary outcomes to ensure that the correct cycle is indeed imposed on the part. Often this will include placement of thermocouples along the part to track its temperature and heating rate throughout the process. Simple, yet powerful, outcomes to track are the minimum and maximum temperatures of the part during the cure cycle. Ensuring these fall within an acceptable threshold for an appropriate amount of time is of high value.

Outcomes of the equipment should also be defined and validated. An empty autoclave or oven airflow field represents the best case to achieve the required heating rates and hold temperatures. Therefore, given an oven or autoclave, an appropriate outcome for thermal management would be the temperature history of the air within the empty convective heating system. Generally, if the required temperature history cannot be achieved when the system is void of parts and tools, then the addition of parts and tools will add thermal resistances making it more difficult to achieve the necessary temperature history of the part. An exception to this is if the heat released due to the exothermic nature of the material is enough to achieve the required temperature for the part itself.

In an ideal world, manufacturers would measure performance outcomes directly. However, this is not feasible if the measurement is destructive to the integrity of the part. As such, proxy outcomes are often used to represent the intended outcome. For example, one common method to measure the degree of cure (DOC) of a part is differential scanning calorimetry (DSC). However, this requires that a sample be taken from the final part. Not only is this time and labour intensive, but it is destructive to the part. Non-destructive techniques for measuring DOC may be employed, such as using Fourier-transform infrared spectroscopy (FTIR). However, this is an advanced technique that again increases labour and equipment costs. As such, part temperature history is a common outcome for thermoset resins because it can be used as a proxy for the final degree of cure of the part, which is the true outcome of concern.

Related pages

Page type	Links
Introduction to Composites Articles
Foundational Knowledge Articles
Foundational Knowledge Method Documents
Foundational Knowledge Worked Examples
Systems Knowledge Articles	System interactions - A109 System parameters - inputs and outcomes - A108
Systems Knowledge Method Documents
Systems Knowledge Worked Examples
Systems Catalogue Articles
Systems Catalogue Objects – Material
Systems Catalogue Objects – Shape
Systems Catalogue Objects – Tooling and consumables
Systems Catalogue Objects – Equipment
Practice Documents
Case Studies
Perspectives Articles

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.

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.

The allowable range of temperature, pressure, and vacuum values at specific times in the cure cycle. These limits are usually defined as process requirements.

A central processing theme in the manufacturing cycle. This theme is concerned with managing the thermal response of materials during storage and handling or parts/tools when they are subsequently heated.

Pre-impregnated (prepreg) material refers to fibre that is already combined with resin. It is the most common material form used in aerospace.

During prepreg production, (e.g. fibres are run through a resin bath), prepreg is heated and partially cured to B Stage (< 5 % degree of cure). Thermoset prepregs (e.g. epoxy prepreg) have to be kept in a freezer at around -20 °C. At room temperature, the epoxy starts to cure.

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.

Manufacturer's Recommended Cure Cycle (MRCC).

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")

Volume fraction of either matrix or fibres with respect to total composite volume (matrix + fibre).

Polymerization of thermoset resins is an exothermic reaction and heat is generated during the curing process. A thermosetting resin has the potential to release a certain amount of energy while curing. This is called the total heat of reaction, H_R, with a unit of J/g (SI units).

The heat of reaction during polymerization is measured using a Differential Scanning Calorimeter (DSC) equipment measuring much energy/heat comes out of the reaction for a small resin sample.

The glass transition temperature (T_g) is the temperature region where the polymer transitions from a hard, glassy material to a soft, rubbery material. It is one of the most important properties of any amorphous polymer.

Thermosets are a class of polymer that undergo polymerization and crosslinking during curing with the aid of a hardening agent and heating or promoter. Initially they behave like a viscous fluid. During curing, they change from viscous fluid to rubbery gel (viscoelastic material) and finally glassy solid.

If heated after curing, initially they become soft and rubbery at high temperatures. If further heated, they do not melt but decompose (burn)

Comes in two parts: part A (resin) and B (hardener). When mixed, curing reaction starts and is not reversible.

Examples include epoxy or polyester.

A class of polymer, some common examples include polypropylene and polyethylene.

They soften and melt upon heating (i.e. potentially recyclable), high viscosity when melted, therefore difficult to saturate fibres. Usually needs a lot of pressure and heat to process.

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

A central processing theme in the manufacturing cycle. This theme relates to management of internal stresses that occur as the material undergoes differential thermal and physical phase change volume changes and viscoelastic property development.

The commonality of material level properties at all scales of the ‘building block’ (coupon level to production scale structures) in terms of chemical–physical–mechanical states such as degree of cure/degree of crystallization, fibre volume fraction, residual stresses, etc.

The failure mode characterized by a partial or full separation of adjacent laminae. Delamination is unique to laminated structures, and can be induced by impact load, fatigue load, and quasistatic load. (Also known as interlaminar separation)

The continuous material phase that binds the reinforcement together, maintains shape, transfers load, protects the reinforcement from environment and damage, and provides the composite support in compression.

Desirable characteristics:

• Moisture/chemical resistance
• Low density
• Processability

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).

Differential Scanning Calorimetry (DSC).