Troubleshooting scale-up issues from thin to thick parts - P140

 

Troubleshooting scale-up issues from thin to thick parts
Practice document
Document Type	Practice
Document Identifier	140
Themes	Thermal management
Tags	Cure and crystallization Size scaling Thermoset cure kinetics
Objective functions	Cost Maintain Rate Maintain Quality Increase
MSTE workflow	Troubleshooting
Prerequisites	Effect of equipment in a thermal management system Effect of shape in a thermal management system Effect of tooling in a thermal management system Effect of material in a thermal management system Heat of reaction System interactions Thermal behaviour

Q: "I am trying to make thicker parts than I have before with a given resin system and am experiencing large exotherms and associated part quality issues. How can I adjust my process to handle these larger parts?"

A: This question is explicitly related to thermal management of thick parts. It is a common effect to reach higher maximum temperatures in a thick part as compared to a thin part because of the difference in volume to surface area of the part. Larger part volume (which generates heat during cure) and the same surface area for conduction and convection of heat away from the part tends to cause higher part temperatures (all else being equal). A fundamental concept in materials science is that process history leads to microstructure evolution, which in turn determines quality as well as mechanical and functional properties. Ultimately, it is necessary to adjust some aspects of the thermal transformation process step to ensure that thick parts have similar thermal histories as thin parts.

Considering that you already have the material, part shape, tooling & consumables, and equipment defined. The options for mitigation are somewhat more limited than if you were strictly in the development phase; however, the similar development practice document is also worth referencing (Ensuring quality during curing of thick parts). Mainly, the mitigation measures you can follow are:

• Decrease temperature ramp rates for the cure cycle (if elevated temperature cure is used)
• Insert intermediate hold temperatures (if elevated temperature cure is used) to allow the cure reaction to advance further while the part is still colder
  • This allows the exothermic heat to be released and raise the part temperature without raising it to the point of thermal degradation; however, it can have implications for flow and consolidation management and residual stress and dimensional control management
• Increase the convective heat transfer coefficient (see Convection) to improve heat transfer away from the part
  • This is particularly useful for room-temperature curing resins where there will be a substantial difference in temperature between the surrounding air and the hot part
  • This typically means increasing air velocity over the part and tool and may involve modifying the sub-structure of the tool to allow for increased airflow
• Adjust the chemistry of the resin system to reduce the rate of the cure reaction
  • Typically only possible for polyesters and vinylesters where the reaction is started by an initiator that controls the amount of free radicals, which in-turn affect the rate of the reaction (epoxies cannot be adjusted in this way)
• Increase the thermal mass of the tool
  • More thermal mass in the tool will reduce the heating rate of the part near the tool (effectively reducing the temperature ramp rate) since more energy is required to heat the tool-part assembly
  • More thermal mass in the tool means that it can absorb more heat energy from the exothermic cure reaction without the temperature increasing as much near the tool
  • Increasing thermal mass during troubleshooting likely means attaching additional material to the backside of the tool rather than rebuilding the tool (case dependent)
• Increase the thermal diffusivity of the tool
  • A tool material with high thermal diffusivity will be better able to transfer energy from where the thick part section is to where the colder thin section is; effectively improving heat flow away from a thick section (only useful where there are regions of the part with different thickness)
  • Active cooling such as water-cooled tooling is one way to effectively redistribute heat to the rest of a tool, or to an external heat sink/heat exchanger (at the expense of complexity)

Overview

Interaction between material, shape, tooling & consumables, and equipment

A fundamental concept in materials science is that process history leads to microstructure evolution, which in turn determines quality as well as mechanical and functional properties. As explained in Systems Knowledge, composites processing is a complex interaction between material response, part shape and dimensions, tooling choices, and equipment behavior. Any change to the MSTEP collection may affect the manufacturing outcomes. Ultimately the goal is to control the interactions of the material, shape, tooling & consumables, and equipment (MSTE) so that the process history in the thick parts is as close as possible to how the process history was in the thin parts. When going from thin to thick parts, the material and shape will generally stay the same, so this means adjusting only tooling & consumables and equipment in order to keep the process history of the thick part as close as possible to the process history of the thin parts.

Before deciding what to change for thick parts, it is best to first understand the extent of the problem. Since you are troubleshooting and already have a physical process to test, the most useful way to do this is to perform an experimental thermal profile. In this way you can use temperature sensors to record the actual temperatures that you are reaching in the thick part. Alternatively, you could do the same using a numerical thermal profile, but this will have more uncertainty and is better utilized at the development stage to reduce the likelihood of needing to troubleshoot an existing problem. Temperatures such as the part surface, toolside temperatures (at the part-tool interface), tool temperatures, and temperatures at the midplane of the part (where temperatures due to the exotherm are usually highest) are useful to record. It is then best to compare these with similar measurements from a thin part that has known good quality. This way, you have a reference for both a good and bad part. You can then proceed with modifying the paramerters of your manufacturing process in order to bring the thermal profile of the thick part closer to that of the thin part. The following sections will provide additional details on what effect different decisions will have.

Thermal management considerations

Link to thermal management

From a thermal management perspective, the first thing is to ensure that the material in the thick parts sees a thermal history comparable to the thin parts. This is a necessary but not sufficient requirement (you may still have other defects, such as voids and porosity, resin rich/poor regions, fibre wrinkling and waviness, and so forth).

Remembering the MSTEP framework, evaluate the thermal history relative to the good quality thin parts. You can do this using:

1. Thermal Simulation
2. Thermal Test
3. Combination of thermal simulation and test

Simulated cure temperature profile of a composite part in an oven or autoclave (convection heating). Air temperature is shown in blue and the part temperatures are shown in green. The thermal lag of the part can be seen during warm-up, and an exothermic peak temperature is seen to exceed the air temperature. The specification bounds of the temperature profile for the part are shown in red.

An example of thermal history evaluation using thermal simulation is shown in the figure on the right. In this figure it can be seen that as the air temperature (blue line) is increased, the part temperature (green lines) also increases, but with some lag owing to thermal resistances related to convection and conduction, as well as the thermal mass of the tool and part itself. Once the cure reaction takes hold, the heat of reaction adds energy directly into the composite part, resulting in an exotherm, which in this case causes the part temperature to exceed the applied air temperature. However, provided that the temperatures do not fall outside of the process specification (shown by the red lines), the quality of the part should be acceptable. Of course, it is preferable to keep the actual thermal profile as similar as possible to the thermal profile of the good quality thin parts so that the process history and microstructural evolution is as similar as possible. In practice it is impossible to achieve the exact same thermal history in all regions of the part (particularly through the thickness of thick parts), but steps outlined below can be taken to minimize discrepancies.

Effect of Material

Link to effect of material in a thermal management system

In general, changes to the material system being used are not desirable since it can affect many outcomes such as physical and mechanical properties. However, it may be possible to alter the concentrations of components in the resin chemistry to slow down the cure reaction for some resin systems. For example, the initiator type and concentration added to polyester and vinylester resins can be used to modify the rate at which the cure reaction takes place. The same in not true for epoxy resin systems, where the final properties of the matrix are very sensitive to the specific ratio of epoxy resin to curing agent.

Effect of Shape

Link to effect of shape in a thermal management system

The shape of the part is representative of its geometry, including internal, external, local, and global features. The part geometry plays a key role in influencing heat transfer into and out of the part. Moreover, the dimensions and local features of the tool-part assembly can redirect and even block airflow in a convective heating system.

Change in shape can take many forms including:

• Change in part thickness
• Change in part size in contact with tool
• Change in configured shape of part (i.e. curvature instead of just flat parts)

In this particular case, you are mainly concerned with changes in part thickness, but the shape of the part and tool can also have an effect on heat transfer.

Change in part thickness

The change that will often have the most effect on thermal management is changes in part thickness. The through-thickness direction is often the dominant direction for heat transfer owing to its often smaller dimensions relative to the rest of the structure and larger thermal gradients. According to the conduction equation, \(q_x = -k\tfrac{\bigtriangleup T}{\bigtriangleup x}\), this results in higher heat flux through the thickness of the part.

For resin systems that require the application of heat to cure (see figures below), the temperature of the material in the middle of the through-thickness direction will initially lag further behind that of the exterior. As the cure process progresses however, the energy that is generated form the cure reaction can cause this thermal profile to invert for thick parts since heat cannot be removed fast enough. Whereas in thinner parts it might be possible to keep the temperature difference between the surface and midplane of the part minimal; thicker parts will have larger differences in temperature during cure between the surface and midplane.

When changing a geometry of a part to one with a much greater thickness, it is likely that excess heat could cause issues with part quality. If the excess heat due to cure cannot be dissipated, then a local exotherm will occur, leading to local over-cure and possible heat damage.

For high temperature processing, a larger part thickness results in a greater thermal lag between between the part surface and its midplane. It also results in a larger exotherm which may bring the part above its temperature specifications. This is true for convective and conductive heating environments.

10mm part	50mm part
Convective heating - heating rate of 3°C/min	Convective heating - heating rate of 3°C/min
Conductive heating - part surface temperature and applied temperature are practically equivalent	Conductive heating - part surface temperature and applied temperature are practically equivalent

For resin systems that cure at room temperature (such as polyester resins), the more massive (thick) the part is, the more heat will be trapped within the part and the further the cure reaction will be advanced. If the part is large enough, the heat added by the part will dominate the cure reaction more so than the ambient air temperature. Due to this effect, a thicker part can actually reach the same degree of cure at a lower ambient air temperature as compared with a thinner part. This phenomenon is shown in the figures below. In particular, the right hand figure shows that at an ambient temperature of 20°C, an 8mm part reaches a DOC of 85%, whereas a 3mm part only reaches a final DOC of 76%. This difference becomes more stark the higher the ambient temperature. In contrast, at very low ambient temperatures, the thermal energy is not sufficient to significantly increase either the 3 or 8mm part to a high DOC. The green line represents the theoretical degree of cure of pure resin ignoring its mass and looking strictly at the specific (per mass) material properties. In other words, thickness of the resin layer is not taken into account. Hence why both the 3 and 8mm laminates display a higher final degree of cure as ambient air temperature increases.

For low temperature processing materials, thicker parts generally result in a higher degree of cure due to an increase of heat released during cure.

Part temperature	Degree of cure

Change in part size in contact with tool

Owing to the dominant direction of heat transfer in a part being through-thickness. The change in part size in contact with the tool usually does not have a significant effect on thermal management considerations, but is more important to other considerations such as residual stress and dimensional control.

Change in configured shape of part

In convection heating systems, the largest effect of shape on thermal management considerations will likely be the influence of the shape on the airflow in the oven or autoclave as shown in the picture below. The interaction of the shape of the part (and its tooling) will alter the airflow and change local heat transfer coefficients (HTC). The change in local HTC alters the flow of heat into and out of the part locally. If the HTC is low, that region of the part will be slower to heat up initially, and slower to remove exothermic heat during cure. Conversely, regions with high HTCs will heat up more quickly, possibly resulting in higher exothermic temperatures, but will also be able to remove exothermic heat into the airflow more rapidly if the surface temperature of the part exceeds the air temperature.

The complex interaction where the tool-part assembly may block gas flow in a forced convection environment.

Effect of tooling

Link to effect of tooling in a thermal management system

Tooling can be a major contributor to the thermal mass of the system and also the geometry that interacts with the equipment. Therefore it plays an important role in the thermal history of the part.

Tooling material and thickness

Increased tool thermal mass acts to damp out the effects of the exotherm caused by the heat of reaction of a curing laminate, but the magnitude of the effect is a complex interaction between part material and thickness, tool material and thickness, and the air temperature history (for equipment using convection). High tool thermal mass can also be an issue if it prevents certain areas of the part from ever reaching their necessary cure temperature for the required amount of time, and so this balance must be carefully considered, and the geometry of the tool designed appropriately based on the amount of composite material and heat transfer to the surrounding environment in certain regions. Both the tooling dimensions and the tooling material will affect the thermal mass.

Other material properties such as thermal conductivity and thermal diffusivity are also important. Thermal conductivity refers to how well the material transfers heat energy, while thermal diffusivity is a measure of the rate of this heat transfer, taking into account the object's conductivity and thermal mass. Tools with high thermal diffusivities, such as aluminum, show more uniform temperature distributions. Tools with low thermal diffusivity tend to show more severe temperature gradients across their surface.

The following table gives a qualitative comparison of typical tooling materials. Note that depending on the application, and local geometric features, thermal mass could be a benefit or a hindrance.

Qualitative comparison of tooling materials

Tooling material	Cost	Durability	Weight	Thermal mass	Thermal conductivity	Coefficient of thermal expansion (CTE)	Thermal diffusivity
Invar	$$$	Excellent	Heavy	High	Moderate	Low	Low
Steels	$$$	Excellent	Heavy	High	Good	Moderate	Moderate
Composites	$	Low	Light	Low	Low	Moderate	Low
Aluminum	$$	Good	Moderate	Low	Excellent	High	High

A selection of physical and thermal material properties are also shown in the table below to give you a better idea of how different the various tooling materials are.

Typical material properties for tools. Note that the properties for the composite tool are dependent on the fibre volume fraction and orientation.

Tooling material	Density (kg/m3)	Specific heat capacity (J/kg-K)	Thermal conductivity (W/m-K)	Coefficient of thermal expansion - CTE (x10-6/°C)	Thermal diffusivity (x10-6m2/s)
Invar	8000	515	11.0	0.6-1.5	2.67
Mild steel	7850	510	55	11	13.7
Carbon-epoxy composite	1580	870	0.7 (through-thickness)	-0.5 (in-plane) 22.5 (through-thickness)	0.5 (through-thickness)
Aluminum	2710	896	167	23	68.9

Tooling shape

As discussed above in effect of shape, the shape of the tooling can also significantly alter how the part and tool interact with any equipment that uses convection as the method of heat transfer. Depending on your application, you may desire high convective heat transfer coefficients (usually the case for oven or autoclave curing, or room temperature curing of thick parts) or low convective heat transfer coefficients (often the case for room-temperature cure of thin parts or in low ambient temperature conditions).

Effect of equipment

Link to effect of equipment in a thermal management system

Generally speaking, changing the type of equipment (i.e. oven, autoclave, heated platens, room temperature, etc.) used for the thermal transformation step of a thick part to be different from that of a thin part is not an acceptable change as the boundary conditions and the applied temperatures around the part/tool will be drastically different than what was used for similar parts. However, it frequently occurs that a manufacturer may have several models of the same type of equipment (perhaps purchased at different times) with different geometries (size, configuration of internal baffles, etc.) and perhaps different heating methods (electric, gas, etc.), but cannot economically restrict a given part to only being produced with a particular piece of equipment.

When changing between equipment of the same type, two primary areas of focus should be:

1. Temperature control
2. Control of the interface between equipment and the tool-part assembly

Temperature control

Typically, accurate temperature control of the equipment itself (e.g. air temperature within an oven or autoclave) is often well controlled by the equipment. However, improper configuration of the equipment such as poorly tuned PID controllers, or improper pressure differentials could be a cause of temperature variations in the equipment. For example, ovens or autoclaves with poorly tuned PID's may oscillate significantly around the temperature set point, even when at steady state. If the oscillations occur slowly enough, the part temperature may also oscillate undesirably. The temperature may also undershoot or overshoot at the end of ramp segments, which can also contribute to poor thermal history in the part. Additionally, ovens that have a lower air pressure inside compared to the surrounding building can suck in cold air through gaps, resulting in local cold spots nears gaps (e.g. near doors). Ensuring a slight positive pressure and good sealing of gaps can prevent the introduction of air with the wrong temperature. In general, ensuring that the equipment is properly maintained to meet the original specifications of the manufacturer is important to preventing issues such as these.

In room-temperature cure applications, the building itself is the equipment. This can be very challenging in locations where the ambient outdoor temperature changes significantly with the seasons, or even within a day. Since the air temperature around the part is a very important boundary condition for room temperature cure parts, the environment should be controlled as best as possible with HVAC systems and/or staff should understand how the temperature will affect the timing of the process and how to mitigate the effect of temperature changes. For example, see the case study on troubleshooting of room temperature processes for large recreational and industrial parts.

Equipment and the tool-part interface

Airflow velocity distribution in an autoclave with flow originating from the bottom. The position of the tool will impact the magnitude of air velocity that the tool and part experience. A different autoclave may have an entirely different pattern of airflow compared to this one, depending on how its convection system is designed

When changing between equipment of similar type, it is common to encounter changes to the interface between the equipment and the tool-part assembly. In convective heating systems this means changes to the airflow around the tool-part assembly and a resulting change in heat transfer coefficient.

Given that the goal is to keep the thermal history of the thick part as similar as possible to the thin part, this is not in and of itself an issue if it serves to cancel out effects from changes in part shape and tooling, and results in a thermal history closer to that of the thin part. However, if the change in airflow combines with changes in part shape and tooling to make the thermal history of the thick part even more different than that of the thin part, then this is an issue. Ultimately, the combined response of the system composed of material, shape, tooling & consumables and equipment (MSTE), must be considered and adjusted so that the thermal response of the thick part is as close as possible to that of the thin. This can be achieved through numerical simulation (see How to perform a numerical thermal profile) or experimental testing (see How to perform an experimental thermal profile).

Material deposition management considerations

Link to material deposition management

Content coming soon.

Flow and consolidation management considerations

Link to flow and consolidation management

Content coming soon.

Residual stress and dimensional control management considerations

Link to residual stress and dimensional control management

Content coming soon.

Diagnose

When considering how to troubleshoot a problem such as this, it is important to keep a fundamental concept in materials science in mind: The process history leads to microstructure evolution, which in turn determines mechanical and functional properties. Therefore, to obtain the same quality in thick parts as we were able to achieve in thin parts, it is important to keep all parameters inside the thick part during manufacture as close as possible to how they were in the thin parts. Some key outcomes to look at are:

1. Degree of cure

Determine if your parts are showing signs of over-cure. The following table lists some common methods of diagnosis for degree of cure:

Methods of diagnosis for degree of cure

Problematic outcome	Diagnosis method	Indicators
Over-cure	Visual Inspection	part may look burnt brown/golden appearance
	Tactile inspection	part may be "dusty" if extensive degradation has occurred surface ash rubs off
	Differential scanning calorimetry	degree of cure is higher than a properly cured resin system Note: Due to reduced polymer chain mobility after vitrification of the resin (diffusion constrained), thermoset polymers do not typically reach a degree of cure of 1 during a normal cure cycle (see Cure of thermosetting polymers). Advancement of cure close to a degree of cure of 1 only happens when temperature increases above the recommended cure temperature, restoring polymer chain mobility, and allowing for polymer chains to continue reactions. Further addition of energy will cause chemical bonds to break (degradation/burning)
	Fourier-transform infrared spectroscopy (FTIR)	Spectroscopic signature does not match the signature of a properly cured sample

2. Thermal profile

Determine how different the thermal profile of regions in your thick parts are to the thermal profile experienced by your thin parts. This can be achieved through numerical simulation (see How to perform a numerical thermal profile) or experimental testing (see How to perform an experimental thermal profile).

Remediate

Depending on your diagnosis, use your understanding of the systems approach to composites manufacturing in order to mitigate the quality issues by tuning the:

• Chemistry of the resin material (where appropriate)
• Shape of the part-tool assembly,
• Tooling & consumables, and/or
• Equipment and how it interfaces with the part-tool assembly

Some common mitigation methods for common problematic outcomes related to thick parts are shown in the table below:

Thermal mitigation strategies for tooling

Theme	Problematic outcome	Mitigation strategy
Thermal management	Maximum temperature too high	Ensure good airflow across this area of the tool (convective heating) Consider whether you can rearrange the part or parts inside your equipment, or modify your equipment to redirect the air where needed Ensure good contact in this area with the heat source (conductive heating) Increase local tooling thickness in problem areas (increase thermal mass) e.g. locally add aluminum blocks Change tooling material to one with higher thermal mass Change tooling material to one with higher thermal conductivity/diffusivity Modify temperature history of cure cycle to reduce temperature ramp rates or introduce intermediate temperature holds Examine equipment for temperature uniformity Modify initiator concentration or type to reduce the rate of reaction For polyester and vinylester chemistries only
	Uneven temperature/large thermal gradients across tool	Ensure good airflow across the tool (convective heating) Consider whether you can rearrange the part or parts inside your equipment, or modify your equipment to redirect the air where needed Use tooling with an open substructure (convective heating) Reduce the amount of structure blocking airflow Redesign the substructure of the tooling to optimize airflow given the orientation of the tool in the equipment and the direction of airflow within the equipment Ensure good contact with the heat source (conductive heating) Change tooling material to one with higher thermal diffusivity Consider the addition of local heat elements onto/into tools in regions that are thermally massive so that all regions of the part warm up more uniformly Providing additional local heat during the heat-up stage can improve temperature uniformity throughout the part Consider using active redistribution of thermal energy in the tool (liquid cooling) Examine equipment for temperature uniformity
	Over-cured part (thermally degraded/burned)	Ensure good airflow across the tool (convective heating) Consider whether you can rearrange the part or parts inside your equipment, or modify your equipment to redirect the air where needed Use tooling with an open substructure (convective heating) Reduce the amount of structure blocking airflow Redesign the substructure of the tooling to optimize airflow given the orientation of the tool in the equipment and the direction of airflow within the equipment Ensure good contact with the heat source (conductive heating) Change tooling material to one with higher thermal mass Increase local tooling thickness in problem areas (increase thermal mass) e.g. locally add aluminum blocks Examine equipment for temperature uniformity Modify initiator concentration or type to reduce the rate of reaction For polyester and [A101|vinylester]] chemistries only

Related pages

Page type	Links
Introduction to Composites Articles
Foundational Knowledge Articles	Cure of thermosetting polymers - A162 Degree of cure - A104 Thermoplastic polymers - A161 Thermoset polymers - A105
Foundational Knowledge Method Documents	How to measure curing time and degree of cure - M100
Foundational Knowledge Worked Examples
Systems Knowledge Articles
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	Decreasing Cure Cycle Time - P121 Maintaining quality when scaling-up from coupons to production-ready parts - P124 Practice for developing a thermal transformation process step - P105 Troubleshooting scale-up issues from thin to thick parts - P140 Troubleshooting scaling issues from small to large parts - P136 Troubleshooting scaling-up issues from coupons to parts - P134
Case Studies	Troubleshooting of room temperature processes for large recreational and industrial parts - C100
Perspectives Articles	Composite materials engineering webinar session 3 - Constituent materials - Resin - A122 Composite materials engineering webinar session 4 - Thermal management and resin cure - A123 Composite materials engineering webinar session 5 - Manufacturing processes - Introduction - A124 Composites Process Simulation: A Review of the State of the Art for Product Development - A283 Dr. Anoush Poursartip's cdmHUB global composites expert webinar - A136 Effect of cure on mechanical properties of a composite (Part 1 of 2) - A319 Effect of cure on mechanical properties of a composite (Part 2 of 2) - A320 Heat Transfer in Composites Processing - A321 Introduction to the processing of thermoplastic composites - A322 Resin Behaviour During Processing: What are the key resin properties to consider when developing a manufacturing process? - A257 Understanding Polyester Resin Processing: The Effect of Ambient Temperature on the Final Part - A286

About	Help

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.

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.

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.

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.

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

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.

A quantitative measure of how a material will respond to transient thermal conditions. It is defined as the ratio of thermal conductivity to the volumetric heat capacity of the material (density times the specific heat capacity).

Materials with large thermal diffusivity have a high thermal conductivity relative to their capacity to store energy (heat capacity) and will rapidly distribute the thermal energy throughout its volume and rapidly change temperature to reach the new equilibrium temperature.

Materials with small thermal diffusivity have a low thermal conductivity relative to their capacity to store energy (heat capacity) and will have a sluggish temperature response to a change in thermal conditions (large temperature lag). This is because it takes more energy per unit volume to change the temperature, and the low thermal conductivity means it takes longer to distribute the thermal energy throughout its volume.

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

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.