Effect of equipment in a thermal management system - A110

Effect of equipment in a thermal management system
Systems knowledge article
Document Type	Article
Document Identifier	110
Themes	Thermal management
Relevant Classes	Equipment
Tags	Equipment Equipment parameters System control Thermal properties Thermal transformation Thermal management interaction
Prerequisites	Degree of cure Equipment Heat of reaction Heat transfer System interactions Thermal and cure/crystallization management (TM)

Introduction

In any thermal system, in order to understand what the part is doing, we must understand the whole system. The part is typically small, compared to the tool, and it is the equipment that applies the thermal boundary conditions to the part-tool combination. It is the equipment, for example the autoclave, or the press, that defines the boundary conditions for the tool-part assembly. If the part and tool are directly exposed to the environment, as in room temperature cure, then the equipment is basically the room. The equipment interacts with both tool and part, but since the tool is typically significantly more thermal massive than the part, the equipment interacts indirectly with the part, with the tool serving as the medium of heat exchange between equipment and part, at least on one side of the part. In all cases, the equipment parameters significantly influence the overall thermal response of the system.

Scope

This article discusses the impact of the equipment on a thermal management system, remembering that what matters primarily is the part thermal response. The effect of various equipment parameters are investigated. The page links out to foundational knowledge content and brings in physics-based simulation to demonstrate the behaviour of equipment and how altering their parameters impacts the part thermal response. While the content presented here is applicable to thermal management in all factory cells, the focus of this page is on the thermal transformation cell as that is the predominant cell associated with thermal management.

Significance

It is the equipment that provides heat (and cooling) to the system and dictates the final state of the part. Grouping equipment according to the boundary conditions they impose on the part-tool combination is a convenient classification approach. Moreover, it provides a practical way to analyze thermal systems. Understanding the effect of various boundary conditions in the context of thermal management allows for one to determine heat flow through the tool-part assembly, assuming all other parameters of the tooling and part are known. Because the equipment serve as major constituents of factory cells and can represent significant expenses for companies, their effect on the system should be well understood.

Prerequisites

Recommended documents to review before, or in parallel with this document:

• Degree of cure
• Heat of reaction
• Heat transfer
• Thermal management
• System interactions
• Equipment (system class)

Overview

The most important equipment parameters from a thermal management perspective are the applied temperature, applied heating/cooling rate, and for convective heat transfer systems, the heat transfer coefficient (HTC). The former two parameters manifest themselves in the imposed temperature cycle. The latter parameter (HTC) is only applicable for a convective heating system and is dependent on several other equipment parameters; namely, air/gas flow velocity and pressure. Moreover, the HTC can vary locally within the equipment due to the streamline direction, geometrical features of the tool/part, and the location of the tool/part. In general, the applied temperature and heating rate are not indicative of the actual temperature or heating rate experienced by the part, due to interactions with tool and part parameters.

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.

The equipment provides the boundary conditions which interface either directly with the part or indirectly, by first interacting with the tooling and consumables. Therefore, external heat transfer to the part comes from either the equipment or the tooling/consumables. Similarly, internal heat generation within the part must transfer to either the equipment or tooling/consumables. External heat may be applied to the system in different ways, each of which constitute a different boundary condition. These boundary conditions are dependent on the equipment used. Fundamentally there are three modes of heat transfer, conduction, convection, and radiation. Any one of these may be considered in the processing of composites.

In many cases, it is enough to analyze the system using a 1D thermal assessment. That is, assuming heat flows through the thickness of the tool-part assembly. Therefore, the conduction or convection boundaries imposed by the equipment act upon the top and bottom surface of the tool-part assembly. In reality, heat transfer occurs in three dimensions and the boundaries along the front, back, and sides of the assembly affect the heat transfer to and from the stack. Typically the thermal gradient in these other dimensions is not significant however, and can therefore be ignored. Depending on the equipment, the boundaries on the top and bottom of the assembly may not be the same. If the equipment uses a fan to blow hot air (or other fluid) to heat the stack, then the equipment boundary is one of forced convection, such as in an autoclave or convection oven. If the equipment heats air to then heat the stack but does not mechanically move the air (such as using a fan), then it is using natural convection, such as in a traditional oven or in room temperature curing. The heat transfer mechanisms in forced vs natural convection are the same, with the difference being the magnitude of the heat transfer coefficient. Finally, if the equipment uses a heated platen (or other object) in direct contact with the tool-part assembly, then it is utilizing a conductive boundary condition. In some cases, the tool may be a part of the equipment and apply heat directly to the part. Alternatively, the equipment may directly heat the tool which in turn heats the part, such as cooking food (part) in a pan (tool) on a stove (equipment).

Natural convection	Forced convection	Two sided conduction	One side conduction with natural convection

Recall the 1D form of the heat transfer equation:

\(\frac{\partial}{\partial t}(\rho C_pT)=\frac{\partial}{\partial z}\Bigl(k_{zz}\frac{\partial T}{\partial z}\Bigr)+\dot{Q}_{r}\) 1D form of the heat transfer equation, where:<br />\(t\) = time,<br />\(\rho\) = density,<br />\(C_p\) = specific heat capacity,<br />\(T\) = temperature,<br />\(z\) = through-thickness distance (z direction),<br />\(k_{zz}\) = thermal conductivity in z direction, and<br />\(\dot{Q}_{r}\) = rate of energy given off by material during exotherm.

Where,

\(\dot Q_r = \frac{d\alpha}{dt}(1-V_f)\rho_r H_R\) Internal heat generation during polymerization (exotherm), where:<br />\(\dot{Q}_{r}\) = rate of energy given off by material,<br />\(\alpha\) = degree of cure,<br />\(t\) = time,<br />\(V_f\) = fibre volume fraction,<br />\(\rho_r\) = resin density, and<br />\(H_R\) = heat of reaction of resin

In order to solve this equation to determine the temperature of the part, the boundary conditions of the system must be known. As mentioned above, these may be either convective or conductive. A convective boundary is one where the heat flux due to convective heat transfer is equivalent to the conductive heat flux. A conductive boundary condition implies that the surface temperature of tool-part assembly is known. This may also be referred to as a constant surface temperature boundary (or simply temperature boundary). Mathematically speaking, these two boundaries can be represented as follows:

Convective boundary: \(-k{\partial T \over \partial z} = h[T_\infty - T(0,t)]\) Convective boundary condition. The left hand side refers to conduction into/out of the surface as represented by Fourier's Law. The right hand side refers to convection at the surface as represented by Newton's Law of Cooling.<br />\(k\) = thermal conductivity,<br />\(T\) = temperature,<br />\(z\) = through-thickness distance (z direction),<br />\(h\) = heat transfer coefficient (HTC),<br />\(T_\infty\) = ambient temperature, and<br />\(T(0,t)\) = temperature at a position of z = 0 (i.e. the surface) and time = t (i.e any time).

Conductive boundary (constant surface temperature): \(T(0,t) = T_s\) Constant surface temperature indicative of a conductive boundary. \(T(0,t)\) = temperature at a position of z = 0 (i.e. the surface) and time = t (i.e any time). \(T_s\) refers to the magnitude of the surface temperature.

Convective heating

Temperature application

In a thermal management system, one of the most important functions of the equipment is to apply the environmental conditions that the tool, part, and material are subject to. This includes the applied temperature and the applied heating/cooling rate. While it may seem obvious that the equipment controls such aspects as heat input, the control over such features and their effect on the system can be crucial in achieving the necessary part quality. Arguably, the most important instance of controlling the applied temperature and heating rate occurs during the thermal transformation step of the factory. Here, heat is applied to convert the part into its final form with the as-designed mechanical properties. The manner in which heat is applied may be as simple as leaving the part in a room with no additional heat added (i.e. room temperature cure), or as complex as subjecting the part to multiple stages of heating and isothermal holds in order to meet material specifications. The manner of heat application, including the duration of time under which the part is subject to specific temperatures, is known as the cure cycle for thermoset polymers.

High temperature processing - cure cycle

Changes in the cure cycle can have significant effects on the part's thermal response. As such specific cure cycles are often defined based on the material and can be found on many material supplier data sheets. These are known as the manufacturer recommended cure cycle (MRCC). While the MRCC captures the material response, it typically does not capture the system response. That is, the effect of equipment, tooling, and part parameters are not considered. Therefore, following the MRCC is a good starting point and may be sufficient to obtain an appropriate degree of cure, however, system effects should not be ignored. It may be that because of poor airflow, thick tooling, or geometrical complexities in the part that the MRCC does not result in the material achieving the appropriate degree of cure.

With regards to high temperature processing, the two most common ways to alter the cure cycle are to change the heating rate (also referred to as the ramp rate) and the number of isothermal holds. Performing either action can result in differences in the thermal lag and the maximum temperature experienced by the part. Reducing the ramp rate, increases the time for the system to respond to the increasing temperature. The result of this is that the thermal lag between system components is reduced. That is, the temperature distribution across the part, between the part and tool, and between the part and environment. A reduced ramp rate may also reduce the maximum part temperature experienced during the exotherm. This is because the cure reaction kicks off during heat up, as opposed to during the hold at maximum temperature. Therefore, when the exothermic reaction begins, the air temperature is lower. Once the air temperature reaches its maximum value, less residual heat from the part is available to be released, reducing the exothermic overshoot. The lower the ramp rate, the more time the tool and/or environment has to draw heat away from the part, thereby further reducing the overshoot. If timed and managed appropriately, an exotherm that occurs during the ramp phase can help bring the part to temperature without a serious overshoot. In doing, so the part meets temperature specifications, while reducing some of the heat up time.

Increasing the number of isothermal holds has a similar effect to reducing the heating rate; it gives the system time to respond to the imposed conditions. Furthermore, it allows for a fraction of the material's exothermic heat to be released at lower temperatures. It is important that intermediate holds occur at lower temperatures than the intended processing temperature. In doing so, the polymerization reaction advances at lower temperatures. Thus, some exothermic heat is given off prior to reaching the final processing temperature, meaning the amount of exothermic heat released at high temperature is reduced. The longer the intermediate hold is, the more the polymerization reaction will advance and the more exothermic heat will be given off. This does, however, assume that the hold temperature is sufficient to supply enough energy to notably advance polymerization. Similarly, increasing the temperature at which the intermediate hold occurs will further increase polymerization and give off additional exothermic heat, thereby reducing the residual heat to be released when the part reaches its intended processing temperature. This may be beneficial in reducing the maximum part temperature. However, if the intermediate hold temperature is too high, then the part may exotherm above the temperature specification during the hold, thus defeating its purpose.

Decreasing the ramp rate and increasing the number of isothermal holds has a combined effect of reducing thermal lag and decreasing the part's maximum temperature. However, this also increases the cure cycle time, especially for long isothermal holds. Therefore, while a series of slow ramp rates followed by isothermal holds may allow for manufacturers to meet temperature specifications, it may also drive their costs up, by reducing throughput.

For high temperature processing, reducing the heating rate and adding in isothermal holds can decrease thermal lag and reduce the maximum part temperature.

Heating rate of 3°C/min vs 1°C/min with no intermediate isothermal holds	Heating rate of 3°C/min vs 1°C/min with a 60 min isothermal hold at 110°C
Heating rate of 3°C/min vs 1°C/min with a 120 min isothermal hold at 110°C	Heating rate of 3°C/min vs 1°C/min with a 60 min isothermal hold at 140°C

Low temperature processing - ambient air

In the case of low temperature processing where no heating rates are introduced, the ambient air temperature plays a significant role in the part's thermal response. Higher ambient air temperatures allow the cure reaction to advance rapidly, giving off high levels of heat and resulting in a higher part temperature. As a result, the degree of cure (DOC) also advances. While higher degrees of cure are desirable, one must be weary not to exceed the maximum part temperature as defined by the material requirements. Large exotherms can result in over-curing or degrading the part, which ultimately reduce the final mechanical properties.

For low temperature processing, the ambient air temperature can play a significant role in the maximum part temperature and the DOC.

Heat transfer coefficient (HTC)

The heat transfer coefficient (HTC) describes how well heat is transferred to and from a fluid by convection. In the case of composites processing, the HTC dictates how well heat is transferred from the environment to the tool-part assembly and vice versa. There are a number of factors that may influence the HTC. These include, air velocity, air density, pressure, temperature, part shape, part location, and equipment volume. If the surrounding air (or other fluid) is moving rapidly, convective heat transfer is much more efficient than with stagnant air due to the increased number of molecular interactions. Hence, a higher airflow velocity translates to a higher HTC. Of course, in order for the airflow to be truly efficient in heating/cooling the part, it should impinge equally along all sections of the part. This means having a well distributed airflow field or a well placed part. This is particularly a problem if the tool, part, or equipment have geometrical features that block or reduce airflow in certain areas. Similarly, if multiple parts are being cured together, one tool-part assembly may block airflow from reaching another. The next factor affecting the HTC is pressure. In a pressurized system under constant volume (such as an autoclave), increasing the air pressure increases the density of air in the system. Therefore, there are more molecular interactions between the tool-part assembly and the environment, resulting in more efficient heat transfer (represented by a higher HTC). For non-pressurized systems, such as in an oven or room temperature setup, air density is affected by elevation. Parts cured at a higher elevation are subject to a lower air density and therefore a lower HTC. Lastly, decreasing the volume of the thermal system reduces spacial temperature variations in the system, thus allowing for a more even HTC distribution across the part.

Typical HTC values are provided in the table below. It should be noted that these values depend on the conditions in which they were measured according to the aforementioned factors. It's possible that values outside these ranges exist under different system conditions.

Typical HTC values for different equipment setups

Stagnant air	Outdoors with wind or indoors with airflow	Oven	Autoclave
2-10 W/m\(^2\)K	>15 W/m\(^2\)K depending on airflow velocity	15-50 W/m\(^2\)K	60-200 W/m\(^2\)K

High temperature processing - HTC

The higher the HTC, the better the heat transfer. This means that a higher HTC will reduce thermal lag between the part and environment. Not only does this assist in heating the part during high temperature processing, but increasing the bagside HTC also assists in reducing the exothermic overshoot. The figure below provides three HTC values. Namely, 5 W/m\(^2\)K (indicative of stagnant air), 30 W/m\(^2\)K (indicative of good airflow in an oven), and 100 W/m\(^2\)K (indicative of good airflow in an autoclave). If the air is stagnant the thermal lag between the air and part temperature is significant. Increasing the HTC to 30 or 100, greatly reduces this thermal lag. Furthermore, for an HTC of 5 W/m\(^2\)K, the thermal lag persists throughout the exotherm. This increases the time at which the part is above the maximum applied temperature, thereby significantly increasing the chance of thermal degradation. The increase in thermal lag between a poor airflow (low HTC) and good airflow (high HTC) heating environment also affects the part DOC. Although in all three cases, the final DOC of the part is more or less the same, the time it takes to reach a high DOC is much longer for stagnant air. For many epoxies, gelation occurs around 50% DOC. In such a situation as the one presented below, a part in an environment with an HTC of 5 W/m\(^2\)K would take 3 hours to reach its gelation compared to under 2 hours for an HTC of 30. Reaching a high DOC early can help in increasing throughput in the factory, by reducing cure cycle.

Increasing the bagside HTC not only reduces thermal lag and the timespan of the exothermic overshoot, but also decreases the magnitude of the overshoot itself. This is because as soon as the part temperature is above that of the air, the air acts to cool the part. A higher HTC is more efficient in doing so and therefore reduces the amount by which the part temperature increases above that of the air. This is, in fact, how the HTC behaves for low temperature processing as well.

Effect of heat transfer coefficient (HTC) for high temperature processing. Heating rate is 1.5°C/min.

While increasing the bagside HTC does help in reducing the part exotherm, inreasing the toolside HTC does not necessarily have the same effect. For tools with a high thermal mass relative to the part, an increased toolside HTC will actually increase the exotherm of the part. Conversely, if the thermal mass of the tool is low or comparable to that of the part, then a high toolside HTC does assist in reducing the part exotherm. The above figure assumes the same HTC for both bagside and toolside, but the nuances between the two should be recognized.

To learn more about how the tool's thermal mass and toolside HTC interact to influence the part temperature, refer to the following link: effect of tooling in a thermal management system - substructure and HTC effects.

Low temperature processing - HTC

The exothermic heat generated during low temperature processing is often desirable to advance the cure reaction. That's why for low ambient temperatures a low HTC may be preferable. An HTC of 0 indicates the tool and/or part is perfectly insulated from the environment, meaning there is no convective heat transfer with the surroundings. Conversely, a high HTC indicates significant convective heat transfer with the surroundings. If the air temperature is lower than that of the part, then the HTC will act to cool the part according to the second law of thermodynamics. Similarly, if the air temperature is higher than that of the part, the HTC will act to heat the part. Hence, insulating the part may be desirable for low temperature applications, while for high temperature applications, the opposite may be true.

Consider the figures below. The ambient air temperature is 15°C and the toolside/bagside HTC varies from 0/0 to 30/30 respectively. At higher HTC values, the maximum part temperature decreases and the final DOC along with it. This is because the high HTC is acting to cool the part. An analogy can be drawn to a fan blowing cold air over a hot object. Conversely, at low HTCs the maximum part temperature and DOC increase. This is because the part is being insulated from the environment. It is no longer, or at least less, exposed to the cool air. Therefore, it is allowed to heat up to a higher temperature and reach a greater DOC. In this case, the part can be compared to an object being heated internally while also insulated from its surroundings.

Effect of heat transfer coefficient (HTC) at a low ambient air temperature (15°C).

In the above figures, for both high and low temperature processing, it was assumed that the top and bottom HTC of the part were equivalent. In many situations, this is not the case. Depending on the airflow distribution in the equipment and the geometry of the tool-part assembly, the HTC may vary along the tool-part surfaces. This can lead to inconsistencies in the temperature of the part and its final DOC if it is not taken into account. To learn more about how the tool or part may affect the HTC, visit the following pages:

• Effect of tooling in thermal management system - tooling configuration
• Effect of shape in a thermal management system - part configuration

For measuring or modelling airflow, and determining the HTC, refer to the following links:

• How to measure airflow in a thermal system
• How to simulate airflow in a thermal system
• How to experimentally determine the HTC
• How to back calculate the HTC using simulation

To learn more about convective or conductive heating equipment, visit the equipment page in the Systems Catalogue volume.

• Equipment

Conductive heating

Whereas in a convective heating environment the surface of the part may experience temperature gradients across its surface, in a conductive heating scenario the surface temperature of the part is typically uniform. This is because the part is in direct contact with heated tooling. Therefore, if the contact between the tool and part is good, the surface of the part takes on the same temperature as the tooling, which is directly controlled. Thermal gradients may still exist through the part itself, however. These gradients may be small or large depending on the system parameters. For example, if a slow heating rate is used, giving time for effective heat transfer to occur, or if the part is thin, then the thermal lag may be small. In fact, under the same conditions, the thermal lag between the part midplane and heat source in a conductive heating environment is typically less than in a convective heating environment due to improved heat transfer at the surface. The same is generally true for the exotherm. That is, the exotherm is often smaller in a conductive heating environment compared with a convective environment for the same conditions. This is shown in the figures below. Even with an HTC of 50 W/m²k, the thermal lag in the convective system (representative of a high airflow oven) is significant compared to the negligible lag in the conductive system (representative of a hot press). For the conductive case, a heating rate of 100°C/min is used to represent a preheated platen at 180°C coming into contact with a room temperature tool/part. For the convective heating case, this is used to represent a preheated oven at 180°C. It should be noted that an actual ramp rate of 100°C/min is unachievable for most convective heating systems.

Small thermal lag and lower exotherm experienced in a conductive heating setup as compared with a convective heating setup.

Conductive heating	Convective heating
Heating rate of 3°C/min to 180°C	Heating rate of 3°C/min to 180°C
Heating rate of 100°C/min to 180°C - representative of a preheated platen coming into contact with a room temperature tool/part	Heating rate of 100°C/min to 180°C - representative of a room temperature tool/part placed in a preheated oven

In many conductive heating situations such as hot press processing for example, the equipment is often preheated and held at a set temperature before coming into contact with the tool/part. Therefore, the heating rate applied to the tooling surface is essentially infinite. If the tool is thin or has a high thermal diffusivity and low thermal mass (such as aluminum), then the tool experiences negligible thermal gradients and heats up to the equipment temperature very quickly. Assuming the tool and part have good contact, the part surface temperature will match that of the tooling surface it is in contact with. Therefore, the surface temperature of the part is effectively exposed to the same infinite heating rate, matching the equipment temperature almost immediately. This can result in large initial thermal gradients within the part. If the tooling material or thickness is such that the tool takes a while to heat-up, the tool may be preheated to reduce the heat-up time and improve throughput.

The internal thermal gradients within the part depend largely on the part thickness. For a thicker part, larger thermal gradients will exist and a more severe exotherm will occur. Consider the figures below. A fast heating rate is used to simulate a preheated press coming into contact with a room temperature tool/part. The tool and part surface take on the temperature of the equipment. However, within the part notable temperature differences exist. Each line in the figures represents 2mm into the part from the surface to the midpoint. At 80°C, it can take up to 50 minutes for the 50mm thick part to reach uniform temperature. At 180°C, the exotherm experienced by the part is large. This results in an inversion of thermal gradients, where the centre of the part becomes the warmest and the surface the coolest. In fact, the exotherm can be large enough to raise the internal temperature of the part by more than 60°C. Moreover, it takes time for this heat to dissipate which can be detrimental to the integrity of the part. Because of the exotherm, in the 180°C case, the temperature of the part is not uniform even after 60 minutes.

Temperature gradients within a thick (50mm) part in a conductive heating environment. Each line represents 2mm into the part. The surface temperature is the straight-edge line.

Applied temperature of 80°C at the surface.

Applied temperature of 180°C at the surface.

Such temperature distributions within the part should be considered during conductive, and convective, heating. If the variations are large (as typically exhibited in thick parts), and the curing time is short, it is possible that the centre of the part remains uncured while the surface is fully cured. It's even possible for the centre to be uncured and the top to be over cured. An analogy can be drawn to cooking hamburgers, where, if the surface temperature is too hot, the outside of the patty will burn while the inside remains uncooked. Of course, this analogy does not take into account the exothermic nature of polymer curing reactions which is highly important.

For more information on the effect of shape parameters such as thickness, or in performing a thermal profile, visit the links below.

• Effect of shape in a thermal management system
• How to perform an experimental thermal profile
• How to perform a numerical thermal profile

To learn more about conductive or convective heating equipment, visit the equipment page in the Systems Catalogue volume.

• Equipment

Scenarios

1. A practitioner is using a high temperature cure epoxy (Hexcel 8552). They are following the manufacturer recommended cure cycle, yet the maximum part temperature during exotherm is too high (205°C) and does not meet requirements.

By reducing the heating rate from 1°C to 2°C, they reduce the max part temperature from 205°C to 192°C. However, this is still above the maximum temperature requirement and the part fails spec. To further reduce the max part temperature, the practitioner introduces a second intermediate hold at 150°C. By doing so, the part exotherms during this second hold at 150°C, rather than just before the final hold at 180°C. As a result, the exotherm does not bring the part temperature above 180°C, and the temperature requirements are satisfied. The part passes spec.

If timed appropriately, heating up as the exotherm heat is coming down can help bring the part to temperature without surpassing maximum temperature requirements. Not only does this allow the part to pass specifications but also helps in reducing the energy output of the equipment.

Choosing an appropriate cure cycle for a carbon-epoxy part.

MRCC	Reducing ramp rate	Adding an additional hold

2. A boat hull manufacturer is using a low-temperature cure polyester resin, cured outdoors without external heat added. They notice that parts cured on days when it's windy have lower mechanical performance, especially in the fall/winter, but even in the spring/summer.

The reason for this is because the higher air velocity on windy days increases the HTC. For low temperature cures, where the heat given off during part exotherm is significantly greater than the ambient air temperature, an increased HTC acts to cool the part by absorbing this heat. Therefore, the max part temperature and the duration at which the part remains at elevated temperature is reduced. As a result, the final degree of cure of the part is also reduced. This in turn, impacts the final mechanical properties of the part. This effect of cooling by an increased HTC becomes less drastic as the ambient air temperature is increased. Hence why the manufacturer noticed a particular drop in mechanical performance during the fall/winter, when ambient air temperatures are low.

In order to mitigate this loss of heat, an effective strategy is to insulate the part, thereby reducing the part's exposure to the wind and thus reducing the HTC at the part's surface. The scenario behaves much the same as a person experiencing wind chill. The person's body generates heat, which is drawn away from them when they are exposed to the wind. As the wind speed increases, the HTC increases and heat is drawn away quicker. To combat this, people wear jackets which reduces the exposure of their skin to the elements, thus reducing heat transfer by convection.

Relating ambient air temperature and HTC to degree of cure.

Complicating factors/edge cases

It should be noted that while this page demonstrates the general effects equipment has in a thermal system, the figures generated are material specific. Results will differ between materials, even for seemingly similar material systems (i.e. one polyester resin based system versus another). Similarly, changes in other processing parameters such as part thickness, tooling thickness/material, and others. Finally, in the generated curves, consumables were not considered for simplicity. Depending on the consumables used and their thickness, they may have a notable effect on the part response. The results shown here are intended only to highlight the importance of the equipment from a thermal management perspective.

Related pages

Page type	Links
Introduction to Composites Articles
Foundational Knowledge Articles
Foundational Knowledge Method Documents
Foundational Knowledge Worked Examples
Systems Knowledge Articles	Effect of equipment in a thermal management system - A110 Effect of material in a thermal management system - A155 Effect of shape in a thermal management system - A154 Effect of tooling in a thermal management system - A142
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	Introduction to Tooling for Composite Materials Processing - A340

References

References

About	Help

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.

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.

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

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.

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.

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.

Manufacturer's Recommended Cure Cycle (MRCC).

The use of multiphysics models to predict the outcome of real-world scenarios. May be analytical (closed form, "hand calculations") or computational (implementation on computers is required due to the large number of calculations involved. e.g. finite element method, finite difference method)

Most often in composite materials engineering, modelling refers to either:

• Process modelling (predicting manufacturing outcomes, or the inverse problem of predicting manufacturing parameters to achieve a desired outcome), or
• Performance modelling (predicting the stiffness/strength/suitability of a structure, or the inverse problem of the material properties and dimensional requirements to achieve a desired stiffness/strength/suitability of a structure)

(same as "Simulation")

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.