How to perform an experimental thermal profile - M102

 

How to perform an experimental thermal profile
Systems knowledge method
Document Type	Method
Document Identifier	102
Themes	Thermal management
Tags	Thermal properties Method

Overview

Experimental thermal profiling is a practice in composites manufacturing where material/tooling temperatures and temperature rates are measured using thermocouples. This activity is performed to ensure that all material points in the part satisfy the material’s thermal specifications. Typical thermal specifications may include minimum/maximum heat up and cool down rates and minimum length (duration) of temperature holds. Experimental thermal profiling is often a necessary step to qualify a manufacturing (MSTE) system before moving it to production.

Scope

This method document provides a step-by-step workflow on how to perform an experimental thermal profile. The current practice consists of instrumenting the part to verify that the thermal history of the part meets the material's thermal specifications. The accepted workflow as described below does not take advantage of manufacturing simulation and is purely experimental. However, manufacturing simulation can be introduced into this workflow as an enabling tool. The use of process simulation is not covered in this method document. To learn how process simulation can be used to predict the tool/part thermal history, refer to how to perform a numerical thermal profile.

Significance

The thermal history of the material throughout the manufacturing process play a critical role in the outcomes of the part. If the material is not stored at the proper temperature, or if its temperature during thermal transformation does not follow the material’s thermal specifications, the part's performances may be impacted. For instance, quality issues such as reduced mechanical properties, non-uniform consolidation, volatile entrapment, and dimensional non-conformity, can manifest if the thermal history of adjacent zones differ significantly. Therefore, verification of the material’s thermal history is important to establish a link between process conditions and part performances. Experimental thermal profiling is a procedure used to verify that the thermal history of the material is compliant with the material's thermal specifications such that the desired as-manufactured properties can be achieved.

Instrumented part and tooling prior to bagging and thermal profiling (photo courtesy of CCMRD).

You might consider this activity when:

• You are undergoing production development and are in the final stages of locking down your manufacturing process.
• You are exploring the design space with respect to material, shape, tooling, or equipment and would like to optimize your process.
• You are troubleshooting and reacting to a need to make fundamental changes to your material, shape, tooling, or equipment, in order to meet thermal requirements.

Prerequisites

• Thermal management
• General material properties
• Temperature sensors

Workflow

The step-by-step workflow described below merely consists of measuring the temperature of the part during the manufacturing process and, more specifically, during thermal transformation at a number of discrete points. To ensure a good part, all measured temperatures must conform to the material's thermal specifications. In practice, this is often limited to a few locations within the part in order to measure the maximum (lead) and minimum (lag) temperature. The lead and lag temperature are used to ensure that the temperature profile of all material points satisfy the upper and lower bounds of the thermal specifications. Note that in production, the part thermal history can be monitored by measuring the temperature of the tooling or equipment instead of the part itself. Representative locations identified through thermal profiling analysis are used as proxies for the part temperature.

Thermal profiling workflow.

1. Define Part-tool Thermal Profiling Plan (PTTP)

1.1 Identify manufacturing system

Thermal profiling at the pre-production phase is either performed on the real part, a representative part, also called a charge, or a representative part when the production includes a family of parts. The first steps of the workflow aim to define them, and also to identify the zones of interests where to measure the thermal history. The objective of the following first step is to define the real system from a thermal management perspective.

The parameters listed below are required to identify a representative manufacturing system when needed, to define the thermal profiling plan and complete the thermal profiling workflow; they describe the manufacturing system from a thermal management perspective. A fully defined system should include a value for each parameter, however, in practice it is not always practical or even necessary to spend the time and effort to obtain an accurate value. What is important, is to be aware of these parameters and how they affect the engineering decisions made in subsequent steps. The table serves as a checklist to expose gaps in information and ensure that all parameters are considered. Information on how to define them can be found in the Foundational Knowledge volume.

a) Consider each parameter in the table below (see also Thermal management),

b) Document those that are known,

c) Consider those that are not known, and

d) Determine if testing should be conducted to obtain an accurate value.

MSTE parameters relevant to thermal management

Class	Parameter
Material	Type
	Heat of reaction
	Cure kinetics
	Heat capacity
	Thermal conductivity
	Density
	Temperature cycle
	Pressure cycle
Shape	Geometry (i.e., flat, curved, etc.)
	Feature (i.e. inserts, holes, etc.)
	Architecture (i.e. solid laminate, sandwich laminate, etc.)
	Dimensions (i.e. thickness, width, length)
Tooling	Geometry and configuration (i.e., type of substructure)
	Dimensions (i.e. faceskin thickness, width, length)
	Material
Equipment	Type (i.e. autoclave, hot press, oven, etc.)
	Dimensions
	Heat transfer mechanism (i.e. type and amplitude)
	Temperature range and uniformity, controllable or not
	Pressure range, controllable or not

1.2 Define representative manufacturing system

Thermal profiling at the pre-production phase can either be performed on the real manufacturing system if available or on a representative system. For example, at the pre-production phase, you might be performing a thermal profile on:

• A family of parts and are only considering evaluating the thermal response of one representative part, or
• A representative charge rather than the real part.

The decision of using a representative system is driven by cost considerations and availability. Any deviations of the representative system from the real MSTE system have to be documented and the thermal equivalency of the representative system demonstrated.

The objective of this step is to define the representative system to be used for performing the thermal profile. Skip this step if the the real system is available and you decide to use it to perform the thermal profile evaluation. Otherwise, define the representative system and clearly identify any deviations from the real system

a) Define a representative shape if thermal profiling is not performed on the real part(s)

Thermal management parameters for defining a representative charge

Class	Parameters
Shape (representative)	Geometry (i.e., flat, curved, etc.)
	Feature (i.e. inserts, holes, etc.)
	Architecture (i.e. monolithic laminate, sandwich laminate, etc.)
	Dimensions (i.e. thickness, width, length)

b) Define a representative part if thermal profiling is not performed on all real parts

• Divide tooling into a family of tools that have similar features, including tool materials, thermal mass, thickness, and substructure configuration.
• For each family of tools, divide parts into groups of similar cure cycles and materials.
• Further divide parts into laminates and sandwich panels
• Organize laminates by their thickness and sandwich panels by their skin thickness.
• For each family, choose the laminate with the largest thickness or the sandwich panel with the largest skin thickness as the representative part.
• Decide between thermal profiling of a representative part or charge(s) given the cost considerations.
• Define the representative charge(s) if thermal profiling is not performed on a representative part(s):

Thermal management parameters for defining a representative part

Class	Parameters
Shape (representative)	Geometry (i.e., flat, curved, etc.)
	Feature (i.e. inserts, holes, etc.)
	Architecture (i.e. monolithic laminate, sandwich laminate, etc.)
	Dimensions (i.e. thickness, width, length)

c) Define the representative tooling and equipment if not the same as the real equipment

Thermal management parameters for defining a representative equipment

Class	Parameters
Tooling (representative)	Geometry and configuration (i.e. type of substructure)
	Dimensions (i.e. faceskin thickness, width, length)
	Material
Equipment (representative)	Type (i.e. autoclave, hot press, oven, etc.)
	Dimensions
	Type (i.e. autoclave, hot press, oven, etc.)
	Dimensions
	Heat transfer mechanism (i.e. type and amplitude)
	Temperature range and uniformity, controllable or not
	Pressure range, controllable or not

1.3 Define zone(s) of interest

The purpose of the following step is to identify the zone(s) where the temperature has to be measured to capture the thermal history of the part. Thermal lead, lag, and exotherm locations are usually considered to demonstrate that a part's thermal history satisfies the thermal specifications. Thermocouples are installed at these critical locations to measure the manufacturing system response. Lead and lag locations are defined as being the first and last locations to reach the temperature hold respectively. Uncontrolled exotherms are mostly considered for thick parts. The critical locations to be monitored are, in general, determined empirically. The articles contained under Thermal management explain and illustrate how the manufacturing (MSTE) parameters affect lead, lag, and exotherm. Manufacturing simulation can also be used to develop a first understanding of the part’s thermal response and identify the most likely lead and lag locations (see How to perform a numerical thermal profile). When defining the zone(s) of interest, consider that:

• For flat and small parts, when high temperature, convection-based equipment (i.e. oven or autoclave) are used, temperature typically leads on the bagside of the composite and lags on the toolside. It is also possible that a singular point on the part switches from being the lag to lead location, or vice versa, as the cycle advances. The largest exotherms occur at the thickest sections of the part. Parts made on tools that act as effective thermal sinks help mitigate the magnitude of this exotherm. A tool heat survey can be performed to assess the temperature uniformity of the tool and guide the identification of additional zones of interest. A tool heat survey is an optional step used to support the thermal profiling plan.
• For complex, non-symmetrical, or large parts, when high temperature, convection-based equipment (i.e. oven and autoclave) are used, the non-uniformity of the air flow must be considered. In this case, a tool heat survey can also be performed to assess the non-uniformity of the heat transfer coefficient (HTC) and guide the identification of zones of interest.

a) Define exact location and categorization of all thermocouples (i.e. lead, lag, exotherm, part, proxy, etc.) for each zone of interest previously defined.

b) For convection-based equipment, define the tooling’s location and orientation (see task 3.2)

c) For non-ambient temperature equipment, document equipment’s temperature cycle

2. Approve PTTP

3. Conduct PTTP

At this stage, thermal profiling is implemented and the part's thermal history is assessed.

3.1 Instrument the part or representative part or charge. Instrument as described in the thermal profiling plan. Particular attention should be paid when placing the thermocouples to reduce the risk of apparent failure (i.e. a false negative). It is recommended to test the thermocouples before running the thermal profiling experiment.

3.2 Place the load in the thermal transformation cell. Note that, when convection-based equipment are used, the heat transfer coefficient variations are usually not considered while defining a thermal profiling plan. The tool location is not specified and documented in a typical thermal profiling plan. However, it is recommended to document the tool location and surrounding conditions. The robustness of the manufacturing process can be assessed by repeating this evaluation step for different tool locations.

3.3 Run temperature cycle.

3.4 Collect temperature measurements for assessment.

4. Analyze and troubleshoot PTTP

The last part of the thermal profiling workflow provides guidance on how to assess the results of the evaluation done previously and on how to optimize and troubleshoot the system if the specifications are or are not satisfied, respectively.

The main purpose of the following steps are to decide whether to run the production, optimize the system to reduce cost or to satisfy other outcomes, or troubleshoot the system to satisfy the specifications.

4.1 If specifications are satisfied, move to the final task and evaluate cost and other manufacturing outcomes.

4.2 If specifications are not satisfied, assess mitigation strategies detailed in Practice for troubleshooting a thermal transformation step.

5. Document PTTP

Limitations

Typically, each of the basic steps in the experimental thermal profiling workflow is based on empirical approaches. For instance, expertise or processing trials are required to ascertain where the thermal lead and lag locations on a part occur. Thermocouples are installed at these critical locations to measure the MSTE system response.

A common biased judgment is to think that the lead (fast cure point) always occurs at the thinnest location on the composite and that the lag (slow cure point) always occurs at the thickest location. This might be an incorrect assumption depending on the equipment used. In general, when convection-based equipment are used, temperature leads on the bag side of the composite and lags on the tool side, and it is possible to obtain a thermal lead and lag at the same point. Also, having a thermal lag distribution in your part or tool may actually help to mitigate the occurrence of an exothermic overshoot since:

• Heat can be drawn out of the part and into the tool (since the tool generally lags the part),
• Heat could be drawn out of a hot zone and into a cooler zone within a composite at any given point, if the thermal distribution is significant.

Manufacturing simulation can be introduced into the current practice to enable significant improvements in efficiency, effectiveness and robustness. To learn about thermal simulation, refer to how to perform a numerical thermal profile.

About	Help

Experimental thermal profiling is a typical practice where part/tool temperatures and temperature rates are empirically measured using temperature measurement devices (typically thermocouples). This activity is performed to ensure that representative locations in the part of interest satisfy the cure window with respect to minimum/maximum heat up and cool down rates and length (duration) of temperature holds.

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.

With regards to modelling & simulation, verification confirms the implementation of a computational model to ensure that it represents the mathematical model and equations used to describe it (requirements are met).

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.

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

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

The 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, simulation refers to either:

• Process simulation (predicting manufacturing outcomes, or the inverse problem of predicting manufacturing parameters to achieve a desired outcome), or
• Performance simulation (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 "Modelling")