Transitioning tooling styles between different thermal transformation equipment - P139

 

Transitioning tooling styles between different thermal transformation equipment
Practice document
Document Type	Practice
Document Identifier	139
Themes	Thermal management Material deposition management Flow and consolidation management Residual stress and dimensional control management
Tags	Equipment Equipment parameters Tooling Tooling aids Tooling parameters
Objective functions	Cost Maintain Rate Increase Quality Maintain
MSTE workflow	Troubleshooting
Prerequisites	Effect of equipment in a thermal management system Effect of tooling in a thermal management system

Q: "I am attempting to transition tooling styles, from autoclave to press cure, to increase production rate and am experiencing new quality problems."

A: The problem here is two-fold. The first is that there is a significant change in equipment, from a convection-based one to a conduction-based one. The second is that in order to accommodate the change in equipment, a change in tooling must also be done, from one-sided tooling in the autoclave to two-sided tooling in the press. Often, a hot press will have its platens preheated prior to cure or will heat up quickly in comparison to an autoclave. This shortens the overall cure cycle time, allowing for increased production rate. However, it also subjects the part to a faster heating rate. As a result, the part may experience a much larger exotherm, especially if it is thick. This may thermally degrade the part. Whereas in an autoclave, the part can heat up/cool down from the air, in a hot press, the heat going to and from the part must pass through tooling, which very quickly heats up to the temperature of the platens. Therefore, an increase in tooling thermal mass has less of an effect on reducing the exotherm as compared with a convection-based system.

Other potential sources of quality problems are the change pressure. Hot presses often apply significant pressure to the part. Material systems with relatively low viscosity may experience excessive flow as a result. This can lead to dry spots, poor resin volume fraction, and increased porosity. Moreover, if the part is not designed to be subject to such high pressures during manufacturing, then the part may deform or even break. Another potential source of defects is the lack of vacuum application in hot press curing.

Overview

Depending on the equipment used for thermal transformation, the tooling style may change. For example, in hot press curing, two sided tooling is used. That is, both the top and bottom surfaces of the part are in contact with tooling. In an oven or autoclave, however, typically only one sided tooling is used to support the bottom surface of the part (known as the toolside surface). The top of the part is typically covered in consumables, such as release film, breather, and a vacuum bag. This is referred to as the bagside surface. In this latter scenario, the part is subject to vacuum in addition to any external hydrostatic pressure that is applied (in an autoclave for example). In the former scenario (i.e. the hot press cure), no vacuum is applied and all pressure comes from the force of the platens compacting the part. From a systems perspective, the difference between the two scenarios is that the boundary conditions have changed. This includes the part interface boundaries as well as the boundaries of the tool-part assembly as a whole. In the oven/autoclave scenario, the tool-part assembly is subject to convective heating, whereas in the hot press scenario, the tool-part assembly is subject to conductive heating.

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

When troubleshooting quality issues arising from a change in equipment and tooling, the root cause of the defects may be from the equipment, the tooling, or the combination of both equipment and tooling.

If the change in equipment is suspected to be the cause of the quality issues, visit the following page for troubleshooting tips:

• Troubleshooting quality issues during cure for different equipment types

If the change in tooling is suspected to be the cause of the quality issues, visit the following page for troubleshooting tips:

• Troubleshooting tooling to achieve part quality

For both cases, recall that the thermal transformation step (as with all process steps) involves interactions between the material, part shape, tooling & consumables, and equipment. While troubleshooting the tooling (for example) may lead to a mitigation strategy to resolve the quality issues, the ramifications of implementing such a strategy, with regards to the interactions with other components, should be understood. This will prevent further quality issues from arising down the line and allow for a more robust process definition.

Thermal management considerations

Link to thermal management

Thermal management considerations for the equipment and tooling are detailed in the links above. A few general points to keep in mind are:

• For the same thermal conditions, a conduction-based environment will exhibit smaller thermal lags than a convection-based environment. See Conductive heating.
• Because conduction-based equipment often use fast heating rates, or have their platens preheated (eg. a hot press), along with two-sided tooling, the exotherm potential may be quite high - especially for thick parts.
• For high temperature processing, the higher the heat transfer coefficient (HTC), the quicker the heat up time and the less the thermal lag. See HTC, high temperature
• For low temperature processing, a high HTC acts to cool the part. See HTC, low temperature
• The shape and position of the tool/part in a convective heating system affects the local HTC. See Tooling configuration and Part configuration.
• Increasing the bagside HTC reduces the part exotherm.
• Increasing the toolside HTC increases the exotherm if the tool is thermally massive compared to the part. If the tool is not thermally massive compared to the part, increasing the toolside HTC reduces the exotherm. See Tooling substructure
• Tooling with a higher thermal conductivity/diffusivity will present a more uniform temperature distribution and reduce the part exotherm (assuming the same thermal mass).
• The higher the thermal mass of the tool/part, the longer it will take to heat up
• Increasing the thermal mass of the tool will increase thermal lag but reduce part exotherm

To learn more, or to explore these points in more detail, visit the following pages:

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

Material deposition management considerations

Link to material deposition management

Content coming soon

Flow and consolidation management considerations

Link to flow and consolidation management

Flow and consolidation considerations for the equipment and tooling are detailed in the links above. A few general points to keep in mind are:

• An altered thermal profile of the part may also alter the viscosity profile. This, in turn, will alter the flow characteristics of the material.
• The pressure applied by a hot press is often much higher than that applied by an autoclave. For low viscosity systems, this may lead to excessive resin flow.
• Lack of vacuum application in a hot press may lead to increased porosity if the part has entrapped gas that does not dissolve into the resin under pressure.

Residual stress and dimensional control considerations

Link to residual stress and dimensional control management

• A large discrepancy in the coefficient of thermal expansion (CTE) between the tool and part may result in significant residual stresses in the part. This may lead to warpage upon demoulding.
• Generally speaking, a tool with a small CTE is desirable for carbon fibre-epoxy parts.
• Hot presses often have a minimum force which they can apply. If this force is too high for the part design, it can lead to part deformation or damage.

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 RSDM system - A278 Effect of tooling in a RSDM system - A277
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	Developing production scale tooling - P129 Ensuring quality during production of variable thickness parts - P127 Ensuring tooling choice meets part quality metrics - P128 Maintaining part quality when changing curing equipment - P122 Transitioning tooling styles between different thermal transformation equipment - P139 Troubleshooting a cure cycle for improved production rates - P132 Troubleshooting quality issues during cure for different equipment types - P141 Troubleshooting tooling to achieve part quality - P142 Troubleshooting when changing production tooling material - P130
Case Studies	Conducting a thermal tooling survey on three complex tools of different materials - C105 Developing for Future Production Scale Up - C113 Use of Right Sized Simulation to Aid Decision Making in Thermoplastic Composite Manufacturing Systems - C114
Perspectives Articles

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In composites processing, viscosity is an indicator of how easily the resin matrix will mix with the reinforcement and how well it will stay in place during processing. The lower the viscosity, the more easily resin flows. Resin viscosity ranges considerably across chemistries and formulations.

By scientific definition, viscosity is a measure of a material’s resistance to deformation. For liquids, it is in response to imposed shear stresses.

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.

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

A process whose outcomes are relatively insensitive to variability of manufacturing parameters.

Carbon fibres are composed of large aromatic sheets similar to those in graphite. These graphitic layers form the basic structural units in the shape of ribbons. The structure of carbon fibre ribbon is believed to be a columnar arrangement of disoriented graphite crystallites parallel to the ribbon length. The idealized tetragonal crystallites are stacked above one another, with slight disorientation between the crystals in the direction of fibre axis, trapping sharp needle like voids, where the boundaries between the stacks represent the disordered regions.