Maintaining part quality when changing curing equipment - P122

 

Maintaining part quality when changing curing equipment
Practice document
Document Type	Practice
Document Identifier	122
Themes	Thermal management Materials deposition and consolidation management Residual stress and dimensional control management
Tags	Equipment Equipment parameters Development to production
Objective functions	Cost Maintain Rate Maintain Quality Maintain
MSTE workflow	Development
Prerequisites	Effect of equipment in a thermal management system

Q: "I am making good quality parts in one cure vessel and am considering moving to another, seemingly similar, vessel where I will use the same cure program. How do I ensure that my part quality does not drop?"

A: The key to maintaining the same part quality is to ensure that the temperature, pressure, and vacuum history of the part remains the same in either cure vessel. First question is whether the heat transfer to the part and tool is sufficiently similar in the two cure vessels. If not, ensuring the same part temperature history, might require altering the vessel temperature cycle. However, if that is locked under specifications, then changes can also be made to the tooling in order to adjust thermal response of the part. If pressure is being applied in one equipment and not the other, you won't be able to ensure the same pressure history. The problem them becomes whether pressure has a significant impact on your part quality. To address this concern, you will need to understand how pressure influences flow, consolidation, and void evolution of your material and whether you will achieve acceptable quality with or without external pressure being applied. The same goes for vacuum. To learn more, visit ensuring appropriate resin flow for a new material. Finally, if you are changing from conductive heating to convective heating, or vice versa, this is a significant change and it is likely your temperature, pressure, or vacuum history will have changed. You will likely need to implement mitigation strategies to ensure quality remains the same. Continue reading to learn more.   

Overview

Changing equipment changes the environmental conditions of the system. This means that the boundary conditions on the tool/part change. This includes parameters such as temperature, pressure, airflow, and vacuum. If such changes are not considered, then part quality may be reduced owing to an altered thermal, flow, and/or stress response of the part. In order to maintain quality when changing curing equipment, alterations to the processing conditions likely have to be implemented. This may include altering the cure cycle, tooling design, or even the part design (i.e. changing the part material or shape). This latter activity is a significant change that should be avoided if possible.

Thermal management considerations

Link to thermal management

From a thermal management perspective, changing the equipment alters the mode of heat transfer, the applied temperature, and the heat transfer coefficient. Each of these parameters in turn will influence the part temperature and its final degree of cure (DOC). Poor control of the DOC can lead to many equipment-related quality issues during cure. It is important to consider how a change in equipment may have affect the DOC of the part. If switching equipment results in uneven part temperature, it's possible the part DOC will not be consistent across the part. Moreover, if the part temperature climbs too high, it is possible for the part to thermally degrade, thus reducing its mechanical properties.

During cure, the mode of heat transfer is typically either conduction or convection based. In the former case, heat transfers to and from the tool/part assembly by direct contact with a temperature-controlled, solid surface. It is the temperature of this surface and how well it contacts the tool/part that is important. In the latter case, heat is transferred to and from the tool/part assembly by motion of a surrounding fluid (such as air or nitrogen gas). In this case, the temperature of the fluid and the heat transfer coefficient are important. In both cases, the initial temperature of the tool/part are important and should be known.

Common conductive heating devices:

• Heat blanket
• Heated tooling
• Hot press

Common convective heating devices:

• Room temperature (or ambient air temperature) cure
• Oven
• Autoclave

To investigate the thermal profile of parts in various equipment, any of the following methods may be used:

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

Similarly, to measure the HTC of specific equipment experimental tests, simulations, or a combination of experiments and simulations may be performed. To learn about these methods, click on the appropriate link below:

1. How to experimentally determine the HTC
2. How to back calculate the HTC using simulation

Materials deposition and consolidation management considerations

Link to Materials deposition and consolidation management (MDCM)

While controlling part temperature falls under thermal management, the part temperature also plays a role in resin flow. As part temperature increases, the resin viscosity decreases to a point. Eventually, gelation of the resin occurs and the material no longer flows. Therefore any equipment-related effects on part temperature will not only impact quality due to control of the DOC, but also due to the flow behaviour of the material. It is also important to consider whether changing equipment changes the applied pressure and vacuum acting on the part. Some equipment don't apply external pressure (such as an oven) and rely on vacuum pressure for consolidating the part. Pressure will also impact resin flow and must be considered. To learn more about managing resin flow, navigate to the following page, ensuring appropriate resin flow and part consolidation for a new material.

Changing to an equipment that does not apply pressure may also decrease part consolidation and increase porosity as a result. Similarly, changing the vacuum conditions of the part may introduce defects as well. For example, a hot press does not typically use vacuum. Therefore, void removal may be challenging and porosity may still be an issue even with high applied pressure. Conversely, applying vacuum where the part previously was not subject to vacuum may result in volatiles being released, again leading to porosity. This latter issue can usually be solved by debulking the part prior to cure and/or increasing the applied pressure to keep the volatiles in solution.

Residual stress and dimensional control management considerations

Link to residual stress and dimensional control management

Management of the tool/part temperature is also important for dimensional control. If changing equipment results in an uneven temperature distribution of the part, or if the difference in thermal expansion between the tool and part is significant, part warpage or other dimensional changes may occur.

Typical equipment parameters

Hot press:

Link to hot press page

• Mode of heating: Conduction
• Temperature: Can control temperature to required value for most composite applications
• Gauge pressure: High pressure, minimum force on the order of tons
• Vacuum pressure: Vac = 0
• HTC: N/A

Room temperature cure:

Link to room temperature cure page

• Mode of heating: Convection
• Temperature: Temperature not controlled. T = 20-25°C
• Gauge pressure: P = 0
• Vacuum pressure: Vac = 1atm
• HTC: 5-10 W/m²k, unless windy or fan implemented

Oven:

Link to oven page

• Mode of heating: Convection
• Temperature: Can control temperature to required value for most composite applications
• Gauge pressure: P = 0
• Vacuum pressure: External vacuum, Vac = 1atm
• HTC: 15-50 W/m²k

Autoclave:

• Mode of heating: Convection
• Temperature control: Can control temperature to required value for most composite applications
• Gauge pressure: P = 0 - 7atm
• Vacuum pressure: Internal vacuum, Vac = 1atm
• HTC: 60-200 W/m²k

Changes to part quality

To learn about how equipment parameters may influence the part temperature and its DOC, refer to the following link, effect of equipment in a thermal management system. To read more about specific equipment, click on the links above.

If changing from a conduction to convection-based equipment, the first thing to consider is how the mode of heating may affect the part's thermal response. In conductive heating, the surface temperature of the part is controlled fairly well, due to being in direct contact with a hot object. In contrast, convective heating relies on airflow to heat/cool the part. This is generally less efficient. As a result, in a high temperature, convective heating environment, the tool/part may lag the air temperature and take longer to heat up. Moreover, local variations in the HTC may cause different areas of the tool/part to heat up at different rates. If the cure cycle is not adjusted to account for these differences, it may result in poor or inconsistent DOC, poor flow leading to higher porosity or dry spots, and/or residual stresses leading to warpage upon demoulding.

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

On the flip side, if switching to a conductive heating system there may be other quality concerns. In conductive heating, the tool must heat up before the part begins to heat up. Therefore, the temperature of the part is largely dependent on what the tool does. This is important for heating the part quickly, but also in drawing heat away from the part when it exotherms. The same is true for a convective heating environment, except that in a convective environment, usually only one side of the part is against the tool. Because two-sided tooling is used in a conductive environment, the heat going to and from the part must pass through tooling, which very quickly heats up to the temperature of the platens (in a hot press). Therefore, an increase in tooling thermal mass has less of an effect on reducing the exotherm as compared with a convection-based system. Moreover, often a hot press (for example) will have its platens preheated prior to cure or will heat up quickly in comparison to convection-based equipment. This subjects the part to a faster heating rate. As a result, the part may experience a much larger exotherm, especially if it is thick.

Another factor to consider is that there may be no application of vacuum in a conductive heating system. For example a hot press does not typically have vacuum control. As a result, it may be difficult to evacuate entrapped gas, leading to porosity once the part cures. Moreover, the high pressure experienced in a press may not be suitable for some materials or parts. It may lead to excessive resin bleed out, resulting in dry spots, or even deform/damage the part. Finally, while a press can accommodate complex part shapes, it is limited by the mould shape. Convective heating equipment are not as limited in this regard.

If keeping with the same mode of heating, it is important to consider that the parameters may vary drastically between equipment. For example, the heat transfer coefficient in an oven is much less than in an autoclave. Therefore, the part will take longer to heat up and may experience a larger exotherm. If the cure cycle is not adjusted to account for this increased heat-up time, the final DOC may be less. Similarly, if the part exotherm cannot be managed, the part may thermally degrade and have reduced mechanical properties. Moreover, an oven does not supply external pressure. As a result, the porosity in the part may be higher. Conversely, an autoclave, which does supply pressure, may result in part deformation due to core movement/crush.

Another aspect to consider is that low temperature curing (i.e. ambient air temperature cures) and high temperature curing do not necessarily present the same challenges. In high temperature curing, the part relies on external heating to bring it to temperature. It is also important to manage the exotherm to prevent thermal degradation of the part. In contrast, while low temperature curing is heavily influenced by the ambient air temperature, the exotherm is responsible for bringing the part to temperature and allowing it to fully cure. Since the ambient air temperature is cool, increasing airflow actually reduces the part temperature and may result in a poor final DOC. Therefore, reducing airflow is generally desirable.

In all cases, defects such as poor DOC, thermal degradation, porosity, resin rich/starved areas, poor consolidation, warpage, part deformation, and others are likely to lead to reduced mechanical properties. In general, quality issues associated with a change of equipment parameters are as follows:

Quality issues associated with a change in equipment. HTC effects do not apply to conduction-based equipment.

Change in equipment parameters	Potential outcomes	Associated problems/defects
Reduced HTC (high temperature curing)	Larger thermal lag between air and part temp Higher exotherm	Longer heat up time Poor DOC Thermal degradation
Increased HTC (low temperature curing)	Reduced exotherm	Poor DOC
Variable HTC (due to airflow differences across tool/part)	Inconsistent temperature profile across part	Inconsistent DOC Inconsistent flow, leading to porosity and/or resin rich/starved areas Part warpage
No pressure	Reduced part consolidation	Increased porosity Delamination
High pressure	Excessive resin bleedout Core movement/crush or deformation/damage to other areas of the part	Dry spots Reduced mechanical properties Part deformation
No vacuum	Void entrapment	Increased porosity
Vacuum without debulk	Volatiles being released	Increased porosity

Mitigation techniques

Mitigation strategies for the following problems are listed below. It is assumed that changes to the part (i.e. changing the part material and/or shape) are not feasible. Therefore the suggestions only deal with changes to equipment or tooling parameters.

• To learn about how equipment parameters may influence the part temperature and its DOC, refer to the following link, effect of equipment in a thermal management system
• To learn how tooling choices may influence the part temperature and its DOC, refer to the following link, effect of tooling in a thermal management system.

Mitigation strategies associated with equipment-related defects

Problem/defect	Mitigation strategy
Thermal lag (high temperature cure)	Implement slower heating rate and/or intermediate isothermal holds Change tooling material one with lower thermal mass Reduce tooling thickness Increase airflow (convective heating) Ensure tooling/part is not restricting airflow (convective heating) Use tooling with an open substructure (convective heating)
Large exotherm (high temperature cure)	Implement slower heating rate and/or intermediate isothermal holds Change tooling material to one with higher thermal mass and/or thermal conductivity Increase tooling thickness Increase bagside airflow (convective heating) Ensure tooling/part is not restricting airflow (convective heating)
Low maximum temperature resulting from a reduced exotherm (room temperature cure)	Do not cure near doors - especially if they lead to the outside Insulate part from the environment Increase ambient air temperature Use HVAC (or other) system to control environmental conditions If curing outdoors, ensure weather is similar between cures
Inconsistent part surface temperature (convective heating)	Change tooling material to one with higher thermal diffusivity Increase airflow (high temperature curing) or reduce airflow/insulate part (low temperature curing) Ensure tooling/part is not restricting airflow (high temperature curing)
Inconsistent part surface temperature (conductive heating)	Ensure good contact between part and tool change tooling material to one that has a higher thermal diffusivity
Poor consolidation	Increase applied pressure Use vacuum when curing
Poor resin distribution	Ensure consistent part temperature (see above) Ensure pressure application is not too high to cause excessive resin bleedout - important for low viscosity resins
High porosity	Perform a debulk prior to curing Apply vacuum when curing (if debulk performed) Increase applied pressure

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
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	Ensuring quality during production of variable thickness parts - P127 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
Case Studies
Perspectives Articles

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.

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.

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.

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.

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.