Troubleshooting quality issues during cure for different equipment types - P141

 

Troubleshooting quality issues during cure for different equipment types
Practice document
Document Type	Practice
Document Identifier	141
Themes	Thermal management Material deposition management Flow 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	Troubleshooting
Prerequisites	Effect of equipment in a thermal management system

"Q: I am making good quality parts in one cure vessel but in another, seemingly similar, vessel part quality has dropped or become inconsistent, despite using the same cure program."

A: To ensure the same part quality between cure vessels, the temperature, pressure, and vacuum history that the part experiences should remain the same. If external pressure or vacuum are not applied in one vessel but are in another, the pressure and vacuum history of the part will have changed. If this is the case, then it is important to ascertain whether it is necessary to apply pressure and/or vacuum to the material in order achieve quality metrics. This comes down to managing flow, consolidation, and void evolution. To learn more, visit ensuring appropriate resin flow and part consolidation for a new material. Regarding the part's thermal history, even if the cure cycle is the same, the temperature that the part experiences may not be equivalent in both cases. In convective heating systems, a common culprit for this is the heat transfer coefficient (HTC). The HTC may vary widely between cure vessels and even depends on the tool/part position within the vessel. As in all cases, the tooling also plays a significant role in influencing the part's thermal response. If tooling changes between curing equipment, then this may also be a contributor to the part's reduced quality. Finally, switching from convection-based equipment to conduction-based equipment, or vice versa, is a significant change. Applying the same cure program in both cases will likely not result in the same part quality and mitigation strategies will have to be introduced. Continue reading below 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 the cure cycle is not altered to reflect these changes, and their ramifications, then part quality may be reduced owing to an altered thermal response of the part. In order to troubleshoot quality issues that arise during cure due to switching equipment, the defects should be categorized and the root cause determined. From there equipment parameters may be adjusted to mitigate such defects. In some cases, this may mean reverting back to the original equipment choice if the defects cannot be mitigated.

Note that while this page shares many similarities with mainting part quality when changing curing equipment, the workflows follow different steps.

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 is a common culprit for many equipment-related quality issues during cure. When troubleshooting, it is important to investigate how a change in equipment may have resulted in a poor DOC. Aside from the DOC, the part temperature (which is directly related to the DOC) is another factor that should be considered during troubleshooting. If the part temperature reached higher levels than previously experienced, due to the equipment change, it is possible that the part has thermally degraded, 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

Material deposition management considerations

Link to Material deposition management

Content coming soon

Flow and consolidation management considerations

Link to Flow and consolidation management

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. When investigating equipment-related defects it is also important to consider whether there were any changes to the applied pressure and vacuum that the part experienced between the two equipment. 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 switching equipment resulted in an uneven temperature distribution across the part, or if the thermal response of the tool/part changed, then part warpage (or other dimensional control issues) may occur as a result of mismanaged thermal expansion and/or cure shrinkage.

Troubleshooting Steps

1. List defects

The first step is to list what defects are observed after curing the part using new equipment. Examples of defects may include poor mechanical properties, debonding, cracking, poor dimensional control, discolouration, etc. By listing these and relating them to changes in the equipment parameters, the root cause of the defects can be determined. Because defects are arising during cure following an equipment swap, it is likely that the temperature response of the part has changed. Although managing part temperature is a thermal management issue, the effects can be widespread, resulting not only in thermal defects (such as poor DOC), but also in material deposition, flow and consolidation, and residual stress and dimensional control defects. Aside from part temperature, other big factors could be pressure and vacuum. If the application of pressure or vacuum changes between equipment, this may result in significant differences to part quality.

2. Classify previous and new equipment according to their mode of heating

This means identifying whether the previous and new cure equipment apply heat by means of conduction or convection. In general, changing the mode of heating, i.e. moving from conduction to convection or convection to conduction, is a bigger change than remaining with the same mode of heating and just changing the equipment type (i.e. moving from an autoclave to an oven). That said, problems can still arise if changing equipment type but keeping the mode of heating the same.

3. List important parameters with approximate values for the equipment in question

Below is an example of parameters for multiple equipment types:

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

4. Relate defects to changes in equipment 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 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

5. 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 set of steps/procedures that are intended to provide guidance in manufacturing and/or decision making activities.

There are two types of workflows:

• Standard workflows are intended to formalize practices where the manufacturing science base exists. The focus is to provide guidance using manufacturing simulation as an enabling tool (e.g. design activities/decisions relating to thermal management).
• Complex workflows are intended to reduce the level of effort in practices where the existing manufacturing science base is not sufficiently mature to support production scale problems. The focus is to provide guidance using simulation based thinking and/or checklists (e.g. design activities/decisions relating to porosity management).

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.

The act of combining reinforcement fibre, resin to shape on a mould (tool).

There are three main methods to combine resin and fibre to deposit onto a tool (i.e. material deposition):

• Wet Processes: Mixing liquid resin and dry fibre directly in the mould, e.g. Wet Compression Moulding process for parts in BMW 7 series
• Liquid Composite Moulding (LCM) processes: Infusing dry fabric with resin using a pressure gradient, e.g. High Pressure Resin Transfer Moulding process (HP RTM) for parts in BMW i3
• Prepreg Processes: Laying up using prepreg where resin and fibre are already combined, e.g. Automated Tape Laying (ATL) process for some of the parts in Boeing 787.

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.