Troubleshooting a cure cycle for improved production rates - P132

 

Troubleshooting a cure cycle for improved production rates
Practice document
Document Type	Practice
Document Identifier	132
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 Rate Increase Quality Maintain
MSTE workflow	Development

Q:"I am finally making good quality parts but am not meeting my production rate. How can I cure them faster to make better use of my capital equipment?"

A:You can increase production rate by: 1) optimizing the cure cycle, 2) curing multiple parts at the same time, or 3) implementing a post-curing step. These three approaches might or not be all applicable to your production and you have to assess them to understand at what cost they can increase your production rate while maintaining the same quality. They are explained in a systematic manner below with an emphasis on their impact on quality.

Overview

Thermal transformation is one of the most critical manufacturing steps, which most manufacturing outcomes depend on. It allows the solidification of the resin by controlling or monitoring the resin’s temperature. During a cure cycle, a thermoset resin undergoes significant transformation and evolves from a liquid viscous state, which allows forming and consolidation, to a final solid state before demoulding. If a cure cycle is not well developed (see practice for developing a thermal transformation process step), it can be the cause of bottlenecks in a factory, thus decreasing production rate.

Due to the high cost of capital equipment for thermal transformation, it is desirable to maximize the use of these equipment. This means increasing throughput. There are several techniques for increasing throughput without making major material, tooling or equipment changes:

1. Optimize the cure cycle and reduce the overall cycle time. This is the simplest solution but may have detrimental effects on part quality if the material’s thermal specifications are not well defined. It is key that while increasing production rate, quality metrics are still satisfied. The gain in productivity is also limited by the material’s thermal specifications.
2. Curing multiple parts at the same time. This is a practical solution when an oven or an autoclave is not used at full capacity. However, this is not without risks as it is likely that the parts will be exposed to significantly different gas flows and so will have different thermal histories (see practice for increasing throughput by curing parts simultaneously.
3. Reducing the cycle time of expensive thermal transformation equipment, such as an autoclave or hot press, by implementing a post-cure using a separate piece of equipment (see optimization of a hot press process for bike components). This increases the utilization of expensive capital equipment by increasing throughput. When developing a post-cure step, it is critical to understand the resin’s cure kinetics and vitrification behavior.

Other solutions may include changing the tooling to one that heats up quicker. Changing the material to one that is more suited to fast cures is also an option. However, changing the material is often a big change with several consequential effects, wherein a complete redesign of the factory and process steps may have to be performed. Generally speaking, changing the material to increase throughput is the least preferable option.

Cure cycle optimization

Thermal management considerations

Link to thermal management

From a thermal management perspective, optimizing the cure cycle is a major change which can not only impact thermal management outcomes but other manufacturing outcomes related to material deposition management, flow and consolidation management, and residual stress and dimensional control management.

To reduce the overall cycle time, faster heat up and cooling rates, as well as higher cure temperatures, can be implemented according to the material’s thermal specifications to allow for the shortest cure cycle. To maintain the same level of quality, the part’s thermal response must fall within the material's thermal specifications, i.e. specifications on the lead and lag temperatures, exotherm, etc. For example, as explained in Systems Knowledge - effect of equipment in a thermal management system, increasing the heating rate has the combined effect of increasing the temperature gradients across the part and the thermal lag between the part and the curing environment, and may also lead to a larger exotherm. On the other hand, increasing the cure temperature has the effect of increasing the lead temperature and exotherm which might exceed the material's thermal specifications.

To ensure thermal conformance as you are optimizing a cure cycle, you can evaluate the part’s thermal history by using:

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

If you find that the part’s thermal history no longer meets the material's thermal specifications, you will have to continue optimizing the cure cycle or keep the problematic cure cycle and assess if by changing the tooling and/or equipment the given thermal specifications can be satisfied.

For example, you might consider changing the thickness, substructure, or material of the tooling. As illustrated in Systems Knowledge - effect of tooling in a thermal management system, an exotherm can be mitigated by increasing the tool's facesheet thickness with the trade-off of increasing thermal lag. Replacing an aluminum tool with an invar one, for example, has such an effect.

Material deposition management

Link to material deposition management

Content coming soon.

Flow and consolidation management

Link to flow and consolidation management

From a flow and consolidation management perspective, optimizing the cure cycle also represents a major change as it might affect the amount of resin flow. This should be taken into account by the material’s thermal specifications which should be defined in such a way to guarantee an acceptable amount of resin flow to achieve consolidation. A convenient approach to quantifying the resin flow difference between two cure cycles is to calculate a flow index (FI), defined as\[FI=\int{1 \over \mu} dt\]

Where the viscosity (μ) history can be determined either by simulation using a cure kinetics/viscosity model evaluated for the actual temperature history, if it exists for the material, or by a rheometry test using the actual temperature history.

If pressure is being applied, the pressure-modified flow index (PFI) can be used\[PFI=\int{p \over \mu} dt\] Where the pressure (p) is the applied pressure, and it is assumed that the resin sees this full pressure. In reality, where the part is complex, the pressure seen by the resin is often not the applied pressure. This is particularly true where the tooling is rigid or semi-rigid on both sides. This is true in any closed mould process, but also in open mould processes where soft tooling (caul plates) are used on the bag side.

Here, you must take two steps. First, determine the FI or PFI for the existing cure cycle which is your baseline. Then determine the FI or PFI for the optimized cure cycle. Compare the two. Are they significantly different? If so, it is highly likely that you will encounter quality issues. Note that even if they are similar, you may encounter difficulties due to downstream effects due to other differences, such as the reinforcement behaviour, the resin propensity to form defects, and so forth.

Residual stress and dimensional control management

Link to residual stress and dimensional control management

Content coming soon.

Load optimization

Thermal management considerations

Link to thermal management

From a thermal management perspective, curing multiple parts simultaneously in a forced convection oven or autoclave can represent a major challenge. Even if the parts are similar and are sharing the same cure vessel, this does not mean that they are exposed to the same curing conditions. The placement of each individual part and their geometry influence the surrounding gas flow and consequently the local heat transfer coefficient (HTC). These local variations in HTC can create a distribution in the thermal history of parts, which might deviate from the specified thermal history defined during material qualification, leading to a production state with non-uniform part quality.

To ensure thermal conformance, it is recommended to conduct a thermal profile on the new load using either:

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

Independently of the method used, a thermal survey is often done on only one, or a few, of the part/tool(s) in order to save time and effort. The results of the survey are then applied to the all the parts.

In that case, the thermal profiling activity starts by identifying one, or a few, representative part(s) and identifying the zones of interests, i.e. most likely locations of lead and lag. The lead and lag locations are the hottest and coldest areas at any given point of time in the cycle. When the thermal survey is done experimentally, determining these two locations mostly relies on engineering judgment. The thickest and the thinnest parts are typical candidates for having the largest lead and lag temperatures. Similarly, within the same part, extremes in any feature (such as min/max thickness locations) may lead to the highest lead, lag, or exotherm location. To understand more about the effect of part shape or material on the thermal response of parts visit Systems Knowledge - effect of shape or Systems Knowledge - effect of material.

Note that the lead and lag locations may change during the cure cycle and can depend on the cure cycle itself. A switch in the lead/lag location during a cure cycle is often due to the exothermic nature of the thermoset resin. One may have to identify multiple lead or lag locations, or choose proxy thermocouples (i.e. thermocouples placed on the tool) that lead/lag more than the part in order to bound the part temperatures. Attention should be given to identify the location with the slowest heating rate. This can be different from than the lag location and may need to be tracked as well (depending on part specifications, and other requirements).

If you find that the representative part(s) does not meet the given thermal specifications, you will have to implement step-by-step mitigation strategies. You might consider changing:

• The temperature cycle and/or heat transfer capability of the equipment as discussed in in Systems Knowledge - effect of equipment in a thermal management system,
• The tool locations and configurations as discussed in Systems Knowledge - effect of tooling in a thermal management system.

Material deposition management

Link to material deposition management

Content coming soon.

Flow and consolidation management

Link to flow and consolidation management

Content coming soon.

Residual stress and dimensional control management

Link to residual stress and dimensional control management

Content coming soon.

Post-cure development

Thermal management considerations

Link to thermal management

From a thermal management perspective, changing a cure cycle to include a post-cure to increase production rate represents a major change. The goal is to reach full consolidation and partial cure with the existing thermal transformation equipment and then use a second, less expensive equipment to finish the cure (see optimization of a hot press process for bike components).

The first curing step must be designed to ensure an equivalent consolidation and, depending on the process, allow the part to be demoulded for a free-standing post-cure. The second step, the post-cure, must be defined to meet the specifications for the final degree of cure (DOC) and, in the case of a free-standing post-cure, do so without inducing any post-deformation and dimensional control issue.

In practice, the existing cure cycle might be entirely revisited or simply interrupted and followed by a post-cure step. In both cases, the two-step cure cycle must allow the part(s) to meet the material's thermal specifications. In addition, before the post-cure step, the material's consolidation must be completed. Therefore, the resin must have reached gelation. Also, if the part is demoulded before the post-cure, the cure must be advanced enough so that the resin is vitrified at the demoulding temperature. Gelation and vitrification are key thermo-chemical transitions to take into account while developing a two-step cure cycle. For a free-standing post-cure, the cure cycle must be allowed to reach the specified final degree of cure while preventing any deformation. This might constrain the post-cure heating rate and hold temperature depending on the resin's cure kinetics, the equipment's heat transfer capability, and the part's weight and geometry.

Material deposition management

Link to material deposition management

Content coming soon.

Flow and consolidation management

Link to flow and consolidation management

From a flow and consolidation management perspective, the first curing step must allow for the same amount of resin flow to ensure the same level of consolidation. As explained above, a simple measure of the resin flow and resulting consolidation is the flow index.

To ensure that the interruption of cure does not significantly affect resin flow and material consolidation, you can determine the FI or PFI for the baseline cure cycle. Then determine the FI or PFI for the two-step cure cycle. Compare the two. Are they significantly different? If so, it is highly likely that you will encounter difficulties. Note that even if they are similar, you may encounter difficulties due to downstream effects from other differences, such as the reinforcement behaviour, the resin propensity to form defects, and so forth.

Residual stress and dimensional control management

Link to residual stress and dimensional control management

Content coming soon.

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

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.

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.

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.

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.

A central processing theme in the manufacturing cycle. This theme concerned with managing changes in the physical response of parts/tools when the resin is predominantly in a liquid phase (e.g. pre-gelation, pre-solidification) and the prevention of manufacturing defects.

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.

An engineering design activity that relates to the determination of material level properties that will be used in the structural design and certification.

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.

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.