Optimizing a cure cycle for improved production rates - P144

 

Optimizing a cure cycle for improved production rates
Practice document
Document Type	Practice
Document Identifier	144
Themes	Thermal management Material deposition management Flow and consolidation management Residual stress and dimensional control management
Tags	Cure and crystallization System optimization and robustness Cure cycle optimization
Objective functions	Cost Maintain Rate Increase Quality Maintain
MSTE workflow	Optimization

Q:"I am making good parts but I am looking to shorten the cure cycle. How do I ensure I maintain the same level of quality?"

A:You can shorten a cure cycle by 1) increasing its heating and cooling rates and 2) increasing the hold temperature for the cure while shortening its duration. To maintain the same level of quality, the shorter cure cycle should not compromise the consolidation and cure of the material and should not induce the formation of larger residual stresses. You can ensure equivalency by assessing if the optimized cure cycle allows the parts to meet the material's thermal specifications given by the material's manufacturer or specifically developed for your parts.

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. As this happens, care must be taken to ensure that the level of cross-linking or cure is in the appropriate range: the resin must be neither under-cured nor over-cured. Given the exothermic nature of the cure reaction, it is possible to overheat a part, above the applied temperature, thus leading to heat damage. The part's consolidation and formation of residual stresses are also affected by the resin's solidification process. Optimizing a cure cycle therefore represents 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.

Due to the high cost of capital equipment for thermal transformation, it is desirable to maximize the use of these equipment and shorten the cure cycle. There are several techniques for increasing throughput and optimizing the cure cycle is the simplest solution (see other techniques in Troubleshooting a cure cycle for improved production rates). Gain in productivity can be made without making major material, tooling or equipment changes. However, shortening the cure cycle may have detrimental effects on part quality as explained in more details below.

Thermal management considerations

Link to thermal 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.

Related pages

Page type	Links
Introduction to Composites Articles
Foundational Knowledge Articles
Foundational Knowledge Method Documents
Foundational Knowledge Worked Examples
Systems Knowledge Articles
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	Increasing throughput by curing parts simultaneously - P143 Optimizing a cure cycle for improved production rates - P144
Case Studies
Perspectives Articles	Case Study: Optimizing a Press Moulding Process - A324 Composites Process Simulation: A Review of the State of the Art for Product Development - A283 Understanding Polyester Resin Processing: The Effect of Ambient Temperature on the Final Part - A286

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 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.