Ensuring appropriate resin flow and part consolidation for a new cure cycle - P119

 

Ensuring appropriate resin flow and part consolidation for a new cure cycle
Practice document
Document Type	Practice
Document Identifier	119
Themes	Thermal management Materials deposition and consolidation management Residual stress and dimensional control management
Tags	Resin flow Consolidation
Objective functions	Cost Maintain Rate Maintain Quality Maintain
MSTE workflow	Development
Prerequisites	Viscosity (resin)

Q: "I have defined a baseline cure cycle leading to a satisfactory level of consolidation but am looking at moving to a different one. How do I ensure that the amount of resin flow of my new cure cycle is neither too little nor too much when compared to the baseline cure cycle?"

A: Controlling the amount of resin flow is a complex task involving the material, part shape, tooling, and equipment for a given process (MSTEP collection). At a minimum, to ensure consistency in flow for different cure cycles, the flow index should remain the same between cure cycles. The details of flow index are outlined below. In many ways, this problem is similar to ensuring appropriate resin flow and part consolidation for a new material, except the issue is not a change in material, but a change in the cure cycle. Generally speaking, this is an easier problem to solve, as a change in material likely has several other consequential effects including a potential change in specifications.

Overview

Resin flow and part consolidation are driven by the interaction between the material's viscosity, the applied pressure/vacuum, the part shape, and the boundary conditions on the part. In turn, the resin viscosity is dependent on the part temperature and the degree of cure of the resin. An increase in temperature, in itself, results in a decrease in viscosity due to thermal thinning: the molecules move apart from each other due to increased thermal vibration. But time at temperature leads to cure advancement in a thermoset. As the degree of cure increases, the molecules become longer and start to cross-link, and so the viscosity increases. Thus there are two competing effects when heating a thermoset: a decrease in viscosity due thermal vibration and an increase in viscosity due to cross-linking. Initially, the thermal effect dominates and overall the viscosity decreases. At some point, however, the degree of cure effect becomes dominant and the viscosity starts to increase. This trend accelerates until the material reaches a sufficient level of cure, called the gel point, beyond which the material can no longer flow, and consolidation is complete. Generally speaking, the higher the applied pressure, the more the material flows (until gelation) and the better the consolidation. Too much applied pressure, however, may cause excessive resin bleed out, resulting in potential dry spots and increased porosity in the finished part.

Thermal management/flow and consolidation management considerations

Link to Thermal management

Link to Materials deposition and consolidation management

The thermal history of the part determines the viscosity history of the resin. All else equal, a similar viscosity history should lead to a similar resin flow and thus resin volume fraction throughout the part. A simple measure of the effect of viscosity history on resin flow and resulting volume fraction is the flow index (FI)\[FI=\int{1 \over \mu} dt\]

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 bagside.

Once the resin gels, the material typically stops flowing. To learn how to measure gelation, refer to the following link:

• How to measure gel time

Materials deposition and consolidation management considerations

Link to Materials deposition and consolidation management (MDCM)

Content coming soon.

Residual stress and dimensional control management considerations

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	How to measure gel time - M101
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	Ensuring appropriate resin flow and part consolidation for a new cure cycle - P119 Ensuring appropriate resin flow and part consolidation for a new material - P120
Case Studies
Perspectives Articles	Composite materials engineering webinar session 7 - Manufacturing processes - Liquid composite moulding - A126 Resin Behaviour During Processing: What are the key resin properties to consider when developing a manufacturing process? - A257

About	Help

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.

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.

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.

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.

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.

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