Increasing throughput by curing parts simultaneously - P143

 

Increasing throughput by curing parts simultaneously
Practice document
Document Type	Practice
Document Identifier	143
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 Rate Increase Quality Maintain
MSTE workflow	Optimization
Prerequisites	Effect of equipment in a thermal management system

Q:"I would like to increase the utilization of a cure vessel and increase production rate by curing multiple parts simultaneously in it. How do I establish cure families for a given cure vessel where all the parts in the family can be cured together and achieve acceptable quality and/or be cured in a reasonable amount of time?"

A:You can cure multiple parts together and maintain quality as long as the parts have a similar thermal behavior. Key features which need to be consistent while grouping parts and defining a cure family include: the thickest and thinnest location of the tool, the thickest and thinnest location of the part, the reinforcing structure of the tool, and the core sections and special features of the part or tool.

Overview

As explained in Systems Knowledge, composites processing is a complex interaction between material response, part shape and dimensions, tooling choices, and equipment behavior. Any variation in the MSTEP collection may affect the manufacturing outcomes. When curing multiple parts simultaneously in the same cure vessel, variations between MSTEP collections must be accommodated to meet the material and process specifications. For example, the parts might be made of different materials, have different shapes and a wide range of thicknesses. In addition, their tool might be made of different materials, have different substructures and thicknesses. Finally, even if the parts are sharing the same cure vessel, this does not mean that they are exposed to the same curing conditions. In a forced convection oven or autoclave, the placement of each individual part and their geometry influence the surrounding gas flow and consequently the local heat transfer coefficients. All these variations combine to determine the distribution of parts’ thermal histories, which might deviate from the specified thermal histories defined during material qualification. To ensure thermal conformance, current practice consists of grouping parts into families as described below.

Thermal management considerations

Link to thermal management

From a thermal management perspective, curing multiple parts simultaneously in a forced convection oven or autoclave represents a major challenge. The oven or autoclave must use a cure cycle which is valid for all parts in the load. This can be achieved by grouping parts with similar thermal features. Usually, increasing throughput by curing parts simultaneously is applied to parts which look almost identical and only differ by slightly different size or shape. The manufacturing of fuselage frames is a good example of a bus-stop autoclave production where multiple parts are cured simultaneously. Fuselage frames are made of many types of parts, all having a common architecture, similar tooling, size, and shape which make their concurrent cure possible.

First, the parts must be grouped by processing specifications and further by the specific cure cycle. Then the following features within a family need to be consistent and in the same relative location:

1. The thickest and thinnest location of the tool
2. The thickest and thinnest location of the part
3. The reinforcing structure of the tool
4. The core sections and special features of the part or tool.

These criteria can be used to group different parts together in a single load where each part/tool has a valid thermal profile using a single specified cure cycle.

Finally, you can evaluate the thermal history of a family of parts by using:

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 whole family. 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 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 the part, specification, 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.

Depending on how advanced you are in the development process and what the thermal specifications are that you are failing, you might also consider altering or changing:

• The part family (i.e. changes to the material or shape of the part),
• The thermal specifications (i.e. a bridge program).

Material deposition management considerations

Link to material deposition management

Content coming soon.

Flow and consolidation management considerations

Link to flow and consolidation management

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
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

Material and process (M), Shape (S), Tooling and consumables (T) & Equipment (E) - all have an interlinked effect on the Process step (P). (See MSTE factory ontology)

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

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.

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.

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.

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.

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.