Ensuring quality during production of variable thickness parts - P127

 

Ensuring quality during production of variable thickness parts
Practice document
Document Type	Practice
Document Identifier	125
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 Quality Maintain
MSTE workflow	Development

Q:"I am developing a part with a wide range of thicknesses. How do I ensure that I get good part quality in all part regions at the same time?"

A:You can ensure quality when producing a part with a wide range of thicknesses by assessing the thermal history of your part and more specifically of the thinnest and thickest sections and comparing them. When significantly different, sections might meet the thermal specifications and be of acceptable quality while others might fail. If this is the case, you have to reconsider your equipment and tooling to mitigate the thermal gradients between sections of different thicknesses.

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. Producing a part with a wide range of thicknesses not only brings material deposition challenges but also leads to more complex and non-uniform flow and compaction, thermal, and residual stress and dimensional responses.

Thermal management considerations

Link to thermal management

From a thermal management perspective, changing the part thickness is a major change. As explained in Systems Knowledge - effect of shape in a thermal management system, the thermal response of a part depends on its thickness. Therefore, a part with a wide range of thicknesses develops a non-uniform thermal response during its manufacturing which in turn might impact its flow and compaction and the development of residual stresses.

First, the part thickness defines its thermal mass and so how much energy must be transferred in-or-out of the part to heat or cool it. For example, the thicker the section, the larger its thermal mass, and therefore the more heat is required to increase its temperature. As the thermal mass increases, it not only takes more energy but also more time as the heat needs time to travel in-and-out of the part. Ultimately, this means that the thickest sections have larger thermal lags (i.e. larger temperature difference between the part and the equipment) and larger through-thickness temperature gradients as compared with thinner parts.

Second, a thermoset part releases heat during cure. The thicker the section, the longer the path is for the heat of reaction to travel through the part and escape. This means that more heat of reaction is trapped within a thick section than a thin one which contributes to increase its temperature and might lead to an exotherm.

Thermal management outcomes such as thermal lag, through thickness temperature gradients, and exotherm might differ significantly between thin and thick sections of a part, creating hot and cold spots. These additional in-plane gradients between thin and thick sections are, to some extent, compensated by the in-plane conduction of the part, but this is limited by the low thermal diffusivity of composites even along the fiber direction.

While developing your manufacturing workflow, the sooner you consider these issues (at the conceptual screening stage and at the preliminary selection stage) the better. If you wait to confirm that all is well during final production, then you are essentially in troubleshooting mode. You are now constrained by the choices you have made, and the cost and effort to change can be significant.

You can evaluate the thermal history of a variable thickness parts by using:

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

If you find that the thermal history of your part no longer meets the given thermal specifications, you will have to change the MSTEP collection.

For example, you might consider changing:

• The temperature cycle of the equipment and introduce an intermediate hold as explained in the NASA ATCAS program case study.
• The thickness, substructure, or tooling material. 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 lags. Replacing an aluminum tool with an Invar one, for example, has such an effect.

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

• The equipment to maximize the heat transfer coefficient. This allows you to decrease thermal lags but might come with the trade-off of increasing the exotherm when the tool is lagging more than the part (see Systems Knowledge - Effect of equipment in a thermal management system).
• The part design to reduce its thickness variation.
• The material system if the exotherm is an issue which cannot be addressed with the above mitigation strategies.

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	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 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.

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.

Polymerization of thermoset resins is an exothermic reaction and heat is generated during the curing process. A thermosetting resin has the potential to release a certain amount of energy while curing. This is called the total heat of reaction, H_R, with a unit of J/g (SI units).

The heat of reaction during polymerization is measured using a Differential Scanning Calorimeter (DSC) equipment measuring much energy/heat comes out of the reaction for a small resin sample.

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)