Effect of tooling in a RSDM system - A277

Effect of tooling in a RSDM system
Systems knowledge article
Document Type	Article
Document Identifier	277
Themes	Residual stress and dimensional control management
Relevant Classes	Tooling and consumables
Tags	Thermal properties Tooling Stress development Consumables Heat transfer

Introduction

The residual stress and dimensional change of a part is very closely linked to the thermal history the part experiences during manufacturing. Because tooling plays a crucial role in a thermal management system, it can significantly influence the residual stress and dimensional change of a part. Tooling material, shape, size, the construction, and the design of the sub-structure can all affect the thermal behavior of the tool-part assembly, which can consequently affect the residual stress and deformation of a part. In addition, the tooling surface condition can also affect the coupling between the part and tool, as it can facilitate geometrical locking, leading to residual stress build up in the part.

From a manufacturer's perspective, these parameters should be considered during the designing phase of the tool. The tool should be designed to work with the equipment as a system to produce parts that meet to specifications.

Scope

This article discuss the impact of tooling on residual stress and deformation of composite parts. The influence of tooling material and their A272(CTE), heat capacity, substructure and surface condition are discussed. The direct impact of tooling on thermal management and its relationship to RSDM is also explained. Please visit Effect of tooling in a thermal management system for more details.

Significance

Once a tool is manufactured, the tool undergoes a thermal profiling/characterization stage, where the tool's thermal behavior such as temperature uniformity, and dimensional stability at high temperature are characterized. At this stage, if the tool is unable to produce parts that conform to process specifications (including dimensional tolerance), it is very expensive and time consuming to modify the tool.

Prerequisites

Recommended documents to review before, or in parallel, with this document:

• Heat transfer
• Specific heat capacity
• Thermal conductivity
• Thermal diffusivity
• Thermal management
• System interactions
• Tooling and consumables (system class)
• Effect of tooling in a thermal management system

Analysis

Effect of tooling material on RSDM

Since most advanced composites are cured at elevated temperatures in ovens or autoclaves, A272(CTE) of the tool plays an important role in residual stress and deformation, especially in tool-part interaction. The difference in CTEs causes tools made of different material to expand different amounts under the same cure cycle. For example, from the table below, an aluminum tool will expand more that a carbon-epoxy tool which will expand more than an invar tool. Because the composite part usually gels at elevated temperature (partway through the cure cycle), they will take the shape of the hot tool. Adding to the complexity, the composite part is frictionally coupled to the tool and has a different CTE.

CTE values for common tooling and part materials

Material	CTE (x10-6/°C)
Aluminum	23
Steel	11
Invar	0.6 to 1.5
Epoxy	45 to 62
Polyester	60 to 200
Vinylester	100 to 150
Carbon fibre (longitudinal)	-0.2 to -0.5
Carbon fibre (transverse)	10 to 15
E-glass fibre (longitudinal)	5
E-glass fibre (transverse)	5

Tool part interaction causing warpage is illustrated in the figure below: a) tool expands more than the laminate in in-plane direction upon heating, inducing tensile stress in the laminate close to interface (typically the first and second ply). b) Some tensile stress is released via inter-ply slippage, resulting in a through thickness stress gradient. When the resin gels, the through thickness stress gradient is locked in. c) Part warps upon demoulding.

It is worth noting that while some studies agreed that the higher the tool material CTE, the higher deformation, others did not observe the impact of tooling materials on part deformation . Aside from the A272, tool material density, thermal conductivity and diffusivity play important roles in achieving uniform temperature distribution in a tool, which can affect residual stress development. Please visit Effect of tooling in a thermal management system for more details on how these material properties can affect the temperature uniformity.

Tooling thermal management and RSDM

During cure, a thermal gradient across the tool surface can lead to residual stress and deformation of the part. Depending on the tooling material, the unevenly heated tool can expand such that it is out of tolerance, producing a deformed part. A thermal gradient across the tool surface can also cause areas of the part to cure ahead of other areas, generating residual stresses. Because the tool is adjacent to the part, a thermal gradient across the part and the tool in the through-thickness direction (aka the pancake effect) can also generate residual stresses. In general, thermal management is more complicated for parts cured at elevated temperatures because of the additional heat-up stage. For room temperature cured parts, the heat transfer between the part and the tool only occurs when the part exotherms to a higher temperature during polymerization and the tool acts as a heat sink.

As mentioned many times in the Thermal and cure/crystallization management (TM), the interaction between the tool, part, equipment is a complicated system level problem. In the context of residual stress and dimensional control management, ultimately what matters are the actual tool shape (likely at elevated temperature), which shapes the part and the thermal uniformity the part experiences. Please visit Effect of tooling in a thermal management system for more details.

Effect of tooling surface condition on RSDM

Tool-part interaction depends heavily on the friction between the first ply of the composite and the tool. The first ply of the part may stick well to a tool with a rougher tool surface. When the tool expands due to thermal expansion, it can impose strain on the ply. The use of non-stick FEP (Fluorinated Ethylene Propylene) films have proven to decrease the amount of deformation on flat laminates compared to using chemical release agent . Sensibly, higher processing pressure also results in greater deformation due to tool-part interaction.

If a peripheral grinding process is used to finish the tool surface, the surface roughness might be different in the grinding direction and the lateral direction. The roughness in the grinding direction was measured to be four times smaller than the lateral direction. As the tool is being used throughout its life, the directional roughness difference will decrease from resin and release agent residual filling the surface.

Effect of tooling size on RSDM

Tooling size can affect RSDM in two ways: thermal uniformity and the amount of thermal expansion/contraction. In general, the large the tool, the higher the thermal mass, the harder it is to execute good thermal management and achieve uniform temperature across the tool. Hot presses are typically not suitable for very large parts so large ovens and and autoclaves with more powerful circulation systems maybe required. Larger tool will also expands more when heated. The larger the tool, the larger the distortion due to tool-part interaction for flat parts or parts with mild curvature .

Effect of tooling on RSDM after gelation

The aforementioned sections mainly focused on the effect of tooling on RSDM during the heat-up stage of a cure cycle, mainly before the part gels. As mention in Residual stress and dimensional control management (RSDM), the majority of the residual stresses such as cure shrinkage stress and thermal stress form after gelation when the resin is able to bear stress. The tool behaves as a constraint during the post-gelation stage which prevents the part from deforming freely. Depending on the boundary conditions (surface condition, shape etc.), the tool can impact the final part residual stress levels to different extends.

Another important parameter is the resin modulus. When the resin just reaches gelation, the modulus is relative low. As the cure progresses and resin eventually cools down to room temperature, the modulus increases while experiencing cure shrinkage stress and thermal contraction stress and being constrained by the tool at the same time. When the part is demolded from the tool, it is at a higher modulus compared to when it gelled and the part is allowed to deform freely. The constraints (or boundary conditions) imposed by the tool has large impacts on the final part residual stress and deformation, a flat tool with two layers of FEP will not matter as much as a male C tool with release agent that creates geometrical locking. On a similar note, if a part is removed from the tool once it reaches gelation and proceeds with a free standing cure (while remaining vitrified to prevent viscoelastic deformation), the residual stress and deformation will be different than if it had been kept on the tool past gelation.

Effect of tooling during de-moulding

De-moulding can be difficult if the part deforms in a way that stuck to the tool. For example, a composite cylinder made on the outer surface of a mandrel can shrink (radius decrease) onto the mandrel, making it difficult for the mandrel to slide out. A C-shape made on a male (convex) tool can also spring-in and clamps onto the tool. Please refer to Effect of shape in a RSDM system for schematics of the geometries. Therefore, the de-moulding strategy should be considered during tooling design. Proper draft angles, collapsible tooling, dissolvable tooling, release agent or films etc. may be employed to de-mould parts that are susceptible to dimensional changes. More information can be found on Shape Development.

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 tooling in a RSDM system - A277
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	Developing production scale tooling - P129 Ensuring tooling choice meets part quality metrics - P128 Transitioning tooling styles between different thermal transformation equipment - P139 Troubleshooting tooling to achieve part quality - P142 Troubleshooting when changing production tooling material - P130
Case Studies	Conducting a thermal tooling survey on three complex tools of different materials - C105 Developing for Future Production Scale Up - C113 Use of Right Sized Simulation to Aid Decision Making in Thermoplastic Composite Manufacturing Systems - C114
Perspectives Articles

References

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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.

Engineered materials (designed to have specific properties) made from two or more constituent materials with different physical or chemical properties. The constituents remain separate and distinct on a macroscopic level within the finished structure.

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.

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.

A central processing theme in the manufacturing cycle. This theme relates to management of internal stresses that occur as the material undergoes differential thermal and physical phase change volume changes and viscoelastic property development.