Developing production scale tooling - P129

 

Developing production scale tooling
Practice document
Document Type	Practice
Document Identifier	129
Themes	Thermal management Materials deposition and consolidation management Residual stress and dimensional control management
Tags	Tooling Tooling aids Tooling parameters Development to production Production scaling
Objective functions	Cost Maintain Rate Maintain Quality Maintain
MSTE workflow	Development
Prerequisites	Effect of tooling in a thermal management system Practice for developing a thermal transformation process step

"Q: As part of my development strategy, I am considering transitioning from temporary development tooling to more durable production steel tooling with the exact same shape and bagging. Can I still cure good quality parts, with the same cure program in the same cure vessel?"

A: Depending on how big the change from temporary to production tooling is, you will likely have to alter your cure cycle. Production tooling is often more thermally massive, meaning it will take longer to heat up and will induce a larger thermal lag between the air and tool and the air and the part (for a convective heating system). In order to compensate for this, a slower ramp rate or longer hold times may have to be introduced. The benefit of a more thermally massive tool is that it will assist in absorbing the exothermic heat generated during cure.

Overview

Changing to more durable tooling for production (often referred to as "hard tooling") will allow for more use out of the tool before it has to be retired. This is beneficial for high production volumes, where the tooling must withstand thousands of cure cycles. Steel and steel-nickel alloys, such as invar, are common materials used for high volume production tooling . The downside to using such tooling materials is that they are heavy and expensive. Moreover, a change in tooling material also means a change in the system parameters. In particular, the thermal properties of the tool will have likely changed. Some potential major changes to consider are the thermal mass (as mentioned), thermal diffusivity, and the coefficient of thermal expansion (CTE). Changing the thermal properties of the tool can have a significant impact on the final quality of the part.

Thermal management considerations

Link to thermal management

Transitioning from soft to hard tooling will come with an increase in thermal mass of the tool. As a result, the tool will take longer to heat up and thermal lag will increase. However, the increase in thermal mass will also help absorb the exothermic heat generated, reducing the chance of thermally degrading the part during high temperature processing. The other thing to consider is the thermal diffusivity of the tool. If transitioning to metallic tooling for production, it is likely that the thermal diffusivity is relatively good. However, even among metals, thermal diffusivity may vary, with aluminum presenting a high diffusivity. An increased thermal diffusivity allows for more uniform temperature distribution across the tool surface. This is important for preventing hot and cold spots, allowing for a more even temperature distribution across the part as well. Moreover, improving thermal diffusivity allows for better conduction of heat through the tool, thus improving the efficiency at which the tool may draw heat away from the part during exotherm. To see a practical example of how a change in tooling material affects the thermal response of the tool, visit the following page.

The energy required to heat the tooling is determined by its thermal mass, as governed by the following equation:

\(Q=mC_p\Delta{T}\)Energy required to raise an object's temperature, where \(Q\) = energy, \(m\) = object mass, \(C_p\) = specific heat capacity, \(\Delta{T}\) = change in temperature from applied energy. Note that \(mC_p\) is the thermal mass of the object.

Thermal diffusivity can be calculated using the following equation:

\(\alpha=\frac{k}{\rho C_p}\)Thermal diffusivity, where \(\alpha\) = thermal diffusivity, \(k\) = thermal conductivity, \(\rho\) = density, and \(C_p\) = specific heat capacity.

The tables below provide a quantitative and qualitative list of typical thermal properties for tooling materials. Properties for composite tools are dependent on the fibre volume fraction and orientation. Note that in the second table, high and low thermal mass are shaded both red and green. The reason for this is that either can be beneficial or detrimental. A high thermal mass increases thermal lag and heat up time, but it decreases part exotherm. A low thermal mass decreases thermal lag and heats up quickly, but increases part exotherm.

Typical material properties for tools. Note that the properties for the composite tool are dependent on the fibre volume fraction and orientation.

Tooling material	Density (kg/m3)	Specific heat capacity (J/kg-K)	Thermal conductivity (W/m-K)	Coefficient of thermal expansion - CTE (x10-6/°C)	Thermal diffusivity (x10-6m2/s)
Invar	8000	515	11.0	0.6-1.5	2.67
Mild steel	7850	510	55	11	13.7
Carbon-epoxy composite	1580	870	0.7 (through-thickness)	-0.5 (in-plane) 22.5 (through-thickness)	0.5 (through-thickness)
Aluminum	2710	896	167	23	68.9

Qualitative comparison of tooling materials

Tooling material	Cost	Durability	Weight	Thermal mass	Thermal conductivity	Coefficient of thermal expansion (CTE)	Thermal diffusivity
Invar	$$$	Excellent	Heavy	High	Moderate	Low	Low
Steels	$$$	Excellent	Heavy	High	Good	Moderate	Moderate
Composites	$	Low	Light	Low	Low	Moderate	Low
Aluminum	$$	Good	Moderate	Low	Excellent	High	High

Verifying that the tool operates as expected is critical for evaluating the performance of the tool. Attaching thermocouples to the surface and under structure of the tool when performing the initial runs will ensure a better understanding of the heat transfer occurring in the tool during production. See image below for an example.

Invar autoclave tool instrumented with thermocouples for surveying the heat transfer of the tool during production.

The other aspect to consider when changing tooling is how it may influence airflow in a convective heating system. However, in this case, if the shape of the tool is not being changed, then the airflow around the tool should remain the same. In order to ensure that airflow, and therefore the heat transfer coefficient (HTC), has not changed, the tool-part assembly should be placed in the same location in the equipment during production as it was during qualification.

To learn more about how tooling choices may affect the thermal response of the part, visit effect of tooling in a thermal management system.

Materials deposition and consolidation management considerations

Link to Materials deposition and consolidation management (MDCM)

Tooling may influence the flow characteristics of the material by altering the thermal profile of the part. This, in turn, may alter the material's viscosity evolution. To learn about ensuring appropriate resin flow, visit the following page: ensuring appropriate resin flow and part consolidation for a new material.

Residual stress and dimensional control management considerations

Link to residual stress and dimensional control management

The coefficient of thermal expansion (CTE) is an important factor that should be considered when changing tooling material. If the mismatch in CTE between the tool and part is considerable, and this is not mitigated for, then the part may deform during cure or upon demoulding. The best way to mitigate for this is to match the tool CTE with the part CTE. Generally speaking, composite materials have a low CTE while metallic tools have a much higher CTE. The exception is invar, which has a very low CTE and is therefore a preferable option for advanced tooling choices. However invar is also very expensive, heavy, and thermally massive. Composite tools, may also offer a close match in CTE to composite parts. However, because the resin and fibres of a composite material exhibit different CTEs, the overall CTE for a composite part is dependent on the fibre volume fraction and orientation of the fibres. Therefore, it can't be assumed that all composite parts, even if made with the same materials, have the same CTE. The same is true for composite tooling.

Another way to mitigate CTE-induced deformation at the tool-part interface is to include a low-friction film between the two surfaces. This may be a release agent, release film, teflon film, or other consumable.

General CTE values for common tooling materials are provided in the table below:

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

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 quantitative measure of how a material will respond to transient thermal conditions. It is defined as the ratio of thermal conductivity to the volumetric heat capacity of the material (density times the specific heat capacity).

Materials with large thermal diffusivity have a high thermal conductivity relative to their capacity to store energy (heat capacity) and will rapidly distribute the thermal energy throughout its volume and rapidly change temperature to reach the new equilibrium temperature.

Materials with small thermal diffusivity have a low thermal conductivity relative to their capacity to store energy (heat capacity) and will have a sluggish temperature response to a change in thermal conditions (large temperature lag). This is because it takes more energy per unit volume to change the temperature, and the low thermal conductivity means it takes longer to distribute the thermal energy throughout its volume.

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.

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

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.