Ensuring tooling choice meets part quality metrics - P128

 

Ensuring tooling choice meets part quality metrics
Practice document
Document Type	Practice
Document Identifier	128
Themes	Thermal management Material deposition management Flow and consolidation management Residual stress and dimensional control management
Tags	Tooling Tooling aids Tooling parameters Development to production
Objective functions	Cost Maintain Rate Maintain Quality Increase
MSTE workflow	Development
Prerequisites	Effect of tooling in a thermal management system

Q: "As part of my development program, I am considering different tooling materials such as carbon-fibre reinforced polymer (CFRP) or steel for example. How do I ensure that my tooling choices do not create quality issues?"

A: The tooling choice will impact the temperature history your part sees during cure, as well as the final dimensions of your part. The key is to implement tooling that is appropriate for your material, intended part shape, and thermal cycle. If your intent is to increase part temperature quickly or decrease thermal lags, then you will need tooling with a low thermal mass (such as CFRP). If your temperature specifications are tight and you're worried about the part exotherming and degrading, increasing the thermal mass of your tool will help - both steel and invar are suitable for this application. If instead you want to ensure that your tool heats up evenly, a tool with a high thermal diffusivity, such as aluminum, is an ideal choice. Another important aspect to consider is the coefficient of thermal expansion of your tool (CTE). In order to ensure that your as-manufactured dimensions are close to your as-designed dimensions for your part, it is common to use a tools with a low CTE, such as invar or CFRP. Alternatively you can compensate your tooling for dimensional changes in both the tool (what are the dimensions and shape of the tool at cure temperature?) and part (how does the part distort during cure?). Tooling compensation can be done empirically or by simulation. With whichever tooling material you choose, there will be tradeoffs. The key is to balance the pros and cons in order to achieve the intended quality metrics for the process.

Overview

Tooling plays a critical role in composites manufacturing and can affect the system response in several significant ways. This includes influencing part's shape, thermal response, flow and consolidation, dimensional control, and others.

Thermal management considerations

Link to thermal management

From a thermal perspective, some major ways in which the tooling influences part quality are as follows:

1. The tool thermal mass is typically significant compared to the part thermal mass. This means that considerable energy is spent in heating the tooling. At the simplest level, you may be able to consider just the thermal mass of the tool skin. An increase in thermal mass of the tool will result in the tool taking longer to heat up and an increase in thermal lag within the system. 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 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.

2. Thermal 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. 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.

3. The tooling substructure plays an important part in controlling the airflow under the tool skin, and thus determines the toolside heat transfer coefficient (HTC). If the airflow is different, this can change the HTC enough to change the part temperature history.

The tables below provides 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

To see a practical example of how the tooling material affects the thermal response of the tool, visit the following page. Remember that if the thermal history changes significantly, then all downstream outcomes may also be affected, see other outcome management considerations.

Refer to effect of tooling in a thermal management system to learn more about tooling considerations from a thermal perspective.

Material deposition management considerations

Link to material deposition management

Content coming soon

Flow and consolidation management considerations

Link to flow and consolidation management

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 selecting 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

About	Help

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.

The use of multiphysics models to predict the outcome of real-world scenarios. May be analytical (closed form, "hand calculations") or computational (implementation on computers is required due to the large number of calculations involved. e.g. finite element method, finite difference method)

Most often in composite materials engineering, simulation refers to either:

• Process simulation (predicting manufacturing outcomes, or the inverse problem of predicting manufacturing parameters to achieve a desired outcome), or
• Performance simulation (predicting the stiffness/strength/suitability of a structure, or the inverse problem of the material properties and dimensional requirements to achieve a desired stiffness/strength/suitability of a structure)

(same as "Modelling")

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.