Decreasing Cure Cycle Time - P121
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| Document Identifier | 121 | ||||||
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| MSTE workflow | Development | ||||||
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Q:"I need to ensure that I have the fastest cure cycle possible as I need to maximize use of my cure vessel resources. How do I develop the fastest cure cycle possible for a given combination of cost and quality?"
A:You can develop the fastest cure cycle possible by selecting the highest heating and cooling rates and cure temperature which allow you to meet the material's thermal specifications. For room temperature curing when the cure temperature is not actively controlled but a function of the ambient air temperature, the material's thermal specifications should take into account air temperature fluctuations to effectively constrain the duration of the material deposition step, and the thermal transformation or cure step before demoulding. The material's thermal specifications should also provide guidance on how to develop a post-curing step using active heating when the ambient temperature is too low to reach an appropriate final degree of cure or to meet the rate requirement by shortening the ambient cure.
Overview[edit | edit source]
Thermal transformation is one of the most critical manufacturing steps, which most manufacturing outcomes depend on. It allows the solidification of the resin by controlling or monitoring the resin’s temperature. During a cure cycle, a thermoset resin undergoes significant transformation and evolves from a liquid viscous state, which allows forming and consolidation, to a final solid state before demoulding. As this happens, care must be taken to ensure that the level of cross-linking or cure is in the appropriate range: the resin must be neither under-cured nor over-cured. Given the exothermic nature of the cure reaction, it is possible to overheat a part, above the applied temperature, thus leading to heat damage. The part's consolidation and formation of residual stresses are also affected by the resin's solidification process. The development of a cure cycle therefore represents a major development step, which not only dictates thermal management outcomes but also impacts other manufacturing outcomes related to Materials deposition and consolidation management, and residual stress and dimensional control management.
Due to the high cost of capital equipment for thermal transformation, it is desirable to maximize the use of these types of equipment. There are several techniques for increasing throughput of the thermal transformation step (see other techniques in Troubleshooting a cure cycle for improved production rates) and developing the shortest cure cycle is the simplest one. Increases in productivity can be made by selecting the highest heating and cooling rates and cure temperature but this comes at the cost of increasing the risks of failing the material's thermal specifications and quality requirements as explained in more details below.
The development of a cure cycle is an iterative process during which an understanding of the thermal response of the part/tool assembly is developed. This understanding is necessary to define an appropriate cure cycle for the thermal transformation equipment in order to meet the material's thermal specifications. The material's thermal specifications are defined in the first place during the material development phase. The material's thermal specifications constrain the part's thermal history during processing and cure to ensure that the level of quality is in the appropriate range. Thermal specifications typically limit the lead and lag temperatures during a ramp as well as the maximum and minimum temperatures during a hold. If not available, they must be defined first.
Thermal management considerations[edit | edit source]
Fast heat up and cooling rates, as well as high cure temperature, can be selected according to the material’s thermal specifications to allow for the shortest cure cycle. How fast the ramp rates or how high the cure temperature can be are ultimately defined by the material's thermal specifications as the cure cycle must allow the part's thermal history to meet the material's thermal specifications. For example, as explained in Systems Knowledge - effect of equipment in a thermal management system, increasing the heating rate has the combined effect of increasing the temperature gradients across the part and the thermal lag between the part and the curing environment, and may also lead to a larger exotherm. On the other hand, increasing the cure temperature has the effect of increasing the lead temperature and exotherm which might exceed the material's thermal specifications. If the cure cycle is not defined properly, the part's thermal history will fail the material's thermal specifications. It is recommended to start the iterative development process with low ramp rates and cure temperature and to gradually increase them and the complexity of the cure cycle.
To ensure thermal conformance as you are developing a cure cycle, you can evaluate the part’s thermal history by using:
- Thermal Simulation
- Thermal Test
- Combination of thermal simulation and test
If you find that the part’s thermal history no longer meets the material's thermal specifications, you will have to continue the iterative development process and define and evaluate alternative cure cycles. For instance, if the exotherm is too large, you might consider lowering the cure temperature or the heating rate. Depending where you are in your development process, you might also decide to keep a problematic but shorter cure cycle and assess if by changing the tooling and/or equipment the given thermal specifications can be satisfied. For example, you might consider changing the thickness, substructure, or material of the tooling. 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 lag. Replacing an aluminum tool with an invar one, for example, has such an effect.
Materials deposition and consolidation management considerations[edit | edit source]
Resin viscosity affects both flow dynamics during deposition and the subsequent consolidation step. A lower viscosity resin facilitates better flow and can lead to shorter cycle times. However, an excessively low viscosity may compromise the resin's structural integrity and final part quality. Higher viscosity resins may slow down the deposition and consolidation process, potentially elongating cure cycle times.
The fiber bed permeability dictates the ease with which resin can flow through the reinforcement. Higher permeability allows for better resin infiltration into the fiber bed and can reduce cure cycle time. However, very high permeability can cause defects, such as porosity.
For more information, see: Materials deposition and consolidation management (MDCM)
Residual stress and dimensional control management considerations[edit | edit source]
A faster cure cycle may limit the time available for proper material consolidation and adherence to desired shape. High heating and cooling rates can also cause high gradients of viscosity and degree of cure in the part and exasperate the development of residual streses and distortions due to non-uniform material properties. The rapid changes in temperature and viscosity gradients can result in uneven curing and cooling, leading to internal stresses that may impact the mechanical properties and shape of the final product.
For more information, see: Residual stress and dimensional control management (RSDM)
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Understanding Composites Processing
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The way that the material, shape, tooling & consumables and equipment (abbreviated as MSTE) interact with each other during a process step is critical to the outcome of the manufacturing step, and ultimately critical to the quality of the finished part. The interactions between MSTE during a process step can be numerous and complex, but the Knowledge in Practice Centre aims to make you aware of these interactions, understand how one parameter affects another, and understand how to analyze the problem using a systems based approach. Using this approach, the factory can then be developed with a complete understanding and control of all interactions.
Interrelationship of Function, Shape, Material & Process
Design for manufacturing is critical to ensuring the producibility of a part. Trouble arises when it is considered too late or not at all in the design process. Conversely, process design (controlling the interactions between shape, material, tooling & consumables and equipment to achieve a desired outcome) must always consider the shape and material of the part. Ashby has developed and popularized the approach linking design (function) to the choice of material and shape, which influence the process selected and vice versa, as shown below:
Within the Knowledge in Practice Centre the same methodology is applied but the process is more fully defined by also explicitly calling out the equipment and tooling & consumables. Note that in common usage, a process which consists of many steps can be arbitrarily defined by just one step, e.g. "spray-up". Though convenient, this can be misleading.
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