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Tooling and processing for efficient and high tolerance prepreg composite manufacturing
KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics.ORCID iD: 0000-0001-5567-1037
2022 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Composite use is normally motivated in aircraft design by improved weight-specific mechanical properties as compared to metals. However, fatigue and dielectric properties may also be critical for material selection. In the modern world the environmental footprint, with reduced emissions in focus, will be lighter with the successful use of composites. Flight safety is the most important requirement for the aerospace industry. In order to benefit from the important properties which ensure the necessary performance of the aircraft, part materials must be according to material specification. This is not unique to the composite materials. However, since the composite material is often created simultaneously with the composite part, the manufacturing processes used are even more critical for a composite part than for a metal part. Tooling is required for most of the process steps in composite part manufacturing. The large number of tools required to complete a composite part and the relatively small series involved in most composite applications render tooling cost a major contributor to final part cost. This is especially true in the aeronautical field considering the high-quality requirements as concerns shape accuracy and laminate quality, as well as low production rates with annual production ranging from a few dozen to the rare thousands of parts. Since tooling adds a substantial contribution to the part cost, it is imperative to be aware of the different options available for each process step and how these affect process stability and cost. Göte Strindberg, a former technical director at Saab, often said “Ninety-nine per cent right can be one hundred per cent wrong in a composite manufacturing process”. Finding the correct parameters for the last per cent is key to composite part success. This has been the overall goal of this project. The beginning of the project focused on both process and tooling related improvements for hot-forming of prepreg laminates enabling new opportunities and improved ergonomics. The research results in improvements in laminate quality, for example, reduced thickness variation, and a significant increase in process efficiency. One conclusion is that an 80% reduction of cycle time can be achieved with maintained, or even improved, laminate quality. A novel method for compensation of forming tools is also presented. Research then continued with cure tool related issues. Shape fidelity and durability for composite cure tools was studied with the combined outcome of an in-depth understanding of composite cure tool performance at the elevated temperature and moisture conditions present during the use of the tools.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2022. , p. 161
Series
TRITA-SCI-FOU ; 2022:10
Keywords [en]
Tooling, Composite Manufacturing, Mould, Cure tool, Hot-forming, Compensation, Moisture, Shape distortion
National Category
Composite Science and Engineering
Research subject
Aerospace Engineering
Identifiers
URN: urn:nbn:se:kth:diva-312115ISBN: 978-91-8040-246-0 (print)OAI: oai:DiVA.org:kth-312115DiVA, id: diva2:1657504
Public defence
2022-06-08, https://kth-se.zoom.us/j/61052452396, F3, Lindstedtsvägen 26, Stockholm, 09:00 (English)
Opponent
Supervisors
Funder
Swedish Foundation for Strategic Research, ID15-0042
Note

QC 220512

Available from: 2022-05-12 Created: 2022-05-11 Last updated: 2022-06-25Bibliographically approved
List of papers
1. Geometric compensation of convex forming tools for successful final processing in concave cure tools – An experimental study
Open this publication in new window or tab >>Geometric compensation of convex forming tools for successful final processing in concave cure tools – An experimental study
2019 (English)In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 116, p. 187-196Article in journal (Refereed) Published
Abstract [en]

To enable shimless composite part assembly it is desired to manufacture composite spars in concave, female tools. However, due to restricted access this process is often associated with a high degree of manual operations or expensive highly specialized production equipment. This study investigates a sequential manufacturing operation where the material is automatically laid up flat and hot formed onto a convex forming tool. Thereafter it is placed in the concave tool for curing. The study investigates four different ways to geometrically compensate the forming tool to simplify seating and enhance part quality. All compensations require material movement in-between forming and curing; this movement was tracked using Micro-CT. Micrographing and optical shape measurements show that a correct compensation provides high quality parts without adding labour intensive manufacturing steps.

Place, publisher, year, edition, pages
Elsevier BV, 2019
National Category
Composite Science and Engineering
Identifiers
urn:nbn:se:kth:diva-312050 (URN)10.1016/j.compositesa.2018.10.030 (DOI)000451491700020 ()2-s2.0-85056199790 (Scopus ID)
Funder
Swedish Foundation for Strategic Research
Note

QC 20220510

Available from: 2022-05-10 Created: 2022-05-10 Last updated: 2022-06-25Bibliographically approved
2. Using tailored temperature variations to obtain flawless forming of multi-stacked unidirectional prepreg
Open this publication in new window or tab >>Using tailored temperature variations to obtain flawless forming of multi-stacked unidirectional prepreg
Show others...
2020 (English)In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 54, no 26, p. 3999-4009Article in journal (Refereed) Published
Abstract [en]

In this article, tailored temperature zones are used to obtain improved quality during rapid, high pressure forming of multi-stacked unidirectional prepreg. Particularly in aerospace applications, commonly used forming processes for multi-stacked unidirectional prepreg are often considered a bottleneck in production since the forming cycle requires both heating and cooling ramps and consequently takes long time—often about 1 h. It is possible to speed up the process by using elevated pressure and temperature. However, higher pressure and temperature also increase the influence of pressure gradient-driven, in-plane material movement (squeeze flow). This typically appears as radius thinning when forming a C-spar geometry on a male mold. Decrease of lay-up temperature will decrease radius thinning, but due to obstructed interply slippage, instead bending-induced wrinkles appear on the spar flange. In this article, tailored temperatures at the radius and in the flange area are introduced by using a hot lay-up and a cold mold. The results show that temperature differences of 6℃–10℃ between the radius area and the flange edge of the lay-up decreases radius thinning while still avoiding bending-induced wrinkles. Except from the radius temperature also the stacking sequence and the choice of prepreg system showed a significant influence on the radius thinning.

Place, publisher, year, edition, pages
SAGE Publications Ltd, 2020
Keywords
Composites, forming, prepreg, Aerospace applications, Molds, Elevated pressure, Heating and cooling, High-pressure forming, Pressure and temperature, Stacking sequence, Temperature differences, Temperature variation, Temperature zone, Flanges
National Category
Vehicle Engineering Aerospace Engineering
Identifiers
urn:nbn:se:kth:diva-287997 (URN)10.1177/0021998320924714 (DOI)000533916800001 ()2-s2.0-85085496405 (Scopus ID)
Note

QC 20220530

Available from: 2020-12-29 Created: 2020-12-29 Last updated: 2022-06-25Bibliographically approved
3. Vacuum-Assisted Hot-Forming Using Tailored Laminate Temperature
Open this publication in new window or tab >>Vacuum-Assisted Hot-Forming Using Tailored Laminate Temperature
2021 (English)In: Applied Composite Materials, ISSN 0929-189X, E-ISSN 1573-4897, Vol. 28, no 2, p. 247-269Article in journal (Refereed) Published
Abstract [en]

A non-isothermal vacuum assisted hot-forming process using tailored laminate temperature is introduced. By using process simulation and manufacturing experiments, improved laminate quality is achieved compared to the standard hot-forming process. Furthermore, it is also shown that the manufacturing time in the clean room can be reduced to one tenth of the standard process time. In this study 8.4 mm thick quasi-isotropic laminates from unidirectional prepreg were laid up flat with an automatic tape laying machine and hot-formed to a U-shaped laminate. The laminates were then cured in a concave mould with standard bag on the inside. A complete tailored temperature hot-forming cycle of 7.5 min produced a very good final laminate quality with a total thickness variation as low as 4.0% and without wrinkles or indications of porosity. With a 4 min hot-forming cycle the thickness variation was also acceptable at 8%.

Place, publisher, year, edition, pages
Springer Nature, 2021
Keywords
Forming, Prepreg, Simulation, Thickness variation, Wrinkles, Ceramic materials, Composite materials, Hot forming process, Laminate quality, Laying machines, Manufacturing time, Process simulations, Quasiisotropic laminates, Total thickness variations, Hot stamping
National Category
Production Engineering, Human Work Science and Ergonomics
Identifiers
urn:nbn:se:kth:diva-304439 (URN)10.1007/s10443-020-09858-8 (DOI)000609050600001 ()2-s2.0-85099543034 (Scopus ID)
Note

QC 20220503

Available from: 2021-11-08 Created: 2021-11-08 Last updated: 2022-06-25Bibliographically approved
4. The true shape of composite cure tools
Open this publication in new window or tab >>The true shape of composite cure tools
2020 (English)In: Journal of Manufacturing Processes, ISSN 1526-6125, Vol. 59, p. 279-286Article in journal (Refereed) Published
Abstract [en]

The cure tool is a key factor for successful composite part manufacturing since the tool shape is directly reflected in the moulded part. A composite cure tool is an alternative to metal tools. Lead time is often lower and rate tools less expensive. The recurring cost per produced part in the tool is often also lower, for instance from faster autoclave cure time. However, durability is commonly considered less favourable. Beyond all, a composite cure tool is often motivated as being the better option since the thermal expansion is close to that of the part which simplifies compensation of process-induced shape distortion. But is this really accurate? Perhaps, but only if the true mould shape at cure temperature is known. This paper includes both experiments and simulation showing actual composite cure tool shape at cure temperature. Also, simulation of stress levels in the tool laminate indicate an important characteristic of durability.

Place, publisher, year, edition, pages
Elsevier Ltd, 2020
Keywords
Anisotropy, Composite cure tool, Process simulation, Tool deflection, Tool design, Autoclaves, Durability, Thermal expansion, Autoclave cure, Composite parts, Cure temperature, Mould shape, Moulded parts, Recurring costs, Shape distortions, Stress levels, Curing
National Category
Composite Science and Engineering
Identifiers
urn:nbn:se:kth:diva-287943 (URN)10.1016/j.jmapro.2020.08.052 (DOI)000590952800004 ()2-s2.0-85092127195 (Scopus ID)
Note

QC 20220530

Available from: 2020-12-21 Created: 2020-12-21 Last updated: 2022-06-25Bibliographically approved
5. Moisture In Composite Cure Tools
Open this publication in new window or tab >>Moisture In Composite Cure Tools
2022 (English)In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 158, article id 106984Article in journal (Refereed) Published
Abstract [en]

Composite cure tools are preferable to metal tools regarding in-plane thermal expansion, quicker part processing using the tool and potentially lower cost. However, durability is often questioned and moisture in the tool is often assumed to be a negative factor. In order to increase understanding of the durability of composite cure tools sorption and swelling, two high performance prepreg tooling materials using Benzoxazine and Bismaleimide resin reinforced with carbon fibre fabric were measured over a six-month period. An Epoxy-based prepreg was also studied. The experimentally acquired material characteristics were used in simulation of water content and swelling in a composite cure tool during use. It was concluded that gradient moisture content and swelling will always be present which presents a considerably different shape distortion compared to isotropic water content. The data presented may contribute to informed selection of composite cure tool material and facilitate optimisation of tool maintenance.

Keywords
Tooling, Sorption, Swelling, Simulation
National Category
Composite Science and Engineering
Research subject
Aerospace Engineering
Identifiers
urn:nbn:se:kth:diva-312107 (URN)10.1016/j.compositesa.2022.106984 (DOI)000805466500001 ()2-s2.0-85130416861 (Scopus ID)
Funder
Swedish Foundation for Strategic Research, ID15-0042
Note

QC 20220511

QC 20220707

Available from: 2022-05-10 Created: 2022-05-10 Last updated: 2024-03-18Bibliographically approved

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