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Cores & inserts - A202

From CKN Knowledge in Practice Centre
Cores & inserts
Foam core stack-3v8MuTnf.jpg
Document Type Article
Document Identifier 202
Relevant Class



Introduction[edit | edit source]

Composite structures frequently utilize lightweight core materials within components for a variety of reasons. The most popular use is to construct sandwich panels. The core material increases the moment of inertia by spacing the high strength material away from the neutral axis where the tensile or compressive loads are high in bending. Core materials can also provide a layer with high permeability to assist in resin flow during a liquid moulding process. Common core materials are wood, metallic and non-metallic honeycomb, and open and closed cell foams[1].

Scope[edit | edit source]

This section will discuss the manufacturing processes and common use for honeycomb, wood, and foam core materials. The core material properties and the selection of the core material when designing a composite product are also discussed. A list of core materials and insert suppliers are included.

Significance[edit | edit source]

Sandwich panels are widely used in the aerospace sector and increasingly in transportation, energy, marine sectors, etc. It is crucial to understand the properties of the core materials in order to select the right material and design and manufacturing sandwich panels that meet performance requirements.

Prerequisites[edit | edit source]

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

Core material types[edit | edit source]

Wood[edit | edit source]

Common wood core materials include balsa, mahogany, spruce and poplar. While any type of wood can be laminated between two face sheets and form a sandwich structure in principle, balsa (Ochroma pyramidale) wood is the most common wood core material in composites applications due to its low cost, low density, relatively high mechanical properties and other properties. Balsa wood is a cellular material consisting of fibers (66-76%), rays (20-25%) and vessels (3-9%) [2]. To be used as a core with composite materials, balsa wood is cut transversely to its grain direction, and re-joined (by adhesive) side by side such that the grain direction is perpendicular to the core surface; therefore called end-grain balsa. [pic] Density of end-grain balsa core can range from 100 to 300 kg/m^3 [3]

Eng-grain balsa

Aside from the advantages mentioned above, end-grain balsa cores have high compression and shear strength, good fatigue performance and are easy to machine. Depending on the face sheet material and manufacturing process, faces sheets can also be directly laminated onto the core without using adhesives. Disadvantages of end-grain balsa cores include low conformability, high variability in properties (since the core is made from smaller blocks bonded together) and high moisture sensitivity. Instead of bonding the small blocks side by side using adhesive, balsa can be produced in flexible sheet form where the small blocks are held by a fabric scrim backing, allowing the core to conform to curvatures to some extend. Moisture sensitivity can be mitigated if the core is properly sealed[3].

End-grain mahogany or spruce have also been used as high strength inserts to close out the sandwich panels. However, aluminum extrusions are more recent replacements. In general, the thermal conductivity of wood along the grain direction is higher (about 2.5 times) than that in the direction parallel to the grain.

End-grain cores are widely used in marine, sporting goods applications. Major manufacturers of balsa core include:

  • Alcan Baltek Corp
  • DIAB
  • Nida-Core

Honeycomb[edit | edit source]

Please visit Honeycomb

Foam[edit | edit source]

Please visit Foam

Core material properties[edit | edit source]

General properties[edit | edit source]

The density of core are typically measured in pounds per cubic feet (lbs/ft3) of grams per cubic centimeter (g/cm3).

Mechanical properties[edit | edit source]

Mechanical properties of the core heavily depend on the core material and density. For honeycomb cores, cell size, configuration and manufacturing process can also affect the core mechanical properties. Mechanical properties for honeycomb cores are usually directional.

Sandwich panels under bending can fail by face sheet yield, face sheet buckling (wrinkling), core shear failure, or face sheet indentation. Depending on the failure mode, Young's modulus, shear modulus, yield strength and shear yield strength of the core or a combination of these mechanical properties can play roles in the failure of sandwich panels. Values for the compressive modulus and strength are typically provided by suppliers. In general, honeycomb has better shear and compressive strength and modulus than foam, see table [4]

Thermal properties[edit | edit source]

Depending on the functions of the sandwich panel or the stand alone core, the specific heat Cp and thermal diffusivity of the core should be taken into consideration when selecting a core material. The core, face sheets and adhesive must also have compatible coefficient of thermal expansion to withstand thermal cycles during service and provide dimensional stability.

Other properties[edit | edit source]

Other properties to consider when selecting a core material include:

  • Chemical and corrosion resistance
  • Moisture resistance
  • Flammability
  • Acoustic damping
  • EMI/RFI shielding

Core material processing[edit | edit source]

Cleaning and drying[edit | edit source]

Core materials should be clean and free of contamination or solvent residual prior to be bonding or assembled into sandwich panels. Contamination that occurs during transportation, receiving, storage, forming or machining can heavily impact the bond strength. Dust from machining, for example, can be wiped with clean cheese cloth or blown by compressed air. Grease or wax on metallic honeycomb can be cleaned by solvent vapor or bath. Depending on the core material and contamination type, different cleaning methods may be employed.

Moisture within the core prior to bonding can have detrimental effects on the core-face sheet bond quality. During thermal transformation, the steam pressure can result in blisters between the core and face sheet or in the case of open cell cores (such as honeycomb), damage or destroy the core. To remove moisture, cores can typically be dried in an oven. Depending on the core material, the drying temperature can range from 60 °C to 121 °C (140 °F to 250 °F). Drying time is usually 120 minutes at minimum. Storing conditions should also be specified to avoid moisture absorption. During material deposition, the core laid into a tool should be covered with adhesive film or prepreg within 24 hours from being taken out of the storage. Exposed core should be cover with vacuum bags sealed to the tool.

Forming[edit | edit source]

Curvature and geometrical complexity can be formed into cores prior to the material deposition step. In general, the denser and thicker the core, the harder it is to form. Some common core materials and their forming methods are:

  • Metallic honeycombs can be mechanically formed with or without heat
  • Non-metallic honeycombs are typically thermal formed (heat)
  • Foam core are typically thermal formed (heat)
  • Wood core can be formed with heat and moisture. Wood becomes pliable after soaking in hot water. Wood can then be formed into shape and dried

Thermal forming involves heating the core to the forming temperature then forcing the core into the desired shape (with vacuum, mechanical weight, dies, etc). Thermal forming can alter mechanical properties and may have a tendency to spring back.

As an extra processing step, forming the core may involve extra tooling and equipment. The increase in time and cost to manufacture a product and should be carefully considered during design.

Splicing[edit | edit source]

Smaller core sections can be spliced together to construct a larger core with more complicated geometries. Different core materials can also be spliced together to take advantage of the different properties. It is not uncommon to splice/insert a core into another core such that it is completely surrounded to alter the local properties. When performing such insertion, the core being inserted is typically cut slightly larger than the nominal such that the fit would be snug. When splicing cores together, it is also a good practice to match the cores' directions. For example, matching the ribbon directions of two honeycomb cores.

When splicing two or more different core materials together, it is important to make sure the final sandwich panel will be flat without steps at the splice. If one core material is more compliant, it can be slightly thicker to "match" the compressive modulus of the others. The spliced cores can also be machined smooth before being deposited into a sandwich panel.

Splicing adhesives should be carefully selected while considering the entire thermal history of the sandwich panels. If the sandwich panel is being co-cured, the adhesive needs to be compatible with the face sheet and adhesive curing requirements. Certain foaming characteristics might be required when splicing honeycomb cores.

Machining[edit | edit source]

Cores can also be machined to achieve complex geometries and contours. During core machining, it is very important to avoid contamination so that any subsequent core bonding is not affected. No oil or other types of lubricants should be used unless specified and have a plan for removing them before bonding. It is also common to have equipment and work cells dedicated to core machining to eliminate contamination.

The machined surface finish also can have an impact on the subsequent bonding. For example, the burr/fuzz on honeycomb cell edges after machining can either facilitate the bonding by "biting" into the adhesive or prevent proper fillet formation. Evaluation should be conducted on a per case basis.

Very low density or light weight honeycomb cores may need to be stabilized by taping or potting before machining. Similar to core forming, machining involves extra tools and equipment, adding cost and complexity to product manufacturing.

Explore this area further

Related pages

Page type Links
Introduction to Composites Articles
Foundational Knowledge Articles
Foundational Knowledge Method Documents
Foundational Knowledge Worked Examples
Systems Knowledge Articles
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
Case Studies
Perspectives Articles


  1. [Ref] Campbell, F.C. (2004). Manufacturing Processes for Advanced Composites. Elsevier. doi:10.1016/B978-1-85617-415-2.X5000-X. ISBN 9781856174152.CS1 maint: uses authors parameter (link) CS1 maint: date and year (link)
  2. [Ref] Borrega, Marc et al. (2015). "Composition and structure of balsa (Ochroma pyramidale) wood". 49 (2). doi:10.1007/s00226-015-0700-5. ISSN 1432-5225. Cite journal requires |journal= (help)CS1 maint: extra punctuation (link) CS1 maint: uses authors parameter (link)
  3. 3.0 3.1 [Ref] Astrom, B.T. Manufacturing of Polymer Composites. ISBN 9780748770762.CS1 maint: uses authors parameter (link)
  4. [Ref] Bitzer, Tom (1997). Honeycomb Technology Materials, Design, Manufacturing, Applications and Testing. 1997 Springer Science+Business Media Dordrecht. doi:10.1007/978-94-011-5856-5. ISBN 978-94-011-5856-5.CS1 maint: uses authors parameter (link) CS1 maint: date and year (link)

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Welcome to the CKN Knowledge in Practice Centre (KPC). The KPC is a resource for learning and applying scientific knowledge to the practice of composites manufacturing. As you navigate around the KPC, refer back to the information on this right-hand pane as a resource for understanding the intricacies of composites processing and why the KPC is laid out in the way that it is. The following video explains the KPC approach:

Understanding Composites Processing

The Knowledge in Practice Centre (KPC) is centered around a structured method of thinking about composite material manufacturing. From the top down, the heirarchy consists of:

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.

The relationship between material, shape, tooling & consumables and equipment during a process step

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:

The relationship between function, material, shape and process

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.

The relationship between function, material, shape and process consisting of Equipment and Tooling and consumables


The KPC's Practice and Case Study volumes consist of three types of workflows:

  • Development - Analyzing the interactions between MSTE in the process steps to make decisions on processing parameters and understanding how the process steps and factory cells fit within the factory.
  • Troubleshooting - Guiding you to possible causes of processing issues affecting either cost, rate or quality and directing you to the most appropriate development workflow to improve the process
  • Optimization - An expansion on the development workflows where a larger number of options are considered to achieve the best mixture of cost, rate & quality for your application.