Shape Development - P151.0

 

Shape Development
Practice document
Document Type	Practice
Document Identifier	151
Tags	Integrated product development Shape
Objective functions	Cost Maintain Rate Maintain Quality Maintain
MSTE workflow	Development
Prerequisites	Integrated Product Development Practice Practice for Developing a Product

Introduction

There are many geometrical considerations for composite part shape development. There have been many instances where a developer inexperienced with composites generated a product that was not manufacturable or extremely costly to manufacture. For example, when a part features vertical surfaces with negative draft angles or die-lock geometric features, it typically means that multi-piece tooling and post-processing may be required, both of which increases costs & complexity to ensure that the component is manufactured with the quality originally intended.

For a composites product to be manufacturing-friendly and cost-effective, there are many geometric considerations such as draft angles, size of radii, complex curvature, etc. to consider.

In some instances, an original metallic component is redesigned for composites without the adequate geometric considerations for a completely different manufacturing process. In these cases, a common misconception of composites is to consider them as “black metal”, and to follow a methodology that directly replaces the metallic component with the dark color carbon fibre reinforced plastic (CFRP). This is rarely the best case or optimized scenario and should be avoided. Geometries that are common and easily produced using metallics are often difficult, expensive, or even impossible to manufacture out of composites. Therefore, the misconception that a developer can simply replace a metallic component directly with composite laminates is a mistake that will yield a part without the desired results.

Scope

This section introduces several general shape considerations and the connection to manufacturability when developing a composite product. These considerations include corner radii, curvatures, draft angle, die lock, deep draw, ply build up and assembly. The items noted are considerations commonly encountered in composite development activities but do not represent an exhaustive list and are presented at a high level.

A composite product can generally be classified into a solid laminate, sandwich construction or a stiffened laminate. The applications of the shape considerations to those classifications will be discussed in this section and its subpages. The significance of individual considerations will be dependent on the specific part functional requirements, manufacturing process, and tooling strategy.

Significance

The development of composite structures' shape can heavily impact manufacturability. Decisions made during the development stage can impact factors such as manufacturing cost, repeatability, tooling complexity, and manufacturing defects. Decisions made with consideration to manufacturing limitations can reduce or prevent potential fabrication issues. This article highlights common considerations when developing composite structures. The applicability of specific development considerations are situation dependent and may not be suitable in all cases. Where possible, completion of subscale manufacturing trials concurrent with design of components can be used to validate manufacturability or identify potential areas of concern prior to finalization of a composite part (see Processing science).

Shape Considerations

Solid laminate vs Sandwich construction vs Stiffened laminate

The selection of a solid laminated, a sandwich construction or a stiffened laminate depends on the functional requirements, product shape, manufacturing process and cost. A solid laminate is a composite part built up of plies oriented in different directions to produce a laminate. A sandwich construction includes some form of core material between two thin laminates. Lastly, a stiffened laminate is a solid laminate that has a bonded spar or cross beam to improve stiffness, similar to an I-beam.

Functional requirements include weight, load requirements, energy absorption, insulation and EMI/RFI shielding etc. For example, sandwich construction and stiffened laminate are more efficient compared to solid laminate in the case of beam bending. Given a specified bending load and deflection, a sandwich panel or a stiffened laminate can meet the functional requirements while use less material than a solid laminate. For example, aircraft control surfaces have strict stiffness requirements and consequently are commonly made from sandwich constructions.

Product shape constraints can also impact whether to use a solid laminate, sandwich construction or a stiffened laminate to achieve the functional requirements. If a product is intricate and possess complex geometries or curvatures, it is usually difficult and expensive to utilize a sandwich construction because most core materials have limited conformability around curvatures (see Sandwich Panels for more details). Intricate products may also not have the space for stiffeners. For example, a CFRP bike frame is typically made from solid laminates.

It is typically most economical to utilize sandwich construction in flat or gently curved regions than in highly-curved regions. For example, the floor panels on an airplane which are essentially load bearing flat plates, are usually made from a sandwich construction. The use of rigid cores such as foam in complex curvature geometry would require the foam to be either moulded or machined to match the complex geometry, thus significantly increasing costs and manufacturing complexity.

From the processing perspective, a solid laminate is typically simpler to manufacture because the sandwich construction involves the deposition of adhesives and core materials whereas a stiffened laminated will either be co-cured, co-bonded or assembled mechanically. Extra tools, consumables and jigs might be required for stiffened laminates which can significantly increase cost. If a solid laminate is thick, the excess heat from the exothermic resin curing process must be considered (see Effect of material in a thermal management system for more details). Whereas for sandwich constructions, engineers are commonly concerned with heat transfer (see Heat transfer for more details) and whether cure is uniform for face sheets and at different locations of the laminate if there is ply buildup or drop off.

Radii and Sharp Corners

Composite materials are reliant on maintaining a specific ratio of fibre and resin across a part to have consistent mechanical properties. In components where continuous or long-fibre reinforcement are utilized, there are limitations on conformability of the materials. With very small radii (<1/8”) the reinforcement may not be able to conform to the part geometry, resulting in resin-rich areas or voids in the final component. The ability of a material to conform to tight radii is influenced by a number of factors including reinforcement type (carbon, glass, etc.), material orientation, fabric architecture, and pressure applied during the manufacturing process. Manufacturing processes utilizing high processing pressure and/or non-continuous reinforcement (i.e. SMC), can allow for smaller radii to be consistently produced successfully.

Transitions in geometry can create difficulties in part fabrication if the reinforcement is unable to conform to match it. A number of metallic components produced in a forming operation will include features such as corners with multiple faces converging. Many forms of composite reinforcement will not be able to easily conform to this type of geometry resulting in fibre misalignment, fibre breakage, or other manufacturing defects. Manufacture of components with corners may require ply splicing and additional time and care during the layup process. Smooth transitions in part geometry can simplify the manufacturing process and should be employed where possible.

A sharp corner or small radius on a composite moulded part presents manufacturing issues due to the limited geometry in these corners to allow for reinforcement material to conform into. In these instances where the reinforcement material is unable to properly conform, defects and issues such as the following may occur:

• Material bridging / Resin-rich region
• Air voids
• Stress concentration

These defects and issues may impact the aesthetic quality, weight, and mechanical properties of the end product. A properly developed corner should consist of a generous radius that is consistent on both the external and internal surfaces. For best results where the part thickness remains consistent along the radius, the external radius should always be the sum of the internal radius and the part wall thickness.

Complex Curvature

A composite moulded part with complex curvatures needs to be considered for its ease of manufacturability during the development phase in relation to conformability and/or flow of the material. There are varying degrees of conformability and drapability limitations when working with fabric fibre reinforcements, depending on the material. When the complex curvatures of the part are beyond these limits, manufacturing issues, complexity, and costs arise. The material may need to be spliced and darted at strategic locations in order to conform and drape over the complex surfaces. Where possible, gentle curvatures without abrupt changes should be used in composite structures. To reduce manufacturing risk, it is recommended that material samples are obtained from supplier early on in the development process to assess their ability to conform to specific part geometries.

Draft

Draft on a composite moulded part is the angle (typically measured in degrees) of a surface relative to the part demoulding direction. A negative draft angle indicates that the component cannot be demoulded and this is known as “die-lock”. Note that many manufacturers also consider a zero degree draft to be a “die-lock”. A sufficient draft allows a part to be removed from the mould easily after cure without causing unnecessary wear and damage on both the mould and the part surfaces. In most cases a minimum of two degrees of draft is recommended to avoid difficulties during demolding or requiring the tooling to be made in multiple pieces. Composite components with negative draft can be produced utilizing multi-piece tooling, or tooling systems using inflatable, dissolvable, or shape memory materials. In many cases this adds to cost and complexity of the tooling and manufacturing process.

Draft angle

Die-Lock / Undercuts

Most composite manufacturing processes are completed using a single tool where the part can be demoulded in a single direction. An undercut on a composite moulded part is a geometric feature such as an indentation, protrusion, or return flange that prevents the part from being able to be demoulded from the tool. Geometries with undercuts are considered die-locked and will result in parts that most often cannot be demolded in a single direction and will require the use of multi-piece, collapsible, or specialized tooling systems that can add to cost and complexity and may not be suitable for some manufacturing processes. Alternatively, components can be manufactured as multiple sub-components with die-locked features added during the assembly process. An example of this is bonding a return-flange onto a component as a secondary operation. Where possible, components should be developed to avoid die-locked conditions to simplify tooling and part manufacture. Although it is not always possible to eliminate an undercut feature, it is important to consider the increases in upfront mould costs, mould complexity, and mould maintenance in addition to the increase in part costs. If considered early in the development process, there is a higher likelihood of reducing or eliminating undercut geometries.

Die-lock/undercut

Deep Draws

A deep draw for a composite moulded part is a deep vertical surface relative to the demould direction and the parting line. Depending on the composite manufacturing process, a deep draw geometry may create issues such as:

• inaccessibility / reachability
• resin-pooling at bottom of vertical surface
• inaccurate material positioning on vertical surface
• lack of clearance for machining for mould manufacture
• lack of accessibility for material deposition equipment

One option for alleviating the effects of deep draws is to increase the draft angles of the vertical surfaces. The mould can also be developed such that the part orientation and demoulding direction reduces the vertical effects of a deep draw.

Deep draw

Ply Buildups and Transitions

Regions in composite structures are often locally reinforced by adding additional plies to select locations. To be able to complete this accurately, personnel performing material deposition must be able to quickly and accurately locate where plies will be positioned. When developing a component with local reinforcement, consideration should be given to how ply locations will be identified during the layup process. When plies are located near a feature on the part or tool that can easily be referenced, this process may be relatively easy. If the plies are not near a feature that is easy to reference, this can prove to be a difficult and time-consuming task. The use of equipment such as laser or similar projection systems are often used to assist with ply positioning. When such equipment is not available, other forms of ply locating aids such as mylar templates may be used. For most manufacturing processes, composite components are produced with excess material past the trim line and trimmed to net shape so the part edges cannot easily be used as a reference featuring during layup. The benefits of using local reinforcement rather than additional continuous plies over the entire component should be weighed against the additional labour time and potential for error of adding material locally. In some cases, reinforcing patches are bonded onto the part exterior as a secondary operation rather than included in the original component layup.

Assembly Considerations

Interface and Clearance

A successful composite product always requires detailed considerations for assembly interfaces and clearances. Failure to consider these may lead to assemblies that do not fit, fit poorly, or simply cannot be installed/assembled due to lack of clearance. Interfaces with hardware typically need to be flat, smooth, and rigid. Interface development should always be completed with CAD model assemblies of all interfacing components or representative mock-ups interface surfaces in CAD environment. Actual estimated part thicknesses should be used in the CAD models and based on reliable information on the specific material and manufacturing process used. It is recommended that installation tools & equipment CAD models are also incorporated into the assembly CAD model to check for access clearance.

During the composite manufacturing process, thermal expansion/shrinkage, resin shrinkage during cure, and tool/part interaction during the manufacturing process can result in internal stresses and final part geometry that does not match the geometry of the tool. The discrepancy can be increased by additional processing such as post-curing components in an un-supported state. In some cases, utilizing a balanced and symmetric laminate design can help mitigate the potential for warpage during processing. In some situations, this may not be sufficient to meet final geometric requirements and analytical tools can be utilized to predict process induced warpage and mitigate issues through modifications to the part shape, tooling geometry, or manufacturing process parameters. See Residual stress and dimensional control management (RSDM) for more information.

B-Side Surfaces

The B-side surface of a composite moulded part may or may not be part of a moulded tool surface depending on the manufacturing process. In both cases, considerations for this surface should be secondary to the A-side (toolside) surface but should never be ignored. The B-side surfaces are often used for interfacing with attachments, brackets, and other installation hardware. For areas where there is hardware interfacing, the B-side surface should be flat and smooth. Manufacturing processes that do not utilize a tool to control the B-side surface may exhibit considerable surface finish variability between the A-side and B-side of the finished component. As a result, part thickness might also be not well controlled. For composite moulded parts within an assembly, careful considerations must be given to the part thickness and the allowable clearance between mating parts in the assembly. Where possible, part thickness used when developing a component or assembly should be validated using similar materials and processing conditions to the anticipated manufacturing process. Incorrect assumptions on final part thickness and/or B-side geometry can result in issues during the assembly process, leading to cost increases or re-design of components.

Explore this area further

• Shape Development - P151.0
  • Sandwich Panels - A325

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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.

Carbon fibres are composed of large aromatic sheets similar to those in graphite. These graphitic layers form the basic structural units in the shape of ribbons. The structure of carbon fibre ribbon is believed to be a columnar arrangement of disoriented graphite crystallites parallel to the ribbon length. The idealized tetragonal crystallites are stacked above one another, with slight disorientation between the crystals in the direction of fibre axis, trapping sharp needle like voids, where the boundaries between the stacks represent the disordered regions.

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.

With regards to manufacturing, risk is the combination of the probability and consequences of undesirable manufacturing outcomes. Manufacturing risk can lead to technical issues, program/schedule delays, and cost overruns.