The Quest for Lightness
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THE MAGAZINE DEVOTED TO NICKEL AND ITS APPLICATIONS |
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| BOEING'S NEW 777 aircraft is designed with several major components made of lightweight,
non-metallic composite materials. |
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| NON-METALLIC COMPOSITE components can be created in electroformed nickel tools, such as this. The
consistent thickness of electroformed molds enables the molds to heat evenly and quickly in an
autoclave. |
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| THE IDEAL SHAPE for an electroformed nickel tool is a J-shape, such as this helicopter engine
cover. |
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| ELECTROFORMED NICKEL tool-making techniques are ideal for creating shapes such as this BD100 tail
cone. |
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| SIDE VIEW OF the BD100 tail cone tool which is also pictured above. |
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| A CO-ORDINATE MEASURING machine is used to compare the surface contours (in this case, a C17 flap track
fairing tool) with the design values to give finished parts the proper dimensions. |
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| A CLOSE-UP of the CMM machine also pictured above. |
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PDF of this article (460 kB) |
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Read another Nickel Magazine article on the use of nickel alloys in the
making of composite aircraft components here. |
Electroformed nickel molds are helping Airbus and Boeing create more fuel-efficient aircraft. By
Carroll McCormick
Nickel magazine, March, 2004 -- Airplane manufacturers on both sides of the Atlantic
are increasing the use of non-metallic composites, rather than alluminum alloys, to reduce the weight and
therefore increase the fuel efficiency of their commercial aircraft. While this may be bad news for the
aluminum industry, it is good news for the nickel industry.
The reason is that the molds (or "tools," as they are sometimes called) that are used to make the major carbon-fibre and other composite components are mostly made of either K93600 (or Invar [TM]) or a thin layer of pure nickel deposited on a metal substrate in a process known as "electroforming." Each has its own unique application.
For example, Ex-Press Plastics (Process Equipment) Limited, in Norfolk, England, makes nickel electroformed tools that are used to fabricate the landing-gear doors of the Airbus A380. The company has also made an electroformed mold, 12.5 metres long, for the tail elevator of Boeing's 777. The company uses about eight tonnes of nickel per year.
Invar (which is 36% nickel and 64% iron) has a near-zero co-efficient of thermal expansion and is typically used to build tools from welded sheets or tooled blocks (for details, see the February 2003 issue of Nickel Magazine). However, electroformed nickel tools are created by depositing nickel on plastic "bathmasters," which are epoxy forms corresponding to the size and contours of an aircraft part.
Electroformed shell tools are lightweight and their consistent thickness enables them to heat evenly and quickly in an autoclave. "If you are running an autoclave at, say £60 an hour, it is important how long it takes to heat up the tool,"explains Michael Shead, technical sales manager with Ex-Press Plastics. "You get more throughput in one of your more expensive resources. The [production] bottleneck is usually the autoclave. When you are busy, your autoclave can be running twenty-four hours a day, and your lay up department is not,"
In a process familiar to people in the electroplating business, the surface of a bathmaster is sprayed with a layer of silver nitrate just a few microns thick, then lowered in a nickel sulphamate solution. The silver in the silver nitrate acts as an initial conductive layer for the nickel to be deposited on. "Then the nickel itself becomes a conductor and you can increase the current loading on the bath mandrel," says Shead.
The finished thickness of the nickel is a normally five millimetres, though, says Shead, "we aim for a deposit of four to six millimetres." Any thicker and the tool takes too long to heat up in the autoclave. Moreover, at these thicknesses, the surface of the tool can be adjusted with a co-ordinate measure machine (CMM) to give the finished parts the proper dimensions.
Normally, a nickel electroformed tool will last for as long as a manufacturer needs to make components. Therefore, if production rates are high, more than one tool is needed. In the case of the Airbus landing gear doors, four tools were ordered.
Shead says tool-making eliminates the vacuum leaks sometimes found in welded-up tools. Moreover, the tool surface is easier to adjust if components exhibit unacceptable "spring back" when removed from a tool.
"With Invar, if the component has spring-back, you can sometimes accept that, or you may have to re-machine the tool," Shead explains. "With electroformed nickel, you can adjust it to compensate for the spring-back of the carbon component. The shell is attached to a steel structure by studs that you can adjust to pull or push the nickel shell into position. We use a CMM five-axis head that feels its way around on the surface of the tool and compares its measurements with the CAD data." Technicians then adjust the studs to fine-tune the shell shape.
Designers use a compensation factor in the CAD file to offset differences in the co-efficient of thermal
expansion between the shell tool and the composite material.
Invar finds application in deep-drawn tools, including, for example, a U-shaped box with almost 90[degree]
corners. "That would be the worst case for a electroformed tool," says Shead. "The tool would crush the part
as it cools. An ideal shape for an electroformed tool is a J-shape, which has plenty of draft angle on the
side and big radius on the bottom. The leading edge of wings and elevators and engine nacelles are good
examples. The minimum radius (you can achieve) is the thickness of the shell; smaller, internal radii will
reduce the shell thickness."
A shape such as an engine nacelle would challenge an Invar mold maker, however, electroformed shell tools are well-suited for components with 3-D curves.
Carroll McCormick is a Montreal-based freelance writer.
PHOTOS: Ex-Press Plastics Limited
 Mike Shead |
