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Manufacturing report on Car Bonnet
Anuj Kamble (S0555276)
AM-2,MBA&E
WS2015

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 Specification: Bonnet is a part of subsystem of a car “closures”, which also contains
doors and tail gate. It is fundamentally a reinforced skin panel with many safety and
quality requirements. Bonnet usually do not directly open onto the passenger cell which
greatly reduces the importance of certain functional requirements normally associated
with closures such as passenger safety, air tightness, low cycle fatigue strength.
 Application: Bonnet is hinged to the cowl part. It is surrounded by weather strips along
its periphery resting on fender. Bonnet is an access panel to the engine compartment to
enable maintenance of power train, drive belts, battery, fluid levels and lamp units.
 Features:
 Pedestrian Safety
 Aerodynamic improvement
 Styling
 Noise reduction
Product Definition

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Bonnet Assembly and child parts:
1. Inner Panel
2. Outer Skin panel
3. Hinge reinforcement –LH
4. Hinge reinforcement-RH
5. Lock reinforcement
6. Acoustic padding
7. Gas spring
Product Specification
1
2
4
3
5

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Functions of child parts of Bonnet assembly:
 Inner Panel: Provides stiffness to bonnet acts as a supporting frame to bonnet assembly.
 Outer Skin panel: Provides covering to inner panel assembly, improves visual quality
 Hinge reinforcement –LH/RH: Support hinges for bonnet lifting mechanisms
 Lock reinforcement: Support locking mechanism
 Acoustic padding: Provides noise reduction
 Gas spring: Provides ease in lifting of bonnet

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Design Aspects : Following aspects must be considered while designing of the bonnet.
 Pedestrian Safety: It is a non technical requirement but now a days it is one of the most
important legal requirement in many countries. According to vehicle safety norms while
considering Pedestrian Safety following points must be considered:
 Child Head & hip impact Criterion
 Adult Head & hip impact Criterion
 Bonnet leading edge geometry
 Space must be created between the bonnet and engine components underneath in order
to create a safe bonnet that will absorb some of the energy in a controlled manner to
prevent injury.
Technical & Non Technical Requirements
Source:‘The Aluminium automotive manual- The
Design case study on bonnet & boot lid ’Version
2011 © European Aluminium Association
(auto@eaa.be)

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 Vehicle Quality:
 Torsional Stiffness
 Flutter/ Bending Resistant
 Dent Resistant
 Corrosion Resistant
 Low mass ( For easy opening , Low mass contribution in total Vehicle mass)
 Engine Noise Reduction
 Low maintenance
 Aerodynamic outside panel
 Effective inside engine cooling.
Technical & Non Technical Requirements
Source:‘The Aluminium automotive manual- The
Design case study on bonnet & boot lid ’Version
2011 © European Aluminium Association
(auto@eaa.be)

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 Styling:
 Tight radii on hemmed panel edges
 Excellent painted surface quality
 Stretch flanges
 Good Shape ( Low strain doubly curved surfaces)
 Manufacturing:
 Inner Panel: Excellent Weldability , Galling Resistant , Paint drain feature
 Outer Panel:
 Dent resistance during assembly
 Clean Sheared edges
 No visual defects resulting from stretch forming.
Technical & Non Technical Requirements
Source:‘The Aluminium automotive manual- The
Design case study on bonnet & boot lid ’Version
2011 © European Aluminium Association
(auto@eaa.be)

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Desired characteristics and manufacturing techniques:
Material Characteristics:
Corrosion resistant
Elastic Limit
Total Elongation
Surface Hardness
Stretch-Strain defects
N-Value
R-Value
Surface Quality pot forming
Surface roughness
Pint bake response
Hem flange capabilities
Fatigue Resistance
Low density
Lubrication Coefficient
Shape Characteristics:
Swages
Section Collapse initiation
Feature
Stretched outer skin panel
Drawn inner panel
Flanged holes
Flat hem flanged front edged
skin panel
Rope hem flanged upper edged
skin panel
Flanged panel edges.
Manufacturing:
Tooling cost
Stamping
Blanking & Punching
Spot welding
Self pierce rivets
Adhesive bonding
Finishing
Anti corrosion coating
Painting
Cycle time
Source:‘The Aluminium automotive manual- The
Design case study on bonnet & boot lid ’Version
2011 © European Aluminium Association
(auto@eaa.be)

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Material Selection criteria for Bonnet Components:
 Stiffness of Bonnet due to inner panel:
• E and Yield strength requirements for global vehicle stiffness. Global vehicle torsional and
bending stiffness is decoupled from the bonnet system.
• Bonnet stiffness is obtained through section properties and material thickness. Vibration
stiffness is obtained from careful design of bonnet inner panel and anti flutter mastic
connecting it to the outer surface. Mastic properties and bead size are chosen so as to
avoid print through or witness marks on the visible painted surface. The resulting stiffness
is designed to be high enough to avoid visible flutter resulting from aerodynamic effects.
Materials Selection:
Source:‘The Aluminium automotive manual- The
Design case study on bonnet & boot lid ’Version
2011 © European Aluminium Association
(auto@eaa.be)

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 Dent Resistance for Outer skin Panel of Bonnet :
 Dent resistance is split into two categories; quasi-static and dynamic.
 Quasi-static (very low speed) dent resistance is necessary to avoid plastic deformation
from handling operations in the assembly plant and from palm or finger tip pressure during
normal opening and closing operations.
 Dynamic dent resistance is necessary to limit or eliminate local dents from high speed
impacts of free road surface stone chips and other flying objects.
• Quasi-static (QS) dent resistance is the more important of the two because it directly
influences manufacturing costs and scrap rates of skin panels.
Source:‘The Aluminium automotive manual- The
Design case study on bonnet & boot lid ’Version
2011 © European Aluminium Association
(auto@eaa.be)

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Source:‘The Aluminium automotive manual- The
Design case study on bonnet & boot lid ’Version
2011 © European Aluminium Association
(auto@eaa.be)

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 From the Prioritization matrix there are following materials which can be selected for
the bonnet.
 For inner panel: AL 6XXX , Laminated 5/6/7XXX Sheet steel, High performance
Plastics/composites
 For outer panel: AL 6XXX , Laminated 6/7XXX Sheet steel
 For my bonnet , I am selecting AL 6000 material.
 For inner panel; sheet thickness should be 0.7-0.8 mm
 For outer panel; sheet thickness should be 0.5-0.6 mm

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 To make 100,000 car bonnets per year; I have to adapt automation process.
Automation in Bonnet manufacturing can be achieved by using Industrial robots for
process like Spot welding, Roller Hemming Process and bonnet painting process.
 For Bonnet assembly manufacturing following process will be automated:
 Spot welding
 Hemming
 Painting
 Following processes will be manual:
 Bonnet fitment to car body
 Acoustic padding & Lock fitment to bonnet
 BIW quality checks, Welding quality tests , Paint quality test, Corrosion tests
 Final product Inspection
Manufacturing process for Bonnet:

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Assembly plan /Manufacturing diagram for Bonnet:
Inner panel
assembly of
bonnet
Robotic
Spot
Welding
Outer skin
panel of
bonnet
O/P from press shop with
stretch drawing Stamping,
Flanging processes
Robotic
Roller
Hemming
Bonnet
assembly
Bonnet
Fitment to
BIW
Robotic spray
Painting of
whole car body.
Anti
Corrosion
coating (KTL)
Assembly
shop
Acoustic
padding for
noise
reduction+
Lock fitment
Final Bonnet
Assembly
Welding
Quality
check
Gap &
Flushness test
Paint
quality
check.
Pedestrian
Head/Hip
impact test.

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 Robotic Spot welding cell: In this robotic welding cell inner panel of bonnet assembly
will be spot welded. This robotic welding cell contains Industrial Robot, Spot welding gun,
Robot controller, Welding controller, Welding fixture, Safety fences, Light curtain (sensors
to sense operator presence), Child parts storage bins , Final inner panel assembly storage
trolley. Output of this spot welding cell will be inner panel assembly of bonnet which will
then be input to the hemming cell.
 Robotic roller hemming cell: There are several automation processes for hemming are
available like die-press hemming, table top hemming but for bonnet hemming I am
selecting robotic roller hemming process to have flexibility in automation so that I can
hem different variants of bonnet by just changing hemming bed and robot program.
Hemming cell contains Industrial robot, Robot controller, Hemming bed, Safety fences,
Light curtain, Child parts storage bins , Final Bonnet assembly storage trolley. Output of
this hemming cell will be bonnet assembly. Which will be fitted to the car body at fitment
station.
Manufacturing process for Bonnet:

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 Bonnet fitment station: At this station Bonnet is fitted to the car body. Bonnet assembly is
lifted with the help of pneumatic jigs and then fitted to the car body manually. Bonnet is
hinged to the car body and gas springs are provided for easy opening of the bonnets. Once
bonnet is fitted to the car body gaps and flushness are checked at all the areas with gauges.
 Painting station: Anti corrosion coating is applied to bonnet by KTL process. After KTL
process color of bonnet changes to whitish brown. At this station 3 layers of paint is applied to
the bonnet. Robotic spray painting process is used to have uniform thickness of paint at all
over the bonnet. With robotic spray painting less paint is wasted and cycle time is also
optimized. Different colors of paint can be applied to bonnet with this setup. Paint quality and
paint thickness are checked after painting.
 Assembly station: After painting quality checks acoustic padding and lock is attached to the
bonnet assembly.
Manufacturing process for Bonnet:

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 Spot welding Process: Spot welding is a process in which contacting metal surfaces are
joined by the heat obtained from resistance to electric current. Work-pieces are held
together under pressure exerted by electrodes. The process uses two shaped copper alloy
electrodes to concentrate welding current into a small "spot" and to simultaneously clamp
the sheets together.
Manufacturing process for inner panel:
Source:http://www.siliconcontrols.ca/How-Spot-
Welding-Works.jpg

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Manufacturing process for Bonnet inner panel:
 Bonnet inner panel assembly is mainly comprised of stamped child parts such as inner
frame, Hinge reinforcement, Lock reinforcement. All these child parts are joined together by
spot welding process. Welding fixtures are used where operator loads the inner panel and
then all respective child parts on the fixture to their desired location.
 While deciding location of spots on the panel I have considered that geometry of all the child
parts are fixed on inner panel. The distance between two spots is not less than 30mm to
have better spot quality.
 The welding fixture is designed considering ergonomic conditions of operator, ease in
accessibility for welding gun ,Fixing Geometry and satisfying all geometric and design
tolerances for inner panel assembly.
 While deciding spot welding sequence cycle time optimization and full robot utilization is
considered. While designing layout for spot welding robotic cell operator safety is considered.

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Spot plan for welding of inner panel:
FIXTURE
11
2
56
47
8 3
9
10

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Schematic Layout for robotic spot welding cell:
Robot X-Gun
Bonnet
inner panel
Welding
controller
Robot
Controller
Operator
Safety
Fence
Welding
Fixture
Laser
curtain
Storage area for
Child part bins
Storageareafor
finishedinner
paneltrolley

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 Hemming process: A hem is a bent edge of a
metal sheet. Hemming is a process by which
that edge is bent. In an outer part (orange) is
bent around an inner part (black) by hemming.
It gives a neat and a compact joint.
 Bonnet assembly is manufactured by hemming
process. Inner Bonnet panel assembly and outer
bonnet panel are hemmed together by robotic
roller hemming process.
 Rope hemming is done on the windshield side
area of the bonnet. And all other sides are flat
hemmed.
(a) Rope Hemming
(b) Flat Hemming
Manufacturing Process Bonnet Assembly:
Image Source: Jonkers, B , “Simulation of the Robot
Roller Hemming process.pdf”, November 2006

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Robotic roller hemming process for Bonnet
Assembly:
 Adhesive sealant is applied on inner panel of the bonnet. This process is done manually.
Operator follows the sealant plan and applies sealant on inner panel. Sealant acts as an
adhesive and reduces relative motion between inner and outer panel of bonnet.
Sometimes sealant also used to reduce vibrations in bonnet.
 After application of sealant inner panel assembly is loaded on outer panel.
 A robot guides a roller parallel along the flange of outer panel. Complete bonnet is
hemmed with same roller.
 In between the hemming steps the orientation (angle) of the roller is changed as shown
in previous figure.
 The hemming bed is designed considering ergonomic conditions of operator, ease in
accessibility for roller, Fixing Geometry and satisfying all geometric and design tolerances
to achieve good hemming quality.
 Robot programming is done according two 3 hemming steps and robot path is optimized
to achieve cycle time. While designing layout for robotic hemming cell operator safety
and material storage areas are also considered.

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Schematic Layout for robotic roller hemming cell:
Robot
Hemming
Roller
Bonnet
Assembly
Robot
Controller
Operator
Safety
Fence
Hemming
Bed
Laser
curtain
Storage space for
inner and outer
panel trollies
Storagespacefor
bonnetassembly
trolly

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 Welding Quality/ Strength Test: Destructive welding quality test: ISO 14324:2003
specifies test procedures for fatigue testing spot welds, at ambient conditions, under
repeated tensile loading to produce either shear or cross-tension loading of the spot weld, in
steel of sheet thicknesses of 0,5 mm to 6 mm.
Product inspection:
Image Source: Google images

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 Ultra sonic testing ( Non destructive test):If the weld is improperly formed, it is
possible that the two parts may not fuse completely, or that the area of the nugget may
be smaller than what is required for a strong joint. With proper equipment and technique,
ultrasonic spot weld inspection can quickly and nondestructively provide valuable
information about weld quality.
 To check spot welding quality of bonnet inner panel I will use destructive as well as
ultrasonic testing because spot quality problems are seldom visible to optical inspection,
and destructive pull tests are inefficient and limited to a few samples.
Source:http://www.ndt.net/article/dgzfp03/paper
s/p01/p01.htm

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 Visual inspections:
 After Hemming process: Visual inspections are carried out to detect hemming waviness
and dimensional qualities after hemming process.
 Gap & flushness test: After attaching bonnet to the car body gaps and flushness are
checked all over the area. This is done by inserting standard gauges in the gaps between
bonnet and car body.
 Visual inspections are also carried to detect dents on bonnet surface and for other
manufacturing defects .
 Corrosion Tests*: Corrosion tests are carried out on bonnet to test effects of different
atmospheric conditions on bonnet. This can be conducted in controlled environment.
Bonnet paint quality is tested under different atmospheric conditions and then probable
corrosion affected areas are determined.
*Source:http://www.astm.org/Standards/D1654.
htm

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 Measuring thickness of paint: Thickness of paint is checked all over the bonnet to
check uniformity of the paint. Thickness of paint is checked with a portable instrument
and probe combine the magnetic induction method and phase –sensitive Eddy current
method.
Source:http://www.qualitymag.com/articles/910
06-measure-automotive-coating-thickness

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 Pedestrian Hip/head impact test: Evaluation of pedestrian safety for a bonnet must be
carried out in the context of its surrounding elements:
 Vehicle styling, size (wrap around distance “WAD”) and under bonnet clearance to other
elements (considered as hard points).
 Local bonnet stiffness is influenced by mounting point stiffness such as hinges, bump
stops and latches.
 This is better understood by superimposing the kinematics of a dummy onto the test
conditions.
Bonnet
Head impact area
Hip impact area
Wind shield
Source:‘The Aluminium automotive manual- The
Design case study on bonnet & boot lid ’Version
2011 © European Aluminium Association
(auto@eaa.be)

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• Thum ,Manufacturing_Project_AM2_V1.pdf
• ‘The Aluminium automotive manual- The Design case study on bonnet & boot
lid ’Version 2011 © European Aluminium Association (auto@eaa.be)
• A. AMBROZIAK, M. KORZENIOWSKI, ‘Using resistance spot welding for joining
aluminum elements in automotive industry’ , Wrocław University of Technology,
Łukasiewicza 5, 50-371 Wrocław, Poland
• Rsw_of_aluminium_V4.pdf ( Developments towards high-volume resistance spot
welding of aluminium automotive sheet component , International Automotive
Research Centre , Warwick Manufacturing Group, the University Of Warwick, CV4 7AL
UK
• Jonkers, B , “Simulation of the Robot Roller Hemming process.pdf”, November 2006
• http://www.iso.org/iso/catalogue_detail.htm?csnumber=24088
• http://www.ndt.net/article/dgzfp03/papers/p01/p01.htm
• http://www.astm.org/Standards/D1654.htm
• http://www.kuka-systems.com/de/technologies/
• http://www.arotechnologies.com/en-GB
• http://www.qualitymag.com/articles/91006-measure-automotive-coating-thickness
References:

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Thank you!!!