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Cost Advantages of the Adhesive Bonding Technique - Sikaflex



Introduction

In most cases, adhesive bonding is more economical and less labour intensive than conventional fastening methods. It is also cleaner and uses less energy. Manufacturing costs are directly proportional to the number of components to be fastened together. Adhesive bonding helps to reduce the number of individual components and encourages a modular approach to design and construction. Major assemblies such as complete deck elements can be pre-assembled with their interior linings already in place. A case in point would be the construction of a GRP leisure boat which had its Deck-to-Hull bonded using flexibilized adhesive. The thick-layer adhesive method is particularly effective in keeping down production costs and this method eliminates the high mechanical stresses that results when a deck is fixed using screws.

Also, by simply increasing the thickness of the adhesive layer, larger manufacturing tolerances can easily be accommodated.

The boom in transportation has led to the widespread use of adhesive bonding as a joining technique. This trend has been accelerated by the increased demand to reduce costs. The shipping companies want vessels that are inexpensive, both to purchase and to operate. So coupled with the low purchase price, there is immense pressure to reduce the weight of vessels. The desire for weight optimization leads to a material mix of steel, aluminum and reinforced plastic parts. This design approach requires the joining of different materials in various joining zones and here adhesive bonding shows enormous advantages, compared to welding, screwing, riveting or bolting.

The fact that adhesive-bonded joints are less prone to corrosion is an added bonus in helping to keep operating costs down. In addition, where dynamic forces are applied to the joint, the layer of elastic adhesive also acts as a sealant, preventing ingress of air, water, saltwater, dust or other corrosive media.

Application Example 1 - Fitting a Railing Segment

In order to keep weight to a minimum, a railing segment is made from aluminum tubes in the form of a lattice. The ends of the tubes must be fixed to the steel structure of the ship. In the example shown here, each railing segment has 15 connection points onto the steel structure of the ship (5 pillars and both ends of the 5 crossways).

Using traditional methods, welding aluminum to steel requires the use of specially made bimetal plates that are costly and difficult to manufacture. First of all, the two metals must be bonded together using compression rolling and electrolysis. Then the edges of the aluminum need to be milled away to expose the steel, enabling the plates to be welded to the steel deck. The aluminium tubes are then welded onto the aluminum side of the plates. The complete process must be carried out with care to ensure a quality finish.

Using the alternative bonding process with Sika adhesives, the aluminium of the railing segment is equipped with aluminium plates that are to be bonded onto the steel deck. Surface preparation is necessary before bonding.

The strength of the bonded fixing was tested in the European Research Project 'Bondship'. This showed that a fracture appeared in the weld of the aluminum tube, whereas the adhesive bond remained in sound condition.



The fracture occurs on the aluminum weld; the Sika adhesive holds true (Left)

(Right) The fracture occurs on the aluminum weld; the Sika adhesive holds true


Table 8 shows the cost comparison between bonding and welding methods. The cost for the traditional welding method is three times higher than for bonding and, providing the bond area is adequate, the durability is sufficient for most applications.

Table 8. Cost Comparison of Attaching a Railing Segment to a Vessel using Two Methods Methods
Bonding with Sikaflex or SikaFast Material Costs Cost
$ (US)
Primer and adhesive bond area 1380 cm2, thickness 1 cm) 57
Working Time Time Taken
(minutes)
Cost
$ (US)
Cleaning and ventilating 20 11
Grinding bond face 30 17
Cleaning and ventilating 20 11
Primer application and ventilating 30 17
Adhesive application 30 17
Positioning and bonding 60 33
Overall Time Taken 190 106
Total 163

Cost saving approximately 200%

Welding Material Costs Cost
$ (US)
14 bimetal plates with milled edges 139
Weld material for 5.24 m 49
Workign Time Time Taken (minutes) Cost
$ (US)
Positioning, adaptation and fixing 300 165
Welding the crossways 150 83
Welding the pillars 90 50
Total 486

All amounts are based on a labour cost of $33 per hour

Application Example 2 - Installation of a Gutterway on a Deck

Section through a gutterway

Various types of gutterways are needed on the decks to drain away various liquids. The special challenge for bonding is the indeterminate mixture of chemicals that a bonded joint is exposed to under service conditions. The joint of the gutterway must be tight all the way around. The length of the gutterway is 900 mm and the width 230 mm, so the overall joint is 2,260 mm2. The comparison in table 9 shows that bonding costs are less than half of the welded solution.

(Left) Adhesive is applied to the frame
(Right) The gutterway is placed in position

Table 9. Cost Comparison of Fitting a Gutterway on Deck using Two Methods


Bonding with Sikaflex

Material Cost
Cost
$ (US)
Primer and adhesive 2 x length 223 cm, width 2 cm, thickness 1 cm 45
Working Time Time Taken (minutes) Cost
$ (US)
Cleaning and ventilating 10 6
Grinding bond face 20 11
Cleaning and ventilating 10 6
Primer application and ventilating 30 17
Adhesive application 30 17
Positioning and bonding 30 17
Overall Time Taken 130 74
Total 119

Figures are based on a labour cost per hour of $33
Welding

Material Cost
Cost
$ (US)
Weld material for 2 x 2,23 m 128
Working Time Time Taken (minutes) Cost
$ (US)
Positioning, adaptation and fixing 120 66
Welding 120 66
Overall Time Taken 240 132
Total 260

Cost saving approximately 120%



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