C OVER STORY

A ssembly and Joining Technolo gy

Injection Adhesives for
Vehicle Body Construction
New lightweight concepts for reducing CO2 emissions from vehicles and the trend toward electricvehicles are two global trends that call for new vehicle designs. The use of aluminum profiles andcastings is particularly increasing in these areas. Castings are often used as so called nodes forjoining profiles. In particular, the bonding of these components by means of adhesives presents
new technical challenges. Sika has now developed a new adhesive solution for this purpose.

10AUTHORS

Michael Gutgsell
is Department Manager Researchand Development Body-in-White
Adhesives at Sika Technology AGin Zurich (Switzerland).

Noah Munzinger
is Department Manager Researchand Development Body-in-White
Foams at Sika Technology AGin Widen (Switzerland).

© SikaTECHNICAL CHALLENGE

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In order to further reduce the CO2 emis­
sions from vehicles, lightweight designs
are an issue that automobile manufac­
turers approach with different concepts.
The most important one is certainly themixed material concept. This involves
the optimal combination of different
materials such as steel, aluminum andplastics according to their properties
and using them in the same car body.
The different thermal expansion coeffi­
cients of the individual materials canlead to problems during cooling and,in extreme cases, to failure of the bond.
In order to solve this problem, Sika devel­
oped the SikaPower MBX technology
some time ago. Not only the use of differ­ent materials can cause problems withlightweight structures, but structural
hurdles also have to be overcome.

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C OVER STORY

A ssembly and Joining Technolo gy–SikaReinforcer

High Strength Bonding

SikaPower SmartFlow

Concept

Structural part withintegrated adhesive
foam

Structural part glued inwith adhesive

Structural glued in with
injectable adhesive(Crash) resistance+++++

Process+++++TABLE 1 Comparison of concepts (© Sika)

FIGURE 1 Bonding ofprofiles with castings orother profiles (© Sika)

An important application in this context
is the bonding of cast aluminum nodes
with aluminum profiles. In skeletal bod­ies, better known as space frame struc­tures, profiles are connected via nodes,
FIGURE 1.
As with push-in connectors, the bond­ing of joints such as these is usually noteasy to solve. Due to the low gap dimen­
sions, most of the adhesive previously
applied to the profile or casting is removed
during bonding. As a result, these types
of joints are now largely mechanicallyjoined, such as by riveting. Modern elec­
tric vehicle architectures have a particu­
larly high proportion of profiles, for exam­ple in battery frame structures. This ispartly due to the fact that the size of thebattery restricts the structural space andthat it tends to require use in a straightline. The more intensive use of profiles
in car body construction has increased

the need for a suitable adhesive solution.
Sika has developed the SikaPower Smart­
Flow injection adhesive for bonding suchpush-in connectors.
Another area of application con­cerns the bonding of plastic reinforce­
ment parts into the car body, FIGURE 2.
Glass fiber-reinforced polyamide parts,
known under the trade name of Sika­
Reinforcer, are generally used for thispurpose. There are several concepts forjoining these structural parts to the carbody. The structural parts can be hooked
into the body. During hot curing theadhesive foam expands to the metal
and thus creates a bond. Alternatively,
such structural parts may also be glued
directly into the cavity with a structuraladhesive. At Sika, this process is referred
to as High Strength Bonding (HSB). This
procedure offers various advantages
from a mechanical perspective, TABLE 1.
In contrast to foam (SikaReinforcer),bonding the structural part with anadhesive (HSB) leads to a stiffer andmore (crash) resistant structure. Depend­ing on the process, however, manufac­
turing challenges may arise. In particu­
lar, the reliability of the nesting process
for individual parts cannot always beensured. The SikaPower SmartFlow

injection adhesive provides a remedy forthis. In this case, the bonding adhesive
can be partially or fully injected directly
into a metal structure which has alreadybeen joined. This method is particularly
suitable for reinforcing A and B pillars
and for reinforcing battery frame struc­tures in vehicles. For this application,
the stiffening part can be designed sothat the adhesive flows from the injec­tion point to all four sides and thus
ensures optimum bonding of the struc­tural part to the metal structure.
INTELLIGENT INJECTION ADHESIVE

Either high temperatures and/or highpressures are required to inject stan­
dard body adhesives into a cavity. Inaddition, it is necessary to provide guid­ance of where the adhesive is to flow.
This is usually achieved when the cav­
ity is closed and only needs to be filled.
However, targeted injection becomes
particularly difficult when the cavity
is not closed. The assemblies describedat the beginning are usually not form-fit
or sealed, which means they are open
on at least one side. When an adhesiveis injected into a bond that is open on
the side or not form-fit on the side, theFIGURE 2 Reinforcement of cavities by bonding structural parts (left: general view, right: detailed view) (© Sika)

12Adhesive injection

FIGURE 3 Injection and distributionof SikaPower SmartFlow (© Sika)

Adhesive flows
into the channel provided

adhesive spreads not only in the desired
direction but in all directions. The spec­
ification of a channel in a component
provides very little help. In comparison,
the SikaPower SmartFlow adhesive onlyspreads in the spe­c ified channel, even if
the channel is open at the side, FIGURE 3and FIGURE 4. This is a property which
proves to be particularly advantageous

Adhesive does not flow
into the adjacent unsealed area (shoulder)in the case of the tolerances prevailingin bodywork. To simplify matters, theinjection cavity in this report is referred
to as the channel, and the lateral sur­face, which does not have a form-fit
bond, is referred to as the shoulder,
FIGURE 3. The height of the shoulder
indicates the distance between the twojoined parts in the area of the shoulder.

The new bonding technology has
been tested and validated on various
laboratory models. In all illustrations
shown in this article, the channel depth
is 3 mm, and the shoulder height is
1 mm. Although the difference is quite
small, the adhesive flows only in thechannel and not in the shoulder. Studies
have shown that the ratio of channel

FIGURE 4 Adhesive distribution with SikaPowerStandard (left) and SikaPower SmartFlow (right)(© Sika)
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A ssembly and Joining Technolo gy

FIGURE 5 Injection into various channel widths (© Sika)

FIGURE 6
Flow behavior withlarge channels withSikaPower Standard(top) and SikaPower
SmartFlow (bottom)(© Sika)14

depth to shoulder height is decisive, andthe design must take this into account.
At a given channel depth of 3 mm, it
has been shown that the greater theshoulder height chosen, the worse thespread of the adhesive in the channel
and the better the flow of electrocoating
can be guaranteed.
If electrocoating is not absolutely
necessary, a shoulder height of <1 mm
has proven to be optimal. If perfect elec­
trocoating must be guaranteed, a shoul­
der height of 1 to 2 mm is necessary.
However, this does not pose a problem,
because up to a shoulder height of 3 mm,
the wash-out resistance in pretreatment
and dip coating is good, as is the stabil­
ity during heat-curing. A certain shoul­
der width is also necessary to preventan overflow of the adhesive from thechannel into the shoulder. A shoulder
width of > 5 mm has proven to be ideal.
Extensive injection tests have shownthat both narrow and wide channels
can be filled without problems, FIGURE 5.
Channel widths of up to 20 cm werefilled during the tests without any­problems. Taking these specifications
into account, the channel can be inte­
grated either directly in the structuralpart or, in the case of push-in connec­
tors, in the casting.
It is important to select the correct
injection speeds to prevent the adhesivefrom overflowing at the edges. An injec­tion speed that is too high tends to leadto overflowing. However, if the speed is
too low, the adhesive may not spreadthrough the channel as intended. Basi­cally, a low, controlled inflow of adhesive
into the shoulder is preferred. The tem­
perature also has an influence on theinjection result, as the viscosity of theadhesives depends upon it. Using fully
automated application systems, the mostimportant injection parameters, such asextrusion speed and temperature, canbe specifically controlled. This allows
consistent, comparable results to beachieved. The injection point should also
be carefully selected. It should be notedthat the volume of the cavity must beapproximately the same in all flow direc­
tions, so that overflow can be prevented
at the points close to the injection point.
Several test bodies were set up inorder to test the flow behavior of theadhesive as realistically as possible.
The gluing of a reinforcing part into asteel structure was to be illustrated with

FIGURE 7 Multiple injections with SikaPower Standard (top) and SikaPower SmartFlow (down) (© Sika)–SikaPower-497

SikaPower SmartFlow

Lap-shear strength30 MPa25 MPaImpact resistance

40 N/mm

35 N/mmT-peel resistance

10 N/mm

9 N/mmOpen timeGoodGood

Application temperature range

50 to 60 °C

50 to 60 °C

TABLE 2 Comparison of important material properties (© Sika)

the selected test piece. This geometry
was selected primarily because of thelarge channels for the tests. The rele­vant structural parts were produced
using a 3-D printer, FIGURE 6. A suitably
shaped Plexiglas housing was usedinstead of the metal structure to opti­
cally track the flow behavior. The tests
have confirmed that large channels
can be filled without overflowing at
the edges and the shoulders. It was also
shown that typical elements such asholes or recesses for such components
do not pose a problem for the spread of
the adhesive. In such a case, the adhe­sive flows around the obstacle and thenflows together again without leaving
any air pockets behind, FIGURE 6. If it isnecessary to fill even larger channels or
to shorten the filling time, the adhesive
can also be injected in several places at
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the same time, FIGURE 7. In this casethe flow fronts flow together seamlessly.
The innovative adhesive is based on
the well-known SikaPower adhesives
which are intended for use in the bodyshop. Thus, the most important productproperties of the new adhesive are also
comparable with those of known sys­tems, TABLE 2.
SUMMARY

The adhesive SikaPower SmartFlow
allows new bonding solutions. Thismeans that push-in connectors suchas those found in electric vehicles orlightweight designs can be reliably
bonded. The same adhesive can also
be used to bond structural parts overthe entire surface, resulting in higher
stiffnesses and strengths.

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