Applying the Adhesive

TM 750-245-4

assembly to which the compound is applied to prevent

the surfaces. Remove oils, greases, and most similar

damage to heat-sensitive compounds or materials.

contaminants with chemical cleaning agents such as

solvents and detergents. Make sure no solvent remains

b. Heat. To apply heat, use a forced-air-circulating

within the mold cavity.

oven, if possible, capable of maintaining temperatures

from 100 to 500 degrees F. within 10 degrees F.

9-6. Applying the Adhesive

When heat lamps are used, be careful not to develop

Apply adhesives with a spatula, brush, or syringe-type

hot spots by concentrating heat in small areas. Rotate

extruding gun (sealant gun).

Apply only enough

the work on a turntable or by hand to prevent these hot

adhesive to insure complete contact between the

spots during heatlamp cure.

adhering areas.

Apply two-part adhesives to one

c. Pressure. Pressure is seldom required during

surface only (the smaller, if possible). Apply one-part

the cure of most adhesives. If necessary, apply just

adhesives to both surfaces. The solvent in one-part

enough pressure to insure complete contact.

elastomeric adhesives must be allowed to dry until tack-

Elastomeric adhesives do not exude from the glue line.

free before mating the parts. Since the various types of

Do not attempt to force the parts together so hard as to

solvents require different drying periods, observe strictly

cause exuding. Resinous or two-part adhesives require

the instructions on drying time. After the parts have

enough pressure on the bonded parts to exude a slight

been firmly secured, remove excess adhesive around

amount of adhesive at the glue line. Clean off excess

the glue line with MEK solvent. Do not use excessive

adhesive from the glue line.

solvent, the solvent may retard the curing of the

d. Single-Part Adhesives.

Thoroughly blend

adhesive if allowed to penetrate the glue line.

single-part adhesives, such as solvent dispersions,

Thoroughly clean the tools with MEK and wipe them dry

before application. Shake the unopened container, or

with a clean cloth.

open the container and stir the con tents, or both.

9-7. Using Fixtures in Bonding

9-9. Final Inspection

a. Aline the parts to be bonded and place them in

The quality of an adhesive bond can best be determined

contact as instructed in the maintenance document.

by assuring that the correct adhesive was used and then

Hold and align the bonded part during adhesive cure

by monitoring the entire bonding process, from cleaning

with fixtures such as C-clamps, spring clamps, weights,

to curing. If samples can be taken for testing to

masking tapes, molded fixtures, or combinations of

destruction, it is obvious that such a test is the best

these devices in accordance with the specific system

measure of the adhesive bond. In most instances,

documentation.

Avoid damaging the assembly or

however, inspection must be accomplished without

extruding excess adhesive at the glue line when

damage to the adhesive joint.

The following

applying pressure with a fixture.

nondestructive tests will serve to reject most of the

b. Release agents may be used on fixtures, but

defective bonds:

avoid excessive use. Excessive release agents can

a. Visual Examination. Check the joint for warpage

contaminate the operator's hands or may enter the

and improper alinement. The edges of the joint should

bonded area. Make sure the release agent will have no

show some flash (extruded adhesive) to indicate that the

damaging effect on the bonding operation or succeeding

proper amount of adhesive was used. The flash should

operations.

Apply the release agent according to

be hard and nonporous to indicate proper curing. Soft

instructions on the container.

and tacky shows undercure; burnt and bubbly shows

overcure. Visual examination is the most effective test.

9-8. Curing the Adhesive

b. Tapping. On large bonded areas, the joint may

a. Cure Conditions. All adhesives require some

be tapped with a hard object. Voids and blisters sound

curing period. The time required varies with the heat

hollow compared to well-bonded areas.

and pressure applied.

Observe the temperature,

c. Proof Loading. Load the joint with the same

pressure, and cure time specified by the instructions for

type of stress anticipated in service. The magnitude of

each particular adhesive. The time and temperature of

the stress should be greater than normal but should not

cure must be consistent with thermal limitations of the

exceed 125 per cent of anticipated load.

9-2