Using Fixtures in Potting

TM 750-245-4

accidentally enter the adherent areas as a result of poor

c. Extrude potting material into an open mold by

workmanship.

placing the nozzle at the bottom of the mold and

retracting the nozzle as the cavity fills; keep the nozzle

c. While preparing adherent surfaces, consider

tip just below the level of compound in the mold. Be

which areas do or do not require adhesion. In potting,

sure to use access holes in closed molds, if they are

the mold or container becomes a part of the finished

provided.

product; therefore, adhesion to the mold is essential. In

embedding, however, a release agent must be applied

d. Before the potting compound solidifies, examine

to the mold before potting. Often, electrical wires with

any wires or other movable protrusions for alinement,

nonadherent surfaces are encountered. Do not attempt

and realine them as necessary.

to abrade or clean electrical wires or sleeved junctions

e. Special instructions or precautions which must

near the potted area, since the abrasion may cause

be observed while applying potting compounds covered

short circuits.

in (1) and (2) below:

d. Check each unit to be processed to insure that

(1) Staking sealants. Staking sealants are

components are in place and ready for potting or

noted for the controllability of their adhesive strengths to

encapsulating. Correct any discrepancies. Aline molds

metal substrates. These compounds remain liquid

and components in the fixtures and center all wires and

indefinitely until they are confined between closely fitting

components protruding from a mold uniformly in the

metal surfaces in the absence of air; then they change

cavity.

to an infusible, insoluble state. These compounds are

e. After the potting compound has cooled to room

used in sealing threaded fasteners, plugs, and threaded

temperature, proceed carefully to remove fixtures and

fittings against fluid pressure; in locking such threaded

molds, if required. Trim flash areas as required. Store

parts against working loose under shock vibration; and

fixtures and molds are required to keep them in good

in retaining existing or replacement ball bearings in worn

operating condition.

housings, thus eliminating the need for a press fit.

These compounds are supplied in color coded strengths

and two viscosities for each color. The metal substrate

9-15. Curing the Potting Compound

which the compound contacts is considered the catalyst.

a. Cure Conditions. All potting compounds require

(2) Urethane foams. Access holes in the

some curing period. The time required varies with the

mold should be no less than 1/2 inch in diameter, so that

heat applied. Observe the cure time and temperature

the mixed foam can be poured quickly into the part to be

specified by the instructions for each particular material.

potted. Bleeding vent holes should be provided if

The time and temperature of cure must be consistent

possible, and if not, the excess foam may exude from

with thermal limitations of the assembly to which the

the access hole when the foam rises. Make sure the

compound is applied to prevent damage to heat-

mold is heated according to the foaming instructions.

sensitive components or materials.

All foams are exothermic; the heat of reaction causes

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

the chemicals to form gas pockets and cause foaming.

oven, if possible, capable of maintaining temperatures

If the heat from the reaction is used up in warming the

from 100 to 500 degrees F. within 3 10 degrees F.

mold, poor foaming action will result and the rise will be

When heat lamps are used, be careful not to develop

insufficient. A general rule, if mold-heating information

hot spots by concentrating heat in small areas. Rotate

is not provided, it to heat the mold from 100 to 120

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

degrees F. for one hour just before foaming.

spots during heatlamp cure. Do not exceed curing

temperatures or the potting material, the potted part, or

9-14. Using Fixtures in Potting

both may be ruined.

a. Aline parts to be potted as instructed in the

c. Special Instructions. Special instructions or

technical documentation. Hold and aline the potted part

precautions which must be observed in the cure of

during the cure of the potting compound with fixtures

potting compounds are covered in (1) through (3) below:

such as molds, C clamps, spring clamps, weights,

(1) Silicones. A minimum of 20 percent

masking tapes, molded fixtures, or combinations of

relative humidity is necessary to cure silicone materials.

these devices in accordance with the specific

(Heat does not accelerate their cure.) Because of this

instructions.

moisture requirement, compounds applied thicker than

b. Release agents may be used on fixtures at

one-eighth inch require longer to cure than do thin

times, but avoid excessive use. Excessive release

coatings. As a general rule, for each one-eighth inch

agent can contaminate the operator's hands or can

thickness, allow 24 hours at room temperature for cure.

Post cure silicones for 15 to 60 minutes at 10 degrees

9-4