S AND MINERAL FILLED EPOXY MOLDING COMPOUNDS
Processing Guide


Chemistry and Features

Epoxy molding compounds were
commercialized in the early 1950s. They
have become the most widely used material
for encapsulation and have established the
performance standards against which other
materials often are evaluated. Epoxy
molding compounds are desirable in
electrical applications for the following
reasons:
Low
Shrinkage Excellent adhesion to a wide variety
of materials Good electrical properties Resistance to moisture and
chemicals Compatible with most other
materials used in electrical
apparatus.

There are several classes of epoxy resins:
1. Novolac epoxies, particularly the
epoxy cresol and epoxy phenol
novolac resin with o-cresol or
phenol and formaldehyde with
epichlorohydrin. This material is
used where good thermal
properties, high resistance to
solvents and chemicals are
required. (Figure 1) This is the
typical type of epoxy that SBNA
manufactures.
2. The cycloaliphatics epoxy resins are
important where arc track and
weather resistance are required.
3. Another class of epoxy resins is
based on aromatic diamines. These
resins have good mechanical
properties, high thermal capabilities
and fatigue resistance. Epoxy resins
must be cured with cross-linking
agents (hardeners) or catalysts to
develop the desirable properties.
The epoxy and hydroxyl groups are
the reaction sites through which
cross-linking occur. Agents include
amines, anhydrides, Alde Hyde
condensation products, and
novolacs and acid catalysts.
4. Aromatic amines are used where
high heat deflection temperatures
are required and elevated curing
temperature is acceptable.
5. Anhydride resins are used where
low exothermic, good adhesion,
electrical and thermal properties are
required. Long pot life is achieved
with anhydride resins.

Figure 1: Epoxidizied Cresol Novolac Resin

Most epoxy molding compounds exhibit low
dielectric loss. High insulation value, low
shrinkage and high mechanical strength that
are maintained under severe moisture and
high temperature. Epoxies range from slow
burning to self extinguishing. These benefits
have made epoxies one of the most popular
plastics used in electrical/electronic field.
Epoxies are highly chemically resistant,
being affected only by strong acids and
ketones. They are non-outgassing, flame
resistant and have ease of flow with
relatively low molding pressure.

Flow Rheology

Compounds based on Epoxy resins are
designed to process well in all types of
molding equipment. Compounds are
available in a variety of flow ranges, from
very stiff for compression molding to very
soft for injection molding. Their plasticities
are normally measured by a special transfer
molding procedure known as Mesa Spiral
Flow (Figure 2) and Emmi Spiral Flow
(Figure 3). This test provides an indication
of the relative ability of the compound to flow
and fill molds. Brabender torque rheometer
measurements are sometimes utilized to
characterize both flow and curing properties.

Figure 2: Mesa Spiral



Figure 3: Emmi Spiral


Storage

Compounds based on Epoxy resin possess
greater reactivity, and therefore cold storage
of these materials is recommended. Normal
storage life is about 6 months when kept at
or below 40°F (4°C). The material should
be brought to room temperature before
containers are opened to prevent the
introduction of condensed moisture from the
atmosphere.

Preforming or Screw Plasticating

Epoxy compounds can be charged directly
into the mold cavity in granular form. This
practice may be desirable in the case of
single cavity compression molds for large
components, as well as with presses
utilizing automatic powder loading. The
compound should be accurately weighed or
volumetrically metered for each molding
cycle.

For the production of small and medium-
sized parts with multi-cavity compression
molds or transfer molds, preforms are
generally preferred. Preforms can be made
in many shapes and sizes by compaction of
the molding material in a die. Preforms are
easier to handle and can be more readily
preheated than loosely packed bulk
material.

Granular Epoxy compounds are easily
preformed by any of the usual types of
hydraulic or mechanical preforming presses
or screw plasticating machines. Screw
plastication combines both preforming and
preheating in one process.

Automatic preformers require free-flowing
granular compounds in order to maintain
weight consistency and rapid cycles.
Materials with the lowest bulk densities,
such as those containing long glass fibers,
cannot usually be preformed automatically in
conventional equipment. These often
require auxiliary feeding equipment or a
manual weighing and pressing operation.

Preheating

The molding compounds should be heated
before being loaded into the mold. This
reduces the time required to bring the
material to a plastic state and allows shorter
molding cycles than are possible with cold
material. Preheating can increase
productivity by 20 to 40% depending on the
geometry of the part and the material being
molded. The preheated material also flows
more easily and uniformly and generally
requires less pressure. Other benefits of
preheating include smoother molded
surfaces and enhanced physical properties.

The most commonly used preheating
methods are dielectric (HF) heating of a
densely packed perform or frictional heating
using a screw plasticator.

Preheat temperature is one of the most
critical molding parameters. The
temperature should be optimized to provide
the easiest flow. This usually means raising
the temperature of the compound until
obvious precure or short shots occur in
molding, and then dropping back 10 to 15°F.

Molding Methods

In general, Epoxy compounds can be
processed by all conventional thermoset
molding methods, such as compression,
transfer or injection molding. Most
thermoset molding machines are suitable for
these materials without the need for special
equipment.
Suggested Start-Up Conditions


Transfer or plunger molding is a
predominant method used with Epoxy
compounds. This method is particularly
useful for parts that contain molded-in
inserts. Strength may be
Molding conditions may vary depending on
the shape of the part, mold design, and
composition of the material. Suggested
start-up conditions and approximate curing
times are provided in Figure 4 and 5.



Figure 3: Injection Molding Presses
Postcuring and Degassing

Molded parts normally never attain complete
cure under commercial conditions. In most
cases, conditions are established which
produce an acceptable degree of cure and
attendant physical properties when the parts
are removed from the mold. It is not
necessary to postcure molded parts unless
a specific application requires that
resistance to high-temperature or chemical
exposures be maximized or that the by-
products of the curing initiators be driven off.

somewhat lower and shrinkage somewhat
higher than that of parts molded by
compression. The magnitudes of these
differences are dependent on runner and
gate design plus the flow pattern in the mold
cavity.

The retention of physical properties at
elevated temperatures is governed by the
T
g
. For maximum thermal stability and
retention of physical properties at high
temperatures, molded parts can be
postcured. This should be performed in a
forced-draft oven having good temperature
control and outside venting.


Formulations are available to take full
advantage of the high productivity of
injection molding. They offer excellent
barrel and nozzle stability with fast cure in
the mold.

Optimum postcuring conditions are
determined by the specific application
requirements. When determining the oven
temperature cycle, it is important to note that
lower temperatures generally result in less
thermal stress and less discoloration of the
part. Most cycles begin at 275°F (135°C)
and the temperature is gradually raised to
avoid exceeding the T
g
. The rate of
temperature rise should be as fast as
permissible without visual signs of
degradation such as blistering or cracking.
A final oven temperature of 175°C is usually
sufficient to complete the crosslinking
reactions without significant discoloration.
For applications requiring minimum
outgassing, the final oven temperature
should be at least equal to the maximum
service temperature and the parts should be
baked from 4 to 6 hours at this final
temperature or until the rate of weight loss is
negligible.

In injection or transfer molding, a sound
molding requires complete cavity filling and
adequate packing before gelation. Mold
filling time should neither be too short, to
avoid trapping air or overheating material
through friction, nor too long, to avoid
gelation before the cavity is completely
packed. If molding conditions do not allow
sufficient packing pressure, part strength will
be compromised. Other properties including
dimensions are affected by c