Aesthetically Critical Bulk
Molding Compound:
Material Composition, Mold Design
And Processing Techniques
Introduction:
Bulk Molding Compound (BMC) is a
composite material well known for its creep
resistance at temperature, flame retardant
qualities and dielectric strength. As a
member of the thermoset family of plastics,
BMC has been specified for mature
applications such as circuit breakers,
headlamp reflectors and other high heat,
load bearing items. These parts are
generally coated or restricted to the nonvisual areas of an end use application. BMC
materials, although functional, have been
somewhat rudimentary in their approach to
ingredient content, mold design and molding
processes. The results of these “basic”
techniques have been both positive and
negative. BMC is popular as a low cost
alternative for non-aesthetic, high heat
applications. The downside to a “no frills
workhorse” reputation is that BMC has little
association with the higher margin, aesthetic
markets.
The requirements of the high heat/cosmetic
marketplace have been met traditionally by
engineering thermoplastics and substrates
treated in secondary operations.
Thermoplastic resin has a long history of
being user friendly in surface critical
applications. BMC has not been
competitive with aesthetic materials and
coatings until recently because of its
traditional performance limitations. A basic
BMC formulation can produce very little or
highly inconsistent gloss when molded. The
surface profile may magnify the flow and
fill of a part with visible ripples and
arbitrary shrink marks. The color of choice
might be mottled or separated. Regardless
of the cost savings, the need for components
exhibiting consistent profile, gloss and
pigmentation has compelled some designers
to look away from BMC when considering
high heat, cosmetic materials.
Through the innovative use of traditional
raw materials and new chemical additives
the most recent generation of BMC
represents a low cost alternative to high heat
thermoplastics and secondary coating
operations. Smarter mold designs used in
conjunction with better processing
techniques deliver the full aesthetic potential
of these BMC materials to the end use
application. The inherent performance
qualities of BMC can be specified in
applications with gloss and color as
additional features.
Material Composition:
BMC is an agglomeration consisting of
mineral filler, liquid resin, fiber and
specialty additives. Manufactured using
high and low sheer mixing processes BMC
begins with the independent blending of all
liquid components. Liquid polyester resins,
low profile thermoplastic additives, peroxide
catalyst systems and other performance
enhancers are added at this stage of
production. This blend experiences a high
sheer mixing operation lasting several
minutes. The result is a combination called
the premix (Illustration #1). Many premix
compilations resemble warm molasses in
viscosity and tackiness. The premix is
metered to a specific weight into low sheer
mixing equipment. Added to the premix are
the fillers and fibers of choice. The
resulting product is filler and fiber saturated
and bound by the premix. This dough like
material is packaged in its bulk format or
extruded into a limitless number of sizes and
shapes to meet the particular handling needs
of the molder (Illustration #2).
Filler
A highly filled material, BMC can contain
as much as 80% mineral filler by weight.
Important for engineering, cosmetic and
economic reasons, filler is not a useless
consumer of space in an independently
functional resin system. Early research
developed filler systems with selfextinguishing characteristics for high heat or
flame contact scenarios. This technology
led to the use of BMC materials in electrical
enclosure and support applications: a sizable
market for over twenty years.
Research focusing on surface quality has
illustrated the expanding importance of filler
and its role in meeting the demands of the
marketplace. Fillers of various mineral
derivations can be manufactured with
reasonably tight control over particle size.
Simple, yet functional, some BMC materials
can be designed with a single filler and one
standard particle size. Parts molded with a
material containing one filler may
experience “typical” BMC surface traits.
Profile quality can be significantly affected
by inadequate particle packing.
Particle packing is one of the techniques
used to make BMC more attractive and less
costly. The goal is the most efficient and
aesthetically effective use of resin possible.
Resin pockets that exist between particles in
molded BMC are larger when there is no
engineered variety in the particle sizes
(Illustration #3). Particle packing is
achieved by adding different sized filler
grinds to a BMC system. An effective
interface between filler particles reduces
microscopic inconsistencies in shrink. This
phenomenon results in a smoother surface
profile. The quality of a component’s
profile will affect how light is reflected.
Consistent light reflection is critical to the
visual interpretation of gloss (Illustration
#4).
Resin is one the most expensive chemicals
utilized in a BMC system. In addition to
improved aesthetics, the skillful
replacement of resin with filler keeps
material costs in control.
Low Profile Additives
Low profile additives are included in BMC
to reduce and control shrink. Thermoplastic
is cut with styrene (or other compatible
monomers) and brought to a liquid state
similar in viscosity to the polyester base
resin. Thermoplastic is chemically
incompatible with thermoset polyester.
When blended under high sheer conditions
the thermoplastic remains chemically
independent of the polyester in dispersed
microscopic globules. It is the
incompatibility of these components that
make them so effective together in a BMC
material.
Once exposed to appropriate heat and
pressure the polyester element of BMC will
begin to contract or shrink with cure. The
thermoplastic component has an inverse
reaction to heat. By expanding with the
melt process thermoplastic globules resist
the shrink of curing polyester (Illustration
#5). When the BMC component is
demolded and cools the thermoplastic
returns to its solid state having served its
purpose.
The ingredients blended to make BMC have
different coefficients of thermal expansion.
On a microscopic level, some separation
takes place between these ingredients during
the molding process. Areas that are resin
rich will shrink more than areas having a
greater quantity of inert filler and fiber.
Thermoplastic additives reduce this
inconsistency in shrink and enhance surface
profile. BMC with little or no thermoplastic
additive may exhibit shrink related ripple
and flow marks. As with filler (outlined
above) the resin components in aesthetic
BMC are engineered to resist
inconsistencies in shrink to provide critical
reflective properties.
Low profile additives and their base
polymers must be considered carefully
however, as they have negative as well as
positive effects. Thermoplastic shrinks
more than BMC after the molded component
is allowed to cool. Throughout the part
thermoplastic globules draw away from the
polyester and a subsurface micro-voiding
occurs. The coefficient of thermal
expansion of the base thermoplastic must be
understood. An “overly expansive” additive
can leave micro-voids that are large enough
to disturb the subsurface refraction of light.
properties (including gloss) through specific
acid and alcohol selections, material ratios
and process alterations. With the addition of
styrene monomer (or other available
monomers) the base resin is prepared for use
in the BMC product. Resin is produced in a
facility that specializes in reaction type
manufacturing methods. Most producers of
BMC purchase their polyester resins from
outside sources. Newer resins have been
made available over the past five years that
have greatly improved gloss factors in
properly formulated BMC products.
Subsurface micro-voids are opaque and can
cause a part to look hazy even though it has
a good surface profile. Low profile
additives must be carefully selected when
developing a deeply pigmented product.
Pigment will absorb all colors of light
except for its own which it reflects. Opaque
micro-voids will scatter light and cause a
deeply pigmented material to look milky,
washed out or under mixed (Illustration #6).
Surface profile is the foundation of a
cosmetic material. A considerable amount
of research goes into engineering BMC that
will successfully shoulder the gloss of
polyester resin. Synergy between filler, low
profile additive and polyester is the key to
making BMC that will mold with
competitive cosmetic qualities.
There are chemical tradeoffs involved with
formulating cosmetic BMC. A smooth
profile must be obtained without distorting
visual interpretations from below the
surface.
The development invested into a BMC
material offering cosmetic properties
represents only 33% of the aesthetic part
equation. Equal in importance to the BMC
formula is the mold design and method of
molding. Traditionally, BMC molds have
had rather broad design guidelines. Molds
that will manufacture aesthetic parts should
comply with more specific methods of
design. Polyester must be drawn to the
surface of a part chemically and
mechanically to obtain smooth profile and
gloss. Cosmetic BMC requires more
accurate (and in many instances greater)
heat, pressure and speed to drive gloss
factors to market worthy levels.
Resin
Thermosetting polyester resin that contains
no filler or fiber can be cast into
configurations for specialty applications.
These castings do not provide the yield or
properties expected of BMC. Neat polyester
will provide an excellent surface profile and
very high gloss factors, however. Polyester
is what gives BMC many of the outstanding
heat and creep resistant properties it
traditionally displays in end use. Also, it is
the resin that is driven to the surface of a
molded part and (with a proper profile) is
responsible for gloss.
Polyester resin is manufactured by initiating
a condensation reaction between various
acids and alcohols. Manufacturers of resin
are capable of manipulating end use
Mold Design:
Unlike compression molds designed with
vertical sheer edges, injection molds
represent a significant challenge in the effort
to obtain and maintain optimized cavity
pressure. One of the most common trouble
spots for an injection mold is the land area.
When molding BMC for a glossy and/or
deeply pigmented application, lands or
vents that inadvertently release cavity
pressure will always promote scrap.
Land Area
The first step to insuring a mold is landing
cleanly and completely is to check
parallelism using a common bluing agent.
The parallelism of a mold should always be
checked at operating temperatures. It is
helpful to check a mold’s parallelism in an
accurate spotting press as well as the
machine the tool will be molded in. Presses
have a tendency to be overlooked as a cause
of cavity pressure loss and should be
checked as well. Vents should also be
reviewed periodically to insure they have
not become worn beyond their original
specifications.
In some cases the tool steel is (or has
become) flush with the mold base. This
dramatically reduces the pounds per square
inch available at the parting line. The
vented core or cavity should be pre-loaded
approximately .050” when designing the
mold. If the mold is existing in a “flush
condition” the base should be reduced by
.050” on the side of venting. Atmospheric
and “pool” venting can be achieved with
superior control and reduced flash buildup
using a pre-loaded design. A .050” land
elevation should be reviewed and
maintained over the life of a program. By
insuring unencumbered contact, the mold
land is receiving all of the press clamp
energy and the pressure seal around the part
is far greater.
Venting
Venting a mold is often a “per
configuration” or trial and error study. A
helpful practice is to have an initial mold
trial with no vents whatsoever. By
reviewing the first parts and noting the areas
of non-fill, burn and gas accumulation
accurate vent locations can be selected.
Over-venting a mold can cause pressure loss
leading to “pockets” of dull surface. A
standard BMC vent design recommends a
depth of .001” to .003” and widths ranging
from .125” to .250”. Vents that exceed a
depth of .004” can cause slight chipping or
gloss to delaminate during the deflashing
process. These parameters may need to be
modified based on the volume of the part
and the extent of gas entrapment. Vents
require excellent polish and should be
chrome plated to allow flash to pull cleanly
from the mold with each cycle and prevent
steel erosion.
The most common vent designs carry gas
completely out of the mold to atmosphere.
Success with “pool” type vents has gained
some popularity for molding high gloss
BMC. Pool vents comply with the standard
parting line depths and polish outlined
above. .250” to .500” away from the parting
line, however, pool vents are cut to a depth
of .006” to .008” for an additional .250” to
.500” length. The vent then terminates
without an atmospheric exit. The advantage
of a pool design is the reduced risk of cavity
pressure loss. Many configurations will
vent adequately with no atmospheric exit
(Illustration #7).
Another method that has met with success is
the overflow channel. An overflow channel
is cut to the width of the part plus .250”.500” per side at the end of flow. (The extra
is to insure corners vent well). Most designs
follow the parting line depth and polish
recommendations outlined above. The
length of an overflow is generally .250” to
.500”. BMC flash is quite weak and longer
overflows may promote unwanted material
accumulating on the mold face. In larger
parts pools or atmospheric release channels
may be added to the overflow to allow more
gas and activated “front resin” to flow
completely away from the part (Illustration
#8).
Venting blind pockets is necessary to allow
the escape of gas from the mold and ensure
proper part fill out. Knockout (vent) pins
should be placed in areas likely to entrap
gas. Gas related voids and burns on boss
tips or ribs may not be critical to the “Bsurface” of a part. This entrapped gas may,
however, escape from the mold by baffling
back across the internal detail just prior to
complete mold fill. This phenomenon can
cause significant surface flaws on the “A
side” of a part.
Vacuum systems have been used to assist in
the venting of blind pockets incapable of
receiving ejector pins. Although helpful in
this instance, vacuum systems are not
equivalent in effectiveness to a properly
vented mold.
Sprues, Runners and Gates
For ease of flow, reduced waste and
minimum cycle times mold runners should
be fully round and as short as possible.
Unlike some thermoset products BMC
works well with extended nozzle systems.
Shorter sprues can be specified to further
reduce material waste and fiber degradation.
Parts designed with multiple wall thickness
should be gated into the thickest portion of
the component (to avoid thin to thick flow
rippling). If a mold is designed without a
gate cutting mechanism, the gate should be
located in a region of the part that is not
aesthetically critical. A gate cutting system
or compression molding is required for parts
without a non-critical surface. The depth
and width of a gate should be as generous as
possible. Tapering a gate thickness slightly
away from the part will promote breaking at
the point of minimum thickness rather than
into the part. Gates thinner than .030”
should be avoided as sheer will degrade
fiber and promote precure conditions in the
part.
Standard gate areas should be greater than
.0008 square inches per gram of part
weight.
Mold Heat
Mold heat should be carefully considered to
promote as much control across the flow of
the material as possible. Plus or minus ten
degrees F is a maximum variation
recommendation for aesthetic BMC
applications. Electric heater cartridges are
preferred and should be tapped
perpendicular to the longest flow route of
the material. By inserting heaters
perpendicular to flow, zone control can be
established at multiple intervals along the
fill of the part. Layouts designed to run
parallel to flow leave temperature variations
caused by uneven steel thickness and
inconsistent cartridge heaters difficult to
remedy. If possible at least two zones of
heat should be established per part on both
the mobile and stationary halves of the mold
(more for large parts).
Strategic placement of thermocouples with
consideration for the location of the heaters
affected is critical.
Thermocouples should be tapped into the
actual tool steel and not located “nearby”
in the mold base. The closer
thermocouples can be placed to critical
surface locations, the better.
Thermocouples tapped from the rear of the
mold and located in the center of their
respective zone are the best for mold heat
control (although clearly more difficult to
service).
Enhancements
It is critical to design a mold for aesthetic
BMC parts that is capable of consistently
maintaining accurate processing conditions.
Although there is no replacement for a good
initial design, there are some enhancements
that can be made to further optimize
processing conditions. Pressure transducers
have been available for years. They are one
of the most effective, affordable, yet under
utilized technologies available to aesthetic
BMC molders. Cavity pressure is
potentially the most important detail
involved with consistently molding glossy
BMC. Pressure transducers can control
critical press function using real time data
being generated inside the mold.
Repeatability is the key when a robust
parameter outline has been developed.
Initial mold design, no matter how
important, cannot overcome variations in
material or machinery. A “smart mold”
capable of generating pressure driven
signals can alter a process on a shot to shot
basis and correct variations that may
otherwise cause scrap.
Processing Techniques:
Ejector pins have assumed many roles
including part ejectors, in-mold vents and
pressure indicators. In an effort to drive
cavity pressure to a greater level for a longer
period of time ejector pins have shown
promise as “pressure pins”. By reducing the
stops on the back of the ejector plate the
pins can be retracted by the press to a point
below the molding surface. Shortly after
material has filled the mold the ejector pins
are then driven forward by the machine
flush to the molding surface. Return pins
prevent the “pressure pins” from over
stroking and scoring the mold cavity.
Molding equipment does not have to be
new to be effective. Effective equipment
must provide variable process control,
reliable data and the accurate repetition of
manufacturing set points.
The stroke distance for this pressure
enhancement must be selected based on the
size and needs of the respective part.
Increments of .020” are recommended when
reducing ejector plate stops. Larger
diameter pins are preferred, as they require
less movement to displace more material.
Narrow pins may require too much
movement to displace an adequate volume
of BMC. This can produce irregular profiles
on a part’s “A side”.
Molds with gate cutting mechanisms benefit
from “pressure pin” technology. Gate
cutters are blades that drive forward
severing gates from parts before the material
has cured. When the cutter blade is
engaged, cavity pressure from the gate is
discontinued.
If “pressure pins” are driven forward
simultaneously with gate cutters, desired
cavity pressure can be maintained from
inside the mold.
Temperature, speed and pressure represent
the three most significant factors in molding
cosmetic BMC. Control over these
parameters is the difference between
efficiency and high scrap rates. The
machinery chosen to mold BMC is as
important to a program as the material and
the mold.
To understand the importance of
temperature and speed for cosmetic results
using BMC, a brief description of catalyst
function and cross-link density is necessary.
Cross-Link Density
Themoset molding materials contain various
additives that decompose when exposed to
heat. This reaction causes a chemically
irreversible, molecular cross-linking to
occur in the resin ingredients. BMC is
catalyzed using various peroxide-based
additives that decompose at different
temperatures. The greater the molding
temperature the faster a catalyst will reach
its point of decomposition and initiate crosslinking. The efficiency with which catalysts
and resin systems link polyester molecules
together directly affects the molded
component’s cross-link density (CLD). The
more efficiently and completely a BMC
cross-links, the greater its CLD will be.
BMC that has not cured properly will have
lower CLD. An inappropriate balance of
catalyst and resin can cause LOWER CLD.
Molding conditions that do not allow the
catalyst to decompose at an appropriate
temperature or point in fill will also
participate in lower CLD. Microscopic pits
or craters are symptoms of polyester with
low CLD. These flaws result in poor
surface quality. BMC must achieve high
CLD to produce cosmetic results. The
respective functions of the engineered
components (filler, low profile and resin)
will be hampered by a premature or
inadequate cure situation.
Most BMC molds in the 270 to 390 degree F
range. Generally, aesthetic BMC molds in
the middle to upper temperatures in this
range. By molding with higher temperatures
BMC will begin to cure quickly, most
especially at the surface where the mold and
material are in direct contact.
Consistency in surface cure is paramount
to a uniform high gloss appearance. For
this reason it is critical that mold heat be
consistent and recover rapidly from shot to
shot.
Speed
The goal when processing aesthetic BMC is
to project material across the molding
surface quickly enough that cross-linking
not begin in earnest until the mold is filled.
BMC material should be stationary and
under pressure before significant crosslinking is initiated. The CLD of a BMC is
clearly affected by material movement after
catalyst decomposition begins. The greater
the mold temperature can be (within reason)
without premature cross-linking, the greater
potential BMC has for superior surface
consistency and gloss. Injection/closure
speeds for aesthetic BMC are almost always
(comparatively) fast to compensate for
elevated mold temperatures.
Several cosmetic BMC components
currently in production utilize mold
temperatures in excess of 370 F and fill
times less than one second.
Pressure
The cavity pressure placed on cosmetic
BMC must be adequate in order to drive
polyester resin to the surface. Pressure
requirements vary from configuration to
configuration and must be diagnosed with
trial efforts. As with temperature and speed,
the pressures necessary to obtain cosmetic
results with BMC are generally greater (both
PSI and duration) than non-cosmetic
applications. Clamp pressure should
maintained at 2000 PSI or greater.
Compression processing with a “tight”
vertical sheer edge provides substantial and
immediate cavity pressure across the molded
surface. Injection molded components
derive their cavity pressure from the screw
or plunger at the gate. Material movement
after fill is more prevalent using the
injection process. High pressure from the
gate can cause excess material to “leak”
from atmospheric vents and the land area
after mold fill.
Variations from standard injection
parameters must be made available for
cosmetic BMC molding. Profiled holding
pressure is a method used for overcoming
unwanted post-injection flow. Immediately
after BMC has been injected into a mold
holding pressure does not need to be fully
applied. By including a brief (two to six
second) low-pressure hold segment, vents
and land flash will cure. The pressure
recommended for this initial hold setting
will vary per configuration. The key is to
quickly produce a stationary material
condition under pressure. By curing thin
vent and land sections a processor can, in
effect, dam unwanted flow. The gate is
much thicker than vent and land material
and will remain uncured longer. Greater
pressure than previously possible can then
be applied to the material in the “sealed”
mold (without shifting the curing surfaces).
Cosmetic BMC components have been
manufactured with as little hold pressure as
100 PSI and as much as 1000 PSI+. Hold
pressure times vary in much the same way.
Larger parts generally require more pressure
and time to obtain consistent surface values.
A material “cushion” is recommended
during the hold pressure segment. Screws or
plungers with no excess material at the tip
during hold will “bottom out” on the internal
nozzle wall or pot floor. Bottomed out
screws or plungers will not provide accurate
hold pressure data or desired cavity
pressure.
Conclusion:
Bulk Molding Compound offers properties
that are requisite for high end engineering
applications. Recognized for its durability
in harsh environments, BMC has a
reputation for being able to “take the heat”.
Cosmetic applications that require the
property values exhibited by BMC have
traditionally gone to engineering
thermoplastics and coated substrates,
however.
BMC can compete in the aesthetic
marketplace. The manufacture of cosmetic
parts using BMC is expanding in popularity.
The molding of glossy, deeply pigmented
BMC components can be executed
productively by implementing modern
techniques in formulation, mold design, and
process technology.
References:
Doug Fallis, President-PDF, Inc. Interview
August 1998.
***Special thanks to Doug for his
research (and patient instruction)
involving innovative molding
techniques.***
Larry Nunnery, President-Bulk Molding
Compounds, Inc. Bulk Molding Compounds,
Modern Plastics Encyclopedia.
Jody Riddle, Laboratory Manager-Bulk
Molding Compounds, Inc. Interview
November 1998.
.