Steve Kolste
Bemis Mfg.
Sheboygan Falls, Wis.
Chris Scarazzo
Solvay Advanced Polymers LLC
Alpharetta, Ga.
It’s a processing technique that’s been
around since the early ’70s. But coinjection
molding and its offspring technologies
have never been more in vogue. Their
prominence comes from parallel advance
in multibarrel, coinjection molding and
the seemingly weekly breakthroughs in
materials.
These molding techniques address a
problem with high-performance plastics.
They are often too expensive to serve as
the sole material. But multibarrel coinjection
molding lets molders economically
put these advanced materials on the outside
of the part while using less costly or
recycled materials inside.
The process uses one machine, one tool,
and a single cycle to mold two materials,
separate and distinct, or combined into
one product. Coinjection basically laminates
the materials to create a structure
that is often stronger and more stable than
would be possible with a single material.
Lower costs and expanded
material options
have been the key drivers
behind the spread of coinjection molding.
Designers traditionally used the process
for aesthetic applications. There is less
chance a consumer would get hurt if, for
example, the colorful soft touch grip delaminated
from a toothbrush. Likewise,
tactile accents used on handles and grips
provide ergonomic benefits such as reducing
vibrations and fatigue, but often play
little or no structural role. Therefore, in a
profession that prides itself on caution and
precision, critical components would see
no untried material.
But designers jumped at the opportunity
to evaluate new plastics that could
give aesthetics over those possible from
metal or composites. This was especially
true where molded-in-color could replace
commercial painting.
However, designers are widening plastics’
structural role as metal prices continue
to rise and new plastics deliver better
mechanical properties and heat and
chemical resistance.
For example, Ixef polyarylamide from
Solvay Advanced Polymers L.L.C., Alpharetta, Ga., is highly prized for
aesthetic looks and excellent stiffness.
It has found its way into a wide
array of consumer products including
cell phones and PDAs. However,
there’s a premium price for those
good looks and performance. But
coinjection molding with, for example,
PBT regrind in the core of a part
can address that cost issue.
Designers are now embracing
coinjection molded or multishot
parts for more than aesthetics.
Consider a part with a cross-sectional
area of 1 sq in. If this part
were made from a material that
had a tensile strength of 30 kpsi,
it would theoretically have a load
bearing capability of 30,000 lb.
However, in practical terms it is
nearly impossible to mold a part
with such a large cross-sectional
area without defects. In the
molding process, the outer
perimeter of the part is the
first to freeze or cool. The
inside is still in its melted
state though the perimeter
has been established. As
the part cools, the inside
wants to shrink. Because
the outside of the part has
a firm dimension, the inside sees
stresses that ultimately causes
defects such as sink marks, internal
stresses, cracks, or voids.
Sink marks are easy to spot on a
part surface, but voids and cracks
are internalized and can go unnoticed.
Internal defects, however,
significantly detract from the overall
mechanical properties of the
part. These defects act as stress
risers and are the points of failure
initiation. The reality of such a part
is that it will typically only have
around half the design load-bearing
capability.
Traditionally, designers have
addressed this problem by coring
out or removing material from
the cross section of the part using
a gas-assist molding process.
This results in a hollow part with
uniform walls that can be molded
without defects. It is here that the design engineer sometimes clashes with the
product designer. That’s because the product
designer may be more concerned with the aesthetics
of a part that “feels” hollowed out, or
lacks the bulk or mass normally associated with
a critical, structural part.
In contrast, coinjection molding lets designers
mold an aesthetic, high-strength “skin”
on a part while the core is molded in a lowerperforming,
less-costly material that can also
be foamed. Foaming the core produces a uniform
cell structure which can have reasonably
good mechanical properties, yet be free of the
stresses and defects of a fully densified part.
Consider that 1-sq-in. cross section again.
Assume a 50-50 breakdown of skin and core
material. If one has a core material having a
tensile strength of 15 kpsi with an exterior material
that has 30 kpsi, the practical strength
will be 50% greater than in a part made from a
single material.
Further, the part is lighter and its materials
cost less. Regrind from the molding process
can also be incorporated into the core
material, further improving efficiency without
sacrificing appearance.
In the past decade, demands to reduce
weight and cost, have pushed designers to
consider polymers for complex and critical
components such as gears, bushings, bearings
and even support structures such as seating.
New advances in processing, including gas
assisted, multimaterial molding and new
grades of high-performing plastics have
made this possible.
New plastics stand up to extreme temperatures
and demanding environments that were
once only the domain of metals and composites.
In addition, emerging tiers of “ultra” polymers
can deliver compressive performance
once possible only in a metal like titanium.
Just think about a typical riding lawn
mower. A model made in 1990 was bulky, boxy,
heavy, and by now, probably rusty. By contrast,
today’s models are sleek, user friendly,
easier to maneuver, and more fuel efficient. No
design engineer worth his or her salt would
have ever considered grill, lighting, or enginepanel
components made of plastic 15 years
ago. That just isn’t the case today.
Such trends come from the ever expanding
performance profile of plastics and the evolution
of cost-effective processing techniques
such as multibarrel molding.
Make Contact:
Bemis Mfg., (800) 558-7651, bemismfg.com
Solvay Advanced Polymers LLC, (800) 621-4557, solvayadvancedpolymers.com