Increasingly, injection molding is being used as an assembly
system in which components are combined without use of adhesives or other
fastening methods.
Most often, this concept is referred to as in-mold assembly
in which more than one injection barrel is used to form two or more materials
in an injection mold that rotates or indexes to form different parts of a
component. It's an assembly operation because it eliminates a secondary
operation of welding, or adhesive gluing, or some other fastening method.
One example is a mold called the twin cube, which is a mold
comprising two rotating cubes between its two mold halves. They function like
two different molds. Different materials can be injected into each cube, and
different processes take place in each cube, eliminating the need for several
assembly operations that would normally occur following the molding cycle.
"Where previously two machines, two molds and assembly
facilities were required, now one twin cube machine equipped with new, patented
twin cube stack turning mold does the job," says CEO Rainer Armbruster of Foboha, a German mold
manufacturing company that is now part of Advaltech.
There are now 12 different twin-cube systems operating,
including one with three injection machines. "We have a couple of applications
that are round flip-top disc-top closures in the US, and we also have a much
larger oval disc top closure system," says David Nolan, a US-based sales
engineer for Foboha. "We are running an automotive speedometer needle-and hub assembly
in Europe.
The disc-top closures are molded in two colors and are used
for body care product packaging like shampoo, shower gel, or body lotions.
Assembly operations in the process can include placement of inserts, labels, or
decorative foils, as well as inspection or part removal without any increase to
the overall cycle time.
Snapped Together
Two components are snapped together automatically in the
middle parting line between the two twin cubes. In-mold assembly is achieved by
a precise core pull movement-not by the closing movement of the machine, as it is
sometimes done. The exact positioning of the parts is done by the mold. This
step eliminates a secondary assembly operation.
Each cube has 4 x 48 cavities. In one of the cubes the body
is molded, and in the other the cover of the polypropylene closure is molded.
The substrate material is produced in the first parting line
and the second component is injected after a 180 degree rotation of the center
plate on a vertical axis. As the second material is being injected, the
substrate for the next shot takes place.. In addition to incorporating assembly
steps, the turning mold technology provides double the cavitation, and double
the productivity, with one molding machine.
Viewed another way, the twin cube approach provides the same production
quantities with half the clamping force. Cost of the twin cube tooling is up to $1 million.
The next step will be a four-sided cube mold with 90 degree
rotations.
"We see tremendous
potential in the packaging industry for high volume production of ‘convenience
packaging' articles," says Jörg Dassow, head of applications engineering at Ferromatik Milacron, a machinery OEM that
works on in-mold assembly projects. "Consider the push-pull closures which are
replacing conventional screw-top caps for sports drinks, or the many types of
closure mechanisms used for body care or cleaning products." To reinforce the
point, Ferromatik Milacron, owned by Milacron of Cincinnati, OH, established an
Applications & Systems Business Unit to develop more business that moves
assembly operations into the molding process.
Several other mold builders, such as MGS Mfg. Group, are teaming with machinery
builders, such as Engel, to also
develop systems that can dramatically cut costs and improve component
part-to-part repeatability. The big current buzz among machinery OEMs at events
such as Fakuma in Germany
is about energy savings because of the rapid rise in hydrocarbon prices this
year. But the long-term technology that will really impact productivity
undoubtedly is this new emphasis on in-mold assembly.
At the
Fakuma exhibition in German this month, Arburg
demonstrated electric machines with
a high-speed in-mold labelling (IML) application. IML eliminates the requirement to adhesively
adhere packaging labels in a secondary operation. Six yogurt pots are produced-with
labels- in an overall cycle time of four seconds. In order to meet the
requirements, the Arburg machine is equipped with integrated hot runner
control, a high-performance plasticizing cylinder and a mold featuring a
pneumatic needle shut-off system. The complete IML automation equipment - feed,
separation and placement of the labels, as well as removal and cavity-specific
stacking of the pots - are provided by Waldorf Technik.
Metal Matrix Composites
A different approach to an assembly technology is a metal
matrix composite developed by CPS Technologies
of Norton, MA, in which a silicon
carbide powder is molded as a net-shape porous component that is subsequently
infiltrated with aluminum for use in packaging and electronic thermal
management applications.
The aluminum-silicon carbide (AlSiC) assembly competes
against traditional thermal management hermetic packaging materials, such as copper
paired with molybdenum (CuMo) and copper/
tungsten (CuW). Components made from
silicon carbide infused with aluminum have a significantly lower density, and
are 1/5 to 1/6 lighter in weight than CuMo and CuW, respectively, yet have
similar thermal conductivity and expansion coefficient (TCE). AlSiC is also lower in cost.
Another cost element of CuMo and CuW is the need to machine
all but the simplest shapes. AlSiC
designs can be cast to the final shape, requiring no secondary machining - a
significant cost improvement. The
net-shape casting capability also allows the direct creation of functional
design features, such as pockets for circulators as well as cavities or
pedestals for the die.
Mark A. Occhionero, vice president of marketing and
technical sales at CPS, likens the final structure to a molded Jell-O with
fruit. "It's essentially like the grapes
being the silicon carbide and the Jell-O being the aluminum. In typical
applications the silicon carbide is at 63 percent (by volume) and the aluminum
is at 37%."
CPS competes against Japanese OEMs such as Sumitomo and
Denka for three types of applications: hermetic packaging, based plates for
high power silicon switching devices, and application specific integrated
circuits.
"What's important about our materials is that in comparison to
competitive materials, they have very good controlled thermal expansion, they
have a reasonable thermal conductivity, but most importantly they are
lightweight," says Occhionero. They have
the weight of aluminum and the strength and stiffness of steel."
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