hot runners, mixed materials molding
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1
HOT
R
UNNERS
T
HE
K
EY
TO
M
ULTIMATERIAL
A
PPLICATIONS
Multimaterial molding has been familiar to
automotive molders for years. Machines with up
to four injection units can be found throughout the
car lens industry, adding complexity to both mold
and machine. The technology has helped to
increase productivity by reducing assembly time
of multicolored parts. The standard method for
molding multicolored car lenses has pioneered the
multimaterial molding process, extending itsapplication uses to a wide range of other markets.
Multimaterial molding has emerged as a way to
give consumer and industrial products a distinct
look, feel, or functionality. For example, a
multimaterial solution to the ink wearing off your
computer keys might be to mold the letters right into
the plastic keys, using a different plastic material. In
the automotive industry, PA and TPE combinations
have become very common because they eliminate
costly and complicated assembly steps.
The molds and machines for such applications
are more complex than are those for single-
material molds; however, the cost saving achieved
by consolidating or eliminating assembly
procedures makes the technology economical.
In multimaterial applications, parts must either
be transferred from one cavity to the other or core-
back movements (see the following example) open
up cavity space for the material following the
initial injection. Technologies used for cavity
transfer include robots and mold- or machine-
mounted turntables. A new technology for cavity
transfer was introduced at the K-show, where
cavities rotate on a vertical axis in the mold (i.e.,
as opposed to the usual horizontal turning axis).
The cavities can rotate faster due to the lower mass
moved. Core-back technology is typically not ascomplex, but there are limitations to part
geometries that can utilize such technology.
Multimaterial molding would be much more
difficult without the use of hot runner technology.
The use of cold and insulated runners is limited.
Part transfer is simplified if a robot does not have
to deal with a cold runner because parts need to be
moved from one cavity to the next at the same
time. Three plate molds cannot be used to eject
two or more separate cold runners from thestationary half of the mold. In other instances the
melt channels coming from the injection units
may overlap on the way to the cavity gate,
requiring one manifold to host multiple channel
systems for various colors.
Two examples of multimaterial hot runner
technology will now be explored.
A
DVANCED
T
ECHNOLOGIES
M
ARTIN
B
AUMANN
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Society of Plastics Engineers
2
M
ULTICOLOR
A
PPLICATION
C
OSMETIC
P
ART
(12 + 12 C
AVITY
M
OLD
)
A shampoo company sought to differentiate its
bottle from the competition by using two different
colors for the base and the lid of the closure
(Figure 1). The shape of the closure and the risk of
flaking ruled out painting or stickering the parts
after molding. In addition, the parts production
needed to be as cost effective as possible, which
meant keeping mold and machine costs to a
minimum.
Two hot runner manifoldsa 12 drop and a
six drop (Figure 2)had to fit into a tight mold
frame and provide balanced part filling. A vertical
injection unit provided the material for the lid
[polypropylene (PP), 3.1 g], and a centralhorizontal injection unit provided the material for
the base (PP, 8.4 g). A computer simulation
provided the necessary input for the channel
layout, considering optimum diameters for
balanced filling and fast color changes. To achieve
a balanced flow, the outer manifold (12 drop)
features four melt channel level changes and five
different melt channel diameters. The inner, less-
complicated manifold (six drop) has one level
change and four different melt channel sizes. Such
effort is necessary to achieve balanced filling.
Cost optimization was achieved by utilizing
core-back technology. A small moveable core in
the hinge area of the base cavity retracts after the
base material is injected and cooled. The retract
core provides the necessary interface between
base and lid cavity. The lid material is then
injected directly via a hot tip nozzle. This simple
approach avoided the use of a rotary table or a
robot to move the base in order to mold the lid
onto it. As a final step, the two-colored closures
are closed prior to part ejection via a mechanicalcam and are ready for assembly onto the bottle.
M
ULTIMATERIAL
A
PPLICATION
F
ASTENER
PATPE(4 + 4 C
AVITY
M
OLD
)
Two hot runners in On top of the other
formation (Figure 3) were required to mold a
small automotive fastener. The part is made up of
two very different materials. A polyamide (PA)
base provides the necessary structural integrity for
the part as well as the fastening function.
Figure 1. Shampoo closure.
Figure 2. Shampoo closure hot runner. Figure 3. On top of the other formation.
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Advanced Technologies
3
Thermoplastic elastomer (TPE) material is
molded on to it to provide a waterproof seal. The
4.5-g PA base is directly gated via a hot tip
nozzle. The TPE seal is gated via two valve-gated
nozzles and a short cold runner. Because of the
close pitch spacing (
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Society of Plastics Engineers
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Is the Machine Stack Ready?
A stack mold can virtually double the output
of an injection molding machine. The hot runner is
one of the key technologies for all stack
applications. It is responsible for delivering the
melt to the center section of the mold. The
importance of having a stack-ready, optimizedmachine for stack applications is often
overlooked; not every machine is built to accept a
two- or four-level stack mold. The injection
molding machine must be able to accept a mold
that typically has twice the weight and shut height
of a single-face mold that is molding the same
parts. To minimize mold wear, special attention
has to be given to the robustness of the machine.
There are also a variety of options available for
driving and supporting the center section with the
hot runner of the mold.
There are Some Items to Consider:
Clamp Opening.
Stack mold systems with
two mold openings require approximately
double the shut height and opening stroke
relative to single-face applications.
Injection Capability.
The machine must
provide double the shot weight, plasticating
ability, and injection rate than that required
for a comparable, singe-face mold.
Tie Bar Support for Center Section.
Machine tie bars must provide sufficientsupport for the hot runner and cavity plates
as well as accurate, repeatable alignment
during mold open and close to prevent
damage to cores and cavities.
Sprue Break Capability. The injection unit
of the machine must retract from the sprue
bar enough to clear it in order to prevent
collision and possible damage between the
injection nozzle and the sprue bar upon
mold close.
Injection Unit Shut-off Nozzle
. This
allows
recovery and prevents resin drool while the
mold is open. Although it is possible to run
stack molds without it, the shut-off nozzle
will help improve cycle time (recommended).
A V
ARIETY
OF
C
HOICES
FOR
S
TACK
M
OLDING
N
EEDS
Standard Stack Hot Runner
The standard stack hot runner features a center
sprue bar for melt delivery to the center section.
The parts produced on both the injection and
clamp sides of the mold are the same resin and
part weight. These systems come available as two-
level molds (two mold faces) or four-level molds
Figure 5. Harmonic linkage (4 16 stack).
Figure 6. Harmonic linkage provides precisewear-free center section movement.
Figure 7. Tie bar support assures minimizeddeflection, decreasing potential mold wear.
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Advanced Technologies
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(four mold faces). A rugged center section
support is key to minimizing wear on cavity and
core shut-offs. The guided sprue bar features an
antidrool bushing to reduce drool. On systems
like this, parts typically freefall from the mold.
Some application examples are: closures,
packaging, and housewares.
Offset Stack Hot Runner
On an offset stack hot runner, the sprue bar isoffset to the side of the system. Offsetting the
sprue bar allows plastic to be fed into the hot
runner at a location other than the center. This is
ideal for large, flat parts. Robot access is
simplified because the offset sprue bar can be
placed where it will not interfere.
Some application examples are: washing
machine components and large houseware
containers.
Family Stack Hot Runner
These specialty stack systems are used to
make related parts in the same mold (e.g., to
produce a lid and base). The challenge with a
family mold is the balancing of the two different
parts for equal filling, which is critical to
producing quality parts. Cavity filling studies are
applied to determine proper filling.
Some application examples are: part
assemblies like the one shown in Figure 3.
Stack Platen
The stack platen is designed to run two
identical, independent, single-face molds in a
single press, thereby doubling the production of the
machine. The attraction for molders is that it allows
Figure 8. Standard stack hot runner
showing center sprue bar.
Figure 9. Stack hotrunner with offset
sprue bar.
Figure 10.Flow pattern for
family stackhot runner.
Figure 11. Stack platen.
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Society of Plastics Engineers
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the existing single-face molds to operate in a stack
configuration. This approach permits flexibility
because the molds could also be run independently.
Another benefit of the stack platen is that molds for
different parts can be used in the same carrier aslong as they have similar projected areas.
Some application examples are: any in which
productivity needs to be increased on existing
injection molding machines.
Stack Carrier
The stack mold carrier rides on the machine
bed and has the ability to support very heavy
molds. Because the linkage uses bearings instead
of gears, wear is minimal. This technologyeliminates the need for individual, mold-mounted,
center-section supports. The stack mold carrier
facilitates quick mold changes and automatic part
removal because the mold can be lowered in from
the top and bolted on.
Some application examples are: very heavy
stack molds that require a lot of center section
support.
Figure 12. Stack mold carrier.
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