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When considering the use of stack molds, especially for fast cycling products,
special measures must be planned to remove the huge amounts of products.
Depending on the shape of the product, these measures could be any of a
number of controlled ejection methods, conveyors, stackers, packing ma-
chines etc.
Figure 4.70 shows a typical cup stacker. The stacker is positioned at the rear
of the molding machine. It consists of an unscrambling vibratory conveyor
(A) that receives the randomly ejected, scrambled containers from under
the mold. The cups are then elevated to the orienting section (B) and moved
to the stacking section (C), where they are counted to the required stack
length. The stack is moved to the staging table (D). From there the stacks are
removed by hand and packed into boxes. Production, depending on product
size, ranges from 2,400 to 12,000 pieces/hour.
Figure 4.71 shows a typical lid stacker for round lids, in a range from 3 to
6.5 in. diameter. The randomly ejected lids are moved by in-press conveyors
onto a twin-disc unscrambler (A). The lids are then single-filed from each
disc and oriented ready for stacking in two lanes up the elevating conveyor
(B), to the stacking system “waterfall” (C). Next, the flow is split again and
four lanes (D) enter the spin bar stacking section (E). Stacks are cut to length
ready for transfer to the tray loading section (F). The automatic tray feeder
(G) moves the cardboard trays under the stacked lids, four stacks at a time,
and moves the loaded boxes to the unloading station (H) from where they
are hand packed into cartons.
Figure 4.72 shows a (CHSV) retrieval, transfer, and stacking system, seen
from the side, where the molding machine would be located. Arms enter
between the open 2
× 8 container stack mold. Eight suction cups face the
cores in each level. The ejected containers are held in these cups, which then
retract out of the molding area. Another array of suction cups in the
Figure 4.70 Typical cup stacker
(Courtesy: CBW)
A
B
C
D
E
F
G
H
Figure 4.71 Typical lid stacker for round lids
(Courtesy: CBW)
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4 Mold Selection
“out-side” position picks up the cups from the transfer arm. After two shots,
the receiving station rotates and stacks the cups to a predetermined number.
From there, the 16 stacks per level move on a conveyor to the packing station.
4.1.9 Semi or Fully Automatic Operation?
With extremely rare exceptions, every mold can run automatically. “Auto-
matic” in this context means that the mold cycles without an operator’s
intervention, such as opening the safety gate to remove the products, or
lubricating (spraying) the molding surfaces, and so forth. The following
reasons make it desirable for the mold to run automatically:
Safety considerations. Every time an operator needs to open the gate and
reach into the molding area, there is a safety hazard, even with all the
safety features on today’s machines. Safety features have been known to
fail and, worse, they have been deliberately tampered with or voided to
save costs to repair a problem. The best and safest policy is to have no
operator near the operating parts of the machine. Note that in general,
mold operators employed to “run” a machine are often not educated in
machine operations and are therefore more difficult to train in safe
procedures around a machine. Usually, well-trained setup personnel are
employed to install the molds and to get the machine up to production.
They are better educated and well trained on the job and are less likely to
run the machine in an unsafe condition or to condone unsafe practices.
Figure 4.72 CHSV retrieval, transfer, and
stacking system (Photo courtesy: CBW, USA)
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Quality of product. Uniformity of the quality of product depends much
on the uniformity of the cycle time. Some operators are more skilled
than others, but even with the same operator, the time to open the safety
gate, to remove the product, and to close the gate before starting the next
cycle can vary greatly, depending on the time of the day or night, the
length of time working on the machine, and is affected by fatigue and
boredom. If the product tends to hang up in the mold, or worse, if a
portion of the piece sticks in the core or the cavity and must be removed
with a tool (e.g,. with a pointed piece of brass wire), it may take a while
before the mold can be restarted. These undesired stops and variations
in the molding cycle affect the melt quality and the cooling temperatures
in the mold and can result in unequal quality of the product. Also, if
such stoppages are too long, the extruder and any plastic in the mold
may have to be purged, before the mold can be restarted.
Absenteeism. This is often a serious problem, especially with the available
labor force required to operate the molds. To be operated most eco-
nomically, molding machines should be run uninterrupted, 24 hours a
day, for at least 5 days a week. Many plants operate 7 days a week and
never stop a machine, except for mold changes and scheduled mold and
machine maintenance. The problem is that absenteeism can create labor
shortages, which are difficult to control and affect the continuous
operation of a machine
Labor cost. The elimination of machine operators is often quoted as the
reason for running fully automatic, but in fact this reason is much less
significant than the others given above. With very few exceptions, every
mold can be designed so that it will run fully automatically. The problem
is sometimes that the mold has been designed correctly to run auto-
matically, but was installed and set up poorly so that it will not eject
properly, as intended.
4.1.10 Insert Molding
There are many applications where “inserts” must be used in a mold. Here,
we will highlight a few examples (see Fig. 4.73). The real challenge is to design
these molds to run automatically.
Inserted labels at the bottom of products. There are different methods of adding
labels to the mold before it closes. Label insertion is always done automatically,
often with multi-cavity molds. They use automatic label dispensers reaching
into the molding area and placing the label into the cavity or on the core
while the mold is open for ejection
Inserted labels at the sidewalls of products. More difficult is the application of
labels on the sidewalls of containers, which are usually tapered, with little or
large draft angles. Figures 4.74 and 4.75 show a typical system for applying
labels on the side of containers. Figure 4.74 shows a view (from the rear of
the machine) of an automatic inserting mechanism, attached to a standard
Figure 4.73 Variety of plastic lids with
labels applied automatically in the molds
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4 Mold Selection
A
B
C
D
E
Figure 4.74 View (from the rear of the
machine) of an automatic inserting
mechanism (Courtesy: Hekuma)
Operator side
Fixed plate
Inserting labels
Taking out cups
2 cavities 2 cavities 4 cavities 8 cavities
Movable plate
Takeout-head
of EDAT for
cups
Inserting-head
of EDAT for
labels
Cavities arrays: cups
663890
3087
3750
Cup stacking
Pivoted chute
for manual
packaging
Horizontal
stackingstation
Taking over
labels
High speed side entry robot
HELI 1-1700
560
1420
6388
Electric cabinet
Overhead cable trunking
Label separation
Label magazine
refilling position
Label preforming
Push and wind device
(Patent depending)
Non-operator side
Label magazine
refilling position
1000
Ferromatic
K 155 D
Figure 4.75 Schematic of the system
shown in Fig. 4.74 (Courtesy: Hekuma)
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ABCD
Figure 4.76 Molded box
molding machine. The mold (A) can be seen behind the tie bars. In the
foreground, left, is the dispensing mechanism (B) for the pre-formed cup-
shaped label blanks (C). Vacuum in the transfer arm (D) picks up the inserts,
ready for the next cycle. After the mold arrives in the open position, the arm
moves the inserts into a position opposite the cavities (left) and the vacuum
changes to air pressure to push the inserts into the cavities. At the same time,
the opposite face of the transfer arm receives the finished products as they
are ejected from the cores and holds them with vacuum. When the transfer
arm arrives in the OUT position, the finished products are then air ejected
and stacked to the right (E).
Figure 4.75 schematically shows the same system as in Fig. 4.74, including
other features, which are not visible in the photo. It shows the label magazines,
the label separating, and the label pre-forming devices, and the stacking of
the finished products. The top right corner gives layouts of molds suitable
for this operation with 2, 4, 6, or 8 cavities.
Printed strips. Some molds feed printed strips through the cavity side of the
mold and cut the label as the mold closes. This too is performed in a fully
automatic mold.
Paper and plastic containers. Some products (e.g., low-cost boxes) consist of
a printed cardboard blank in a shape that, when folded, forms a box. As the
mold closes, the core folds the cardboard while pushing it into the cavity.
Plastic is then injected so that the cardboard along the open joints and corners
is sealed by narrow, thin, molded plastic. These molds run also fully auto-
matically, with a dispenser supplying the blanks to the open mold.
Figure 4.76 shows a molded box (A), consisting of printed cardboard bottom
and sides, but with a molded plastic rim (B) and plastic edges (C). Also shown
is the flat cardboard blank (D), which is fed automatically into the mold.
Metal inserts. The inserts can be loaded with independent robots or with
dispensing and inserting attachments to the mold. Unfortunately, there are
still molds for which the inserting is done by hand or with suitable loading
boards, requiring an operator at the machine.
Wire inserts. Molding plastic over electrical connectors and electrical wires is
a special industry. This is done manually by using shuttle molds with at least
two cavities (or cores), depending on the design of the product and the mold.
While one mold half with the inserts is in the molding position, an identical
mold half is outside the molding area, where an operator places the inserts
into their respective locations. This is done mostly in vertical clamp machines,
so that the shuttle can slide horizontally in and out of the molding area.
This is relatively safe, because the (sitting) operator never needs to reach
into the molding area. The molding cycle is usually fairly long and the operator
has ample time to place the inserts before the next shot is ready; the finished
products are removed when the other shuttle reaches the OUT position,
before reloading.
4.1 Selection of an Appropriate Mold
Figure 4.77 Metal insert over-molded parts
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4 Mold Selection
4.2 Summary
Up to this point, we have learned about the things to be considered by the
decision maker in order to make the proper selection of a mold, i.e., how to
look critically at the product, the importance of the expected production
and productivity, and how to select the type of mold (and molding machine)
most suitable for the job. All this applies to the technology of injection
molding as it stands today, but is also valid for any future developments in
this field. The most important principles are always common sense, simplicity
and safety, and the target to produce the best product, at the best quality and
the lowest cost.
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. with
labels applied automatically in the molds
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4 Mold Selection
A
B
C
D
E
Figure 4.74. of the product and the mold.
While one mold half with the inserts is in the molding position, an identical
mold half is outside the molding area, where an