147
A stack mold can make identical products in both levels or have stacks for
matching products in each level. This offers the possibility for perfect color
match, for assembling the products inside or near the mold, and/or for
controlled ejection for subsequent assembly or other post-molding opera-
tions, as explained in Section 4.1.2.2.
Figure 4.63 shows a 2
× 4 family stack mold for two different rectangular
tubs (modular construction). Air ejection and cam-operated actuators with
suction cups (A) remove the products and drop them into chutes (B) on the
side of the center section. This mold is built for a machine specially equipped
for stack molds. On the side of the center section (C) are provisions (D) to
mount the center section to the machine supports. Cycle time 6 s; no MO
time; productivity: 4,800 tubs/h.
Figure 4.64 shows a 2
× 8 stack mold for containers (modular construction).
Air ejection and cam-operated actuators with suction cups (A) remove the
products and drop them into chutes (B) on the side of the center section.
This mold is built for a machine specially equipped for stack molds. The side
of the center section has provisions to mount the center section to the machine
supports. Cycle time 6 s; no MO time; productivity 9,600 pieces/h.
Notable advantages of stack molds are as follows:
For the same output there is only about one half of the plant space
required, compared with two machines making the same product on
single-level molds.
The same clamp size can be used for twice the number of cavities. In fact,
it is required to have about 10% more clamping force than for a similar,
one-level mold, as explained [5]. However, it is important that the
machine has at least twice the shot capacity than that required for a single
C
A
B
B
D
Figure 4.63 A 2 × 4 family stack mold
for two different rectangular tubs
(Courtesy: Husky)
4.1 Selection of an Appropriate Mold
1281han04.pmd 28.11.2005, 11:14147
Previous Page
148
4 Mold Selection
level mold for the same product. If not, the mold cannot be filled. Also,
the plasticizing capacity must be large enough to supply at least twice the
amount of plastic.
The mold usually costs approx. 1.8 times of two single-level molds.
There is less handling of materials and products, because there is only
one point of production instead of two; also, fewer services are required.
Disadvantages of stack molds include:
Any breakdown of mold or machine can have twice the effect on
production. If one of two single-level molds stops producing, there will
be at least the output from the other mold.
The molding machine may have to be modified for longer tie bars, more
stroke, to provide good support for the tie bars and the floating center
section of the mold, and more travel to retract the injection unit. It may
even need a larger injection unit. The safety gates protecting the point
where the machine nozzle sits on the sprue bushing may have to be
extended to protect bystanders.
Sometimes, more heat controls are needed, because now there is the
equivalent of two molds controlled from one control panel.
When planning ahead for new installations with the intention of using more
stack molds in the future, we must consider two possibilities.
1. A planned stack mold can be a complete (self-contained) mold, with the
supports for the floating (center) section and the actuating mechanisms
(racks and pinion, or levers) as part of the mold. This mold can be
operated in any suitable machine, but is more expensive because of the
A
B
B
Figure 4.64 A 2 × 8 stack mold for containers
(Courtesy: Husky)
1281han04.pmd 28.11.2005, 11:14148
149
E
ABCD
added components; at today’s rates (year 2005) approx. US$15,000 per
stack mold.
There is a safety concern, which must be considered. While the floating
plate can sometimes be well supported during operation with (especially
long and strong) mold leader pins, there is the danger of dropping the
center plate, if it is not safely secured during handling and installation. It
is better to have the usually heavy mass of the center section independently
supported on the machine frame (using the well supported tie bars or
ways on the machine bed) to ensure alignment with the other mold plates.
2. The supporting and operating components (standard accessories) can
be added to the molding machine as a one-time machine investment of
about US$25,000. The cost of these accessories is readily recovered after
the second mold used in such a modified machine. The following figures
show how much simpler the mold can be if the machine is already
equipped with all the necessary hardware to support and move the center
section of the mold.
Figure 4.65 shows a 2
× 2 stack mold for large flower pot trays, installed in a
machine modified for use with two-level stack molds. The machine features
random air-ejection. Carrier (A) for center section (B) glides on lower tie bars
(C), which, in turn, are well supported (D) on the rigid base. This machine
uses a different design of rack and pinion drive (E) for moving the center
section. The racks are part of the machine. The pinions are hidden, above
left, behind the safety gate. Cycle time 8 s; productivity: 1,800 trays/hour.
Figure 4.65 A 2 × 2 stack mold for flower
pot trays (Courtesy: Husky)
4.1 Selection of an Appropriate Mold
1281han04.pmd 28.11.2005, 11:14149
150
4 Mold Selection
ABCDEF
Figure 4.66 A 2 × 12 stack mold for
stadium cups (Courtesy: Husky)
Figure 4.67 Typical growth pattern of
productivity of molds and machines
Figure 4.66 shows a 2 × 12 stack mold for heavy containers, installed in a
machine equipped for 2-level stack molds. The mold features random air-
ejection. Carrier (A) for center section (B) glides on tie bars (C), which are
well supported (D) on the rigid base. This machine uses “propellers” (E) and
levers (F) for the actuation of the center section of the mold. The levers are
part of the machine. Cycle time 10–12 s; productivity: 7,200–8,640 pieces/h.
Figure 4.67 shows a bar chart for a typical growth pattern of productivity of
molds and machines. The increase is the result of the development of better
molds and better performing machines used for this product, a “409” (4–9/
16 in. diameter) series container. Note that this is the productivity of one
machine and that the footprint of the 1,000-ton machine is only about
1.5 times that of the 225-ton machine.
One machine today can produce the
output of 7–10 machines that are 30
years old
1281han04.pmd 28.11.2005, 11:14150
151
4.1.8.2 Multi-Level Molds for Very High Production Rates
Theoretically, there is no limit to the number of levels. In compression
molding of thermosets, the use of up to ten levels is old technology and
quite common, e.g., for rubber floor mats.
Figure 4.68 shows a 4
× 8 stack mold for lids, installed in a machine equipped
for 4 level stack molds with random air-ejection. The (standard) small
propellers (A) and links (B) are part of the mold; the large propellers (C)
and links (D) are part of the machine. Cycle time 5.5 s, productivity: almost
21,000 lids/h.
For practical reasons, the limit in injection molding today is four levels, mainly
because of the size (the length) of the mold and machine required for multiple
stacked levels, but also because of the required output and injection capacity
of the injection unit. Dozens of four-level molds have been built and are in
production since the early 1990s.
The following illustrations are of 4-level molds built for machines specially
equipped for these molds. Note that the red colored parts are parts of the
machine, not the mold, and can be used for other molds on the machine.
Figure 4.69 shows a 4
× 8 stack mold for containers, installed on a machine
equipped for 4 level stack molds with air ejection and swing chute design:
Servomotors (A) move swing arms (B) with suction cups over the cores to
receive and remove the cups, which are then dropped into the chutes (C)
when the mold is closed. The actuation of this mold is of a different design,
with one propeller (D) and different levers (E) and (F) for the motion of the
three moving sections. All red parts are part of the machine. Cycle time:
4.7 s; productivity: 24,500 containers/h.
ABCD
Figure 4.68 A 4 × 8 stack mold for lids
(Courtesy: Husky)
For the same product and the same
number of cavities per level, a 4-level
stack mold essentially quadruples
the output of single-level mold.
The stack mold requires only one
rather than four molding machines.
A specially built machine provides
the required high shot and
plasticizing capacity
4.1 Selection of an Appropriate Mold
1281han04.pmd 28.11.2005, 11:14151
152
4 Mold Selection
4.1.8.3 Provisions to Remove Molded Products
This is an area often overlooked when planning for a new and better
production method. Even when molding with single-level molds, handling
the quantity of products coming from the mold can become a serious
problem. I remember in the 1960s, the first 2-cavity disposable drinking cup
mold running at 30 shots per minute. Because of the large number of cups
falling out of the mold (60 cups per minute) we had to provide conveyors
out and away from the molding area, toward automatic stackers. If this
handling system stopped for any reason, the molding machine was literally
covered with cups within minutes and had to be stopped to prevent damage
to mold and machine.
The problem is: “What to do with the molded products?” This problem must
be solved before even considering any high-production mold. How will we
get rid of the products in time to clear the molding area?
Example 4.1
A 2 × 4 stack mold for a margarine tub will run at a 5 s cycle and produce
2
× 4 · 60 s/min ÷ 5 s = 96 tubs per minute. Even when properly stacked,
they represent quite a volume, but being ejected at random will create
quite a mountain. One of the 4-level molds used was for a PE lid of 150 mm
(6 in.) diameter with 8 cavities per level, at a 5.5 s cycle, it yielded
4
× 8 · 60 ÷ 5.5 = 349 lids per minute, or 21,000 per hour!
AB C
DEF
Figure 4.69 A 4 × 8 stack mold for containers
1281han04.pmd 28.11.2005, 11:14152
Next Page
. 2 × 2 stack mold for flower
pot trays (Courtesy: Husky)
4.1 Selection of an Appropriate Mold
1281han04.pmd 28.11.2005, 11:14149
150
4 Mold Selection
ABCDEF
Figure. and
plasticizing capacity
4.1 Selection of an Appropriate Mold
1281han04.pmd 28.11.2005, 11:14151
152
4 Mold Selection
4.1.8.3 Provisions to Remove Molded Products
This