Tài liệu Mold Selection P5 doc

This mold is built for a machine specially equipped for stack molds.. This mold is built for a machine specially equipped for stack molds.. Notable advantages of stack molds are as follo

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

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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

Figure 4.64 A 2 × 8 stack mold for containers

(Courtesy: Husky)

E

A

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

A B C D

E F

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

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

A

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

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!

D E

F

Figure 4.69 A 4 × 8 stack mold for containers