Tài liệu Mold Selection P4 docx

The stack mold requires only one rather than two molding machines, provided the shot- and plasticizing capacity of the machine can provide for the increased number of products made on th

gate must be opened to access the mold for servicing The rear gate is usually either screwed locked, or does not have safety features such as drop bars Opening the rear gate often stops the operation of the machine completely, even shutting down the hydraulic pumps Over the years, with these safety features in molding operations, the number of personal injury-accidents has greatly decreased

Referring back the Fig 4.54, plates (a) and (c) must be locked together solidly even though they are clamped together during injection by the clamping force of the machine If plastic is injected (inadvertently or deliberately) while the clamp is open, these plates could see a large separating force; the plastic could escape through a gap between these two plates and spray into the open, causing injury to bystanders With any such molds, to ensure that these plates are held together securely, pairs of large, solid screws or clamps are provided

on the operator’s side of the mold and at the rear of the mold To pull the plates (a) and (c) apart is relatively easy; after unlocking them, cavity plate (a) can be latched to the core plate (b); the clamp is then opened so that plate (a) moves with the core plate (b)

As seen above, for startup of an insulated runner mold it is necessary to open the mold between plates (a) and (c), remove the runner, re-close and lock the plates together, and restart after closing the safety gates This presents

a serious problem Since the screws or clamps are on both sides of the mold and this whole “operation” of unlocking, opening, cleaning, and re-locking must be accomplished within about 15 seconds; therefore, operators are required on each side of the machine This can be very unsafe, unless other safety measures for the rear gate than the ones mentioned above are provided There is always a risk of having more than one operator starting up any machine, but here, they are also being rushed, while only one of the operators

is at the controls of the machine

The challenge is to find a method to allow locking the two plates (a) and (c) together from the operator’s side only, with only one operator used for startup Once a method of safe operating conditions is created, this very economical system can be used for many applications

4.1.8 Single- or Multi-Level Molds?

“Stack” (multi-level) molds have a long history Back in the 1950s, a stack mold was used for making matched color toilet seats and covers This was an ideal application: both pieces are very heavy-walled and require similar, very long cooling times In addition, the projected area of each piece was about the same and the color match is (inherently) perfect

Later, stack molds were used for smaller products, using elaborate systems of hot nozzles and cold runners; today, mostly thin-walled products are made

on stack molds and practically every stack mold uses a hot runner system The cycle times are often in the order of 5–8 s and the output of these molds

For the same product, and the same number of cavities per level, a stack mold essentially doubles the output

of single level mold The stack mold requires only one rather than two molding machines, provided the shot- and plasticizing capacity of the machine can provide for the increased number of products made

on the stack mold

A

Figure 4.57 A 2 × 1 cavity stack mold for a large container

can be enormous The theory and design features of stack molds are explained

in much detail in [5] Here, we are more concerned with why and when to

use them Understanding the following will allow the decision maker to make

the right choice between single- and two-, or more-level molds

4.1.8.1 Two-Level Stack Molds for High Production

A 2-level stack mold consists of two essentially identical, conventional molds,

placed back to back, but with one, common hot runner system feeding the

two cavity plates The two core sections are mounted on the stationary and

moving platen, respectively The center section of the mold, also called the

“floating” mold section, (two cavity plates and the hot runner assembly in

between them) is supported (directly or indirectly) on the machine base or

on the tie bars It is moved, usually half the length of the machine stroke, by

levers, gears, or other mechanical methods in synchronization with the clamp

motion Alternatively, hydraulic cylinders could move the center section,

independently of the clamp motion The core half on the moving platen can

be exactly like it would for a single-level mold and can use the existing

machine ejection mechanism The core half mounted on the stationary platen

presents two problems:

(1) In most stack molds, the sprue supplying the center hot runner system

passes through the center of the (stationary) core half, so that there must be

a passage large enough for a long, heated sprue bushing to move through so

that the cavity stacks closest to the center may have to be spaced farther apart

With the above mentioned toilet seat and cover mold, this problem was

resolved by placing the open gap of the horseshoe-shaped seat on the side

nearer the injection side of the machine, so that the sprue bar could pass

through it and the seat could fall freely without hanging up on the sprue bar

Figure 4.57 shows a 2 × 1 cavity stack mold for a large container This mold

is built for a machine equipped with the actuating mechanism and with

special supports for the floating center section Note the sprue bar (A) of the

hot runner system (only partly shown) is located at the side of the cavity to

engage in a special, offset hot runner manifold behind the core plate near

the stationary platen (not visible in this photo)

4.1 Selection of an Appropriate Mold

If the sprue bar cannot be in the center of the mold, as, e.g., in Fig 4.57, the solution is to relocate the sprue bar outside the molding area This requires

an additional hot runner system in the mold half, located on the stationary platen The machine nozzle enters at the usual center of the mold into the (additional) hot runner manifold leading to outside of the molding area where the long sprue can supply the melt to the center section from outside the molding area In this arrangement, the main hot runner system is supplied near its end instead of the usual point in the center of the manifold This method will allow molding large products even on that side of the mold where the sprue bar is located This method is also useful for example for a

2 × 2-cavity mold or a 2 × 6-cavity mold, with the center cavities arranged

one above the other, for free-fall ejection, so that the upper products will not

hit the hot sprue bar when falling down This off-set sprue bar arrangement

is also useful when extracting the molded products with robots, should more clearance for the path of the robot arm be required

Injection-molding machines, as a rule, don’t have ejection systems on the in-jection side; actuators must be added either to the core section located on the stationary platen, or to the stationary machine platen The ejection system can also be mechanically linked to the moving platens or to the operating mecha-nism moving the floating section This is sometimes done (very crudely) by connecting the ejector mechanism (stripper plate or ejector plate) with chains

or (somewhat better) with lost-motion links to the center section of the mold

A better solution is to connect the ejector mechanism with links to the “pro-peller” that actuates the center section One disadvantage of these methods is that the machine stroke must be closely controlled to avoid damage to the links

In addition, the ejection takes place only close to the end of the opening stroke, and MO time is necessary to allow the products to clear the molding area

A newer design avoids these problems Figure 4.59 shows a stack mold for lids The profile of the rocker arm (A) is designed so that, as soon as the roller (B) on the cavity-side is engaged, the shorter arm of the rocker pushes

on roller (C) to move the stripper plate (D) forward to eject This can take place early during the opening stroke so that the products have enough time

to clear the molding area before the mold closes again Usually, no MO time

is required and the mold will cycle faster, for higher production Note that springs (not visible) inside the core plate return the stripper plate as soon as the rocker (A) leaves the roller (B) This method also makes the mold independent of an accurate opening stroke of the machine, because there are no fixed links between the mold sections The rocker arm system is also less expensive (by about $10,000) than an independent ejection system on the mold half mounted on the injection side of the machine

The ideal solution (where possible) is to use only air ejection from the cores, requiring no mechanical ejection system at all; but this only is possible with certain product shapes and plastics, see [5]

Figure 4.60 shows an early design (year 1975) of a self contained stack mold with center section supported and aligned on the upper tie bar (A), actuated

by levers (B) and (C)

Figure 4.58 Typical air eject stack mold

(2 × 4) for a round container

(Courtesy: Topgrade Molds)

stack mold for threaded lids The lids are

stripped off the cores The mold is equipped

with rocker arms (A) in each corner of core plate and the matching rollers (B) and (C) for the actuation of the arms and motion of the stripper plate Note the BeCu core caps (E); random ejection; product mass: 14 g;

cycle time: 11.0 s; productivity: 21,000 pieces per hour

A

B

C C

Figure 4.60 Self contained stack mold

(Courtesy: Husky)

4.1 Selection of an Appropriate Mold

Figure 4.61 shows a recent self-contained 2 × 4 stack mold for dairy containers (modular construction) It features special supports for the center section (A) and rack and pinion actuation (B) for its motion The mold is equipped with servomotors (C) and suction cups (D) to remove the air-ejected containers The mold runs without MO time, at a 2.75 s cycle, for a pro-ductivity of 10,473 pieces/h

Figure 4.62 shows a recent self-contained 2 × 8 stack mold (modular con-struction) for fairly heavy (55 g) stadium cups The mold features rack and pinion actuation (A), air ejection, random free-fall Note the center sprue (B) and how the machine tie bars (C) are supported (D) on the base to take the mass of the heavy center section (E) The mold has a cycle time of 10 s, for a productivity of 5,760 cups/h

A B

C

D E

Figure 4.61 Self-contained 2 × 4 stack mold

for dairy containers

A

B

C D

E

Figure 4.62 Self-contained 2 × 8 stack mold

(modular construction) (Courtesy: Husky)