4.1.7.2 Cavity SpacingNote that the cavity spacing in 2-plate molds for most products is usually greater than in either a 3-plate or a hot runner mold because of the space required for t
Trang 1Because of the better cooling, the, mold will cycle faster for higher
productivity
Interchangeability of components is easier to achieve and repairs are less
costly
Heat expansion is not a problem; by having the cores mounted to float a
limited amount to self-align with the cavities, regardless of any
tempera-ture difference between the plates, the size of the mold (number of
cavities) is not limited
If this design is used for molding products that can only be ejected with
air, it becomes a really simple mold, with very few mold shoe components
This method is of special advantage in stack molds, which are then much
simpler
Disadvantages of the modular mold design are:
The cavity spacing is larger than a comparable retainer plate design,
because the wall thickness of the cavity must be strong enough to
withstand the injection pressures Therefore, the mold will be larger, for
the same number of cavities and more expensive
Because the mold will be larger than a comparable mold with retainer
plate design, the cost of the mold will be somewhat higher, but this
additional cost can be easily recovered by the increased productivity and
lower maintenance cost
Modular molds cannot be used for 2-plate molds, because the cold
runners would be interrupted between the modules However, they can
be used for 3-plate molds
Today, most molds for small containers (dairy, etc.) are of modular construction
Trang 2138 4 Mold Selection
A
A B C
E
F
G
K
L
M
N
O
O
G
U P P
V
R
L T S
Figure 4.49 Modular 6-cavity mold (Courtesy: Husky)
Figure 4.50 Underside of core backing plate (Courtesy: Husky)
Figure 4.49 shows a modular 6-cavity mold for a container with core lock design The cavities (A) are set into the cavity retainer plate (B), the hot runner system
is in the plate (C) behind the cavities In front is a cavity (A) with cavity bottom (E) The core assemblies (F) are mounted with float, on top of the core backing plate (G) that is also the frame for the ejector mechanism shown in Fig 4.35 The core assembly (F) consists of the core (H), a BeCu core tip (I), the core retainer block (K), and the stripper ring (L) All water supply to the cores and the cavities is cross-drilled in the cavity retainer plate and the core backing plate Some of the water connections can be seen at the bottom (M) The three leader pins (N) are just for mold handling and to protect the cores from damage; the final alignment is
by the individual tapers (O) between cavi-ties and cores Note the numerous venting groves and channels, cavities, and stripper rings
Figure 4.50 shows the underside of core backing plate (G) in Fig 4.34 Note the substantial supports (P) under the cores and the center of the mold Note also the three guide pins (R) and bushings (S) for supporting the stripper ejector plate The stripper rings (L) are driven by four pins (T) each, which are guided in bushings (U)
in the plate (G) There is no need for a mold mounting plate The mold can be mounted directly by screws into the tapped holes of the machine platen or by clamps, entering the recesses (V)
Trang 34.1.7.2 Cavity Spacing
Note that the cavity spacing in 2-plate molds for most products is usually
greater than in either a 3-plate or a hot runner mold because of the space
required for the runners between the cavities This means that for the same
number of cavities, a somewhat larger mold shoe will be required; in addition,
more clamping force will be needed because of the additional projected area
of the runner system
Very small cavities, flat or with little depth, can often be located very close
together and can take less space than hot runner molds, even when adding
the space required for the runners But hot runner technology has
con-siderably advanced by providing drops to feed into more than one gate,
therefore the products can be spaced much closer together
4.1.7.3 Hot Runner Edge Gates
There are two other gate configurations not mentioned earlier in the section
on gates, because they are used only in hot runner systems with two or more
cavities
The hot runner edge gate (HREG) is an open hot runner gate into the sidewall
of the product The principle (and construction) of the HREG is simple and
explained in detail in [5] Gates from one drop from the manifold can feed
into just one cavity, two cavities (most frequently used), three, or even four
small cavities There are standard HREG nozzles commercially available It
is very important that the design suggestions by the manufacturers are closely
followed The system is very reliable and trouble-free, but must be used with
absolutely clean, preferably virgin, plastic material to avoid plugging the small
gates by dirt Molds with HREG are frequently used in cases where gating
into the top surface of the product is not acceptable, because the end use of
the product or appearance reasons prohibit it Typical examples are earlier
molds for Petri dish bases and covers that need top surfaces with optical
clarity, but also for clear small boxes for packaging delicate items, such as
jewelry, cosmetics, and so forth As stated above, drops can feed more than
one cavity For example, for an 8-cavity mold, rather than using a standard
hot runner system with eight drops (one per cavity), a HREG mold can be
selected with four drops only, each feeding two cavities This could represent
a considerable saving in mold and hardware cost
Note: HREG molds can be more expensive to design, build, and maintain
Today, Petri dishes are often molded using valve gates, placed near the edge
of the top surface The problem of hot runner gating small products, or where
two products are placed closely side by side, can also be solved by using hot
runner drops and nozzles as shown in Fig 4.53 This method is less expensive
than the use of hot runner edge gates, but will leave a gate vestige on top of
the product, near its edge There are also similar nozzles with three or even
four gates
Figure 4.51 Schematic of hot runner edge
gate
Figure 4.52 Typical hot runner edge gate
vestige (A) on the side, near the bottom of the product
Figure 4.53 One “drop” feeding more than
one gate This picture shows two gates, but there are also drops for 3 or 4 gates
Trang 4140 4 Mold Selection
4.1.7.4 Insulated Runner Molds
Insulated runner molds are a further development of the through shooting principle It is based on the insulating properties of plastics, as we have already seen in the through shooting nozzle (Section 4.1.6.2) Using large diameter flow channels, in the order of 16–19 mm diameter, the plastic layers closer to the walls will freeze, but the center of about 5–10 mm diameter remains molten to allow an effective flow of hot plastic toward the gates
While the principle of operation is simple (Fig 4.54), the start-up of an insulated runner mold needs a certain amount of skill At the first shot into the empty, clean mold, the sprue (d) and the rather large runner (e) are filled, together with some or all of the cavity space If the first shot is insufficient to fill all cavities, after quickly removing the incomplete shot, a second shot immediately following will be usually sufficient to have the whole system filled After removing this second, by now probably complete shot, the mold is ready for production If not, a third shot may be required The main problem with insulated runner systems is the startup of the mold: after the first injection that usually does not fill the runner and all the cavities immediate action is of the essence The incomplete first shot must be removed
if it has not properly ejected, and the mold must be started again quickly so that the plastic in the sprue and runner does not have enough time to freeze
If the plastic does freeze, the mold must be opened between the plates (a) and (c) to remove the plastic in the runner; after closing and locking these plates together, the startup will be repeated By then, the plates have warmed
up a bit and will make the following startup shot(s) easier Preferably, the cooling water is turned off during startup Once the mold can run on cycle automatically, the molding conditions (pressures, temperatures, and times) can be adjusted for best productivity With PS, the time frame available is approx 15 s, for PP and PE it is approx 30 s, before the runner will freeze The main requirement for successfully running these molds is that the ejection method must be absolutely reliable to avoid any delays due to failure to eject Once the mold runs on cycle, it could run without stopping until the production run is completed Molds for 2, 3, 4, 6, 8, and even 12, or 16 cavities can be quite easily made and operated A typical example is a 16-cavity mold for PE chair leg protectors that runs on a 25 s cycle; once started, the molder did not stop it at all for two consecutive years This is not necessarily practical with other molds that need interruptions (time for maintenance), but it shows how well these molds can run
The advantages of this system are
Very simple and inexpensive construction
There is no need for any heaters in the system, although there are variants
to the “true” insulated runner mold where pointed nozzle heaters inside the drop keep the gates from freezing This is used with plastics such as
PS that tend to freeze easier because of their better heat conductivity Both Fig 4.55 and 4.56 show the use of such heater probes with insulated runners
Figure 4.54 Schematic of cross section
through a two-cavity insulated runner mold;
(a) cavity plate; (b) core plate; (c) runner
plate; (d) sprue; (e) insulated runner; (f ) gate
A B
C D
Figure 4.55 Section through a 2-cavity
insulated runner “slug”
D
E
A B C
Figure 4.56 Partial section through an
8-cavity insulated runner “slug”
Trang 5Figure 4.55 shows a section through insulated runner “slug” of a 2-cavity
mold for nylon gears (A) Note the flow of the last injection (blue, B) inside
the white, older plastic (C) Note also the cylindrical hollows (D) where
electric heated “torpedoes” keep the melt hot in each drop, especially with its
pointed torpedo tip extending right into the gate Such heated torpedoes are
suggested for all molds requiring longer cycles and with plastics other than
PP or PE
Figure 4.56 shows the removed insulated runner of an 8-cavity mold for PVC
products (A) The cut-open section of one runner branch clearly depicts the
flow of the dark plastic (B) within the surrounding, colder plastic (C) For
illustration purposes, the color of the plastic was changed from white to
dark Note the pockets (D), where the heated torpedoes are located (see also
note with Fig 4.55)
Color changes are easy; there are two methods:
1 Stop the mold, remove the runner, close the now clean mold, and restart
with fresh new color after the extruder has been purged This takes some
time and effort
2 An even easier method is to remove the old color plastic from the machine
hopper but to continue to run the extruder while the machine is
pro-ducing pieces of the old color When the new color is fed into the hopper;
it will gradually replace the old color in the extruder and after 15–20
shots products with the clean new color should be ready The production
of the shots during the changeover will have a color mix and will have to
be scrapped, unless the plastic can be reground and used where the color
mix does not matter The runners inside the mold will change their hot
inside core (where the plastic flows) to the new color, while the frozen
plastic near the outside of the runners is still of the old color This can be
clearly seen in the two photos above
Note that with conventional hot runners, color changes are always done by
gradually purging through the manifold while the mold is producing A good
hot runner design will permit changing from a lighter to a darker color in
about 50 shots, and longer when changing from a darker to a lighter color
Safety Considerations with Insulated Runner Molds
With all these substantial advantages of insulated runner molds, why are
they not used more frequently?
As these molds are built today, they are inherently unsafe as will be explained
below, and with the rapid development of reliable hot runner systems and
the associated standard hardware, the insulated hot runners have been put
on the sidelines and have been either completely forgotten or are being
avoided
Any “regular” mold can be operated from the operator’s side of the machines,
without the need to open a safety gate, except during startup when the front
Insulated runner molds are ideal for molds requiring multiple color changes a day
Insulated runner molds seem to be ideal – so why are they not used more frequently?
The reason is that these molds need special skills for start-up and are inherently unsafe, as explained in the text