Tài liệu Mold Selection P2 docx

When injecting into single cavity molds, there is usually no problem, because the runner system the sprue is relatively small compared to the mass of the product; the plastic inside the

Self-degating is somewhat more expensive than edge gating and while an

insert for the gate area adds some cost, it may be necessary for long running

molds to avoid expensive repairs later The somewhat higher mold cost may

be well worth it in the long run There is a number of different designs for

tunnel gates [5] The size of the tunnel gate is determined the same way as

the size for any other gate; however, there is a limit to the size that the can be

sheared off cleanly If the gate is too large, hard and brittle plastics may shear

poorly (very rough and uneven) and can easily damage the cavity wall where

the gate is located

Submarine gating is another method of self-degating, if the piece is shallow

and no gate vestige is permitted either on top or on the sides [5] Submarine

gates are more difficult to build than tunnel gates and are therefore more

expensive to manufacture; the mold will also cycle slower The vestige is similar

to that of a 3-plate mold gate, but is on the underside of the product

Note: Dirt in the plastic can easily plug any cold runner gate, but such dirt is

molded into the plastic as it freezes; it will be ejected with the runner so that

the following cycle will see again a clean, open gate As the runner is ground

up before reusing the plastic, hopefully, the dirt will also be ground up;

otherwise, sometimes later it could again plug a gate

4.1.4.3 Inherently Self-Degating Molds

3-plate and hot runner molds are inherently self-degating, i.e., the gates break

off as soon as the cavities and cores (with the products held on them) separate,

provided the product stays reliably on the core as the mold opens Gates for

3-plate molds are usually very small (“pin point gates”) and often take

advantage of the fact that the plastic will heat up due to shear as it passes

through the gate Gates as small as 0.5 mm in diameter are quite common

for small bottle caps, among others There is an upper limit to the size If too

large, the vestige can become very rough and unsightly or too hot, causing

stringing The gate may even break the top of a thin product as the mold

opens

4.1.5 Hot Runner Molds

Today, hot runners are a fully accepted technology and the preferred method

of gating; they are replacing more and more of the older runner methods,

especially the 3-plate systems In fact, older, existing 3-plate molds can often

be quite easily rebuilt into hot runner molds In the earlier years of the hot

runner technology – occasionally even today – mold makers design their

own hot runner systems, either based on their own ideas or by copying other

systems that gradually came on the market (see Fig 4.30)

Today, there are many well established companies specializing in hot runners,

who sell either the basic hardware (manifolds, nozzles, heaters, etc.) or

assembled hot runner systems, complete with all associated plates and other

Figure 4.30 Typical hot runner, section view

(Courtesy: Husky)

hardware ready to be joined to an otherwise complete mold built by the mold maker All that is required for a purchase order is to specify the important interface dimensions and any production data such as plastic to

be used and the mass of the product These hot runner suppliers mass produce the hardware items and use specialized methods and equipment to produce better quality system parts at lower costs Such hot runners are then guaran-teed to work in the new mold and eliminate the need for the mold maker to experiment and waste time and money trying to get a “home made” system

to work

There are still some molds for which the advantages cannot be justified economically, especially for low production items In these cases, the older systems, especially cold runner 2-plate molds, are still much in use

4.1.5.1 Degradation of the Plastic in Hot Runner Systems

Another important consideration is the amount (mass) of plastic injected into each cavity Each plastic has a limited total time that it can remain exposed

to high temperatures before it will start to degrade and lose at least some of its properties A “temperature and time” graph can be obtained from the materials suppliers Some plastics, and most of the commodity plastics, such

as PS, PE, and PP, have a high tolerance for heat and can stand long exposure

to high temperatures, much longer than many other so-called “heat sensitive” plastics But even the commodity plastics will sooner or later degrade If they are exposed too long to high temperatures, they too must be purged from the injection unit before good quality products can be produced again When injecting into single cavity molds, there is usually no problem, because the runner system (the sprue) is relatively small compared to the mass of the product; the plastic inside the hot machine nozzle and inside the sprue are replaced at every shot With multi-cavity molds, where we need a heated distribution system – the hot runner manifold and the drops through which the plastic must flow to the cavities – the plastic can reside there for a considerable time

If the products are large and the channels in the manifold are relatively small, there is little concern, because the plastic residing in the manifold (the

“inventory”) is replaced quickly (after one or a few shots), especially if the molding cycles are short But small channels in the manifold cause a large pressure drop, especially if high pressure is required to fill the cavities

A compromise – an optimal condition – must be reached The hot runner manufacturer uses computer calculations, based on the information on the product, its mass and shape (wall thickness), the cavity spacing, the plastic, etc., so that a manifold with the optimal channel sizes can be supplied Manifolds for multi-cavity molds for very small products with little mass are more difficult, because the amount of plastic required at every shot is small and the plastic advances only slowly through the manifold channels, especially with longer molding cycles This means that the plastic is exposed to the melt temperature within the hot runner system for a long time Any, even minor, stoppage can bring the plastic close to or over the permissible

Make sure to use clean plastic when

operating hot runner systems with

open gates

A good rule of thumb is to have less

than three shots of plastic in a hot

runner

temperature/time limit at which it starts to degrade If this is expected to be

a problem, a cold runner mold should be selected or a better hot runner

system may have to be developed, maybe together with a specialist in this

field

4.1.5.2 Open Hot Runner Gates

There are two distinct styles of open gates, the circular pinpoint and the

annular ring gate Both gates function on the same principle, by (1) freezing

off at the end of the injection cycle to avoid drooling while the mold is open

for ejection, and by (2) opening up again, triggered by the pressure of the

hot plastic as it is injected during the next cycle

The functioning of both styles of these gates depends entirely on (a) the

operating conditions such as melt temperature, the nozzle temperature, the

injection pressure, the timing, and (b) on the design characteristics of the

system used, such as the cavity cooling, the size and shape of the gate, and

the design of the hot runner system Because of the small size of the gate (a

small round hole in one case, a very narrow, circular gap in the other case), a

serious drawback of the open gate hot runner system is its sensitivity to “dirt”

(paper, wood, tobacco, metal chips, etc.) in the plastic Unlike with the cold

runner gates, any dirt fully or even only partially blocking the small passages

will cause the cavity not to be fully filled Additional dirt will remain there

until the mold is stopped and the dirt removed by opening and cleaning the

hot runner system With good mold designs, the cavity plate can be pulled

while the mold is in the machine and the obstructed gate or gates can be

cleaned There will be an interruption in molding, resulting in lost production

of maybe one hour or maybe a whole day, which may nullify any savings

from utilizing the system

Note that molding at lower melt temperatures is of greater advantage even though it requires higher injection pressures to fill the cavities

Figure 4.32 Schematic of circular

open gate The gate opens and

closes based on thermal cycling and

control of temperature and pressure

in the gate (Courtesy: Husky)

Figure 4.33 Schematic of annular

open gate This gate also opens and closes based on thermal cycling and control of temperature and pressure in the gate (Courtesy: Husky)

A hot runner should not consume any more than 25% of the available injection pressure

Figure 4.31 Hot runner maintenance while

in the press is an important feature

4.1.5.3 Valve Gates (Hot Runner)

This is a method to control the opening and closing of the gate, either by mechanical means or by electrically controlling the melt temperature within the gate The mechanically controlled gates use a moving pin that opens and closes the gate as required during the cycle Today, the valve pin is actuated mostly by compressed air and occasionally by hydraulic pistons

Figure 4.34 shows a schematic of one of several typical valve gates The principle here is that a pin (A) is mechanical moved into and out of the gate (B) on every cycle to open and close the gate It allows for faster cycles and higher quality gates

The main advantages of valve gates are:

 The gate can be much larger than the openings possible with pinpoint

or circular open gates Gate diameters of 4 mm (5/32 in.) or even larger are quite common

 The vestige of the gate is a circular mark, similar to that of an ejector pin mark Because of the large opening, dirt is much less of a problem Most dirt will pass through the gate when open and end up being encapsulated into the product, which may – or may not – be accept-able

 The cycle time is shorter than with a comparable open gate diameter because of the longer time required to freeze a larger open gate There

is an exception to this: with very thin-walled products, the gate area with a valve gate would be hotter than with an open gate and could slow down the otherwise faster molding cycle possible, because the thin walls cool faster

The main disadvantages of this system are

 Larger costs compared to open hot runner gates (approx 40%)

 Possibly added space requirement for the valve actuating mechanisms This can affect the spacing between the stacks, especially if the products are small

4.1.5.4 Combination of Hot and Cold Runners

In some molds, often for smaller products and with a large number of cavities, but also with larger ones, as the example shown in Section 4.1.3.2, a combination of cold and hot runner systems can be of great advantage:

 It can eliminate a large portion of the cold runner and thereby significantly reduce the mass of plastic to be reground or lost

Figure 4.34 Typical valve gate

(Courtesy: Husky)

A

B

 There is much less pressure drop between the machine nozzle and the

gates, because the pressure drops in the hot runner manifold is smaller

than in a (long) cold runner

 It can be used when very small cavities cannot be located very close to

each other, at a “pitch” (distance) for which there are no standard-spaced

hot runner nozzles available, or where it is not possible or practical to

use hot runner gates Typically, a cluster of 2–6 (or even more) very small

products can be gated from a small runner or a disk, which is fed from a

hot runner drop

 It will shorten the cycle time Large distributing (cold) runners take much

longer to cool than the final runners feeding the cavities Especially if the

products cool rapidly, such heavy runners significantly slow down the

molding cycle The cold runner portion in such cases can be treated as

any cold runner mold; it could be a 3-plate arrangement (rarely used) or

a 2-plate system with edge or tunnel gates into the product

4.1.6 Single Cavity Molds

Many products, and in particular large products, are molded in single cavity

molds This allows the use of the simplest mold construction, with simple

injection methods as well as simple methods of ejection In addition, large

products are often not required in very large quantities, and if they are, it is

usually more economical to use two or more machines, each with single

cavity molds Such machines can later be used for other molds and give the

plant more flexibility Very large machines (15,000 kN or 1,500 tons and over)

are usually dedicated for molds for specific, very large products, which cannot

be fitted into a smaller machine because of the physical size of the mold, the

clamping force required, and the required large shot capacity and plasticizing

capacity of the injection system

4.1.6.1 Single Cavity Cold Runner Molds

Single cavity molds have been used since the beginning of the

injection-molding era, for any product size from small containers to large pails As

explained in Section 4.1.3.2, edge gating a single cavity is often not practical

or even possible; therefore, we will consider only gating into the outside on

top of the flat or deep product In a typical mold, the plastic enters the cavity

space from a sprue inside a sprue bushing The machine nozzle presses against

the sprue-bushing seat while injecting; the sprue bushing must be well cooled

to ensure that the plastic within is stiff enough for ejection before the mold

opens up Unfortunately, this cooling time for the sprue is often longer than

the cooling time required for the product and will unnecessarily increase the

cycle time

The mass of the tapered sprue increases as the length of the sprue increases

(Fig 4.36) This can be easily improved by using a shortened sprue bushing,

Figure 4.35 Typical 16-cavity mold; top:

cold runner layout; bottom: hot runner supplying 4 drops to small cold runner, each supplying 4 cavities

Figure 4.36 Cold sprues (top) standard

length, (bottom) shortened, into top of product or into runner system

and providing the molding machine with a longer nozzle Shortening the nozzle length by half will reduce the mass by almost 75% of the mass of the longer one, resulting in shorter cycles In addition, the gate vestige, after the sprue is trimmed from the product, is much smaller This type of gate is used frequently for large parts, as long as they can be filled from only one gate

4.1.6.2 “Through Shooting” System

Through shooting is a better method than using a sprue; however, this method

is only applicable, if (a) the cycle time is short and (b) if the L/t ratio is such

that one nozzle alone will be sufficient to fill the cavity space (this method is really a hot runner, in its simplest form) It can also be called “single cavity insulated runner method” (see Fig 4.37)

The principle of this sprue is simple and the method can be used for most types of plastic It is particularly suitable for plastics, such as PE and PP, but also for PS and other plastics if the cycle is short enough The machine nozzle seats on a sprue bushing, with a large (approx 15 to 18 mm diameter) “well”, deep enough to reach the short, open, circular, tapered gate, which leads to the cavity or to a runner system The well can be as deep as 75 mm (3 in.), and even longer wells have been used successfully, but it is recommended to keep it shorter (approx 25–30 mm) As the plastic is injected the first time, the well is filled with a plastic “slug” The outer surface of the slug, in contact with the cooled cavity steel, will freeze, but because of the good heat insulating properties of most plastics, the melt around the axis of the slug stays hot long enough that even though the gate will freeze, the plastic injected during the next shot will easily traverse the still hot center of the slug The heat of the incoming plastic will then melt the frozen gate and the cavity will be filled for the next shot

This system of runner and gate works very well for most plastics if the cycle time is 15 seconds or less; with PE and PP, molds with cycles as long as 30 s are running successfully If a stoppage is long enough to completely freeze the plastic slug, it can be easily removed by pulling back the injection unit about 15–20 cm (6–8 in.) and then pulling the slug out with a heated, hooked wire or some special tool made for the purpose It is suggested to provide the walls of the well with a draft of at least 3° per side and good polish so that the slug slides out easily As soon as the slug is removed, the injection unit can move forward into the molding position, and production can resume This through shooting system is very inexpensive to make and very reliable Color changes are easy: As soon as one color is finished, the slug can be removed and the new color finds a clean mold The gate vestige is small; it looks similar

to the vestige of a 3-plate gate or an open hot runner gate

Other benefits of this method are: (a) there is no sprue to regrind or to scrap and (b) even though any dirt in the plastic larger than the diameter of the gate will plug the gate, this dirt is easily removed together with the slug and the interruption in production is just the short time required to remove the slug with the molded-in dirt and to restart

Figure 4.37 Two examples of the through

shooting system; long slug (top) and short

slug (bottom)

The size of the gate is determined like any other gate, from a small pin point

gate up to a gate of about 6 mm diameter Larger gates are possible but they

might not freeze and the gate could drool while the mold is open for ejection

of the product If a larger gate is required because of the large amount of

plastic that must enter the cavity space, either a cold sprue as described in

Section 4.1.6.1 or a single cavity, hot runner system (“hot sprue”) will be

required, as described in the following

4.1.6.3 Single Cavity Molds for Large Products

There are several options for single gating a large product, apart from the

old fashioned standard cold sprue (for multiple gates into one product see

Section “Single cavity mold, multiple gated” see p 134)

(1) Use a short, cold sprue, as described in Section 4.1.6.1

Since the product is large, the cycle will probably be long (25 s or more)

The sprue can be cut after molding, e.g., during stacking, assembling or

packing, without adding labor cost If the gate is large and needs to have

a good appearance, it may have to be milled in a fixture Many molders

and designers overlook this simple, and inexpensive solution when

making such large products and select more expensive methods

(2) Use the through shooting method, as described in Section 4.1.6.2

This method will work well, as long as

– The cycle time fits in the time frame described in Section 4.1.6.2,

– The gate size is smaller than approx 2 mm diameter

This method could yield an even lower product cost because (a) there is

no gate cutting required, and (b) the cycle time can be less than with a

cold sprue, because the sprue may take longer to freeze than the product

thus controlling the cycle time

(3) Use a hot sprue, as described in Section 4.1.6.4 below

A “hot sprue” is essentially a heated cold runner sprue The melt within

the sprue is kept hot with electric heaters The gate could be an open gate

as shown in Figs 4.33 and 4.34 or a valve gate as shown in Figs 4.41 and

4.42

Hot Sprue, Center Gating

The hot runner suppliers sell hot sprues of various designs as standard

hardware, ready for incorporating into the mold It is important to follow

exactly the interface dimensions and tolerances specified by the

manufac-turers, as well as the operating instructions to ensure trouble-free operation

Hot sprues with an open gate (annular or circular) or with a valve gate can

be purchased in a variety of lengths They are most suitable when it is

necessary to gate inside a deep product, too deep to reach for even a very

long extended machine nozzle

Figure 4.38 Single-cavity pail mold with a

hot sprue (Courtesy: Topgrade molds)

Figure 4.39 shows a hot sprue with heater bands for the top (A) and for the nozzle extension (B) The nozzle insert (C) can have different configurations, determined by the requirements of the mold, the type of plastic and the shot volume

Figure 4.40 depicts the schematic of the open hot sprue There are heater bands for the top (A) and for the nozzle extension (B) The nozzle insert (C) can have different configurations, determined by the requirements of the mold, the type of plastic and the shot volume

A great advantage of a valve gated hot sprue is that the gate can be of any reasonable size, 4 mm diameter and even larger; the gate vestige is circular Hot sprues are quite expensive (in the order of $3,000.00) and need heat controls, wiring, and air pressure lines and controls for the actuating mechanism However, in some molds, these expenses are warranted

Hot Runner Manifold with Offset Gate(s)

In some single cavity molds it may not be acceptable, or even possible, to use

a center gate into the product; often, for appearance reasons Two typical examples:

 The cavity must be edge-gated, as shown in Section 4.1.3.2, where several drops from a hot runner manifold are located to feed one or more cold runner systems outside the circumference of the product

 The gate could be positioned away from the center, but still within the outline of the parting line (or the circumference) of the product (see examples in Fig 4.45)

Figure 4.39 Hot sprue with heater bands

A

B

C

Figure 4.40 Schematic of the open hot sprue

Figure 4.41 Valve gated hot sprue Figure 4.42 Schematic of the valve gate hot sprue (all: Courtesy: Husky)

Molds for Very Large Products and Limited Production

Single-cavity molds, such as for large automotive products, can weigh many

tons, and because the products change almost yearly with the model changes,

it is important that all possible shortcuts be taken to keep the mold cost low,

while still guaranteeing the best quality of the product

Because of the large mass of plastic entering the mold, today, all such molds

use several valve gates, located usually where suggested by flow analysis This

may appear to be expensive but it is necessary to ensure the quality of the

product and for achieving a reasonable molding cycle

On the other hand, great savings can be achieved by proper selection of the

mold steels used The cavities and cores are often cut right out of steel blocks,

each of which could weigh several tons “Conventional” mold plates are rarely

used There is no need for expensive mold steels or pre-hardened steels,

especially if the specifications of the product do not require high gloss finish

Mild steels are often acceptable, but there could be inserts required for places

where wear is expected However, expensive beryllium-copper inserts are

used frequently in locations where it is important to provide better cooling

to reduce cycle time Other good mold making practices, such as cross drilling

for cooling channels, are replaced by the use of flexible hose connections

from channel to channel Simple horn pins, wedges, or hydraulic actuators

can move side cores Alignment is provided with leader pins and bushings

Even the parting line match of the usually complicated shape of the products

need not be perfect Excessive but reasonable gaps can provide good venting

for the large flow of plastic entering the mold, and ensure proper filling

Any unwanted flash occurring could be scraped off by hand The large,

sometimes unwieldy products are often removed from the molding area by

robots, but additional handling is usually done by hand and, if necessary,

any excessive or unsightly flash can be removed at that time

Figure 4.44 shows a mold for a rear door of a car The mold weighs more

than 17,000 kg and runs on a 2,000 ton machine The wall thickness of the

panel (in TPO) is approx 3 mm and the molding cycle is approx 40 s for a

productivity of 700,000 pieces per year Injection is with two Synventive

sequential valve gates

The parting line matches perfectly, therefore no scraping is required Finish

is SPI #4 on the inside, and SPI #2 on the visible outside Ejection is by ejector

pins, with hydraulic actuators inside the mold The panels are removed by

robot, directly to a conveyor

In these cases, either a standard size (listed in a catalogue) hot runner manifold

can be selected or a specially designed manifold will be required The drops

(nozzles) to the product or the cold runners could be open or valve gates

Note that today, many hot runner manufacturers offer standard sizes of many

manifolds and all other hot runner hardware, from stock, at lower cost and

faster deliveries, than special sizes

Figure 4.44 Mold for a rear door of a car

(Courtesy: Accurate Molds)

Figure 4.43 Large automotive mold for

bumper fascia (Courtesy: Accurate Molds)

There is an often overlooked and much less expensive solution for products such as the one shown in Fig 4.45 (top)

This solution is depicted schematically in Fig 4.46: Instead of using an expensive offset hot runner, this large product with an opening in the center can be easily filled from a cold sprue as shown in Fig 4.36 (or a through-shooting sprue as in Fig 4.37) that both feed into a cross-shaped or multi-spoke runner or even into a shallow disk in order to edge-gate or to feed a continuous gate all around the inside of the opening Now we will turn our attention to the (small) cold runner and the problems possibly associated with it:

(a) To make sure that the runner stays with the product from where it will

be broken off or cut by an operator, (b) We will have some scrap, which may represent a very small percentage of the product weight, depending on the size of the opening

Economically, both the cost of the runner and the labor of removing it, and regrinding the scrap, may still be less than the cost of the otherwise necessary hot runner system This can be easily calculated If the total number of products from the mold is small, the cold runner system is preferable If the number is very large, the added cost of a hot runner can be easily justified

Single Cavity Mold, Multiple Gated (Hot Runner or 3-Plate Mold)

The following applies mainly to hot runner molds, but also to 3-plate molds

In a large single-cavity mold, several gates are often used to provide better

plastic flow into the cavity This approach must be selected if the L/t ratio for

a single center gate would be too high or if one gate would not allow enough plastic to flow into the cavity By choosing suitable locations for two or more gates, far enough spaced from each other, the L/t ratio per gate can be much reduced A product that otherwise could not be filled at all, or only with very low viscosity (very hot) plastic and with very high pressures, can be filled much easier from several gates Special care must be taken to ensure that the cavity space is well vented where the streams from the various gates are expected to meet, e.g., by placing ejector pins or vent pins there

One problem with multiple gating with open hot runner gates is that if one

of them freezes more solidly than the other(s), the incoming stream at the next cycle will not be able to dislodge the frozen slug in that gate The plastic will then not use all gates as intended; this can result in unfilled products The most effective solution in such cases is to use valve gates The advantages

of valve gating have been described earlier The disadvantage is the added cost of the valve gate systems and more controls in the machine

When using multiple gating for 3-plate molds, typically when very small (pin point gates) are desirable for appearance, it is important to use very clean plastic; if one gate gets plugged, the other gate(s) will not be large enough

to fill the cavity space There is also a method of sequentially programming

Figure 4.45 Schematic illustrations of two

large, single-cavity molds with offset hot

runner gates within the outline of the

products

Figure 4.46 Large product with center

opening, with cold runner gating