Tài liệu Mold Selection P1 pdf

4.1.2.2 Family Molds for Small or Medium-Sized Technical Products Family molds for small or medium-sized technical products are used when a number of different sizes of similar, rather s

4 Mold Selection

Once a good product design has been achieved and it is decided where the

product will be made and how many cavities are required, we must consider

the available alternatives for the molds

4.1.1 Dedicated Mold, Universal Mold Shoe

“Dedicated mold” means a complete mold that is used for one purpose only

After use, the mold is put into storage until it is used again This is the most

common type of mold Occasionally, especially with molds with 2–8 cavities,

the same mold shoe can be and often is used for more than one set of cavities

and cores In principle, there is nothing wrong with this concept, provided

the molding shop is well organized (good record keeping and proper storage

facilities for the loose stack parts) and the personnel is capable of making

the switch from one product to another without the need for high-priced

mold makers It may take a few hours to switch from one set of stacks to

another and there is always the risk of damage to the mold components in

handling and during assembly The question is whether it is worthwhile to

switch molds, especially if it is done frequently If the mold shoe is quite

simple, it would be better (safer and more economical) to have a dedicated

mold But there are cases where the mold shoe is large, complicated, and

relatively expensive; if the stacks for the various (preferably similar) products

are designed from the beginning so that they can be easily interchanged, this

is a very good and economical solution

Typical examples are 4- or 6-cavity molds for a series of round containers,

with none or only small differences in diameters, but with large differences

in height, as would be the case with small tubs for dairy products, e.g., in

sizes from 0.25 liter to 1 liter capacity Such molds can be designed and built

with all the advantages of a dedicated mold, but saving the cost of several

mold shoes

“Universal mold shoes” are used mainly for low production runs, for which

only small numbers of cavities are required They are based on the principle

that stack inserts can be easily and quickly interchanged by the molding

technicians or setup personnel, often even without removing the mold shoe

from the machine The stacks do not necessarily have to be for the same or

even similar products They are usually designed for one cavity per insert If

there is space, two or more cavities and cores could well be placed within

one insert The disadvantage is that, because the stacks are designed for easy

interchangeability in the mold shoe, it may not be possible to provide them

with the best cooling layouts (facilitating faster cycles) of a dedicated mold

In addition, the product requiring the longest cooling time governs the cycle

Figure 4.1 A 4-level mold designated to

quickly switch to different sets of inserts (Courtesy: Stackteck)

Dedicated molds are usually preferred However, the use of a common mold shoe with different sets of stacks can often be more economical

For small products and low quantities, universal molds can often

be the most economical solution

time; however, for low production, short cycle times are not as significant for the unit cost as is the lower mold cost

4.1.2 “One-Product” Molds or “Family” Molds?

“One-product mold” is a mold built for one specific product The best layout for minimum mold size, space (stack location), cooling, ejection, etc., can be achieved with a dedicated (one-product) mold

A “family mold” is a dedicated mold, in which more than one shape of product

is made during the same injection, which will be of the same material and color A very serious disadvantage of all family molds is that the cycle time of the mold is governed by the product (and the mold stack) that is most difficult

to cool This difference can be substantial, and should be seriously considered, particularly with products as described in Section 4.1.2.1 and 4.1.2.2 For all family molds producing pieces of different size, we must make sure that the mold is laid out so that the clamp forces are balanced as well as possible, i.e., that the sum of all projected areas is about equal in each of the 4 mold quadrants In other words, the projected areas of the cavities above and below the horizontal center line of the mold must be nearly equal and so must be the sum of the projected areas to the right and the left of the vertical center line of the mold (see Fig 4.2)

4.1.2.1 Family Molds for Composite Products

For composite products, such as toys and games, it may be desirable to make all the components of the toy in one shot Often, the various pieces are kept

on the runner system of a 2-plate mold and are packed and shipped together with the runner; it is left to the user to take the pieces off the runner during assembly of the toy The production runs are usually relatively small; therefore, this is a most effective method of producing with low cost molds (don’t forget

to include the cost of the runners in the cost of the product)

Occasionally, a product, e.g., a toy car, may have two or more colors It could

be a car with a blue body, red wheels, and yellow bumpers, etc By molding equal production runs of first blue, then red, then yellow parts, 3 sets of cars can be produced, in the three combinations of colors In this case, the runners are not shipped with the product This method is also used occasionally for technical products

4.1.2.2 Family Molds for Small or Medium-Sized Technical

Products

Family molds for small or medium-sized technical products are used when a number of different sizes of similar, rather small products, such as washers

or seals, are molded in one mold But such molds can also be used for larger products, which are required as a “set” in production, as they are used, e.g., for home appliances, among others Any type of mold can be used (hot runner

Figure 4.2 Schematic of symmetrically

balanced cavities in relation to the

centerline of the clamp

Figure 4.3 Stack family mold for container

and lid (Courtesy: Husky)

Figure 4.4 Cavity view of 72-cavity cutlery

mold; 24 forks, 24 spoons, and 25 knives are

molded every shot (ca 8–10 s)

or cold runner, 2-plate or 3-plate) There are two main disadvantages of this

type of mold:

(1) Except for edge-gated 2-plate molds, the products fall out of the mold all

mixed together and must be separated before storage or use

(2) Stock and production control can have serious problems when some of

the products are used up (e.g., wear) faster than others, and must be

available as spare parts It may then be necessary to run the mold to

produce the full shots while only some of the items are required This

problem can be overcome by blocking off the runner system ahead of

the unwanted cavities and running the mold only for the products

required; this means to run the mold less efficiently

4.1.2.3 Family Molds for Perfect Color Matching

Any plastic, and especially colored plastic, whether colored in-house or bought

already colored from the supplier, comes in batches Within each batch, the

plastic can be considered uniformly mixed and colored These batches are

supplied in bags, or in large carboys, or in truckloads, etc Even though the

specifications to make the batches were identical, there are mixing tolerances

in manufacturing and small variations from batch to batch are unavoidable

It is better to work with large batches, which will yield large numbers of

matching-colored pieces, but this is not always practical or economical If a

product pair (or assembly) must have a perfect color match, the answer is to

make the matching pieces in one shot, which is of course supplied by the

same injection unit, at the same time A typical application for this is a “lady’s

compact”, consisting of a base (for the face powder) and a matching lid (for

the mirror) But there are other applications, some of them in the technical

field It is quite common to build molds that have the same number of each

of the products that require the perfect color match If the pieces are required

in pairs and their projected areas are about the same, a mold layout is rather

easy and the stacks can be laid out symmetrically A problem is that the pieces

are ejected together and must be separated after molding; also, they must

also be stored so that the matching colors are kept together and are not mixed

with products from another color batch (this can also add costs to the

product)

4.1.2.4 Family Molds for In-Mold Assembly

Family molds for in-mold assembly are more sophisticated molds, usually

for very high production volumes and are only rarely used A multiple of

two different but matching pieces is molded in the same mold; they are

assembled during the ejection time, using special motions, which are part of

the mold or during the mechanical removal (with synchronized take-offs or

robots), so that already assembled pieces are ejected to a conveyor or carried

away under controlled conditions Such assembly methods may require longer

ejection times but can save subsequent assembly equipment, and time

Perfect color matching can easily be achieved with family molds

Figure 4.6 Color matched parts for personal

care products

Figure 4.5 View of ejected array in robot end

of arm tooling

4.1.2.5 Family Molds Using Controlled Ejection for Subsequent

Assembly

This method is almost exclusively used for products where the required annual quantities are very large and virtually no changes are expected for years In these cases, a number of pairs of matching pieces, usually of the same projected area or with only small difference in area, are molded in one (single level or stack) mold and then removed either by take-off or by other methods, which maintain the orientation of the matching pieces so that they can be easily assembled in a specially designed machine or mechanism, usually adjacent to the molding machine Typical examples are Petri dishes, video and audio- cassettes, CD “jewel boxes,” and so forth

Figure 4.7 shows a Petri dish system taken from the rear of the clamp, which

is protected by guards (A) The bottoms and the tops of the Petri dish are molded on each face of a 2 × 4, 2 × 6, or 2 × 8 stack mold Guide rails transport

the molded parts by conveyor (B) to an assembly station (C); from there the assembled dishes move to a stacker (D) and the stacks of assembled Petri dishes are then moved to an (open) “sleeving” station (E) where plastic sleeves are manually pulled over the stacks for boxing and shipping to a sterilizer; cycle time: 3.5 s, productivity (with 2 × 8 mold): 8,200 assembled dishes/hour

4.1.3 Where to Gate

The next issue to consider is the location of the gate The gate is the point

where the plastic enters the cavity space In some cases, the product designers will indicate where they believe the gate should be They may select this

location because of the function and strength of the product and in some

A

B

C

Figure 4.7 Petri dish system

(Courtesy: Husky)

Figure 4.8 Petri dishes and CD jewel boxes

are typically molded using family molds

Figure 4.9 Cutlery is also often molded

in family molds

cases, because any projecting gate vestige may be bad for appearance or even

harmful to the user However, such suggested location may not always be the

best for filling the cavity space or for the best strength properties of the

product At this point of the development, the input by a molder or the mold

designers could be very valuable and a dialogue between the product and

mold designers should be encouraged to find the best location for the gate

These days, computer aided mold filling simulation packages can accurately

predict the fill patterns of any part This allows for quick simulations of gate

placements and helps finding the optimal location

4.1.3.1 Cup- or Box-Shaped Products

In general, for cup- or box-shaped products, outside center gating is most

desirable, because it ensures more evenly distributed flow from the gate

towards the rim or edge However, center gating (except for single-cavity

molds) implies the use of either 3-plate or hot runner molds, both of which

are more expensive than 2-plate molds Note that the gate area is always an

area of inherent weakness; molding conditions such as higher melt

tempera-tures, longer molding cycles, and higher cooling temperatures can improve

the strength there and this must be considered as a factor affecting the cycle

time and cost of the product It should also be noted here that hot runner

valve gating reduces the stresses in the gate area

The foregoing does not imply that 2-plate molds cannot be used for cup- or

box-shaped products; in fact, 2-plate molds are used for many such products,

but usually only those with larger wall thickness

4.1.3.2 Flat Products

“Flat” in this context means relatively flat, as opposed to “cup-shaped.” It

includes really flat pieces (in one geometric plane) but also curved products,

such as automotive panels, trays, etc of all shapes Flat products are preferably

gated from the edge of the product, because the flow away from the gate (or

gates) will result in a stronger product; it also ensures that there are no

unsightly gate marks in the middle of the product Here also, it is much better

if the incoming stream of plastic will be directed against a solid portion of

the core or at least a projection of the core and not to flow into an open space,

such as a rib or an open surface Thin-walled, round products, such as lids

for containers and trays, should be center-gated for faster filling and to reduce

possible distortion when ejected early to gain cycle speed; however, they can

also be edge-gated when the center of the lid must not show a gate mark

Figure 4.12 shows a selection of typical automotive products The quantities

are usually small compared with the huge numbers molded for packaging

and medical products and the molds are usually small cavitations (1 or 2)

But even so, most of these products are molded with hot runners, because it

is easier and more effective to control the quality of the products and there is

often less labor required than with cold runner molds Also, the use of regrind

is sometimes not possible, which makes the justification of a hot runner easier

Figure 4.12 Selection of typical automotive

products

Figure 4.10 Mold filling analysis is very

useful for finding the best gate location

Figure 4.11 Typical bottom center-gated

parts

Because in most molding machines the injection unit is in line with the center

of the machine platens, it is not possible to edge-gate a single-cavity mold, unless either there is a large enough opening near the center of the product, from where a cold or hot runner system can feed one or more edge gates (see Fig 4.13), or a hot runner system is used with drops outside of the profile of the product, feeding into cold runners (see Fig 4.15)

An alternative is to have the cavity located completely to one side of the centerline of the machine, which could be possible for any product small enough to fit there This however could leave to a severe unbalance in the clamp This is not recommended

There are ways of balancing the clamp forces, e.g., by doubling the size of the mold and providing a second, similar cavity if the cavity is not too complicated and expensive, or by adding a pressure pad in a location on the platen symmetrically opposed to the cavity (see Fig 4.14) For more details on this subject, refer to [5] Chapter 6

If the product is very large, edge-gating into a single cavity can be achieved with a 3-plate mold (now rarely used for this purpose) or by using a hot runner system, which enters one or several cold runner systems outside the edge of the product From there, cold branch runners can lead to edge or tunnel gates into the side of the product, just like in a regular 2-plate mold This method is used for large, mostly flat products, such as automotive panels, and so forth (see Fig 4.15)

Figure 4.7 shows a flow model of a large automotive panel with three gates (A) from a hot runner system (B), but without the use of cold runners as in the schematic of Fig 4.6 The runners are shown schematically, superimposed over the photo Note that here again, the gates are near the edge of the panel for greater strength

Figure 4.13 Two examples of gating

into the center of an open product.

The sprue could be a cold sprue or a

hot (runner) sprue

Figure 4.14 Balancing of mold

clamp-ing forces; (left) added second cavity;

(right) added balancing pressure pad

Figure 4.15 Schematic of large, single-cavity

mold with hot runners feeding cold runners; (a) product (a large panel); (b) sprue; (g) hot runner channel; (h)”drop” to cold runner; (j) cold runner; (i) gate

Fill time

= 0.9408 [s]

0.9408

0.7056

0.4704

0.2352

0.0000

[s]

A

A

A

B

Figure 4.16 Flow model of an automotive

panel

4.1.3.3 Deep, Large Cup-Shaped Products

Products in this category are large pails, boxes, garbage containers, large crates,

children’s bathtubs, and so forth It is always desirable to use one center gate,

if the L/t ratio is low enough (200 or less.) Today’s machines with high

injection pressures have made it even possible to mold large industrial pails

with an L/t ratio of up to 500 with only one gate However, most large products

(tubs, boxes, etc.) have two or more gates in the bottom for faster,

lower-stress filling and to reduce the L/t ratio for each gate Large industrial

containers, crates, pallets, etc may have four or more gates It is important to

provide venting where the streams coming from the gates are expected to

meet to avoid the risk of air enclosures or even holes at the predicted weld

lines Such molds with one gate can use a cold sprue (simplest mold) or they

can have a hot sprue If two or more gates are required, a hot runner system

must be used 3-plate molds, although theoretically possible, are almost never

used in this arrangement, because of the large size and mass of the cavity

block that would have to move (float) between the moving and stationary

platens to allow ejection of the runner

Figure 4.17 shows a typical heavy crate (A) for bottles with separators (B)

for individual bottles This design requires a mold with side cores for the

deep engravings (C) and the openings (D) in the sides There are also two

baskets (E) with openings (F) in all 4 sides Because the sides are angled, the

openings can be produced by so-called “shut-offs” between core and cavity

contacting in each opening, thus not requiring side cores Such a mold is

much less expensive and can cycle much faster than the mold with side cores

The other picture illustrates a large box (G) with matching, flat snap-on lid

(H)

Figure 4.18 shows 10 and 20 Liter industrial pails Depending on the ratio of

flow length to wall thickness (L/t ratio), they use either a single gate in the

center or three gates near the rim to facilitate filling

4.1.3.4 Elongated Products

For maximum strength it is always better to gate near the end of the product

(cold runners) or on the top surface (A) near the end of the product (3-plate

or hot runners) Gating into the top may be undesirable for appearance, but

proper function of the product should always be the first consideration A gate

mark at the top surface can often be hidden, for example, inside a letter or

and ornament on such surface, or by creating a “fake vestige” in a location

symmetrically opposite the gate (see also Section 2.8.3, Witness Lines)

Figure 4.19 shows typical elongated products (tooth brush, safety razor

handle), which must be gated near the end for maximum strength Similarly,

other products (not shown), such as cutlery (disposable or not), must also

be gated at the end If these parts were to be gated in the middle they would

break at the gate

H

G F E

A

B

C D

Figure 4.17 Typical crates and baskets

Figure 4.18 Very large industrial pails

Figure 4.19 Typical elongated products

4.1.3.5 Inside Center Gated Parts

Cold Runner 3-Plate and Hot Runner Molds

In all gate locations mentioned so far, the gate is always located on the outside (top or side) of the product, i.e., in the hollow (concave) portion of the cavity This is good practice because

 It is the shortest path for the plastic between the machine nozzle and the gates

 The product will stay with the core from where it can be easily ejected by any conventional method

 The best cooling is on the core where the product shrinks on, to ensure proper ejection

However, there are cases where a gate on the outside of the product is not desirable, mostly because of required esthetic appearance Typically, this is the case with high-quality closures (for perfume bottle caps, some in-mold labeled products, etc.) or some spray bottle or over-caps, where a gate vestige on top would “cheapen” the appearance of the package But there are also some technical products and enclosures for which inside gating is preferred

Figure 4.20 shows over-caps (A) with inside center gating It requires long nozzles (B) and, as can be seen, there is not much space for core cooling These molds cycle 2–3 times longer than outside center-gated molds, but have no gate vestige on the outside This example shows clearly how little space there is to provide good cooling, a gate insert, and good heat insulation from the nozzle tip

There are some basic drawbacks with inside center gating (ISCG) (see Fig 4.21):

 The sprue (or drop of a hot runner system) is much longer than with outside gating to reach the bottom of the product

 Because the cavity is on the moving mold half, the product cannot be ejected easily The product will most likely shrink onto and stay with the core from where it is injected; therefore, an ejection system must be incorporated into the injection side of the mold and, if necessary, into the stationary platen of the machine Ejection by air would be best, but is often not possible because of the shape of the product and/or the plastic processed Therefore, mechanical ejectors (strippers or ejector pins) must

be on the injection side of the mold, which also carries the cores and the runner system This ejector system is either air actuated or driven by mechanical links connected to the moving platen Most likely, this ejection mechanism adds still more length to the sprues or drops These difficulties are even greater with unscrewing molds, with the cores on the injection side and the sprue inside the cores

Figure 4.21 Outside center gated (top),

and inside center gated mold (bottom)

A

Figure 4.20 Over-caps (A) with inside

center gating (Courtesy: Husky)

 The cooling on the inside of any core contributes always more than 60%

of the cooling efficiency of a mold But with ISCG, inside the core is also

the sprue of a 3-plate system or the drop of a hot runner system For

shortest molding cycles, we need to cool the core efficiently to remove

the heat both from the product and the sprue in 3-plate molds; however,

in hot runner molds, we must remove the heat from the product while

keeping the hot drop well insulated from the cooled core so that the

plastic in the drop will not freeze Both these conditions mean that there

is very little space to provide good cooling for the product and much

slower cycles will be unavoidable compared to a similar product gated

from the outside

 The need to provide an ejection system on the injection side makes it

difficult to position the runners and cooling lines in either 3-plate or hot

runner mold While the moving mold half with the cavities becomes

very simple and relatively small, the injection side with the cores will be

very complicated and large

With these problems, an ISCG mold is always considerably more complicated

and about 25% more expensive to design and build and will cycle two to

three times slower than a comparable mold with outside gated cavities

4.1.3.6 Slender Products

Round, thin-walled products such as vials, syringes, etc are best (outside)

center-gated, using either 3-plate molds or, preferably, hot runner systems,

Fig 4.22

Core Shift

Slender products (length over diameter ratio of more than 2.5 : 1) have the

problem of “core shift”; in fact, the core is not shifting but a bending of the

core is caused by differences in the plastic injection pressure It is practically

impossible to gate exactly concentric between cavity and core; even with the

closest practical tolerances, there will always be some minute misalignment

between the center of the gate (at the closed end) and the center of the cavity

space between the cavity and the core, Fig 4.23

Such misalignment will allow more plastic to flow into one side of the core

than into the opposite side and as the cavity space fills, the core will deflect

because of a pressure differential in the plastic between the sides and remain

deflected to some extent until the product is ejected After ejection, the core

returns (elastically) to its original straightness The effect of such core

deflection can be measured in the wall thickness but can also be seen easily

by rolling the molded piece on a flat surface; its easily recognizable “banana

shape” is caused by the different shrinkage conditions of the thicker and the

thinner side of the vial (the thicker side takes longer to cool and thus bends

the product after ejection) The thinner the walls are, the worse is the problem

and the more precision in mold making will be required, adding to the cost

Figure 4.22 Gating for vials; (top): center

gating (hot runner or 3-plate); (bottom):

cold runner 2-plate gating A and B show top view of gating at 180° and at 120°

of the mold On the other hand, thicker walls require more plastic (cost!) and longer cooling (more cost!) When center-gating, there is no problem with venting because the plastic flows toward the parting line where good venting is easy to achieve

To overcome core deflection, there are various methods (some of them patented) of stabilizing the core inside the cavity and/or selecting a stiffer

core material with a greater modulus of elasticity (E) than mold steel to reduce

the deflection of the core Some tungsten-carbide alloys exhibit a modulus

of elasticity 2.5 times greater than steel; however, they have little shock resistance and are expensive to manufacture

There are mold makers specializing in these products (vials, syringes, pen barrels, etc.) who have the experience and skills to overcome the problems and provide good molds

An older method for making these products is to use cold runner gates (either self-degating or not) into the side of the barrel (see Fig 4.22) at or near the open end, and to use two gates located at 180°, or three gates at 120° around the circumference of the barrel, or to provide a continuous ring gate all around the opening The ring gate will then be machined off The advantage of any

of these methods is that the plastic enters the cavity space from two or more symmetrically opposed gates and flows in parallel streams, which tend to hold the slender core in center One serious problem with this method is that the cold runner from the sprue is located in the same plane as the stripper Using floating stripper rings is not possible because of the gap required for floating between the rings and the stripper plate and positioning the stripper rings exactly in line with the core is very difficult to achieve and very costly Still, there are many multi-cavity molds built this way The second serious problem with this method is the venting of the air as it is pushed ahead of the inrushing plastic With vials, there is no opening at the closed end and vent pins at the top (dome) are absolutely necessary A composite cavity, with a separate part forming the dome will permit vent gaps between the dome and the cavity portion forming the sides of the vial Products such as syringes have a hole in the dome and can be vented there

Note that uneven filling because the center of the gate is not in line with the centers of the cavity and the core cannot only happen with slender products

as schematically shown in Figs 4.22 and 4.23 Figure 4.25 shows a fairly stubby container with ribs and outside center gating The lower part of the photo shows a complete shot Because the gate is (unintentionally) off-center, the cavity space fills unevenly, as shown clearly with the 6 short shots, from right

to left By progressively increasing the shot size it can be clearly seen how the plastic gradually fills the cavity space and produces an air entrapment that causes much difficulty when molding

Figure 4.26 shows the short shot of a thin-walled cap with a “corrugated” sidewall The corrugations have slightly different radii in the cavity and on the core so that the outer tips of the corrugations are somewhat thicker than the sidewalls, permitting the plastic to flow easier through the thicker tips

Figure 4.24 Typical long slender products

Figure 4.23 Schematic of effect of

misalignment of gate and center of the core