Tài liệu The Plastic Product P4 docx

Gaps Between Cavity and/or Core Parts and Inserts Mold parts and inserts can be dimensioned for locational clearance fits Standard Geometrical Tolerancing; that is, they will probably al

Parting Line Flash

Good molds have venting around most of, if not the whole length of the

parting line As long as the stack and the vents are designed properly, there

should never be flash on the parting line If flashing does occur despite a

properly designed and built mold, there could be a number of other causes:

 The selected molding machine has not enough clamp force (tonnage)

for the projected area of the cavity and the required injection pressure or

was not adjusted properly This is a rather common occurrence, especially

when the mold is installed in a low-tonnage machine (clamping force)

because a suitable machine was not available

 Flashing at the parting line could be caused by a poorly maintained and

out of alignment molding machine, which could be the result of poorly

adjusted tie bars, worn tie bar bushings, worn or bent machine platen

surfaces, poor hydraulic system, and so forth Only too often a mold is

blamed for flashing when the real reason is a poorly maintained or a too

low tonnage machine

 There may be (hard to see) plastic caught between the mold faces

Gaps Between Cavity and/or Core Parts and Inserts

Mold parts and inserts can be dimensioned for locational clearance fits

(Standard Geometrical Tolerancing); that is, they will probably always have

a clearance, but this clearance must never exceed a size where plastic could

flash into it This means that the so-called standard fits are not always suitable

with larger mold components and inserts Smaller tolerances than given with

“standard fits” must be specified for some mold dimension This modified

method of tolerancing fits is quite suitable for the assembling of molds

Because gates wear out in time and get easily damaged, often during startup

of the mold, molders prefer that the gate be in an insert, see Fig 2.58, that

can be replaced when necessary, rather than rebuilding the entire cavity

Welding a worn gate is sometimes successful, if done by experienced welders,

but in most cases, even the best weldment will be visible on the product,

especially on a highly polished surface This kind of repair is generally not

recommended Any gate insert will leave a circular witness line It must be

pointed out to the customer before beginning the mold design that the mold

would be less expensive to build without an insert, but the costs of upkeep

will be much higher, especially with high production requirements, which in

the long run will affect the cost of the product

Figure 2.59 shows a modular cavity (A), the matching cavity bottom (B),

and the beryllium-copper alloy gate insert (C) for the cavity bottom The

witness line will be a small circle around the gate

It is also important to realize that a properly designed gate insert permits

better cooling around the gate which is very important for achieving a faster

molding cycle The added cost of the insert can be easily justified with the

Figure 2.58 Typical cavity with and

without gate inserts

Cavity

Gate insert

Cavity bottom

Figure 2.59 Modular cavity

A common misconception is that if a mold fits a machine it should be able

to run on it

Good cooling Witness line

42 2 The Plastic Product

increased productivity of the mold The cost of a gate insert may range between $200 and $1,000 each

Clearance of Ejector Pins and Ejector Sleeves

Ejector pins are always produced by mold part supply manufacturers with specified, standard, close tolerances so that when the mold is built with the bore dimensions for these pins, as suggested by the pin manufacturer, they will not flash and they will always leave a witness line The clearance between ejector pin and bore is actually desirable, because it creates a good, self-cleaning vent Ejector pins (and sleeves) and the bore wherein they slide will wear over time Especially when abrasive plastics are molded, but also, frequently, when the mold is poorly designed or built, the pins are allowed to rub on the side of the bores If the ejector plates are properly aligned, with their mass suitably supported by guide pins (see Fig 4.51 in Section 4.1.7.1) and the ejector pins (or ejector sleeves) are allowed to float freely in their retainer plates, there is little wear to expect But even under the best conditions, after the mold is in operation for a long time, the clearance in the bores will increase, and the plastic will begin to flash around the pin Such flash may also create points of hang-up and prevent fully automatic ejection

The proper way to repair the mold is to increase the bore size and replace the

ejector pins with a larger size Standard oversize pins are commercially

available, usually 0.13 mm (0.005 in.) larger in diameter than the original, nominal standard sizes Allowing space for oversize bores should be con-sidered when designing the mold; this is sometimes difficult with small technical products, where small pins barely clear the space between walls or under ribs

Note that some molders use double or even multiple ejection strokes to overcome the problem of hanging up due to flashing; however, this adds considerable time to the cycle, adds wear to the machine and mold, and is not recommended

Hiding the Gate Mark

Occasionally, to improve the appearance of the product, it may become necessary to use certain “tricks” to hide the gate marks A pinpoint gate mark (vestige) from a 3-plate or a hot runner mold can be hidden inside an ornament or inside the closed portion of a letter appearing on the same surface (typically, inside a letter O, P, D, etc.); in other cases, an ornament could be created around a gate, such as several concentric circles, with the gate as the center In some cases, where the vestige of a valve gate is on the visible top surface near one end of the product, a similar “fake vestige” near the opposite end can be created with an inserted round pin, which serves no other purpose than to make the product appear symmetrical, which is usually more pleasing to the eye

Figure 2.60 Gate mark hidden in the letter

Y of this Playdoh lid

Consider the locations and sizes of

the ejector pins carefully In many

molds, fewer pins of appropriate size

in more suitably placed locations

could save a lot on mold costs and

improve operation

2.8.4 Weld Lines

Weld lines are created when two semi-molten streams of plastic within the

cavity space intersect They appear on the surface as more or less pronounced

lines These streams may originate from two separate nozzles in the same

cavity, or when a projecting pin or other obstacle breaks the flow into two

separate streams In some products, these weld lines can be objectionable for

aesthetic reasons; in others, they may be acceptable When the plastic surface

is to be printed after molding, such lines could present an unacceptable visible

flaw Note that a weld line also weakens the product, because the plastic

streams – by the time they rejoin – are already colder and may not fuse

sufficiently Higher mold and melt temperatures can sometimes improve or

even eliminate a weld line; however, this means longer cycle times (less

productivity) When molding pigmented (colored, particularly metallic)

plastics, weld lines can show up very distinctly Plastic always finds the easiest

path (“path of least resistance”) within the mold Typically, a heavier rim or

ribs allows more and faster flow than the adjoining thinner walls, so that the

plastic, after rim or ribs are filled, will fill the remainder of the cavity space

from there (“back-fill”) and create weld lines where these flows meet This is

an important consideration for locating the gate(s) If weld lines are created

by such conflux of plastic it is important to provide vents at these spots to let

any trapped air escape These features add cost to the mold

Location of Weld Lines

Usually, the location of weld lines can be quite accurately predicted by plain

logic or with the help a flow modeling computer program If the area where

the weld line is expected to form is critical for strength, a change in the gate

location may be a way to change the location of a weld line In addition, it

may be possible to facilitate the plastic flow to a certain path (e.g., by slightly

increasing a portion of the cross section of the flow path) so that the plastic

reaches the critical point and flows around it before the other stream is met,

thereby relocating the weld line to an area where it will do less harm to the

strength of the product Such slight increase in the thickness may be hardly

noticeable and, while increasing the amount of plastic insignificantly, it

ensures a stronger product Since this is a change to the product, the approval

of the product designer may be necessary

In many cases, a weld line is unavoidable Only its location can be influenced

by the location of the gate and flow leaders/ribs and changes in thickness of the

part To improve the appearance and strength of a weld line, typically a molder

will increase mold temperature, injection speed, melt temperature, or venting

2.8.5 Surface Defects (Flow Marks, Splay, Record

Grooves, Haze, Jetting, Hooks, and Ripples)

Figure 2.63 shows four examples of typical surface defects that are essentially

defects caused by improper settings of the molding machine

Figure 2.61 Weld lines on cosmetic case.

The plastic flows from the gate through the grill and rejoins afterwards, forming the weld line

Figure 2.62 This lid for an ice cream

container has a cardboard insert and the plastic is molded in a ring around it The weld line is formed opposite the gate location

44 2 The Plastic Product

The surface may show unsightly flow marks as the plastic flows away from the gate This is often the case when injecting into the open surface of a product A simple remedy is to ensure that the plastic coming from the gate

is immediately broken up by “shooting” against the core if that is possible This is always the case with hot runner molds and with cold runner, 3-plate molds where the gate is opposite the core With 2-plate molds, if there is no sidewall of the product, it is best to shoot against a core pin located in the cavity space (a pin or projection either in the cavity or in the core) Products with many thick and thin sections may also show such flow defects Slower injection and adjusting the temperatures and pressures may be the only remedy This will increase the cycle time and affect the cost of the product

2.8.6 Identification of the Molded Piece

Every molded piece should – and in some cases must – have certain

identi-fications molded into it The product designer will sometimes specify the location of some of the engravings, but often leave it to the mold maker to decide their location, size, and style The mold designer must know before starting, which engravings will be required Some of the markings could be created by special, often standard inserts (e.g., the date – week, month, year – when the product was molded); they should be located so that the date can

be changed easily without major work on the mold These markings are often mandatory for food packaging, pharmaceutical, and other products Typical markings, in addition to any specific instructions, are:

 Identification of the seller of the product

 Identification of the molder, if not the same as above

 Resin type identification for recycling

 Cavity (and mold) numbering (position, location)

 Patent number, if applicable

 Made in (identification of country of origin)

 Date of manufacture

 Special information: “Food grade”, “Microwave safe”, “Dishwasher safe”, etc

 Others as could be required by law

 Logos

 Product identification

 Bar code

Figure 2.64 Identification of a molded piece

Drool

Figure 2.63 Typical surface defects

Excessive sink marks

Record grooves

In some countries, some of these markings are mandatory, and it is up the

product designer to specify what is obligatory and what is optional in the

countries where the product will be used and/or sold

Occasionally, there is resistance to have cavity numbers shown, but it is

absolutely necessary for the molder to know from which cavity, and from

which mold a piece has originated to be able to find the cause of any defects

If consecutive numbers are not desired, some code could be used for

identification of the stack from which a faulty piece has been ejected

It is always difficult to add engravings to a mold or to make revisions to

existing engravings If it is anticipated that changes are likely to come, the

mold designer should design the mold with the necessary inserts so that

changes can be made easier, or put it on an inexpensive part of the stack It

may add some costs at the time of building of the mold, but will be much

less expensive than making changes to a finished mold This is often the case

when the same mold could be used for products sold to different customers

Every mold insert should be marked with the mold and cavity number to

ensure that after disassembling the mold for maintenance or repairs, the

various mold parts are assembled in the same relationship as before

dis-assembly

2.9 Product Strength Requirements

Plastic products must not only “look good,” but they must also be strong

enough and suitable for the application for which they are intended The

molding material (the plastic) is usually specified by the product designer to

create a product with all the desired properties This material selection can

affect the selection of features to be built into the mold Not every mold can

be used for every type of molding material A mold that is ideal for one type

of plastic may not be suitable for another, or at least not perform as well For

example, air ejection could be used for a product made from PP, but the

same product in PS would require a stripper ring ejection A different plastic

from what was originally intended to use may also present difficulties in

filling and packing

The addition of fillers to the plastic can increase the strength of the product

but will also affect the shrinkage factors, usually reducing them This means

that the size of the product will be affected when molding a material different

from the one for which the mold was built Fillers can be abrasive and wear

parts of the mold sooner than unfilled plastics This can affect the mold

design and construction, therefore mold parts subject to erosion should be

easily replaceable when worn It may also require the use of better wearing

steels This is particularly important for gates in hot runner molds and all

other areas where the hot, abrasive plastic flows at high speed over the molding

surface, but this kind abrasive action can also be the case in molds with cold

runner gates These provisions will increase the cost of the mold

Every molded part should have a unique identification such as a cavity number

Every insert should be marked with mold and cavity number

Typical wear points usually can be foreseen and the affected mold parts made easily accessible for servicing and replacement

46 2 The Plastic Product

2.9.1 Gate Location to Increase Product Strength

Gate location is important for the ability to fill the mold Some large cavities may require two or more gates to properly fill, which of course affects the cost of the hot runner system

The gate location is also important because it determines the direction in which the plastic molecules will be oriented within the molded product to a large extent In general, the strength of plastic in the direction of flow is greater than in the direction across the flow Near the gate, when the plastic flows in all directions, the strength is relatively low and can be compared with the low strength at a weld line Therefore, if maximum longitudinal strength is required in an elongated product, gating near the end of the product is very important (see Fig 2.65) Thin-walled products are especially vulnerable to this difference in strength, because the thinner plastic cools faster and does not have a chance to re-align the molecules randomly for better strength in all directions, as is the case with heavier-wall products where the plastic stays hot longer between the cooled outer layers and gives the inside core of the plastic time to re-align

It is important to recognize the importance of the gate location It may be easier (and cheaper) to locate the gates so as to employ the shortest runner length (cold or hot runner), but this could result in lower strength of the product, particularly with elongated products, which are subjected to bending forces in use There are also other issues that affect the gate location

 The gate needs to be hidden or recessed, so that any protruding gate mark cannot hurt (scratch) anyone using the product (this is very important in medical products)

 A protruding gate mark could also be in the way of a paper label, which will be applied there

 Some protruding gate marks are protected with gate rings for strength, typically at the bottom of large containers (see Fig 2.66)

2.10 Special Features

A big advantage of molding products from plastic is that different parts of a product can be consolidated into one molding and that many features can

be molded in Whether this is done for better appearance or for better functioning and lower assembly costs of the final product, this makes it sometimes more difficult for the design of the cavities and cores (see Fig 2.68)

In addition, some designers, when redesigning a product which was originally made from metals, do not appreciate or understand the intricacies and problems of mold design and molding, especially if they are used to designing for metals or other materials It is therefore always suggested to scrutinize the drawing for the proposed product to see if its design is appropriate for plastics

Figure 2.65 Typical elongated products,

gated at or near the end for maximum

strength

Figure 2.66 Gate protection rings are used

to increase a pail’s impact resistance

2.10.1 Holes and Counter Bores for Assembly Screws

or Rivets

In general, it makes for more pleasant appearance if mounting screws, used

for example to fasten a cover to a base, are recessed, i.e., the screw head will

be below the surface of the cover If the counter-bore for the screw head is

deep and its diameter is small, the core pin creating this counter-bore becomes

very difficult to cool efficiently and a redesign may be necessary in this area

(see [1] Section 5.2.4 for suggestions in this area)

Plastics lend themselves well for the use of self-tapping screws The design

(dimensions) of screw holes must be to the specifications provided by the

manufacturer of such screws and the screws must be selected to suit the

plastic material with which they are used The holes for the (usually small

size) screws are molded with very small core pins; these pins are very difficult

to cool or cannot be cooled at all Because the hubs or walls where the screw

holes are located are much heavier than the adjoining product walls, the

cooling time will be longer than expected for the rest of the product This

problem may result in grossly underestimating the molding cycle time and

lead to a mold with less productivity than expected Many molds have been

built with excellent cooling in areas for which it was easy to provide cooling

However, because there was no effective cooling at all near critical points,

such as some small core pins, they were a waste of effort and money In other

words, despite the expensive (and unnecessary) good cooling for most areas,

such molds ended up having a very slow molding cycle because of poor

cooling in one area

2.10.2 Hinges and Snaps for Assembly

Snapping two molded pieces together (or snapping one molded piece over a

metal piece) is an ideal application for plastics products

Typical examples are containers for dairy products using a PE snap-on lid

over a PP container, a PE lid on metal can (coffee can), cell phone assemblies,

automotive applications, DVD cases, and many more Some of these snaps

have a long, successful history and the dimensioning is well documented

However, the designer is cautioned that such “snaps” depend on the amount

of shrinkage experienced (a) in the mold during cooling and (b) after ejection,

while cooling outside the machine Also, from experience, a mold can produce

perfect (as expected) products in one machine, but not in another, which

could result in wrong snapping (holding) forces More modern machines

can use higher injection pressures and speeds and can therefore also cycle

faster; in doing so, the shrinkage values will change For example, when

receiving a repeat order for a mold (e.g., a lid), it will be correct to assume

that the steel dimensions of cavities and cores for the new mold will be the

same provided that the mold will run on the same or a similar machine

However, if a more recent or better injection-molding machine will be used,

it will most likely be necessary to establish new core and cavity dimensions

Concentrate on the mold parts that are difficult to cool They determine the productivity of the mold

Figure 2.67 DVD cases with hinges

Never locate a gate in a position that will cause a weld line on a hinge

48 2 The Plastic Product

This can be costly and must be considered in estimating the mold cost In the long run, faster cycles can save a lot of money

Figure 2.68 shows several samples of small and large over-caps for containers with integral, hinged snap-on lids that snap close over the opening in the over-cap Some of these over-caps are produced with simple “up and down” molds (A), some need intricate side cores (B) and much more complicated molds Also shown is a very large box (C) with snap-on lid, with an integral, molded hinge connecting the two parts of the product

C

Figure 2.68 Examples of over-caps for containers