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It is important to understand the design of that portion of the mold that is actually in contact with the plastic the “stack”, i.e., the cavity, core, and any other mold components, whic

1 Introduction and Planning

Today, injection molding is probably the most important method of

pro-cessing plastics in the production of consumer and industrial goods, and is

performed everywhere in the world

Once the decision has been made to use injection molding for a new product,

a number of difficult choices are ahead which will be addressed later in more

detail:

 Number of cavities

 Stack design, which is the purely technical aspect of how to mold the

pro-duct It is important to understand the design of that portion of the mold

that is actually in contact with the plastic (the “stack”), i.e., the cavity, core,

and any other mold components, which determine how the final product

will be shaped and how the plastic will enter the cavity space

 Method of ejection, i.e., how the product will be ejected from the mold

 What machine should be used?

 Automation – will it be required?

With new, possibly difficult shapes, these decisions are usually left to the

ingenuity of a mold designer More frequently, precedents from earlier molds

are used and reapplied However, the mold designer must be aware of and

evaluate new ideas, new methods, and developments, which when applied

would lead to better quality, higher productivity, simpler molds, and savings

in the cost of the molded products

After the design of the basic stack and before proceeding with any mold design,

the mold designer must understand what kind of mold should be selected;

in other words, which features will be most suitable for the application to

achieve the most economic overall manufacturing method for the product.

This means not just to specify the number of cavities that will be required

for the expected output, but also the selection of mold materials and the degree

of sophistication of the mold Any planned automation, especially in product

handling after molding, can affect the mold layout, particularly spacing and

orientation of the stacks The mold designer must never lose sight of the

ultimate goal: The cost of the product must be the lowest possible, while still

achieving all specified requirements The most important information is to

know beforehand the quantities to be molded, a piece of information,

particularly with new products, often very difficult to obtain.

It should also be pointed out that of the total cost of almost all plastic

pro-ducts, the cost of the plastic material alone constitutes the greatest component

Mold designers must not be stuck in

a comfortable rut

The ultimate goal of a mold is to produce an acceptable quality product at the lowest possible cost

Often, the most important informa-tion is the most difficult to obtain

2 1 Introduction and Planning

The most sophisticated, best designed mold will not lower the cost of the product by as much as the reduction of just a few percent of the amount of plastic material, if it could be removed from the product without affecting its quality or serviceability Most often, unnecessarily heavy wall thickness and ribbing affects the cost more than anything else Chances are that the lowest weight will be achieved with the highest quality molds

In my long experience, I have had numerous occasions when the client insisted

on having his way When I strongly believed it was the wrong thing to do, I suggested to them to have this mold built somewhere else Almost all came back sooner or later for other business, and acknowledged that they should have listened to me

The foremost intent of this book is to present various alternatives available

to the mold designer or decision maker when planning a mold for a new product or planning to increase the productivity for a product for which a mold exists It raises many of the questions that must be asked by anybody who needs a mold built Some of these “questions” may appear obvious and not worth mentioning, or their pursuit may be thought a waste of time, but

I like to point out that any input could significantly affect the productivity as well as the cost of a mold For an experienced mold designer, the answers for many of these questions often come automatically, without him or her being aware of the fact that a decision has been made But even the most experienced mold designer can gain important information by systematically investigating all areas that can affect the design and the complexity of the mold and even the most experienced designers overlook some obvious facts

In this book an attempt has been made to explain why certain mold features should be selected, considering the planned productivity and expected costs There will also be occasionally references to other books on this subject, such

as “Mold Engineering” [5] and “Understanding Product Design for Injection Molding” [1]

Since in many mold shops the mold designer is also involved in estimating the cost of the mold to be quoted, the book also intends to discuss various ways of how to estimate mold costs Properly estimating mold cost is probably the most difficult part of running a successful mold making operation Regardless of how well a shop is equipped with machine tools and other mold making equipment, and how high the level of experience is of the machinists and mold technicians (mechanics), if the mold cost is not adequately quoted it will be impossible to stay in business We must never forget that the primary purpose of any business is to make money, and there

is nothing easier than to lose money by poorly estimating and quoting There

is no magic formula to estimate a mold cost, but good understanding of the principles will lead to better cost estimates

We must not forget in dealing with

the customers who require a new

mold that it is not what they want

but what they need

Figure 1.1 Typical mold-making factory

using automated equipment

1.2 Oversimplification

In the early 1950s, when I was an R&D engineer at a large electrical

manu-facturer, I had just submitted a request of appropriation for a mold for a new

product when the vice-president of sales stormed into my office, and said:

“Why do you always need so much money for a mold? What is a mold? Isn’t

it just an upper and a lower half?”

This was in the heydays of compression molding, before the injection molding

technology gained importance dramatically A compression mold was exactly

what the VP implied: a lower half with one or more cavities, and an upper

half with the matching cores (see Fig 1.2) The plastic was hand-fed into the

lower (open) cavity; there was no or little sophistication with heating (these

molds were processing thermosets and therefore needed to be heated, not

cooled) Often, there was no ejection mechanism at all, or it was relatively

simple

Of course, what the VP failed to understand was the complexity and accuracy

of the work required to build the various components of these “halves”, the

strength required to resist the high molding forces, the time required for

machining and good polishing, and other features required for even a “simple”

mold Unfortunately, even today, many years later, this attitude of

oversimpli-fication is frequently encountered when discussing a required mold and its

cost

Since that time, thermosets (“compression”) molding has become quite

sophisticated, and is using injection molding technology occasionally, but is

still mostly using the vertical machine arrangement, because of the original

loading method of the plastic material by gravity This was also the time

when injection molding took over the molding market from small beginnings

But for a number of reasons it soon became more convenient to use horizontal

machines, although today again, some vertical injection molding machines

are used for certain applications But regardless of the type machine used,

the most important part of the molding system is still the mold

1.2.1 Definitions

Before continuing, here is a list the various terms used:

 Product: an injection molded plastic piece

 End product: an assembly, of which the product is a part

 User (end user): persons using the product or end product

 Customer: the person or company interested in buying the injection mold

 Mold maker: the person or company engaged in making injection molds

 Mold designer: the person responsible for designing the mold

a b c d e f g h

a Upper platen (stationary or moving)

b Heating platen

c Upper mold half

d Core

e Cavity

f Lower mold half

g Heating platen

h Lower platen (stationary or moving)

Figure 1.2 Schematic of a compression

mold for a plate

4 1 Introduction and Planning

 Product designer: the person responsible for designing the product to be molded

 Molder: the person or company engaged in injection molding plastic products

for this Product?

Before proceeding, we must ask: “why was injection molding selected for the job?”

The molder may have a financial or other interest in preferring to have the product made by injection molding, but we must keep an open mind Have alternative methods or product designs been considered or investigated, employing other manufacturing processes using the same or a similar materials, or using other materials which may permit a similar end product, possibly even with better quality, and/or at lower cost?

A few typical examples of possible manufacturing alternatives for injection molding:

 Thermoforming

 Foam molding

 Coining and die stamping (blanking)

 Extrusion blow molding

 Machining, forming of sheets

 Some other, maybe yet to be developed methods and materials Another possibility is not to use plastics at all, but rather use:

 Paper (cardboard), wood, cloth

 Metals (steel, aluminum, etc.) Injection molding has many advantages, particularly low mass, achievable accuracy, good strength-to-weight ratio, good appearance and surface definition, and numerous specific physical properties But injection molded

products always suffer from the fact that the initial capital outlay for molds

and machines can be very high But we must never forget that on a per unit

basis, especially whenever large quantities are considered, the contribution

of the cost of the equipment (mold, machine, etc.) to the cost of the product

is small and often almost negligible

Figure 1.3 Typical injection molded parts

The relatively high capital cost of a

mold is often almost negligible when

evaluated on a per-molded-part

basis

1.4 The Injection Molding Machine

We will not discuss the advantages and disadvantages of the various injection

molding machines that are on the market, but rather introduce the reader

who is not too familiar with this industry to the various terms that will be

used from time to time if a subject under discussion will have special reference

to a machine element or feature The accuracy of molding, and especially

when molding so-called thin-walled products, is very dependent on the

quality of the molding machine, its mechanical rigidity, accuracy of alignment,

parallelism of platens, the quality of its controls, and the state of maintenance

Every good injection-molding machine consists of these basic elements

1 A rigid base

2 A rigid clamping unit, consisting of two platens, for the mounting of the

mold halves and provisions for guiding the platens (tie bars or ways)

3 Provision for moving the platens, preferably fast, relative to each other,

for opening and closing the mold; the speed of motion is usually

adjustable

4 Provision for clamping, i.e., holding the mold shut against the force of

the injection pressures within the mold (in some machines, provisions 3

and 4 are combined)

5 Provision for ejecting the molded product(s) from the mold

6 Provision to transform the raw plastic (pellets, etc.) into an injectable

melt (the plasticizing unit)

7 Provision for injecting the melt into the mold (in most machines,

provisions 6 and 7 are combined in one unit)

8 Provision for heating the plastic in the plasticizing unit

9 Cycle controls (sequencing logic, timers, etc.) and a command post for

manual operation and for mold setup

10 Heat controls for all heaters in machine and molds Some machines have

a limited number of heat controls and additional controls could be

required for the molds, especially with larger hot runner systems This

point must be considered when estimating the mold cost

11 Safety gates to protect operators and bystanders from all hazards when

operating the machine

12 Mechanical safety drop bar(s) to prevent closing the machine when gates

are open, in case of failures of the other (electric and hydraulic) safety

measures

13 Provision for cooling water distribution to the mold

14 Provision for compressed air, for auxiliary actions required in the mold

Even the best machine – if poorly maintained – will not perform as it should

The mold designer who believes that the product considered could be made better by other methods has a duty to discuss this with the

customer, even if it could mean lost business, this time

6 1 Introduction and Planning

There are other features available, e.g., for the convenience of quick mold installation, setting up and operation of the mold and machine; these features are often offered as options which can be bought with the machine or added

on later

1.4.1 The Right Machine for the Mold

Often, the mold cost will surpass the cost of the machine It does not matter how ell a mold is built if the machine cannot meet the molding requirements

to produce quality products While considering the purchase of an injection mold, it is always important to make sure the machine can do the job Some

of the basic considerations are:

 Tie bar spacing

 Stroke and shut height

 Injection speed (average and peak)

 Available injection pressure

 Recovery rate capability (throughput)

 Platen rigidity (are the platens rigid/robust enough to carry the mold weight?)

 Available clamp tonnage

 Platen parallelism

 Clamp speed requirements

 Shut-off nozzles

 Screw design

 Accuracy and repeatability of controls

 Operator access

 Mold protection capability

As the machine and mold act together as a system, it is fair to say that the system will perform only as well as its weakest component If an existing machine is to be used, the machine should match the machine's capability The mismatched machine can easily destroy the new mold in a matter of months, resulting in costly rework

To determine the right machine, the following information on the mold is required:

 Mold width, length, and height

 Opening stroke required (usually 2.5 × part height)

There is no point in buying a

premium priced mold to run it in an

out-dated machine

 Ejector rod locations

 Locating ring size

 Part dimensions, including wall thickness

 Flow length (length of flow from gate to longest flow path)

 Part weight

 Runner weight (if cold runner)

 Cavitation

 Nozzle radius

 Material (including color and additives, viscosity)

To the customer or entrepreneur not familiar with the problems of molding

and mold making who wants to make a new product, the price of a mold

may seem to be high, occasionally even outrageous; it is often difficult to

convey that the mold price constitutes only a very small portion of the product

(piece) cost, and depends much on the expected production of the mold

1.5.1 What Is an Injection Mold?

A (plastics) injection mold is a permanent tool, i.e., a tool that, if properly

designed, constructed, and maintained will have a life expectancy (useful

life) well beyond the time where the product itself becomes obsolete This

differentiates it from a “one-time use” mold such as a sand-casting mold, as

used in metal foundries A mold can be used to make products in a virtually

infinite variety of shapes, made from injectable plastics Common to all molds

is the condition that it must be possible to remove the product after molding,

without the need to destroy the mold (as is the case in sand-castings)

There is an exception to this, the so-called “lost-core molding”: There are

injection molds for intricate products, such as intake manifolds for internal

combustion engines, previously made from cast iron, which have an outside

shape that can be molded with conventional (permanent, “open and close”)

molds but where the intricate inside shape is made from a molded, low melting

point metal composite which is inserted into the mold before injection, and

then ejected together with the molded product; the metal is then removed

by heat at a temperature above the melting point of the insert, but of course

below the melting point of the plastic used for this product; the molded

metal insert is thereby destroyed, but the metal will be reused

A basic mold consists of two mold halves, with at least one cavity in one

mold half, and a matching core in the other mold half These two halves

It is important to understand that it

is not the mold cost but the piece (unit) cost of the product, which is important

8 1 Introduction and Planning

meet at a parting plane (parting line) As the mold opens – after the injected

plastic (now in the shape of the desired product) is sufficiently cooled and rigid – the product can be removed by hand or be automatically ejected Because injection-molding machines are mostly built with the injection on the stationary platen side, there is, typically, no built-in ejection mechanism

on this side If ejection from the injection side should be required – always

the case in stack molds, and occasionally required in single level molds – any

required mechanism must be added to the mold, and occasionally to the machine; in either case, this adds complexity and increases costs Only molds designed for using only air ejection do not require any external ejection mechanism

Most products are removed (ejected) from the core There are also many molds, which need special provisions to allow the products to be removed from either the cavity or the core This is the case with products having severe undercuts or recesses on the inside and/or the outside of the product, such

as screw threads, holes, ribs or openings in the sides of the product, etc., or molds for insert molding

Some of these design features of the product may require moving side cores, which are either inserts or whole sections of the cavity that move at an angle which is 90° to the “natural opening path” of the mold Others may require special unscrewing mechanisms, either in the core or in the cavity side The mold may require split cavities (or “splits”), i.e., the cavity consists of two or more sections, which are mechanically or hydraulically moved in and out of position, and then clamped together during injection In some cases, the mold may require collapsible cores, or retractable inserts, which are all quite complicated (and expensive) methods

Any of the above special features can add considerably to the mold cost when compared to a simple “up and down” mold where the products can be readily ejected with the machine ejectors during the mold opening stroke or after the mold is open, without the need for any of these complicated mold features Note that in this book, the term (simple) “up and down” molding is used, which comes from the earlier vertical molding machines, even though, today, most general-purpose injection molding machines are horizontal and the mold opens and closes in a horizontal motion

Example 1.1

To illustrate how different mold features affect the mold cost, we assume that a single face mold with air ejection of the products costs X dollars

A similar mold, but with mechanical ejection, costs about 1.2 times X

A similar, air- ejected 2-level stack mold will be about 1.8 times X

An unscrewing mold for a similar size mold and product will cost about

2 times X

Almost any shape can be molded –

but at what cost?

1.5.2 Elements of an Injection Mold

Most readers will expect to see some illustrations (photos or schematics) of

injection molds at this point However, we must not forget that this is not a

book about mold design, but about the relationship between productivity

and cost of molds, as well as the cost of the products to be made There will

be, however, a number of photos of molds accompanying the text where

deemed useful

There are books that show designs of numerous, specific molds but it is

virtually impossible to show every possible configuration that may be

required It is more important for the designer, and any person requesting a

new mold, to understand that a mold consists essentially of a number of

elements from which to choose for the most appropriate design for the

purpose

Every injection mold consists of the following basic elements:

1 One or more matching cavities and cores, defining the cavity space(s)

(today, there are molds with anywhere between 1 and 144 cavities)

2 A method, or element, to duct the (hot) plastic from the machine nozzle

to the cavity spaces:

There is a choice between

– Cold runners (2-plate or 3-plate systems)

– Hot runners (various systems)

– Insulated runners, through shooting

– Sprue gating (cold or hot)

3 Provision to evacuate air from the mold (venting):

There is a choice between

– Natural venting

– Vacuum venting

4 Provision to cool the injected hot plastic sufficiently to allow ejection of

the molded product

5 Provision to eject the molded product:

There is a choice between

– Manual product removal

– Ejector pins and sleeves

– Stripper s (stripper rings or bars)

– Air ejection

– Random ejection

– Various methods of in-mold product removal methods

– Robotic product removal

6 Provision to attach (interface) the mold to the molding machine:

There are several methods to consider

– Mold is for one machine only

Product quality, productivity, and mold cost depend heavily on the proper selection of the runner system

10 1 Introduction and Planning

– Mold to be used on several, different machines – Quick mold change methods (various designs)

7 Method of alignments of cavities and cores:

There are several methods to consider – No alignment feature provided in the mold – Leader pins and bushings (2, 3, or 4) – Leader pins and bushings between individual cavities and cores – Taper fits between individual cavities and cores

– Taper fits between plates – Any combination of the above

8 Any number of (mold) plates to provide the necessary for carrying and backing the above elements

But molds could have additional features, which will also be discussed in the following Each of these features can add (often considerable) costs to the mold but in many cases can increase the productivity of the mold and reduce the cost of the product They may or may not all be necessary and must be carefully considered when deciding on the type of mold most suitable (and most economical) for the job on hand

Features such as serviceability of the mold may affect the mold cost; for example, the access to hot runners for cleaning plugged gates or making minor repairs, such as changing a nozzle, a burned-out heater, or a faulty thermocouple at a hot runner drop will cost more in the initial mold, but this will be easily recouped by reducing the down time necessary to accom-plish such repairs By designing easy access to these components in the machine (without the need to remove the whole mold, or part of it, to the bench), such repairs can be made in less than an hour, instead of taking several hours This work can also be done by the mold setup staff rather than getting the (expensive) mold makers involved

Another area where valuable maintenance time can be saved is to design and provide easy access from the parting line to screws holding modular mold parts to their mounting plates, while the mold is in the machine

On the other hand, in my experience, many molds, particularly molds for lower production quantities, have been vastly over-designed and much money has been wasted

The main purpose of this book is to discuss the various elements or features listed above and to facilitate the selection and the decision making Defining

what is really required considering the shape and complexity of the product

and the required production quantities will enhance mold productivity In addition, the book should facilitate investigating if, even minor, changes to the product shape could lower the mold cost and improve the productivity

of the mold or the whole system

Figure 1.4 Mold maintenance in the press

is important Here, the operator is changing

a nozzle tip while the mold is in the press

(Courtesy: Husky)

Easy serviceability of the mold is

important but often overlooked It

adds some mold cost, but saves

much more in future servicing costs

and downtime

Even minor changes to the part can

dramatically lower or increase mold

costs