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1 Introduction and Planning
1.1 Introduction
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

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

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
1.2 Oversimplification
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

 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
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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)

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
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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

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
1.5 The Injection Mold
It is important to understand that it
is not the mold cost but the piece
(unit) cost of the product, which is
important
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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?
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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:

proper selection of the runner
system
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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