Citation
Analysis for the manufacturing of injection molded parts using CAD/CAM techniques

Material Information

Title:
Analysis for the manufacturing of injection molded parts using CAD/CAM techniques
Creator:
Bajrai, Waleed Hussein
Place of Publication:
Denver, Colo.
Publisher:
University of Colorado Denver
Publication Date:
Language:
English
Physical Description:
vi, 67 leaves : illustrations (some color), 6 folded ; 29 cm

Thesis/Dissertation Information

Degree:
Master's ( Master of Science)
Degree Grantor:
University of Colorado Denver
Degree Divisions:
Department of Mechanical Engineering, CU Denver
Degree Disciplines:
Mechanical Engineering
Committee Chair:
Sanchez, L. Rafael
Committee Members:
Gerdeen, James C.
Adkins, R. Wayne

Subjects

Subjects / Keywords:
CAD/CAM systems ( lcsh )
Injection molding of plastics ( lcsh )
CAD/CAM systems ( fast )
Injection molding of plastics ( fast )
Genre:
bibliography ( marcgt )
theses ( marcgt )
non-fiction ( marcgt )

Notes

Bibliography:
Includes bibliographical references (leaves 66-67).
General Note:
Submitted in partial fulfillment of the requirements for the degree, Master of Science, Mechanical Engineering.
General Note:
Department of Mechanical Engineering
Statement of Responsibility:
by Waleed Hussein Bajrai.

Record Information

Source Institution:
University of Colorado Denver
Holding Location:
Auraria Library
Rights Management:
All applicable rights reserved by the source institution and holding location.
Resource Identifier:
37846983 ( OCLC )
ocm37846983
Classification:
LD1190.E55 1997m .B35 ( lcc )

Full Text
ANALYSIS FOR THE MANUFACTURING OF INJECTION MOLDED PARTS
USING CAD/CAM TECHNIQUES
by
Waleed Hussein Bajrai
B.S.M.E., University of Colorado at Boulder, 1995
A thesis submitted to the
University of Colorado at Denver
in partial fulfillment
of the requirements for the degree of
Master of Science
1997
AL


This thesis for the Master of Science
degree by
Waleed Hussein Bajrai
has been approved
by
R. Wayne Adkins
Date


Bajrai, Waleed Hussein (M.S., Mechanical Engineering)
Analysis for the Manufacturing of Injection Molded Parts using
CAD/CAM Techniques
Thesis directed by Assistant Professor L. Rafael Sanchez
ABSTRACT
The primary objective of this project was the analysis of the
manufacturing of injection molded parts using CAD/CAM techniques.
Injection molding is one of the most popular methods for producing
plastic parts. In order to achieve the consistency and reliability
required, the mold must be built with a high degree of precision.
Until recently, this process typically required considerable manual
dexterity. With the advent of CAD/CAM techniques, moldmakers have
simplified their tasks, improving their quality and reduced their costs.
The analysis was performed according to the following steps:
- Design of the mold for the part
- Analysis of the mold design
- Manufacturing of the mold
- Cost analysis to manufacture the mold and the plastic part
AutoCad 13 was used to design the mold. In addition EZ-mill
software was generated to fabricate the mold using a 3-axis CNC
111


(Computer Numerical Control) Milling Machine. The prototype for the
analysis was based on a plastic part typical of those found in the
industry (courtesy of Matrix Co.). The part consisted of a top and a
bottom cap that form an enclosure to host a circuit board. The
produced mold for the part had to meet the standard specifications.
The results of this study show the costs and benefits associated with
the use of modern CAD/CAM techniques applied to injection molding
part design.
This abstract accurately represents the content of the candidates
thesis. I recommend its publication.
Signed1
L. Rafael Sanchez
IV


CONTENTS
1. Introduction.....................................................1
1.1 Project Description.............................................7
1.2 Device Description..............................................7
2. Injection Mold and Material required.............................9
2.1 Injection Mold..................................................9
2.2 Materials for Mold Fabrication................................11
2.2.1 Ferrous Material.............................................12
2.2.2 Non-ferrous Material.........................................13
2.2.2.1 Copper Alloys..............................................13
2.2.2.2 Zinc Alloys................................................15
2.2.2.3 Aluminum...................................................15
2.3 Plastic Material...........................................16
2.3.1 General Characteristic of Plastic Materials..................20
2.3.2 Choosing the Right Material.................................22
3. Mold Design and Fabrication.....................................25
3.1 Plastic Part Design............................................28
3.2 Mold Design Description.......................................30
3.2.1 The Cavity Plate.............................................32
3.2.2 Core Plate...................................................36
3.2.3 Ejector Plate................................................37
3.2.4 Ejector Housing..............................................38
3.3 Mold Fabrication...............................................38
v


3.4 Manufacturing the Plastic Part...........................41
4. Cost Analysis..............................................45
4.1 Mold Cost.................................................45
4.2 Part Manufacturing Cost...................................46
5. Analysis of the Flow.......................................48
5.1 Geometry of Single Screw..................................48
5.2 Feed Zone.................................................51
5.3 Plasticating Zone.........................................52
5.4 Metering Zone.............................................53
5.5 Overall Calculation for Output............................55
6. Conclusion.................................................57
7. Recommendations............................................58
8. Appendix...................................................59
9. References.................................................66
vi


1. Introduction
Since its birth in the 1800, injection molding has grown steadily to
become a thriving industry. Injection molding has become the method
of choice throughout the manufacturing industry. This is enhanced by
the ease of with which injection molding delivers parts for industrial
applications with high precision and quality. Injection molding has
found use in the fabrication of plastic parts such as combs,
automotive parts, toys, medical products, plumbing, and packaging.
In general, injection molds require high precision so that they can
yield high quality products. Since the mold serves as the building
block for parts that need to be manufactured, it must be reliable, very
precise, and provide a long service life with minimum maintenance
and problems.
Injection molds are usually made from high-strength metals such as
steel because of the high pressure and heat that the mold must
operate under during its service. The selection of the mold material is
heavily dependent upon the material of the part to be manufactured
and the manufacturing process. At present, one can use alloys and
material upgrades to rapidly build very light molds. For example,
because of their light weight, ease of use, and availability, aluminum
alloy with a surface hardness 188 BHN and its alloys have become the
material of choice among mold makers to fabricate high precision
molds. Similarly, other materials such as Beryllium copper, i.e.
Surface hardness 28-42 Rockwell hardness, compared to steel-
carbonizing with surface hardness of 60-65 Rc, have been used
successfully to reduce the time it takes to fabricate a mold so that
l


products can reach the market place faster. Molds have also been
fabricated from plastics such as epoxies with low surface hardness to
yield parts with high precision and quality. Table 1.1 shows the
surface hardness and core hardness for the different materials used
to fabricate molds.
Table 1.1: Spectrum of Materials Used in Building Molds-Arranged in Order
of Surface Hardness.
Material Class Surface Hardness Core Hardness
Carbides 68-75 Rc 68-75 Rc
Steel, nitriding >68 Rc >38 (Varies)
Steel, carbonizing 60-65 Rc 20-42 Rc
Steel, water hardening 67 Rc 40-55 Rc
Steel, oil hardening 62 Rc 40-60 Rc
Steel, air hardening 60 Rc 60 Rc
Nickel-cobalt alloy 45-52 Rc 45-52 Rc
Steel, prehardened 44 Rc 44 Rc
Beiyllium copper 28-42 Rc 28-42 Rc
Steel, prehardened 28-32 Rc 28-32 Rc
Aluminum bronze 188 BHN 188 BHN
Steel, low alloy & carbon 180 BHN 180 BHN
Kirksite (zinc alloy) 80-105 BN 80-105 BN
Aluminum alloy 60-95 BN 60-95 BN
Brass 50 BN 50 BN
Sprayed metal <50 BN <50 BN
Epoxy, metal filled 85 RM 85 RM
Epoxy, not filled 80-110 RM 80-110 RM
Silicone rubber 15-65 Shore A 15-65 Shore A
Legend: Rc = Rockwell hardness = C scale; RM = M scale; BHN = Berinell
number 3000 kg;
BN = Brinell number 500 kg load. (Courtesy of Stokes-Trenton, Inc., Trenton, NJ)
In general, the injection mold consists of two main parts and
two secondary parts; the force, the cavity, the ejector plate, and the
ejector housing, respectively. Figure 1.1 shows the parts in a standard
The force or core gives the manufactured part the inside details and
the cavity gives the part its outside shape.
2


1 SPRUE BUSHING
2 LOCATING RING
3 TOP CLAMPING
PLATE
4 FRONT CAVITY PLATE
5 REAR CAVITY PLATE
6 SUPPORT PLATE
7 EJECTOR HOUSING
8 EJECTOR RETAINER
PLATE
9 EJECTOR PLATE
10 EJECTOR PINS
11 CORE INSERT (male
section)
12 CAVITY INSERT (female
section)
13 SPRUE PULLER PIN
14 WATER LINES
Figure 1.1: Various Components of a Two-Plate Injection Mold.
(Courtesy Dow Plastic Co., Midland, MI)
In general, injection molding machines possess the same
standard parts which include the following: a clamping or movable
end which can be moved by either a hydraulic or electric system, a
stationary end which provides a nozzle protrusion and retention for
the fixed half of the mold, and plasticizing and material feed units.
Figure 1.2 shows a schematic diagram of a standard injection molding
machine. The mold can be directly mounted on the platen of a
molding machine, where the molten is injected inside it through the
sprue to produce the part.


FEEDSCREW
FIXED PLATEN
Figure 1.2: Basic Components of Injection Mold Machine.
The cylinder shown on the left side of Figure 1.2 serves to open
and close the mold and provides the clamping pressure which closes
the mold when injection of the material takes place. The right
cylinder shown in Figure 1.2 provides the force necessary for the
screw to act as a ram and push the melted material inside the mold.
Injection machines can operate either horizontally or vertically.
However, machines which operate horizontally are more widespread.
In contrast, machines that operate vertically are more desirable when
one desires insert or lose coring types of molding.
The injection machines, which are designed to operate
continuously at high speeds, are equipped with numerous valves,
timers, heating controls, and safety features. Today the injection
presses have become so sophisticated that versions using computer
numerical control (CNC) are available. Such presses can monitor the
4


whole molding process through the injection pressure, the velocity
feed of filling, the back pressure on the screw, and the time and
temperature measurements. They also offer features for storing the
parameters of a given molding condition or for future use. Further,
todays presses include self-diagnosis tools to detect and correct
problems that might occur during the molding process. Note the
capacity of the injection mold machine is rated in ounces of
polystyrene molded per cycle, or in cubic inches of material.
A considerable amount of material is needed to proceed with the
injection molding process. The material is held in the heating
chamber. The temperature in the chamber is controlled by the heating
bond. The temperature increases until it reaches the required level so
that the plastic material reaches its plastic state and can be pushed
into the mold. The mold is usually kept at a temperature below that of
the material so that it allows the compound to harden after it is
injected. When the hardening process is finished, the ejector pins help
the part to exit the mold.
The selection of materials for the manufacturing of a given part
is dictated by the particular design, the material properties, the
manufacturing process, and costs of the process and material. Since
the part that we fabricate is plastic based, we will concern ourselves
mainly with this type material. In general, plastics materials are
synthetic materials built up from monomeric building blocks that are
joined together to produce high molecular weight polymers. Plastic
materials are considered to be so versatile that their properties can be
very well controlled to match requirements of any specific end user
application. The plastics characteristics are listed in Table 1.2, shown
5


in comparison with metals. The properties of plastics can be varied
by either altering the basic chemical composition (using different
monomers, CO-monomers, polymerization conditions, etc.) or by
using different additives that are melt blended with the polymer to
enhance specific properties lacking in the base polymer. Hence, it is
mandatory that one understands the chemistry of polymers in order
to be well qualified to work with plastics materials.
Table 1.2: Characteristics of Plastics when Compared to Metals
Characteristic Advantage / Disadvantage
Low melting point High elongation Low density Low thermal conductivity Electrical resistance Optical clarity Easily colored Solvent sensitivity Flammable Ease of processing/Lower useful thermal range Low brittleness/Higher creep and lower yield strengths Lightweight products/Low structural strength Good thermal insulation/Dissipates heat poorly Good electrical insulation/Doesnt conduct electricity Usefulness as a clear material/Degradation by sunlight Use without painting/Difficult to match colors Can be applied as a solution/May be affected by solvents Waste can be burned/May cause fumes or fire hazard
Note that some characteristics of a given material can be
advantageous in one application, but present a disadvantage in
another. For example, the low melting point of plastics renders
processing easy since the molding can be done at lower temperatures.
However, it yields a narrow useful thermal range because plastics lose
more of their beneficial mechanical properties at lower temperatures
than do most metals. Therefore, the selection of material requires
careful study of the material and the part to be designed.
6


1.1 Project Description
This paper will present a detailed overview of the injection process.
Further, the specific steps will be discussed which are required to
design and manufacture an injection mold for a plastic enclosure that
consists of two parts, i.e. a top cap and a bottom cap (see Figure 1.3).
The completed plastic enclosure will host a circuit board. The
enclosure includes an extremity which enables the user to clip the
electronic device to their belt.
Figure 1.3 The enclosure parts
1.2 Device Description
Matrix Corporation was contracted by PEAK Engineering &
Automation Company to produce an injection mold that will be used
for manufacturing a part that will host a circuit board. The
specifications of the mold are as follows: the mold should be built
7


from Aluminum 7075, which represents the hardest aluminum alloy
that is suitable for the fabrication of prototype molds. The mold base
should be 11 inches long and 9 inches wide. Such dimensions are
standard for making the final plastic part. The fabricated mold will
have two cavities that will produce the top and bottom parts of the
enclosure. All parts of the mold must be fitted and the produced part
is required to be well within the specified tolerance range. When
completed, the mold will be tested in a trial run, and the resulting
part must meet all the requirements.
8


2. Injection Mold and Material required
2.1 Injection Mold
Injection molds for thermoplastic materials depend on the type of
material used for molding, the design of the feature gating, and the
ejection system in order to fulfill the manufacturing requirements
with maximum economy. Moreover, the selection of materials for the
manufacturing of a given part is dictated by the particular design, the
material properties, and the manufacturing process. The costs of the
process and material dictate the mold size, amount of mechanization
required, and the cycling efficiency. Based on their criteria, one could
classify molds as follows:
1. Two plate mold:
For this type of mold, the cavities are assembled to form one plate
which applies a force against the other plate. The stationary half of
the mold will have a central sprue bushing runner system to multiple-
cavity molds or direct center gating to an individual-cavity mold. The
moving half of the mold normally contains the forces and ejector
mechanism and runner system.
2. Three plate mold:
This type of mold is similar to the two plates mold with an
additional movable intermediate plate. The intermediate plate, which
normally contains the cavities for multiple-cavity molds, permits
center or offset gating of each cavity from the runner system
connected to the central sprue bushing.
9


3. Loose detail mold:
In this type of mold, threads, insert or coring which cannot be
produced by normal operation of the press are processed by separate
mold details which are ejected with the part and removed by hand or
with a disassembly fixture after cycling. This practice is often used
for experimental or small production requirements.
4. Horizontal or angular coring mold:
A particular method has been used in this type of mold that
permits the movement or coring of mold sections. Through the use of
angular cam pins that permit secondaiy mold detail movements, this
method must be actuated by pneumatic or hydraulic cylinders which
are energized by the central press system, cams, solenoids, or an
independent air supply. This type of mold is used for intricate
product production requirements.
5. Mold with rising cam or ejector angular movement:
This mold is used to mold undercuts on the inside of the part or on
the outside when there is no room in the mold to accommodate an
angular cam mechanism. The head of the cam must slide on the
ejector bar as it moves forward, and is usually made in the form of a
T nut.
6. Ejection on nozzle side of mold:
In this mold the ejector plate may be operated by pulling it forward
using a roller chain or pull rods from the rear half of the mold, or by
using air or hydraulic cylinders mounted on the ends of the ejector
bar. This design is used when it is necessary to have the ejection on
the same half of the mold as the sprue.
7. Automatic unscrewing mold:
10


This mold incorporates thread cores or bushings actuated by a
gear and rack mechanism, and moved by a long double-acting
cylinder sequentially within the molding cycle. This type of mold is
used when internal or external threads on product designs require
large volume and low production cost.
2.2 Materials for Mold Fabrication
Mold materials constitute the least important item when
fabricating a mold. However, when the choice of the material is not
appropriate, it results in many hours of labor being lost, which in
turn can add up to a monetary loss. Figure 2.1 details the breakdown
on the cost for building an injection mold.
80%
15%
5%
' Build, finish,
try out,. -
r'shop overhead
jDesign^naricetingi
}*. administration
f^^Material
Moldmaking costs
Moid-using costs
40%
40%
20%
Figure 2.1 Break Down of What it Costs to Build and Maintain an Injection
Mold. (F.T. Gerson Ltd., Toronto, ON, at 1992 SPE Molding and Mold Design
Div. Retec.)
11


There are several factors which determine the selection of
materials for a given mold. These include three main factors: the cost
of manufacturing the mold, the nature and shape of the mold, and the
properties of the mold material. Generally speaking, mold
construction materials can be divided into two groups: ferrous and
non-ferrous materials.
2.2.1 Ferrous Material
Steel and its alloys are the most popular materials for injection
molds. They provide the following advantages: economical machining,
capacity for heat treatment, sufficient toughness and strength, ease of
polishing, resistance to heat and wear, high thermal conductivity, low
notch sensitivity, and corrosion resistance. Through different heat
treatments such as annealing, steel can be tailored to achieve the
required properties for its application. Steel Alloys represent steel
materials which contain other alloying elements in addition to carbon.
These materials are added in precise concentrations to modify the
properties of steel to meet the required specifications. Table 2.1 lists
some elements and alloys used in fabricating molds and gives the
properties peculiar to each.
12


Table 2.1 Effect of Alloying Elements in Steels used for Mold Making
Element Effect
Carbon Increase strength and brittleness; lowers corrosion and conductivity
Manganese De-oxidize, raises strength, ductility, wear resistance
Silicon De-oxidize, raises hardness
Nickel Austenite stabilizer; raiser toughness, strength and through-hardness but lowers thermal and electrical conductivity
Chromium Carbide former; increases hardness, resistance to wear and corrosion; lower conductivity
Molybdenum Increases heat resistance; expands heat treatment range; raises creep strength
Vanadium De-oxidizer; forms hard carbides; raises fatigue strength
Tungsten Form extremely hard carbides; increases hardness and resistance to wear and high temperature; helps to mentain sharp edges (parting lines)
Sulfur Increases machinability (free machining) but lowers corrosion resistance and weldability ands interferes with texturing and plating
2.2.2 Non-ferrous Material
The best known non-ferrous metals for injection molding are
copper alloys, zinc alloys, and aluminum alloys. These materials are
mostly used to fabricate molds when a small number of parts is
required for manufacturing, e.g., prototype or experimental molds.
2.2.2.1 Copper Alloys
Copper Alloys are known for their high thermal conductivity
and ductility. The mechanical properties of copper alloys can be
improved by cold rolling or forming. Beryllium copper is a series of
hard enable copper alloys, with a basic composition of 2.5% beryllium
and 96.5% copper. It has been used in cases where casting is more
effective than engraving to generate odd shapes or fine details. In
13


general, the alloys with 1.7% or more beryllium provide better fluidity,
and therefore reproduce details more accurately. A high Beryllium
concentration yields a high fidelity but increases the cost of the
molding. Table 2.2, shows different Beryllium copper alloys and their
applications.
Table 2.2 Beryllium Copper Alloys; Properties and Applications.! 1]
Designation Composition (Balance Cu) Be Co Others Thermal Conductivity (BTU/ft/hr/F at 68F) HRC Characteristic and Applications
C172000 2.0 0.5 60 40 Good strength and wear resistance with good electrical and thermal conductivity
C17510 0.6 2.5 145 22 High thermal and electrical conductivity but lower hardness. Used where max. heating and cooling rates are essential
C82400 1.7 0.3 58 37 Good strength, hardness, corrosion resistance, and conductivity
C82510 2.0 0.5 0.3Si 56 41 Similar to 82400 above, but with better castability
C82600 2.3 0.5 0.3Si 54 44 As 82400 above, but with improved wear resistance. Used in pressure and Ceramic Casting
C96700 1.2 30.0 Ni 21 50 Highest corrosion resistance, strength, and castability, but lower thermal conductivity. Resists flame retardants, blowing agents, and other corrosive chemicals contained in molding resins
14


Alloys with less than 1.7% beryllium are generally used only for
mold cores and mandrels where high fidelity of reproduction and very
high strength are not required. In this case the cost of the material
remains low and the thermal conductivity is high, which allows for
faster heat extraction and shorter cycles during molding.
2.2.2.2 Zinc Alloys
The properties of zinc alloys are very similar to these of copper
alloys. Zinc alloys have a very high thermal conductivity of 105w\m.k.
Typical compositions for zinc alloys are Zn 92%, Cu 3.5%, and A1
4.0%. The physical properties are Tensile 38.000 psi, elongation 3 to
5%, melting point 717F, and hardness 80 to 105 Brinell. However,
because of their inferior mechanical properties, these alloys are
mainly used to fabricate pre-production prototype molds or for small
production runs. In contrast, high grade Zinc alloys are used in metal
spraying processes where cavities are generated from a male master
or mandrel.
2.2.2.3 Aluminum
Pure aluminum is considered too soft to be used to fabricate
injection molds. However, with recent advances in aluminum alloys,
these alloys are finding use in plastic molds components, especially
for high-pressure injection molds often used in prototypes or on
15


limited production runs. They are widely recommended for expanded
foam, structural foams (low pressure processes), rotational, blow, and
cost molds.
Aluminum alloy has a lifetime in production runs of over
250,000 injection molding cycles in most of the common resins except
those which require corrosion resistance (e.g. PVC). Aluminum alloy
7075 is used for short run injection mold construction because it is
low cost and easy to machine compared to steel. Techniques such as
anodizing can improve the properties of these materials. For instance,
anodizing greatly increases the alloys wear resistance, but limits its
ability to make sharp edges due to its high probability of chipping.
Hence, when selecting a mold material, a tool designer must be well
aware of the requirements of the produced part, material properties,
and process conditions.
2.3 Plastic Materials
This section will discuss some properties and aspects of plastics
materials so that one is able to select the proper material for a given
application. Note that the use of plastics to design parts also requires
a fundamental knowledge of polymers.
Plastics are defined as a synthetically produced material that
can be molded and hardened into objects or formed into films or
textile fiber. Plastics are primarily derivatives of natural gas and oil.
The properties of plastics depends heavily on the size of the molecule
and on the arrangement of the atoms within the molecule. There are
16


two basic types of plastics available from which to produce a product.
These are termed thermoplastic and thermoset. Thermoplastics are
linear or branched polymeric materials that soften when heated, and
resolidify when cooled and are recyclable. Thermoset plastics are
polymers that chemically react during processing to form a cross-link
polymer chain network. Unlike thermoplastics, thermosets are not
directly recyclable. Table 2.3, shows a list of major polymers.
Table 2.3: Major Polymers [2]
Group Name
Plastics Thermoplastic Polyethylene (PE) Polypropylene (PP) Polystyrene (PS) Polyvinyl Chloride (PVC) Polyacetal (POM) Acrylic (PMMA) Polyamide (PA) Polycarbonate (PC) Polytetrafluorethylene (PTFE)
Plastics Thermosets Epoxy (EP) Melamine-Formaldehyde (MF) Urea-Formaldehyde (UF) Unsaturated Polyester (UP) Phenolic (PF) Alkyd Polyurethane (PUR)
One way to classify thermoplastic materials is to use a polymer chain
conformation. This yields the following categorization:
1) Amorphous polymer: Consists of polymer molecules with random
configuration.
2) Semi-Crystalline polymer: In this chemical configuration, polymer
molecules have enough regularity and flexibility built into their
structure. This allows the molecules to form ordered molecular
arrangements. These ordered regions are crystals that form as the
17


thermoplastic cools from the molten state. When this material
reaches the molten state under heat, it has an amorphous
configuration, hence, the name semi-crystalline. Polyethylene,
polypropylene, and nylons are examples of this kind of polymer.
3) Liquid Crystalline: This material, like semi-crystalline
thermoplastics, possesses ordered chain arrangements in the solid
state. In addition, it also exhibits ordered molecular arrangements
in the melted state. Figure 2.2 shows the different chain
configurations for polymers.
Amorphous Semi-crystalline Liquid crystalline Thermosetting
poivmer polymer polymer polymer
Figure 2.2 Plastic Materials are Categorized as being either Amorphous,
Semi-Crystalline, Liquid Crystalline or Thermosetting.[3]
18


Polymers are seldom used alone as plastic materials, but are
usually blended with additives because in most cases designers must
modify the material composition using additives to suit their needs.
The following is a list of additives:
1. Inert particulate fillers (e.g., whiting and china clay used, for
example, in PVC).
2. Reinforcing particulate fillers (e.g., carbon black and fine silicas
which are used mainly in rubbers to enhance tear strength,
abrasion resistance and modules).
3. Fibrous fillers (e.g., glass used widely in engineering
thermoplastics and also carbon).
4. Antioxidants. These reduce the effects of oxygen on aging and at
elevated temperatures. These are widely used in polyethylene and
polypropylene.
5. Stabilizers. These take many forms, but of particular importance
are the additives used to reduce degradation rates in PVC.
6. Plasticizers. These may make a polymer mass flexible, as in the
case of plasticized PVC.
7. Fire retardants.
8. Pigments.
9. Lubricants. There are many types of lubricant. External
lubricants help to prevent sticking of molten polymers to
processing equipment. Internal lubricants are designed to aid flow
without having a plasticizing effect, while materials such as
graphite and molybdenum disulphide are used to reduce friction of
the polymer against other material.
10. Cross-linking systems-used with thermosetting plastics.
19


11. Blowing agent-used to make cellular, (e.g., foamed) plastics.
Additives do have side effects which are sometimes significant,
particularly when the additives are used at high concentrations. For
example, the edition of glass fibers to a material will typically improve
properties such as modulus, strength, and thermal conductivity.
However, the reinforced materials are typically more difficult to
process when considering problems such as equipment/mold
abrasion, achievable part surface finish, and fiber orientation. It is
very important for designers to recognize the fact that commercial
plastic material grades do contain additives, and the effect of these
additives on all aspects should be considered.
2.3.1 General Characteristic of Plastic Materials
This section will discuss some of the important characteristics that
product design is need to know about plastics materials that will aid
in choosing the plastic material for certain applications.
1) Relatively easy to mold into complex shapes: The injection molding
process can be used to produce thermoplastic or thermosetting
plastic parts of very complex geometry.
2) Low specific gravity: Most plastics materials have specific gravity
values that range from approximately 0.8 to 1.8. These values are
much smaller than those of steel. Also, plastic materials have good
strength/weight or stiffness/weight ratios, which give the plastic
materials an advantage in terms of performance because it makes
it possible to produce lightweight products that are durable.
20


3) Relatively low energy requirements for processing: In general, the
overall energy requirements for plastic materials processing are
significantly lower than for metals, glasses, or ceramic. Because
plastic materials in general have lower processing temperatures
and lower specific gravities/part weight.
4) Mechanical performance: Plastic materials are available with
mechanical properties that range from elastomeric to rigid and
stiff. Plastic materials can also be very impact resistance or tough,
and useful in a variety of high abuse/high impact applications.
Plastics mechanical properties can be very sensitive to any
changes in temperature, rate of loading, and relative humidity.
5) Good electrical insulation: Many plastic materials offer outstanding
electrical insulating properties, and as a result, are often used in
electrical insulating applications such as switches and electronic
enclosures.
6) Good thermal insulation: This is important in a variety of energy
conservation applications. On the other hand, the low thermal
conductivity can be a problem in dynamic applications such as
gearing, where frictional heat is generated.
7) Flammable: Almost all plastic materials will burn to some degree,
or decompose when subject to combustion condition.
8) Poor weather resistance: Many plastic materials have poor long
term weather resistance. The long weather resistance of any
polymer can be improved significantly by using ultraviolet
stabilizers and antioxidants as additives. In some cases, coating is
used to overcome the problems associated with long term aging.
21


9) Chemical resistance: Most plastic materials offer good resistance to
corrosion caused by the presence of moisture, salts, weak acids,
and bases. However, most thermoplastics are soluble or will swell
in the presence of specific organic solvents. Also, some chemicals,
particularly organic chemicals, can cause environmental stress
cracking and crazing. So it is critical that injection molded
products be designed and molded properly in order to minimize
their internal stress and avoid environmental stress cracking in
applications where chemical contact is anticipated.
10) Transparent: Some amorphous thermoplastic materials, such as
polystyrene and polycarbonate, are available as transparent
material grades.
2.3.2 Choosing the Right Material
Picking the best polymer to use in a plastic application is
becoming increasingly difficult. The introduction of many new
materials, as well as alloys of these materials and the more common
polymers, has opened up a myriad of possibilities. In choosing the
right material to do a job, one may find several products that will
meet the requirements. They may vaiy in price or in processing
characteristics, but in any case, they present a choice. One should
first list the conditions that the material must meet to fulfill the
requirements of the application, then list all of the materials that
provide the properties needed to meet this criteria.
22


The types of questions that must be answered when looking for the
material:
1. What is the highest temperature to which the application will be
exposed during its life cycle?
2. What is the coldest temperature to which the application will be
exposed?
3. How stiff must the material be to perform in the application?
4. Should the materials be transparent, translucent, or opaque?
5. What flammability resistance is required?
6. With what chemical environment will the application come into
contact?
7. Will the application have to sustain any impact loading?
Once one has answered all seven questions, it should be fairly easy to
choose one or several plastic polymers that will be able to perform in
this application. One can than start considering such things as price,
molding characteristics, and other factors that may help in selecting
the best material. In this case, ABS material was chosen by PEAK
Engineering & Automation Company for their product. Answering the
above questions to make sure that ABS material is the right material
for this product, the following Table 2.4 was created:
23


Table 2.4 Chick List for ABS Material
Requirements Application requirements ABS Property[4]
The highest temperature (UL). The highest temperature the part will be exposed to as high as 40C Temperature index 60-75 C
The coldest temperature. The coldest temperature the part will be exposed to is -30C Glass transition temperature is equal to 100C.
The stiffness of the material in the application. The part should rigid and farm so it holds the circuit board tightly. Widely used where toughness, rigidity and good appearance are important, it has tensile strength of (31- 45 MPa).
Should the materials be transparent, translucent, or opaque? The part should be opaque It comes in dark colors where light cannot go through.
The flammability resistance. The part will not be around fire or very high temperatures Standard grades are considered low burning and usually meet UL HB requirement.
The chemical environment contact. The part will be exposed mainly to sun or normal water Resistant to alkalis and acids but not concentrated oxidizing acids, so it has good chemical resistance.
The impact loading. Low impact Impact strength (Izod): 1-8 ft lbf in'1 notch
Table 2.4, shows clearly that ABS is the proper material for the
produced part. ABS material meets all the requirements that the
enclosure needs. There are different materials is available that can be
used instead of ABS, but regarding price, ABS material considered
one of the lowest cost materials that can be used for a prototype
purpose.
24


3. Mold Design and Fabrication
A general flow chart is shown in Figure 3.1. The chart summarizes
the steps required for injection molding part design and
manufacturing.
When a new part idea is generated, it must go through a series
of steps in order to be manufactured by injection molding. First, the
idea must be reviewed in order to get a clear understanding of the
25


parts function, purpose, and production method. Then a plan must
be developed for the manufacturing of the part. Every manufacturing
department that the part will go through must review and approve the
drawings. Design engineers will create a design for the part, which
will then be reviewed by the mold maker. The mold maker will assess
the moldability of the part and determine the material to be used.
The part design will move to the next stage, which consists of
designing the mold that will form the part. In this stage a mold
designer will create a design to produce the part, taking into account
all the requirements for the part and the process conditions. At same
time, the mold material will be determined. A computer simulation
will be created for the mold to reach optimum performance and to
optimize the fabrication through CNC machines. This design must be
reviewed carefully by engineers and mold makers to make any
changes required before manufacturing the mold. After the mold is
fabricated, a test must be done for the part to assess whether the
mold meets all the specifications for the part and the process. Finally,
the mold is ready to run to produce the part. These steps do not
necessarily follow a sequential order. Good communications between
each department will reduce steps and time delays.
Before describing the process of making the injection mold for
this part, it should be recognized that the design of the product will
ultimately determine the ease of molding and manufacturability, as
will the tooling requirements and costs. Also, it should be recognized
that the overall shape of a product and specific feature details may
need to be altered in order to improve the moldability of the plastic
part. In addition, the properties of the molded plastic part will be
26


greatly influenced by factors such as the tool design and processing
conditions. In order to develop a quality part, the plastic part
designer, the mold maker, the material supplier, and the process
engineer must all work together in an effort to develop a part that is
moldable and fully functional.
We must also consider that the injection molding process involves a
series of sequential steps. The different phases include:
1) Mold Filling: after the mold closes, the melt flows from the injection
unit of the molding machine into the relatively cool mold through
the sprue, the runners, the gates, and then into the cavity.
2) Packing: In this phase, the melt is pressurized and compressed to
ensure complete filling and detailed surface replication.
3) Holding: The melt is held in the mold under pressure to
compensate for shrinkage as the part cools. Holding pressure is
usually applied until the gate solidifies so that the melt can no
longer flow into or out of the cavity.
4) Cooling: The melt continues to cool and shrink with no shrinkage
compensation.
5) Part Ejection: The mold opens and the cooled part is then stripped
from the core or cavity by the ejector system.
Each phase of the injection molding process has an influence on the
design of the plastic part. In order for a plastic part to be considered
moldable, it must satisfy the moldability requirements for each of
these processing phases. These moldability requirements will be
addressed in the description of the process of making the mold.
27


3.1 Plastic Part Design
The project includes the design and fabrication of a case which
will host a circuit board and its battery (Courtesy of Peak
Engineering). The case or box consists of two main components, a top
cap and a base whose dimensions are listed below (see Figure 1.3,
Drawings 1 and 2 in Appendix A):
Outside Dimensions
Base(in) Top(in)
Height 0.65 0.70
Width 3.325 3.625
Length 3.125 3.125
Wall Thickness 0.09 0.09
Corner Radius 0.25 0.25
Note that the error tolerances in the dimensions must fall within the
acceptable limits shown below:
Acceptable Tolerances
.X .03
,XX.010
.XXX .005
Angle 0.5
Surface Finish: 125 //in
In order to minimize loose fits, the circuit board will be fixed to
the box. As a result, the external and internal dimensions of the box
must be identical. To achieve this design for securing the board in the
28


base, the latter must be perforated to provide two holes for the
insertion of aluminum pieces that will attach the board to the base.
As mentioned previously, in normal use the base is clipped to the
users belt. To this end, the base was designed to include a nose-like
support, which serves to clip the device to the user. (See Figure 1 and
2 in Appendix A for detailed dimensions of the top and bottom cap).
Note that in designing the box, it is important to control the wall
thickness to keep it constant throughout, i.e. approximately 0.09
inches. This thickness value was carefully chosen to guarantee good
heat transfer between the material and the cavity of the mold. This
thickness will also minimize the shrinkage of the part that is usually
due to the nature of the chosen material (ABS material) when
undergoing a cooling cycle. Another advantage for selecting this
thickness is to reduce the cooling time of the part. In doing so, one
can increase the part usage per hour and simultaneously reduce the
mold machines energy consumption. The following table gives some
of the ideal wall thicknesses for different materials.
Table 3.1: Wall Thickness Recommendations (inches)[5]
ABS 0.04-0.14
Acetal 0.02-0.12
Acrylic 0.03-0.15
Cellulosics 0.03-0.39
Liquid crystal polymers 0.01-0.12
Long fiber plasticfVertons) 0.08-0.98
Nylons 0.01-0.12
Polyaiylate 0.05-0.15
Polycarbonates 0.04-0.15
Polybutylene terephthalate 0.03-0.13
Polyethylene(LD) 0.02-0.25
Polvethvlene(HD) 0.03-0.2
Polyphenylene sulphide 0.02-0.18
Polypropylene 0.03-0.15
Polysulphones 0.04-0.15
Modified PPO 0.03-0.14
Polystyrene 0.03-0.15
SAN 0.03-0.15
UPVC 0.04-0.15
29


Additionally, all the part edges were to carry a slight trim in order to
provide a gradual flow during mold fillings and avoid any stress
concentration that may occur in the corners, which may lead to a
component failure.
3.2 Mold Design Description
After this part is designed, its moldability is verified by
consulting with the mold maker. Next, the mold that will be used to
fabricate the desired part is designed. To design the different parts of
the mold, AutoCAD Release 13 software package was used. This
software design tool was selected because it is the standard in the
industry for engineering applications.
Aluminum alloy 7075-T6 was chosen as the mold material. This
material is an alloy made out of 5.6% zinc, 2.5% magnesium, 1.6%
copper, and 0.25% chromium. When these proportions of zinc,
copper, magnesium, and chromium are used, the aluminum alloy
heat treats to the hardest level. This aluminum alloy is suitable for
this low production application and fits best the choice of ABS plastic
material processing conditions. Consequently, the mold will resist
very high compressive, bending, or shearing stresses. These stresses
are present when the highly compressed molding compound moves
inside the mold.
The mold is made out of four main components the cavity plate,
core plate, ejector plate, and the ejector housing. All cavities and core
30


dimensions have a shrinkage factor for the ABS material which ranges
from .004 to .007 inch/inch, .004 inch/inch. The shrinkage factor can
be calculated using the following equation:
Mold dimension = Part dimension(l + shrinkage value) (3.1)
The first step in designing the mold for the part is to determine
the parting line location. The parting line is the point where the cavity
plate and the core plate meet, which should be a straight line at the
top, permitting the use of a simple flash-type cutoff. In this case, the
parting line was determined to be at the top center of the mold. All
measurements must be taken with respect to the parting line. In the
second step, it was determined that the mold size base was 11x9
inches. This decision is usually decided based on the part size and
the size of the injection mold machine. Figure 3.2 shows the general
assembly of the mold.
Core Plate_______
Cavity Plate
Figure 3.2 General Assembly of the Mold
31


Circular Cross-section Cross section for Runners
itlll 0= smQX 1,5mm Advantages: Smallest surface relative to cross-section, slowest cooling rate, low heat and frictional losses, center of channel freezes last, therefore effective holding pressure Disadvantages: Machining into both mold halves is difficult and expensive
Parabolic cross-section |-W ~ 5 10" '//W///&i/// W =1,25-0 0 = Smax + 1,5mm Advantages: Best approximation of circular cross section, simpler machining in one mold half only (usually movable side for reasons of ejection Disadvantages: More heat loss and scrap compared with circular cross section
Trapezoidal cross-section ////\////\'/// W = 1,25-0 Alternative to parabolic section Disadvantages: More heat losses and scrap than parabolic section
/Z////Z////z Z/ZZZZZZZ/ZZ/ Unfavorable cross sections must be avoided
Figure 3.3 Cross Sections for Runners. [6]
The runner diameter is determined to be 0.25 inch and the
length is 2 inches long. By choosing this size we are able to rabidly fill
the cavity with material, and the molded part can be easily removed.
A gap of 1/8 inch or larger must be left between the cavity and the
edge of the runner to allow enough space to separate the gate from
the runner in the core plate. Table 3.2 Shows typical runner
diameters for various unfilled materials.
33


3.2.1 The cavity plate
The cavity plate is the male half of the mold, which shapes the
external details of the enclosure. The plate consists of two cavities
located at one line, sprue, runner system, and leader pins.
The sprue, which is located in the center of the plate, allows for
the injection of the material. The selection of the sprue diameter is
determined by the diameter of the machine nozzle. For instance, the
sprue diameter is usually set equal to that of the nozzle or slightly
(1/32 inch) larger. When the diameter of the sprue is smaller than the
opening of the nozzle, this causes misalignment, which results in a
blow-by condition. The Blow-by condition refers to the problem which
occurs when the sprue sticks to the nozzle. Additionally, excessive
shearing of material in that area could also occur. In this case we
used a sprue diameter of 0.25 inch.
The runner system is designed to be a circular duct so that it
has the minimum surface-to-volume ratio. In this fashion, heat loss
and pressure drop are minimized. The selection of the shape and size
of the runner system is crucial to the process, since it influences the
amount of material that flows inside the cavity. Figure 3.3 shows the
most common cross sections of runners and their performance.
32


Table 3.2 Runner Diameters for Unfilled Materials.[1]
Material Runner Diameter
ABS, SAN 3/16-3/8
Acetal 1/8-3/8
Acetate 3/16-7/16
Acrylic 5/16-3/8
Butyrate 3/16-3/8
Fluorocarbon 3/ 16-3/8 Approx.
Impact acrylic 5/16-1/2
Ionomers 3/32-3/8
Nylon 1/16-3/8
Phenylene 1/4-3/8
Phenylene sulfide 1/4-1/2
Polyallomer 3/16-3/8
Polycarbonate 3/16-3/8
Polyester thermoplastic 1 /8-5/16 Unreinforced
3/16-3/8 Reinforced
Polyethylene-Low to Hi-Density type
1,2,3,4 1/16-3/8
Polyamide 3/16-3/8
Polyphenylene oxide 1/4-3/8
Polypropylene 3/16-3/8
Polypropylene-general purpose medium
impact-hi impact 1 / 8-3/8
Polysulfone 1/4-3/8
Polyvinyl chloride(plastticized) 1/8-3/8
PVC Rigid (modified) 1/4-5/8
Polyurethane 1/4-5/16
Another method can be used to determine the runner diameter [6],
i.e.:
D = Srnax+1-5mm (3-2)
D-Diameter of runner (mm)
smax -Maximum thickness of part (mm)
However, since this equation does not take into account geometry of
the part, it can only be used for the determination of the diameter of
the runner in small parts.
The Cavities in the cavity plate are formed directly in the solid
block and are spaced equally from the edge of the runner to reduce
34


the material scrap and yield efficient cooling. All edges have a 0.25
inch radius to eliminate stress concentration and achieve gradual flow
transitions during the mold filling. The cavity is designed with a draft
angle of 1/2, where the draft angle is the amount of taper required to
allow the proper ejection of a molded part from the mold. It is
represented in Figure 3.4.
In most cases, parts tend to stay with the core when the mold
opens because the shrinkage of the plastic martial causes a contact
pressure and normal frictional forces between the part and the core.
Moreover, the material shrinkage through the wall thickness tends to
pull the part away from the cavity wall. Draft angles reduce the effect
of undercuts, eliminate sliding friction and part damage after the
initial break away as the mold opens. Draft angles also facilitate air
movement to compensate for vacuum effects as the tool opens. Draft
angles range from 1/2 to 2, depending upon the depth of the
drawing, material rigidity, surface lubricate, mold surface roughness,
and material shrinkage. The Draft angle is crucial when designing the
35


cavity and the mold core, since it can make the ejection of the part
very difficult.
The plate will have four leader pins of with a 0.624 inch diameter,
which is the standard size pin for this size and mold weight. One pin
is set at each corner so that the mold is well aligned with the core
plate during injection and shifting does not occur. Figure 3 in
appendix A shows the design of the cavity plate.
3.2.2 Core Plate
The core plate will be identical in size to the cavity plate and will
consist of core inserts which form the inside details for the top and
the bottom part of the enclosure. The plate comprises the side of the
runner system and the gates. Each corner of the plate includes
shoulder bushes for the leader pins. Figure 4 in appendix A shows
the dimensions for the core plate. Two inserts are designed for the top
cap and the bottom cap. When designing the inserts, all edges were
rounded to create a smooth transition flow during the injection
process, and to avoid high concentration in the corners. The selection
of the circular shape of the inserts is not mandatory. However, in this
case, it worked best. For other parts, such a shape might not be
suitable, and other geometrical forms might apply.
Similarly, the insert also has a draft angle of 1/2 degrees as the
cavity. The second half of the runner system will be located in the
center of the plate and has the same dimensions as the cavity plate.
36


A tap gate is attached at the extremities of the runner. Figure 3.5
shows the top view of the gate.
Figure 3.5 Top View Of the Runner System
There are different factors which determine the location, shape,
and size of this gate. Some of these factors are the geometry of the
part, wall thickness, material viscosity, shrinkage factor, and
distortion. In each corner there will be shoulder busing pins, located
at the same place where the leader pins are located in the cavity plate
to yield the proper alignment.
3.2.3 Ejector Plate
The ejector plate holds the ejector pins that eject the part from
the core when the mold opens. The number of pins and their
locations are shown in Figure 5 in Appendix A. The pins are
37


positioned in the ejector plate to align well with the holes in the core
plates. The ejector plate contains six holes of 1.33 inch diameter to
allow the support pillars to pass through which to prevent the core
plate from bending.
3.2.4 Ejector Housing
The purpose of the ejector housing is to clamp to the core plate
and the moving parts of the press. It consists of two riser plates and
a back plate. Figure 6 in Appendix A shows a drawing of the ejector
housing as well as its dimensions. The riser has a slot to allow it to
clamp to the machine and screw holes to attach it to the back plate
and the core plate. The back plate has a hole which allows the
knockout rod to reach the ejector plate when the plastic part is
ejected.
3.3 Mold Fabrication
When the mold design was completed, based on the
requirements and the standard process conditions, the press of
building the mold began. Figure 3.6 shows a schematic diagram of
the milling machine. It is a three-axis milling machine with CNC
(Computer Numerical Control) features that can operate with any
typical modern software.
38


-----N OvrmWt
Figure 3.6 Basic Components of the Bridgeport Milling Machine. [7]
EZ-TRAK is an extension of the Bridgeport PCNC (PC-based
Numeric Control) product line, which provides the user of a manual
machine with the power of a CNC. In addition EZ-mill software was
used to generate the G-code to fabricate parts of the mold
automatically.
First, the milling machine was checked to make sure that it is
perfectly trained. This implies that the head of the milling machine is
parallel with its own table. When the milling machine is not
positioned parallel with its table, the proper dimensions of the mold
cannot be achieved with the desired precision. The steps are as
follows:
1. Move the knee of the milling machine to position it to get sufficient
space between the head and the table so that the indicator on the
head is set.
39


2. Make sure the block is flat square, and then place it on the table in
either direction the (x-direction or the y-direction).
3. Move the head down until the indicator touches the block. Next,
read the indicator and then move the block to the other side in the
same direction.
4. Move the head down until the indicator touches the block and read
the indicator.
5. When both readings are within the required tolerance, i.e. 0.0005,
it can be concluded that the machine is in tram.
The same procedure is repeated in the perpendicular direction. When
both readings do not match within the required tolerances, the
machine must be retrained.
Second, square all the aluminum 7075 blocks as follows:
1. First, place the machined surface against the fixed jaw of the vise.
Insert a soft rod between the adjustable jaw and the workpiece.
2. Machine this surface, then check the correct width on the ends of
the workpiece.
3. Place the workpiece in the vise with one of the finished surfaces
against the solid jaw, and the other surface against the bottom of
the vice. Place the rod between the adjustable jaw and the work
piece again.
4. Machine the surface to correct the width.
5. Check the correct thickness of the workpiece.
6. Finally, place the first finished surface in the vise and machine it.
Each time, tap the blocks lightly to seat them on the vise properly.
The flatness and the squarness of the blocks should not be more than
five tenths of an inch. After squaring the blocks, mark them, so that
40


each block can be placed in correct location with respect to the
others. Next, locate the leader pins on the blocks and begin to form
the cavities in the cavity plate. Then use a Paul end mill cutter to
generate the holes for the ejector pins with a 0.0005-0.001 inch
clearance between the holes and the pins. Note that too much
clearance may cause the plastic material to flush. Holes in the ejector
for the support pillars to go through should have a clearance of 1/16
inch. After the ejector pin holes have been made, drill holes for the
water line. Finally, create the runner system and gates. Note that all
dimensions in the mold must be very close within 0.0005-0.001 inch.
3.4 Manufacturing the Plastic Part
When the mold is completed, make sure it is veiy clean. Verify
that all the parts of the mold are fitted together and the ejector plate
is in the right location. When this is not the case, damage might
result in the mold. Second, make sure that the plastic material is
clean and diy, and keep in record the source of the material begin
used as well as how it has been stored. This procedure is known as
the material handling. To guarantee diyness of the plastic material,
place the material in a dryer for a given time depending on the
material. In this case, the ABS material was placed in the dryer for 2-
3 hours under 165F. Note that the time the material remains in the
diyer is specified by the material supplier. Figure 3.7 shows a plastic
material diyer which is some time located above the hopper to provide
the press with the material continuously.
41


Figure 3.7 Plastic Dryer Machine
Drying the plastic material before using is very important for
the quality of the molded part, because moisture that exists in the
material will turn into steam at high temperature during the injection
process. That steam becomes a gas that is trapped in the melt and
travels through the flow path into the cavity. The gas will form
different shape in the part that are considered a defect in the finished
product. Moisture must be reduced to acceptable levels prior to
processing.
Next, install the mold into the press, described as follows:
1. Install the cavity plate into the stationary side of the press,
there are locating rings the cavity plate has to go between it in
order to align with the nozzle.
2. Clamp the core plate into the other side of the press, then move
this side of the press so that the core plate is perfectly aligned with
the cavity plate. Make sure that booth plates are tight enough to
the machine.
42


3. Finally, attach the water line to the mold.
Figure 3.8, shows how the mold should be located into the press.
Figure 3.8 Describe the Mold Position into the Press
After the mold in this case was put into the press, material of
ABS was placed in the hopper, and making sure that the material was
contained so that no contamination occurred. The machine was
turned on to start running to produce the part. Figure 3.9 shows the
machine used to produce the plastic part.
43


Different parameters must be taken into account to obtain best
quality parts. Such parameters are the speed of the screw, applied
temperature, and pressure. These parameters depend on the part
and the material requirements. Thus, experience plays a significant
role in determining the optimum values of these parameters that the
machine should operate under.
44


4. Cost Analysis
4.1 Mold Cost
Matrix Corporation was contracted by PEAK Engineering &
Automation Co. to produce an injection mold for the Tuco Project. A
cost estimate was performed base on the raw material required to
produce the mold and the labor cost. This cost analysis was made
according to the requirements that needed to build this mold by the
moldmaker Gaiy Moorehouse. Table 3.9 shows the breakdown for
the mold cost as follows:
Table 4.1 Break Down for the Mold Cost
Unit Price/Unit Total
Raw material
Aluminum 7075
Block:
1 l"x 9"x 3" 2 115 230
Il"x4"x2" 2 31 62
1 l"x 9"x 1" 1 36 36
1 l"x 4"x .5 2 12.5 25
3.6"x 3.2"x 1.7" 2 20 40
Rod:
1.35" Dim 4.01" Long 6 9 54
Support pillars $447
Steel pins
Ejector pin:
1/4" Dim 12 3 36
3/16" Dim. 2 2.5 5
45


Table 4.1 Cont.
Return pin:
5/8" Dim. 4 10 40 $81
Screw (Socket Head Cap)
1/4-20 (1 1/2" L) 6 0.53 3.18
1/4-20 (2 1/2" L) 8 0.86 6.88
3/8-16 (1" L) 4 0.61 2.44
1/2-13 (5" L) 4 4.64 18.56
1/2-13 (1" L) 4 1.25 5 $36.06
Springs
1" Dim (4" L) 4 3.39 13.56 $13.56
Water pipe (Brass)
1/8-27 NPT (0.8" L) 4 1.19 4.76 $4.76
Labor
120 50 Total cost 6000 $6,000 $6,582.38
All estimates were in accordance of industry standard and usage.
4.2 Part Manufacturing Cost
In order to obtain the cost of producing 1,000 identical plastic
parts, Matrix Corporation provided the following table that shows the
breakdown of the part cost:
46


Table 4.2 : Breakdown for the Part Cost
Cost per part
Material cost: (ABS material) $ 1.50/lb The part weight is 90g 0.30
Machine time: $ 55/hour Cycle time is 30sec/part 0.46
Energy cost: $ 1.50/hour for electricity Machine produce about 120 part/hour 0.0125
Labor: $10 for machine operator ($10/ 120part) 0.083
Total Cost per Part 0.85
This cost analysis dose not included the setup fee, which usually
included dependig on the quantity of the produced parts.
47


5. Analysis of the Flow Mechanics for Single
Screw Injection Process
As stated earlier injection molding process, involves a series of
sequential process steps. These phases are; the molding phase, the
packing phase, the holding phase, the cooling phase, and part
ejection phase. In this section we will give a summary analysis for the
flow mechanics in the molding phase from Mechanics of Polymer
Processing by Pearson[8], where the plastic material is preplasticizied
in order to be injected inside the mold. The quality of the produced
part depend heavily on the quality of the output flow in the molding
phase, and its normally found that the success of the injection
process will largely dependent upon the performance of the extruder.
In most modern injection molding machines, a single-screw extruder
is used for preplasticizing.
5.1 Geometry of Single Screw Extruder
Consider a barrel with inner diameter, dh, as the primary
defining parameter. Figure 5.1 shows a full turn of the flight helix,
whose lead angle at the barrel is h. The axial width of the flight is
wfa and of the screw channel is wca, their sum being the screw pitch
(or lead) ndh\.ax\ surface of the beryl is h{. The depth of the screw channel is hs. These
48


dimensions are sufficient to define the geometry of the screw channel
provided:
(a) the bottom of the screw channel (the screw root) lies on a circular
cylinder coaxial with the barrel;
(b) the top of the screw flight also lies on a circular cylinder coaxial
with the barrel;
(c) the sides of the screw flight are defined by radii through and
perpendicular to the screw/barrel axis to the screw/barrel axis. The
(r, (/>, z) cylindrical polar coordinate system.
Figure 5.1 Screw Geometry
We chose a further frame of reference that is axially fixed but
which rotates with the screw. This retains a fixed axial coordinate
position for feed pocket, but means that the position of the nose of the
screw at any axial position vary with time as the screw retracts. This
explained by Figure 5.2.
49


V,
Figure 5.2 (a) Unrolled Geometry for Single-Screw in Barrel, (b) Motion of
Barrel (vb) and Screw (v^) Relative to New Coordinate System x.
This particular coordinate system simplified the calculations for
quasi-steady operation when vsct is constant and where screw flow
analysis carried out for x* increasing from its fixed value at the feed
pocket. The relevant relative barrel and screw velocities are given by
It is assumed that vb rabidly reaches its set value once the screw-
starts rotating. The kinematical result relating the output of the
screw to its backwards velocity using the nomenclature of [8] is,
v; = (vb cos^b vB cot^b cos0b, Vb sin h vscr cos^b,0)
=(-vKrcosec^b,0,0)
(5.1)
(5.2)
(5.3)
If the moving system is used then
C?T, =0T1 +V*rCOSeC^bM'e/l.
(5.4)
50


Where
wecosec0b = cot jbwa = 7udbwc /(wQ + wt) (5.5)
A reasonable value for (9*, is ivbwchs and from (5.1) and (5.2),
v__ w.h. h
WA ____
dl 3 db
1
(5.6)
Qt, will not differ by much from Qlx when hj db 1 and the effect of
screw retraction will be minor.
5.2 Feed Zone
The frictional analysis can carried out in the X* frame, and vs*
being the relevant unknown velocity of the solid block. The volumetric
delivery of the screw will be
V = V* + cosecbwchs (5.7)
The calculations for V* is in [8].
When the screw is not rotating, heat will be conducted from the barrel
through the lubricating layers to the solid bed. Such melting as
occurs will increase the depth of the lubricating layers. This can be
calculated by a one-dimensional heat-transfer analysis with a moving
melting interface. For our case, we need only an estimate of the depth
of the molten layer if the dwell time is td.
For Sf >>1, Where Sf = pmTmT'!P£l > a linear temperature profile
within the melted layer can be used as a reasonable approximation,
and so
51


With a solution of
to
d/ ~ Sf A
(5.8)
=(2*Jd/Sf)T
(5.9)
For the layer thickness at the end of the dwell time.
5.3 Plasticating Zone
In our case we will consider 6-zone model to get sufficient
analysis. The overall mass balance including the effect of density
changes becomes
The relation obtained in [8] now involves Vb* and Vs*r. The equation
for region C that obtained in [8] need to be changed, which become
dMj dmj
(5.10)
dxj dt
Where
m
(5.11)
(5.12)
PmVCrel34^S kC ) = Ps+ (A ~ Pj^j / 8t (5.13)
= PS^(VA3+-^-)
(5.14)
52


_ i ar
+ V'
m j Crel 4 *
l A
Crel 3
er dr
+
A 4
dx* dt
2t*
A4
OW* . . 5/2,
ST
5x
= a--------- + 77
^ *2 '
A3
^'Crel4
l2
dx*
LA/i3 ;
Sc
71 = PsVl3 + (Ps Pm )^T = AnVC
a/
m Crel 3
(5.15)
(5.16)
Where xmM has been suitably defined. Because dhmc/dt now arises,
initial conditions /7c*(x*4,0) and T have to be prescribed which are
obtained from solution of the heat conduction equation that is
obtained when V*, Vb, and VS*CT are put equal to zero. This becomes
dr d2r
~di~~Km~&Z
With boundary conditions
T'(hs) = Tb r(hs ~hmc)-Tm
And with
dK
dt
4sPn
a-
dT'
dx'p^j
V'fc
Ih-K:-
(5.17)
(5.18)
Giving the time development of h].
5.4 Metering Zone
This zone is the most important in determining the temperature
the melt fed into the reservoir. We introduce Graetz number Gz,in
the form
53


Gzd=/Js2/4tfd (5.19)
If Gzd 1, then thermal equilibrium is reached during the dwell time
and r=Th= Ts everywhere at the beginning of the active phase.
If Gzd 1, then thermal conduction is almost negligible and so the
dwell period will have little effect in the velocity and temperature
profiles during the active phase.
A >> 1, GzdC small
The relevant equation for Tm becomes
Pj
. d

v2 +
dx* dt
V =
dx]
a.
er
dx
+ t1{D\T)D*2
(5.20)
3
The momentum equations remains unaltered but with boundary
conditions reflecting the changes to Vb* and V*CT for Gzcc 1, an
additional term
Pju
f 1'
viv2iy
" l "
Vlv2lA:-
dr
dt
(5.21)
For Gzcc 1, we use (5.20) without the term involving v'2dldx\ which
leads to
a2r
or,
= -Na e
* -r
rsY
l *;J
A >> 1, Gz
d.C.
This is the most relevant situation,
becomes in this case
+
' dv.
i(v+o
>
ar
+
dt
(5.22)
= 0(1)
The relevant equation for T*
54


_ er dr e2r
Gzd.c vi -r-r + Gz
sa;
dt Sc,
2
+ Na*e
-T
rd?;v
+
civ;
V^3 ,
|(v+l)
(5.23)
Where
Gz* = hi / and t t/t
a o m a a
5.5 Overall Calculation for Output to Reservoir
In the calculation we assumed that flowrate 0T is given for the
various zones, but a full analysis must predict 0T so as to satisfy the
boundary conditions on pressure p, or more relevantly in this case the
overall force balance on the screw. In this case the dominant
components of the axial force balance will be the reservoir pressure
pres acting over the entire cross-section of the screw, the viscous drag
at the flight tips caused by the axial movement of the screw and any
resistance to motion, Fmach imposed mechanically or hydraulically at
the drive-end of the screw. An estimate of the total viscous drag is
^drag = 2(rjvm /h()ndblmw{ /(we +w{) (5.24)
Where r\ is evaluated for T -Th, D = (v^ + lhf and /m is the Vetted
length of the screw. If we take the volumetric delivery of the screw to
be half that of the drag flow delivery then the pressure drop over a
(notional) metering section of length /m and depth hs will be
55



(5.25)
Where tj is now evaluated at D = vb / h%.
If we take this to be the major component of the reservoir pressure,
then the force exerted by the melt in the reservoir on the screw will be
^.*2 xndlvjjhl (5.26)
The balance equation for the screw will be
^mach + -^drag = ^res (5.27)
Using the relationship (5.6) for which is consistent with the
volumetric delivery implied by (5.25), we obtain the ratio
^ 7flight K /c Og)
Fm "Hchannel 30^f
Where w{ /(wf + wc) 8. We expect /7flight < ^channd Hence we can expect
^<<^3 (5.29)
56


6. Conclusion
CAD/CAM systems can be used effectively to produce plastic part
using injection molding. A well-applied CAD/CAM system will
increase the quality of the product, reduce costs, and allow for
efficient planning and continued improvement of the product.
Mold design, on the other hand, still requires trial and error, the
amount of which depends on the specific part to be made.
CAD /CAM techniques can still be used advantageously for this
case. The experienced toolmaker can expedite the trial and error
process through the more efficient, cost-effective use of CAD/CAM
techniques.
Innovations in software and advances in computers are making
CAM/CAM programs more affordable, more user-friendly, and
more available to small and medium-size processors. In mold
design software particularly, growing number of processors use
CAD/CAM programs to speed development cycles, reduce
toolmaking costs, and improve productivity. The speed with which
software permits design changes to be made, and the fact that
designs can be tested on a computer without the expense of
cutting steel, complement manufacturing trends.
57


7. Recommendations
The evaluation of simple analytical models for plastic made parts,
vis-a-vis of trial and error experiences is recommended. Tool
makers make few use of analytical techniques. This is most likely
due to the lack of proven analytical tools, simple and efficient,
that the toolmaker can correlate to his/her experience.
More sophisticated techniques, such as Finite Elements Methods,
can be used to more closely study the stability of plastic mold
making responsible for the present need of trial and error .
Through the use of these methods, complemented with
experimental studies, it may be possible to further reduce trial and
error, thereby increasing the efficiency of CAD/CAM techniques.
Structural analysis can be implemented through CAD/CAM
techniques to predict in the design stage the differential shrinkage
that will occur in a injection molded part.
58


8. Appendix


9. References
1. DuBois and Pribbles, Plastic Mold Engineering Handbook.
Chapman & Hall, New York, Fifth Edition, 1995.
2. James C. Gerdeen, Mechanical Design with Polymers and
Composites, Tivoli Copies, fifth Edition, 1995.
3. Robert A. Malloy, Plastic Part Design for Injection Molding, Hanser
Publisher, New York, 1994.
4. J. A. Brydson, Handbook for Plastics Processors, Heinemann
Newnes, Great Britain, First Edition, 1990.
5. P. S. Crackknell and R. W. Dyson, Handbook of Thermoplastics
Injection Mould Design, Blackie Academic & Professional, New
York, First Edition, 1993.
6. Menges and Mohren, How to Make Injection Molds. Hanser
Publishers, Munich Vienna New York, 1986.
7. Bridgeport, EZ -TRAK 3-axis Programming and Operations Manual,
1995.
8. J. R. A. Pearson, Mechanics of Polymer Processing, Elsevier
Applied Science Publishers, New York, Firs Edition, 1985.
9. DouGlas M. Bryce, Society of Manufacturing Engineers, Dearborn,
Michigan, 1996.
9. A. Brent Strong, Plastics Materials and Processing, Prentice-Hall,
Englewood, New Jersey, 1996.
10. Bruce C. Waendle, What Every Engineer Should Know About
Developing Plastics Products, Marcel Dekker, Inc., New York, 1991.
66


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4. MATERIAL TO BE BLACK ABS PLASTIC
TYP
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9.000
TITLE : Analysis for the Manufacturing of Injection Molded Parts Using CAD / CAM Techniques
Tolerances: .X .003 Angle .5 .XX .001 .XXX .0005 Drawn : Waleed Bajrai
Figure A-3 : Cavity Plate


TITLE : Analysis for the Manufacturing of Injection Molded Parts Using CAD / CAM Techniques
Tolerances: .X .003 Angle .5 .XX .001 .XXX .0005 Drawn : Waleed Bajrai
Figure A-4 : Core Plate

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TITLE : Analysis for the Manufacturing of Injection Molded Parts Using CAD / CAM Techniques
Tolerance: .X .003 Angle .5 .XX .001 .XXX .0005 Drawn : Waleed Bajrai
Figure A-5 : Ejector Plate


n ; ^ ~ ~ co
9.000
TITLE : Analysis for the Manufacturing of Injection Molded Parts Using CAD / CAM Techniques
Tolerance: .X .003 Angle .5 .XX .001 .XXX .0005 Drawn : Waleed Bajrai
Figure A-6 : Ejector Housing


11. G. Gruenwald, Plastics How Structure Determines Properties.
Hanser Publisher, New York, 1993.
12. Feirer and Tatro, Machine Tool Metalworking. McGraw-Hill Book
Company, Inc., New York, 1961.
67