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Mold.ppt
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Mold Design
Mold Design
Fundamentals
Fundamentals
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Mold.ppt
Basic Tasks of a Mold
Basic Tasks of a Mold
q
Accomodation and Distribution of the Melt
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Shaping of the Molded Part
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Cooling/Heating and Solidification of the Melt
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Ejection (Demolding) of the Molding
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Mechanical Functions
)
Accomodation of forces
)
Transmission of motion
)
Guidance of the mold components
The mold is probably the most important element of a molding machine. It is a
arrangement, in one assembly, of one (or a number of) hollow cavity spaces built
to the shape of the desired product, with the purpose of producing large numbers
of plastic parts. Thus the primary purpose of the injection mold is to determine
the final shape of the molded part (shaping function).
In addition to give the final shape of the molding, the mold performs several
other tasks. It conducts the hot melt from the heating cylinder in the injection
molding machine and distributes the melt to the cavity (or cavities), vents the
entrapped air or gas, cools the part until it is ejectable, and ejects the part without
leaving marks or causing damage.
The secondary tasks of a mold derived from these primary tasks include several

mechanical functions such as accommodation of forces, transmission of motion,
guidance and alignment of the mold components.
The mold design, construction, the craftsmanship largely determine the quality
of the part and it manufacturing cost.
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Mold.ppt
Functional Systems of the Injection
Functional Systems of the Injection
Molds
Molds
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Melt Delivery System: Sprue/Runner/Gate
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Cavity (with Venting)
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Tempering/Heat Exchange System
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Ejection System

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Guiding and Locating System
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Machine Platen Mounts
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Force Supplier
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Motion Transmission System
An injection mold is composed of several functional units. Each unit performs
one or several task of the mold.
The melt delivery system or runner system performs the task of receiving and
distribution of the melt. The runner system is in fact a set of flow channels that
lead the melt into the cavities.
Forming/shaping the molten material into the final shape of the part is the job of
the cavity. During the filling and packing/holding stages, melt is forced by
injection/holding pressure to completely fill the cavity (or cavities).
Mold tempering or heat exchange system is used to control the mold
temperature, cool down the molten melt (or,if thermosets or elastomer are used,
heat the melt and cross-link the material) uniformly, solidify the molding to an
ejectable state. Mold tempering system design has direct impact to the production
cycle time and the quality of the molded part.
Ejector system is utilized to open the mold and remove the molded part from the
cavity. Mold mounting, alignment, and guiding are accomplished by the
guidance/ locating system and machine platen mounts. Other auxiliary units such
as force supplier and movement transmission unit are essential to accomplish the
functions of an injection mold.
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Mold.ppt
Structure of A Mold Unit
Structure of A Mold Unit
Sprue
Sprue
Primary Runner
Secondary Runner
/Sub-runner
Gate
Part
Cold-Slug Well
Cold-Slug Well
Sprue Ejector Pin
Sprue Bushing
Above figure shows the layout af a typical simple injection mold, which has
four identical cavities. Melt from the nozzle enters the mold via the spure, which
has a divergent taper to facilitate removal when demolding.
Opposite the sprue is a cold slug well, which serves both to accept the first
relatively cold portion of the injected material, and to allow a re-entrant shape on
the end of an ejector pin to grip the sprue when the mold opens.
The melt flows along a system of runners leading to the mold cavities. In

general, for a single cavity mold, only the sprue or primary runner appears in the
mold; whereas for a multicavity mold, secondary runners or subrunners are
needed to distribute the melt into each cavity.
The gates at the entries to the cavities are very narrow passages in at least one
directions, so that the molded part can be readily detachable from the runners
after removal from the mold.
Sometimes additional cold slug wells are added in the end of primary runners to
trap the cold slug during the filling stage.
The mold is aligned with the nozzle on the injection cylinder by means of the
locating ring surrounds the sprue bushing.
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Mold.ppt
Mold Design Issues
Mold Design Issues
mold base
cooling channel/lines
runner (mainfold) system
gate

cavity
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Mold Design
)
No.Cavity
)
Cavity Layout
)
Runner System Design
)
Gating Scheme
)
No.Gate
)
Gating Location
)
Mechanical/Mechanism
Consideration
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Cooling System Design
)
Cooling Channel Layout
)
Special Design
The primary tasks of an injection mold include the accomodation and
distribution of the melt, the shaping and cooling/heating of the molding,
solidification of the melt, as well as ejection of the molded part. Besides, a mold
has to provide mechaincal functions such as accomodation of forces,
transmission of motion, and guidance of mold components.
Hence the primary functional systems of a injection mold include the melt

delivery system ( sprue/runner/gate ), cavity (single-cavity or multicavity),
ejection system, guiding and locating system, as well as mold temperature
control unit (cooling system).
From the view point of mold design, we have to evaluate the suitable size and
layout of runner system and cavity, number of cavity, cooling system, etc.
We will propose a few examples to illustrate how these design parameters
influence the productivity and quality of the moldings.
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Mold.ppt
Determine Number of Cavities
Determine Number of Cavities
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Single Cavity vs. Multicavity Mold
)
Productivity and complecity consideration
q
Determination of Number of Mold Cavities
)

Number of moldings required and period of delivery
)
Quality control requirements (dimensional tolerance,etc.)
)
Cost of the moldings
)
Shape, dimensions, and complexity of the molding (position of
parting line and mold release)
)
Size and type of the injection molding machine machine (shot
capacity, plasticizing capacity, mold release..)
)
Plastics used (gating scheme and gate location)
)
Cycle time (increase in recovery time of plasticating unit,
injection time, pressure drop, and mold opening time)
The multiple mold cavities can produce several article at the same time and
hence has a higher output speeds and improved productivity. However, the
greater complexity of the mold also increases significantly the manufacturing
cost. The problems arising from a multicavity mold includes cavity layout, flow
balance, balanced cooling channels layout, etc.
Theoretically, for the same product, cycle time do not increase prorate with the
number of cavities because th cooling time does not change. However, one often
find that cycle time will increase as the number of cavities increases, for the
following reasons:
-Increase in recovery time of plasticating unit for the next shot and injection
time because the total shot volume is increased. These increases in time are
significant for large shots.
-Increase in pressure drop becaused of the increased flow length from sprue,
through runner system, to each cavity. The pressure drop can be a determining

factor in the evaluation of numbers of cavity.
-Increase in mold opening time because of the increased complexity.
Both the technical and economic criteria have to be considered in determining
the number of mold cavity, such as the numbers of moldings required, the cost
and time of mold construction, the complexity of the molding, cycle time, quality
requirements and the plasticating capacity of the available machine equipment,
etc.
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Mold.ppt
Cavity Layout
Cavity Layout
Layout in Series
Circular Layout
X-style layout
H-style bridge
(branching) layout
When the number of parts produced in each cycle exceeds one, a multicavity
mold have to be used. Many cavity layouts can be adopted in the production.

For example, layout in series has the advantage that there is no space restriction
for each cavity; however, the unequal flow lengths to individual cavities may
lead to unbalanced flow and differential part weights in each cavity.
Circular layout has the advantage of equal flow length and uniform part
quality; however, only limited number of cavities can be accomodated by this
layout.
H-style layout and X-style layout belongs to the so-called symmetrical layout.
They are good in flow balance. Their disadvantage is that more larger runner
volume and much scrap will be generated. Hot runner system can be adopted to
conquer this drawback.
Layout of cavities not only influence the filling pattern and extent of pressure
packing, but also determines the equilibrium of injection force and clamp force
during the molding cycle.
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Mold.ppt
Design of Runner System
Design of Runner System
Piston or

Screw
Screw Chamber
(Reservoir)
Heating Element
Nozzle
Runner
Gate
Sprue
Cavity
Mold Unit
q
Runner System
)
Sprue
)
Runner
(Primary/Secondary)
)
Gate
q
Goal:
)
Accommodates the molten plastics material coming from the screw
chamber and guides/distributes it into the mold cavity
)
Raises the melt temperature to the proper processing range by viscous
(frictional) heating while the melt is flowing through the runner
q
Design Consideration
)

Quality (filling pattern...) & Economics (cycle time...)
A runner system is composed of the sprue, the runner(s), and the gate(s) that
connecting the runner with the cavity.
The primary task of a runner is the delivery and distribution of melt from the
screw chamber into the mold cavity. The runner system must be designed in such
a way that the melt fills all cavities simultaneously and uniformly under uniform
pressure and temperature. This design criterion is referred to as the flow balance
of the runner system.
Melt temperature may be significantly increased as it passes througn the narrow
runner passage or gate due to friction effect. This viscous heating is important in
raising the melt temperature and reducing the flow resistance because of the
shear-thinning character of plastic material.
The runner system has significant impact on the part quality and the economics
of manufacture. Problems such as weld lines, pressure drop, material waste,
removability of moldings, etc.,are related to the design of runner system.
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Mold.ppt
Common Runner Cross Sections

Common Runner Cross Sections
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Circular Runner
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Full Round Runner
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Parabolic Runner
)
U-Type or Modified
Trapesoidal Runner
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Trapezoidal Runner
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Half Round Runner
q
Rectangular Runner
There are several types of cross section can be adopted for a runner. The
selection of the runner cross section depends on its efficiency and ease or
difficulty of tooling.
Circular or full round cross section provides a maximum volume-to-surface
ratio and hence offers the least resistance to flow and least heat loss from the
runner. However, it requires a duplicate machining operation in the mold, since
two semi-circular sections have to be cut for both mold halves and aligned as the
mold is closed.
Parabolic or U-type runner represents a best approximation of circular runner,
although more heat losses and scrab produced (mass is 35% greater), it needs
simpler machining in one (movable) mold half only.
Trepezoidal runner is an alternative modification of circular runner, its
performance is similar to that of the parabolic runner. Trapezoidal runner is
often used in three-plate molds since sliding movements are required across the

parting-line runner face.
Half round and rectangular cross section may lead to larger flow resistance and
are unfavorable in the runner cross section.
Normally, full round or trapesoidal runners are adopted in most practical cases.
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Mold.ppt
Considerations in Runner Design
Considerations in Runner Design
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Part Consideration
)
Geometry, Volume, Wall Thickness
)
Quality (Dimensional,Optical, Mechanical...)
q
Material Consideration
)
Viscosity, Composition, Fillers,Softening Range, Softening

Temperature,Thermal Sensivity, Shrinkage, Freezing Time...
q
Machine Consideration
)
Type of Clamping, Injection Pressure, Injection Rate...
q
Mold Consideration
)
Way of Demolding, Temperature Control...
Key factors affecting the design of a runner are summarized here.
In the aspect of part consideration, the geometric dimensions of the runner
should be such that flow restriction is at a minimum, that is, the runner should
convey melt rapidly and unrestricitly into the cavity in the shortest way and with
a minimum heat and pressure losses. The runner system should allow cavity
filling with a minimum numbers of weld line so that the mechanical and surface
properties of moldings can be improved. The runner should permit the
transmission of holding pressure during the packing/holding stage so that the
dimensional accuracy can be ensured.
In the aspect of material consideration, the flow character and the thermal
properties of material are related to the sizing of runner diameter and the runner
length. Long or small runner should be avoided for material with short flow
length (high viscosity). Runner should be properly sized to minimize material
waste while not cause significant pressure loss.
In the aspect of machine consideration, we should note the allowable injection
pressure, injection rate, type of clamping, etc.
The runner should be design so that demolding and removal from the molded is
easy. Location and number of runner ejectors should be considered in the mold
design phase.
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Mold.ppt
Flow Balance in the Runner Design
Flow Balance in the Runner Design
q
Flow Balance in Multi-Cavity Molds:
)
Increase in recovery time of plasticating unit, injection time,
pressure drop, and mold opening time
PLAY412
Consider the runner system design in the multicavity mold case.
In a symmetric, naturally balanced cavity layout, all flow lengths from the
sprue to each cavity are of the same length. In this ideal case the plastic melt will
fill all cavities simultaneously under the same pressure and temperature
conditions. The molded part in each cavity has the same weight and final
properties.
Unfortunately not all runners can be naturally balanced, especially for large
parts where multiple gating may be needed to produce a proper part. Moreover,
the natural flow balance is difficult for molds with a large number of cavities
and is even impossible for the so-called family mold (combination mold) where

each of the cavities is of different size and forms one component part of the
assembled finished product.
In these cases we have to balance the flow artifically. Balancing ensures
virtually equal flow of plastic through each gate of a multicavity mold, and/or
through each gate (if there is more than one) into each cavity. The melt should
arrive at all gates/cavities at the same time and with the same properties so that
all molded parts have uniform characteristics. This type of runner system is
called the artifically balanced runner systems.
On the other hand, even though the cavity layout is virtually balanced, the
desired balanced flow may not be achieved since the flow depends on the plastic
material used, the process condition setting, the accuracy of machining and the
finish inside the channel, temperature difference due to unbalanced
cooling/heating, , uneven venting, mold surface quality, etc.
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Mold.ppt
Runner Design and Part Shrinkage
Runner Design and Part Shrinkage
Runner cross-sectional Area

Part Shrinkage
Runner Length
Part Shrinkage
The runner system design has a significant impact on the quality of moldings.
For example, the part shrinkage increases as the runner length is increased since
more pressure drop in the runner system and the melt is less packed within the
mold. In general, the runner length should be as short as possible in order to
reduce the pressure drop and amount of scrap. However, the runners must be of
adequate length to satisfy the other conditions such as flow balance
consideration, accommodation of cooling lines and ejector pins, etc.
The part shrinkage reduces as the runner cross section is increased since the
filling process is promoted and the effective holding pressure is higher. However,
increase the runner size also produces more scrap and material waste.
The size of the runner depends on the size of the part and its wall thickness, the
design of the mold and the type of plastic being processed. Plastics with low
viscosity (high melt flow index or long flow length) permit a longer or thinner
runner.
The runner cross section should be as small as possible but still compatible with
the melt flow requirement such as pressure drop consideration.
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Mold.ppt
Design of Runner
Design of Runner
Plastic Materials Recommended
R unner D iamete rs
A B S, S A N
0.187-0.375” (4.7-9.5mm)
Acetal
0.125-0.375” (3.1-9.5mm)
A crylic
0.312-0.375” (7.5-9.5mm)
Butyrate
0.187-0.375” (4.7-9.5mm)
Cellulosics
0.187-0.375” (4.7-9.5mm)
Fluorocarbon
0.187-0.375” (4.7-9.5mm)
Ionomer
0.093-0.375” (2.3-9.5mm)
N ylon
0.062-0.375” (1.5-9.5mm)
Polyamide
0.187-0.375” (4.7-9.5mm)
P C
0.187-0.375” (4.7-9.5mm)
Polyester
0.187-0.375” (4.7-9.5mm)
PE

0.062-0.375” (1.5-9.5mm)
PP
0.187-0.375” (4.7-9.5mm)
PPO
0.250-0.375” (6.3-9.5mm)
Polysulfone
0.250-0.375” (6.3-9.5mm)
PS
0.125-0.375” (3.1-9.5mm)
PU
0.250-0.313” (6.4-8.0mm)
PVC
0.125-0.375” (3.1-9.5mm)
For most thermoplastics, minimum recommended runner size=1.5mm (0.06”)
This table lists the recommended runner diameters for different thermo-plastics
in injection molding industry. For most thermoplastics, the minimum
recommended dimension of runner is 1.5mm (0.06”), too small the dimension
may lead to excessive presure drop and filling difficulty.
The recommended runner size also reveals the flow ability (processability) of
the plastic material. Plastics with low viscosity (high melt flow index or long
flow length) such as polyethylene (PE) permit a smaller runner. Larger runner
should be adopted for plastics that have shorter flow lengths (higher viscosity
values), such as polycarbonate (PC).
This table serves as an initial guess for runner sizing.
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Mold.ppt
Design of Runner
Design of Runner
q
Location and Number of Runner Ejectors
Stiffer Plastics
Ejector Pin
Softer/Flexible/Sticky
Plastics
Both the number and location of ejectors depend on the plastic being processed.
The stiffer the plastic is (at the moment of ejection), the fewer ejectors are
needed; also, the designer has higher degree of freedom to determine the ejector
locations. For example, the ejectors can be placed under the connecting runners
(bridge runners) .
For soft, flexible, or sticky plastics, more ejectors have to be adopted. Care must
be taken in the ejector location so that the part can be ejected without leaving
marks or causing damage. In general, more ejectors lead to an increase in the
comlexicity of mold and the cost of the hardware and of machining.
In the design phase of the runner system, one should consider the ease of
demolding and removal from the molded part. The runner system should provide
sufficient spacing for cavity in order to accommodate cooling lines and ejector
pins and leave adequate cross section to withstand the injection pressure force.
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Mold.ppt
Runnerless Molding Technology
Runnerless Molding Technology
Moldings
Runner System:
•Scrap and material waste
•Pressure drop
q
Runnerless Molding Technology:
)
runners and sprues are kept a molten state during the processing
)
runner systems are never actually ejected with the molded parts.
q
Types of Runnerless Molding Technology:
)
Insulated Runner System
)

Heated/Hot Runner System
The conventional runner systemare referred to as cold runner systems since the
runners solidifies during the cooling phase of the injection molding cycle and is
ejected with the part. During the molding cycle the pressure drop increas as the
runner is cooled down gradually. Degating is required during mold opening (for
three-plate molds) or separately afterwards (for two-plate molds) and the runner
system is regarded as scrap. The runner material may be reground and recycled
again, but it may have some physical properties degraded from the original,
virgin material. For small products the mass of cold runners may be as much as
80% of the mass of the total shot.
On the other hand, the so-called runnerless molding technology has been
developed to circumvent the drawbacks encountered in the cold runner systems.
In these special mold designs the runners and sprues are kept a molten state
during the processing and are never actually ejected with the molded part. There
are no runners to be reground and recycled, thus, savings in material, labor,
and/or overhead are realized.
Typical examples of runnerless molding methods include insulated runners,
heated/hot runner systems.
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Mold.ppt
Insulated Runner System
Insulated Runner System
Molten state melt
Solidified resin shell
Cooling Lines
Emergency
parting line
Parting line
q
Oversized the runner diameter (15~30mm)
q
Insulation effect of frozen skin shell
q
Works for most olefinic resins(PE,PP...) and PS
In the insulated runner system, the runner diameter is oversized (say, 15~30mm)
in order to maintain the molten state of the material. The large diameter runner
allows an inner molten melt to pass through during the molding cycle because of
the insulation effect of frozen skin shell surrounding the melt core.
The insulation runner system has the advantage of extremely simple
construction, low cost tooling, and high efficiency, provided the system can be
left running undisturbed for long periods. This design is suitable for most olefinic
plastics (such as polyethylene (PE), polypropylene (PP)... ) and polystryene (PS).
The disadvantages of the insulated runner system includes:
- it requires fast cycle to maintain molten state within runner (at least 5
shots/min).
- it requires long start-up periods (15-25min) to stabilize the runner temperature
(up to 150
o

C)
- it needs a long color change time
- it needs very accurate gate temperature control in order to have a satisfactory
production rate.
- Additional emergency parting line is required to facilitate the removal of the
frozen runner in the case of prolonged delay in the cycle time.
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Mold.ppt
Internally Heated Hot Runner
Internally Heated Hot Runner
System
System
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Material is heated by the heating element in the center of the runner
q
Annular gap for melt flow
Heater Cartridge
Heated Probe

(Torpedoe)
Part
Melt
Tempertature Profile
Vlocity Profile
In the internally heated hot runner system, the material is heated and kept at a
molten state by the heated probe (torpedoe) in the center of the runner. The melt
is allowed to flow in the cross section of the annular gap of the runner.
The advantages of the internally heated hot runner systems include:
-Less heat loss and lower heating power required since the thermal insulation of
polymer melt
-Less mold components mis-matching problem arising from thermal expansion
-Inexpensive (as compared with the external heated runner system)
-Little space required.
The disadvantages of this design include:
-Higher shear rate and pressure drop since the restricted flow area
-Sophicated heat control required (temperature profile exists in the cross
section of the annular gap of the runner).