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Mold.ppt
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YSTEM
YSTEM
Mold Design
Mold Design
Fundamentals
Fundamentals
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Mold.ppt
Functional Systems of the Injection
Functional Systems of the Injection
Molds
Molds
q Melt Delivery System: Sprue/Runner/Gate
q Cavity (with Venting)
q Tempering/Heat Exchange System
q Ejection System
q Guiding and Locating System
q Machine Platen Mounts
q Force Supplier
q Motion Transmission System

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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

Mold Design
) No.Cavity
) Cavity Layout
) Runner System Design
) Gating Scheme
) No.Gate
) Gating Location
) Mechanical/Mechanism
Consideration
q 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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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
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

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Mold.ppt
Common Runner Cross Sections
Common Runner Cross Sections
q Circular Runner
) Full Round Runner
q Parabolic Runner
) U-Type or Modified
Trapesoidal Runner
q Trapezoidal Runner
q 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

Considerations in Runner Design
q 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.

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
Design of Runner
Design of Runner
Plastic Materials Recommended
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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)
Po
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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”)

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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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

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Mold.ppt
Internally Heated Hot Runner
Internally Heated Hot Runner
System
System
q 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

Externally Heated Hot Runner
System
System
q Material is heated by the cartridge-heating manifold in
the housing of the runner
q Circular cross section for melt flow
Cooling Lines
Heater Cartridge
Heated Manifold
Part
Air gap insulation
Insulation Blocks
Hot Runner
Vlocity Profile:
plug-like flow
Temperature Profile:
constant temperature profile
In the externally heated hot runner system the material is heated by the
cartridge-heating manifold in the housing of the runner. Thus a plug-like flow
profile and an approximately constant temperature profile across over the circular
flow area is developed. Thus the flow resistance is smaller than that of the
internally heated system.
The advantages of this design are:
-More uniform temperature distribution.
-Better temperature control
-Lower melt stresses and pressure drop
-Color/material changes easily
The disadvantages of the externally heated hot runner system include:
-More complicated design
-More Expensive

•geometry
•wall thickness
•direction of mechanical
loading
•quality demands
(dimensions,cosmetics,
mechanics )
•Flow length
Plastic
Material
•viscosity (MFI)
•processing temperature
•flow characteristic
•fillers
•shrinkage behavior
Then gate provides the connection between the runner and the mold cavity. It
must permit enough material to flow into the mold to fill out the cavity, raises
melt temperature by viscous (frictional) heating, and freezes-off when the
holding stage is over. It should be smaller in the cross section so that it can be
easily separated from the molded part (degated).
The type of the gate and its size and location in the mold strongly affect the
molding property and the quality of the molded part. The factors which
determine the gate design is summarized here briefly.
General speaking, the gate should be small, simple to demold and easily
separated from the part. The gate should be connected to the molding in such a
manner that the latter is not distorted (the molding tends to deform concave to the
feed ) and does not exhibit blemishes. Cost of tooling is also a consideration
factor. The location of the gate must be such that weld lines are avoided or
shifted to a less critical position. Molding defects such as jetting, burning,
thermal degradation, short shot, etc. should be avoided in the production.

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and is easy to construct and degate.
The pin gate or pinpoint gate is a kind of restricted gates that are usually
circular in cross section and for most thermoplastics do not exceed 1.5mm (0.06
in.) in diameter. It is generally used in three-plate molds (with automatic gate
removal) and hot runner construction. It provides rapid freeze-off and easy
degating of the runner from the gate. Flexibility in gate location is another
advantage of the pin gate. It can easily provide multiple gating to a cavity for
thin-walled parts. Viscous heating as the melt passing through the restricted
pinpoint gate raises melt temperature and improves the filling process since the
melt viscosity is lowered. Higher pressure drop is a drawback.
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Mold.ppt
Gating Scheme
Gating Scheme

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Mold.ppt
Gating Scheme
Gating Scheme
Ring Gate
Submarine/Tunnel Gate
The ring gate accomplishes the same purpose as gating internally in a hollow
tube, but from the outside. In the ring gate the melt reaches an annular channel
manifold next to the sprue. The gate has a small cross section and acts as a
throttle. Therefore the annular channel fills before melt begins to fill the cavity. It
is adopted in the case that the core cannot be mounted on just one side of the
mold such as in the case of disc gating. The ring gate is used to produce sleeve-
like parts with core mounted at both sides of the mold.The advantages of this
gating scheme include: uniform wall thickness around circumference can be
obtained, applicable for long cylindrical part, as well as easy production.
However, final finishing of molded part is necessary and sometimes slight weld
line may appear.
The submarine or tunnel gate is used mainly for small parts in multicavity mold
where it is possible to locate the gate laterally. This gate is automatically degated

fairly large cavity id filled through a narrow gate (such as a side gate) is used,
especially in the case of low-viscosity plastic melt.
Jetting gives rise a random filling pattern: the melt no longer fills the mold by
an advancing front way but snakes it away into the cavity without wetting the
walls near the gate. Surface defects, flow lines, variations in structure, and air
entrapment are related to the jetting phenomena.
Jetting can be prevented by enlarging the gate or locating the gate in such a way
that the flow is directed against a cavity wall. For example, tab gates (or fan
gates) can minimize the potential of jetting by reducing the inertia of the inlet
melt flow.
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Mold.ppt
Effect of Gating Scheme
Effect of Gating Scheme
Time
Cavity Pressure

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Pinpoint Gate
Film Gate
)Different influence on holding stage and effective holding time
The gating scheme has a significant influence on the holding pressure profile
during the cooling stage.
For exmple, the size of a sprue gate is large so that the holding pressure can be
transmitted without difficulty. The gate freezing-off time is longer due to the
larger gate size, leads to a slower droping in the cavity pressure and a longer
effective holding time. Hence in general a sprue gate is used for part that the

the melt is split by the
obsraction into two fronts
the two streams are
brought back together
the temperature at the weld line
does not differ much
Weld lines or knit lines are formed during the mold filling process where two
melt fronts meet each other. Microscopically, in the weld lines (or weld planes)
the two fronts are made of molecules that are aligned with the front shape and
will meet tangentially. The incomplete molecular entanglement and diffusion,
unfavorable frozen-in molecular (or fiber) orientation, as well as the crack-like
V-notches at the weld surface lead to structural weaknesses in the weld line area.
The presence of weld lines causes reduced mechanical strength for structural
applications and surface visual imperfections in the part. The allowable working
stress would be reduced by at least 15% in the weld line area.
In general, the colder the merging flows of melt, the more these weld lines
become visible and the poor is their strength.
Hot weld lines (or streaming weld line, meldline) is formed in the molds with
obstructions such as core, insert, or pin. In this case the melt front is separated by
cores or obstructions and recombines at some downstream location.
Experimental results indicate that the strength of the weld would decrease as the
distance between the obstruction and the gate increases, since the average flow
front temperature has been reduced.