2mold - cnckhacda.com

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Mold.ppt
CCOREORETTECHECH SSYSTEMYSTEM
Mold DesignMold Design
FundamentalsFundamentals

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Mold.ppt
Basic Tasks of a MoldBasic Tasks of a Mold
q Accomodation and Distribution of the Melt
q Shaping of the Molded Part
q Cooling/Heating and Solidification of the Melt
q Ejection (Demolding) of the Molding
q 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 InjectionFunctional Systems of the Injection
MoldsMolds
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
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 UnitStructure of A Mold Unit
SprueSprue
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 IssuesMold Design Issues
mold base
cooling channel/lines
runner (mainfold) system
gate
cavity
q 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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Mold.ppt
Determine Number of CavitiesDetermine Number of Cavities
q 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 LayoutCavity 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 SystemDesign 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 SectionsCommon 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
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 DesignConsiderations 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.

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Mold.ppt
Flow Balance in the Runner DesignFlow 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 ShrinkageRunner 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 RunnerDesign of Runner
P lastic M a te rials R eco m m en d ed
R u n n e r D iam e te rs
A B S , S A N 0 .18 7-0.37 5” (4.7-9.5m m )
A c etal 0 .12 5-0.37 5” (3.1-9.5m m )
A c rylic 0 .31 2-0.37 5” (7.5-9.5m m )
B u tyrate 0 .18 7-0.37 5” (4.7-9.5m m )
C e llu lo sics 0 .18 7-0.37 5” (4.7-9.5m m )
F lu o ro c arb o n 0 .18 7-0.37 5” (4.7-9.5m m )
Io n o m e r 0 .09 3-0.37 5” (2.3-9.5m m )
N y lo n 0 .06 2-0.37 5” (1.5-9.5m m )
P o lyam id e 0 .18 7-0.37 5” (4.7-9.5m m )
P C 0 .18 7-0.37 5” (4.7-9.5m m )
P o lyes te r 0 .18 7-0.37 5” (4.7-9.5m m )
P E 0 .06 2-0.37 5” (1.5-9.5m m )
P P 0 .18 7-0.37 5” (4.7-9.5m m )
P P O 0 .25 0-0.37 5” (6.3-9.5m m )
P o lysu lfo n e 0 .25 0-0.37 5” (6.3-9.5m m )
P S 0 .12 5-0.37 5” (3.1-9.5m m )
P U 0 .25 0-0.31 3” (6.4-8.0m m )
P V C 0 .12 5-0.37 5” (3.1-9.5m m )
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 RunnerDesign 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 TechnologyRunnerless 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 SystemInsulated Runner System
Molten state melt
Solidified resin shellCooling 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 RunnerInternally Heated Hot Runner
SystemSystem
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
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).

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Mold.ppt
Externally Heated Hot RunnerExternally Heated Hot Runner
SystemSystem
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
-Significant thermal-expansion-induced mis-match problems for various mold
components.

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Mold.ppt
Design of GateDesign of Gate
Generalities
•ease of demolding
•ease of degating
•weld lines
•distortion
•molding defects
•cost
Part Design
•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.
Gating scheme and location of gates are crucial to the quality of the molding.
Filling pattern and cavity pressure profile are closely related to the final
properties of molded parts, such an mechanical properties, cosmetics (surface
properties), dimensional accuracy. A gate should provide appropriate filling
pattern and viscous heating effect, permit effective packing and holding of the
material within the mold. These criteria depend on both part design as well as
physical properties of the plastic material.

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Mold.ppt
Gating SchemeGating Scheme
Direct/Sprue Gate
Side/Edge Gate
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Pin Gate
There are several gate type can be adopted in the mold design, and each has its
own advantage for application.
The direct gate or sprue gate feeds material directly into the cavity. It is used
for temperature-sensitive or high viscosity materials, and is suitable for
producing part with heavy sections. The direct gate can be applied in high quality
part because it allows effective holding (minimum pressure loss) and exact
dimensions can be obtained. However, it is suitable only for single-cavity molds.
Visible gate mark and the high stress concentration around the gate area are the
disadvantges.
The side gate or edge gate is the standard gate for injection molding. It is used
wherever the product can be or must be gated from the parting line and where
self-degating is not required or practical. It is carried out at the side of the part
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
Fan Gate
Film Gate
Tab Gate Disc Gate
The fan or fin gate is a fanned out variation of the edge gate. It is used for large
flat parts (say,over 8cm x 8cm or 3 in x 3 in) or when there is a special reason
such as elimination of weld lines. when the danger of part warpage and
dimensional change exists, the fan gate is often adopted.
The film gate or flash gate involves extending the fan gate over the full length
of the part but keeping it very thin. It is used for flat molded part in the situation
that the orientation of flow pattern in one direction is required, this is important
in the applications of optical parts. It has the advantages that there is no weld
line, reduced warpage and improved part dimensional stability. However,
postoperation for gate removal is required for this type of gate.
The tab gate is used in cases where it is desirable to transfer the stress generated
in the gate to an auxiliary tab, which is removed in a postmolding operation. The
tab gate is capable of preventing the jetting problem during the filling stage. Flat
and thin parts require this type of gate.
The disc gate or its variation, the diaphragm gate, has a conical manifold. It is
used for rotationally symmetrical parts (hollow tubes) with core mounted at just
one half of the mold. The advantage of using this gate system is that there are no
weld lines, and concentricity of the molded part is ensured. This is a important
dimensional requirement for pipe fittings. The cone or diagram region eliminates
stress concentration around the gate since the whole area is removed, but the
postoperation is necessary and more difficulty.

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Mold.ppt
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
as soon as the mold opens, this is the primary advantage of this gate system.
However, it is used for simple part only because of high pressure loss as the melt
passing through the small gate cross section and the runner length. The tunnel
gate can be used only for tough, elastic materials, since the material in the tunnel
has to withstand deformation during mold opening; the tunnel could break and
plug the runner system if brittle materials are used.

23
Mold.ppt
Effect of Gating SchemeEffect of Gating Scheme
Side gate: possibility of jetting
Tab gate: uniform filling, no jetting
The filling pattern of melt flow is largely governed by the location and size of
the gate(s). For example, jetting of the plastic into the mold cavity may occur if a
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.

24
Mold.ppt
Effect of Gating SchemeEffect of Gating Scheme
Time
CavityPressure
Sprue Gate
‧
‧
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‧
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‧
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‧
‧
‧‧
‧
‧
‧
‧
‧ ‧
‧ ‧
‧
‧
‧
‧
‧
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‧
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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
dimensional accuracy is important.
On the other hand, the pinpoint gate freezes early and leads to a shorter
effective holding time. This may cause sink marks and voids in the final part.
The cavity pressure curve of part with film gate is located between that of sprue
gate and pinpoint gate.
In the mold design phase, one have to consider if the gate can provide suitable
filling pattern, viscous heating, as well as its influence on effective holding time.

25
Mold.ppt
Weld line and Gate LocationWeld line and Gate Location
q Hot Weld (Streaming Weld, Meldline)
Weld lines arising from obstructions (core,insert,pin...) in the flow
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.

26
Mold.ppt
Example of Hot Weld linesExample of Hot Weld lines
Melt Front
Average Temperature
a.weld @ 188 oC
c.weld @ 184 oC
b.weld @ 185 oC
Consider a part has one rectangular and two circular inserts obstructing the flow
with the rest of the cavity at an uniform thickness. From the CAE analysis we can
predict the location of weld lines behind each insert.They are hot weld lines since
they are formed due to the exist of flow obstructions and the welding temperature
is high.
The welding temperature at position a,b,and c is 188, 185, and 184 o
C,
respectively. The melt front splits and recombines around each insert. Weld
strengths tend to decrease as the number of flow stream divisions and
recombinations increase. They also decay with the distance from the gating
position because the melt is cooled along the flow path. We can anticipate that
the local strength in each welding position:
Thin sections are particular prone to weak welds because of rapid melt
solidification and less chance for chain diffusion.
σ σ σ1 2 3> >

27
Mold.ppt
Weld line and Gate LocationWeld line and Gate Location
q Cold Weld (Butt Weld)
arise from the impingment of advancing fronts from different
gates in multi-gating molds. Worst welding manner.
Melt fronts traveling in opposite
directions meet, and are almost
immediately stoped after meeting.
the temperature of the fronts has dropped
somewhat at the welding zone
On the other hand, the so-called cold weld lines or butt weld lines present in
multiple gating molds where the impingment of advancing fronts from different
gates may occur.
Cold weld lines are generally considered to be the worst welding manner
because they are formed from melt fronts traveling in opposite directions, the
fronts meet and are almost immediately stoped after meeting. The temperature of
the meeting fronts has dropped somewhat at the welding zone, this leads to a
weak welding condition since the molecular diffusion and entanglement is rather
poor in the low temperature area.
For unreinforced plastics, the tensile strength in the cold weld region can be
reduced to 80%; for fiber-reinforced plastics, this value is reduced to 30% to
40%.
The melt temperature is the most significant process variable in the welding
phenomena. Hotter melt tends to improve the weld strength due to the increased
molecular chain mobility and their coupling. Increase the mold temperature is
another strategy to improve the welding strength. Besides, welding strength can
be improved by good molding venting (avoid air entrapment), high injection
speed (decrease the temperature drop).
Gate design play an important role in the removal or elimination of weld lines.

28
Mold.ppt
Weld line and Gate DesignWeld line and Gate Design
Edge gating will lead to
a weld line opposite the gate
Weld strength is weak when
diameter ( &flow length ) is increased.
Spoke gating will produce
four weld lines with
stronger weld strength due
to shorter flow lengths
Sprue gating at the cup
bottom will eliminate
weld line,gate mark problem
Consider the gate design in an injection-molded cup. This part can be produced
using a single edge gate in a two-plate mold. This gating scheme would result in
a cold weld line opposite the gate. As the diameter of the cup is increased, the
weld line becomes more visible and the welding strength is decreased since the
flow length prior to welding is longer and the welding temperature is lower.
When an internal spoke gating scheme is adopted, although four weld lines will
be formed, however, each weld line is likely to be stronger (compared to the part
with a single edge gate) due to the reduction in melt flow length in the cavity.
Hence the weld line produced by the spoke gate is less visible and the welding is
stronger.
If a sprue gate at the cup bottom is used in this case. No weld line would be
produced in the final part. However, the significant gate mark is a problem and
an postoperation is require to finish the product.

29
Mold.ppt
Location of Weld LineLocation of Weld Line
Possible weld line location
Possible weld line location
PLAY447
Top Cover of Scanner
In general, weld lines would be visually unacceptable, or, since they act as
stress concentrator, may be structurally unacceptable, depend on the product
specification and quality requirement.
Computer analysis is capable of predicting the possible location of weld line.
According to the analysis result we can modify the gate design, part design
(modify the part thickness), or process condition, to relocate the weld lines to
visually or structurally less sensitive areas.
Consider a scanner cover that is produced by three submarine gates as an
example. In multi-gated parts the weld lines are almost unfavorable. From the
CAE analysis result we can predict the possible weld line locations and check if
they occur in critical regions. This precautions from CAE analysis in the design
phase will minimize the risk of part failure.
We can modify the design conditions to see if the weld lines can be relocated to
noncritical regions. When they are unavoidable, venting plays an important role
in improving the weld strength. That is, it is essential that air at the weld should
escape before the melt streams meet. Other techniques to improve weld strength
are to :
- Increase melt temperature (that is, chain mobility and coupling)
- Increase mold temperature (that is, chain mobility and coupling)
- Increase injection pressure (that is, lower the temperature difference)
- Avoid use of external release mold lubricant (avoid the presence of foreign
substances at the weld interface)

30
Mold.ppt
Weld Line and Gate DesignWeld Line and Gate Design
Allow cavity filling with a minimum no. weld lines
more significant
weld line
less significant weld line
more significant
weld line
As a rule, if a single gate can fill the cavity without excessive injection
pressure, use it. Multiple gating always produce extra weld lines in the product.
However, two or more gates per cavity are sometimes required for very large
products (such as automobile products, bottle crates, etc.) where the flow lengths
from a single gate would be too long and/or too high the injection pressure is
required to fill the cavity. In some cases a multiple gating scheme is required to
avoid short-shot (incomplete filling) problems.
Consider the injection-molding of the motorcycle side cover by ABS. If two
gates per cavity is adopted, one weld line is produced in each cavity. However,
the injection pressure required is high and short-shot problem will present in the
end of filling; If triple gating scheme is employed, the cavity can be complete
filled without difficulty, except that there is an additional (less significant) weld
line in the final product.
It is important that the melt arrives at the welds (junction points) hot enough to
form an acceptable welding. Venting problem should not be overlooked in
improving the weld strength.

31
Mold.ppt
AirAir--Trap and Gate LocationTrap and Gate Location
Air Bag Housing
thinner section
(0.5-0.8mm)
thicker section
(>10mm)
Racetrack Effect
Air-trap here
PLAY
When the plastic melt fills the mold, it displaces the air. The displaced air must
be removed quickly, or it may cause burn spot (due to the fast compression of
trapped air pocket by the low-thermal-conductivity polymer melt), or it may
restrict the flow of the melt into the mold cavity, resulting in incomplete filling
(short-shot problem).
Consider the injection-molding of a air bag housing. Notice that the part
consists of a thin central region and a thick rim around it. A single gate is
adopted in the original design. Most of the melt flow along the part side since the
section is thicker and the flow resistance is lower than that in the central thinner
region. That is, the melt races away along the thick rim while the central region is
filling at a slow rate. The filling along the rim is dominant and finally the melt
backfills the central region and cause an entrapment of air there. In this case an
air-trap problem is caused by the racetrack effect of melt flow.
To avoid the buring or incomplete filling associated with the entrapment of air,
proper venting is required. Venting is provided by the clearance between
knockout/vent pins and their holes, parting lines, as well as additional venting
slots (in general, 0.01 to 0.02mm deep and 3mm to 6mm wide).
Gate location is directly related to the consideration of venting location. In
general, the vent is located opposite the gate, area near the end of filling, or in the
air-trap position.

32
Mold.ppt
Viscous Heating and Gate SizeViscous Heating and Gate Size
pinpoint gate (dia.=2mm)
Temperature(oC)
Gapwise Scale
inlet melt
temperature
temperature peak caused by
viscous heating effect
Melt viscosity is reduced and flowability is improved by raising the
melt temperature via viscous heating effect
Temperature raised <15 oC (lower value for thermal sensitive material)
As the melt flows through the restricted gate, the flow velocity is sufficient high
and the melt is highly sheared in the narrow passage. This frictional (viscous)
heating would cause a raising in melt temperature. The temperature change is
related to the melt viscosity and the local shear rate.
The nomial wall shear rate in the gate is greater than 1000 sec-1
and can reach as
high as 105
sec-1
. At this high shear rate the viscosity may be reduced due to the
shear-thinning rheological character of polymer melt. The melt viscosity is
further reduced by the viscous heating in the gate region. The viscosity reduction
as the melt flows through the gate is important in improving the flowability of the
material.
A gate should be properly sized so that it could provide sufficient shearing and
viscous heating in order to achieve the greatest flow length possible. If the gate is
too large, it may freeze permaturely due to the insufficient viscous heating and
the dominant mold cooling effect. On the other hand, if the gate is too small,
filling process is highly restricted, leads to the overheating and thermal
degradation of part.
In general, the temperature change across the gate should be controlled within
the range of 15 o
C; if the material processed is thermal-sensitive, the range
should be smaller.

33
Mold.ppt
Gate Design vs. Part ShrinkageGate Design vs. Part Shrinkage
Gate Size
Part Shrinkage
Higher Packing Lower Packing
Demolding
Less Shrinkage Larger Shrinkage
Differential Shrinkage
Back
Gate design is important not only in controlling the filling pattern of the mold
cavity, but also in the dimensional quality of molded part.
Smaller gates freeze off sooner. Once the gates frozen, there is no melt added
during the holding pressure stage, and the molded part will therefore shrink
more.
On the contrary, larger gates remain open longer. They freeze slowly and melt
continues to feed under holding pressure through the open gate, adding more
plastic as the melt shrinks in the cavity. Longer effective holding time and higher
holding pressure level of larger gates lead to smaller part shrinkage values.
In the mold cavity, the areas closer to the gating position are better packed than
the more remote areas, which may already have cooled down enough to prevent
additional melt to make up for volume contraction through shrinkage. The result
is that the areas near the gate shrink less than the areas farther away.
Besides, during the mold filling stage the polymer molecules undergo a
stretching that results in molecular orientation and anisotropic shrinkage
behavior: plastic materials tend to shrink more along the direction of flow (in-
flow shrinkage) compared to the direction perpendicular to flow (cross-flow
shrinkage), while the shrinkage behavior of reinforced material is restricted
along the fiber-orientation direction.
This differential shrinkage is the primary cause of part warpage.

34
Mold.ppt
Cooling System DesignCooling System Design
molding
cooling
channels
•Layout
•Size
•Distance to Molding
•Coolant Flow Rate
•Coolant Temperature
•Type of Coolant
•Mold Material
The mold of thermoplastics receives the hot, molten plastic in its cavity and
cools it to solidify to the point of ejection. The mold is equiped with cooling
channels or cooling lines that remove heat released from the part via flowing
coolant. The mold temperature is controlled by regulating the temperature of
coolant and its flow rate through the cooling channels. Productivity (cycle time)
and quality (dimensional accuracy) of molded part depend heavily on the design
and efficiency of the cooling system.
High efficiency cooling system may cool down the part uniformly and quickly,
hence the cycle time can be shortened, this leads to an improvement of the
molding productivity.
The cooling channels should be spaced evenly to prevent uneven temper-ature
on the mold surface, they should be as close to the part surface as possible,
taking into account the strength of the mold material. The cooling channels are
connected to permit a uniform flow of the coolant, and they are thermostatically
controlled to maintain a given coolant temperature.
Even mold temperature distribution is important to ensure the dimensional
accuracy of molded part. Uneven mold temperature leads to unbalanced cooling
of part surface. The thermal stresses associated with the temperature profile
across the part thickness result in part warpage or distortion.
Design parameters involved in cooling system involves the type of coolant and
mold material, coolant flow rate, coolant temperature, distance and size of
cooling channels, and their layout.

35
Mold.ppt
Cooling Channel Layout vs.Cooling Channel Layout vs.
Part WarpagePart Warpage
Lower Cooling Rate
Higher Cooling Rate
Unbalanced Cooling
Demolding
Colder surface
Smaller Shrinkage
Hoter surface
Larger Shrinkage
Warpage of Injection-Molded Part
Uniform cooling throughout the part is critical to the dimensional accuracy of
molded part.
Consider the cooling of an injection-molded plate part by a poor-designed
cooling system. The top face of the part is cooled by three cooling channels, the
part surface temperature in higher due to the insufficient cooling; on the other
hand, the bottom face of the part is cooler since it is cooled by four cooling
channels (assume that all cooling channel has the same cooling efficiency).
The hotter top surface of the part will continue to shrink more than than the
cooler bottom surface after the gate frozen off and part ejection. This differential
shrinkage through the part thickness is caused by the differential cooling (
difference in the cooling rate between the cavity and the core) and would cause
the part to warp due to the unbalanced internal thermal stresses and their
associated bending moments as the part is ejected from the mold.
Non-uniform cooling plays a key role in the warpage behavior of molded part,
especially in the cases of flat moldings, such as disks (records, trays, etc). The
differential cooling problem can be minimized with proper mold cooling system
design.

36
Mold.ppt
Wall Thickness and Part DesignWall Thickness and Part Design
q Flow Length/Wall Thickness Ratio (L/t Ratio)
a measure of moldability of the part
( )
L / t Ratio
Maximum Flow Length
From Gate to Rim
Average Wall Thickness
≡
⎛
⎝
⎜
⎞
⎠
⎟
L
t
L/t Ratio
0 100 200 300
Heavy-walled parts
easy to mold
Most parts
relatively easy to mold
Thin-walled part
Difficult to mold,
needs special considerations
Very-difficult-to-
mold part
needs special
equipment
An important measure of the moldability of a part design is its flow length/wall
thickness ratio (L/t ratio). The L/t ratio of a part is defined as its maximum flow
length from gate (the pressure source) to the farthest point (end point of filling),
to its average wall thickness.
A smaller values of the L/t ratio indicate a shorter flow length or thicker part
section, represent a smaller flow resistance and pressure loss, hence the parts are
easy to mold. On the other hand, thin-walled parts or parts with longer flow
length have larger L/t ratios and the molding is more difficult to carry out.
The L/t ratio of a given part can assist the part designer in determining the gate
locations, especially for parts of constant wall thickness. It’s rather difficult to
evaluate this value for a complicated part with variable wall thicknes, this
situation is further complicated by the fact that runner systems can consume a
significant portion of the mold’s pressure drop.
Many factors influence the L/t ratio of a given design, such as plastic materials
processed, melt temperature, mold temperature, maximum injection pressure, and
injection velocity, etc.

37
Mold.ppt
Maximum Flow Length ofMaximum Flow Length of
Plastic MaterialsPlastic Materials
PC,PVC
Acetal
Nylon
Acrylic
ABS
PS
HDPE
PP
LDPE
25
36
38
33-38
45
51-63
57-63
63-70
70-76
0 10 20 30 40 50 60 70 80
PC,PVC
Acetal
Nylon
Acrylic
ABS
PS
HDPE
PP
LDPE
(cm)
Maximum Flow Length in a 2.54mm(0.1in.) thick part
The flow of the plastic melt in the mold depends on various factors, such as the
plastic used, melt temperature, mold temperature, length and diameter of sprue
and runners, gate type, etc. In determining the minimum wall thickness of the
part, all these factors have to be considered.
The L/t ratio achieveable depends heavily on the type of plastic to be processed.
A high viscosity (low melt index) plastic such as polycarbonate (PC),
polysulfone (PSU), acrylic,etc., has a higher resistance to flow because of its
microstructure (cross linking, high molecular weight) and thus has a shorter
maximum flow length. It requires higher injection pressure to fill the mold cavity
with sufficient filling speed. For example, in a testing mold with a thickness of
2.54mm (0.1in.), the maximum flow length of PC is 25cm.
On the other hand, for easy-flow, low-viscosity plastics such as poly-propylene
(PP), polyethylene (PE), the maximum flow length is longer and the minimum
wall thickness that can be filled is smaller than for stiff-flowing materials.
Typical maximum flow length of general purpose grades of thermoplastics,
based on a cavity thickness of 2.54mm (0.1 in.) and conventional molding
techniques, are provided here to illustrate their processing properties. These data
are obtained from the spiral flow length experment and can be used as a reference
of moldability of various resin grades.
The actual maximum flow length of a plastic material depends on part design,
mold design, as well as the process variables.

38
Mold.ppt
Maximum Flow Length ofMaximum Flow Length of
Plastic MaterialsPlastic MaterialsMaximumFlowLength
Part Thickness
increasing
injection pressure
@constant injection speed,
mold/melt temperature
MaximumFlowLength
Part Thickness
increasing
melt/mold temperature
@constant injection pressure
injection speed
The maximum flow length achieveable for a particular plastic grade depends on
molding conditions of the experiments.
For instance, under a constant injection speed/mold temperature/melt
temperature condition, the flow length increases as the applied injection pressure
is increased because of the increasing driving force for mold filling. Thus easy-
to-flow materials require a lower injection pressure to fill the mold cavity with
sufficient filling speed.
Under a constant injection speed/injection pressure condition, the maximum
flow length of a given material increases as the mold temperature and/or the melt
temperature is raised. A plastic material has a longer flow length at higher
temperature because of its thermal-reduced melt viscosity.
These flow length data of plastic materials provide valuable information about
their flow behavior and processing properties. They are available from material
suppliers.

39
Mold.ppt
Wall Thickness of a PartWall Thickness of a Part
Plastics Min. Wall
Thickness (mm)
Max. Wall
Thickness (mm)
Suggested Wall
Thickness (mm)
POM 0.4 3.0 1.6
ABS 0.75 3.0 2.3
Acrylic/PMMA 0.6 6.4 2.4
Cellulose 0.6 4.7 1.9
Teflon 0.25 12.7 0.9
Nylon 0.4 3.0 1.6
PC 1.0 9.5 2.4
Polyester 0.6 12.7 1.6
LDPE 0.5 6.0 1.6
HDPE 0.9 6.0 1.6
EVA 0.5 3.0 1.6
PP 0.6 7.6 2.0
PSU 1.0 9.5 2.5
PPO 0.75 9.5 2.0
PPS 0.75 3.8 2.3
PS 0.75 6.4 1.6
SAN 0.75 6.4 1.6
PVC-Rigid 1.0 9.5 2.4
PU 0.6 38.0 12.7
Surlyn 0.6 19.0 1.6
The nominal minimum, maximum, and suggested wall thickness for various
plastic materials is listed here. The essential issue in determining the wall
thickness of a part is the flowability of polymer melt. The wall of a part should
allows plastic melt to flow properly under appropriate injection pressure. The
wall should permits effective transmission of packing/holding pressure during the
holding stage. Finally, the wall should withstand the internal/external loading
after the part is ejected from the mold cavity.
The allowable minimum wall thickness is smaller for easy-flow, low-viscosity
plastics such as polyethylene (PE) and polypropylene (PP). This value is larger
for polycarbonate (PC) and polysulfone (PSU) that are more viscous and stiff-
flow.
Typically, a thin-walled part can be arbitrarily defined as a part with a L/t ratio
greater than 200 or with wall thickness less than 1mm (t<1mm). In a thin-walled
part mold cooling effect is dominant and the part is rapidly cooled. Cycle time is
usually short (less than 10 sec). The injection pressure needed is higher for
proper filling and short-shot (incomplete filling) can be a problem.
A heavy-walled part can be defined as a part with a L/t ratio smaller than 100 or
with wall thickness more than 2mm (t>2mm). Filling is not a problem in a heavy
wall and the injection pressure needed is lower than that of the thin wall. Cycle
time is long, often longer than 20 sec.
Determining the proper part thickness is important to facilitate the processing
and ensure product strength.

40
Mold.ppt
Wall Thickness of a PartWall Thickness of a Part
q Empirical Equation
t,L in mm
for easy - flow plastics: t = 0.6
L
100
0.5
for fair - flow plastics: t = 0.7
L
100
0.8
for stiff - flow plastics: t = 0.9
L
100
1.2
+
⎛
⎝
⎜
⎞
⎠
⎟
+
⎛
⎝
⎜
⎞
⎠
⎟
+
⎛
⎝
⎜
⎞
⎠
⎟
e.g.,PP,PE,Nylon
e.g.,POM,PMMA
e.g.,PC,PSU
An empirical equation is presented here to give an rough estimate of wall
thickness for a plastic part. For example, if polypropylene (PP) is used as the
molding compound, the wall thickness of a 50-cm long part will be:
wile for the stiff-flow polycarbonate (PC) the required wall thickness is:
the cooling time is about four times that of PP.
t mm= +
⎛
⎝
⎜
⎞
⎠
⎟ =0 6
500
100
05 33. . .
t mm= +
⎛
⎝
⎜
⎞
⎠
⎟ =0 9
500
100
12 56. . .

41
Mold.ppt
Part Wall TransitionPart Wall Transition
flow
t Sharp/Stepped Transition:
poor design
flow
3t
Gradual Transition:
better design
thick-to-thin gating
flow
3t
Gradual Transition:
thin-to-thick gating
(not recommended)
flow
3t
Smooth/Tapered Transition:
best design
For a variable wall thickness part, the wall transition should be gradual to
ensure proper mold filling and part strength.
Consider the sharp or stepped transition case, the wall thickness undergoes a
step change in the part. During the filling stage the melt front chages its filling
velocity suddenly in the wall thickness transition region and a pressure loss is
caused by the flow contraction effect. The filling pattern in this design may result
in air entrapment and stress concentration problems.
A better design is to modify the stepped transition into a gradual transition
(usually tapered a transition length equal to three times the difference in
thickness). The melt velocity undergoes a gradual change as the cross section
contracts gradually. Pressure loss due to the gradual contraction is lower than that
of the stepped transition. High stress concentration around the transition region
can be avoided.
The best design is to vary the wall thickness as smooth as possible, usually a
tapered transition is adopted. Pressure loss and stress concentration can be
minimized in this design.
Note that the melt flow should be directed in the direction from “thick-to-thin”
whenever posible. The thicker section requires more packing/holding to
compensate for volume contraction and should be located closest to the gate. If
the flow direction is from thin section to thick section, the thinner section may
freeze off faster and hinder the packing of the thicker section, poor surface
finishes and sink mark/warpage problems may be caused.

42
Mold.ppt
Wall Thickness and ShrinkageWall Thickness and Shrinkage
local heavy section:poor
sink mark
Shrinkage voids
Use two short thick ribs:good
Use a long thin ribs:betterCore out the heavy section:better
Thin wall parts with heavy boss, ribs, rims, and/or other local heavy cross
sections usually is difficult to molding. Usually the poorly cooled heavy sections
will shrink more because the holding pressure will be ineffective after the thin
walls freeze and block the melt flow to these heavy sections. This can be often
seen by the sink marks on the surface behind these local heavy sections. Also, the
differential cooling and shrinkage of the thin and thick sections lead to warpage
of the molded part. When the cooled outer surface of the part is strong to resist
sinking and the inner hot melt cools and shrinks, shrinkage holes/voids will be
created within the plastic wall.
Thick ribs provide improved structural benefits and are easier to fill, however,
the level of sink associated with the thick ribs can be excessive. The sink mark
and internal shrinkage voids problems are significant if the rib wall thickness is
too heavy and/or if the rib base is wide.
Adopt a long but thin rib is a good strategy to improve the design. In practice,
rib wall thicknesses are typically 40%-80% as great as the wall from which they
extended, with a base radius values from 25%-40% of the wall thickness. The
specific rib designs are material dependent, and are influenced primarily by the
shrinkage behavior of the plastic material.
Alternative better design is to core out the heavy section, uniform wall thickness
can be obtained in this case. This results in cycle time reduction along with an
overall quality improvement.

43
Mold.ppt
Wall Thickness and ShrinkageWall Thickness and Shrinkage
original design
thick section thick section
thin section
rib
sink mark
sink mark
sink mark
sink mark
part warpage
voids
stress concentrationbetter design
Thick walls in a part will fill easily, with less pressure, but will take a longer
time to cool and shrink more; on the other hand, thin walls require much higher
pressure to fill the cavity space at high speed and will not shrink as much as
heavy walls.
Thin wall parts with heavy boss, ribs, rims, and/or other local heavy cross
sections usually is difficult to molding. Problems such as sink marks, warpage,
and shrinkage voids may be caused if the part wall is not properly desinged.
When parts have both thick and thin sections, gating into the thick section is
preferred because it enables packing/holding of the heavy section, even if the
thinner sections have frozen off. The design can be further improved by coring
out heavy bosses and heavy sections, and by using ribs and edge stiffeners to
compensate for the loss in stiffness of a thinner section. A cored out section not
only shrinks less but also takes a shorter cooling time.
A properly design part, with even wall thickness and adequate ribbing, is
usually stronger and stiffer than a part with thicker and/or uneven walls. Saving
of material, reduction in part weight and cycle time, improvement in part quality
, etc., are the advantages obtained if we design the part carefully.

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