Mold Design Fundamentals P2 pot

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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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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
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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Externally Heated Hot Runner
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
-Significant thermal-expansion-induced mis-match problems for various mold
components.
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Mold.ppt
Design of Gate
Design 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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Gating Scheme
Gating 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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Gating Scheme
Gating Scheme
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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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
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.
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Effect of Gating Scheme
Effect 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.
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Effect of Gating Scheme
Effect of Gating Scheme
Time
Cavity Pressure
Sprue Gate
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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.

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Weld line and Gate Location
Weld 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.
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Example of Hot Weld lines
Example of Hot Weld lines
Melt Front
Average Temperature
a.weld @ 188
o
C
c.weld @ 184
o
C
b.weld @ 185
o
C
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
σ
123
>>
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Weld line and Gate Location
Weld 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.
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Weld line and Gate Design
Weld 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.
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Location of Weld Line
Location of Weld Line
Possible weld line location
Possible weld line location

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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)
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Weld Line and Gate Design
Weld 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.