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Design Tips
for Rapid Injection Molding
Volume 1
NOBODY’S FASTER IN THE SHORT RUN.®
Protomold 5540 Pioneer Creek Drive, Maple Plain, MN 55359 (763) 479-3680
Design Tips for Rapid Injection Molding
Design Tips categorized by topic
Page
Table of contents
3
Draft your way to a better part
4
Don’t cut corners! Foci on your radii.
5
Start with the right finish!
6
Thick section shrinking causes warping and sinking
7
The right resin is a material choice
8
Expect to eject
9
Be open to shut-offs
10
Get a feel for texture
11
Living with hinges
12
The hole picture
13
Consider warp factors
14
Know more knit lines
Material
selection
ñ
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Design
guidelines
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ñ
ñ
ñ
ñ
External link to more information
Quality
assurance
Understand
the process
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ñ
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Volume 1
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design matrix
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2
Design Tips for Rapid Injection Molding
Draft your way to a better part
As illustrated in figure 1, draft is the angle
between the direction of ejection of a part
from the mold and the surface of the part. Its
traditional function is to facilitate the removal
of the part from the mold, but in Protomold’s
Rapid Injection Molding process, it also permits
deeper geometries to be milled while reducing
cost and insuring trouble free molding. Drafts of
1.0 degree and larger may result in lower costs
with the Rapid Injection Molding process.
It is important that textured surfaces have
adequate draft to prevent the part from
sticking in the mold and to prevent “drag”
marks. Protomold requires that lightly textured
It is important that textured surfaces have
adequate draft to prevent the part from sticking
in the mold and to prevent “drag” marks.
Top view
Top view
No draft results in sliding
parallel mold surfaces
Draft results in improved
mold shutoff surfaces
Figure 2: Draft can also improve mold life.
surfaces (T-1) have a minimum of 3 degrees of
draft and that more heavily textured surfaces
(T-2) have a minimum of 5 degrees of draft.
Undrafted
Drafted
Figure 1: Draft definition.
©2007 Protomold. All rights reserved.
Another example of the importance of draft
in the manufacturability of your design is
illustrated in figure 2. In this case the draft not
only helps the part eject from the mold more
easily, but it also minimizes the amount of sliding
required between the mold’s telescoping shutoff
surfaces. A minimum of 3.0 degrees of draft
is required for telescoping shutoff features.
Volume 1
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Draft your way to a better part
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3
Design Tips for Rapid Injection Molding
Don’t cut corners! Foci on your radii.
Whenever possible, a part to be injection molded
should be designed with generously radiused
corners to enhance its quality and moldability.
As illustrated in figure 1, corners designed without
radii can cause stress concentrations. These in
turn may reduce the ability of the part to withstand
load and/or cause warping in its geometry.
Figure 2 illustrates how sharp corners might
adversely affect the flow of resin during molding,
potentially causing incomplete fill. They also
tend to cause the part to stick to the mold during
ejection, which can cause a variety of problems.
Figure 1: Corner radii enhance part quality.
And in figure 3 we show how the judicious use
of fillets can also help to improve mold life by
helping to minimize corner stresses at the bottom
of tall, thin cores in the mold (at the entrance to
deep thin holes in the plastic part). These fillets
also help to enhance the ability of the mold to
fill and further reduce internal part stresses.
See the Protomold Design Guide
for more details.
Figure 3: Corner radii can enhance mold life.
Figure 2: Corner radii enhance moldability.
Volume 1
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Don’t cut corners! foci on your radii.
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4
Design Tips for Rapid Injection Molding
Start with
the right finish!
When designing a part for injection
molding, it is important to keep in mind
the relationships between surface finish,
moldability, cost, and lead time.
Table 1 contains the list of standard surface
finishes available through Protomold’s
rapid injection molding service, listed in
order from lowest to highest cost.
“PM” in the table signifies a surface finish
adjusted to fit the rapid injection molding
process where SPI (The Society of the Plastics
Industry) denotes an industry-standard finish.
The photographs shown in figure 1 illustrate
the difference in cosmetic appearance for a
few of these options on some example parts.
If the part will not be visible to the end
user, you will probably choose to specify
either PM-F0 or PM-F1 using the drop-down
menus on your ProtoQuote (see sample).
But many times your design will require a
more cosmetic surface finish. In these cases
there are two key things to keep in mind:
Polishing: Smoother part surfaces are achieved
using manual mold polishing techniques. Consider
a part with tall, thin, and curved ribs which needs
to have an SPI-A2 finish. In this case you should
expect a significant cost increase, because it
is very time consuming to polish deep, narrow
SPI-A2: Grade #2 Diamond Buff, 1-2 Ra
Higher
Cost
PM-T2: Protomold texture, SPI-C1 followed by medium bead blast
PM-T1: Protomold texture, SPI-C1 followed by light bead blast
SPI-B1: 600 grit paper, 2-3 Ra
SPI-C1: 600 grit stone, 10-12 Ra
PM-F1: Low-cosmetic ­— most toolmarks removed
PM-F0: Non-cosmetic — finish to Protomold discretion
Figure 1: Surface finish examples.
slots in molds. And such lengthy polishing times
may also affect the lead time for your parts,
potentially making it impossible for Protomold
to accept your order for our famous 3-day turn.
Texturing: Given the line-of-sight nature of
bead blasting, it may not be possible to texture
the sides of minimally drafted ribs on a part
because the mold surfaces may be inaccessible.
In addition, if the walls of your part are textured,
it may have an adverse effect on the ability of
the part to release from the mold, potentially
resulting in unsightly “drag marks”. For these
reasons we can only apply texture to areas
of the part that are drafted at least 3 degrees
for a T1, and 5 degrees for a T2 texture.
See the Protomold Design Guide
for other helpful design information.
Table 1: Protomold’s standard surface finishes.
Volume 1
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Start with the right finish!
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5
Design Tips for Rapid Injection Molding
Thick section shrinking
causes warping and sinking
As the plastic solidifies in the injection mold,
it freezes from the outside (near the mold
surface) toward the inside. In thick sections
this results in inward pulling stresses (due to
contraction) that can cause sink marks in the
outer surfaces of the part (see figure 1).
and thin sections, resulting in part warpage. So in
the design of parts to be injection molded, it is a
good idea to maintain consistent wall thickness
and avoid thick areas whenever possible.
Figure 2
Figure 3
Thick walls cause sink,
warp & excess shrink.
Thinner walls give
accurate parts
thicknesses helps to avoid sink (see figure 3).
Warpage due to stresses in step transitions
between wall thicknesses can be improved
through the use of a ramp (see figure 4).
The use of gussets can be helpful to provide
support in corners to avoid warping.
See the Protomold Design Guide
for other helpful design information.
Figure 1
Figure 4
Bad Bosses
High stress
concentrations
Reduced stress
concentrations
Thinner wall results in
warpage during cooling
Gussets provide additional
support to reduce warpage
Good Bosses
As Designed
As Molded
In addition, because thinner sections will freeze
faster than thicker sections, there is also the
possibility of stresses building up between thick
©2007 Protomold. All rights reserved.
Thicker and non-uniform wall thicknesses can
often result in sinks in the material due to the
same solidification physics described above
(see figure 2). The use of thinner, uniform wall
Volume 1
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thick section shrinking causes warping and sinking
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6
Design Tips for Rapid Injection Molding
The right resin is a material choice
Application-specific requirements will
always drive the need for particular
material properties like tensile strength,
impact resistance, or ductility.
But successful designs for injection
molded parts are also built on an
understanding of process-related issues
such as the ability to fill the mold,
tendency to flash, ease of part ejection,
and the potential for warp, sink, or void
creation. As noted in prior Design Tips,
part geometry can be used to help address
some of these issues; but just as material
properties are an important factor in
meeting the requirements of a given
application, they should also be considered
to ensure the moldability of the part.
Table 1 lists some commonly used
resins along with their brand names
and a high-level summary of their
material properties, moldability
characteristics, and relative costs.
The complete list of resins is available
on the Protomold Web site , and you
can visit the Protomold Design Guide
for other helpful design information.
Mechanical Properties
Moldability Characteristics
Some brand
names
Strength
Impact
High tempstrength
Warp and
dimensional
accuracy, molded
Fills
small
features
Voids
in thick
Sink in
thick
Flash
High temp
on mold &
ejectors
Relative
cost
Acetal
Delrin, Celcon
Medium
Medium
MediumLow
Fair
Fair
Poor
Good
Good
Fair
Medium
Nylon 6/6
Zytel
Medium
High
Low
Fair
Excellent
Good
Fair
Poor
Fair
Medium
Nylon 6/6, glass filled
Zytel
High
Medium
High
Fair
Good
Good
Fair
Fair
Medium
Polypropylene
Maxxam, Profax
Low
High
Low
Fair
Excellent
Poor
Poor
Good
Low
High density
Polyethylene (HDPE)
Dow HDPE,
Chevron HDPE
Low
High
Low
Fair
Excellent
Poor
Poor
Good
Low
Polycarbonate
Lexan, Makrolon
Medium
High
Mediumhigh
Good
Fair
Fair to
good
Fax
Good
Good
Mediumhigh
Acrylonitrile Butadiene
Styrene (ABS)
Lustran, Cycolac
MediumLow
High
Low
Good
Fair
Good
Fair
Good
Good
Low
Polycarbonate/ABS Alloy
Cycoloy, Bayblend
Medium
High
Medium
Good-excellent
Fair
Good
Fair
Good
Good
Medium
Plybutylene
Terephthalate
Valox, Crastin
Medium
High
Low
Fair
Fair
Fair
Fair
Good
Mediumhigh
Polystyrene
Styron
Mediumlow
Low
Low
Good
Good
Fair
Fair
Good
Low
Thermoplastic Elastomer
Isoplast,
Santoprene
Low
High
Low
Poor
Excellent
Good
Poor
Excellent
Lowmedium
Acrylic
Plexiglass-Acrylite
Medium
Low
Low
Good
Fair
Good
Good
Good
Medium
Poor
Table 1: Resin Selection Guide.
©2007 Protomold. All rights reserved.
Volume 1
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The Right Resin is a material choice
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Design Tips for Rapid Injection Molding
Expect to eject
The Protomold Rapid Injection Molding
process uses ejector pins of various
sizes to push the plastic part out of the
mold after it has solidified. The sizes and
arrangement of these pins are selected to
minimize the impact on your part design.
Figure 1 is an example of the illustration
Protomold provides early in the process of
designing the mold so that the location and size of
both the gate(s) and ejector pins can be approved.
Sometimes it is appropriate to adjust the design
of the part to accommodate the need for ejection
pins to push the part out of the mold. For example,
figure 2 illustrates how an injector pin “landing
pad” has been integrated into the wall of a part
design in order to provide sufficient material
for the full diameter of the pin to meet the
part. Landing pads may also be used to provide
additional support for the ejection of thin curved
walls, and in some cases the pins themselves
can be contoured to fit the shape of the part.
Of course, always remember to provide as
much draft as possible so that the ejector pins
can do their job, especially for applications
where it isn’t possible to use mold-release
lubricants to help the part eject more easily.
Each situation is different, but a good
understanding of the use of ejector
pins is important when designing a
part to be Rapid Injection Molded.
Figure 1 : A typical Protomold-supplied gate and
ejector pin layout submitted for customer approval.
©2007 Protomold. All rights reserved.
Figure 2 : The wall of this part has been
increased to support the full impact of the
ejector pin without damaging the part.
Visit the Protomold Design Guide
for other helpful Rapid Injection
Molding design information.
Volume 1
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Expect to eject
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8
Design Tips for Rapid Injection Molding
Be open to shut-offs
This month’s Design Tip combines elements of
the prior July and November tips to address
the important relationship between the draft
designed into the part, the resulting parting line
of the mold, and the final quality of the parts.
As we reviewed in prior Design Tips, draft is
a critical feature of almost all part designs
to be injection molded because it helps the
part eject from the mold as easily as possible.
But when you are deciding exactly how to
design the draft, it is helpful to understand
how your decisions will effect the mold’s
parting line geometry and shut-off surfaces.
Figure 1 illustrates how the decision to draft
the walls of a simple part can have a major
effect on how the mold will be designed. In
“Design #1,” the walls of the part are drafted so
that they can be ejected from within deep, thin
mold cavities. The issue with this approach is
that deep, narrow slots are a challenge to mill
and polish, so you may not be able to achieve
the desired geometry or final surface quality.
On the other hand, if the walls of the part are
drafted as shown in part “Design #2,” the mold
©2007 Protomold. All rights reserved.
becomes a core/cavity design that is much easier
to mill and polish. And the end result is a part
that can have significantly improved surface
finishes as shown in the photographs in figure 2.
Visit the Protomold Design Guide
for other helpful Rapid Injection
Molding design information.
Part from mold with deep, thin ribs.
Part from core/cavity mold.
Initial
Undrafted
Design
Draft
Design #1
Mold Design Approach #1:
“Deep Ribs”
Design
Decision
Figure 2 : Mold design affects part quality.
Draft
Design #2
Mold Design Approach #2:
“Core/Cavity”
Figure 1 : Draft design affects mold design.
Volume 1
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be open to shut-offs
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9
Design Tips for Rapid Injection Molding
Get a feel for texture
As illustrated in the September 2003
Design Tip, Protomold offers the
following two texture options:
PM-T1: SPI-C1 (600 grit stone, 10-12
Ra) followed by light bead blast
PM-T2: SPI-C1 followed by medium bead blast
If you plan to specify either of these textures via
the drop-down menus of your ProtoQuote®, you
need to be aware of the fact that Rapid Injection
Molding requires a different draft angle on
vertical faces for each of these texture options:
3 degrees for PM-T1 and 5 degrees for PM-T2.
Rib features
poorly suited
to texturing
And also keep in mind that due to the line of sight
nature of the mold texturing process, it may not
be possible to texture rib-shaped areas of the
part design such as those illustrated in figure 1.
Another thing to remember is the effect part
geometry may have on the quality of the desired
texture, even if the mold itself is textured
perfectly. For example, a wall with greater than
nominal thickness will pull away from the textured
mold surface during solidification, resulting in an
untextured area on the surface of the part. And a
wall with less than nominal thickness will tend to
adhere more intimately to the textured surface,
which can often result in a flat, chalky appearance
on the part. Figure 2 illustrates examples.
Sinkage
Thickness
Back side
geometry
issues
Front side
texture
variations
Figure 2 : Examples of texture problems caused
by variations in part wall thicknesses.
So there are even more reasons to pay attention
to the guidance in the October 2003 Design
Tip about using consistent wall thicknesses.
Visit the Protomold Design Guide
for other helpful Rapid Injection
Molding design information.
Figure 1 : Example “rib” geometries
that cannot be textured.
©2007 Protomold. All rights reserved.
Volume 1
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get a feel for texture
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Design Tips for Rapid Injection Molding
Living with hinges
Figure 1A, 1B. Without a living hinge,
this box would require two molds, two
molding operations, and assembly.
©2007 Protomold. All rights reserved.
See figure 3 for a recent example designed
by our customers and manufactured
via Rapid Injection Molding:
Visit the Protomold Design Guide
for other helpful Rapid Injection
Molding design information.
At Protomold we don’t design parts - that’s
your job. But if you happen to be unfamiliar
with the technique called “living hinges”, this
Design Tip may come in handy someday.
As described in detail by Dr. Glenn Beall in
his August 2002 Injection Molding Magazine
article, “By Design: Polypropylene part
design, Part 2 — Living hinges”, in the late
1950s it was discovered that below a certain
thickness, polypropylene molecules oriented
in the direction of flow. And repeated bending
perpendicular to that orientation was possible
without breakage due to the increased strength
Check out both of these excellent
articles for additional technical
design details and illustrations.
Figure 2 : Box with living hinge.
that resulted. The name “living hinge” was given
to this technique and has been used ever since.
Figure 3 : Example designed by our customers
and manufactured via Rapid Injection Molding.
Living hinges are very useful in certain designs for
injection molded parts because you can combine
two or three parts into one. And as noted on
efunda.com (an excellent online engineering
fundamentals resource) in a page dedicated
specifically to living hinges, a well designed
hinge in these materials can last for millions
of cycles. Additional materials with somewhat
less of a life expectancy are Nylon and Acetal.
Volume 1
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Living with hinges
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Design Tips for Rapid Injection Molding
The hole picture
You may have heard the grade school riddle,
“If you dig a four foot hole and fill in two feet
of it, how much of a hole do you have left?
Answer: All of it, because you can’t have half
a hole.” Holes are just one of those things that
are defined by the absence of something else.
When we want to incorporate holes into injection
molded part designs, it can sometimes be
complicated just getting them to happen in the
right place with the right shape. One approach
often used in moldmaking is to use “core pins”
which essentially refers to the use of a cylindrical
piece of metal separately inserted into one
side of the mold to provide the inner surface
of the desired hole, especially if the hole is
deep. In conventional injection molding these
pins are typically made from hardened steel.
However, in rapid injection molding these features
are milled directly into the mold geometry from
a block of aluminum. And because aluminum
has less mechanical strength than steel, it
is desirable to take this into account when
designing your part. Some general guidelines
are provided in the following table and figure.
Draft
Fillet at entrance
Diameter
Through-hole Guidelines for
Rapid Injection Molding:
• T he diameter of the hole should be
no smaller than .060” (1.5mm).
• T he length of the hole should be less
than eight times its diameter.
• T he more draft on the hole - the better
(minimum ½ degree in most cases).
• A
fillet at the entrance to the hole greatly
increases the strength of the core.
• A
deep hole can sometimes be split and
drafted to each side.
Figure 2 illustrates an instance of a throughhole design implemented using Protomold’s
rapid injection molding process.
Visit the Protomold Design Guide
for other helpful Rapid Injection
Molding design information.
Part Design
Mold Core Design
Mold with Core Pin
Mold with Part
Figure 2: Example of a part with through-hole features created with the standard rapid injection molding process.
©2007 Protomold. All rights reserved.
Volume 1
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The hole picture
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Design Tips for Rapid Injection Molding
Consider warp factors
We have another riddle for you to
introduce this Design Tip:
“When is circumference less than 2πr
and when is it greater than 2πr?”
Figure 1
Cone Shape
Potato Chip
Shape
The answer is when you’re talking about a
cone or a potato chip shape, respectively (see
illustrations). What’s the point? Well, if your
injection molded part is filled from the center
and the resin shrinks less in the flow direction
than in the transverse (as with any glass or
carbon filled resin), the part will want to warp
into the shape of a cone. But if the material
has the opposite shrink characteristic, as with
unfilled nylon, then it will tend to warp into
a potato chip shape. Either way, you may not
get the final geometry you’re expecting.
Which brings us to this month’s discussion about
warp - something hard to predict with precision
but usually manageable with the knowledge
of a few fundamentals. We’ve covered part
geometry-based techniques for managing warp in
our October 2003 Design Tip, but two additional
considerations are the characteristics of the
resin and the nature of the gate(s) to be used.
As illustrated in the Protomold Design Guide
resin properties table, the tendency for resins
to warp varies significantly. Good or excellent
dimensional behavior can be expected from
a polycarbonate/ABS alloy (e.g. Cycoloy), but
only fair or poor results should be expected
from materials like thermoplastic elastomer
(e.g. Isoplast) or glass filled nylon (e.g. Zytel). Of
course, there are other considerations to take
into account when selecting a resin, such as
additional mechanical properties and cost.
In addition to looking at alternative materials,
sometimes warp can be reduced by changing
the nature and/or location of the gate(s). For
example, in the case of the disk-shaped
part illustrated above, rather than locating a
single gate in the center of the part, it may be
advantageous to have several equally spaced
gates around the circumference of the part as
illustrated in the figure to the right. Although
this may result in knit lines where the resin
flows meet, the multi-gate approach may
cause the overall stresses to balance and
help to avoid the cone or potato chip effect.
Figure 2
Gate
Gate
Gate
Very seldom can you get everything you
want in one part, which is what engineering
trade-offs are all about. But as a well-informed
designer, you can make a big difference in
our ability to make your parts, and more
importantly, the success of your project.
Visit the Protomold Design Guide
for other helpful Rapid Injection
Molding design information.
Volume 1
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Consider warp factors
n
13
Design Tips for Rapid Injection Molding
Know more knit lines
Know about knit lines? If knot — we hope
this Design Tip will be useful for you.
much as possible about why they happen and
how to reduce their impact on your design.
A “knit line” in a plastic injection molded part
(see figure) is created when two separate plastic
flows meet within the mold and resolidify along
their interface. Depending on the resin, resin
temperature, mold temperature and filling speed,
knit lines can vary from virtually invisible to
something that looks like cracks in the plastic.
And in some cases (e.g. long thin features with
resins like LCP) the knit lines can have reduced
mechanical properties and be a cause of part
breakage. So for reasons ranging from cosmetics
to functionality, it is important to know as
As noted above, the size and shape of the knit
line is affected by the molding parameters, but
its location will be primarily governed by the
geometry of the part. The primary cause of knit
lines is the way the plastic flow rejoins after
it goes around a metal core in the mold. So for
this reason there is a knit line (visible or not)
downstream from every hole that goes through
your part. And for similar reasons there is a knit
line between every two gates on the part.
Figure 1: Example of a knit line occurring
on the backside of a hole.
Protomold mold technicians try to minimize
the appearance of any knit lines, but they
must balance this with other challenges
like avoiding sink or blush, achieving the
desired surface texture, etc. So anything
you can do to help avoid knit lines when
designing the part would be a benefit.
unfilled materials will tend to have stronger
knit lines than filled materials. In fact, knit line
strength will decrease with higher filler content as
well as with longer fibers. In addition, materials
that that tend to outgas a lot (e.g. Santoprene) or
contain additives like flame retardants, lubricants,
and mold releases can further exacerbate the
problem. Third, it may be possible to improve
the situation by working with Protomold to
optimally place the gate(s) so that the knit lines
are minimized or moved to a less critical area.
Visit the Protomold Design Guide
for other helpful Rapid Injection
Molding design information.
Here are a few things to consider. First, thicker
walls will slow down the cooling rate of the resin
and thereby help to improve the appearance
and strength of any knit lines. Second, the resin
you select may make a difference. For example,
Volume 1
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Know more knit lines
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