Design Tips
for Rapid Injection Molding
Volume 7
Real Parts. Really Fast.
WWW.PROTOMOLD.COM
Proto Labs, Inc. 5540 Pioneer Creek Drive, Maple Plain, MN 55359 877.479.3680
Design Tips for Rapid Injection Molding
Design Tips categorized by topic
Page
TABLE OF CONTENTS
3
Keeping files in line
4
When not to draft
5
The #1 rule
7
Woe with the flow and how to avoid it
9
Do the bump
11
Making choices at the resin buffet
13
Texture: When things get rough
15
Ejector pins: Pushing your parts around
17
Matched mating parts
19
Pickouts for interior undercuts
21
Keeping your parts in line
23
Living in the material world
©2011 Proto Labs, Inc. All rights reserved.
Material
selection
Design
guidelines
Quality
assurance
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Understand
the process
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Volume 7
DESIGN MATRIX
2
Design Tips for Rapid Injection Molding
Keeping files in line
It wasn’t long ago that most injection molds
were cut manually by machinists working
directly from drawings. Then CAD and
CNC (computer numerical control) came
along and enabled milling work from digital
input. Today, Proto Labs has added another
layer of automation to mold design and
CNC toolpath generation. However you get
from model to mold, the accuracy of the
original design determines the quality of
the finished product.
One common file format that we cannot use
is STL. This format is designed for stereolithography and, though many CAD packages
offer it as an output option, it does not
contain data that is precise enough for rapid
injection molding. So if you want to submit a
design for rapid injection molding, don’t save
it as STL. Similarly, 2D files, wireframe models
or .dxf (Drawing Interchange Files) do not
contain all the information needed for the
rapid injection molding process.
At Proto Labs, all parts begin as 3D CAD
files, but we’ve learned from experience that
some files work better than others (and some
don’t work at all). Of course, we’re always
willing to work with customers to adapt files
for production, but following a few simple
guidelines will speed up the process and
help you get exactly what you want without
unnecessary delays.
We realize that a part design often requires
rework, but if you must edit a design, it is
better to undo whatever needs changing than
to patch it. For example, if you create a hole
that you later decide you don’t need, plugging the hole is not the same as deleting the
feature and recreating it. Patching can create
internal surfaces, which can be confusing as
there is no way to be certain if the internal
surfaces are errors, parts of an assembly, or a
garbled model.
The file formats we can use include:
■
■
■
■
■
■
■
SolidWorks Native (.sldprt)
Pro/ENGINEER Native (.prt)
IGES (.igs): Initial Graphic Exchange
Standard
STEP (.stp): Standard for the Exchange
of Product model data
ACIS (.sat): Andy, Charles, Ian’s System
(no kidding!)
Parasolid (.x_b or .x_t)
AutoDesk (.ipt and .dwg, 3D only)
©2011 Proto Labs, Inc. All rights reserved.
You can, however, “join” separate parts to
create a single part. If you design a single
part by assembling separate pieces, you
must join them within the software; otherwise, your design will include internal faces
and, when you submit it, you will receive a
message reading:
Assembly Needs To Be Joined
It appears that your part is modeled as several
components. In order to quote your part we
will need the components combined into a
single model.
Conversely, if you are designing two or more
parts and submit an “assembly file,” that is,
a file showing parts that are intended to be
separate, you will receive a message reading:
Assembly Needs To Be Separated
It appears that your part is modeled as an
assembly. In order to quote, we will need a file
for each individual part output in one or more
of the following file formats: SolidWorks native
(.sldprt), Parasolid (.x_t or .x_b), STEP (.stp),
IGES Surface (.igs), or ACIS (.sat).
When you export your design, set export
tolerances as high as possible. 1/1,000th
is good; 1/10,000th is even better. This
ensures maximum accuracy in your final
part. In closing, keep in mind that modeling
software is very complex. For a variety of
reasons, saved files may occasionally appear
incomplete. This can often be resolved by
resaving the file in a different format. If this
appears to be a problem with a file you
submit, you will receive a message reading:
There were some missing and/or incomplete
faces in the model that we received. Often,
saving the model in a different file format will
eliminate these issues.
Some of our customers send their designs
in two different formats just to be safe. But
whatever system you use and whatever you
send, we will do everything we can to get you
the parts you need in a timely manner.
Volume 7
KEEPING FILES IN LINE
3
Design Tips for Rapid Injection Molding
When NOT to draft
We’ve talked so often about the need for
draft in injection molded parts that you may
be shocked to hear that there are features
that not only don’t need to be drafted, but
that work better if they aren’t drafted. It’s
true, and the reason we haven’t mentioned
it before is this is a new capability added to
our injection molding process.
The features in question are typically screw
holes used to connect plastic parts—front
and back halves of a plastic shell, for
example—with thread-forming screws.
The holes are formed by posts in the mold
called “cores” (see Figure 1).
Previously, we created cores in the mold
by directly milling them from the aluminum
mold body. Tall thin cores could “stick” to
the plastic part and break off when the parts
were ejected. To strengthen these cores
and reduce ejection stresses we required
them to be drafted as tall narrow cones.
The resulting tapered screw holes could
be problematic. Unless the screw was also
tapered (like a wood screw), it would become
tighter as it was screwed into the tapered
hole. If it got too tight, it would crack the
part. If it was too loose it could “strip” and
fail to hold.
Long, straight screws, tapered pilot holes,
and knit lines were a bad combination. If the
hole and corresponding screw length were
short, the part could be safely produced, but
designs with deeper holes unfortunately went
back to designers as no-quoted parts.
Figure 1: Example of a core used to form a hole
in a molded part.
©2011 Proto Labs, Inc. All rights reserved.
That’s all in
the past. Now,
Protomold can
produce highaspect-ratio small
diameter holes
using steel core
pins in the mold.
Say, for example,
Figure 2: Sample part with
you are designing
3/4” deep holes.
a part with a 3/4”
deep, 1/8” diameter hole (see Figure 2). You
simply include that feature in your 3D CAD
model; our proprietary software will go to
work and design
the mold with a
cylindrical steel
core pin for
forming the hole
(see Figure 3).
This innovation
changes two
things for you.
Figure 3: Mold with cylindrical
First, we can now steel core pins for part.
mold parts with
deeper, narrower holes. Second—and here’s
the shocker—you don’t have to draft those
features. The reasons are simple. A steel pin is
strong enough to handle the stress of ejection
and its surface is smooth enough to release
cleanly from the part without draft. And, while
there shouldn’t be any cosmetic effect on the
resulting part, if there is, it will be inside the
hole where it won’t be seen.
The size of the hole in your part will be determined by the size of the thread-forming screw
you’ll use for assembly. The hole itself will be
slightly larger than the minor diameter of the
fastener—the diameter of the shaft at the root
of the threads. Typically, the manufacturer will
specify a diameter for the pilot hole in their
screw specs. Finally, note that some screws
will be specified for particular plastic resins,
so if you change your resin during prototyping
you should make sure you’re still looking at
the right type of fastener.
Volume 7
WHEN NOT TO DRAFT
4
Design Tips for Rapid Injection Molding
The #1 rule
The fundamental injection molding design
rules for this decade are the same as those for
the last decade (and several before that). But
we wanted to reiterate them to help keep your
projects on track as we stride confidently into
the two-thousand-tweens.
When Proto Labs Customer Service Engineer
Dave Nyseth proposed the following list of
fundamental injection molding design
rules, we asked him to put them in order of
importance. “They’re all Number One,” he
replied. “Ignore any one of them and it can
stop you in your tracks.” For that reason, we
present Four #1 Rules for Successful Rapid
Injection Molding:
1a) Maintain uniform wall thickness
1b) Maintain appropriate draft
1c) Understand the resins you plan to use
1d) Understand the Protomold
manufacturing process
Uniform wall thickness
Uneven wall thickness is an open invitation
to a multitude of problems. Depending on
gate placement, it can lead to incomplete
mold filling if resin has to pass through a thin
area to reach a thick one. And because resin
shrinks as it cools, thick areas may shrink
more than thin ones, which can lead to warp
in the finished part.
©201 1 Proto Labs, Inc. All rights reserved.
So, if walls are to be identical (or at least
similar) in thickness, what should that
thickness be? If it is too thin, parts won’t be
structurally sound, but if they are too thick
they may shrink enough to cause unsightly,
potentially risky surface sink. Also, because
dissolved gases are released as resin cools,
thick walls can develop bubbles at or below
the surface, weakening the part. The ideal
thickness of a wall will depend on its function
and on the resin used. Information on
acceptable wall thickness for various resins
can be found on the Design Guidelines page
of the Protomold web site. Our ProtoQuote®
design analysis can make some guesses about
wall thickness issues, but since we don’t
require you to select a resin type until you’re
ready to order, you need to factor in resinspecific properties when designing your part.
ensures that the part surface and mold
surface will draw apart instead of being
dragged across one another during ejection.
The required degree of draft needed to
avoid damage depends on a variety of factors
including height, location, and surface texture
of the feature.
Draft is almost always required for surfaces
that are parallel to the direction of mold
opening. In parts with cam-driven side actions,
draft is also required for surfaces parallel to
the direction of cam action. And shutoffs—
surfaces where mold faces meet—that are
parallel to the direction of mold or cam opening require draft as well. Detailed information
on draft can be found at on our website. If a
submitted design needs additional draft, this
will be noted in the ProtoQuote.
If a feature needs to extend above or below
the rest of the part surface, it need not be
thicker than the adjacent areas. Instead, it can
be designed as a cored-out feature rather
than a solid one.
Appropriate draft
For a moment, let’s think baseball. Picture
a base-runner sliding into second base just
under the fielder’s throw. Now imagine the
condition of his uniform (and maybe his hip as
well) as a result of that slide. That’s what can
happen to undrafted surfaces when a mold
opens and the part is ejected. Proper draft
Continued on next page…
Volume 7
THE #1 RULE
5
Design Tips for Rapid Injection Molding
Resin characteristics
Manufacturing process
The characteristics of various resins differ
across too many dimensions to discuss in
detail here, but we want to remind you of the
issues that can affect the molding of your part:
Protomold’s rapid injection molding process
is significantly faster and more affordable than
traditional injection molding while sharing
many of the traditional method’s capabilities.
However, our process does have a few
limitations that users should understand.
These include:
■
■
■
Obviously, mechanical properties such as
strength can be an issue; stronger resins
may require less material to meet your
requirements.
Shrinkage varies among resins and can
definitely affect moldability. This can
be of special concern with filled resins,
which shrink unevenly depending on the
direction of resin flow.
Viscosity, and the ability to fill small
features, also varies among resins.
■
maximum part sizes
■
specific requirements regarding
side actions
■
limits on the use of fine detail adjacent
to steep walls
■
the sharpness of the outside corners
of parts
■
the need to accommodate ejector pins
Details regarding these requirements can be
found under the heading “Protomold Specific
Design Guidelines” in the middle of the page
at Design Guidelines page.
Of course, if you have questions regarding
these or other issues, Proto Labs service
representatives are available to help at
877.479.3680.
Basic information on these and other
characteristics can be found on our website at
www.protomold.com/MaterialSelection.aspx
©201 1 Proto Labs, Inc. All rights reserved.
Volume 7
THE #1 RULE
6
Design Tips for Rapid Injection Molding
Woe with the flow (and how to avoid it)
We’ve written often about issues—sink or
gas bubbles—that can be caused by overly
thick walls in plastic parts, but thin walls can
be problematic too. Obviously a feature must
be thick enough to serve its purpose and
handle expected stresses, but it must also be
thick enough to allow proper resin flow
during molding.
Identifying features that are too thick is
relatively easy; too thin is another matter.
Resin flow is affected by many factors
including resin characteristics, gate location,
wall thickness, and other aspects of
part geometry. Any of these can slow resin
flow, resulting in incomplete mold filling or
structurally weak knit lines. For this reason it
is important to anticipate the flow of resin
into and through a mold. This can be difficult
in complex designs, which is why Protomold
developed ProtoFlow®, a sophisticated
program that models resin flow needed to
produce a customer’s 3D CAD design. Its
analysis is offered free with Protomold’s
ProtoQuotes® online quotes. Developed
using cutting-edge algorithms and run on
Proto Labs’ massive compute cluster,
ProtoFlow considers geometry, wall thickness,
and resin characteristics in simulating resin
flow through a gate and into a mold.
have several factors working in your favor.
One is the ability, within limits, to choose
the location of injection gates. Another is
the ability to apply pressure of up to 15,000
psi to move resin through the mold. At the
same time there are several factors working
against you. First is the fact that resin cools,
thickens, and eventually hardens as it travels
through the mold. Also, whenever resin flows
meet—either coming from multiple gates or
after flowing around an obstacle—they can
form structurally weak knit lines if the flows
have cooled too much to fully meld where
they meet.
To help anticipate and head off problems,
Protomold offers free, computerized flow
analysis. A ProtoFlow analysis is added to
any ProtoQuote in which there appear to be
potential flow issues. The ProtoFlow analysis
is a 3D animated simulation that shows the
temperature and fill pressure of your chosen
resin as it flows through the mold (see Figure
1). If you do not specify a resin, the system
assumes Lustran® 433 as the default.
Figure 1:
The ProtoFlow analysis sample
Continued on next page…
We can view mold filling as a race in which the
goal is to get resin to every part of the mold
before it hardens. In running that race you
©201 1 Proto Labs, Inc. All rights reserved.
Volume 7
WOE WITH THE FLOW
7
Design Tips for Rapid Injection Molding
The ProtoFlow animation includes two
simulations. The temperature analysis shows
cooling of resin as the mold is filled which
can help predict potential warp in the finished
part. The pressure analysis uses both color
and isobars to show pressure at various
points in the resin flow during mold filling.
Greens and blues indicate low pressures,
which are not likely to cause problems; golds
and yellows, along with “crowded” isobars,
indicate areas of high pressure, which can
cause weak knit lines, increased flash, “short
shots,” and gas trapping.
Where problems are indicated, the first step
a designer can take to reduce pressure is to
increase wall thickness in the problem areas.
In some cases, increasing thickness by as
little as 15 percent can resolve the problem.
In other instances, thickness may have to be
increased by 100 percent or more, but this can
be done incrementally allowing you to address
potential fill issues without increasing wall
thickness more than is necessary. In addition
to thickening walls, you can also consider
specifying a resin with a higher flow rate.
Protomold customer service engineers can
help you determine how best to address any
of these issues.
©201 1 Proto Labs, Inc. All rights reserved.
A “successful” ProtoFlow analysis can
significantly reduce the likelihood of flow
problems, but it does not guarantee that
there will be no issues in producing parts
from your model. Also, while a ProtoFlow
analysis is not automatically generated with
every ProtoQuote, you can request a free
ProtoFlow analysis with any ProtoQuote by
contacting customer service at 877.479.3680.
For basic information on minimum wall
thickness for various resins, go to our
recommended wall thickness information
(www.protomold.com/DesignGuidelines_
RecommendedWallThickness.aspx). While
ProtoFlow helps ensure that wall thickness
is appropriate from a moldability standpoint,
only you can ensure that your design is
suitable for its intended purpose.
Volume 7
WOE WITH THE FLOW
8
Design Tips for Rapid Injection Molding
Do the bump!
A bump-off is a small undercut in a part
design that can be safely removed from a
straight-pull mold without the use of side
actions. It works like the snaps on the wind
flap of a parka or the snaps on a western style
shirt—push to close, pull to open. If you look
very closely at the two parts of a snap, you
will see that there is slight deformation of the
material when the snap is opened or closed.
The choice of material and design of the
snap’s components allows that deformation
to take place without damage or significant
wear to the mating parts. This is exactly what
happens during the ejection of a part with a
bump-off feature.
Figures 1 and 2 address the shape of the
undercut feature. In order to be successfully
bumped-off, the leading edge of an undercut
must provide a “ramp” or radius like that in
Figure 1 rather than a hook or sharp edge as
shown in Figure 2. If other factors are correct,
during ejection the ramp-shaped lip inside the
cylinder in Figure 1 can ride up over the edge
of the groove in the mold that formed it, much
as a car rides over a speed bump. Conversely,
the hook in Figure 2 will remain lodged in the
groove that formed it and either prevent
ejection entirely or be torn off when the part
is ejected.
Figure 2
Resin choice is critical to the success of a
bump-off. Depending on the shape of the
part, the resin will need to stretch and/or bend
during ejection and then return to its original
shape and size. A resin like TPE or unfilled
polyethylene is flexible enough to bump off.
Glass-filled nylon, on the other hand, is very
rigid and is not likely to work very well.
In the case of a plastic part in an aluminum
mold, any deformation that takes place at
ejection will be entirely in the plastic part,
not in the mold. A number of factors determine the ability of a part to be “bumped-off”
without damage. These include the shape of
the undercut itself, the resin used to form the
part, the geometry of the area surrounding
the undercut, and the design of the mold.
Figure 1
Continued on next page…
©201 1 Proto Labs, Inc. All rights reserved.
Volume 7
DO THE BUMP
9
Design Tips for Rapid Injection Molding
in Figure 5. Here the raised ring is replaced
by an inward bend in the vertical wall of
the part. Instead of being compressed, the
wall of the part can be “snaked out” during
ejection, requiring only that it bend and
stretch. This is easier for most resins to do
than to compress. You can see an example
of this bump-off in our new Protomold Torus
complex feature sample.
Figure 4: Undercut will have to squeeze out
through rib (harder approach).
Figure 3: Represents 3 different
ways in which a part with an
undercut feature could be made.
Finally, we consider the shape of the part
and the molding technique. If Figure 1 is, for
example, a snap-on cap for a bottle, Figure 3
shows cross sections representing different
ways in which that cap could be made. Figure
3.1 and 3.2 show ways in which the part could
be molded as a rib. In this approach, both the
outside and inside of the cap are formed by
the B-side mold half; the A-side is simply a flat
surface that forms the bottom of the upturned
cap. Figure 3.3 shows the part as it would be
©2011 Proto Labs, Inc. All rights reserved.
A third approach is shown in Figure 6. Here
the part is formed between a cavity in the
A-side mold half and a core in the B-side.
After cooling, the mold opens, leaving the part
on the core. With nothing pressing against
the outside of the part, the part wall is free
to stretch as it is ejected off the B-side core,
releasing the part with the inside ring intact.
Figure 5: Undercut can snake out during part
ejection (easier approach).
produced by core-cavity molding, the outside
formed by the A-side and the inside by the
B-side mold halves.
The approach shown in Figure 4 is problematic because during ejection the bump will
have to squeeze out through the thinner rib
section below it. This requires that the resin
be compressed during ejection, and few resins
other than low durometer TPEs or TPUs are
that compressible. A better approach is shown
Figure 6: Mold can be made core-cavity,
allowing room for the part to “bump-off”
after the mold opens.
If you have questions about the feasibility of
bump-offs for small undercuts in your part
design, Proto Labs customer service engineers
are here to help and can be reached at
877.479.3680.
Volume 7
DO THE BUMP 10
Design Tips for Rapid Injection Molding
Making choices at the resin buffet
Choosing a resin for a plastic part can be as
important (and challenging) as designing
the part itself. You can spend all the time in
the world poring over information in books
or on the web, but you may not know for
certain that the resin you’ve chosen is the
right one for the job until you make and test
some prototypes.
Of course there are times when the choice of
resins is obvious, when there is some special
requirement that overshadows all others and
only one product will do. On the other hand,
there are applications in which the requirements aren’t very challenging and any of a
number of resins would meet the need. More
often, however, the choice of resins is a matter
of balancing competing demands and finding
just the right combination of cost, cosmetics,
moldability, and performance. In such cases
there may be several serious contenders when
you’re ready to start making prototypes.
The challenge is choosing the best all-around
material for the application before you commit
to full-on production.
Statistics on a resin data sheet can help
narrow the choices, but in a competitive
market there’s really no such thing as “good
enough.” And while the difference between
just the right resin and the second-best
choice may be small, when multiplied by a
production run of 100,000 pieces, time and
energy invested in finding the right product
can bring substantial returns. The challenge
is to find a way of effectively comparing the
“finalists” to find the one best resin for your
particular part.
The traditional way to make this decision is
to choose a resin, make some prototypes,
evaluate the resulting parts, and decide
whether to stay with that choice or try
another. The problem is that your first choice
may seem perfectly adequate until you
compare it with something better. And with
deadlines looming, as they usually do, “perfectly adequate” can be tempting, especially
when further exploration costs time and
money. There is, however, an alternative
that won’t squeeze your budget or put you
off schedule.
Using the same 3D CAD model, multiple material prototype part
samples were injection molded for product testing.
Continued on next page…
©2011 Proto Labs, Inc. All rights reserved.
Volume 7
MAKING CHOICES AT THE RESIN BUFFET
11
Design Tips for Rapid Injection Molding
Instead of trying resins sequentially, have
several versions of your prototype made at
the same time and test them side-by-side.
This can be done either by machining or
molding prototypes.
1. To begin, first make sure that your
design is injection moldable before
you move forward with prototyping.
You can do this by uploading a 3D
CAD model to receive a ProtoQuote,
which includes a design analysis that
can help identify potential moldability
problems before you commit to the
cost of prototyping.
this reason, if you need to use a different
resin and its shrink coefficient is different,
a new mold may have to be cut (hence the
above advice to consider Firstcut CNC
machining first).
2. If you want to compare resins
with significantly different shrink
coefficients, you can have prototypes
machined from solid resin by Firstcut.
These will all have the required
dimensions so they can be easily
compared, fitted into assemblies,
etc. This should allow you to at least
reduce the resin choices to a set with
similar shrink coefficients.
The sequential use of Firstcut CNC machining
followed by Protomold injection molding
will help you fully explore your resin options
while avoiding excessive tooling costs, and
the additional production time for molding
parts in more than one resin will be
negligible—probably just a matter of hours.
The information you’ll get can be invaluable.
Specifications cannot tell you precisely how
a resin will function in your particular design,
which is why we make prototypes. For a small
incremental cost you can answer questions
such as: how does your part look in apple red
versus lemon yellow? How opaque is each
resin at a particular point in the design? How
does it stand up to a three-foot fall, a sharp
rap with a ball-peen hammer, or an hour in a
closed car on a sunny afternoon? How much
glass fill does it really need to meet your
strength goals? Will it warp or sink?
If you’ve narrowed down your resins to a set
with similar shrink coefficients or if your
tolerances are not particularly tight, you can
then use Protomold to injection mold your
prototypes. Keep in mind that each mold is
manufactured with a shrink coefficient. For
With parts in several resins you can get right
down to serious comparative testing and stay
on schedule while you let your customers take
a look at them, check their mechanical properties, evaluate their fit with other parts, test
them in different environments, etc.
©201 1 Proto Labs, Inc. All rights reserved.
This approach lets you test resins against
one another, instead of simply against
expectations, and see which one really
delivers the best combination of characteristics. You may find that you can actually
exceed expectations, and that’s always good.
In short, multi-resin prototyping has a lot of
advantages and very small incremental costs.
The only stipulation, as mentioned earlier, is
that truly equivalent parts from a single mold
require the use of resins with similar shrink
coefficients. If, on the other hand, you want to
compare parts made of resins with different
shrink coefficients you can easily do so using
Firstcut’s automated machining.
If you have questions about how to effectively
explore multi-resin molding, feel free to speak
with Proto Labs Customer Service Engineers
at 877.479.3680.
Volume 7
MAKING CHOICES AT THE RESIN BUFFET 12
Design Tips for Rapid Injection Molding
Texture: When things get rough
Surface finish on a plastic part can serve
many functions, from improving grip to hiding
fingerprints to facilitating paint adhesion.
Protomold offers seven finishes on molded
parts: five polishes and two textures. The five
levels of polish are created using manual mold
polishing techniques; the two levels of texture
are achieved by bead blasting the mold
surface after applying a manual base polish.
Protomold’s available textures are light bead
blast (T1) and medium bead blast (T2). Many
factors can influence your choice of texture.
The most obvious is the intended function of
the finished part. Either texture can provide an
attractive, non-reflective surface. If grip is an
issue, that may influence your choice between
light and medium texture. Other factors may
include secondary processes to be applied to
the part and the part’s esthetic fit with other
components of your finished product.
The resin being used can also affect texture
choice. For example, olefin resins such as
polypropylene can have a waxy feel and, in
conditions of high humidity, can become
slippery. An appropriate texture can reduce
these problems. In such cases, the ability to
texture the part’s surface can expand your
range of usable resins.
Another factor linking resin choice and surface
texture is “sink.” In many cases, a slight sinking
of the part surface as the part cools may be
no more than a minor cosmetic issue. A textured surface, however, will create shadowing,
a visible darkening of areas with sink across
a surface. This makes an even slightly sunken
area far more apparent, creating a significant
cosmetic problem. In such cases there are
three possible solutions:
1. Keep the texture but reduce the
thickness of the feature to reduce the
likelihood of sink.
The last and potentially most serious issue
with a textured surface is the possibility of
the part sticking in the mold. A textured
surface that is perpendicular to or significantly
angled away from the direction of mold
opening will not present a problem. On a
surface that is parallel to the direction of mold
opening, however, each little valley in the
textured surface acts as a tiny undercut.
Forcibly ejecting such a part from the mold
can create drag marks on the part surface as
the textured mold face scrapes off the high
areas of the textured surface. To eliminate
drag marks, Protomold requires that surfaces
with T1 texture be drafted by at least three
degrees and surfaces with T2 texture be
drafted at least five degrees. (Of course,
surfaces that are created using side actions
need not be drafted away from the direction
of mold opening; rather, they must be drafted
away from the direction of side action cam
withdrawal.)
2. Keep the texture but choose a resin
that shrinks less to reduce sink.
3. Reduce or eliminate the surface
texture to reduce shadowing.
Continued on next page…
©201 1 Proto Labs, Inc. All rights reserved.
Volume 7
TEXTURE: WHEN THINGS GET ROUGH 13
Design Tips for Rapid Injection Molding
For a variety of reasons, Protomold’s
ProtoQuote® design analysis does not look
for or identify surface texture. Instead, you
will select surface finishes—polish or texture—
after you have received your ProtoQuote.
Once a ProtoQuote interactive quote is
presented (along with moldability analysis),
you can select the surface finish right in the
quote and immediately see the resulting effect
on cost (see Figure 1).
While ProtoQuote does not recognize texture,
it does identify surfaces that fall short of the
draft required for light or medium texture.
Surfaces highlighted in red have less than
three degrees of draft. These cannot support
light or medium texture (see Figure 2). Any
surfaces highlighted in yellow, in a quote,
will have at least three but less than five
degrees of draft and could support light
but not medium texture. In other words, if
your ProtoQuote design analysis has areas
highlighted in yellow, you can add T1 texture
without problems but not T2. If it includes
area highlighted in red, you should either
revise your design to increase draft or skip
texturing this area entirely.
Figure 2: Areas highlighted in red have less than
three degrees of draft and cannot support light or
medium texture.
There are two additional points to keep in
mind regarding texture. First, the ProtoQuote
has provisions for texturing either the entire
A-side, entire B-side, or both sides. Partial
texturing a side can be done in some cases,
but it requires a conversation with a Customer
Service Engineer. Second, areas that cannot
be accessed by the bead-blasting stream
cannot be textured. Proto Labs customer
service engineers, who can be reached at
877.479.3680, can help address either of
these issues as well as any other questions
you may have regarding surface finish.
Figure 1: Pricing is immediately updated as you make selections on your quote.
©2011 Proto Labs, Inc. All rights reserved.
Volume 7
TEXTURE: WHEN THINGS GET ROUGH 14
Design Tips for Rapid Injection Molding
Ejector pins: Pushing your parts around
Ejector pins are the ‘bouncers’ of the injection
molding world. They apply a force to eject a
part from the mold, and in some cases can
leave marks. At Protomold, our goal is to
design and position pins to minimize their
effect on your parts, and while Protomold
typically determines pin placement, customers
get to sign off on pin locations before an order
is finalized (see Figure 1).
Pins are located in the B-side mold half, the
side in which the part will stay when the mold
opens. Once the mold is opened, the pins
extend into the mold cavity, push the part
out, and then retract, allowing the mold to
close and be refilled.
Protomold uses round ejector pins, and their
placement depends on a number of factors.
Obviously the shape of the part is one. Factors
like draft and texture of sidewalls and depth
of walls and ribs can increase the likelihood
that areas of the part will cling to the mold.
Resin choice can also affect pin placement or
size. Some resins are ‘stickier’, requiring more
force for release from the mold. Softer resins
may also require the use of more or wider pins
to spread force and prevent puncturing or
marring of the cooled plastic.
In Protomold’s process, the ends of ejector
pins are flat and perpendicular to the direction
in which the pin moves. To be effective, the
pins need a flat ‘pad’ to push against, and
©201 1 Proto Labs, Inc. All rights reserved.
Because it is in a different plane than the part
surface, the pad may be raised slightly above
the part surface at one edge or recessed
slightly below the part surface at one edge.
Configuring a pad that is slightly recessed into
the part surface is the default configuration
for pins on contoured surfaces.
Figure 1: 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.
the surface of the pad must be perpendicular
to the direction of pin movement. If the part
surface at that location is textured, the smooth
surface of the pad will be apparent. And if the
surface of the part is not parallel to the flat
end of the ejector pin, the cosmetic impact
will be even more obvious.
In a traditional steel production tool it may
be possible to machine the end of the pin to
match the contour of a part surface that is
not perpendicular to the direction in which
the pin moves, producing a contoured pin.
Protomold’s process, however, does not
support the production of contoured pins. If a
pin needs to act on a part surface that is not
parallel to the pin-end, there will have to be a
pad provided that is in the same plane as the
pin-end rather than that of the part surface.
Volume 7
A post gate produces an extreme example of
a raised ejector pad (see Figure 2). In cases in
which an edge gate cannot be used, resin is
injected through an extension of an ejector pin
channel. When the part has cooled, the ejector
pin pushes against the resulting post and, in
the process, clips off the runner. The post is
typically removed from the finished part in a
secondary operation.
Figure 2: A post gate allows resin to be injected
through an ejector-pin hole. When the part is ejected,
a small ‘post’ of plastic is left on the part where the
ejector pin is located.
Continued on next page…
EJECTOR PINS: PUSHING YOUR PARTS AROUND 15
Design Tips for Rapid Injection Molding
In most cases, ejector pads (or the vestiges
left by their removal) are on the non-cosmetic
sides of parts. In some cases, however, this
may not be possible. Take for example the
case of a clip formed using a pass-through
core (see Figure 3). In this case, because the
clip increases the surface area of that side of
the part, the ‘clip-side’ part surface will adhere
more strongly to its mold half. This will make
that mold half the B-side. The clip would
normally be on the cosmetic side of the part,
but its presence requires that ejector pads
also be on that side of the part.
All of the above cases assume that there are
surfaces against which pins can push to eject
parts from the mold. There are, however, some
designs in which there are no such surfaces.
Take, for example, a grate, in which all that
faces into the B-side mold half are the tops of
ribs. If the rib edges do not provide enough
surface area for the pins to push against, the
designer would need to add some bosses to
act as ejector pads.
In most cases, ejector pin placement is a
relatively minor concern in the early phases
of part design. Protomold will propose pin
placement when an order is placed and
present a pin and gate layout for customer
approval. At that time, Protomold will address
any questions or concerns and make changes,
if necessary, to meet customer requirements.
Figure 3: The bottom of the clip’s ‘hook’ and blue face of the clip’s shaft
will be formed by a pass-through core (shown by yellow lines) of the A-side
mold half, which protrudes through a hole in the base of the part. The rest
of the clip is formed by the B-side mold half.
©2011 Proto Labs, Inc. All rights reserved.
Volume 7
EJECTOR PINS: PUSHING YOUR PARTS AROUND 16
Design Tips for Rapid Injection Molding
Matched mated parts
Normally, when we think of a pair of mating
parts we think of left and right, front and back,
or top and bottom, with the two parts being
distinct from one another. There are, however,
situations in which two mating parts may be
exactly the same. In other words, you could
reach into a bin of identical parts, pull out any
two and join them. The advantages of such
self-mating design include lower manufacturing cost—the parts can be made in a single
mold instead of two or more—and lower cost
of maintaining inventory.
Clearly, if mating parts can be made identical,
it makes sense to do so. The problem is that
parts with potential to be self-mating are not
always easy to recognize. Their geometry
can range from very simple to very complex,
but one trait their assembly will always
share is rotational symmetry (as opposed to
bilateral or mirror image symmetry). Wikipedia
describes an object with rotational symmetry
as one “that looks the same after a certain
amount of rotation.”
Take, for example, the assembled box shown
in Figure 1. In its simplest form—a rectangular
box—the part would be both bilaterally and
rotationally symmetrical. With the addition of
hinges at the back and latches at the front,
however, the assembly remains rotationally
symmetrical but is no longer bilaterally
symmetrical. It is the distribution of hinge
parts on the back edge and latch parts on the
front edge that makes this part self-mating.
Figure 1: A single mold can be used to create a box,
using a self-mating design where the assembly is
rotationally symmetrical.
If the designer hadn’t been looking for a way
to make a self-mating part, there could have
been two distinct parts. One would have had
two hinge hooks and two latch tabs, while
the other had two hinge pins and two latch
posts. By designing the part with one of each,
the designer eliminated the need for a second
mold. As designed, when you put two of these
pieces together, the hook component of each
one’s hinge (left rear) engages the pin of the
other half’s hinge (right rear), and the latch
tab on each half (front left) engages the
mating part’s latch post (front right). The
result is a single part that serves as either
top or bottom.
A second example is the elbow shown in
Figure 2. This bent tube is molded in two
halves to allow easy ejection, and the two
parts are then bolted together to form a
finished tube. If the designer wanted the
parts to snap together, this design could have
been modified to have an even number of
connectors with both male and female
connectors on each part. As with the box
in Figure 1, when one part is rotated, its
male connectors would align with female
connectors on the mating part and the two
could be snapped together.
Figure 2: Two parts can be bolted together to form a
finished tube.
Continued on next page…
©201 1 Proto Labs, Inc. All rights reserved.
Volume 7
MATCHED MATED PARTS 17
Design Tips for Rapid Injection Molding
A more complex example is shown in
Figure 3. Posts on the right side of the part
align with holes on the left so that two
identical parts can snap together to form
the completed device.
impacted handling. Designed in two parts, the
walls can be kept thin and easily moldable
while maintaining the rounded shape
for easy handling. And because it is hollow,
the assembled part is light and the cost of
resin is minimized.
Opportunities to design self-mating parts
will be relatively rare. Tops and bottoms and
backs and fronts usually serve different
purposes and are designed accordingly. But
when you have a symmetrical assembly that
needs to be made in two or more parts, often
to allow the inside to be hollow, you may have
an application for self-mating design. The
biggest challenge is usually the mating
surfaces and connectors.
Figure 3: This key-shaped part is half of a medical
device used in a minimally invasive surgical procedure.
It might have been possible to design the
part to be molded in one piece instead of two,
but this would have either left some very
thick areas, violating some basic rules of
design for injection molding. Alternately, the
one-piece part could have been thinned to
prevent those problems, but that might have
©2011 Proto Labs, Inc. All rights reserved.
In all three parts shown above, the mating
edges are in a single plane, but that need
not always be the case. Obviously if there
is a protruding edge at some point on the
part, it will have to meet a recessed edge
on its rotated mate just as male and female
connectors align. Achieving this alignment begins in the drafting phase of development and
ends with testing on real parts.
Volume 7
MATCHED MATED PARTS 18
Design Tips for Rapid Injection Molding
Pickouts for interior undercuts
We’ve written often about the way that
Protomold handles undercuts which interfere
with ejection from a straight pull mold. As
long as undercuts are on the outside surface
of the part, most of them can be formed by
a side action that withdraws during mold
opening to allow the part to be ejected. If,
however, the undercut is on the inside surface
of a core, mechanisms like lifts or collapsible
cores are required but are currently unsupported in Protomold’s process. However, we
are able to use pickouts to form such features.
A pickout is part of the mold during resin
injection and cooling, but it is part of the
molded part during ejection, filling the
undercut feature and providing an
appropriately drafted surface to allow
ejection from the mold. The pickout is then
“picked out” of the part and then reinserted
into the mold to form the next part.
pickouts, the order is reversed; the part is
ejected with the pickout attached to the
plastic part. Second, and perhaps more
significantly, a cam operates automatically,
while pickouts are removed from parts and
reinserted into the mold manually.
For efficiency, a typical pickout operation
will use a single mold but two copies of the
pickout. Immediately after a part and pickout
are ejected, the operator inserts the second
copy of the pickout into the mold. Then,
while that part is being molded, the operator
removes the pickout from the previously
ejected part and is ready to repeat the process
when the next part is ejected. Manual handling
adds to the cost of molding but may be
justifiable if there is no other way to form the
required feature.
There are alternatives to pickouts. For very
shallow undercuts in parts made of flexible
resin, a bump-off may be a better choice.
However, if the undercut is too deep or the
resin is too rigid to use a bump-off, then a
pickout may be a better choice. The
Protomold Torus sample part includes several
features formed using pickouts (see Figure 1).
In the case of the Torus, the direction of
mold opening would be vertical, so the three
undercut features at the top of the part
obviously need some additional treatment.
Under some circumstances these features
could have been formed, at least partially,
using side actions depending on orientation,
proximity to the parting line, and freedom
from interference from other features. In this
case, however, that wasn’t feasible, so we
used pickouts.
A pickout is like a side action in two ways.
First, neither is milled as part of the A- or
B-side mold half. It is its own component
of the mold. Second, like a side action, a
pickout is removed from the finished part in
a direction other than that in which the mold
opens. In other ways, however, pickouts and
side actions are very different.
First, using a cam, a side action that forms
the undercut is withdrawn comlpetely from
the part mechanically as the mold opens, and
the part is then ejected from the mold. With
©201 1 Proto Labs, Inc. All rights reserved.
Continued on next page…
Figure 1: This section of the Protomold Torus shows an o-ring groove
and mushroom stud. They are created using a two-piece pickout.
Volume 7
PICKOUTS FOR INTERIOR UNDERCUTS 19
Design Tips for Rapid Injection Molding
One example might be inward-pointing
clips on a shell to engage mating slots
(see Figure 4).
4. Pickouts must exceed a minimum size,
which is very dependent on geometry.
5. Because plastic shrinks as it cools, a
pickout may not be suitable if the cooled
part will grip the pickout too tightly for
easy removal. Features molded by a
pickout should be drafted.
6. Finally, because the pickout must be
manually removed, there can be no
features in or in front of the pickout
that would interfere with its removal
(see Figure 5).
Figure 2: The two-piece pickout is manually
removed from the part after ejection.
Figure 4: The protrusions at the pink highlighted corners are too large to be formed
using bump-offs, so pickouts could be used
The following should be considered in
determining whether your application is
right for pickouts:
Figure 3: Highlighted areas hold the detachable pickouts, which are manually inserted
into the mold prior to each injection cycle.
Figures 2 and 3 show the actual pickouts and
mold half used to create the features in this
part. In Figure 3, you can see the straight,
drafted surfaces on the A-side mold half
that will allow the pickouts to fall free of the
straight-pull mold when the mold is opened
for the ejection cycle.
Of the thousands of parts submitted to Protomold, relatively few actually require pickouts.
©2011 Proto Labs, Inc. All rights reserved.
1. Undercuts, in general, add to the
complexity of molding and should only
be included in your design if absolutely
necessary.
2. Small undercuts in flexible resin may
be suitable for bump-offs. These involve
some extra work in mold milling but
have no moving parts, so they are
generally less expensive than side
actions or pickouts.
3. Because the molding of pickouts is
labor-intensive, if undercuts are on the
outer surfaces of the part, side actions
may be preferable, especially in longer
production runs.
Figure 5: The circled feature would
interfere with the manual removal of
a pickout.
If you submit a 3D CAD model for a
ProtoQuote and a pickout is required, that will
be indicated in the quote and incorporated
into the pricing. If you have any questions,
as always, feel free to contact Proto Labs
Customer Service Engineers at 877.479.3680.
Volume 7
PICKOUTS FOR INTERIOR UNDERCUTS 20
Design Tips for Rapid Injection Molding
Keeping your parts in line
When you are developing a 3D CAD model to
be injection molded, you may not spend much
time thinking about where its parting line
will be, but it’s worth keeping in mind as the
location can affect your part in several ways.
On some parts the location for the parting line
is obviously right down the middle, while for
more complex parts it may not be so obvious.
Take, for example, a simple cup. The outer face
is formed by one mold half (A-side), while the
cup’s inner surface and brim will be formed by
the other mold half (B-side). The parting line
occurs along the outside edge of the brim of
the cup (see Figure 1).
For other designs, the parting line location is
not so obvious. These tend to be “free-form”
shapes with soft edges. An example would be
the familiar green molded-plastic toy soldiers.
Most are designed to be injection molded in
two-part, straight pull molds, and if you look
carefully, you can see the seam around each
figure where the two halves of the mold meet
(see Figure 2).
Figure 2: The highlighted “seam” is a result of the
parting line (the location where the two halves
of the mold meet). A slight mismatch in the mold
will leave a raised seam.
Figure 1
When a design is a complex shape,
determining where the parting line is can
be a lot more complicated than for the cup
mentioned earlier. You can assume that if
you send an undrafted 3D CAD model of
such a part, Protomold will determine where
the parting line should be. However, in
designing such a part, you might want to
think about the parting line anyway for one
simple reason: designers and molders look
at parts differently. Molders share your interest
in producing the best possible part, but the
focus is on molding it correctly, whereas
designers focus on how it will function after
it comes out of the mold. The location of the
parting line can affect both.
First of all, the location of the parting line
determines the direction of mold opening
and, consequently, the direction in which
features must be drafted for easy ejection.
Second, it affects where any vestiges left by
the mating surfaces of the mold halves will
be, and potentially, how those vestiges will
look. Third, parting line location can also
impact the cost of mold making and the type
and cost of secondary operations needed to
finish the part.
Continued on next page…
©2011 Proto Labs, Inc. All rights reserved.
Volume 7
KEEPING YOUR PARTS IN LINE 21
Design Tips for Rapid Injection Molding
The plastic toy soldier is an obvious example
of a design with a challenging parting line,
but the issue can come up on simpler parts
as well. For example, a straightforward
geometric design with radiused or rounded
edges can also be problematic. The parting
line has to trace the path along which a
tangent to the surface is parallel to the
direction of mold opening. In Figure 3, the
parting line has been placed across an
otherwise smooth surface. Any mismatch in
mold edges will create a fairly obvious seam,
so a parting line in this location would create
a need for tighter tolerances and potentially
increase milling costs. The increased likelihood
of flash could also impact both the cosmetics
and functionality of the part, potentially
making assembly of finished parts more
difficult. If we instead place the parting line
Figure 3: Both mold halves form the parting line, so any
mismatch in the mold will leave a seam, changing the
shape of the part at the parting line. We recommend
placing the parting line on a sharp edge.
©2011 Proto Labs, Inc. All rights reserved.
along a sharp edge, any potential seam
would be camouflaged, and we would avoid
the undesired manufacturing, functionality,
and cosmetics issues outlined above
(see Figure 4).
Whether you get your information from a
CAD package or from Protomold, keep in
mind that the suggested parting line may
not be your only option. Neither your CAD
program nor Protomold’s design analysis
software knows how you intend to use the
part. Look carefully at the suggested
parting line and consider whether its location
will work both cosmetically and functionally.
If not, there may be other options for your
existing design, or you may want to change
the design to allow a more suitable parting
line for your application. If you need
assistance, you’ll find it at 877.479.3680,
where Proto Labs Customer Service Engineers
are standing by to help.
Figure 4: Both mold halves meet to form the sharp
edge of the part.
There are several ways to address parting
line challenges. Simple awareness of the
significance of the parting line is a good start.
As stated earlier, on many parts, that location
is obvious and not a problem. For more
complex parts, you may be able to use tools
within some CAD packages to locate and
evaluate split lines. Or, upload an undrafted
3D CAD model and Protomold will propose
a parting line and suggest appropriate draft
based on that orientation. (Note: our software
may also point out that the part cannot be
made in a straight pull mold and that design
changes, side actions and/or pickouts may
be required.)
Volume 7
KEEPING YOUR PARTS IN LINE 22
Design Tips for Rapid Injection Molding
Living in the material world
Prototypes are for testing whether your
design and the material you’ve chosen for
your plastic part combine to provide the
functionality you need. A prototype lets
you see how the part looks, whether it can
handle an impact, how dimensionally
stable it is, and how it wears. If there are
problems, it gives you the opportunity
to redesign or choose another material
before committing to the cost of fullscale manufacturing or, worse yet, taking
a less-than-optimal product to market.
In some cases, the part’s function is
simple and the choice of material is
not that critical; ordinary, affordable,
easy-to-mold ABS or polypropylene may
be perfectly adequate. But the demands
on many plastic parts today are far
greater, and the thousands of other
engineered resins are out there for a
reason. In some cases, your requirements
are so specific that resins will have to be
custom blended to meet the need, but
even when requirements are less rigid,
material choice can be critical.
You can design your part and then go
shopping in the vast “resin supermarket”
for a suitable material. There are, however,
good reasons to choose your resin, or at
least narrow the choices, as early in the
process as possible, perhaps even before you
begin designing. Designing with resin in mind
can speed the development process, simplify
choices along the way, and reduce or eliminate
the need for rework in the later stages of
the process.
Clearly, functionality will be a major factor
in resin choice, but other factors including
moldability should be considered as well.
Knowing in advance the resin you will use
can make a significant difference in the way
you design a part. If, for example, the resin
you choose is expensive, you can focus on
eliminating unnecessary material in your
design. If you need the strength of a resin
that does not flow well in a mold—glass-filled
nylon, for example—you will have to be
particularly aware of narrow areas that can
lead to voids or knit lines. Or if your part will
serve as a bearing and requires the
lubricity of an acetal resin like Delrin®,
you can begin to design knowing that
this resin is very sensitive to excess
wall thickness.
Of course, design choices are rarely
as simple as a single functional requirement or a single resin characteristic.
Early decisions regarding resin may
be based on a long list of requirements
including strength, durability, hardness,
flexibility, lubricity, electrical resistivity, chemical resistance, UV resistance,
heat tolerance, flammability, color,
transparency, environmental challenges, and cost. Moldability factors
can include ease of flow, tendency to
flash, ease of ejection, and likelihood of
warp or sink. Obviously, not all of these
characteristics will apply to any one part.
But it does make sense to carefully review the
requirements, determine which ones apply,
and then rank them in terms of importance.
Such a checklist can then be used to narrow
the list of usable resins, possibly even
identifying the one best candidate before
even beginning to design the part.
Continued on next page…
©201 1 Proto Labs, Inc. All rights reserved.
Volume 7
LIVING IN THE MATERIAL WORLD 23
Design Tips for Rapid Injection Molding
Once the resin has been identified, the design
and molding process can then be adapted to
that material’s requirements. For example:
■
■
■
■
Polystyrene is hard, clear, and
inexpensive, but brittle, limiting its
application or requiring that steps be
taken to toughen the resin.
ABS is very affordable and impact
resistant but is susceptible to sink,
requiring that thick areas be avoided.
LCP is strong and fills thin features
well but forms weak knit lines, affecting
both part geometry and gate
placement.
Nylon is affordable and strong, but
absorbs water leading to dimensional
and property change. This limits the
applications in which it can be used.
■
Aramid (Kevlar®) fibers add strength,
though not as much as glass, and are
less abrasive than glass.
■
Carbon fiber can strengthen and stiffen
a resin and aid in static dissipation,
but is costly and can lead to warp.
■
Stainless steel fibers are used in electrical housings to reduce electromagnetic
and radio frequency interference.
■
Mineral fillers—talc or clay—can
increase hardness and reduce both
cost and warp.
■
Glass beads and mica flakes add stiffness and reduce warp and shrinkage,
but can be challenging to inject.
Short glass fibers can strengthen a
resin and help prevent high-temperature creep. They can, however, make
a resin more brittle and increase the
tendency to warp as a part cools.
■
Long glass fibers provide greater
strength and creep resistance, but
can impede resin flow, particularly
through thin areas.
©2011 Proto Labs, Inc. All rights reserved.
PTFE (Teflon®) and molybdenum
disulfide, dry lubricants that function
like graphite, can make plastic parts
self-lubricating.
■
UV inhibitors help prevent material
breakdown in outdoor applications.
Keep in mind that, while our Customer
Service Engineers are always willing to
address your questions at 877.479.3680, we
are not resin specialists. We can answer basic
questions, but for detailed information on
standard resins or custom blends, we may
refer you to specialized resources. The final
decision regarding resin choice for a specific
application will be yours.
For information on Protomoldstocked resins and links to supplier
sites, visit our materials page (www.
protomold.com/MaterialSelection.
aspx). For more detailed information
on resins and their characteristics,
see the PolyOne (www.polyone.com)
or RTP Plastics sites (www.rtpcompany.com).
Decisions don’t necessarily end with choice
of resin. For specialized requirements,
various additives can be used to extend
the capabilities of the base material.
■
■
Our online Protomold Resin Guide lists a sampling of resins,
along with their mechanical properties, moldability characteristics, and some brand names.
Volume 7
LIVING IN THE MATERIAL WORLD 24