Design Tips
for Rapid Injection Molding
Volume 5
Real Parts. Really Fast.
Proto Labs, Inc. 5540 Pioneer Creek Drive, Maple Plain, MN 55359 877-479-3680
WWW.PROTOMOLD.COM/PARTS
Design Tips for Rapid Injection Molding
Design Tips categorized by topic
Page
TABLE OF CONTENTS
3
Hot tip solves gating problems
5
Playing it safe
6
The plane truth about rotational draft
7
Eliminating side-actions for fun and profit
8
Protomold: it isn’t just for prototypes anymore
9
Design guidelines for bigger parts
10
Put your parts on a diet
11
Avoid knitting around your boss
13
You’ve got to check this!
16
Don’t be square!
17
Cams: they’re not just for undercuts
19
Look sharp
©2009 Proto Labs, Inc. All rights reserved.
Material
selection
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Design
guidelines
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Quality
assurance
•
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•
Understand
the process
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Volume 5
DESIGN MATRIX
2
Design Tips for Rapid Injection Molding
Hot tip solves gating problems
Consider how much simpler life would be if
we could simply teleport liquid resin into a
mold. Of course, that wouldn’t be injection
molding. And since we presume that the
developers of practical teleportation will
come to us for their plastic prototypes
(and they haven’t), we’re prepared to state
categorically that teleportation molding is
still some time off.
That leaves us with gates as a means of
getting liquid resin from the barrel and
screw of the molding press into the mold.
Unfortunately, gates interrupt the mold’s
surface, and that interruption, unavoidably,
produces a cosmetic defect on the surface
of the part. Depending on the function of the
part and the location of the gate, this may
or may not be a problem. For example, if the
part or surface will be hidden from view, any
vestiges left by the gate probably won’t be a
problem. But if the gate is located on a visible
surface, you’ll want to consider its cosmetic
impact in designing the part.
Tab gates are effective and most common,
but not necessarily pretty. (See Figure 1.)
Fig. 1: Tab Gate
n
n
n
n
n
©2009 Proto Labs, Inc. All rights reserved.
While they are a simple way of getting
resin into the mold cavity, tab gates have
several drawbacks.
First, they carry resin to the mold via a
runner. Because this allows some cooling
and thickening of the resin, tab gates
require a relatively large opening into the
mold cavity. This, in turn, leaves a large
tab to be trimmed, a process that can mar
the finished surface.
Second, the runner leading to a tab gate
takes up real estate within the allowable
mold footprint. This can be a problem if
the part pushes the limits of allowable
mold size.
Third, because resin cools somewhat on
its way to the gate, there are potential
mold fill considerations like uniformity,
concentricity, knit line formation, thin feature challenges, etc.
Finally, tab gates must be located at the
parting line of the mold.
In many cases, the solution may be a “hot tip”
gate. A hot tip gate has a small circular gate
opening in the “A” side of the mold that lets
plastic into the cavity. It’s called a hot tip gate
because there is a thermostatically-controlled
heater bolted to the back of the mold to keep
the resin hot enough (and thus fluid enough)
to pass through the small gate hole.
The hot tip can be thought of as a direct
extension of the molding press’s barrel and
screw. Resin is hotter at the point of injection,
so the opening can be smaller. Because no
runner is required, the part can use virtually
all of the allowable mold X-Y real estate.
Hotter resin also means material may be
pushed further into a thin feature.
Hot tip gates are typically located at the
top center of a part (as opposed to on
the parting line, as is the case with a tab
gate) and are ideal for round or conical
shapes where uniform flow can improve
concentricity. The hot tip gate leaves a small
raised nub on the surface of the part. Adding
a hot tip dimple to your design may help
shift the nub below the surface of the part,
which might allow something like a decal to
be applied over it with little or no need for
trimming beforehand.
Continued on next page…
Volume 5
HOT TIP SOLVES GATING PROBLEMS
3
Design Tips for Rapid Injection Molding
Hot tip dimples are usually 0.125 to 0.375
inches in diameter and 0.010 to 0.030
inches in depth. This can take the shape of
a spherical or cylindrical depression. (See
Figures 2 and 3). To maintain the always
desirable uniform wall thickness, you could
add material to the opposite side of the part
so the material is not restricted in flow.
Also be aware that some materials such
as acetal and glass filled resins are not
compatible with hot tip gates, and that small
volume parts may be problematic because of
the tendency of resin to “cook” in the hot tip
longer, potentially degrading its properties.
When asking for a hot tip gate, you should
also consider the extra expense of installing
a hot tip gate, such as the features that need
to be machined into the back of the “A” side
mold half. In addition, once a hot tip gate is
used, it is much more costly to modify the
mold. For example, there is no simple way to
move a hot tip without completely re-making
the “A” side of a mold.
Fig. 3: Cylindrical Depression Hot Tip Dimple
Fig. 2: Spherical Depression Hot Tip Dimple
©2009 Proto Labs, Inc. All rights reserved.
Be sure to consider that the “A” side of a part
is usually the externally-facing cosmetic side,
which means if you use a hot tip, the gate
vestige (“nub”) is likely to be visible in an
assembly. As noted above, a hot tip dimple is
a common method of trying to help hide the
nub and should be considered when selecting
decal locations on high cosmetic parts.
As with any plastic part, design and resin
both have a great impact on the success of
your molded part. A well-designed part with
a carefully chosen gate type and location that
are compatible with the resin you select will
allow us to achieve the best possible results.
Volume 5
HOT TIP SOLVES GATING PROBLEMS
4
Design Tips for Rapid Injection Molding
Playing it safe
The whole purpose of prototyping is to allow
yourself the option of tweaking your model—
thickening a wall, adding a rib, placing text
or a logo—before locking in a final design
for production. For injection molded plastic
prototypes, tweaks can entail the creation of
a whole new mold. A new mold may be quite
affordable if your prototypes are being made
by Protomold, but if the change can be made
by modifying the existing mold your cost will
be lower still.
There’s one critical fact to remember if you
want to be able to modify your part by
modifying the original mold. It is relatively
easy to remove metal from an existing metal
mold. Adding metal, on the other hand, can be
difficult or, for all practical purposes,
impossible with rapid injection molding. To
look at this from the part perspective, you can
add plastic, but you can’t take it away.
Designing with this in mind is called “steel
safe” or “metal safe,” and doing so can save
you both money and time when you have to
modify your design. For example:
n
You may be able to thicken a wall, but making it thinner requires remaking the mold.
n
You can add features—bosses, raised text,
ribs, pins etc.—but you can’t remove them.
n
You can reduce the diameter of a hole by
adding plastic around the perimeter, but
you can’t increase the hole size.
n
Similarly, you can eliminate holes, but you
can’t add them.
The rule to remember when initially
designing your part is: “Maximize metal and
minimize plastic.” Figure 1, for example
includes a post (properly drafted, of course)
rising from a base. Figure 2 shows the same
part after the designer has decided that the
post needs to be thicker. (Added plastic is
shown in red.) This change was easy to
execute. If the change had gone in the other
direction, however, it would have required the
milling of a new mold.
Fig. 1
©2009 Proto Labs, Inc. All rights reserved.
If you aren’t entirely sure whether a feature is
needed or whether a feature is the right size,
you might want to review your part feature
by feature, asking yourself which ones may
need to be changed in later iterations before
committing your design for prototyping; then
start with the “less plastic” option.
Not sure how thick a wall should be? Start
thin and thicken it later. (ProtoQuote® will
warn you if your wall is too thin for effective
mold filling.)
Unsure whether you’ll need a rib to strengthen
your part or a brace to prevent warp? Leave it
off and add it later if it’s needed.
Have two mating parts that might (or might
not) need alignment pins? Put the holes in and
leave the pins off. If you need them, you can
add the pins in the next iteration. If you don’t,
you can eliminate the holes.
One final thing to keep in mind when you plan
for changes: at Protomold, we can make parts
to tolerances of ±.003” + the shrink tolerance
of the resin.
Fig. 2
Volume 5
PLAYING IT SAFE
5
Design Tips for Rapid Injection Molding
The plane truth about rotational draft
Let’s say you’re designing a plastic part with
rotational symmetry; for simplicity, we’ll make
it a dowel. In your CAD software you would
create the shape of half the cross section—in
this case, a rectangle (see Figure 1)—and
rotate that shape through 360° to create the
solid. So far, so good! However, knowing that
your part is going to be injection molded and
that the parting line of the mold will run
along the length of your dowel, you realize
that, unless you do something, the end faces
of the dowel will be parallel to the direction
of mold opening. In other words, those ends
need to be drafted. There are two ways to
draft those ends, one of which works better
than the other.
The problematic method is also the most
obvious: when laying out the cross section,
you tilt the ends slightly (See Figure 2).
This way, when you rotate the shape it makes
the ends of the finished design shallow cones
instead of flat disks (See Figures 3a and 3b).
This is “rotational drafting”, and it combines
the drafting step with the rotation that creates
the 3D shape.
Sometimes a flat world is just a lot easier
to navigate.
Fig. 3a: Part Design
Resulting from
Rotational Draft Method
Fig. 1: Undrafted Dowel Half Cross Section
Why is this an important topic for rapid
injection molding? Protomold’s 3-axis milling
process plunges in the z-axis only. This makes
a planar draft a simpler, more reliable cut than
a rotational draft, due to the latter’s varying
cut angle. For this reason, the ProtoQuote
design analysis will show these rotational
draft issues as required changes and ask for
increased draft, wall thickness or both. The
easiest fix is to replace the rotational draft
with a planar draft.
Fig. 3b: Part Design
Resulting from
Rotational Draft Method
The preferred method is “planar drafting”
and it is a separate step from the rotation
that creates the basic part. In this case, each
half of the end surface is drafted separately
in a plane angled away from the parting line
(See Figures 4a and 4b). The key difference
between these two approaches is what
happens as the drafted surface approaches
the parting line.
Fig. 4a: Part Design
Resulting from Planar
Draft Method
Fig. 4b: Part Design
Resulting from Planar
Draft Method
Fig. 2: Pre-drafted Dowel Half Cross Section
©2009 Proto Labs, Inc. All rights reserved.
Volume 5
THE PLANE TRUTH ABOUT ROTATIONAL DRAFT
6
Design Tips for Rapid Injection Molding
Eliminating side-actions for fun and profit
Quick, how would you make the part shown in
Figure 1? Well, since by now you’re aware of
the fact that Protomold supports up to four
side actions per mold, the obvious answer
is to use a mold with a side-action to create
the bottomless box with the window. After
all, without a side-action, a mold feature
that protrudes inward from the A-Side of the
mold (or outward from the B-Side) would
be entrapped in the window when the mold
opened, right?
works for one simple reason: the side walls are
drafted. That means that the mold surface will
begin to pull away from the part surface as
the mold begins to open. So if your design can
support the draft and you’d like to save a little
money and skip the side action altogether, it’s
worth taking note. Take a look at Figure 2 to
see how this can be applied.
Fig. 2
Figure 2 shows a cross section of the window
area in the closed mold in which:
Fig. 1
Wrong! This part can be made in a two-part
straight pull mold without side actions. It
©2009 Proto Labs, Inc. All rights reserved.
n
The gray areas are the mold walls
n
The window is being formed partially by
an extension of the mold’s A-Side and
partially by an extension of the B-Side
Volume 5
n
The dotted line represents the plane of the
outside surface of the wall
n
The red line is the shutoff where the metal
faces of the two mold halves meet when
the mold is closed
Note that the shutoff runs at an angle across
the window, dividing the parts of the window
that will be produced by the A and B mold
halves. Because the wall is drafted, the two
mold halves move away from the part (and
from one another at the shutoff) as the mold
opens and the part is ejected. No part of
either mold half is entrapped in the window;
hence, no side action is required.
There is one important consideration when
designing a part using this technique: the draft
angle of the shutoff. To avoid damage to the
mold the shutoff must be drafted a minimum
of 3°. Because the shutoff is angled slightly
relative to the wall itself, the draft of the wall
must be greater than 3° to allow a 3° draft of
the shutoff. The required amount of wall draft
will vary directly with wall thickness
and inversely with window height (“shutoff
height,” as shown in Figure 2). Most CAD
programs can help determine the proper
degree of wall draft to create a minimum of 3°
draft at the shutoff.
ELIMINATING SIDE-ACTIONS FOR FUN AND PROFIT
7
Design Tips for Rapid Injection Molding
Protomold: it isn’t just for prototypes anymore
Protomold has long been the place to
go for fast, affordable, injection molded
prototypes. But for an increasing number
of customers, Protomold is also a source
of production parts in quantities of up to
25,000. Depending on the number of parts
you need, Protomold can be significantly less
expensive than a traditional molder using
steel tooling. And regardless of quantity, you
still get Protomold’s speedy turnaround for
unmatched speed to market.
There are several ways that Protomold can
help control cost and slash production time
on large orders. Multi-cavity molds, which
produce up to eight copies of a part in each
press cycle, increase tooling cost somewhat,
but can more than offset that increase by
reducing press cycle time and cost. The
ideal number of parts per mold depends on
various factors.
Obviously, part size can be a limiting factor,
but even for small parts the ideal number of
cavities per mold can vary. For example, a
customer needing 24,000 identical small parts
might maximize savings by using an 8-cavity
mold. For a customer needing fewer parts,
say 2,000, the lower tooling cost of a 4-cavity
mold might be the better choice. “What-if”
scenarios using the “cavities” option of the
ProtoQuote® (see fig. 1) or Protomold’s sales
or customer representatives can help you
choose the most cost-effective solution.
©2009 Proto Labs, Inc. All rights reserved.
In addition, until the part has been thoroughly
prototyped and tested, you should probably
stick to single cavity molds. Once your design
has been proven, you may elect to convert to
a multi-cavity mold for production.
Fig. 1: ProtoQuote® Cavity Specification
There are a couple of limitations to keep in
mind as you consider the use of multi-cavity
molds:
1.Side actions: Because they tend to use so
much of the mold’s maximum “footprint”, multi-cavity molds don’t leave room for side action cams and must be simple straight-pull molds.
2.Hot tips: Due to mold complexity
issues, multi-cavity molds cannot use
hot tip gates.
Volume 5
Fig. 2: Multi-Cavity Mold
If you have questions regarding the use of
multi-cavity molds for production parts, check
with your Protomold sales representative, who
will be happy to help you find the best and
most cost-effective solution.
PROTOMOLD: IT ISN’T JUST FOR PROTOTYPES ANYMORE
8
Design Tips for Rapid Injection Molding
Design guidelines for bigger parts
With Protomold’s expanded capability
to produce larger parts, there are a few
important considerations to keep in
mind when designing parts to fit the
Protomold process.
n
For starters, our current increased size
capabilities are as follows:
n
Side action cams consume space, reducing
maximum mold size.
n
To see how these limitations specifically
affect your part, upload a 3D CAD model
for a free ProtoQuote® moldability analysis
and quote.
n
Maximum part outline is approximately
8.9 inches by 29.6 inches (the maximum
outline applies only to shallower parts).
n
Increasing part depth reduces maximum
outline. At 1-inch depth, maximum outline
is 7.6 x 16.9; at 2-inch depth maximum
outline is 5.6 x 14.9; at 3-inch depth,
maximum outline is 23.6 x 12.9. Why is
this? It’s because plastic is injected at
pressures as high as 10,000 PSI, and we
need enough mold material surrounding a
deeper part to keep the mold sides from
bending out.
n
Outline notwithstanding, maximum
projected mold area is 175 square inches.
n
Maximum part volume is approximately 59
cubic inches.
n
Maximum mold depth either side of the
parting line is 3 inches, which means a part
can be up to 6 inches tall if the parting
line passes through the middle of the part
(inside and out).
©2009 Proto Labs, Inc. All rights reserved.
Parts should have approximately 1° of
draft per inch of depth from parting line.
Minimum draft, regardless of depth, is 1/2°.
This allows the part to eject from the mold
without drag marks.
Recommended wall thickness varies by
resin, but larger parts should have thicker
walls. Large parts should have walls at
or near the manufacturer’s maximum
recommended thickness for the specific
resin. See the recommended wall thickness
by resin type chart at protomold.com/
designguidelines.
3. Avoid unsupported geometry (see fig. 2).
4. Use core-cavity construction rather than
ribs wherever possible (see fig. 3).
For detailed guidelines, examples, and
illustrations, see Protomold’s Design
Guidelines.
Fig. 1: Radius Corner
ProtoQuote can provide feedback on
the proposed wall thickness of your parts.
Other guidelines are similar to those for
smaller parts:
Fig. 2: Unsupported Geometry
1.Try not to mix thick and thin sections; keep wall thickness as constant as possible.
2. Radius corners to minimize flow
restrictions and prevent stress concentrations (see fig. 1).
Fig. 3: Rib vs. Core-Cavity
Volume 5
DESIGN GUIDELINES FOR BIGGER PARTS
9
Design Tips for Rapid Injection Molding
Put your parts on a diet
They say “Less is More,” and while that may
be a dubious premise in some areas — your
next salary review, for example — it is often
true of molded plastic parts. We’ve written in
prior Design Tips about the consequences of
overly thick features. As liquid resin cools, it
can lead to all sorts of problems — sink at the
part surface, hidden voids within the feature,
or warp as areas of different thickness
solidify and shrink at different rates,
resulting in undesirable bending. In addition,
unnecessary thickness can throw off part
dimensions, reduce strength and necessitate
post-process machining.
In the past, we’ve suggested that large
features can be “cored,” hollowed out from
the bottom, leaving the original surface shape
while reducing wall thickness. Think of a solid
square cube being turned into a hollow box. If
the part doesn’t have a “back side” from which
coring can be done, there’s always the option
of splitting the part in two, molding two
hollow halves and then joining them; but that
can be complicated and expensive.
What we’re suggesting here is a form of
coring done from the outside. As materials
get stronger, you can see this technique being
used in many places. The holes in cinder
blocks reduce their weight and the cost of
concrete without significantly reducing their
effective strength. Sporting equipment made
of high-tech materials — knives and bicycle
parts for example — are increasingly sporting
©2009 Proto Labs, Inc. All rights reserved.
cutouts of various shapes and sizes that leave
the part strong enough for its intended use
but far lighter than a solid part would be. A
common term for this is “skeletonizing.”
Our example from the plastics world is a
screwdriver handle. The shape shown in
Figure 1 is designed to fit the human hand,
but in doing so, utilizes far more resin than is
necessary to transmit the force exerted by the
hand to the shaft of the screwdriver. Coring
out the handle in the traditional sense, besides complicating manufacturing, would
defeat the handle’s purpose by eliminating
material along its central axis, where it
connects to the metal shaft.
producing a design that can still be made
in a two-part mold. The overly thick shape
has been converted in two ways. The first
is a series of relatively thin walls, creating a
rib cage structure. The second is to core out
the mass with a number of pockets. Both
examples retain the shape of the handle while
reducing excess material and the potential for
injection molding process issues. We may also
improve the user’s grip through the addition
of the ridges.
Fig. 2: Externally cored screwdriver handle
Fig. 1: Solid screwdriver handle
A more practical solution is “external coring”
as shown in Figure 2. Here we significantly
reduce the amount of material required, while
This kind of external coring can be easily
accomplished in most 3D CAD programs. Of
course, the surfaces of the individual walls
now have to be drafted in the direction of
mold opening to facilitate ejection, but this
is also easy using 3D CAD. And, last, but not
least, your part now has a cool, modern look.
Volume 5
PUT YOUR PARTS ON A DIET 10
Design Tips for Rapid Injection Molding
Avoid knitting around your boss
Start with a simple fact: resin cools as it is
injected into a mold. This is why the leading
edge of the resin flow within a mold is always
the coolest area of the resin, and, therefore,
the closest to solidifying. In a well-designed
mold, this is generally not a problem. The
exception may occur when the resin flow is
divided by an obstacle and then meets again
on the other side of the obstruction, for
example, the core that creates a rectangular
hole in a cover plate (see fig. 1). When this
happens, you have two surfaces meeting
downstream from the obstruction. Ideally,
they will meld together to form a solid joint,
but if they have cooled too much to meld
completely, the result is a knit line.
A knit line is any line, visible or not, where two
resin flows meet (see fig. 2). Depending on
the design of the mold and the material being
injected, a knit line may present no problem
at all, may be a cosmetic issue, or can cause a
potentially serious structural problem. One of
the deciding factors is the resin being injected
since resins vary in their tendency to form knit
lines. Among the most likely to show lines is
ABS. In many cases, a knit line in ABS is solid
enough that it will not significantly weaken the
part, but it may appear to be a crack in the
finished part.
One area in which knit lines can cause
structural problems is behind a boss. A boss,
of course, is a feature with a hole designed to
accommodate a threaded fastener. (Protomold
doesn’t create internal threads; those would
be cut by a self-threading screw, machined in
a separate operation, or added as an insert.)
The boss is created by a raised core pin inside
the mold around which resin flows. When the
resin faces meet on the back side of the pin,
they form a knit line.
Two factors can makes this particularly
problematic. If the boss is near the edge of the
part, the knit line will be very short, leaving
relatively little surface holding the two faces
together. When you add the “wedge” effect of
a screw being driven into the boss, a knit line
can turn into a crack.
Knit lines will also occur between gates in
a part. Gates are the areas where resin is
injected into your part. When you get your
gate and ejector layout, check it. We don’t
often use multiple gates, but if we do, check
to see if you have any critical cosmetic or
strength requirements about halfway between
every two gates.
Continued on next page…
Fig. 1
©2009 Proto Labs, Inc. All rights reserved.
Fig. 2
Volume 5
AVOID KNITTING AROUND YOUR BOSS
11
Design Tips for Rapid Injection Molding
There is one more factor that can contribute
to problems with knit lines, and that is the use
of filled resins. Picture the flow of a liquid resin
filled with, for example, glass fiber. Obviously,
as the resin front moves through the mold, the
fill material will always be behind the front.
So when two fronts meet and solidify, there
is little or no fiber crossing the meeting line.
This doesn’t necessarily mean that the knit line
will be weak, but it will not have the benefit of
fiber reinforcement.
©2009 Proto Labs, Inc. All rights reserved.
What can you do to prevent problematic knit
lines? You probably can’t eliminate features
like bosses, but you can choose resins that
are less susceptible to knit line formation.
Specifically, you can avoid filled resins in parts
that will have features like through-holes. You
can thicken part walls to slow resin cooling,
being careful not to thicken them enough
to cause sink. And you can place knit-linecausing features farther from the edges of
parts when the design allows.
Of course, finding knit lines in your prototypes
is better than finding them in your production
parts, and that’s what prototyping is for. If you
have critical requirements for strength of knit
lines, please call a customer service engineer
at (877) 479-3680 to discuss.
Volume 5
AVOID KNITTING AROUND YOUR BOSS 12
Design Tips for Rapid Injection Molding
You’ve got to check this!
“Flaps 10 degrees………check, cabin air
intake………check, landing light………check.”
Ever dream about being a pilot running
through your preflight checklist with your copilot? Well, now as a designer, you can have
almost as much fun when you design plastic
parts using the Protomold Plastic Part Design
Checklist. You too can be confident you won’t
miss a single aspect of parts design when you
use this handy resource tool.
Like a pilot’s preflight routine, our checklist
below is a handy reminder of some of the
considerations to keep in mind when you’re
designing injection molded plastic parts.
And, in addition to our checklist, we’ve
catalogued dozens of tips that can aid in
making your parts easier to manufacture,
lighter, stronger and offer improved
performance. Each tip is chock full of great
manufacturability pointers, including coring
out, preventing sink, drafting for easy ejection,
adding text, and more. Some of these useful
design tips are referenced in the designer
checklist; or you can download our complete
Design Tip Compilation Volumes at
protomold.com/designtips.
Also, you can access ProtoQuote® automated
design analyses, which can help to identify
potential design problems once a 3D CAD
model has been submitted. These free services
allow you to quickly see issues that are easy to
overlook. Our suggestions and recommended
design modifications are returned to you
within one day of submitting your part.
Try our Checklist! It’s a practical tool that
allows you, the designer, to spot potential
issues and correct them beforehand.
PLASTIC PART DESIGN CHECKLIST:
Designing with Protomold in mind
Design Considerations
Are the cores and holes drafted toward
the ejector pin side (low cosmetic side) of
my part?
YES
Does my part design include sufficient draft
for part removal from mold?
YES — We strongly advise using at least
0.5 degrees on all “vertical” faces.
Do I need to add draft to my design?
MAYBE — For aluminum molds,
increments of .5 degrees (shallow features) to
3 degrees (deep features) should be added. A
good rule of thumb is 1 degree per inch depth
including cam/side pull cores.
Continued on next page…
©2009 Proto Labs, Inc. All rights reserved.
Volume 5
YOU’VE GOT TO CHECK THIS 13
Design Tips for Rapid Injection Molding
Does my part have a wall thickness that is
greater than .040” and under .180”?
YES
NO — Parts under .040” may produce
a part that has shorts, voids, weak knit lines;
parts over .180” may have excessive sink,
internal voids, warp, poor texture pick-up.
Dimensions
Volume
Does my part have two dimensions that are
larger than .25” and less than one of the
following? (Dimensions (in mold) = X x Y x Z
(depth of part)
Does my part have a volume greater than
.005 in3?
n
Selection of the proper material is crucial
to part production. Designers should
consider the mechanical characteristics,
molding properties, and cost of the resin
used.
n
Uniform wall thickness for all resins is the
best place to start.
n
Glass filled resins are more likely to warp.
YES — proceed
30.5” X 13.5” X .25”
28.5” X 11.5” X 1”
26.5” X 9.5” X 2”
24.5” X 7.5” X 3”
NO — If no, consider the possibility of
redesigning the part so Protomold may be
able to mold your part. Let us know if we can
assist with this.
YES — proceed to Volume
Does my part have a volume less than 59 in3?
NO — My part is over 3”.
*
*
*
*
Materials
What do I need to consider when selecting a
material that is best for my design?
My part is over 3” deep, is my parting line in
the middle of the part?
YES
NO — If no, consider the possibility of
redesigning the part so Protomold may be
able to mold your part. Let us know if we can
assist with this.
YES — proceed
NO — If no, consider the possibility of
redesigning the part so Protomold may be
able to mold your part. Let us know if we can
assist with this.
Surface Area
Does my part have a surface area of less than
175 square inches?
YES — proceed
NO — If no, consider the possibility of
redesigning the part so Protomold may be
able to mold your part. Let us know if we can
assist with this.
Continued on next page…
©2009 Proto Labs, Inc. All rights reserved.
Volume 5
YOU’VE GOT TO CHECK THIS 14
Design Tips for Rapid Injection Molding
Geometry
Is it a straight pull mold?
Is the undercut on my part less than one 8.419”
x 2.377” x 2.900”? (Dimensions (in mold) =
Horizontal x Vertical x Depth of cam pull).
YES — proceed
NO — Off angle geometry may need to be
modified or cams/side actions need to be applied
YES — Protomold will use cam/side action
to mold the feature
NO — Can you use sliding shut offs?
I DON’T KNOW — Call Protomold
Customer Service Engineers (CSE’s)
at 877-479-3680.
Undercuts
Are the undercuts located toward the outside
of my part?
YES — Proceed to next question.
Protomold may be able to use cams/side
actions to mold the undercut.
NO — Pass-through cores or filling in
the undercut geometry will be required.
Protomold can certainly mold your prototype
parts, however may not be able to mold your
production parts. You could cut the undercut
geometry with a secondary operation for
testing or low volume production.
©2009 Proto Labs, Inc. All rights reserved.
Volume 5
YOU’VE GOT TO CHECK THIS 15
Design Tips for Rapid Injection Molding
Don’t be square!
We’ve talked in the past about corners and
reasons to make them rounded (radiused)
instead of sharp. Let’s talk about this in a
little more detail and help distinguish
between inside and outside corners, both
of which should be radiused but for slightly
different reasons.
Example of Sharp Corners
vs. Radiused Corner
If you look at Figure 1, you see the parting line
where A- and B-Side mold halves meet
to form the sharp edge of the part. In
machining molds, there is always some
tolerance, but slight movement of the parting
line to the left or right will not change the
geometry of the edge.
Outside corners
The first thing to keep in mind is that an
outside corner of your part is created by an
inside corner of a mold, and vice versa. One
reason we don’t make parts with sharp outside
corners is because our molds are made by a
vertical milling process that cannot cut a sharp
inside corner. The radius of our inside corner
(your outside corner) cannot be smaller
than the radius of the cutter, which will vary
somewhat with the depth of the cut.
Inside corners
Our milling process can produce sharp outside
corners when making a mold, so you can have
sharp inside corners. The problem is that sharp
inside corners can create serious stresses in
a part as it cools. The reason is simple. The
rate of resin cooling is proportional to surface
area. Any corner will have more surface area
on the outside of the curve than on the inside.
(Think about the advantage of the proverbial
“inside track.”) On a radiused corner, there is
always a difference between the two surface
areas, but if the inside of the corner is square,
it essentially has a surface area of zero, which
maximizes the difference between the inside
and outside surface areas.
©2009 Proto Labs, Inc. All rights reserved.
If the part consisted of two walls meeting to
form an “L” shape, the part may tend to warp
as it cools, reducing the angle between the
two walls. If, however, the corner is in, say, a
box whose shape keeps the walls from moving
in relation to one another, instead of warping,
they’d merely become stressed. The result
could be cosmetic problems; a fracture or
buckled floor.
In addition, because they are sharp, the
outside corners of a mold half can “grab” the
part within which they are forming a core,
either making ejection difficult or risking
damage to the part or mold. And finally, sharp
corners can contribute to sink and weakened
knit lines. So radius those corners!
Now that we’ve hopefully convinced you to
radius corners, let us describe one situation
where you should not radius a corner: at the
parting line. See next page…
Fig. 1
In Figure 2, on the other hand, both the
mold halves form the parting line edge, so
any mismatch in the mold will leave a ledge,
changing the shape of the part at the parting
line. That’s one reason we recommend leaving
the parting line sharp.
Fig. 2
Volume 5
DON’T BE SQUARE 16
Design Tips for Rapid Injection Molding
Cams: they’re not just for undercuts
By moving in directions perpendicular to the
direction of mold opening, side action cams
allow the production of parts with “undercuts”
that could not be successfully made in
two-part, straight-pull molds. But there
are parts without undercuts that can also
benefit from the use of cams. The Protomold
process (vertical milling) requires increased
wall thickness and draft as the part depth
increases. Using cams, we can reduce the
need for draft and wall thickness in
some instances.
Imagine, for example, a thimble, essentially
a cup with tapered sides (see figure 1). The
obvious way to mold such a part would be in a
two-part mold in which the outside is formed
by the A-Side mold half and the inside core is
formed by the B-Side mold half (see figure 2).
Resin would be injected through a tab gate
placed along the parting line at the rim of the
cup. If the walls of the part were thick enough,
this might work. In a thin-walled design,
however, there could be problems.
Fig. 1
Fig. 2
Continued on next page…
©2009 Proto Labs, Inc. All rights reserved.
Volume 5
CAMS: THEY’RE NOT JUST FOR UNDERCUTS 17
Design Tips for Rapid Injection Molding
So how do you identify parts that can benefit
from the use of cams? Tall, thin parts with a
core, like our thimble, are prime candidates.
Another candidate might be a part that, in
a straight-pull mold, would require more
draft than the designer is able to provide. In
that case, side-action cams may completely
eliminate the need for draft as the outside
face is pulled perpendicular to the part instead
of being pushed or pulled from the mold. And,
finally, cams can allow production of parts
with texture on faces parallel to the direction
of mold opening. In simple straight-pull molds,
such texture acts essentially as a field of
undercuts that can prevent the clean ejection
of parts. If there is a design need and we can
get a cam on that face, why not? It just may
be the face that makes your part.
First, in a thin-walled design, the resin could
cool quickly enough to result in a “short
shot,” that is, incomplete filling of the cavity.
Obviously, this would not be acceptable.
Even if the cavity did fill completely there
would be two flow fronts meeting on the side
of the core opposite of the gate, creating
knit lines that could substantially weaken the
resulting part. One possible solution would
be to use a hot-tip gate and inject resin at
the closed end of the cylinder—the bottom
of the cup or top of the thimble. This may
not be possible due to the height of the part,
resin compatibility with the hot tip or other
unforeseen issues. A better solution might be
to lay the design on its side and use a cam to
create the core (see figure 3).
In this case, the core of the thimble is formed
by a retractable cam while the outside
of the part is formed by two straight pull
mold halves. As shown in the diagram, the
parting line runs across the closed end of
the cylinder and down the sides. For the sake
of symmetry, a tab gate is located along the
parting line at the closed end of the cylinder.
Resin is injected through the gate and the
resin flows down the length of the cavity
uniformly.
©2009 Proto Labs, Inc. All rights reserved.
Fig. 3
Basically, cams aren’t just for undercuts
anymore. Need a flat face to bolt up against a
mating part? How about texture? How about
your company’s logo or part numbers? Adding
a cam to the mold may be just what you need.
Volume 5
CAMS: THEY’RE NOT JUST FOR UNDERCUTS 18
Design Tips for Rapid Injection Molding
Look sharp
For several years Protomold has been
expanding the range of prototypes and
production parts we can make by increasing
our mold bases and press sizes. At the same
time we’ve been aware of the demand for
thinner features and Protomold has begun to
incorporate additional technology in our mold
making process to allow your parts to be taller
and narrower.
Once you have received your ProtoQuote, you
can discuss your project with our Customer
Service Engineers (CSE’s) by calling us at
(877) 479-3680. Our CSE’s can help you
understand how your part is going to be
affected by our process and if there are
alternatives within the Protomold process to
manufacture your part.
Bottom line: if you want thinner, less drafted
features on relatively small parts, give us a try.
As always, you can upload 3D CAD models
to ProtoQuote at www.protomold.com/
PartUpload.aspx.
For example, we now offer:
n
Thinner ribs with less draft
n
Smaller, more complex features atop tall
walls
n
Speaking of “bottom line,” this technology
does add some cost to the mold. We do add a
mold advisory to your ProtoQuote under our
“Other Info” column. You can still design to the
“old” Protomold rules for draft, and avoid the
added expense. Give our CSE’s a call at
(877) 479-3680, and they can help you out.
V-ribs for ultrasonic welding
In the past, taller ribs have required more
draft than shorter ones, but with the new
technology, this is no longer always the case.
On small parts, we now have the ability to
consider less draft on deep ribs and sharp
corners on outside edges.
Fig. 1
These new capabilities do not apply to all
parts. As with many of our capabilities, the
applicability will vary based on a number of
factors. The best way to know whether they
apply to your parts is to submit your design to
ProtoQuote®, which will evaluate your model
against our most current capabilities.
©2009 Proto Labs, Inc. All rights reserved.
Volume 5
LOOK SHARP 19