Download - Protolabs Tips 4
-
7/31/2019 Protolabs Tips 4
1/20
Design Tips
NOBODYSFASTERIN THE SHORT RUN.
or Rapid Injection MoldingVlue 4
Print.Frat: Landscape
Pae Se: Fit to page
Bind and save.Bind in presentation
ormat or uture reerence
Prtld 5540 Pioneer Creek Drive, Maple Plain, MN 55359 (763) 479-3680
http://print/http://print/http://print/http://print/http://print/http://www.protomold.com/partshttp://print/ -
7/31/2019 Protolabs Tips 4
2/20
Des Tps or Rapid Injection Molding
007 Protomold. All rights reserved. Vlue 4 n DESign mATRix n
Design Tips categorized by topic
Paemateralselect
Desudeles
Qualtassurace
Uderstadthe prcess
3 Fun with cams
5 Sizing: an in-depth examination
6 Living in the material world
8 The orphan llet
9 What you dont C cant hurt you
11 When you really need to dodge the drat
13 Night o the living hinge 14 Good vibrations ultrasonic welds
15 Resist that sinking eeling
17 The inside scoop on outside threads
19 Sliding shutos (again)
20 When things get rough ...
TaBlE O cONTENTS
Extern ink to more inormtion
-
7/31/2019 Protolabs Tips 4
3/20
Des Tps or Rapid Injection Molding
3
In a previous design tip, we used the example o
a house-shaped box with a mousehole doorway
(See Figure 1). The outside o the house was
ormed by the A-side o a simple straight-pull
mold; the inside was ormed by the B-side o the
mold. A shuto, a raised pad on the surace o
the B-side mold, ormed the doorway. In this tip
were going to complicate the process by turning
the door shown in Figure 1 into a window (See
Figure ). By adding material below the bottom
o the eature, weve created an undercut eature
that cannot be produced in a two-part mold.
Whereas the shuto that created the doorway in
Figure 1 can exit the doorway through the open
bottom o the eature when the mold opens, a
mold eature used to orm the window in Figure
would be trapped when we try to open the mold.
The solution is to create a third mold part that
moves perpendicular to the direction o mold
opening (or parallel to the plane o the molds
parting line). This side-action cam lls the
space that will become the window. When a
side action is used, mold opening drives the
cam out sideways as the two primary halves o
the mold open, ater which the part is ejected.
Sometimes Protomold will add other aces to
the cam to eliminate parting lines on a critical
ace. We have done this with the whole ront
o the house to prevent parting lines belowthe door/window. You can discuss this with
your Protomold customer service engineer.
While a wide variety o parts can be produced in
straight pull molds, side actions literally open up
whole new dimensions in part design. One o the
most common applications is the production o
through-holes, o which the window mentioned
above is an example. Producing a through-hole in
the process o molding saves the time and cost
o a separate operation ater the part has been
molded. In a straight pull mold, through-holes
can be made in the direction o pull. They can
also be made in other directions using sliding
shutos, which work well or some applications,
such as the dormer window in the house. See
our tip at: Creating Through-Holes. When
sliding shutos arent appropriate, side-action
cams can create holes and other eatures other
directions as long as the direction o cam travel
is perpendicular to the direction o mold opening
and the eature is on the outside o the part.Figure 3 shows a part with several eatures that
could only be made using side actions. The tan
circular hole is similar to the house window in
Figure . The purple rectangular indentation can
be thought o as a hole that doesnt go all the
way through the wall. But like a hole, it would
be an unmoldable undercut in a straight-pull,
two part mold. The side-action cam, however,
is well out o the way beore the part is ejected.
Fun with
cas
007 Protomold. All rights reserved. Vlue 4 n FUn WiTH CAmS n
Fure 2 Fure 3Fure 1
http://www.protomold.com/DesignGuidelines_SimpleStraightPullParts.aspxhttp://www.protomold.com/DesignGuidelines_SimpleStraightPullParts.aspx -
7/31/2019 Protolabs Tips 4
4/20
Des Tps or Rapid Injection Molding
4
In all o the previous examples, cams are used
to create small eatures on a larger part, but
this is not the only way they can be used. The
part shown in Figure 4 uses cams to create
the entire circumerence o the part, while the
indented top and bottom are ormed by the
A- and B-side primary mold halves. Alternatively
the entire part could be rotated 90 degrees
making the sides in the diagram with the A
and B mold halves and using side actions to
create what are shown as the top and bottom.
In short, now that weve added side-
action cams to our mold-making tool
kit, Protomold is not just or simple parts
anymore. Here are some guidelines:
We can build up to our separate
side actions into a single mold.
While side actions must all be inplanes parallel to the plane o the
primary mold parting line, they need
not all be in the same plane.
Side actions can be used to produce
eatures on the outside o a part
but not (yet) on the inside.
Like primary mold sections, side
actions may require drating. This was
discussed in the June 006 design tip.
I you have any questions regarding
the application o side actions to your
parts, eel ree to contact us.
Visit thePrtld Des gudeor other helpul Rapid Injection
Molding design inormation.
007 Protomold. All rights reserved. Vlue 4 n FUn WiTH CAmS n
Fure 4
Protomold is not just or sple parts anymore.
http://www.protomold.com/Design_Tips/UnitedStates/2006/2006-06_DesignTips/default.htmhttp://www.protomold.com/ContactUs.aspxhttp://protomold.com/DesignGuidelines.aspxhttp://protomold.com/DesignGuidelines.aspxhttp://www.protomold.com/Design_Tips/UnitedStates/2006/2006-06_DesignTips/default.htmhttp://www.protomold.com/ContactUs.aspxhttp://protomold.com/DesignGuidelines.aspx -
7/31/2019 Protolabs Tips 4
5/20
Des Tps or Rapid Injection Molding
5
Fure 1
Theres a Wall Street saying oten quoted to
those who see no limits to a avorite stocks
prospects: Trees dont grow to the sky. In
other words, everything has its limits. And so
it is with Protomolds molding capabilities. Wecan deliver great parts incredibly ast and at
amazing prices, but due to a number o actors
related to our existing molding equipment we
do have size limitations. We are, o course,
always striving to expand our capabilities.
Until recently, our production was limited to parts
cut no more than two inches deep into each mold
hal. In other words, the depth o a careully
designed part could be a ull our inches, but
only i the depth o the part were divided equally
between the two mold halves. (See Figure 1)
With the addition o new technology, we ca
w prduce parts wth a ttal depth
s ches as long as neither mold hal is cut
more than three inches deep. (See Figure )
Regardless o the depth o the part, its total
volume cannot exceed .1 cubic inches.
The reason is simple: that is the volume o
resin that our largest press can currently
inject into a mold in a single shot.
The next issue is maximum part outline. Imagine
that you sat your part on a fat surace running
parallel to the parts parting line. The shadow o
the part projected downward onto the surace
is the part outline or projected area. (Light
shining through holes in your part doesnt count
toward the projected area.) For parts up to
two inches in depth in each mold hal, the part
outline must t within a rectangle measuring
7.5 x 14 inches. For parts up to three inches in
depth in each mold hal, the part outline must t
a rectangle measuring 6 x 8 inches. The reason
or this limitation is the size o the raw mold
stock we use or molds o dierent depths.
The nal issue is total mold area. This is the
actual area o the opening where the two mold
halves meet, and it cannot exceed 75 square
inches. This limitation is based on the maximum
closing orce our molding presses can exert.
That orce must exceed the injection pressure,
typically measured in psi, o the resin multiplied
by the total mold area or the press will be unable
to hold the mold closed during injection.
T suare the data:
Finally, there is the issue o drat. A good rule
o thumb is that parts should be drated one
degree or each inch o depth cut into the mold
hal. In other words, one inch o depth requires
one degree o drat; two inches requires two
degrees; three inches o depth gets threedegrees. Parts o one hal inch or less require
a minimum o one hal degree o drat.
Visit thePrtld Des gude
or other helpul Rapid Injection
Molding design inormation.
007 Protomold. All rights reserved. Vlue 4 n Sizing: An in-DEPTH ExAminATion n
S: an in-depth examination
Fure 2
2 rom prting ine. 4 totRequires 2 drt
3 rom prting ine. 6 totRequires 3 drt
Maximum depth per mold hal 3
Maximum part outline 7.5 x 14 6 x 8
Maximum projected part area 75 in 75 in
Maximum part volume 15.75 in3 15.75 in3
http://protomold.com/DesignGuidelines.aspxhttp://protomold.com/DesignGuidelines.aspxhttp://protomold.com/DesignGuidelines.aspx -
7/31/2019 Protolabs Tips 4
6/20
Des Tps or Rapid Injection Molding
6
Weve all heard at one time or anotherrom
a parent, a coach, or a teachera reerence to
what youre made o. It probably reerred to
what you could do or withstand, but since were all
made o pretty much the same stu, the meaningo the phrase was more gurative than literal.
When youre an injection molded part, however,
what youre made o literally determines a
great deal o what you can do or withstand.
At Protomold, we keep over 100 resins in stock
and have access to hundreds more. But the three
most oten requested are ABS, polycarbonate,
and 33 percent glass-lled Nylon. This is not
to say that these are the three most widely
used resins or injection molding, just that they
are the most used by Protomold customers.
ABS is a good, inexpensive, general purpose
resin. It is widely used or the cases o hand-
held electronic devices, the housings o power
tools and many other products we use every
day. The material is tough enough to takea licking in everyday use, and while it may
scu rom rough handling, it is less subject
to breakage than a lot o other plastics.
Another plus or ABS is its excellent moldability
characteristics. It is somewhat susceptible
to sink and can be damaged by solvents, but
i you design parts careully, it is possible to
produce well-ormed parts without serious
shrink, sink, or internal stress. It is important
to maintain relatively even wall thickness
in designing parts in ABS, though not quite
as critical as with other, more shrink-prone
materials. In general, ABS is opaque, although
a clear version o the material is available.
Polycarbonate is considered a higher-end
resin. While it does cost more than ABS, it is just
a medium-cost resin. It can be very strong, so
much so that it is used or bulletproo windows.
And while it is oten chosen, because o its
transparency, or use in lenses and light pipes,
it can also be opaque. Because o its high
strength, it is used to make cases and housings
which need a stronger material than ABS.
Polycarbonate does have some shortcomings,
including a tendency to sink. I a polycarbonate
part is not properly designed, the surace o overly
thick area can sink signicantly during cooling. In
some instances, shrinkage may not show on the
surace, but internal shrinkage may cause a void
inside the part, seriously weakening the nished
piece. Proper design and avoiding thick/thin
geometries will help prevent such problems. Also,
polycarbonate is susceptible to petroleum-based
solvents. In some applications, polycarbonate can
007 Protomold. All rights reserved. Vlue 4 n LiVing in THE mATERiAL WoRLD n
Lv in the material world
ABS
Plcarbate
-
7/31/2019 Protolabs Tips 4
7/20
Des Tps or Rapid Injection Molding
7007 Protomold. All rights reserved. Vlue 4 n LiVing in THE mATERiAL WoRLD n
be blended with other resins, like ABS, to achieve
a compromise on both properties and cost.
Glass-lled nylon is the strongest o the three
resins addressed here. Common glass-lled
nylons are medium-cost resins, though some
specialized versions o the material can be very
costly. The material resists many solvents and
hydrocarbons, but is attacked by some acids and
bases. (You should research your application
and environment beore nalizing your resin
choice). And with the addition o glass ber,
nylon is very heat resistant. With up to three
times the strength o polycarbonate, this material
is used or protective or structural parts that
need to withstand a great deal o stress.
On the other hand, glass lled nylon is the most
shrink-prone o the three resins being discussed.
Nylon itsel is very subject to shrinkage as it
cools, and the addition o berglass can cause
dierential shrinkage relative to the direction o
resin fow during mold lling and contributing
to warp. For this reason, i the strength, heat
resistance, and chemical compatibility o this
material are needed, good design is critical in
preventing distortion o the nished parts.
Visit thePrtld Des gudeor other helpul Rapid Injection
Molding design inormation.
glass-lled l
When youre an injection molded part, however,what ure ade literally determines agreat deal o what you can do or withstand.
http://protomold.com/DesignGuidelines.aspxhttp://protomold.com/DesignGuidelines.aspxhttp://protomold.com/DesignGuidelines.aspxhttp://protomold.com/DesignGuidelines.aspx -
7/31/2019 Protolabs Tips 4
8/20
Des Tps or Rapid Injection Molding
8
Fillets, in certain geometries, can be a problem.
Take, or example, the part shown in Figure 1. As
produced by the designers CAD package, the green
aces o this part (including the let and ar sides
that dont show in the diagram) are created by the
A-side mold and drated toward the A-side. In other
words, they taper slightly toward the bottom o thediagram. The suraces shown in blue are created
by the B-side o the mold and are drated toward
the B-side, which means they taper toward the
top o the diagram. The problem is with the llet.
Figure 1 actually shows the 3D CAD diagram as
evaluated by Protomolds ProtoQuote quoting
and analysis sotware. The area noted in red,
the llet, is where ProtoQuote
has identied a
problem. The customers CAD sotware, recognizing
that the llet connects to both an A-side aceand a B-side ace has tried, unsuccessully, to
resolve the confict between two opposite drat
directions. The reason it has been unsuccessul is
that, in reality, this llet can not be part o either
side. In other words, this eature is an orphan.
To be clear, this is a llet which connects
an A-side drated ace to a B-side drated
ace, and is over (or could be under) a fat
surace, which creates undercut geometry.
I this llet were created by the B-side o the
mold, it would have to taper in the same direction
as the adjoining blue ace, that is, toward
the top o the part. The problem is that the
adjoining green ace, which is part o the A-side,
is tapering in the opposite direction, toward
the bottom on the part. The result would be a
misalignment a step along the line where
the llet (red) meets the A-side ace (green).
I, on the other hand, this llet were created by
the A-side o the mold, there would be a problemin the area that appears, in Figure 1, as a small
red triangle on the oot o the part at the base o
the vertical tower. The part o the A-side o the
mold that created the llet would trap the plastic
part under it at that triangle when the mold opens.
(Figure shows the A-side mold itsel and in red
the projecting eature that would trap the part.)
In Figure 1, Protomolds analysis sotware has
attempted, unsuccessully, to resolve the confict
by dividing the llet between the two mold halves.The red hal has been assigned to the A-side, the
blue hal assigned to the B-side. The bright blue
lines indicate the undercut area. Unortunately,
the problem o mold entrapment remains, as can
be seen in both Figure 1 and Figure . Figure
also shows a secondary problem. The area o
the mold that is supposed to create part o the
llet comes to a razor edge. Such an edge
would be subject to extreme wear and, as a
result, allow the ormation o undesirable fash.
There are three pssble sluts t
the prble the rpha llet:
The designer could redesign the part so that
everything was drated toward the top. In
that case, the entire part, llets included,
could be molded in the B-side mold, with the
A-side just orming the base o the part.
The designer could avoid vertical llets that
connect A-side and B-side drated aces. This
would prevent the problem in the rst place.
This part could be manuactured as
designed with the addition o a side-
action cam. Protomold can include up
to our such cams in a mold, but this
would increase the cost o the mold.
Visit thePrtld Des gude
or other helpul Rapid Injection
Molding design inormation.
007 Protomold. All rights reserved. Vlue 4 n THE oRPHAn FiLLET n
The orphan llet
Fure 1
Fure 2
1
2
3
http://protomold.com/DesignGuidelines.aspxhttp://protomold.com/DesignGuidelines.aspxhttp://protomold.com/DesignGuidelines.aspx -
7/31/2019 Protolabs Tips 4
9/20
Des Tps or Rapid Injection Molding
9007 Protomold. All rights reserved. Vlue 4 n WHAT yoU DonT C CAnT HURT yoU n
Start with the simple act that plastic resins shrink
as they cool. Some shrink more than others, but
they all do it. I the shrinkage were perectly even,
we could simply make the mold slightly oversize
and count on shrinkage to reduce them to the
desired size. Unortunately, shrinkage is a more
complicated process. As a result, certain shapes
that are otherwise perectly acceptable can be
dicult to mold because they tend to warp as
they cool. Anything with a C shape, like the part
shown in Figure 1, can be particularly problematic.
O course your choice o resin can contribute
to the problem in two ways. The rst is
variation in the tendency o the resin
to shrink as it cools. For example:
Acrylic shrinks very little
HDPE shrinks quite a bit
Nylon 6/6 alls somewhere
between the two
The second materials issue is specic to lled
materials. As they are injected into the mold, the
ber ller in these materials tends to align with
the direction o resin fow. The resulting grain
causes uneven shrinkage between dimensions
that run with the grain and those running across
the grain. The result is an increased tendency o
parts made o lled resin to warp as they cool.
As ar as shapes are concerned, the
problem with C is actually a problem
with its two right angleL corners.
Figure is a close-up view o the angle o one o
the Ls. You can see that the distance along the
inside o the angle (rom A to B) is shorter than
the distance around the outside o the angle (rom
C to D). As a result, the surace on the outside o
the angle is larger than that on the inside. More
area means aster radiation o heat. As a result,
the C-D side o the angle hardens beore the A-B
side. As A-B continues to cool, it also continues
What you dont
C cant hurt you
Fure 1
Fure 2
As ar as shapes areconcerned, the problemwith C is actually aproblem with its two
right angle L corners.
-
7/31/2019 Protolabs Tips 4
10/20
Des Tps or Rapid Injection Molding
10007 Protomold. All rights reserved. Vlue 4 n WHAT yoU DonT C CAnT HURT yoU n
to shrink, pulling what was designed to be aright angle to something less than 90 degrees.
The solution? I you radius the corner on the
inside and outside as shown in Figure 3 (using
the ormula shown maintains constant wall
thickness), you will minimize the warping
eect. Using a larger radius (but maintaining
constant thickness) will reduce warp more
as it reduces the dierence in mold metal to
cool the inside and outside o the wall.
O course, everything thats true o an L is doubly
true o a C, which increases the magnitude o the
problem because there is more curve, hence more
dierence between the length o the outside and
inside suraces. Whether made up o angles or
curves, the inside o the C will be shorter than
the outside and, as a result, will still be cooling
and shrinking ater the outside has hardened,
pulling the jaws o the C closer together.
There are a number o ways to address the
problem. Turning the C into an O eliminatesthe opening and prevents the ends o the C rom
being pulled toward one another. In essence, the
added part o the circle acts as a brace to help
the part hold its shape. Putting a removable
brace across one o the open sides can also help
counteract the orces trying to close the jaws o
the C until the part has cooled and stabilized.
I none o these is possible, the best way to
reduce the problem is to choose one o the more
shrink-resistant resins. These would include:ABS, Polycarbonate, PC/ABS, PETG Polyester,
Polystyrene, and K-resin Polystyrene butadiene.
And, o course, where shrinkage could distort
your part it is particularly important to pay close
attention to geometry and avoid lled resins.
Visit thePrtld Des gude
or other helpul Rapid Injection
Molding design inormation.
Fure 3 Fure 4
http://protomold.com/DesignGuidelines.aspxhttp://protomold.com/DesignGuidelines.aspxhttp://protomold.com/DesignGuidelines.aspx -
7/31/2019 Protolabs Tips 4
11/20
Des Tps or Rapid Injection Molding
11
Weve spent so much time reminding designersthat parts must be drated to acilitate ejection rom
molds that it seems strange to talk about how to
avoid having to drat parts, but it can sometimes
be done when absolutely necessary. Keep in mind,
however, that drating is still the key to simplicity
o design, ease o molding, and cost control.
As weve said in previous Design Tips, when a
surace is parallel to the direction o mold opening,
it should be slightly tapered toward the mold;
otherwise the mold surace will drag across the
surace as the mold opens, damaging the surace.
Drating causes the part ace to move away rom
the mold ace as the part is ejected, preventing
damage. The slight change in ace angle usuallymakes no dierence in either the unctionality
or appearance o the part. But what i it does?
Probably the most common reason not to drat
a surace is to make it t with other parts
o a nished product. Figure 1 is a bracket
which bolts to a machine. I the mating ace
is drated, the top ace tilts at an angle that
is unacceptable or this application
For a specic requirement, like this one, Protomold
can incorporate cam-driven side actions into a mold.
These are typically used to create undercuts that
could not be molded in a simple two-part mold.
But, because cams move perpendicular to the
direction o primary mold opening they can also
be used to produce suraces that are undrated
in relation to the A- and B-side mold halves.
Imagine a part with a surace parallel to the
direction o mold opening. Lets assume that
we cannot drat the problem surace and mustnd some other way to protect it during ejection
(see Figure 1). Protomold would normally require
drat on this ace as shown in Figure .
I the surace cannot be drated, so as to move
away rom the mold as the part is ejected, an
alternate solution is to have the mold move
away rom the surace. This is achieved using
a side-action cam (brown ace in Figure 3).
007 Protomold. All rights reserved. Vlue 4 n WHEn yoU REALLy nEED To DoDgE THE DRAFT n
When you really needto dde the drat
Fure 1 Fure 2
Drat is still the key tosimplicity o design, ease omolding, and cost control.
-
7/31/2019 Protolabs Tips 4
12/20
Des Tps or Rapid Injection Molding
1
In essence, a side action works like the moving
wall in the Death Star trash compactor in Star
Wars. As the A- and B-side mold halves open or
close along the X-axis, the cam (or cams) move
along the Y/Z axes. Beore ejection, the cam will
withdraw leaving no mold wall next to the problem
ace to cause problems as the part is ejected.
Aside rom undrated aces and the obvious undercut
eatures, there are several other applications or
side actions. Raised lettering on a ace parallel
to the direction o mold opening presents a
problem even i the ace is drated; side actions
solve that problem (brown ace in Figure 4).
Similarly, texture on a low-drat ace, which
might not be reproducible in a straight-pull mold,
can be produced in a mold with side actions.
There is additional cost or each side action, and
there may be some fash between the side action
ace and the rest o the mold. Thereore they
should be considered an option with tradeos, not
a panacea or all undercuts or zero-drat aces.
One more application is the production o decal
recesses. These are shallow undercuts, but they
can simpliy the placement o decals and, i they
all in aces that are parallel to mold opening, are
made possible using side actions. ProtoQuote
now points out areas that can be produced
using side actions, giving users the option o
redesigning their parts or standard straight-pull
molds or using this more advanced capability.
Visit thePrtld Des gude
or other helpul Rapid Injection
Molding design inormation.
Fure 3
Fure 4
007 Protomold. All rights reserved. Vlue 4 n WHEn yoU REALLy nEED To DoDgE THE DRAFT n
http://protomold.com/DesignGuidelines.aspxhttp://protomold.com/DesignGuidelines.aspxhttp://protomold.com/DesignGuidelines.aspx -
7/31/2019 Protolabs Tips 4
13/20
Des Tps or Rapid Injection Molding
13
Night o thelv he
Take a look at a door and the hinges on which itswings. There are probably three or our hinges,
each o which consists o three separate parts
and our to six screws. Do the math, and youll
see that the hinge you take or granted in your
daily comings and goings consists o at least 1
separate components. Lie would be so much
simpler i the number o parts needed or a hinge
could be reduced. The good news is that, at
least in the design o plastic parts, it oten can.
A reduction to three or two or just one part or
a hinge would be notable. A reduction to zero
is truly impressive, and thats exactly how many
additional parts a living hinge requires. Quite
simply, a living hinge is a thin strip molded into
a plastic part to create a line along which the
part can bend. Properly designed and executed,
it can be closed and opened over the lie o the
part with little or no loss o unction. But simple
though it may be in concept, a living hinge must
ollow certain guidelines i it is to work properly.
First, only certain resins are fexible enough to
support the degree and requency o bending
required o a hinge. The best resins or parts with
living hinges are polyethylene and polypropylene.
When a hinge bends, tensile orces are
transmitted to the material along the outside o
the bend. The thicker the hinge, the greater the
stress in the outside surace, so the hinge should
not be too thick or it may crack when it is bent. On
the other hand, i the hinge is too thin it will not
be strong enough to withstand any tearing orces,especially at the ends. The ollowing geometry
(rom eunda.com) works well or hinges made
o either o the two resins mentioned above.
Also, be careul that, when the hinge is
bent ully, there wont be intererence
rom thick edges along the hinge.
Finally, a thin spot in a part (which is what a
hinge is) can be challenging to ll during resin
injection. Success depends on proper gate
placement. A single gate that orces resin through
the hinge area in a mold increases the strength
o the hinge; however, this approach can lead
to sink in areas downstream rom the hinge.
On the other hand, multiple gates may eliminate
the problem o sink, but i resin fows meet at
the hinge (which they will tend to do), they will
usually cause cracking. When you order a mold,
Protomold will propose gate location(s) to optimize
lling o the part including any living hinges.
I this all seems like a lot o trouble, keep
in mind that experts suggest that a well
designed living hinge can be fexed millions
o times. Thats more times than most o
us will walk through doors in a lietime.
Visit the Prtld Des gude
or other helpul Rapid Injection
Molding design inormation.
007 Protomold. All rights reserved. Vlue 4 n nigHT oF THE LiVing HingE n
Fure 1
Fure 2
http://protomold.com/DesignGuidelines.aspxhttp://protomold.com/DesignGuidelines.aspx -
7/31/2019 Protolabs Tips 4
14/20
Des Tps or Rapid Injection Molding
14
Theres no shortage o ways to join
plastic parts. There are bolts and screws,
molded-in clips and snaps, and a variety
o adhesives. But or simplicity and
permanence, nothing beats sonic welding.
The ultrasonic weld joint is a method or joining
two parts. They are designed with a small amount
o extra plastic in the area you want to weld. The
two parts are placed together and in contact
with an ultrasonic generator which causes them
to vibrate many thousands o times a second.
Friction at the joint liquees the extra plastic
and small adjoining areas on both parts. As the
melted material on the mating parts cools, the
two parts become essentially one. I you can live
with its permanence, ultrasonic welding is the
best o all possible solutions. It eliminates the
loose parts and painstaking insertion o threaded
connectors. It avoids the geometric complexity o
molded-in plastic snaps, and it does away withthe chemical problems and mess o adhesives.
The weld is ideal or permanently sealing
maintenance-ree devices like batteries or
assembling a non elastic cup seal to a piston. It
is a solution to encapsulating something within
plastic when overmolding is not allowable.
Sonic welding is also useul or preventing
tampering that could void a warranty.
Protomold does not actually perorm the
sonic welding o parts, but is oten called
upon to mold parts that will be joined using
that process. Weld interaces have dierent
congurations rom simple to complex. Below
are three that work well within the Protomold
process and ultrasonic welding in general.
Visit the Prtld Des gude
or other helpul Rapid Injection
Molding design inormation.
007 Protomold. All rights reserved. Vlue 4 n gooD ViBRATionS ULTRASoniC WELDS n
gd vbrats ultrasonic welds
The Shear Jt is very strong, se-igning joint tht
is prtiury useu or reting hermeti ses nd
right-nge joints. It is ide or rystine mteris
suh s Nyon, PPS, nd P PO, nd n so be used
with rger prts mde o morphous mteris. Note
tht this joint n eve fsh when prts re joined.
The Step Jtis stronger, se-igning joint
tht provides n exeent pperne. It is
suitbe or use with morphous mteris.
The Tue ad grve Jt eimintes fsh used
by the weding proess s the wed ours between
two ws nd is n exeent hoie or hermeti
ses. Not reommended or thin wed prts.
http://protomold.com/DesignGuidelines.aspxhttp://protomold.com/DesignGuidelines.aspx -
7/31/2019 Protolabs Tips 4
15/20
Des Tps or Rapid Injection Molding
15
With a ew notable exceptions (HO or one)
most materials shrink as they cool and solidiy.
This is true, to a greater or lesser extent, o
virtually all plastic resins. Uniorm, predictable
shrinkage would be easy to account or inmaking a mold; we could simply design the
mold slightly larger than its desired size and
the part would shrink to a perect t. In reality,
however, shrinkage is rarely that simple.
Some resins shrink most in the direction
o resin fow within the mold, while others
shrink least in that direction. And, depending
on the shape o the part, shrinkage o later-
cooling areas o the part can pull against areas
that have already solidied, causing sink or
warping. To the extent that these problems
can be anticipated, they can be minimized.
There are our primary actors that
contribute to sink and warping:
Shrk characterstcs the res
resins dier in both tendency to shrink and
shrinkage relative to direction o resin fow.For example, an alloy o polycarbonate and
ABS is very resistant to shrinkage, while
glass lled nylon not only shrinks, but shrinks
less in the direction o resin fow within the
mold than perpendicular to resin fow.
Shape the part thick areas are
particularly prone to sink when the suraces
closest to the mold solidiy and are then
pulled inward as the underlying resin cools
and shrinks. Potential problem areas can beobvious, e.g., a thick wall. Or they can be
subtle, e.g., a boss nestled in an inside corner.
Sudde trasts r thck areas
t th these can result in stress and
warping at the point o transition.
Pr placeet ates pattern o resin
fow can lead to warping o the nished part.
Fortunately there are ways to address all
o these issues and eliminate or reduce
distortion o the nished part.
Kw the shrk characterstcs
ur chse resThese can be ound at www.ides.com.
As a very general rule, shrink under 0.010
inches/inch (or mm/mm) is more orgiving,
higher shrink demands a well designed part.
I the material is too shrink-prone or the
application, consider another material.
Put thck parts a det
Unnecessarily thick parts can sometimes
be put on a diet to prevent sink. I the
unction o the part requires the larger shape,coring out the thick section can produce a
hollow shape with thin walls, which will
serve the same unction (see Figure 1).
007 Protomold. All rights reserved. Vlue 4 n RESiST THAT SinKing FEELing n
Resist that skeeling1
With a ew notable exceptions(H2o or one) most materialsshrink as they cool and solidiy.
2
Fure 1
http://www.ides.com/http://www.ides.com/ -
7/31/2019 Protolabs Tips 4
16/20
Des Tps or Rapid Injection Molding
16007 Protomold. All rights reserved. Vlue 4 n RESiST THAT SinKing FEELing n
Redes ad relcate
I placement o a eature, such as
a boss, results in excess localized
thickness, consider redesigning or
relocating the eature (see Figure ).
Rap trasts ad
usupprted eetr
Transitions rom thick to thin can be
ramped to reduce stress and eliminate
warp. Whenever possible, gussets or some
other 3D structure bracing such corners
can prevent warping (see Figure 3).
Use Radused Crers
Un-radiused inside corners can overheat and
stress the resin fow, causing distortion o
the angles between walls. A radius in the
corner is always good practice (see Figure 4).
Place ates stratecall
Gate placement can help control warp when
using resins characterized by dierential
shrink in the direction o resin fow. The
disk shown here resulted rom a gate in the
center o the disk. Placing gates at the edge
o the disk reduced the warp (see Figure 5).
Visit the Prtld Des gude
or other helpul Rapid Injection
Molding design inormation.
Fure 2
Fure 5
Fure 3
4
5
Fure 4
Depending on theshape o the part,shrinkage o later-cooling areas othe part can pullagainst areasthat have alreadysolidied, causingsink or warping.
6
3
http://protomold.com/DesignGuidelines.aspxhttp://protomold.com/DesignGuidelines.aspx -
7/31/2019 Protolabs Tips 4
17/20
Des Tps or Rapid Injection Molding
17
As every engineer worth his or her sodium
chloride knows, one o the most undamental
mechanical devices is the inclined plane,
and a spirally-threaded cylinder or screw
may be its most commonly used orm.
I you are a plastic part designer, youve probably
incorporated threads into a design or will in the
uture. Wed like to pass along some suggestions
concerning the geometry o outside threads and
the limitations o the rapid injection moldingprocess to keep in mind when the time comes.
Most threads have undercut areas. Its just
a act o the geometry as the suraces o
the screw wrap around, regardless o the
orientation o the screw body. There are
several ways to deal with these undercuts.
The rst method is driven by the primary rule
o engineering: KISS: keep it super simple.
Fortunately, or some threads, we can ignore
the undercuts, machine what we can, and get a
unctional thread. For example, Figure 1 illustrates
a thread design that cannot be machined exactly
as its designed. The blue aces are assigned to
the B-side o the mold, and the green aces areassigned to the A-side o the mold. The thread
aces were split at vertical drat. Unortunately,
some aces overhang others (shown in red as
undercut aces), creating a mold that (even i we
could machine it) will interlock and cant open.
Figure shows the solution. We split the screw
at a horizontal plane that passes through the
axis. Faces above the plane are B-side, aces
below are A-side regardless o whether they
have reverse drat (shown by the dark blue
aces). When we design a mold or this thread
and machine it, we will leave a little extra metal
at the undercuts. When we mold parts, there
will be a little plastic missing in these areas.
The threads will be a little thinner than the CAD
model in those areas, but in most cases you cant
tell the dierence without a close examination.
007 Protomold. All rights reserved. Vlue 4 n THE inSiDE SCooP on oUTSiDE THREADS n
The inside scoop onutsde threads
Fure 1
Most threads have udercut areas.Its just a act o the geometry
Fure 2
-
7/31/2019 Protolabs Tips 4
18/20
Des Tps or Rapid Injection Molding
18
I the rst approach doesnt work, perhaps with an
acme thread or on a large screw, a second method
would be to modiy your design to eliminate the
undercut areas rom your thread. We call this
a hal-thread design. It involves cutting the
threads o the sides o your screw (see Figure 3).
The disadvantages o this are additional
design, less thread strength and intermittent
threads which might be dicult to screw in.
Lastly, i you need the ull thread, the way to
go might be to use cams. With a cam (side
action) on each side o the part, the undercuts
can be pulled and you get the ull strength
o the thread. Figures 4 and 5 illustrate this
approach. Disadvantages o this method
include our parting lines instead o two on your
thread and the additional cost or the mold.
Can we do outside threads? You bet! We
have a whole toolbox o methods or creating
external threads on your plastic part. To
learn more, submit a 3D CAD model or a
quote or call Protomold at 763-479-3680.
Visit the Prtld Des gude
or other helpul Rapid Injection
Molding design inormation.
Fure 4: Us sde acts
(cas) t prduce udercuts
Fure 5: Faces assed
thread us sde actsFure 3: Hal threaded part udercuts
007 Protomold. All rights reserved. Vlue 4 n THE inSiDE SCooP on oUTSiDE THREADS n
http://www.protomold.com/PartUpload.aspx?s=PMDT0807http://www.protomold.com/PartUpload.aspx?s=PMDT0807http://www.protomold.com/PartUpload.aspx?s=PMDT0807http://protomold.com/DesignGuidelines.aspxhttp://www.protomold.com/PartUpload.aspx?s=PMDT0807http://www.protomold.com/PartUpload.aspx?s=PMDT0807http://protomold.com/DesignGuidelines.aspx -
7/31/2019 Protolabs Tips 4
19/20
Des Tps or Rapid Injection Molding
19
Weve mentioned sliding shutos beore, but they
are both important enough and tricky enough to
deserve closer attention. Done right, they can
give you a lot o design fexibility; done wrong,
they can easily destroy a mold. See Figure 1.
The bottom o the clips hook and the blue
ace o the clips shat will be ormed by an
extension (shown by yellow lines) o the
A-side mold hal, which protrudes through a hole
in the base o the part. The rest o the clip is
ormed by the B-side mold hal. See Figure .
In this D diagram, red indicates sliding contact
between metal suraces rom the two mold
halves. (In the actual mold, there would be
three fat aces o the extension rom the
A-side mold hal making sliding contact with the
B-side mold hal.) This is called a sliding shuto,
telescoping shuto or a pass-through shuto.
As you can imagine, i these suraces are
parallel to the direction o mold closing, they
will rub against one another along their entire
length as the mold closes. Since the t o
the two mold halves must be tight to prevent
fash, there will be considerable riction and
wear along these aces as the mold opens and
closes, quickly ruining the mold. This causes
fash on the plastic parts under the clip head,
interering with the operation o your clip.
The solution is to drat the aces by at least three
degrees, so the aces approach one another
as the mold closes but do not actually touch
until the mold is ully closed. See Figure 3.
Sld Shut De
I youd like to see sliding shutos, both
poorly and well designed, in action, click here.
Visit the Prtld Des gude
or other helpul Rapid Injection
Molding design inormation.
007 Protomold. All rights reserved. Vlue 4 n SLiDing SHUToFFS (AgAin) n
Sliding shuts (again)
Fure 1 shws the eature we are
ld: a clp rs r a fat surace.
Fure 2 shws a sect vew
the eature the clsed ld.
Fure 3
Done right, they can give you a loto design feblt; done wrong,they can easily destroy a mold.
http://www.protomold.com/DesignGuidelines_PartRadiusingAndDraft.aspx#slidingshutoffhttp://protomold.com/DesignGuidelines.aspxhttp://www.protomold.com/DesignGuidelines_PartRadiusingAndDraft.aspx#slidingshutoffhttp://protomold.com/DesignGuidelines.aspx -
7/31/2019 Protolabs Tips 4
20/20
Des Tps or Rapid Injection Molding
0
Texture on a plastic part serves a variety o
purposes rom purely esthetic to purely practical.
Whatever the goal, there are a ew things to
remember to ensure that you get the texture you
want and that Protomold can eectively produce
what you speciy. On a surace lying perpendicular
to the direction o mold opening, texture is
relatively simple. (For eatures created by a side-
action, the same is true or a surace perpendicular
to the direction o side-action opening.)
Suraces parallel to the direction o mold opening
are more challenging. Consider what happens
when you drag your knuckles across the surace
o a brick. Thats pretty much what happens
to a plastic part when its surace is dragged
across the textured surace o an opening mold.
The solution, o course, is to drat the surace
so the mold surace moves away rom the part
surace as the mold opens. Thats true o any
surace parallel to mold opening, but even morecritical as the degree o texture increases.
Here are se udeles r
drat tetured suraces:
A 1-inch high rib with a smooth
nish requires 1 o drat.
The same rib with a PM-T1
nish requires 3 o drat.
The same rib with a PM-T
nish requires 5 o drat.
These requirements can impact other aspects o
your design as well. Take, or example, the scoop
shown in Figures 1 and . In Figure 1, the sides
o the scoop are ribs ormed in grooves cut into
the B-side mold hal. The two walls o the groove
must be drated in opposite directions to allow
the part to be ejected. As a result, the side walls
get thicker toward the back wall o the scoop.
In Figure , the side walls o the same scoop areormed between a cavity in the A-side mold hal
and a core in the B-side. In this case, the two mold
suraces that orm the side walls o the scoop are
drated in the same direction, resulting in side
walls that are o even thickness rom end to end.
Both versions o the part have the same side-wall
drat, but the one shown in Figure is the better
design since it maintains even wall thickness.
There are several ther pts t reeberwhe des tetured suraces:
Because texture is typically created by
processes like bead blasting the mold
aces, it may be impossible to texture
ribs ormed in deep, narrow grooves in a
mold. This is one more reason to use the
core/cavity approach rather than a deep-
groove rib approach or orming walls.
Very thick walls may shrink signicantly as
they cool, pulling the surace away rom the
mold ace beore it has ully cooled, thus ailing
to properly texture the surace. I your part
has mixed thick and thin areas there may be
ugly variations in texture. This is a unction
o part design and cant be processed out.
Very thin, textured walls may adhere
too aggressively to the mold ace and
be damaged during ejection.
Note that the design guidelines pertainingto textured suraces are similar to those
or any part, except that, when you add
texture, you must increase drat and pay
more attention to wall thickness.
Visit thePrtld Des gude
or other helpul Rapid Injection
Molding design inormation.
007 Protomold. All rights reserved. Vlue 4 n WHEn THingS gET RoUgH n
When things get
ruh
Fure 1
Fure 2
http://protomold.com/DesignGuidelines.aspxhttp://protomold.com/DesignGuidelines.aspxhttp://protomold.com/DesignGuidelines.aspx