protolabs tips 4

7/31/2019 Protolabs Tips 4

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Design Tips

NOBODYSFASTERIN THE SHORT RUN.

or Rapid Injection MoldingVlue 4

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Pae Se: Fit to page

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ormat or uture reerence

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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


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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
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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.
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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.

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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
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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


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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.

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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


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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.




007 Protomold. All rights reserved. Vlue 4 n THE oRPHAn FiLLET n

The orphan llet

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1

2

3


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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.


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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.




Fure 3 Fure 4


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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.


12/20


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.




Fure 3

Fure 4

007 Protomold. All rights reserved. Vlue 4 n WHEn yoU REALLy nEED To DoDgE THE DRAFT n


13/20


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



007 Protomold. All rights reserved. Vlue 4 n nigHT oF THE LiVing HingE n

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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.




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.


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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/


16/20


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).




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


17/20


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


18/20


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.




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


19/20


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.




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


20/20


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


The same rib with a PM-T


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.




007 Protomold. All rights reserved. Vlue 4 n WHEn THingS gET RoUgH n

When things get

ruh

Fure 1

Fure 2

protolabs tips 4

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