ultrasonic plastic welding basics

7/27/2019 Ultrasonic Plastic Welding Basics

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Ultrasonic Plastic Welding Basics

By Janet Devine, President Sonobond Ultrasonics

This ultrasonic welding primer will help you understand how joint design requirementsand selection of welding machine frequency affect the finished product.

MANUFACTURERS constantly search for equipment that will increaseproduction, reduce reects and otherwise impro!e their efficiency" Ultrasonic weldin# $$

used in the medical, electrical, automoti!e, pac%a#in#, toy, housewares, cosmetics and

other industries $$ achie!es those o&ecti!es"

Ultrasonics can &e used to insert metal fasteners in thermoplastic materials, to form a

plastic ri!et, to de#ate plastic parts and to cut and seal films and fa&rics, as well as to oinplastic parts to#ether"

'ow does it wor%( Ultrasonic !i&rational ener#y at the interface of the plastic parts &ein#

oined causes the plastic to soften and flow in a fraction of a second" )hen the material ispressed to#ether and resolidifies, the &ond is made"

No #lues or sol!ents are needed" Toolin# can &e desi#ned to secure and ali#n the parts"'eatin# is confined to the interface area so the assem&led part is not too hot to handle"

Equipment can &e inte#rated into automated lines"

There are two &asic techniques* plun#e weldin# and continuous weldin#" +n plun#e

weldin#, the parts are placed under a tool or horn the horn descends to the part undermoderrate pressure and the weld cycle is initiated" +n the continuous weldin# process, the

horn may -scan- the part, or, with films and fa&rics, the material is passed o!er or under

the horn on a continuous &asis"


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Principles of OperationEvery ultrasonic unit contains the following five elements:

. A power supply that takes line power at 50/60 cycles and changesit to a high ultrasonic frequency of 20000 cycles per second orhigher!

. A converter or transducer that contains pie"oelectric crystals that

change the incoming high#frequency electrical signal to mechanicalvi$ration of the same frequency!

. A $ooster that transmits the vi$rational energy and increases itsamplitude!

. A horn to deliver the vi$ration energy $y contact with the parts to$e welded!

. An anvil or nest to support the workpiece! %or $onding ofte&tilesthe pattern wheel replaces the anvil!

Fi#" / $ Ultrasonic weldin# equipment such as this Sure)eld line from Sono&ond comes in a !ariety of si0es and styles, includin#

microprocessor$controlled units, modular units and hand$held !ersions"

Equipment

Systems ha!e e!ol!ed into a ran#e of si0es and styles to suit a wide ran#e of applications$ Fi#" /" The most common type is a press" The weldin# press is equipped with a

pneumatic system to supply the necessary contact force and the head is mounted on a

slide so it can &e raised and lowered to contact the part to &e welded" +t is important the

press &e ri#id so &endin# deflections do not affect the weld consistency"

The head is usually acti!ated &y a palm &utton set" The ultrasonic ener#y can &e started

ust &efore the horn contacts the part, after contact &ut &efore full pressure is reached or

when full preset pressure is reached" After the ultrasonic ener#y is stopped, there isusually a short delay &efore pressure is released to permit solidification of the plastic to

occur"

Many units are a!aila&le with microprocessor controls" These permit control of the weld

&y time, ener#y or distance, whereas the con!entional weldin# machine is controlled &ytime only" Microprocessor$equipped systems May also ha!e a port to transfer data to a

printer or computer for stora#e or for further analysis" Some systems can recei!e data

from an e1ternal computer permittin# remote control"

Modular units are a!aila&le, suita&le for incorporation into automated equipment forspecial$purpose machines" Some small units are a!aila&le in handheld !ersions"

2ower le!els for 34$%'0 equipment are a!aila&le at as hi#h as 5444 )" Most equipment

sold falls into the 644$3444 ) ran#e" 7esi#n requirements dictate that the hi#her thefrequency the smaller the unit" This also means hi#her frequency units ha!e less powerhandlin# capacity and are suita&le for the smaller, more delicate or precision parts"

Equipment is a!aila&le with frequencies of 34, 58, 94 and :4 %'0"

For continuous weldin# of film and fa&ric, the ultrasonic system may &e &uilt into a ta&le

that resem&les a sewin# machine" The system is equipped with a rotatin# wheel that canem&oss or cut the fa&ric with a wide ran#e of patterns" ;ther arran#ements include


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multiple head systems used for wide roll #oods these may &e used to quilt, slit or em&oss

fa&rics"

HornsThe horn is the part of the ultrasonic system that contacts the parts to &e oined" The horn

is desi#ned to resonate at the frequency of the ultrasonic system" )hen the horn !i&ratesit stretches and shrin%s in len#th &y a small amount" This motion is referred to as the

amplitude of the horn"

Amplitude is measured as the pea%$to$pea% motion at the face of the horn" +ncreasin# the

!olta#e to the transducer or chan#in# the &ooster or the #eometry of the horn can chan#e

this !alue" Some plastics respond &etter at hi#her amplitude"

The material of which the horn is made must ha!e #ood acoustical and mechanicalproperties" These properties are usually found in low$density materials such as aluminum

or titanium" 'owe!er, steel or nic%el alloys are sometimes used when wear is a factor"

Aluminum can &e chrome plated and titanium may &e car&ide coated or car&ide tipped toreduce wear"

Horn Geometry

The shape and si0e of the parts to &e welded usually dictate the horn face" The

requirement that the horn &e a resonant mem&er of the system dictates some constraints

The most commonly used horns are stepped cylinder or stepped &ar horns

$Fi#" 3" These are simple to machine and capa&le of hi#h #ain


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Sharp transitions and tool mar%s should &e a!oided &ecause they can cause stress risers

that lead to crac%in# of the horn" To pre!ent unwanted !i&ration in the width direction,

horns wider than 5$in"


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minerals, mold release a#ents, plastici0ers and UD sta&ili0ers that can act to lu&ricate the

surface and a&sor& moisture, either of which can ma%e ultrasonic weldin# more difficult"

;n the other hand, if at low le!els, the addition of #lass fi&ers, impact modifiers andre#rind material may increase welda&ility"

Fi#" 9 $ Compati&ility of thermoplastics for ultrasonic weldin#"

Joint Design

To ensure that plastic assem&lies are adequately oined, they should &e desi#ned at

inception with a suita&le oint desi#n" Many factors are ta%en into consideration* the

material to &e &onded, the ultimate use of the product, the cost and ease of moldin# andthe location of the oint surface relati!e to the horn"

The oint surface should &e appro1imately perpendicular to the !ertical a1is of the

ultrasonic system and parallel to the face of the horn" The oint surface should &e in one

plane" The distance &etween the horn face and the oint should &e within /?9 in" "58mm= of the horn face" This is referred to as a near field weld" Far field weldin# is the term

used when the oint is farther away and is only done when the plastic material canefficiently transmit the ultrasonic ener#y to the oint location" +n #eneral, this limits far

field weldin# to ri#id amorphous materials, althou#h some semicrystalline materials can&e welded far field #i!en a fa!ora&le #eometry"

The oint #eometry should &e tailored to the end product use" The parts must not fit so

ti#htly &efore oinin# that they inhi&it the !i&ration needed to induce weldin#" Thin crosssections may crac% under the action of !i&ration, and delicate parts, such as fine wires,

may &ecome dama#ed when their enclosures are welded" ;&!iously, the ideal conditions

are not always attaina&le and compromises can &e made"

The simplest type of oint desi#n uses a trian#ular or, less frequently, a roundedproection called an ener#y director" The function of the ener#y director is to pro!ide a

site to initiate the rapid plastici0ation of the oinin# surface &y concentration of the

ener#y"

Trian#ular ener#y directors ha!e an an#le of >4 or 4 de#rees and a hei#ht from 4"446 to4"494 in"


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centerin# the parts is not present, then the nest or holdin# fi1ture must pro!ide this

feature, althou#h this is not the preferred method"

+f the parts ha!e thin walls that may &ul#e under pressure, it is ad!isa&le to support thepart up to the oinin# 0one"

)hen weldin# with an ener#y director, the idea is to melt and collapse the ener#y

director" +t is important the molded parts, the an!il or the stro%e limitation of the press

does not impede this motion"

A num&er of oint desi#ns usin# a 4$de# ener#y director are shown in Fi#" 8"

These include &utt oints, step oints and ton#ue and #roo!e oints" The 4$de# ener#ydirector is suita&le for most amorphous resins &ut >4$de# is preferred for polycar&onate,

acrylics and semicrystalline materials"

Crystalline materials may recei!e incomplete fusion when an ener#y director is used

&ecause material displaced from the ener#y director may solidify &efore it flows across

the oint to form a seal" For this reason, shear oints are often preferred" A carefully

desi#ned shear oint can also achie!e lea%ti#ht oints"

Fi#" 8 $ @asic ener#y director desi#ns"

A shear oint uses an interference fit &etween the walls of the parts to &eoined $ Fi# >" This necessitates the meltin# and mo!in# of molten plastic at the oint as

the parts are simultaneously su&ected to ultrasonic ener#y and downward force" @ecause

a shear oint melts lar#er amounts of material, it may require hi#her power and lon#er

time than a oint made usin# an ener#y director" The shear oint also tends to impartsideways motion into the part, so the side walls should &e well supported &y the nest or

holdin# fi1ture" +f necessary, the fi1ture can &e split to permit easier loadin# andunloadin# of the parts"

The shear oint should also ha!e a lead$in to pro!ide self$ali#nment of the parts" The

initial contact area at the &ase of the lead$in should &e small" The desi#n shown in Fi#" >

is for a medium$si0e part of up to 3 in"


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+n #eneral, semicrystalline parts up to a&out 5 /?3 in" $ A shear oint desi#n"

Other onsiderations

Part !i"e

2art si0e has an influence on the power le!el required and the frequency of the weldin#

machine selected" For lar#e parts #reater than / /?3 in"


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Ultrasonic te1tile oinin#, primarily in nonwo!ens is used in medical #owns, &ooties,

caps, face mas%s, hy#iene products, incontinence products, &ed protectors, sur#ical

drapes, pillow co!ers and filter media"

ultrasonic plastic welding basics

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