Ultrasonic energy has been used to join ther-moplastics for over 35 years. Ultrasonic weld-ing of thermoplastic materials is by far themost common form of ultrasonic assembly,and is used extensively in all major industriesincluding automotive, appliance, electronic,toy, packaging, textile, and medical. It offersadvantages in speed, economy, and efficiency,and is frequently chosen when parts are toocomplex or expensive to be molded in onepiece.

This bulletin provides guidelines to aid thedesigner during the initial concept stage of anew product design, ensuring optimum pro-duction results. The dimensions given in thedesigns should be used as guidelines only, sincethe specifics of your application may requirevariations. If you have questions or need assis-tance in designing your parts, contact yourlocal Branson representative, regional technicalcenter, or Branson headquarters in Danbury,Connecticut.

PPrriimmaarryy FFaaccttoorrss IInnfflluueenncciinngg JJooiinnttDDeessiiggnnAll of the following basic questions must beanswered prior to the design stage to gain atotal understanding of what the weld jointmust do:

• What type of material(s) is to be used?

• What is the overall part size and configura-tion?

• What are the final requirements of the part?

- Is a structural bond desired and, if so,what load forces does it need to resist?

- Is a hermetic seal required? If so, towhat pressure?

- Does the assembly require a visuallyattractive appearance?

- Is flash or particulate objectionableinside and/or outside?

- Any other requirements?

TThhrreeee MMaajjoorr JJooiinntt DDeessiiggnnCChhaarraacctteerriissttiiccssIn order to obtain acceptable, repeatable weld-ed joints, three general design guidelines mustbe followed:

1. The initial contact area between the mat-ing surfaces should be small to concen-trate and decrease the total energy (andthus the time) needed to start and completemelting. Minimizing the time the vibratinghorn remains in contact with the part alsoreduces the potential for scuffing, and sinceless material is moved, there is less flash.

2. A means for aligning the mating partsshould be provided. Features such as pinsand sockets, steps, or tongues and grooves,should be used for alignment rather thanthe vibrating horn and/or fixture, to ensureproper, repeatable alignment and to avoidmarking.

3. Horn contact and placement must beconsidered to provide proper bearing overthe joint area, to direct the mechanicalenergy and force to prevent marking of thecontact surface.

TTwwoo MMaajjoorr TTyyppeess ooff JJooiinntt DDeessiiggnnThere are two major types of joint design: theenergy director and the shear joint. All otherjoint variations can be classified under thesegeneral categories or as hybrids combiningaspects of both.

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The energy director is typically a raised trian-gular bead of material molded on one of thejoint surfaces. The primary function of theenergy director is to concentrate the energy torapidly initiate the softening and melting ofthe joining surface.

The diagrams in Figure 1 (next page) showtime-temperature curves for a common buttjoint and the more ideal joint incorporating anenergy director. The energy director permits

Part Design for Ultrasonic Welding

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rapid welding while achieving maximumstrength; material within the director generallyflows throughout the joint area. The energydirector is the most commonly-used design foramorphous materials, although it is also usedfor semi-crystalline materials.

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The basic design formula for the energy direc-tor design is illustrated in Figure 2. It is veryimportant to remember that the size and loca-tion of the energy director on the joint inter-face are dependent upon:

• Material(s)

• Requirements of the application

• Part size.

The peak of the energy director should be assharp as possible; energy directors that areround or flat at their peak will not flow as effi-ciently.

When using a relatively easy-to-weld resin(e.g., polystyrene which has high modulus[stiffness] and low melt temperature), a mini-mum height of 0.010 inch (0.25 mm) is sug-gested, whereas a semi-crystalline or high-tem-perature amorphous resin (e.g., polycarbonate)

would require a minimum height of 0.015inch (0.4 mm).

In the case of semi-crystalline resins (e.g., acetal,nylon) with an energy director, the maximumjoint strength is generally obtained only fromthe width of the base of the energy director.

As a rule, it doesn’t make any difference whichhalf of the part contains the energy director. Inspecial situations (as in combinations of differ-ent materials), the general practice is to placethe energy director on the part with the mate-rial that has the highest melt temperature andstiffness.

The energy director design requires a means ofalignment such as pins and sockets, aligningribs, tongue and groove designs, or fixturing.Knockout pins should not be placed in theweld area.

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The basic energy director design can be incor-porated into joint configurations other thanthe butt joint to gain additional benefits.Examples of joint design variations utilizing anenergy director include the following alternatives:

Step Joint — The step joint is used for align-ment and for applications where excess melt orflash on one exposed surface is objectionable.(Figure 3.) Note that 0.010 to 0.025 inch (0.25to 0.64 mm) has been added to the gap sur-rounding the perimeter of the part. This adds afeature called a “shadow line” to the design.

When welding is completed, a 0.010 to 0.025inch (0.25 to 0.64 mm) gap will be formedaround the periphery of the part, creating amore appealing appearance, since part-to-partvariations will be less noticeable. If the gapwere completely closed, it is likely that flashwould be formed in some outside areas, withslight gaps in others; whereas with the shadow

Figure 1. Time-Temperature Curves: Butt Joint vs. Energy Director Design

*

*

Figure 2. Basic Energy Director Design Formula

Figure 3. Step Joint

*NOTE: Typically a 90o included angle is used for amor-phous resins, while a 60o included angle is used for semi-crystalline resins. The included angle may vary dependingon materials, fillers, part geometry, or requirements. Foryour specific application, please contact your local salesengineer or regional office for recommendations.

line, minor variations in the parts are less like-ly to be noticed.

The design of the energy director uses the samebasic design thought process used in the buttjoint energy director (i.e., material, require-ments, part size). Note that a minimum wallthickness of 0.080 inch (2 mm) is recom-mended for this design.

Tongue and Groove — The major benefits ofusing this joint design are that it prevents flash,both internally and externally, and providesalignment. Containment of the materialenhances the attainment of hermetic seals. Theneed to maintain clearance on both sides of thetongue, however, makes this more difficult tomold. Also, because of the reduced weld area, it isgenerally not as strong as a step joint. (Figure 4.)

The design of the energy director uses the samebasic design thought process used in the buttjoint energy director (i.e., material, require-ments, part size).

Textured Surface** — This feature is exclu-sively used in conjunction with an energydirector. Molding a textured surface on the

mating part tends to improve the overall weldquality and strength by enhancing frictionalcharacteristics and melt control. (Figure 5.)

Usually the texture is 0.003 to 0.006 inch deep(0.076 to 0.152 mm), and is varied based onthe height of the energy director. In most cases,

the advantages include increased weld strength,reduced flash or particulate, reduced weldtimes, or lower amplitude requirements.(Branson TechnoLog TL-4 provides details onthis concept.)

Criss-Cross — This design incorporates energydirectors on both mating sections that are per-pendicular to each other, and provides mini-mum initial contact at the interface whileallowing a potentially larger volume of materi-al involvement. This can result in increasedstrength in the weld. (Figure 6.) Each energydirector should be dimensioned at approxi-mately 60% of the size that would be used in astandard single energy director design, with anincluded angle of 60o versus the standard 90o.

If an air- or liquid-tight seal is required, it isrecommended that the corresponding energydirectors be continuous, like a saw tooth.(Figure 7.) The corresponding saw-tooth ener-gy directors must be located on the part thatwill be contacted by the horn. Note that thisdesign generates a very high material flow;therefore, containing flash should be addressedin part design (e.g., use a tongue and groove orstep design).

Energy Director Perpendicular to the Wall —Used to gain resistance to peeling forces and toreduce flash. (Figure 8.) This design should beused when only a structural seal is required.

Interrupted — Used to reduce the overall areaand subsequent energy or power level required,or to minimize part marking. Use only where

*

Figure 5. Textured Surface

Figure 4. Tongue and Groove Joint Design

Figure 6. Criss-Cross Energy Director

Figure 7. Criss-Cross Design for Air- or Liquid-Tight Seals

Branson/MoldTech PatternTexture Designation Depth

Branson 300/11040 0.003''

Branson 450/11050 0.0045''

Branson 600/11050-0.006 0.006''

**This design is covered by Branson Patent No. 4,618,516. License toutilize this design is granted via the purchase of Branson equipment.

*See note on previous page.

structural (non-hermetic) seals are needed.(Figure 9.)

Chisel Energy Director — Typically used whennominal wall thickness is 0.060 inch (1.524mm) or less. (Figure 10.) If a standard energydirector is used, it will be too small (less than0.010 inch/0.254 mm tall), thus resulting inlower weld strengths. The knife edge can be0.015 inch to 0.020 inch (0.381 to 0.508 mm)tall and should utilize a 45o angle. As weldstrength will be limited to weld width, a tex-tured surface should always be added whenusing this design.

Specialized Joints — In order to achieve a her-metic seal in less easily welded resins or irregu-lar shapes, it may be necessary to use a com-pressible seal or a convoluted path for meltflow. Figure 11 shows a joint design incorpo-rating an O-ring. It is important to note thatthe O-ring should be compressed a maximumof 10 to 15%, only at the end of the weld. Pinsand sockets (stud welding) can also be usedsuccessfully with an O-ring design. (See datasheet PW-5.)

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An energy director type of joint design in somecases may not produce the desired results withsemi-crystalline resins such as nylon, acetal,polypropylene, polyethylene, and thermoplas-tic polyester. This is due to the fact that semi-crystalline resins change rapidly from a solid toa molten state, and back again, over a relative-ly narrow temperature range. The moltenmaterial flowing from an energy director,therefore, could re-solidify before fusing withthe adjoining interface. The weld strength in asemi-crystalline resin could be limited to thebase width of the energy director. A shearjoint configuration is recommended for theseresins where geometry permits.

With a shear joint design, welding is accom-plished by first melting the small, initial con-tact area and then continuing to melt with acontrolled interference along the vertical wallsas the parts telescope together. (Refer to Figure12.) This allows a strong structural or hermet-ic seal to be obtained as the molten area of theinterface is never allowed to come in contactwith the surrounding air. For this reason, theshear joint is especially useful for semi-crys-talline resins.

The strength of the welded joint is a functionof the vertical dimension of meltdown of thejoint (depth of weld), which can be adjusted tomeet the requirements of the application.

Note: For joint strength meeting or exceedingthat of the wall, a depth of 1.25X the wallthickness is recommended.

IMPORTANT: The shear joint requires rigid sidewall support to prevent deflection during weld-ing. The walls of the bottom section must besupported at the joint by the holding fixture,which conforms closely to the outside configu-ration of the part. The top part should be ofsufficient structural integrity to withstandinternal deflection.

Figure 9. Interrupted Energy Director

Figure 11. Joint Design with O-Ring for Hermetic Seal

Figure 10. Chisel Energy Director

0.040"(1.016 mm)

Figure 12. Shear Joint Design

Figure 8. Energy Director Perpendicular to the Wall

For a midwall joint, the tongue and groove vari-ation shown inFigure 13 is pre-ferred. It is alsouseful for largeparts. Interferenceon one side onlyis recommended.

Figure 14 shows variations of the basic shear jointdesign.

It should benoted that ashear joint isnot recom-mended forparts with am a x i m u mdimension of3.5 inch orgreater, sharpcorners, or

irregular shapes. This is due to difficulty inholding the molding tolerances necessary toobtain consistent results. An energy directortype joint would be suggested for parts fallingoutside of these parameters (i.e., 3.5 inch,sharp corners, irregular shapes).

When welding parts that need a structuralweld only (strength or air-tight seals are notrequired), use the shear joint design with inter-rupted vertical energy directors shown inFigure 15. This design reduces the overall areaand subsequent energy or power required toweld the parts. The potential for part markingis also minimized.

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The following table gives general guidelines forinterference and part tolerance in relation tomaximum part dimension.

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Sharp corners localize stress. When a moldedpart with stress concentration is subjected toultrasonic mechanical vibrations, damage(fracturing, melting) may occur in the highstress areas. This can be remedied by having agenerous radius (0.020 inch/0.508 mm) oncorners, edges, and junctions. At a minimum,all corners or edges should be broken. (Refer toFigure 16.)

Holes or voids like ports or other openings inthe part being contacted by the horn can createan interruption in the transmission of theultrasonic energy (Figure 17). Depending onthe type of material (especially semi-crystallineresins) and the size of the hole, little or nowelding will occur directly beneath the open-ing. When a hole or bend exists in the part,the resin dampens the transmission of energy,making it difficult to pass vibrations from the

Figure 15. Interrupted Shear Joint Design

Figure 14. Shear Joint Variations

Figure 16. Radius of Corners to Reduce Stress

Figure 13. Tongue and Groove Variation

Figure 17. Interference with Energy Transmission

Maximum Part Interference per Part Dimension Dimension Side (Range) Tolerance

Less than 0.75" 0.008" to 0.012" ±0.001"(18 mm) (0.2 to 0.3 mm) (±0.025 mm)

0.75" to 1.50" 0.012" to 0.016" ±0.002"(18 to 35 mm) (0.3 to 0.4 mm) (±0.050 mm)

1.50" to 3.50" 0.016" to 0.020" ±0.003"(38 to 90 mm) (0.4 to 0.5 mm) (±0.075 mm)

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horn contact point to the interface of the partsto be welded, especially in shear.

Careful attention must be given to the designof parts to avoid such problem areas. Air bub-bles within the plastic, due to improper vent-ing in the mold tool, will also dampen thetransmission of vibrations or cause the materi-al to blow out in these areas.

Near-field vs. far-field welding—Near-fieldwelding refers to a joint 0.250 inch (6.35 mm)or less from the horn contact surface; far-fieldwelding to a joint more than 0.250 inch fromthe horn contact area. (Figure 18.)

As mentioned above, because of their structuresemi-crystalline resins dampen the transmis-sion of energy, making them more difficult toweld far-field. In amorphous plastics, the ran-dom arrangement of molecules allows thevibrations to pass through easily with littleattenuation. Dampening also takes place inlow-stiffness resins (low modulus). Care mustbe taken when designing parts to allow ade-quate transmission of energy to the joint area.

Appendages, tabs, or other details molded onto the interior or exterior surfaces of the mold-ing can be affected by the mechanical vibrations,resulting infracturing.(See Figure19.) The fol-lowing arerecommend-ed to mini-mize or elim-inate this:

• The addition of a generous radius to thearea where the appendages intersect themain part.

• Dampening of flexure through externalmeans.

• Increasing the material thickness.• Evaluation of other frequencies.

Diaphragmming—an “oil-canning” effectwith related burn-through—typically occurs inflat, circular parts in thin-walled sections of thepart (e.g., a livinghinge) contacted bythe horn. (See Figure20.) This can be cor-rected by one or acombination of thefollowing:

• Shorter weld time• Higher or lower amplitude• Amplitude Profiling TM

• Nodal plunger on horn• Thicker wall section• Internal support ribs• Evaluation of other frequencies.Horn contact and placement can play amajor role in the successful welding of moldedparts. In general, the horn should be largeenough to overhang the perimeter of the parts,so it is bearing directly over the joint area.(Refer to Figure 21.) Doing this helps directthe mechanical energy and prevent marking ofthe contact surface. The surface of the horn orpart can also be raised over the weld area toprovide better contact; this will improve theconsistency of energy transmission. Note: Thearea of horn-to-part contact must be largerthan the total weld area. Failure to do thiscould lead to surface marking.

Figure 18. Near-Field and Far-Field Welding

Figure 19. Part Surface Detail Design

Figure 21. Proper Horn Contact and Placement

Figure 20. Diaphragmming