Plastic Assembly News

Ultrasonic Weld Joint & Part Design

Energy Director Design

While it is possible to ultrasonically weld plastic materials without specific joint design details, the weld process is greatly enhanced by adding proven features that aid in the process.  For example, without a means of alignment built into the joint it is impossible to predict where the parts will be positioned after welding.  This is due to the nature of the vibratory process created by ultrasonic welding.  This means of alignment is accomplished through the use of step joints, tongue and groove joints, pins and sockets, raised walls, ribs or other features that are used to keep the vibratory process under control to maintain the desired alignment of the parts after welding. 

 

raised-wall-joint14step-joint2

tongue-and-groove21                               

A  feature that is shown in these joint alignment pictures is the molded-in triangular shape of material called an energy director.  This molded-in triangular ridge of plastic is very effective at reducing the cycle time to achieve a weld and in compensating for non-uniform wall surfaces.  The energy director design has been used for years as a means of focusing the energy to improve weld strength and reduce cycle time.  The energy director is typically  placed only on one half of the part and runs along the surface to be welded.  Without this energy director the weld quality would be suspect for many applications. The peak of the energy director should be sharp with a triangular shape formed from a 60º or 90º included angle.  A 60º angle is generally used with crystalline materials and a 90º is used with amorphous materials.  Material types by polymer structure are illustrated below:

std-energy-director2 

Amorphous Resins 90º Angle

 

Semi-Crystalline Resins 60º Angle

ABS-Acrylonitrile Butadiene Styrene

PA- Polyamide  (Nylon)

ABS/PC-ABS/Polycarbonate

 

PBT-Polybutylene terephthalate (Polyester)

ASA-Acrylonitrile Styrene

 

PE-Polyethylene

Acetate

PEEK-polyetheretherketone

PC-Polycarbonate

PET-Polyethylene terephthalate (Polyester)

PEI-Polyetherimide

PMP-Polymethylpentene

PES-Polyethersulfone

POM- Polyacetal

PMMA-Acrylic

PP-Polypropylene

PPO-Polyphenylene Oxide

PPS-Polyphenylene sulfide

PS-Polystyrene

 

PSU-Polysulfone

 

PVC-Polyvinyl Chloride (Rigid)

 

SAN-Styrene Acrylonitrile

 

SBC-Styrene Block Polymers

 

 

   

energy-director1Depending upon the wall thickness and the application, the energy director typically varies in height in a range from .010 to .035 of an inch.  The minimum height recommended is .010 for most amorphous materials, .020 for semi-crystalline materials and the amorphous polycarbonate material.  While we’ve seen energy director heights of .060 of an inch tall, most energy directors don’t exceed .035. Using a 90º energy director design, the height of the energy director is often determined by the width of the wall where the width of the wall is divided by eight, so that we have an energy director height equal to W/8.  

Without some designed in approach to controlling flash, an energy director by itself has the potential for molten material to flow beyond the wall creating flash outside the weld joint.  If flash is a problem the use of step joints, tongue and groove joints, flash traps, raised walls and energy director placement have all been employed as a means of controlling the material flow.   

Two types of joints commonly used to control flash and provide alignment are the step joint and the tongue and groove. 

step-joint-21 tongue-and-groove

Both of these joints provide excellent flash control and the alignment necessary for a good ultrasonic weld joint design.  The tongue and groove design provides the added benefit of being as an excellent reservoir for the melted material. This pooling of plastic material helps contain the material and increases the likelihood of a hermetic seal. The additional height of .010 to .025” added as a gap around the periphery of the part provides a shadow line that helps to hide variation with part tolerances and melt flow during the ultrasonic welding process.  This is a feature that we highly recommend because it helps to eliminate potential flash caused by the mating perimeter surfaces touching and it becomes imperceptible to the eye when the weld melt down is not perfect. 

 energy-director2

 

 

 

 

 

Recent advancements with the energy director design have resulted in the use of the criss-cross energy director design. Essentially, the criss-cross energy director design utilizes the standard energy director shape where a triangular shaped bead of material is molded into the plastic wall. The energy director has typically been placed only on one half of the part and runs along the surface to be welded.  Although it usually doesn’t matter which of the parts to be welded incorporate the energy director, it is probably more common to find the energy director on the half that the horn contacts.  With the use of the criss-cross design, additional energy directors are added to the mating part, which increases the amount of material interaction.  On the mating surface opposite the perimeter energy director, a series of perpendicular energy directors are molded-in to mate with the perimeter energy director.  See the illustration above. 

When a hermetic seal is desired these additional energy directors should take on a saw tooth pattern with each energy director repeating from the base of the proceeding energy director.  Because there are energy directors on both mating surfaces, the energy director height on each half should be reduced to prevent excessive material flash during welding.  Typically, it is recommended that the criss-cross energy director height be approximately 60% of the standard design.   

The criss-cross energy director joint design is particularly effective when used in combination with a tongue and groove.  Because of the increased material flow with the criss-cross energy director design, it is recommended that a tongue and groove joint be used to capture the additional plastic and contain the flash. 

This criss-cross design certainly increases the mold cost, but we’ve seen applications where the weld strength has far exceeded expectations and hermetic bonds have been achieved that might not have been achieved using the standard energy director on one mating surface.   

Shear Joint Design

shear-jointdoc

shear-joint-interference-guidelinesAlthough the energy director design can work satisfactory for welding semi-crystalline plastic materials, there are circumstances that warrant the use of another type of joint design.  When welding materials like nylon, polyacetal, polyester, polyphenylene sulfide, polypropylene or polyethylene and a hermetic seal is required, the shear joint has proven beneficial for many applications.   Therefore, the shear joint is frequently used when a hermetic seal is required on semi-crystalline plastic materials.  

The reason for using a shear joint relates to the way the semi-crystalline material flows when introduced to heat. These types of plastics have a very sharp melting point and once the heat is reached to create a melt flow, the material immediately becomes liquid and flows rapidly.  Semi-crystalline plastics also re-solidify within a small temperature variation.  When an energy director is used with semi-crystalline plastics, the molten material gets exposed to a lower temperature when outside of the melt zone and can re-solidify before the desired bond is achieved.  Because the melt zone of a shear joint is held along a vertical wall, the temperature variation is reduced within this melt zone since the material is not easily exposed to the surrounding air.  

The shear joint is not the answer for all applications.  We recommend caution when applying the shear joint to polypropylene or polyethylene due to the lack of rigidity in these materials.  The lack of stiffness with these materials often results in pushing past the shear joint instead of melting and welding the material at the joint interface.  The shear joint requires high tolerance, so we also recommend caution incorporating a shear joint design on larger parts.  The shear joint should not be used when a part is larger than 3 ½” in diameter or has sharp corners or unusual shapes.  When you are welding semi-crystalline parts under one or more of these caution conditions, the 60º energy director should be considered.  If a hermetic seal is required on a product made from semi-crystalline resins and an energy director should be used because of the material, size, shape or tolerance requirements, the use of the criss-cross energy director design has proven successful for specific applications.  Reference the information above for welding semi-crystalline material with an energy director.

 

PLASTIC MATERIAL REFERENCE GUIDE FOR ULTRASONIC WELDING

The chart below includes recommended amplitudes for ultrasonic welding. These values are reference points for the required gain needed to weld the plastic material. The amplitude may need to be adjusted to achieve the best results for a particular application. (The amplitude level presented in the chart is displayed in microns-to convert to inches, 25.4 microns is equal to .001 of an inch)

Material Welding Insertion Staking Near Field Welding Far Field Welding Ease of Staking Trade name(s
AMORPHOUS RESINS
ABS-Acrylonitrile Butadiene 30-70 20-50 30-80   1 1 1 Cycolac, Lustran
Styrene                
                 
ABS/PC-ABS/Polycarbonate 70-100 50-70 80-120   2 2 2 Cycoloy,  Pulse
                 
ASA-Acrylonitrile Styrene 30-70 20-40 70-90   1 1 3 Centrex, Geloy, Luran
Acetate                
                 
PC-Polycarbonate 50-90 40-70 50-90   1 2 3 Lexan, Calibre, Novarex
                 
PEI-Polyetherimide 70-100 40-70     2 4 5 Ultem
                 
PES-Polyethersulfone 70-100 40-70     2 4 5  Ultrason
                 
PMMA-Acrylic 40-70 30-60 70-90   1 3 3 Acrylite, Plexiglass, Zylar
                 
PPO-Polyphenylene Oxide 50-90 40-60 60-90   2 3 2 Noryl
                 
PS-Polystyrene 20-40 20-40 70-90   1 1 4 Dylark,   Styron
                 
PSU-Polysulfone 70-100 40-70 90-120   2 3 3  Udel
                 
PVC-Polyvinyl Chloride (Rigid) 40-80 20-50 70-100   2 4 2 Novablend, Ultrachem
                 
SAN-Styrene Acrylonitrile 30-70 20-40 70-90   1 1 3 Lustran, Styvex
                 
SBC-Styrene Block Polymers 50-90 30-50 80-100   2 3 2  K-Resin
                 
SEMI-CRYSTALLINE RESINS
PA- Polyamide  (Nylon) 70-120 40-80 60-120   2 5 3 Celstran, Ultramid,  Zytel
                 
PBT-Polybutylene terephthalate  70-125 40-80 90-120   3 5 4 Celanex, Ultradur,  Valox
(Polyester)                
                 
PE-Polyethylene 70-120 40-80 40-120   2 5 2 Aspun, Clysar, Dowlex
                 
PEEK-polyetheretherketone 60-125 40-80     3 5 5 Arlon
                 
PET-Polyethylene terephthalate  80-120 40-80 90-120   3 5 4 Mylar, Rynite, Cleartuf
(Polyester)                
                 
PMP-Polymethylpentene 70-120 40-80 90-120   4 5 2 TPX
                 
POM- Polyacetal 75-125 40-80 50-100   2 3 4 Acetal, Celcon,   Delrin
                 
PP-Polypropylene 70-120 40-80 40-120   2 5 2 Astryn, Fortilene, Marlex
                 
PPS-Polyphenylene sulfide 80-125 40-80     3 4 5 Fortron, Ryton, Supec

 

Notes: 

   Far and near field welder refers to the distance from the ultrasonic horn contact

            surface to the weld joint.  Generally, any distance in excess of 6.35 mm or .250″ is

            considered far field. 

  –The ease of welding and staking guide is rated from 1 to 5 with 1  

    equaling the easiest and 5 equaling the most difficult. 

 – The ability to weld plastic is based upon a lot of factors including joint design,    

    material fillers, process variables prior to welding, part geometry, good part design 

    practice, part size, amplitude, fixturing, properly designed ultrasonic horns and the 

    material flow during energy transfer. This chart displaying the ease of welding is meant to  

    serve as a guide only.

Criss-Cross Energy Director Design For Ultrasonic Welding

CRISS- CROSS ENERGY DIRECTOR DESIGN

 

 

The use of the criss-cross energy director design has proven to be beneficial for many ultrasonic plastic welding applications. This weld joint has been used in the medical, electronic and cc-energy-directorautomotive industries and has been a good joint design for achieving strong bonds with hermetic seals.  Essentially, the criss-cross energy director design utilizes the standard energy director shape where a triangular shaped bead of material is molded into the plastic wall. This molded-in triangular ridge of plastic is very effective at reducing the cycle time to achieve a weld and in compensating for non-uniform wall surfaces.  Depending upon the wall thickness and the application, the energy director typically varies in height in a range from .010 to .035 of an inch.  The peak of the energy director should be sharp with a triangular shape formed from a 60º or 90º included angle.  The energy director design has been used for years as a means of focusing the energy to improve weld strength and reduce cycle time.  The energy director has typically been placed only on one half of the part and runs along the surface to be welded. Without this energy director the weld quality would be suspect for many applications. The criss-cross design adds additional energy directors to the mating part, which increases the amount of material interaction.  On the mating surface opposite the perimeter energy director, a series of perpendicular energy directors are molded-in to mate with the perimeter energy director.  When a hermetic seal is desired these additional energy directors should take on a saw tooth pattern with each energy director repeating from the base of the proceeding energy director.  Because there are energy directors on both mating surfaces, the energy director height on each half should be reduced to prevent excessive material flash during welding.  Typically, it is recommended that the criss-cross energy director height be approximately 60% of the standard design. 

The joint is particularly effective when used in combination with a tongue and groove joint design.  The tongue and groove design provides the alignment necessary for a good ultrasonic weld joint and the groove serves as an excellent reservoir for the melted material. This pooling of plastic material helps contain the material and reduces the likelihood of a leak path.

tongue-and-grooveBecause of the increased material flow with the criss-cross energy director design, it is recommended that a tongue and groove joint be used to capture the additional plastic and contain the flash.

This criss-cross design certainly increases the mold cost, but we’ve seen applications where the weld strength has far exceeded expectations and hermetic bonds have been achieved that might not have been achieved using the standard energy director on one mating surface.   As always, each application is unique and should be evaluated thoroughly before implementing a joint design.       

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