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ULTRASONIC WELDING
ULTRASONIC WELDING APPLICATIONS
  
Ultrasonic welding is first introduced in early 1960's. It is now a widely accepted process today. High-frequency friction energy is introduced to two contacted plastic surfaces and thus the plastic is melted due to the high heat generated from great friction forces. By the clamping of upper and bottom dies, the melted plastic is formed after cooling. And then two plastic parts are joined together without any glue or adhesives. Producing time and cost are signficiantly saved and these are the main reasons that ultrasonic welding is greatly adopted.

PLASTIC WELDING

Standard plastic welding

Energy Director is a V-shaped elevation on one of the two contact surfaces. It causes a line contact as the ultrasonic vibrations begin to melt the plastic. 

The line contact focuses the ultrasonic energy. As the melt progresses, the energy director turns fluid and flows to fill the space between the two parts. 

The downward force of the horn causes the melted material to spread over the entire contact surface.

Fusion forming

In addition to traditional ultrasonic welding, fusion forming by means of ultrasonic offers a very wide range of possible applications:
  • Riveting
    The horn transfers the mechanical oscillation energy to the rivet spigot. It is the riveting tool while at the same time being worked on the face side to the desired rivet headshape. Within allowable area, it is possible to make several rivets with one shot. Or multiple press head system is adopted for riveting of huge parts, such as bumpers or dashboards of automobiles.

  • Flanging
    The flanging technique is known from metal-working. Additional plastic parts can also be fixed to main body by Flanging. This process is even adoptable for join parts in different materials. The horn surface has different design depending on the required tasks.

  • Embedding
    Threads, screws, or other metal parts are capable to insert into plastic by Ultrasonic Embedding. Sustain torque and stability of embedded parts differs from the size and shapes. Even the parts are not in the same level, it is also possible to complete embedding with one shot by composite horns.


These processes considerably extend the use of ultrasonic. They offer the possibility of formlocked combining of thermoplastic synthetics with other materials – metals, glass or dissimilar plastics. Unlike welding, in the case of fusion forming only one plastic part is locally plasticized and shaped in its viscous state.

Forming by ultrasonic has important advantages over other techniques. Because the forming takes place in the melting phase, only negligible stresses arise in the shaped parts – provided the machinery is correctly adjusted. The problem of stress relaxation is practically non-existent. Fixed connections with no play in them are achieved, coming up to very exacting demands, even in their longterm behaviour.















          

METAL WELDING

Ultrasonic welding of non-ferrous metals, such as copper and aluminium, has been a tried-and-tested process in the industry for years. The welding is completed very rapidly, taking 1 to 3 seconds, depending on the size of the weld node.

Compared to other welding processes, the joined parts are heated up less so that they do not reach their melting point. This results in a number of advantages compared to other jointing technologies. Other materials directly adjacent to the weld, such as the insulation of a wire, is not damaged. Furthermore, the jointed material does not become brittle at the transition to the solid material. The strength of the weld is created by the relaxation process of the first two atomic layers of the parts to be welded.



      

CUTTING

Ultrasonic cutting processes involve any kind of cutting or separating of connected parts. The pressure on the item to be cut can be reduced due to the high number of frequencies per second. This creates a clean cut face. In practice there are two different processes:
  • Cut & Seal; mostly used in the field of textiles, foils and fleeces
  • Cutting; used for food stuffs as well as for solid materials such as rubber, foam and artificial leather



 

 

DESIGN PRINCIPLES OF PLASTIC WELDING
  

Basic concerns

Here are some factors to consider in order to achieve the best results with ultrasonic welding:
  • Strength requirements of the welded joint
  • Is a hermetic seal required?
  • Appearance requirements
  • Is flash permissible
  • lf there is a degree of flexibility on plastic resin, which resin will meet the design requirements and be best for ultrasonic welding?
  • Positioning and type of joint design
  • Parts design for ease of assembly before welding
  • Fixture design to hold the parts in position during welding
  • Will it be possible to make a horn to get the required ultrasonic energy into the joint?

Design of part to be welded

Requirements for transmission of ultrasonic energy

The parts should be rigid if the energy must travel some distance to the joint. Softplastics do not transmit the energy well. The walls must be thick enough to prevent deformation of the welding zone. Thin walls tend to break due to mechanical force and ultrasonic exposure during the cycle.

  

Corners and Edges

All corners and edges should be radiused if possible. Ultrasonics can cause stress concentrations in sharp inside edges which can cause stress cracks or melting of the material.


 

Potential Troubles in Part Design

Extended part areas such as ribs, brackets, and studs may break during welding because of vibration and overheating. Inserted components such as springs or wires on electric components are also at risk. lt is best to use generously rounded edges and corners and short welding times at low amplitudes. lf necessary, potting compound such as silicon can be used to dampen the oscillation of electronic components, springs, etc.


 

Mating of Parts to be Welded

ldeally, one part should mate with the other, and the parts should not be free to slide with respect to each other during the welding process. The fit should be close but not so tight that force must be used to put the parts together before welding. The ideal clearance will fall between 0.05 and 0.1 mm, depending on the size of the parts. The distance that one part should mate into the other should be at least one mm if possible. lf it is not possible to design the parts so that they mate in the weld area, it is also possible to hold the parts in alignment by the fixture or horn.

Recommended gap a=0.025~0.05mm; b= min. 1.0mm

 

 

  

Uniform Energy Delivery

Part design can influence the uniformity of energy delivery to the joint area. For instance, bends, sloping faces and openings in the energy path can reduce the strength of ultrasonic vibrations in the weld area.

Poor or no welding at all zone X


 

Position of Joint Surface

lt is best if the joint area is all in the same plane and the plane of the joint area is parallel to the horn surface. lf the joint area has a step and is not in one plane, the unequal distances from the horn surface may produce uneven welding.

Unequal distances L1 and L2; zone "a" not at right-angle to horn surface.


 

Contact Surface between Horn and Part

Preferably, the contact surface between the horn and the part should be as large as possible and be flat. Some contouring of the horn surface is often possible. lf the surface of the horn is smaller than the contact area between the parts to be welded, welding may still be accomplished, but some of the energy may not get to the weld area. Also, higher force may be necessary to get a good weld. The higher force may lead to marking of the part surface under the horn.

Horn surface area of contact should be as large as possible; for instance, "a" should be larger than "s" if possible.


 


Near Field Welding

The ideal part construction and design would place the contact surface of the horn as close to the welding area as possible. The term "Near field welding" applies if the distance from the horn to the weld area is 6 mm or less. Applications which permit nearfield welding have the least problems although far field welding can also be very satisfactory if care is used.


Near field welding, L less than 6 mm

      

Far Field Welding

In far field welding, the contact surface of the horn is greater than 6 mm from the welding area. The walls of the upper plastic part transmit the ultrasonic vibrations to the weld area, similar to a transmission line. Rigid, amorphous thermoplastic parts are excellent, low loss ultrasound conductors and best for far field welding. Rigid, semi-crystalline thermoplastics will absorb some energy, making far field welding somewhat more difficult. Soft, semi-crystalline thermoplastics have a high mechanical loss rate. Ultrasonic vibrations are therefore absorbed if the distance between horn and weld area is too great. Welding is difficult and melting of the contact area of the horn is likely to occur.


Far Field Welding, L more than 6 mm

Fixture

The parts to be welded can be mounted, centered and kept in place by a fixture. The part to be placed in the fixture should be close fitting, not tight and not loose. If the fit is too tight, energy will be lost to the fixture and welding will be difficult. lf the walls of the parts to be welded are thin and not strong enough to support the force of the horn, the side walls may be supported in the welding zone with a temporary insert.


The two most important welding joints: ED; Energy Director & Shear Joint

 

Energy Director; ED

The Energy Director is a V-shaped elevation on one of the two contact surfaces. It causes a line contact as the ultrasonic vibrations begin to melt the plastic. The line contact focuses the ultrasonic energy. As the melt progresses, the energy director turns fluid and flows to fill the space between the two parts. The downward force of the horn causes the melted material to spread over the entire contact surface. At this moment, the ultrasonic energy supply is shut off and the joint area, still under pressure, cools down in a short period of time, completing the weld.

Energy Director ED Height h = s/10 to s/5


Amorphous thermoplastics do not have a well defined melting point but have a relatively wide softening range. Therefore, welding parameters are less critical and there is reduced chance for thermal damage if the ultrasonic energy is on longer than necessary to obtain a good weld.

Semi-crystalline thermoplastics, on the other hand, have a clearly defined melting point. Most of these materials are sensitive to heat at temperatures above their melting point. Even short times at higher temperatures may cause thermal damage. There is air contact with molten plastic when the energy director is melting and spreading sideways; therefore, the material can crystallize before there is enough heat to weld the entire surface of the joint. Crystallized zones can crack and flake off from the welding area. The air contact may also cause oxidation of the plastic resin. Due to these possible problems, energy directors are not suggested for semi-crystalline materials.

      

Stud welding is a special type of shear welding. lt is an inexpensive solution for weldings where a hermetic seal is not required. A plastic stud molded on one part slides into a hole in the second part with bottom interference. The weld joint also provides positioning.

Shear Joint

Shear joints are good for achieving a hermetic seal and for semi-crystalline thermoplastics in general. A shear joint is obtained with a step and little contact surface. The small surface and the resulting high energy flow cause rapid melting. The two parts slide into each other, forming a vertical weld joint. The sliding of the two melting surfaces prevents bubbles and limits air contact. The weld is homogenous and usually free of leaks. The regular and even welding procedure is easy to control. Since there is little air contact, cooling is slower and crystallization and flaking of the material is impossible.

The integrity of a shear weld is influenced by the amount of overlap of the two parts. The walls of the lower part must be supported in the welding zone to prevent bulging due to welding pressure, especially if the walls are thin.

Shear Joint Tolerance a = 0.025 to 0.05 mm, Positioning height b = 1.0 mm min., Penetration h = s/3 –1.5 s, Overlap = s/10 – s/5


Stud Welding

Stud welding is a special type of shear welding. lt is an inexpensive solution for weldings where a hermetic seal is not required. A plastic stud molded on one part slides into a hole in the second part with bottom interference. The weld joint also provides positioning.


              


 

DESIGN PRINCIPLES OF FUSION WELDING
  
In addition to traditional ultrasonic welding, fusion forming by means of ultrasonic offers a very wide range of possible applications:
  • Riveting
  • Flanging
  • Embedding

These processes considerably extend the use of ultrasonic. They offer the possibility of formlocked combining of thermoplastic synthetics with other materials – metals, glass or dissimilar plastics. Unlike welding, in the case of fusion forming only one plastic part is locally plasticized and shaped in its viscous state. In this way effective use is made of the heat energy between the horn surface and the surface of the plastic part.

Forming by ultrasonic has important advantages over other techniques. Because the forming takes place in the melting phase, only negligible stresses arise in the shaped parts – provided the machinery is correctly adjusted. The problem of stress relaxation is practically non-existent. Fixed connections with no play in them are achieved, coming up to very exacting demands, even in their longterm behaviour.


Riveting

The horn transfers the mechanical oscillation energy to the rivet spigot. It is the riveting tool while at the same time being worked on the face side to the desired rivet headshape. This recess corresponds to the volume of the shaped plastic. Particular attention must be paid to the wear on horn tips, especially when working with abrasive materials. Plastics with mineral fillers or glass fibres require the use of suitable horn materials. Hardened tool steels of hardnesses above 60 HRc, or a suitable coating are recommended.

Thin metal parts can be excited by ultrasonic vibrations and there is a tendency for the parts to climb up against the horn. A clean bond is not guaranteed. Clamping down devices will help. The vibrations can also lead to the break up of exposed parts. Such problems are solved by using sound-compensating materials, possibly combined with clamps designed for the purpose.

lf metal parts are fixed with several rivet heads, all rivet heads should be shaped in one working cycle. lf rivet joints are made individually, the sound energy is conducted through the metal part to the already shaped rivet heads and can lead to breakage.

The horn must not touch the part to be attached. The plasticized material must solidify under pressure during the cooling time. This procedure can be compared with the stress and cooling time for injection moulding. lf the horn lies on the upper part, the pressure on the rivet head is reduced. The result is a non-homogeneous structure with resultant loss of strength.

When metal parts are being riveted, this problem is solved very neatly in the form of a contact breaker. A suitably equipped absorption tool, connected electrically to the controls, causes cut off of the ultrasonic energy if the horn touches the metal part. A welcome secondary phenomenon with this system is that component tolerances are automatically compensated for.

Structure

The general shape of a rivet joint is known from the machine construction. The fixing of the rivet pin should in all circumstances be provided with a ringshaped undercut, with a radius or at least with a bevel. In either case the part to be riveted on must of course be recessed.

            


      

  

Head shapes

The simplest head shape, as in illustrations A and B, is used chiefly for rivet pins up to approx. d = 4 mm. Partically crystalline thermoplastics can in certain circumstances be difficult to work in these forms, because no particular care is taken over melting of the material. A proven, modified shape helps here. The horn has a central tip. The ultrasonic energy is thereby heavily concentrated and greatly assists melting of the material. As a result short welding times and good strength values are achieved.


           


Head shape E shows an alternative to the central spike. Here melting of the material is assisted by suitable shaping of the spigot. lt is important for this tip to be sharp-edged or shaped with a maximum radius of 0.2 mm. This shape is favourable for working with glass fibre reinforced materials.

 





Shapes C and D are suitable for all thermoplastics and rivet spigots where d = approx. 2 – 8 mm.











Head shapes F and G are not defined, these applications are limited to places which are not visible on the finished product.

For partially crystalline thermoplastics and larger spigots, steps must be taken to assist with the melting. A rhombic shaping (Kourl pattern) of the horns has proved very successful. Quite understandably, these two variants do not meet any special requirements for strength. They are used in preference for the fixing of metal parts in electrical engineering.

For the larger spigot diameters, from about 6 mm upwards, the use of hollow spigots as in illustration H is recommended. Accumulations of material and therefore sink marks on injection moulded parts can thus be avoided. The quantity of material to be shaped is reduced, which is beneficial in terms of the welding time and the energy requirement.

The suggested standardization represents approximate values. These can of course be varied and adapted to individualrequirements.




Flanging

The flanging technique is known from metal-working. The most important characteristic in the case of ultrasonic flanging is that the material is plasticized by the ultrasonic energy and shaped in the viscous melting phase.

A typical application is shown as picture. The designer has a relatively free choice in shaping jointed flange connections, though the parts being shaped must exceed the volume calculation.

Even if such joints meet very high specifications, they can never be airtight because of the unequal thermal expansion of both parts. lf airtightness is essential, a separate sealing element must be inserted. The other pictures shows an airtight flanging joint where an 0-ring is used.

When soft materials are being welded, unacceptable welding ridges often occur. Here flanging offers an alternative to traditional welding.




Embedding

Ultrasonic embedding is a very efficient but little used method of joining different form-locking parts to each other. Wall thicknesses and ribs on the synthetic part are plasticized by the horn and pressed into recesses, undercuts and holes. In this way electrical contact elements, for example, can be bedded into plastic housings, plastic parts can be fixed radially and axially on to steel shafts etc.



      

 


DESIGN PRINCIPLES OF EMBEDDING

 

A method known from earlier days of working with Duroplast, for bonding metal and plastic parts together is pressure coating or extrusion coating of metal inserts. The process is also used in thermoplastic injection moulding. lf the physical properties of the thermoplastics are considered when processing and for their longterm behaviour, the result is often unsatisfactory from both an economic and a quality point of view.
  • the metal parts must be pre-heated
  • loading the parts into the injection mould tool is very costly, whether by hand or by robot
  • extended – and in the case of manual loading – irregular cycle-times for the injection moulding adversely affect the quality of the plastic parts
  • the injection moulds are subject to extra wear and tear in the area of the insert loading
  • the manufacturing tolerances of the loading parts must be within unrealistic narrow limits

This always results in very high tangential stresses, which often lead to formation of cracks. As a rule one tries to absorb these stresses with excessive wall strengths surrounding the embedded metal part. Such accumulations of material are unhelpful for achieving reasonable cooling times for injection moulding.

This always results in very high tangential stresses, which often lead to formation of cracks. As a rule one tries to absorb these stresses with excessive wall strengths surrounding the embedded metal part. Such accumulations of material are unhelpful for achieving reasonable cooling times for injection moulding.

Plastics with a high stress-strain ratio, such as for example Standard Polystyrene, are particularly susceptible to stress fractures. All other thermoplastics too, though, can fail in their longterm behaviour under the influence of weathering or chemicals which trigger off stress fractures. One reason for using ultrasonic embedding, which should not be ignored, is the considerable saving in energy.


Design principles:

  • Reference diameter (A), which has the task of positioning the metal part precisely in the hole of the plastic part. The height of this zone must be sized large enough so that in this area the plastic part does not melt under the effect of the ultrasonics. This would result in movement away from the embedding axis.
  • Undercut (B), one or several notches, into which the molten plastic flows, to fix the insert in the axial direction. In this way high pull out forces are achieved.
  • Knurling (C) or lengthways grooves take care of the torsional hold on the insert. Sharp edges encourage stress fractures and must at all costs be avoided.
          

      

  

The standard range of an ultrasonic insert manufacture in general covers three types:
  • standard threaded insert
  • threaded insert with flange
  • insert with set screw
      



 Embedding procedure

In most cases the sonotrode acts directly on the metal part. The part to be embedded is to be considered as an extension of the sonotrode, i.e. it is excited by the sonotrode into vibration with practically equal frequency and amplitude. In this way the heat energy between the surface of the part to be embedded and the surface of the plastic part becomes molten.

 





To avold unnecessarily high pull out forces being applied to the threaded inserts, they should stand slightly above the surface of the plastic. In this way the pull out forces on screwing down are supported on the top surface of the insert and not on the plastic part

 


The insert sinks into the molten plastic under the combination of the amplitude and force applied by the ultrasonic system. The molten plastic flows into the profile of the insert and quickly solidifies when the ultrasonics are switched off. The volume of affected plastic should be equal to or greater than the volume of the profile in the insert. Blind holes should be about 2 mm deeper than the insert so that any surplus molten plastic is forced down into the hole.
    

 







This requirement can be adhered to very easily by using inserts with a flange, and taking appropriate measurements. Also the tendency to protruding flash is significantly less because the flange forms a barrier against the rising molten material.

lf shafts, axles or other unfavourably shaped parts have to be embedded, it is advisable to locate the metal part inside the fixture, and allow the ultrasonic energy to act upon the plastic part. The points in the Design Principles for Ultrasonic Plastic Welding described under near and far field welding must also be taken into account. Marking must be expected on the coupling surface. By using a protective foil between the sonotrode and the plastic part, this can be avoided.


 



















  


Notes

Horns are subject to a high rate of wear and tear by the metal-to-metal contact when embedding is done. For this reason the horn tips are either treated with a coating of hard material or manufactured from hardened steel. Any repair work on worn horns should in principle be left to the manufacturer. Abrasion of metal must be expected at the embedding points. To avoid damaging threads when embedding is taking place, they should have a suitable counter-bore.

Thermoplastic joints can also be produced by ultrasonic embedding. Inserts made of thermoplastics with higher melting points and of lower deformability than the surrounding material can be processed very well.

The excitement of a metal part by ultrasonics generally leads to development of a high noise level. With 20 KHz systems the frequency level of this noise is within the audible range. The stength may reach levels which can damage hearing. The use of hearing protection devices is strongly recommended. This table shows the standard values for the pull out forces of inserts. When filled materials are used (glass fibres, minerals, etc.), the values are generally speaking higher still. They are significantly influenced by the processing conditions, and may deviate upwards or downwards accordingly.



      

 

ULTRASONIC WELDING v.s. AMPLITUDE
  

The amplitude is the movement on the horn surface. The amplitude is depending on the output power of the power supply, Generator setting, Gain of the Booster and Gain of the horn. The amplitude is chosen depending on the material to weld.

The amplitude is measured in micro meter. The value is peak. Typical amplitude values are 10 – 60 micro meter.



Compatibility of plastic materials


This picuture show the compatibility of common plastic materials. Some amorphous materials, especially Styrene is allowed to be welded to different materials. For semi-crystalline, only the same materials can be welded together.
 

 

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