Ultrasonic plastics assembly is a process of joining or reforming thermoplastics using heat generated from high frequency mechanical vibrations. The electrical energy is converted into high frequency vibrations which create frictional heat at the joint area. The plastic in the joint area melts, creating a molecular bond between the plastic components. The high frequency sound vibrations cause an increase in temperature, causing the material to melt and either bond to the adjoining part or take the shape of the tool. The main difference between ultrasonic assembly and heat welding or forming is how the heat is introduced to the desired location, as ultrasonics transmits high frequency sound vibrations which travel through the material and cause heat to develop in the material itself.
How Ultrasonic Assembly Is Done?
Ultrasonic assembly converts high-frequency electrical energy into high-frequency mechanical motion by using a piezoelectric transducer and applied to the parts with force, causing the plastic to melt and form a molecular bond between the components. The process of ultrasonic assembly results in a homogeneous bond between the parts as the plastic cools.
Ultrasonic welding is the most prevalent ultrasonic assembly application. In ultrasonic welding, the high frequency vibrating energy from a horn is applied to a work piece along with pressure, which causes the joint area between two parts to heat up due to frictional heat. The plastic material melts, and the parts are bonded together when the vibrations stop, and the plastic solidifies. The process of ultrasonic welding is used to join two thermoplastic parts together by applying a controlled pressure to the parts and vibrating a titanium or plated aluminum horn vertically to generate frictional heat at the joint interface. The plastic is melted and then allowed to cool, and the clamping force is maintained during the hold time to improve the joint strength and hermeticity. Once the melted plastic has solidified, the clamping force is removed, and the two parts are joined as one.
Advantages of Ultrasonic Assembly:
Ultrasonic assembly has numerous benefits such as being a fast, clean, efficient, and repeatable process that creates strong and integral bonds. The process consumes minimal energy and does not require solvents, adhesives, mechanical fasteners, or external heat. The process is adaptable and versatile, with the ability to change tooling quickly and being applicable to difficult materials. The low-cost investment in ultrasonic equipment combined with its reliability, long life, and consistent performance makes it the preferred method of assembly. Ultrasonic assembly is widely used across various industries including automotive, medical, electrical and electronic, communications, appliances, consumer products, toys, and textile and packaging. It can significantly increase production and reduce assembly costs.
System Components and Functions:
The four major components of an ultrasonic assembly system are the generator (power supply), transducer (converter), booster, and horn (acoustic tool). The generator converts standard electrical power into electrical energy at the required frequency for the system (most commonly 15, 20, 30, or 40 kHz). The electrical energy is then sent to the transducer, which changes it into mechanical vibrations. A press and fixture are also required to complete the ultrasonic assembly system, but are not considered major components. The process is flexible and efficient, producing strong integral bonds while consuming very little energy. The booster is used to increase or decrease the amplitude of the vibrations to match the required application. The amount of increase or decrease is expressed as the gain, which is the ratio of the output amplitude to the input amplitude. The vibrations are then transmitted to a horn of the appropriate size and shape to deliver the vibrational energy to the workpiece. The horn may further increase the amplitude of the vibrations.
Techniques for Applying the Energy to the Work:
The type of ultrasonic plastics assembly system used depends on the application, with options ranging from hand-held convert-a-probe systems, press systems for more critical applications, rotary index parts handling systems for high production speeds, ultrasonic units with stack assemblies for very high-speed assembly, and thrusters for custom installations where a press system may be too large. Each type offers different levels of control, repeatability, and ease of use for different production requirements. Different systems are shown in the image.
Polymers are chemical compounds formed by combining two or more elements into a large molecule. There are two types of polymers: thermosets and thermoplastics. Thermosets are not suitable for ultrasonic assembly because they are hard and brittle, while thermoplastics are ideal because they can be softened upon heating. Thermoplastics are classified as either amorphous or semi-crystalline, with the molecular structure determining the physical properties and melting and welding characteristics. The figure shows the molecular structure of amorphous and semi-crystalline materials. Amorphous thermoplastics have a gradual softening process, while semi-crystalline materials have a sharp melting point. Semi-crystalline materials are more difficult to weld than amorphous materials because of their high energy requirement for melting and their ability to absorb vibrational energy. The image shows the differences in molten states for amorphous and semi-crystalline materials.
Compatibility of Materials:
When bonding two thermoplastic parts, it is important that the materials have compatible chemical properties. If the materials are not chemically compatible, even if they melt together, there will be no molecular bond. For example, polyethylene and polypropylene cannot be welded together. Similarly, only similar amorphous polymers have a good chance of welding to each other, while semi-crystalline materials can only be welded to themselves. Other factors such as hygroscopicity, mold release agents, and fillers can also affect the weldability of the parts.
It is the tendency of a material to absorb moisture, affects the weldability of thermoplastic parts. Materials like polyamide, polycarbonate, polycarbonate/polyester alloy, and polysulfone are hygroscopic and therefore, if moist parts are welded, the water trapped within the material will boil off when the temperature reaches the boiling point, creating a foamy condition at the joint interface which makes it difficult to achieve a hermetic seal and compromises the strength of the bond. To avoid these issues, hygroscopic parts should be welded immediately after molding or stored in polyethylene bags with a desiccant to protect against moisture.
Mold release agents:
The use of mold release agents on molded parts can negatively affect weldability as it reduces surface friction between the parts and the chemical contamination of the resin can inhibit the formation of a bond. The best choice for mold release agents are paintable/printable grades as they interfere least with ultrasonic assembly and often require no pre-assembly cleaning. Zinc stearate, aluminum stearate, fluorocarbons, and silicones should be avoided if possible as they are detrimental to ultrasonic assembly.
The use of lubricants such as waxes, zinc stearate, stearic acid, aluminium stearate, and fatty esters can negatively impact the ultrasonic bonding process. These lubricants improve the flow of the resin, but since they cannot be removed and reduce the friction at the bonding interfaces, they can defeat the entire ultrasonic process.
Plasticizers Plasticizers increase the flexibility and softness of a material but can weaken the bond or joint over time due to migration. FDA-approved plasticizers are preferred but experimentation is advised before production.
Fillers such as glass fiber, talc, carbon fiber, and calcium carbonate are added to resins to change their physical properties. Common mineral fillers can enhance the weldability of thermoplastics by improving the transmission of vibrational energy. However, a direct relationship between filler addition and weldability exists only within a specific range. Excessive filler content can result in agglomeration at the joint, making it difficult to achieve a consistent weld, and can also cause excessive wear on tooling and require more powerful ultrasonic equipment.
Flame retardants are added to plastics to change their flammability properties and prevent combustion. However, these retardants can weaken the strength of the final joint in thermoplastic welding and require the use of high-powered equipment operating at higher amplitudes to achieve sufficient strength.
Regrind refers to recycled or reprocessed plastic material that is added to the original resin. Ultrasonic assembly allows the use of regrind without introducing any foreign substance. However, to achieve the best results, it is recommended to keep the regrind percentage low and ensure that the plastic has not been degraded or contaminated.
The addition of colorants or pigments to plastics does not significantly affect the weldability of the material, except when the proportion of colorant to resin is excessively high. White and black parts may require more pigments, and different colours of the same part may require different setup parameters. It is advisable to experiment with the pigments before full production.
The grade of the resin used in an application can significantly impact its weldability. Different grades of the same material can have varying melt temperatures, leading to poor welds or compatibility issues. It is recommended to use materials of the same grade for best results in ultrasonic assembly.
Joint and Part Design
The design of the mating pieces in assembly is crucial to achieve optimal results. Different joint designs exist, each with its advantages, and the choice of a particular design depends on factors such as the type of plastic, part geometry, and weld requirements. There are three essential requirements for joint design: a uniform contact area, a small initial contact area, and a means of alignment. A uniform contact area ensures that the mating surfaces are in full contact around the joint, and the joint should be in one plane if possible. A small initial contact area minimizes the energy and time needed to start and complete the meltdown between the mating parts. A means of alignment prevents misalignment during the welding operation and can be achieved through molded alignment pins, sockets, channels, and tongues. It is best not to use the horn and/or fixture for alignment. A flat butt joint, which only welds at the high points, results in inconsistent and erratic welds. Increasing the weld time enlarges the original weld points and causes excessive flash outside the joint which is also shown in the figure. Bringing one of the surfaces to a point creates welds that have good appearance but little strength. When strength is improved, excessive flash spoils the appearance of the weld. Figure shows the issues encountered with pointed wall parts.
The Energy Director:
The energy director is a triangular bead molded into the joint interface that provides a specific volume of material to be melted, resulting in good bond strength without excessive flash. This joint design is recommended for amorphous polymers. The energy director concentrates ultrasonic energy at the apex, causing a rapid buildup of heat and melting of the material which forms a molecular bond with the mating surface. The energy director meets two of the three basic requirements for a joint design, providing a uniform and small initial contact area. A means of alignment and flash control must be incorporated into the part design. The common joint design with an energy director is the butt joint, with the width of the base being 20-25% of the wall thickness. If the wall is too thick, two smaller energy directors should be used. This design produces a weld across the entire wall with some flash visible at the joint. Parts should be designed with alignment features or the fixture should provide them. Amorphous materials are easier to achieve hermetic seals with, but the mating surfaces should be as flat and parallel as possible.
The butt joint with an energy director is a well-suited design for amorphous resins in ultrasonic welding due to its ability to produce a molecular bond and achieve a small initial contact area. However, it is not ideal for semi-crystalline resins, as the material displaced from the energy director often solidifies before flowing across the joint, reducing strength and making hermetic seals difficult. The figure shows the energy director for amorphous and semi-crystalline resins. When an energy director must be used with semi-crystalline resins, a larger and steeper design is recommended to imbed into the mating surface, reducing premature solidification and degradation. Experimentation has shown that this design is also superior for polycarbonate and acrylics, even though they are classified as amorphous materials. The use of an energy director in the butt joint design results in a faster melt temperature and stronger weld than a plain butt joint. Following is the time and temperature graph for butt joints.
The Step Joint:
The step joint design is a variation of the energy director joint design, which meets two of the basic requirements of joint design: uniform contact area and small initial contact area. A step joint also provides alignment. The strength of the step joint is less than that of the butt joint with an energy director, and the recommended minimum wall thickness is 0.080” to 0.090”. The step joint is used when the cosmetic appearance of the assembly is important, as it eliminates flash on the exterior and produces a strong joint. The height and width of the tongue in the step joint should each be one-third of the wall thickness, and the width of the groove should be slightly greater to ensure no interference. The depth of the groove should be slightly greater than the height of the tongue to create a slight gap between the finished parts for cosmetic purposes.
The Tongue-and-Groove Joint:
The tongue-and-groove joint is another type of energy director joint. It meets the three requirements of joint design: a uniform contact area, a small initial contact area, and a means of alignment, and also prevents flash on both sides of the interface. This joint is ideal for applications where self-location and flash prevention are important and for low pressure hermetic seals. However, it has a lower potential for weld strength compared to other joint designs, and a minimum wall thickness of 0.120” to 0.125” is recommended. The energy director is the same as the one used in the butt joint. The tongue should be one-third the height of the wall and clearance should be maintained on each side to allow for molten material flow. The groove should be 0.004” to 0.008” wider than the tongue and 0.005” to 0.010” less deep. A slight gap is left between the finished parts for cosmetic purposes.
The Shear Joint:
The shear joint is used when a strong hermetic seal is needed and is best suited for semi-crystalline resins. A certain amount of interference is designed into the part for welding to be accomplished. The smearing action of the two melted surfaces at the weld interface eliminates leaks, voids, and exposure to air, resulting in a strong structural weld. A fixture is necessary to provide rigid sidewall support and prevent part deflection during welding. The fixture should conform to the shape of the part and be split for easy removal. The shear joint meets the three requirements of joint design: lead-in provides alignment and self-location, properly designed and molded parts ensure a uniform contact area, and the small initial contact area occurs at the base of the lead-in.
The design of the parts must take into account various factors beyond just the basic joint design.
Near Field vs. Far Field Welding:
The location of the joint in relation to the area of horn contact is critical in ultrasonic welding applications. Near field welding, where the distance between the horn and joint interface is 1/4” (6mm) or less, is preferred over far field welding. Far field welding, which requires higher amplitudes, longer weld times, and higher air pressures, is generally only advised for amorphous resins that transmit energy better than semi-crystalline resins.
The optimum weld is achieved when the joint interface is on a single plane parallel to the horn contact surface. This allows the ultrasonic energy to travel the same distance through the plastic part to reach the joint. Additionally, the surface that the horn contacts should also be on a single plane parallel to the joint.
Other Part Design Considerations:
The sharp corners in plastic parts can lead to high stress and increase the likelihood of fracturing or melting under ultrasonic vibratory energy. To prevent this, it is recommended to have a generous radius on all corners and edges to reduce stress.
Holes or Voids:
Holes, voids, angles, and bends hinder the transmission of ultrasonic energy, which may result in low or no welding in these areas. To optimize welding, sharp angles, bends, and holes should be eliminated where possible.
Protrusions on plastic parts can cause stress and degating (falling off) when subjected to vibratory energy. To minimize this, a generous radius can be added at the junction, a light force can be applied to dampen the flexure, the appendages can be made thicker, or 40 kHz equipment can be used if possible.
Thin flat circular parts can bend and flex under ultrasonic energy, leading to melting or burning due to the heat generated. To prevent this "diaphragmming," making the affected areas thicker is recommended.
Other assembly techniques:
Staking is a process of mechanically locking two parts together by melting and reforming a stud. This process is used when welding is not possible due to dissimilar materials or when mechanical retention is enough. Staking has advantages such as short cycle time, tight assemblies, good process control and repeatability, and eliminates the need for consumables like screws or adhesives. The most common application is attaching metal to plastic by molding a stud into the plastic part and then using a vibrating horn with a contoured tip to create localized heat and reform the head of the stud. The integrity of the staked assembly depends on the design and ultrasonic parameters used during the process, which should minimize the contact area and focus the energy for a rapid and controlled melt.
The Standard Rosette Profile Stake:
The standard rosette profile stake is a basic staking design that can satisfy most requirements and is used for staking flat-headed studs with a diameter of 1/16 inch or larger. It creates a head with twice the diameter of the original stud and is recommended for non-abrasive rigid and non-rigid thermoplastics.
The Dome Stake:
The dome stake is used for smaller studs or when horn alignment is a challenge and is also recommended for glass-filled resins to prevent horn wear. The stud end should be pointed to ensure a small initial contact area, and horn and stud alignment is less critical than with the standard rosette profile stake.
The Hollow Stake:
The hollow stake is used for studs larger than 5/32 inches in diameter and helps prevent surface sinks and internal voids during molding. It reduces the ultrasonic cycle time by melting and reforming less material and produces a strong, large head. In case of repairs, the formed head can be removed and reassembled by inserting a self-tapping screw in the hollow stud.
The Knurled Stake:
The knurled stake is a simple and fast staking method that can be used with all thermoplastics. It allows for multiple stakes to be made without worrying about precise alignment or stud diameter and is not concerned with appearance.
The Flush Stake:
The flush stake is used when a raised stud head above the surface of the attached part is not allowed. It requires a tapered stud design and the hole in the part to be attached is countersunk to allow the melted stud to fill that area and secure the attached part in place.
Ultrasonic stud welding is an alternative to staking and is used to join plastic parts of similar material at single or multiple points. It is useful in applications where other techniques are not feasible due to material, size, or complexity. In this process, a stud is driven into a hole and welding occurs along its circumference to form a shear joint.
Ultrasonic insertion is a process of embedding a metal component in a thermoplastic part. A hole is pre-molded into the thermoplastic part that is slightly smaller than the O.D. of the insert. The ultrasonic energy applied to the insert generates frictional heat which melts the plastic, allowing the insert to be driven into place. The process is completed in less than one second and the insert is surrounded by the melted plastic. The advantages of ultrasonic insertion include short cycle times, no stress on the plastic around the metal insert, no mold damage, reduced molding cycle times, the ability to drive multiple inserts simultaneously, and high repeatability and control.
Swaging and Forming:
Swaging is a process of assembling two materials by melting and reforming a ridge of plastic to capture another component, typically a dissimilar material like glass. It is a fast and efficient method of assembly without creating a molecular bond. Swaging requires special tooling and consideration of the material properties involved. The shape of the swage is determined by the horn face, which controls how the plastic melts and flows. The swage can be continuous or segmented. Swaging has the advantage of providing a tight finished assembly, fast cycle times, and eliminating the need for fasteners or adhesives.
Ultrasonic spot welding is a method of joining two like thermoplastic components at specific points with no pre-formed hole or energy director. This process produces a strong bond and can be used on large parts, sheets of thermoplastic, and parts with complex shapes or hard-to-reach surfaces. The basic guidelines for spot welding include using a rigid support, medium to high amplitude, and low pressure. Spot welding can be done using a hand-held transducer and is a fast and efficient assembly process without the need for extra fasteners or special fixturing.
Ultrasonic degating is a process used in separating injection molded parts from their runner systems. It involves applying ultrasonic energy to the runner in an out-of-phase manner, causing the parts to melt off at the gate. It is best used with rigid thermoplastics such as ABS, styrene, or acrylics and has advantages such as quick operation, low stress on parts, and a clean break at the part surface. The guidelines for ultrasonic degating include having a small and/or thin gate area and having the horn contact as close to the gate as possible.
Scan welding is a high-speed ultrasonic welding process for flat thermoplastic parts that are conveyed beneath a stationary or rotary horn and anvil. It can be used for both large and small parts with at least one flat surface for horn contact, and is suitable for rigid thermoplastics and some fabric or film applications. The joint design should be self-locating, such as tongue and groove, step, or pin and socket.
Bonding and Slitting:
Two common ultrasonic assembly techniques are ultrasonic bonding and ultrasonic slitting.
Ultrasonic bonding is a technique used in the textile, apparel, and nonwoven industries to assemble two or more layers of nonwoven materials by passing them between a vibrating horn and a rotary drum. The high frequency mechanical motion and compressive force between the horn and drum create frictional heat, bonding the materials together at the horn/material contact points. This process requires less energy than thermal bonding, and results in a soft, breathable, and absorbent material, suitable for applications in the medical and clean room industries. No consumables, adhesives, or fasteners are needed in ultrasonic bonding. The following figure illustrates ultrasonic bonding.
Ultrasonic slitting is a process used to seal the edges of a thermoplastic material by passing it between a vibrating horn and a rotary cutting wheel (anvil). This process also seals the edges of woven fabrics and can be used to slit and melt together two or more layers of woven and/or nonwoven materials. Ultrasonic slitting has a fast speed of operation and can be performed using the continuous, plunge, or traversing method, depending on the requirements and the material manufacturing process involved.
Major Component Design
Ultrasonic generators are electrical devices that take standard AC electrical power and transform it into electrical energy at a specific frequency. The frequency of the output can range from 15 kHz to 40 kHz and the output power levels can go up to 4,800 watts. The generators use a power amplifier that switches transistors on and off at a rate of 20,000 or 40,000 cycles per second. This sends a high-powered signal to the transducer, which vibrates and sends a feedback signal in the form of a sine wave back to the amplifier. In earlier designs, an operator had to adjust the frequency of the power amplifier to match the frequency of the transducer. The conventional soft-start circuit reduced start-up problems by applying half amplitude to the load during the start-up period, but still had inherent problems. Ultrasonic generators have design improvements that reduce the start-up problems and eliminate stress in the stack assembly.
The digital timer in ultrasonic assembly systems acts as the "brain" of the system, controlling the up and down movement of the press/thruster slide and turning on and off the ultrasonic energy produced by the generator. It stores lists of instructions, or modes that can be altered to select variations in the weld cycle by adjusting weld times, hold times, and system parameters. The microprocessor control technology offers multiple operating modes for precise process control, repeatability, and weld consistency, resulting in better part quality and fewer rejected parts.
Transducers are used to convert electrical energy received from a generator into mechanical energy in the form of high frequency vertical vibrations. They use piezoelectricity, which generates electricity or electric polarity by compressing a crystalline substance. The heart of a transducer is the piezoelectric ceramic elements which expand and contract dimensionally when exposed to alternating electrical energy. Ultrasonic transducers consist of a steel back slug, ceramic elements, and an aluminum front slug for directing as much vibrational energy to the booster and horn as possible. The nodal point, an area of little linear motion but radial expansion and contraction, is used for mounting the transducer in a shell. If a solid mounting is desired, resonant mounts can be used with thin metal walls tuned to vibrate at the transducer's natural frequency. However, this increases manufacturing cost. Transducers have a variety of frequencies with varying peak to peak amplitudes, with the highest frequency being 50 kHz and the lowest being 15 kHz. The peak to peak amplitudes range from 31 microns (.0012”) to 8 microns (.00031”). Boosters and horns are used to multiply the amplitude to provide the necessary vibration for the transducer to perform useful work.
Boosters serve two main purposes: providing a second mounting point for the stack assembly and amplifying or reducing the amplitude. There are two types of boosters: standard boosters with split mounting rings and patented resonant boosters with no "O" rings for solid fixed mounting. Boosters can be made of titanium or aluminium and come in different gain ratios to adjust the stack amplitude to the requirement of melting the plastic in each application. The optimum booster size for each application should be used and the generator amplitude setting should be left close to 100% with only small adjustments made when necessary. The recommended max gain booster size is stamped on the horn.
The ultrasonic press is a machine that applies ultrasonic energy to the workpiece to create consistent and satisfactory welds. The effectiveness of the press depends on the stability and reliability of its structure, with steel components providing greater structural integrity than aluminum ones. Flexure should be minimized to prevent inconsistency and reduce the number of rejected welds.
Slide Assembly Design:
The design of the slide assembly in an ultrasonic press plays a crucial role in determining the efficiency and longevity of the machine. Some designs, such as those with bronze bushings, can experience wear and migration issues due to cold flow properties. These issues can be prevented by using different means to hold the bushings in place and maintaining a tight tolerance between the bushing and rod. Linear ball slides are a more durable and accurate option, with no clearance between the bushing and rod, minimizing operating friction. The rail-type linear ball motion system offers these benefits in a compact design, allowing multiple welding heads to be placed closer together.
The ultrasonic press is typically operated using air pressure from an air cylinder, which is controlled by a gauge and regulator. Some ultrasonic welders offer an electronic option to set and monitor pressure levels using an ultrasonic process controller, load cell, force transducer, and electronic pressure regulator. The speed of the downstroke can be adjusted using a flow control meter for incoming air. The force applied to the work during the ultrasonic activation and the speed of the downstroke can both affect the final welding results. Basic presses have a mechanism to vary trigger force dynamically.
The triggering sequence for the ultrasonic press is as follows:
• The horn comes into contact with the workpiece, causing the die springs to compress as they are captured between the slide and the air cylinder.
• The microswitch and the adjustable target make contact, activating the ultrasonic energy.
• The die springs ensure a constant force is applied to the workpiece as the plastic material melts.
The dynamic trigger mechanism in ultrasonic press systems has three main functions: to allow the user to control the amount of force applied to the workpiece, to initiate the ultrasonic energy with a switch closure, and to maintain constant force through die spring reaction. With advances in technology, more precise methods of triggering ultrasonic energy have been developed, such as using a strain gauge or piezo load cell instead of a mechanical switch. Digital control systems can also trigger ultrasound through load cell input, changes in velocity, or differential pressure. The force applied by the air cylinder can be controlled through manual or electronic air regulators, allowing for remote programming and multiple force setpoints. These advancements lead to improved and more consistent welds. Following figure shows dynamic triggering.
The Bottom/Mechanical Stop:
A bottom/mechanical stop adjustment is typically provided on a press to prevent the horn from accidentally hitting an empty fixture or to repeatably weld parts to a finished height. However, variations in dimensional part tolerances can cause the amount of meltdown to vary, even if the stroke is stopped at the same height each time. In critical applications, the bottom stop can be used to control height, but it is difficult to set the weld timer to end at the desired height. Instead, it is recommended to use a microprocessor-controlled generator with an Absolute Distance operating mode, or an end-of-weld limit switch, to terminate the ultrasound based on height rather than time, ensuring consistent finished part heights.
Integrated vs. Modular Press Systems:
There are two types of press systems available: the integrated system and the modular system. The integrated system is a self-contained unit with the generator housed inside the press, which is less costly to manufacture, takes up less space, and has no external cables. However, this design subjects the generator to shock and vibration during each weld cycle, which can affect its reliability over time. If a problem occurs, the entire unit must be shut down, causing inconvenience and halting production. On the other hand, the modular system has the generator housed in a separate chassis connected to the press with cables. This allows for greater component reliability and longevity, as well as flexibility in moving the generator if necessary. If a problem occurs, the defective unit can be quickly replaced, minimizing interruptions in production.
Other Press Controls:
In ultrasonic press systems, there are additional controls available to enhance the welding process for specific applications. These include a pre-trigger control, hydraulic speed control, dual pressure, and an end-of-weld limit switch. The pre-trigger control allows for activation of the ultrasound by horn travel distance rather than pressure or time. The hydraulic speed control provides precise control of the down speed of the slide assembly during welding, while dual pressure improves plastic melt and flow. The end-of-weld limit switch terminates the ultrasound at a predetermined distance from the home position of the slide, providing an alternative method for determining absolute weld distance.
Pneumatic and Servo Comparison:
Ultrasonic welders include both pneumatic and servo-driven press systems. Pneumatic presses apply force between parts using an air cylinder and the amount of force is controlled using a pressure regulator and one or more valves. Advanced pneumatic systems have the ability to measure distance and control the weld and hold distances. Servo systems, on the other hand, use an electrical servo actuator instead of a pneumatic cylinder, and control the speed of the horn during the weld and hold phases for higher precision and repeatability. Process control parameters for servo systems include ultrasound amplitude, weld distance, weld speed, hold distance, hold speed, and static hold time.
The horn plays an important role in ultrasonic assembly as it helps to transmit the ultrasonic vibrational energy to the workpiece, allowing for the localization of the melt in the desired area. The design of the horn is crucial and must be customized based on the specific requirements of each application.
A horn in ultrasonic assembly may have a gain factor, meaning it can increase the amplitude of vibration received from the transducer-booster combination. The gain and nodal stress of a horn depend on its cross-sectional shape. A straight-sided horn has no gain and little stress, while an exponential horn has low gain and low stress, a catenoidal design has medium gain and medium stress, and a step horn has high gain and high stress. The following images show straight-sided, exponential, catenoidal and step horns.
Horns are used for various applications and are made of different materials like aluminium, titanium, or steel depending on their acoustical properties, fatigue strength, and surface hardness. Aluminium is a low-cost and easily machinable material, making it suitable for prototypes or complex machining, but its poor surface hardness and moderate fatigue strength limit its use for high-wear applications. Titanium is preferred for its good fatigue strength, excellent acoustic properties, and good surface hardness but is expensive and difficult to machine. For severe wear applications, CPM10V hardened steel horns are used but are more brittle and limited in size. The use of CPM10V steel was found to be more reliable than D2 steel after experimentation and consultation with metallurgists.
The displacement amplitude at the horn face may vary with the increase in length and/or width. To reduce the internal stresses and ensure uniform amplitude on the horn face, slots are machined into horns beyond a 4.0" (101.6mm) diameter or a 3.5" (88.9mm) length. These slots break the large horn into smaller individual horns, improving the stability and reducing the risk of failure.
A composite horn, also known as a compound horn, is a combination of individual horns (horn tips) attached to a coupling horn to form a single, full-wavelength unit. This configuration allows for higher amplitude to be built into the horn without causing excessive stress. Composite horns are used to solve amplitude or wear problems in large welding applications, providing greater part coverage and sometimes eliminating the need for multiple welders. When designing a composite horn, it should be treated as a tuning fork and have a symmetrical and balanced design. The coupling horn is usually made of aluminium or titanium while the individual horns can be made of titanium or steel. Following is the image of full wave composite horn.
Contoured horns are a common type of horn design that is used to maximize the energy transfer to the part being welded. They are designed using computer 3-D design software and are shaped to surround the part to be welded. The design of the horn and fixture work together to ensure the part is held securely in place during the welding process, while maintaining its integrity. The use of precision horns and careful design helps produce superior welding results. Following image show contoured horns.
In some staking and spot welding applications, a replaceable tip made of titanium may be used for the horn. The tip threads into a one-half wavelength horn that is also made of titanium. This allows for the replacement of worn tips without having to replace the entire horn, reducing expenses. It should be noted that the use of replaceable tips is not recommended for 40 kHz horns.
High Frequency (50, 40, 30 kHz) Horn Design:
When designing horns for 50, 40, and 30 kHz equipment, there are more restrictions to consider compared to 20 kHz horns. The velocity of energy at 50, 40, and 30 kHz is higher than at 20 kHz. Using replaceable tips is not recommended due to heat build-up issues and more critical tuning requirements. Following figure shows Horns Threaded to Accept Replaceable Tips.
A horn's operating frequency is determined by the length of the horn and can be calculated using the formula of wavelength. The operating frequency is usually either 20 kHz or 40 kHz and is displayed electronically on a horn analyser. The length of the horn is machined to the correct frequency and the tuning process is done using a horn analyser. The frequency of 40 kHz horns is higher so they are shorter in length compared to 20 kHz horns. Following image shows a horn analyser.
Finite Element Analysis (FEA):
Finite Element Analysis (FEA) software is used by ultrasonic tooling engineers to test horn designs before manufacturing. This helps optimize horn design and performance by identifying and analyzing stress points, minimizing amplitude irregularities, and optimizing the horn to operate with a 20 kHz ultrasonic signal. Using FEA analysis, the engineers can reduce stress and amplitude variations and optimize the horn design. Below image shows horn design to reduce stress and to minimize amplitude variation.
A fixture is a crucial component in ultrasonic assembly applications that serves two purposes: alignment and support. It aligns the part under the horn to ensure repeatability and supports the joint area for efficient energy transmission. Following image shows the fixturing an energy direct part and fixturing a shear joint part. There are two main types of fixtures: resilient and rigid. Resilient fixtures are used for welding rigid amorphous materials and minimize part marking, but absorb more energy. Rigid fixtures are used for flexible materials and semi-crystalline materials and are made of aluminium or stainless steel. The design of the fixture should consider factors such as ease of loading, part material, joint design, welding technique, and cost. It is recommended that the horn and fixture be manufactured by experienced ultrasonic engineers.
CAD Process from Data:
The design process for fixtures and horns involves data sharing between the company and its customers. The process starts with the customer sending part data to the tooling staff, which is used as the basis for the design of the fixtures and horns needed for the project. As parts become more complex in shape and contour, the requirements for horn and fixture design become more involved. The horn must accurately contact the part's energy director while the fixture must provide proper alignment and support, as well as any necessary clamps, slide mechanisms, or other devices to securely weld the part.
What Is Process Control?
Process Control is a continuous improvement method for product and process quality. It involves four steps: operating a process with a requirement, measuring a variable against the requirement during operation, comparing the result to the requirement, and taking corrective action if needed. Following image shows process control model.
Process control is a method of evaluating and improving product and process quality by using a closed-loop system. It involves operating a process with at least one requirement, measuring a variable against its requirement, comparing the result to the requirement, and taking corrective action if necessary. The process generates feedback by comparing a measured variable to its requirement and using this data to maintain or correct output at a desired level. This system is different from an open-loop system, which does not have feedback for verification or correction.
Open-Loop (Time-Priority) Welding Systems:
Open-loop, time-priority welding is a method in ultrasonic plastics assembly where parts are welded for a predetermined time duration. The horn descends and the ultrasound is turned on for the preset time, but there is no process data provided about the work being done. This method assumes that if the length of time the part is exposed to the sonic energy is always the same, then the strength of each assembly will be consistent, but this assumption is incorrect due to inconsistencies in the parts. This method does not provide a way to determine part quality or control the process.
Closed-Loop (Energy-Priority) Welding Systems:
Closed-loop, energy-priority welding is a method of ultrasonic plastics assembly that welds parts based on the amount of energy they absorb, instead of a pre-set time duration. The horn descends and touches the part, then the ultrasonics is turned on and remains on until the pre-set energy level is reached. Once the energy level is reached, the ultrasonics is turned off and the head retracts. This method addresses the inconsistencies and lack of feedback found in open-loop, time-priority welding, leading to more consistent results. Following image shows close loop welding system.
The closed-loop, energy-priority method in ultrasonic plastics assembly aims to address the inconsistency and lack of feedback in the open-loop, time-based method. The system determines energy by multiplying the power drawn by the part by the exposure time and adjusts the exposure time to reach a pre-set energy level. The method assumes that constant energy will lead to a consistent melt. However, this assumption is invalid due to inconsistencies in the parts and energy losses in the ultrasonic equipment. The energy lost at the interface between the horn and the part and between the part and the fixture can vary greatly and impact the results. Therefore, constant energy does not guarantee consistent results.
With the advent of computer-controlled ultrasonic welding equipment, the process of welding plastic parts became more precise and efficient. The use of microprocessor technology allowed for the gathering and logging of process data for statistical process control analysis, more efficient automation and system integration, reduced setup time, and control and monitoring of process variables on a cycle-by-cycle basis. The feedback data from welding process variables enabled higher finished product quality, fewer rejects, and documented process results, which was not possible with the old time-based welding methods. The development of high-performance plastic resins and the refinement of assembly techniques continue to drive the need for precise and controllable joining methods in ultrasonic plastics assembly.
Ultrasonic System Features – Pneumatic:
Computer-controlled ultrasonic welding equipment offers advanced features beyond simple time- or energy-based welding. The following paragraphs explain some of the enhancements to the welding process:
Electronic Pressure Regulation:
Electronic pressure regulation uses an electronic pressure regulator and a pressure transducer to provide precise control and monitoring of press air pressure during the welding process. The electronic pressure regulator converts an electronic signal into air pressure, while the pressure transducer converts pressure into an electronic signal for monitoring. The ultrasonic welder logs pressure measurements and can generate pressure vs. time graphs for analysis and better understanding of the welding process.
Load Cell (Force Transducer)
The ultrasonic welder uses a load cell or force transducer to measure the force applied during the welding process. This device converts mechanical force into an electronic signal which the ultrasonic welder uses to trigger the ultrasonic energy at precise user-defined force levels.
Remote Setup Switching
The ultrasonic welder has the capability to change welding setups remotely, through a signal from an external source such as a programmable logic controller.
Sequencing is a feature of the ultrasonic welder that allows it to change setups after a specified number of welding cycles or based on inputs from external sources, such as a programmable logic controller or sensor. This is useful in situations where multiple sets of operations are required to be performed on a part, each with different setup parameters.
Weld by Distance
The ultrasonic welder process controller is capable of precise distance measurement with the use of a linear encoder. This enables the system to join plastic components with a specific weld depth through "weld by distance." The horn descends until it contacts the part, marking the distance as the reference point, which eliminates tolerance variations. The ultrasonic welder then welds the pieces together to a user-defined distance, providing consistent and repeatable results each time. The reference mark resets the ultrasonic welder distance register each cycle to maintain accuracy.
The Dual Pressure technique in the ultrasonic welder process controller allows for more flexibility in the welding process by offering the option to weld at one pressure and hold at a second pressure, or to weld at two different pressures and hold at the second pressure. This method improves the results of applications that were previously not possible.
Ultrasonic System Features – Servo:
The ultrasonic welder servo systems offers a different approach to traditional joining techniques, providing improved quality welds and consistent results with its patented features.
Sensing Start Distance and Sensing Speed:
The ultrasonic welder servo systems feature several pre-weld options that are critical for setup and form part of the triggering sequence. These options include Sensing Start Distance and Sensing Speed, which determine the speed at which the press system's horn moves towards the part to be welded. The horn slows down as it approaches the Sensing Start Distance and reaches the trigger value (force or power), at which point the ultrasonic signal is turned on and the horn moves through the weld cycle at a programmed speed.
Start Motion at Force Drop
The Start Motion after Force Drop feature in the ultrasonic welder servo system identifies a specific point in the welding cycle, marked by the detection of a programmed drop in force. This drop in force signals that the parts have started to melt, and the servo motion restarts to continue the compression of the parts for a predetermined distance and speed. This feature is critical to ensuring quality and consistent results in the welding process.
Weld Motion Type:
This feature refers to the ability to customize the speed of the horn during the welding cycle. The servo ultrasonic welder has two options for speed distribution: Constant and Profile. Constant is a constant speed throughout the entire cycle, while Profile speed allows customization in up to 10 segments. This can be useful for fine-tuning the speed during the welding process, especially when working with an unfamiliar material or when an intricate energy director is involved.
The hold feature is a post-weld operation that allows for compression of the molten polymer as it solidifies to prevent residual stress and voids in the bond area. The servo ultrasonic welder offers three hold sequences: Dynamic, Static, or a combination of both. Dynamic hold involves continuing the compression after the ultrasonics have turned off, while Static hold involves holding the final weld position for a set amount of time without movement. Dynamic hold allows for fine-tuning for different material groups, while Static hold may be useful for thin-walled parts.
Teachable Top of Stroke Position:
The servo ultrasonic welder is equipped with a feature that allows the welder's thruster to return to the top of stroke position after a welding cycle. The top of stroke position can be directly programmed or taught using dual opto-touch switches and a computer interface that displays a "TEACH" button. The operator can jog the press into position and use the "Set This Position" function to automatically store the information as the top of stroke position.
In addition to the top of stroke position, other positions such as Sensing Start Distance and Mechanical Bottom Stop can also be taught during the welder's setup phase. The software guides the operator through all of the programming options using pop-up screens and TEACH buttons.
A setup on the servo ultrasonic welder includes all the programmed parameters for a specific welding process and can be saved digitally in memory. All settings, including the home position, speed control, top of stroke, and others, are part of a specific setup. However, if the thruster needs to be repositioned on the column, a mechanical adjustment may be required. If the press is moved, Sensing Start Distance and Mechanical Bottom Stop need to be reset when switching to a new program. For high volume welding with a focus on repeatable results, it's likely that multiple machines will be used simultaneously.
Duplicating Process on Multiple Machines (clones)
The servo ultrasonic welder allows for multiple machines (welders) to be programmed with the same welding process, but this presents a challenge for pneumatic systems due to the minor mechanical differences between each welder.
Ultrasonic Welding Servo System Benefits
The servo ultrasonic welder has several unique features compared to pneumatic systems, which offer advantages over pneumatic systems. These benefits are explained in the text.
Weld and Hold Collapse Distances Control
The servo ultrasonic welder system offers more precise control of weld and hold collapse distances than pneumatic systems, thanks to its direct control method. In pneumatic systems, the collapse distance is controlled indirectly by releasing pressure from the air cylinder, but this can result in variations due to factors such as the limited rate of compressed air release. On the other hand, the servo system directly controls the distance through closed-loop servo position control, resulting in highly precise and repeatable results.
Rapid Speed Change
In ultrasonic welding, profiling the speed during the weld can improve the weld quality by matching the natural rate of melt of the material. The servo ultrasonic welder, with its capability of changing speed quickly and its 50 in/s2 acceleration rate, enables meaningful weld profiling which allows for independent programming of up to 10 different segments of the weld, and dynamically senses when melt is initiated, is a key advantage over pneumatic systems. Although some pneumatic systems can vary the force during the weld, the rate of change is limited by the time required to move air in and out of the cylinder. The servo system's ability to make rapid speed changes also allows for greater flexibility in achieving hold speeds that differ from weld speeds.
The servo ultrasonic welder system is versatile and has advantages over pneumatic systems in handling difficult welding applications. One example is the sealing and cutting of thin film media, where precise distance control is required for quality welds. The servo system's precise distance control capability makes it capable of achieving successful results in these types of applications, which may be difficult or even impossible on a pneumatic welder.
The servo ultrasonic welder offers improved control during the hold phase of welding, which has two stages: dynamic and static. In the dynamic stage, the parts are collapsed further after the ultrasound is turned off. In the static stage, the servo maintains its final position to allow the solidification process to finish. This enhanced control capability provides more precise and repeatable results during the hold phase.
Ease of Calibration
The servo ultrasonic welder is easier to calibrate compared to pneumatic systems as it eliminates pneumatic components. This means that the setup and maintenance of the system is simpler and more straightforward, leading to a more efficient and cost-effective operation.
Welder cloning is Easier:
With servo, ultrasonic machines (welders) can be programmed to have the same performance, making it easier to achieve repeatable results when welding a high volume of parts. This is because the digital process control eliminates the need to compensate for minor mechanical differences between welders, which can be a challenge with pneumatic systems.
The use of servo ultrasonic welder system leads to a decrease in the number of rejects due to its high degree of process repeatability, resulting in improved yields and a higher value of assembled parts.
Smaller Maintenance Cost
The servo system eliminates the need for a compressed air system, leading to cost savings in terms of operating a compressor and lower maintenance costs. The servo actuator has a long lifespan of over 200 million cycles.
Fewer Accidental Changes
The servo ultrasonic welder is designed to maintain process repeatability and calibration by eliminating adjustable mechanical operator controls, thereby preventing accidental or unauthorized changes.