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Ultrasonic Plastic Welding Basics

By Janet Devine, President Sonobond Ultrasonics
This ultrasonic welding primer will help you understand how joint design requirements and selection of welding machine frequency affect the finished product.

This ultrasonic welding primer will help you understand how joint design requirements and selection of welding machine frequency affect the finished product. MANUFACTURERS constantly search for equipment that will increase production, reduce rejects and otherwise improve their efficiency. Ultrasonic welding -- used in the medical, electrical, automotive, packaging, toy, housewares, cosmetics and other industries -- achieves those objectives.

Ultrasonics can be used to insert metal fasteners in thermoplastic materials, to form a plastic rivet, to degate plastic parts and to cut and seal films and fabrics, as well as to join plastic parts together.

How does it work? Ultrasonic vibrational energy at the interface of the plastic parts being joined causes the plastic to soften and flow in a fraction of a second. When the material is pressed together and resolidifies, the bond is made.

No glues or solvents are needed. Tooling can be designed to secure and align the parts. Heating is confined to the interface area so the assembled part is not too hot to handle. Equipment can be integrated into automated lines.

There are two basic techniques: plunge welding and continuous welding. In plunge welding, the parts are placed under a tool or horn; the horn descends to the part under moderrate pressure and the weld cycle is initiated. In the continuous welding process, the horn may "scan" the part, or, with films and fabrics, the material is passed over or under the horn on a continuous basis.

Principles of Operation
Every ultrasonic unit contains the following five elements:
A power supply that takes line power at 50/60 cycles and changes it to a high ultrasonic frequency of 20,000 cycles per second or higher.
A converter or transducer that contains piezoelectric crystals that change the incoming high-frequency electrical signal to mechanical vibration of the same frequency.
A booster that transmits the vibrational energy and increases its amplitude.
A horn to deliver the vibration energy by contact with the parts to be welded.
An anvil or nest to support the workpiece. For bonding of textiles,the pattern wheel replaces the anvil.

Fig. 1 - Ultrasonic welding equipment such as this SureWeld line from Sonobond comes in a variety of sizes and styles, including microprocessor-controlled units, modular units and hand-held versions.

Equipment
Systems have evolved into a range of sizes and styles to suit a wide range of applications - Fig. 1. The most common type is a press. The welding press is equipped with a pneumatic system to supply the necessary contact force and the head is mounted on a slide so it can be raised and lowered to contact the part to be welded. It is important the press be rigid so bending deflections do not affect the weld consistency.

The head is usually activated by a palm button set. The ultrasonic energy can be started just before the horn contacts the part, after contact but before full pressure is reached or when full preset pressure is reached. After the ultrasonic energy is stopped, there is usually a short delay before pressure is released to permit solidification of the plastic to occur.

Many units are available with microprocessor controls. These permit control of the weld by time, energy or distance, whereas the conventional welding machine is controlled by time only. Microprocessor-equipped systems May also have a port to transfer data to a printer or computer for storage or for further analysis. Some systems can receive data from an external computer permitting remote control.

Modular units are available, suitable for incorporation into automated equipment for special-purpose machines. Some small units are available in handheld versions.

Power levels for 20-kHz equipment are available at as high as 3000 W. Most equipment sold falls into the 800-2000 W range. Design requirements dictate that the higher the frequency the smaller the unit. This also means higher frequency units have less power handling capacity and are suitable for the smaller, more delicate or precision parts. Equipment is available with frequencies of 20, 35, 40 and 70 kHz.

For continuous welding of film and fabric, the ultrasonic system may be built into a table that resembles a sewing machine. The system is equipped with a rotating wheel that can emboss or cut the fabric with a wide range of patterns. Other arrangements include multiple head systems used for wide roll goods; these may be used to quilt, slit or emboss fabrics.

Horns
The horn is the part of the ultrasonic system that contacts the parts to be joined. The horn is designed to resonate at the frequency of the ultrasonic system. When the horn vibrates it stretches and shrinks in length by a small amount. This motion is referred to as the amplitude of the horn.

Amplitude is measured as the peak-to-peak motion at the face of the horn. Increasing the voltage to the transducer or changing the booster or the geometry of the horn can change this value. Some plastics respond better at higher amplitude.

The material of which the horn is made must have good acoustical and mechanical properties. These properties are usually found in low-density materials such as aluminum or titanium. However, steel or nickel alloys are sometimes used when wear is a factor. Aluminum can be chrome plated and titanium may be carbide coated or carbide tipped to reduce wear.

Horn Geometry
The shape and size of the parts to be welded usually dictate the horn face. The requirement that the horn be a resonant member of the system dictates some constraints

The most commonly used horns are stepped cylinder or stepped bar horns -Fig. 2. These are simple to machine and capable of high gain (gain is ratio of input amplitude to output amplitude).

Horn geometries include circular flat-faced horns, circular hollow horns, rectangular horns and compound horns -Fig. 3. A compound horn is made up of a rectangular or circular block horn with extender horns attached by a stud to the block horn face. This allows access to recessed weld sites or clearance for protrusions on the part to be welded.

Sharp transitions and tool marks should be avoided because they can cause stress risers that lead to cracking of the horn. To prevent unwanted vibration in the width direction, horns wider than 3-in. (7.62 cm) may need to be slotted. Slots are located symmetrically across the horn with the slot ending at least 5/8 in. (15.875 mm) from the face and from the threaded stud hole. Slots are radiused at the ends and edges to avoid stress risers.

After a horn is made, its resonant frequency is checked. Horn analyzers are available for this measurement. The frequency can be adjusted by removing material from the horn. The horn at idle should pull very low current from the horn analyzer. If it does not, it may be cracked or out of frequency range. Most welding machines will indicate a fault if the horn is defective.

Fig. 2 - Stepped cylinder or stepped bar horns are the most commonly used horns.
Fig. 3 - Horns come in a variety of geomtetries, including hollow cylinder horns, cylinder horns with rectangular faces and compound horns.

Weldable Materials
The term plastic may be used for thermoplastic or thermoset materials. The latter will burn when heated and cannot be ultrasonically joined. Only thermoplastic materials are candidates for ultrasonic welding. Thermoplastics can be further categorized as amorphous or crystalline.

Amorphous resins exhibit random, spaghetti-like structure. They do not greatly dampen energy introduced into the material. As heat is applied, they soften and do not have a sharply defined melting temperature. Amorphous resins include ABS, acrylic, polycarbonate, polystyrene and polysulfone. Crystalline resins have an orderly pattern, like coiled springs. Just as metal springs dampen vibration, so do crystalline materials. They also have a well-defined melting temperature. Crystalline materials include acetal, nylon, polyester, polyethylene, polypropylene and polyphenylene sulfide.

Alloys/blends are combinations of amorphous and/or crystalline polymers and the combinations seem endless.

Many trade names, e.g., Lexan (a polycarbonate), are available in differing formulations usually indicated by a number or alphanumeric designation. Most plastics manufacturers offer extensive technical advice.

Differing materials can be ultrasonically welded if their melting tempera- are within 30F and their composition is compatible. Figure 4 indicates weldability and compatibility of various plastics.

The use of fillers and additives selectively extends the performance and properties of thermoplastics. These include colorants, flame retardants, lubricants, minerals, mold release agents, plasticizers and UV stabilizers that can act to lubricate the surface and absorb moisture, either of which can make ultrasonic welding more difficult. On the other hand, if at low levels, the addition of glass fibers, impact modifiers and regrind material may increase weldability.

Fig. 4 - Compatibility of thermoplastics for ultrasonic welding.

Joint Design
To ensure that plastic assemblies are adequately joined, they should be designed at inception with a suitable joint design. Many factors are taken into consideration: the material to be bonded, the ultimate use of the product, the cost and ease of molding and the location of the joint surface relative to the horn.

The joint surface should be approximately perpendicular to the vertical axis of the ultrasonic system and parallel to the face of the horn. The joint surface should be in one plane. The distance between the horn face and the joint should be within 1/4 in. (6.35 mm) of the horn face. This is referred to as a near field weld. Far field welding is the term used when the joint is farther away and is only done when the plastic material can efficiently transmit the ultrasonic energy to the joint location. In general, this limits far field welding to rigid amorphous materials, although some semicrystalline materials can be welded far field given a favorable geometry.

The joint geometry should be tailored to the end product use. The parts must not fit so tightly before joining that they inhibit the vibration needed to induce welding. Thin cross sections may crack under the action of vibration, and delicate parts, such as fine wires, may become damaged when their enclosures are welded. Obviously, the ideal conditions are not always attainable and compromises can be made.

The simplest type of joint design uses a triangular or, less frequently, a rounded projection called an energy director. The function of the energy director is to provide a site to initiate the rapid plasticization of the joining surface by concentration of the energy.

Triangular energy directors have an angle of 60 or 90 degrees and a height from 0.008 to 0.040 in. (0.2 to 1.02 mm), depending on the material, the wall thickness, the requirements for the joint and the likelhood that excess melted plastic, call "flash" will occur. The energy director is usually centered on one of the parts to be joined and may be discontinuous around the edge of the part. In some cases, usually when the wall thickness is wide, two energy directors are used, side by side, or staggered.

The choice of which molded part carries the energy director is usually not important. When welding materials of different rigidity, the energy director is usually placed on the softer part.

The molded parts should be designed such that they retain their relative position during welding. This can be done by a centering provision designed into the parts. Clearance between the two parts should be small but at least 0.002 in. (0.05 mm). If provision for centering the parts is not present, then the nest or holding fixture must provide this feature, although this is not the preferred method.

If the parts have thin walls that may bulge under pressure, it is advisable to support the part up to the joining zone.

When welding with an energy director, the idea is to melt and collapse the energy director. It is important the molded parts, the anvil or the stroke limitation of the press does not impede this motion.

A number of joint designs using a 90-deg energy director are shown in Fig. 5. These include butt joints, step joints and tongue and groove joints. The 90-deg energy director is suitable for most amorphous resins but 60-deg is preferred for polycarbonate, acrylics and semicrystalline materials.

Crystalline materials may receive incomplete fusion when an energy director is used because material displaced from the energy director may solidify before it flows across the joint to form a seal. For this reason, shear joints are often preferred. A carefully designed shear joint can also achieve leaktight joints.

Fig. 5 - Basic energy director designs.

A shear joint uses an interference fit between the walls of the parts to be joined - Fig 6. This necessitates the melting and moving of molten plastic at the joint as the parts are simultaneously subjected to ultrasonic energy and downward force. Because a shear joint melts larger amounts of material, it may require higher power and longer time than a joint made using an energy director. The shear joint also tends to impart sideways motion into the part, so the side walls should be well supported by the nest or holding fixture. If necessary, the fixture can be split to permit easier loading and unloading of the parts.

The shear joint should also have a lead-in to provide self-alignment of the parts. The initial contact area at the base of the lead-in should be small. The design shown in Fig. 6 is for a medium-size part of up to 2 in. (5.08 cm) in diameter. Smaller parts should have reduced interference fit [0.008-0.012 in. (0.2-0.3 mm) per side] and larger parts an increased interference fit of up to 0.020 in. (0.5 mm) per side.

In general, semicrystalline parts up to about 3 1/2 in. (8.89 cm) in diameter can be welded with a shear joint, but amorphous materials may be larger.

A combination shear joint and energy director is sometimes used to produce a high-strength, leaktight weld.

Fig. 6 - A shear joint design.

Other Considerations


Part Size
Part size has an influence on the power level required and the frequency of the welding machine selected. For large parts greater than 1 1/2 in. (3.81 cm) in diameter, or with any welding dimension longer than 2 in., select a 20-kHz welding machine. The power level--typically available at about 1000, 1500, 2000 or 3000 W--depends on the size of the weld area, type of joint and the material to be welded.

Smaller parts may use a 35- or 40-kHz welding machine. These are available in power ranges of 400-1000 W.

For very small welds, a 70-kHz, 100 W system is available in press form or a hand-held version. This system has been used for high-speed ultrasonic staking of plastic projections for 35-mm film cartridges and for single-use cameras.

Part Features
Sharp corners may fracture or melt when exposed to ultrasonic vibration. To reduce such stress fractures, corners and edges should be radiused.

Projections or tabs may fracture and even fall off. This tendency is reduced if the junction between the tab and the body of the part is radiused. Sometimes it is necessary to thicken the part, lightly clamp it or, if possible, use a higher frequency welding machine (35 or 40 kHz) to reduce breakage.

Sizable holes, sharp angles or bends within the part may also create problems because the ultrasonic energy may be deflected, leaving a section with little or no fusion.

Thin, unsupported sections on a part may vibrate or diaphragm. If the flexing is severe, it may cause a hot spot in the material, even causing a hole in the part. Increasing the section thickness or switching to a higher-frequency welding machine may reduce this problem. Sometimes reducing the amplitude of vibration will help.

Applications
Ultrasonic welding is widely used for various component assembly applications in the medical, chemical and electrical industries as well as others. These include liquid bearing vessels, IV components, hearing aids, filter assemblies, monitors and diagnostic components.

Ultrasonic textile joining, primarily in nonwovens is used in medical gowns, booties, caps, face masks, hygiene products, incontinence products, bed protectors, surgical drapes, pillow covers and filter media.

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