Plastic Welding Techniques

(Source: NearEMPTiness/CC-BY-SA-3.0)Plastic welding has replaced other joining methods in industrial manufacturing as it offers superior joint strength and reduced cycle times. To select the proper welding technique, weld strength, production volume, part size, cycle time, material composition and capital expenditure must be considered. The most common techniques to be evaluated for industrial fabrications include ultrasonic, vibration, hot plate, laser, and spin welding.

Ultrasonic

In ultrasonic welding, high-frequency low-amplitude acoustic vibrations are applied to workpieces under pressure to produce a solid-state weld. The workpieces are sandwiched between an anvil and an acoustic horn referred to as a sonotrode. Acoustic frequencies ranging from 15 kilohertz to 70 kilohertz interact at the interface of the adjoined workpieces enabling a highly concentrated melt zone.

Ultrasonic welding takes place with minimal thermal absorption, allowing for a fast weld that requires minimal set time or finishing. The largest market for ultrasonic welding is in high volume production of small- to medium-sized consumer electronics, automotive parts and disposable medical thermoplastic components. In these applications, it has proven to be an economical alternative to glue, screws, and snap fittings when production volume overcomes the overhead associated with part fixturing.

Vibration

Linear vibration welding, although similar to ultrasonic welding, operates at lower frequencies, higher amplitudes and greater clamping forces. The reciprocating motion allows for a friction weld of large parts and complex shapes. The technique produces strong, air-tight welds. It is easily automated, accommodates multiple parts per cycle and requires lower tooling costs.

Hot Plate

Hot plate welding is a fusion technique in which each half of a thermoplastic part is heated by a platen. The platen heater is often coated by a release agent to prevent adhesion of the melt phase when being operated at lower temperatures. Once the joint surfaces have been plasticized by the heater they are pressed together to form a molecular, permanent and often hermetic bond.

Hot plate welding offers precise control over melt temperature, offers exceptional weld strength and accommodates large complex parts. Due to the ease of attaining a hermetic seal, it is best suited for welding batteries, fuel tanks and washer bottles.

Spin Welding

Spin welding is the preferred technique when joining parts with a circular axis joint. One part is held stationary in a holding fixture, while an electric or pneumatic drive rotates the adjoined pieces at speeds up to 16,000 revolutions per millisecond.

Spin welding is a low cost technique that produces a hermetic seal with excellent weld strength. It does not handle thin walled components or complex surfaces, although it is the most economical technique when handling circular joints that are not easily joined by ultrasonic methods.

Laser Welding

Laser welding, also referred to as infrared welding, encompasses the most diverse range of capabilities, as nearly all thermoplastics can be welded by a laser. In laser welding, a focused beam of light is absorbed at the weld seam to produce a molten pool that cools to form a fused joint. It offers the highest precision, is highly efficient, and is considered the least invasive technique.

The highly localized heat and stress field makes the technique suitable for sensitive assemblies in medical devices as well in industrial and consumer electronics. Recent advances in femto-second pulsed lasers have even allowed for an increased role in nanotechnologies as operators are able to further reduce the heat zone due to ultrashort laser pulses that can deliver very high peak laser intensity with reduced photon energy.

Conclusion

Ultrasonic welding has been very economical for high volume production of small- to mid-sized thermoplastic components, but it does not address the needs of joint strength and has limited capabilities when dealing with lower production volumes and larger parts. Linear vibration welding and hot plate welding can accommodate large and complex shapes with exceptional joint strength, while the finished component may require secondary finishing. Spin welding is particularly well-suited when joining arts with a circular axis joint, but it is still limited when handling thin-walled components. Laser welding is the most versatile process that can scale to production volumes, although the considerable overhead associated with equipment costs will continue to limit its market penetration.

Resources:

Plastic Welders

What is Plastic Welding?