Better bonds: plastics welding gets smarter, faster, stronger.

Stronger and more consistent welds - even with difficult materials - are a sure bet with today's 'smart' assembly systems.

Over the past few years, more sophisticated controls have resulted in improved results across a range of welding techniques. In ultrasonic welding, for example, better welds are produced by controlling energy and depth, in addition to strictly time-based parameters. Better power controls are resulting in faster and more repeatable welds. Equipment suppliers are also focusing attention on non-contact welding techniques, particularly for welding tough-to-bond materials such as thermoplastic elastomers.

MORE HANDLES ON THE PROCESS

Although the ability to monitor critical welding parameters such as time, energy, and distance traveled has been available for some time, users today are more likely to make use of this capability than they used to, says Janet Devine, president of Sonobond Ultrasonics. Not only are the controls getting better and easier to use, but the current focus on quality is prompting more users to communicate welding data to a PC for SPC recordkeeping.

Increasing emphasis on precision, control, and meeting close-tolerance specifications is a continuing trend in ultrasonic welding, according to Richard Berger, marketing v.p. at Sonics & Materials. For that reason, the company recently added SPC and quality-assurance capabilities to its microprocessor-controlled ultrasonic sealers.

Another trend in ultrasonic welding is a switch from strictly time-based control to energy- and depth-controlled welding, according to Ronald Woolfrey, sales manager of Ultra Sonic Seal. "To accurately control the welding operation, you need to regulate how much energy you put in," he says. "You do that through a microprocessor control unit that measures the load and the energy going out." One implication of this trend, says Woolfrey, is a move toward automatic tuning or frequency control and away from external manual adjustments of the power supply to match the set frequency of the tool.

Sylvio Mainolfi, marketing director of Branson Ultrasonics, agrees, saying that microprocessor controls have allowed welding to free itself from strictly time-based control. "With the old analog setup, you basically turned the power on for a set time and that was it. With the advent of microprocessor control, you can get back to the physics of ultrasonic welding and look at the primary equation, which involves energy plus power plus time."

The ability to track each critical parameter allows users to design control around the process, says Mainolfi. "You can weld by total energy, rather than just based on time. By using total energy, you would see that your weld time would vary a little, but each weld would have the same amount of energy going into the part." He notes that other alternatives include time-energy compensation - setting windows around time and energy and putting a window around the final process; welding to distance - either by collapse or by finished part height; and closed-loop control of amplitude.

Today's ultrasonic welding equipment controls the three main variables of time, energy, and clamp force, says Mainolfi. The microprocessor gives complete digital control over time. In the last two years, Branson introduced a product line that controls amplitude to within [+ or -]2%, improving part-to-part consistency. Control over clamp force with improved pneumatics and proportional valves eliminates variance based on air-line pressure changes.

Combining intelligent software with closed-loop controls on key welding parameters such as force and amplitude can help reduce rejects, says David Quebbemann, general manager of Herrmann Ultrasonics. His company has closed the loop on both force and amplitude. For better force control, Herrmann has just added more set-points for its servo-driven pneumatic valve.

The new Vario welding machine from Herrmann varies the welding force according to time increments that can be programmed by the operator. The ability to vary force in a controlled fashion results in a 15-30% stronger bond, according to Quebbemann. He adds that controlling the welding force also helps to eliminate marking of thin-walled parts while achieving the same weld strength.

Sonics & Materials has replaced pneumatic cylinders on its benchtop ultrasonic welders with an electric drive that automatically regulates the speed and force. It eliminates the need for spring-loaded follow-up to maintain the required force on the plastic part during welding (PT, Dec. '94, p. 44).

And for more precise control of travel, the company recently came out with a digital linear encoder that is said to provide welding accuracy as close as 0.001 in. (PT, Feb. '95, p. 73).

PREDICTING PART STRENGTH

"People are looking at ways to make the process easier and more reliable," says David Quebbemann. To that end, Herrmann Ultrasonics recently developed software that is able to predict part strength and other weld-quality criteria without destructive testing. The intelligent software reads key information from the process - weld time, distance, energy, and peak power. Through a statistical model, the software can relate the weld data to part strength in order to predict the performance of the weld.

The ultimate goal of these developments is a smart welding machine capable of having a dialogue with the operator in order to create a good part. Herrmann is building a database of part size, material characteristics, and size of weld, to help the operator pre-select parameters to make a good weld. The company expects to offer the capability to selected customers sometime this year.

SCREENING OUT 'NOISE'

Historically, the electronic interference or "noise" that permeates industrial environments has hampered the development of reliable in-process weld-sensing controls for electromagnetic induction welding, according to Steve Chookazian, business manager of the Emabond Systems Business of Ashland Chemical. This month, the company enhanced its microprocessor controls with electronic sensors for weld displacement and temperature.

The new controls use electronic filters to screen out interference. A laser gauge measures the weld displacement by sensing the pre-weld and post-weld heights of the part. "It's exciting because we are actually measuring the motion of the part at the weld joint, which is telling us a lot," says Chookazian. At the same time, an infrared sensor measures the temperature of the weld. He says the new controls make electromagnetic welders competitive with friction and thermal welders, which already have such control features.

WELDING DIFFICULT MATERIALS

Welding of thermoplastic elastomers is receiving more attention lately from equipment suppliers. Remarks Chookazian, "In automotive interior and under-the-hood applications we are seeing a much greater emphasis on TPEs." Conventional welding methods using friction or thermal energy are difficult to apply to TPEs because the energy is absorbed in the substrate, says Chookazian. In his view, electromagnetic induction welding overcomes those barriers by placing a thermoplastic compound filled with particles of a metal "susceptor" at the weld interface and subjecting it to a high-frequency magnetic field.

Other suppliers are exploring focused infrared techniques for welding TPEs. According to Mainolfi of Branson (which offers focused-IR equipment), "Infrared might be very viable for TPR or TPE. These materials are very resilient and don't absorb energy in the joint area where you need to achieve the melt. You need something that is going to heat the surface, rather than count on transmitting the energy through the structure itself."

Richard Bruno, owner of Bryant Assembly Technologies, says that its scanning IR welding technique heats only a microlayer on the surface of the part, allowing the product to get hot where it needs to be hot and stay cool where it needs to be cool. "It allows you to achieve a very strong bond with a hermetic seal," he adds. Bruno says the technique has potential for welding TPEs as well as other materials - such as nylons, polyesters, and engineering resins - that are prone to degradation from high heat exposure.

LASER WELDING DEVELOPMENTS

A non-contact process that is showing some promise for rigid plastics is laser welding. At K'95, Bielomatik of Germany showed a prototype laser welder that is expected to be commercially available within six to 18 months. Bielomatik says its laser provides a weld that in some cases achieves up to 100% of the strength of the parent material, making it at least as good as current vibration and hot-plate welders. Welding speeds up to 32 ft/min can be achieved, according to Robert M. Casey, v.p. and general manager of Bielomatik Inc. in the U.S. The laser reportedly can be used to weld any thermoplastic material, as long as one face of the weld has a reflective rather than transparent base.

Although Casey doubts that the laser will ultimately replace ultrasonic, vibration, or hot-plate welding processes, he does expect it to fill a niche for jobs that are difficult for other processes.

For example, on thin films laser welding can offer sealing speeds up to four times faster than ultrasonic welding, according to Colin Snook, president of Industrial Laser Source. The laser is capable of producing seals that are stronger than the parent material at processing speeds of up to 50 in./sec, he claims. The non-contact nature of the process eliminates substrate deformation and induced stress, and allows for simple processing of non-self-supporting materials, he says.

Snook's firm is also doing preliminary work with on-the-fly cutting and welding, in which the laser tracks the line speed of an extruded product, does a spot weld, and then cuts the product to length. He adds that new designs will help bring down the cost of lasers, which can be twice that of ultrasonic welders.

A NEW SPIN ON WELDING

According to Wolfgang Weber, president of KVT Technologies, the trend in the automotive industry to purchase pre-assembled modules is increasing the interest in spin welding. He says that new developments are overcoming limitations of spin welders in parameter documentation and angular positioning. A new series of spin welders from KVT can be configured for a large variety of welding tasks, including those requiring angular positioning. The new welder provides angular accuracy of [+ or -]0.5 [degrees] by means of a continuous drive system coupled with an optical encoder. Other features include a dual precision guide system for height adjustment; a non-contact distance-measurement system, and a weight-compensated force-measurement system that includes a load cell and proportional valves.

ADVANCED POWER SUPPLIES

Newer developments in ultrasonic generators are resulting in improved process consistency and reduced cycle times, say suppliers. One trend is toward ensuring consistent amplitude. "If another high-powered machine in the facility turns on, available input voltage may fluctuate, potentially contributing to welding problems," explains Jeffrey Hornstein, training manager of Dukane Ultrasonics. To prevent such variation, Dukane offers line-voltage regulation, which provides consistent amplitude regardless of voltage fluctuation.

Another development is power-factor correction. This feature keeps voltage and current in phase so that electrical usage is much more efficient, Hornstein adds. This can help reduce power costs.

On RF-sealing equipment, Richard Zcach, district sales manager of Kabar/Cosmos, says that regulation of power output can help to reduce sealing times. The company offers four levels of computer controls on its RF heat sealers, which automatically keep output power within a range set by the operator. One version of the power control puts more power at the beginning of the seal and controls the power output throughout the seal. The result is to reduce the sealing time by as much as half, according to Zcach. "Because now we have better control of the power going into the goods, we can reduce the total sealing time."