Walking the weld line: Pitfalls, processes and improvements

It seems Canada is always trailing the U.S. like a little brother, and the joining of thermoplastics using laser welding proves to be no exception.

“The Canadian market is about two to three years behind the U.S. technology-wise,” said Paul Subject, president of Stanmech Technologies Inc., in Burlington, Ont.

The culprit, Subject said, is economics. As a result, processors tend to use laser welding only for high-volume applications.

“These [laser welding] machines cost US$100,000 plus. You’d need to process many hundreds of thousands of parts, because you’d want to get the cost [of welding] down to $1 a part. That adds up to about half a million parts per year,” he said.

Comparatively, the cost of ultrasonic welding technology typically ranges from $15,000 to $30,000, allowing processors to more quickly and easily achieve the $1 per part cost benchmark, he added.

ULTRASONIC BOOM

Mark Caldwell, National Sales Manager for Sonics & Materials, said he has seen an increased demand for 20 and 40 kHz ultrasonic welding component kits from OEM systems integrators.

“This has proven to be an effective and popular production strategy, because the kits cost less, offer the OEM maximum flexibility in terms of size and configuration within their existing systems, and provide the ability to PLC sequentially switch the power supply board’s RF signal to several converter weld locations,” according to a trend report from Sonics.

This trend has cut across several different industries, including automotive, medical and packaging because it provides customers with some flexibility, it said in the report.

For example, a single power supply kit could be used to fire eight ultrasonic weld-point converters in a sequence, and if cycle time was an issue, two power supply kits could be used with each firing four ultrasonic converters in sequence.

Caldwell said Sonics is also noticing an increased demand for rigid ultrasonic converters to replace O-rings.

“Benefits of this technology include improved precision and accuracy, extremely low power loss, minimized heat gain, and in most cases, elimination of the need for air-cooling,” the report noted.

Although ultrasonic welding is a widely used, processors should avoid one particular common error — making their own horn.

“If someone who is not an expert in the process makes a horn, it will never tune properly. Horns have to be machined and tuned to match the frequency of the system,” Caldwell said.

But ultrasonic welding has its limits.

“Ultrasonic welding is good for small parts, like lids and inserts” said Dr. Phil Bates, professor, Canada research chair in polymer processing and joining at the Royal Military College in Kingston, Ont.

BIGGER AND BETTER

If you want to weld large parts, the big workhorse is vibration welding, Bates noted, but the process has its pitfalls. These include limiting part design and hogging valuable floor space, he added.

“It really impacts the design of the part because you have to figure out where you’re going to slice it; you have to slice it on a plane,” he explained. “It’s also going to impact design because it’s going to generate flash. So you have to design the parts with these geometric traps where the flash gets sealed.”

Bates said Branson has a unique approach to vibration welding that results in the flash appearing more esthetically pleasing on the finished part. This is accomplished by heating the welding surface with an infrared lamp prior to vibration welding. This provides a good alternative for applications where you can’t get the flash into a flash trap, for example, a headlamp lens, he added.

Hot plate welding is another technique for welding large parts, but hot plating welding nylon-based air intake manifolds (AIMs), for example, could be a disaster waiting to happen.

“It lends itself well to polyolefins because of their low melting point,” he said. “If you have a material like nylon with a higher melting point, it takes longer, requires more energy, more heat and then material degradation is possible because nylon has a small processing window.”

Bates is leading a team of researchers developing both laser and vibration welding techniques that are more effective for joining large automotive parts, specifically AIMs, which are generally comprised with a glass-filled nylon.

AIMs are usually manufactured either through lost core injection molding or are molded in several pieces and vibration welded together.

With vibration welding Bates has been looking at the effect of weld geometries on part quality. Since some defects can result or be detected during welding, Bates and his team developed a new quality control measure involving placing sensors on the weld line. First, they measured the force created during vibration welding and used that data as a quality control measure. Then the sensors measured the force versus the time for every weld after that.

“When we see the force go higher or lower we know something funny happened,” Bates explained.

Vibration welding is ideal for AIMs, which are generally made from glass-filled nylon, because imperfections in the weld area can be easily fixed because high points are rubbed off during the process. And imperfections are common with glass-filled nylon because of the warpage that occurs with the material during molding, he said

THE ACHILLES HEEL

Additionally, Bates is working to minimize the effect of nip lines through laser contour welding — the Achilles heel of all welding processes, he said.

“The minute you get glass-reinforced, or mineral-reinforced, or God Forbid, nano-reinforced, the bulk material will generally be strong because the fibres are stronger than the matrix,” he said.

So when contour welding, if there are any gaps that are bigger than a certain thickness, the light will travel through, hit the light-absorbent material and melt it. But if there’s a gap, heat won’t be transferred back to the weld.

Now all Bates and his team have to do is determine the critical gap size, and if successful, this research could make way for new designs and processors improve part quality.