Making the Connection

When it comes to welding and bonding, more Canadian manufacturers are choosing to offer the secondary operation in-house: the 2007 Canadian Plastics Injection Molders Benchmark Survey found that nearly 60 per cent of respondents make welding and bonding capabilities available to their customers. That’s compared to 44 per cent in 2006, and just 40 per cent in 2005.

Welding equipment manufacturers have long argued that their product offerings are superior to glues and adhesives, and often require smaller capital investments.

“Glues and adhesives and even solvents add additional costs of buying those materials,” noted Mark Caldwell, national sales manager for welding equipment manufacturer Sonics & Materials.

But once a processor decides to make an investment in welding equipment, how does he go about choosing the right weld process for his application? In general, major suppliers say that you should pay attention to the type of material being welded, the shape of the joint, and the size of the welded part.

UNDERSTANDING ULTRASONICS

Often cited as one of the most common type of welds, ultrasonic welders use the frictional heat created by applied force and mechanical motion to create a bond. Generally speaking, the ultrasonic method can be used to weld most thermoplastics. However, when working with “softer” materials such as polypropylene and polyethylene, look for a model with a lower frequency.

“Ultrasonics require a rigid material, you have to have the ability to project mechanical motion into the part,” explained Dukane Corporation’s national sales manager Mike Johnston, recommending 15 kHz models for PP/ PE applications. “If that part is very flexible, mechanical motion is absorbed.”

When working with ultrasonic welding, high filler contents such as glass or fibre should also be scrutinized.

“Generally anything more than 33 per cent glass or fiber-filled does raise a question,” said Sonics and Materials’ Caldwell. The high filler concentration can create resin-starved areas, which in turn can cause pinhole leaks. In addition, the filler can impede the friction at the joint, resulting in longer machine cycle times and power requirements. Filler content is less of an issue with vibration and spin welding, but materials with more than 40 per cent filler may raise concerns.

Ultrasonics have a part size limitation as well: according to Caldwell, these types of welders can be used to weld a 100 square inch area at the most. Vibration welders are better suited for processors working with larger parts and weld areas.

GOOD VIBRATIONS

Much like ultrasonic welding, vibration welders use frictional heat — one part is held stationary and the second part is vibrated in a linear direction against it. However, unlike ultrasonics, vibration welders can be used to join larger parts. The process is often used to weld automotive components such as tail lamp assemblies, door panels, and glove boxes.

Vibration welding allows processors to weld larger parts and work with more fibrous materials, but there is one important caveat: your application must allow for linear movement between part halves.

“If the joint area or the part does not allow for linear movement back and forth, we won’t be able to weld,” said Caldwell.

Weld experts note that joint designs or part geometries that restrict linear movement can affect the quality of the bond.

“The joint can be up to 10 degrees off, but anything over that in the direction of vibration is very hard to do,” said Dukane’s Mike Johnston.

TAKE IT FOR A SPIN

True to their name, spin welders are designed to join cylindrical and spherical thermoplastic parts.

“You could also have a square part, as long as the joint area is round,” said Johnston, noting that vacuum ports are often spin welded onto automotive intake manifolds.

According to Johnston, off-the-shelf production spin welders can be used to weld parts as large as a paint can, but custom machines can be designed to accommodate big components such as the inside of a washing machine tub.

LASER “POINTERS”

Aside from the cost — laser welders can be an estimated 20 per cent more expensive than vibration welders — processors who invest in laser technology face one major hurdle.

“The top surface has to be optically transparent to the laser,” noted Paul Subject, president of Sta nmech Technologies Ltd. “The laser sees through the clear surface, and it is absorbed through the dark surface.”

Laser welders do offer some key advantages over their counterparts. For instance, when working with a tail light assembly that has a complex shape and multiple compound curves, friction-based welders wouldn’t be up to the job.

“Also, unlike ultrasonic and vibration welding, you can have high-tech parts which have very fine electronics attached to them,” added Stanmech’s Subject. “Laser eliminates the vibration that could damage fine components.”

HOT AIR, HOT PLATES & HOT TECHNOLOGY

Paul Subject notes that hot air fusion welders are more common than ultrasonic welders and the like, but the welders are designed for a different scale of parts.

“If you’re doing many thousands of small parts and you want the joint the two parts together — such as a spray nozzle — they would be ultrasonically welded,” he explained. If you are working with a part like a big tank or fabricating a one-off part, hot air welders may be more suited to your application.

Stanmech provides a full line of handheld hot air tools, where a welding rod is held to the joint groove and hot air is aimed at the joint configuration at the base of the welding rod. The company also provides hot air extruders for bigger parts. With extruders, the plastic weld material is fed in a rod format, heated up, melted, and then extruded onto the surface.

Hot plate welding allows processors the flexibility to weld semi-crystalline and amorphous thermoplastic materials, using radiant or direct heat contact. A heated plate is moved in between two plastic parts and then comes in direct contact, heating up the joint area.

“It’s a slow and cumbersome process, and relies on constant heat and energy,” noted Sonics & Materials’ Caldwell. He does note, however, that the process can be handy when welding complex shapes where there is no relative motion between the joints.

According to Johnston, infrared welders are similar to the hot plate concept, and are used to weld similar types of applications.

“The main difference between current hot plate and infrared is that the heated platen is a different material,” he continued. “The beauty of infrared is you can do it without coming in contact with the part, and you can focus it very accurately.”

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RESOURCE LIST

Dukane Corporation (St. Charles, Ill.); www.dukcorp.com; 630-797-4900

P&J Distribution Services Inc. (Etobicoke, Ont.); www.pjdistribution.com; 416-798-8440

Sonics & Materials Inc. (Newtown, Conn.); www.sonics.com; 800-745-1105

STANMECH Technologies Inc. (Burlington, Ont.);www.stanmech.com; 888-438-6324