According to Murphy's Law, whatever can go wrong, will. It's an important maxim to keep in mind for the end stages of plastics production. In other words, the fact that a part has been molded or extruded to spec doesn't mean that the chances for mishaps are over. Many parts still have to go through a last step: ultrasonic welding, a process that involves the use of high-frequency mechanical vibrations to generate a frictional heat build-up at an interface between thermoplastic parts. Sounds simple, right? But while ultrasonic welding may be the last step, it's definitely not the least -- there's a whole host of problems just waiting to arise.
Here are solutions to some of the most common ultrasonic weld problems, provided by several leading suppliers of ultrasonic welding equipment. The problems have been grouped into two categories: problems that can afflict new applications, and problems that can develop with long-running applications.
PROBLEMS WITH NEW APPLICATIONS
USING IMPROPER/UNEVEN AMPLITUDE
Usually the result of either processor error or damage to the horn (the part of the ultrasonic system that connects the parts to be joined; amplitude is the motion of the horn during welding, measured as the peak-to-peak motion at the horn's face). "Processors may be using the wrong material for a certain amplitude, or they may have started out with one material and then changed to nother that requires more amplitude," said Miranda Marcus, applications engineer with Dukane Corporation.
"One solution is to use a larger ratio booster to increase amplitude," Marcus said. "Processors must also understand that the length of the weld will affect the amplitude to be used."
A second solution lies in employee training. "Training is one of the biggest areas that I try to promote," said Brian Gourley, technical services manager with Sonics & Materials, Inc. "Amplitude is big variable within the welding process, and it's very difficult to understand without proper education."
High stress levels will create premature cracks in a horn, Gourley continued, making it difficult, if not impossible, to get a proper level of amplitude. "Processors can either have the horn repaired or replaced, depending on the degree of damage," he said.
PART NOT SUPPORTED PROPERLY
Depending on their thickness, parts require either a little or a lot of support during welding (Rule of thumb: the thicker the part, the less support it needs). Thin, unsupported sections of a part may vibrate or flex during welding. If the flexing is severe, it may cause a hot spot in the material, or even a hole in the part.
"Without proper part support, the welder loses control of the process," said Brian Gourley. "To solve this problem, we prefer to build not only the horn, but also the nest and the fixture at the same time. By doing the complete package, the customer will have a greater chance of getting a repeatable process."
If the parts have thin walls that might bulge under pressure, it's advisable to support the part up to the joining zone. Reducing the amplitude of vibration can also help with thin parts.
Also bear in mind that shear joints -- which involve trying to melt the material together on the edges of the part -- generally need more support than energy directors, Miranda Marcus said. "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," she said. "If necessary, the fixture can be split to permit easier loading and unloading of the parts."
¿ PROBLEM: DIFFICULT PART FEATURE
Sharp corners may fracture or melt when exposed to ultrasonic vibration. Also, sizeable holes or bends within the part can create difficulties because the ultrasonic energy may be deflected, leaving a section with little or no fusion.
For small, easy welds, processors can try to fix the problem by using more amplitude. (Warning! Too much amplitude can crack the horn, so be careful.)
For tougher problems, corners and edges can sometimes be radiused to make them easier to weld, according to Brian Gourley. "In some cases, it's necessary either to thicken the part, lightly clamp it, or, if possible, use a higher frequency welding machine to reduce breakage," he added.
¿ PROBLEM: USING REGRIND OR "DIFFICULT" RESINS
Problems can arise when processors use a percentage of regrind that's too high for the allowable process. Also, different virgin resins create different ultrasonic energy requirements; some materials are just tougher to bond ultrasonically, period.
Know your resins, and how they react to ultrasonic welding. It sounds simple, but processors don't always understand this, welding equipment suppliers say. "As far as the ultrasonic process goes, there are two basic resin categories: amorphous and semi-crystalline," Brian Gourley said. "Amorphous is tolerable of ultrasonics, and can withstand different conditions and still weld easily." Semi-crystalline resins in general require higher amplitude and energy levels, Gourley continued, due to polymer structure, higher melt temperatures, and heat of fusion. "Semi-crystalline is harder to weld; the tolerance for welding is very narrow, and the procedure is generally more difficult," he said.
Beyond this, it gets really tricky. For example, impact modifiers such as rubber can affect the weldability of a material by reducing the amount of thermoplastic available at the joint interface, foaming agents can reduce a resin's ability to transmit energy, and mold release agents can inhibit welding by interfering with surface heat generation and fusion. Regrind, meanwhile, should be regulated to +/- 10 per cent per part, for proper control.
Rather than memorize all of these variables -- and more -- a simple way to avoid resin-related difficulties in the first place, Miranda Marcus said, is by involving both the equipment supplier and the resin supplier in any new part application requiring ultrasonics.
PROBLEMS WITH LONG-RUNNING APPLICATIONS
¿ PROBLEM: DECREASED WELD STRENGTH
In an weld application that's been running flawlessly for a period of time, a loss of weld strength is usually caused either by wear and tear or damage to the welder or horn. It's an unseen problem, and tip-offs include increased wattage draw, a change in the sound of the weld, and overloading.
Four words: record keeping and maintenance. "The first step in eliminating unseen problems is to record your welding setup," Miranda Marcus said. "Make a 'Weld Process' sheet that includes information such as your weld parameters, manual settings, and the critical dimensions of your part. Also include photos of the welder, showing the alignment and design of the horn and fixture. Refer to this document when problems arise; it may save you a lot of time and trouble."
Ultrasonic welding isn't always gentle, making good maintenance crucial. "If a processor has a vibrating horn against hard, glass-filled plastic, it will wear the face off rather quickly," said Brian Gourley. "But if processors are conscious of preventative maintenance -- which involves calibrating the equipment on an annual basis -- their welders can last for decades."
Also, don't forget that problems with the upstream equipment will come to light during ultrasonic welding, if not sooner: one of the most frequent causes of problems in a long-running process is wear on the mold that produces the parts to be joined. "This is a slow, but sure, event in any molding process," Marcus said.
¿ PROBLEM: ENVIRONMENTAL CHANGES
Not only is climate change tough on the ice caps, fluctuating temperatures in your factory can be bad for your resins, too, eventually causing problems for ultrasonic welding. "Humidity is a particular concern if you're using hydrophilic material such as nylon, polycarbonate or polysulfone," said Miranda Marcus. "Very cold temperatures can cause polymers to become brittle, which might cause them to crack rather than weld at a normal welding pressure. High heat can lead to longer solidification times, causing problems if you're working with short hold times."
"Some materials are less sensitive to process changes," said Miranda Marcus. "Try switching to an easily welded material, like ABS, to achieve greater consistency in your process."
If the material can't be substituted, the processor can try to protect it from the elements. "If the humidity level is high, for example, we advise customers to keep parts from being exposed by storing them in double lined polybags with desiccant material," said Brian Gourley. "If they can't do that, we usually recommend that they increase the amplitude or pressure to try to overcome that moisture." CPL
Dukane Corporation (St. Charles, Ill.); www.dukcorp.com/us; ; 630-797-4900
P&J Distribution Services Inc. (Toronto); 416-798-8440
Sonics & Materials Inc. (Newtown, Conn.); www.sonics.com; 1-800-745-1105