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June 1, 2017

Servo controlled Ultrasonic Welding overcomes design challenge and reduces scrap

Schweitzer Engineering Laboratories (SEL) invents, designs, and builds digital products and systems that protect power grids around the world. Their technology prevents blackouts and enables customers to improve power system reliability and safety at a reduced cost. SEL offers a variety of fault indicators for use on subsurface or pad-mounted transformers, subsurface or pad-mounted switchgear and sectionalizing cabinets, junction boxes, and splices.  Recently, SEL was facing difficulty in assembling one of the many Reset Fault Indicators they offer. The product is made of Polycarbonate-Makrolon 2607 and project was to weld the clear lens display screen to the body of assembly. Since the product is designed for use in harsh, submersed, and corrosive environments, the hermetic seal of the ultrasonically welded housing is tested to sustain temperature ranging from -40 deg C to +65 deg C.

 

Ultrasonically Welded-SEL Fault Indicator 

 

Unacceptably low process yield using pneumatic ultrasonic welder:

SEL was experiencing unacceptably low yield from their pneumatic ultrasonic welding process. The welds were inconsistent, resulting in leaking non-hermitic welds.  Welding process, DOE’s (Design of Experiments) were conducted over period of three years always pointing to the same small welding process window with high down speeds and very short weld times. Engineers at SEL were thinking to redesign the plastic injection molds for even flatter welding surface.  However, before making costly changes to the injection molds they brought multiple sets of parts to Dukane’s Applications Lab.

Problem identified using iQ Servo Ultrasonic Welding Graph feature:

Dukane’s 20 kHz. frequency iQ Series Servo controlled Ultrasonic Welding system with Melt-Match®  technology was used to perform tests on sample parts. iQ Servo welder graphs showed the part was acting like a spring because of a hollow area under the weld zone and was collapsing faster than the horn movement resulting in inconsistent and leaking welds. Therefore, it was necessary to use varying weld speeds.

Servo driven Ultrasonic Welding precise control & repeatability produced 100% yield:

Ken Holt, Sr. Application Engineer, Dukane states, “This grade of Polycarbonate, Makrolon 2607, welds very well when the initial weld speed is slow and is gradually increased over the weld distance. Although old pneumatic systems are capable of varying the force during the weld, the rate of change is restricted due to the time required to move air in or out of the air cylinder. Whereas, the servo system is capable of accelerations of 50in/s2, which is equivalent to changing speed by 1in/s in 0.020s. Dukane’s patented Melt-Match® technology further expands this capability by allowing 10 discrete velocity values during the weld process.”

iQ Servo’s ability to precisely control and vary the weld speed (velocity of the collapse of the weld joint) proved the key factor for welding leak proof parts. Numerous set ups were attempted, until a varying of the weld velocity speed was optimized.  The final speed profile setting from .020”/sec. to .090”/sec. proved to be very robust and allowed for variations on weld distance and amplitude without adversely affecting weld results.

Melt-Detect™ also known as Force Drop is an important component of Melt-Match® technology, which was enabled to ensure complete initiation of melt before prompting the horn to initiate downward movement. The overall “U” shape of the force curve (fig1) indicates that high force was applied at trigger; horn movement was initiated when force dropped to a pre-set value signaling that the part had begun to melt and collapse; finally, force and horn movement was increased gradually providing consistent strong bonding. The gradual increase of the distance vs. time curve (fig. 1) shows no abrupt changes as the weld progresses.

iQ Servo Ultrasonic Welding Fault Indicator
iQ Servo Ultrasonic Welding Fault Indicator

Rick Lewis, Manufacturing Engineering Supervisor, Schweitzer Engineering Laboratories, Inc states, “We had an old 1980’s era pneumatic welder that needed to be replaced.  A new servo welder was the obvious choice because of its greater capability’s. The new servo welder is inte

grated into a robotic cell and is working flawlessly. The repeatability of t

he servo welder and the welding and graphing functions make it easier for setting up new welding programs.  It reduces the engineering time and the number of scrap parts required to set up a new welding process.” he adds, “The ability to use the application lab in St Charles and meeting Ken Holt, at Dukane provided a wealth of knowledge that will be used on all of our future welding projects.”

Thus, by using Dukane’s technology SEL was able to get hermetic seal on their existing part and did not have make any costly design changes to their fault indicators.