ROLE OF FIXURES IN ULTRASONIC PLASTIC WELDING
The use of CAD/CAM technology has greatly improved the manufacturing of contoured fixtures to support the part during ultrasonic assembly. Exact mirror replicas of the part can be created to provide excellent support to the part. Due to complex contours seen on many parts, it is recommended that the horn and fixture be purchased as a set, so proper alignment is achieved between the two. The fixture used for ultrasonic assembly is often overlooked, but usually very critical to the success of the application.
The use of a fixture that is not properly designed will usually result in very poor welds or no weld at all. Typically you want the fixture to support the part half to be welded that has the deepest draw. The horn will contact the other part to be assembled, so that the distance from the horn contact surface to the weld joint is minimized. The primary purpose of the fixture is to hold the part secure, in position and in alignment to the ultrasonic horn that will descend upon the assembly. It is important to support the bottom of the part to prevent it from deflecting under the pressure applied by the ultrasonic welder. If the part does not fit correctly, alignment can be impacted and energy that is needed for the weld can be dissipated into locations not necessary for the weld. A sloppy fit can actually create a condition in which the parts vibrate in unison instead out of phase with each other. It is preferred that the part in the fixture be held sufficiently to allow the mating piece to vibrate against the fixtured part and create a melt flow. If the parts vibrate in unison during ultrasonic vibration, the frictional activity is decreased resulting in less melt flow.
Many different types of materials have been used to build fixtures for ultrasonic assembly including plastics, stainless steel, tool steel, aluminum, cork, poured urethane and silicone rubber. All of these can play a role depending upon customer’s needs, but the most common fixtures include aluminum, steel and poured urethane. Poured urethane fixtures are frequently used for supporting rigid amorphous plastic parts. Urethane fixtures are especially advantageous for providing good support with rigid parts and eliminating or reducing part marking that can occur during ultrasonic assembly. Urethane fixtures have also been used successfully as a part support for insertion and staking applications. Urethane fixtures have been used successfully for insertion applications with 8-32 or smaller inserts and staking applications with boss diameters less than 1/8 of an inch. Historically, urethane pours were not uniform and often used too much urethane in areas for supporting the part. Due to the resiliency of urethane material, this inconsistency of the poured urethane would result in varying support at different locations around the part. The advent of new thin poured urethane fixtures provides good consistent support and a resistance to burning that was seen with the thicker poured urethane fixtures that were used in previous generations of ultrasonic welding. The use of the thinner poured urethane fixtures can be advantageous to welding, reduce part marking and provide good support to the part allowing for good transfer of ultrasonic energy to the part and not the fixture. The most commonly used nest material is aluminum. Aluminum provides good rigid support and can easily be reshaped if the plastic parts come in oversized. It is used for both crystalline and amorphous parts. Aluminum is almost always the material of choice for supporting parts made of crystalline type plastic materials. Aluminum fixtures may need to be hard coated if the plastic material includes abrasive fibers, but the material is generally clear anodized or chrome plated to prevent part marking. Part marking is a real concern with a rigid aluminum nest, particularly if the part does not fit the nest well. If the contours don’t match, the vibratory energy can result in the rigid fixture marking the plastic part creating an unsightly scar. Steel is also used as a fixture choice for ultrasonic assembly. It is often chosen for its improved wear characteristics, particularly when welding highly abrasive materials. It is also selected for use as anvil fixturing to provide knurling and various stitch patterns for textile weld and slitting applications. No coating is required if the material is stainless, but black oxide coating is frequently used for other tools steels.
The deciding choice for fixture materials usually comes from the type of support required and the type of weld joint used. The fixture usually supports the lower part up to the parting line to prevent part marking during the ultrasonic assembly process. If the part has a deep draw it is probably advantageous to use aluminum material to improve loading of the part into the nest and unloading of the part out of the nest. On crystalline material applications, a joint called a shear joint is frequently employed which causes the part to expand at the weld joint as the material melts. With this type of joint, you would want to make sure the fixture material is very rigid to suppress deflection and make sure the melt occurs along the vertical wall of the shear joint. Due to the resiliency of urethane material, it would not be a good material choice for a shear joint welding type of application. When a part is welded using an energy director joint where there is no force against the outside wall, most of the support is needed on the part in the area under the energy director. With the energy director joint the most common choices are aluminum, stainless steel and poured urethane.
The fixture material choice will also affect how the plastic material flows at the joint interface. A soft resilient type of material will create a different flow rate than a rigid metal nest. Different fixture materials will create various frictional reactions at the weld joint. An experienced ultrasonic tool maker can assist with choosing the appropriate fixture material. Most of the time they will be right based upon their application experience and exposure to the broad choice of plastic material choices used to manufacture plastic products. Fortunately, if they select wrong it is rarely a life altering event.
Most fixtures include a slotted aluminum subplate for mounting to a welder base or automation system. The slotted subplate allows for ease of alignment between the part and the ultrasonic horn. Many fixtures will incorporate leveling features on the aluminum subplate to adjust the nest if the fixture in not parallel to the machine and the ultrasonic horn.
Just about any type of attachment can be added to an ultrasonic fixture to improve alignment, enhance part loading and unloading, eject the part, clamp parts, provide slides to move the part or the fixture, etc. Servo and pneumatic drive motion control systems have also been used as fixture attachment devices to lift the part to a stationary vibrating ultrasonic horn. Common used fixture attachments include the use of clamps and slides. A clamp is often used to provide support to the part, but allow a split fixture to open wide for part loading. This clamp approach allows the fixture to provide excellent support at the joint, but still open up for part loading. This fixture clamping approach is often necessary due to limited stroke capacity of the ultrasonic weld machine. Slides are also incorporated into fixture designs to move the part into a position out from under the ultrasonic horn to allow for part loading. Slides are also used for welding at different locations on the part, to move a fixture component into position for improved support and to scan along a weld seam. Slides have been used to move a part along an X-Y axis to aid in installation of multiple inserts or to allow the horn to stake posts at multiple locations. With the use of slides one finds other fixture attachments like stops and detents to control the motion position. The use of clamps and slides can be activated manually or automatically depending upon requirements for volume, ease of use and cost considerations. Mechanical part ejection attachments are often added to fixtures to enhance part removal after the ultrasonic welding process. This eject feature is generally required if there is a deep draw to the part or the part imbeds itself into the nest during the assembly process. The mechanical eject attachment components can be as simple as a manual lever lift or a little more complex like a lift driven by a pneumatic cylinder. Smart fixtures with plc controls have been used as a fixture attachment to provide sensors to ensure part placement before the weld cycle and to activate automatic part ejection after the weld cycle. It is feasible to envision that these smart fixtures could also incorporate part loading.
Customers that need rapid tool change are constantly requesting quick change fixtures. For quick change tooling, the use of dowel pins and bushings are commonly found fixture attachments. Coupled with quick tool change features for the ultrasonic transducer, booster and horn assembly, the end user has the capability to change jobs much faster providing a substantial reduction in setup time.
Dukane Advances the Ultrasonic Delivery System
Ultrasonic welding has been in existence as an assembly process for over 40 years. As a method of bonding plastics, it has become one of the most accepted processes because it is clean, energy efficient and fast.
The birth of ultrasonic welding was discovered quite by accident, as many modern marvels are similarly invented. In the early years of the technology, the energy was applied to the plastic part by an operator who would manually pull the lever arm of an arbor press, which held an ultrasonic transducer and horn. In those days, the pressure would vary during the process and the amplitude would droop tremendously as the load was applied to the material. The process control was crude, but an industry had been launched.
During the late 1960’s and early 1970’s, ultrasonic machines were produced with pneumatic delivery systems and manufacturers called these components, presses or actuators. As a means for delivering the converter and the horn to the plastic, these systems were significantly advanced when compared to the previous hand controlled choice. In the 1980’s and 1990’s new products were produced that controlled the amount of energy delivered to the plastic. Ultrasonic welding machines were developed allowing welding by distance. Other significant electronic advances were made to the power supplies to control amplitude and stay current with the digital revolution. However, the delivery system for bringing the vibrating horn to the plastic has continued to be the standard actuator or press comprised of pneumatic components.
Back in the mid 90’s I predicted that I would be surprised if by the year 2000 manufacturers of ultrasonic plastic welding equipment had not incorporated the servo controlled technology into their standard product line. It just made sense. These innovative machine motion control systems provide the ability to control and profile force with the acceleration and deceleration features embellishing the welding process. I thought that servo controlled ultrasonic systems would become as common place as servo controlled injection molding machines.
Wow was I way too early with my prediction. However, one manufacturer has finally seen the light. Dukane has developed a new delivery system that looks like it could provide users a degree of control not previously realized in the industry. I am sure it is expensive and not meant for all applications, but for those companies looking for precise control of the process it is probably worth investigating.
Gary Clodfelter
Plastic Assembly Technologies, Inc.
Remote Palm Buttons for Branson Welders
Plastic Assembly Technologies has a line of remote palm buttons for Branson 900 and 2000 series ultrasonic welders. PAT’s Remote Palm Buttons are used to initiate the start of your Branson 900 & 2000 series welding equipment. These ERGO friendly palm buttons are easy to install and mount on most tables. The palm buttons are often used to allow removal of the welder from an automation line and cycle the welder independent of the automation.
- Features of the PAT Palm Button Sets:
- PAT palm buttons initiate ultrasonic welder cycle
- All pre-wired for direct plug in to welder
- Includes emergency stop button to terminate cycle
- Includes 9’ cable and D shell connector
- Includes mounting hardware
- Built from aluminum tubing and aluminum plate
- PALM-1 includes light touch mechanical switches
- PALM-2 includes capacitive touch switches
- Easily adapt to Pat’s slim line welder table
Palm Button Set Models
PALM-1 Palm button set with (2) light touch start buttons and emergency stop button
PALM-2 PALM button set with (2) capacitive start buttons and emergency stop button
Plastic Assembly Technologies, Inc., located in Indianapolis, Indiana, has a complete line of accessories for ultrasonic welding.
Aluminum & Titanium Ultrasonic Boosters
Plastic Assembly Technologies has introduced a new line of aluminum and titanium rigid mount ultrasonic boosters without spanner wrench holes. Ultrasonic boosters without spanner wrench holes result in a reduction of stress location points on the booster and provide for boosters that are less prone to fail. Additionally, these boosters are all of the same length, allowing the user to change the amplitude of the converter, booster and horn assembly without affecting the length of the assembly. Therefore, the amplitude can be mechanically changed without requiring significant adjustment in the welder setup. The rigid mount design reduces deflection during the ultrasonic welding process providing for less potential variation during welding. The boosters are priced to reflect the cost of conventional boosters, but provide a host of additional benefits. The ratios of gain changes available are 1:1, 1.5:1, 2.0:1 and 2.5:1. The boosters are designed to fit in Branson or Dukane ultrasonic welders using 3.250″ diameter ring mounts. Plastic Assembly Technologies has developed a stack vise to ease the changeover.
Full Wave Ultrasonic Horn Resolves Problem
Full wave ultrasonic horns are known to have resolved horn failure issues when half wave designs have proven unsuccessful. Most ultrasonic horns are manufactured based upon a half wave design. The half wave design is used to reduce material and machining costs. However, there are applications and specific design situations that warrant the use of a full wave ultrasonic horn.
One example that justifies consideration of a full wave ultrasonic horn is an application that requires a deep pocket in the working face of the tool. When a deep center pocket is placed in a half wave ultrasonic horn, the result is usually more stress on the tool than when the pocket is placed in a full wave ultrasonic tool. This is because deep pockets in half wave horns can result in secondary frequencies, indicating that there are undesirable flexural or bending motions in the tool. These flexural or bending directions of vibration are not in the desired axial direction of motion and can result in increased stress, which can cause a horn to fail prematurely.
Horns are designed to resonate in an axial mode of direction. Deep pockets in a half wave horn are so close to the nodal area of the horn that the axial mode is contaminated by the proximity of the pocket to the back mass of the tool. When an ultrasonic horn is driven at ultrasonic frequencies, it is driven from the center element of the tool. When a half wave tool has a deep pocket in the center element, the horn has to do more work to drive the center element at the desired frequency and this results in undesirable bending or flexural motions. By making a full wave tool, solid mass is added to the center element and this additional mass pushes the center element with more force. This additional mass driver results in a purer direction of motion on the tool and drives the tool more uniformly in the desired axial motion, reducing the flexural motion and stress.
Plastic Assembly Technologies has solutions to your ultrasonic horn problems. Contact Us
Who Spec’d This?
A History of Specifications
When you see a space shuttle sitting on the launch pad, there are two big booster rockets attached to the sides of the main fuel tank. These are the solid rocket boosters, or SRBs.
The SRBs are made by Morton Thiokol at a factory in Utah.
Originally, the engineers who designed the SRBs wanted to make them much fatter than they are. Unfortunately, the SRBs had to be shipped by train from the factory to the launch site in Florida and the railroad line runs through a tunnel in the mountains. The SRBs had to be made to fit through that tunnel. Now, the width of that tunnel is just a little wider than the U.S. Standard Railroad Gauge (distance between the rails) of 4 feet, 8.5 inches.
That’s an exceedingly odd number. Did you ever wonder why that gauge was used? Because US railroads were designed and built by English expatriates, and that’s the way they built them in England.
Okay, then why did the English engineers build them like that?
Because the first rail lines of the 19th century were built by the same craftsmen who built the pre-railroad tramways, and that’s the gauge they used.
I’ll bite, why did those craftsmen choose that gauge? Because they used the same jigs and tools that were previously used for building wagons, and you guessed it, the wagons used that wheelspacing.
Now I feel like a fish on a hook! Why did the wagons use that odd wheel spacing?
Well, if the wagon makers and wheelwrights of the time tried to use any other spacing, the wheel ruts on some of the old, long distance roads would break the wagon axles. As a result, the wheel spacing of the wagons had to match the spacing of the wheel ruts worn into those ancient European roads.
So who built those ancient roads?
The first long distance roads in Europe were built by Imperial Rome for the benefit of their legions. The roads have been used ever since.
And the ruts?
The initial ruts, which everyone else had to match for fear of destroying their wagons, were first made by Roman war chariots. And since the chariots were made by Imperial Roman chariot makers, they were all alike in the matter of wheel spacing.
Well, here we are. We now have the answer to the original question. The United States standard railroad gauge of 4 feet, 8.5 inches derives from the original specification for an Imperial Roman army war chariot.
Specs and bureaucracies live forever.
That’s nice to know, but it still doesn’t answer why the Imperial Roman war chariot designers chose to spec the chariot’s wheel spacing at exactly 4 feet, 8.5 inches.
Are you ready?
Because that was the width needed to accommodate the rear ends of two Imperial Roman war horses!!!
Well, now you have it. The railroad tunnel through which the late 20th century space shuttle SRBs must pass was excavated slightly wider than two 1st century horses’ butts.
Consequently, a major design feature of what is arguably the world’s most advanced transportation system was spec’d by the width of a horse’s behind!
So, the next time you are handed a specification and wonder what horses’ rear end came up with it, you may be exactly right. Now you know what is “behind” it all.
~Author Unknown~
CompuWeld© Program for Ultrasonic Welding
CompuWeld© Software is used for data acquisition with Branson 900 and 2000 series ultrasonic welders for monitoring weld data and providing real time SPC of the critical weld data from the ultrasonic welding process. CompuWeld© SPC or Statistical Process Control involves collecting data from a welder for the purpose of monitoring the process through the use of statistical tools, such as XBar and RBar control charts. Analyzing the output data is done to maintain control of the process and to improve performance of the process. In ultrasonic plastic assembly, the information available about the welding process comes in the form of data about WELD TIME, WELD ENERGY, PEAK POWER, WELD COLLAPSE DISTANCE, TOTAL COLLAPSE DISTANCE AND ABSOLUTE DISTANCE OR FINAL POSITION.
WELD TIME is the duration that ultrasonic energy is on during the weld cycle. ENERGY is the total value of the watts used during the cycle multiplied by the actual time used during the cycle. PEAK POWER is the largest percentage of watts used at a single point in time during the cycle. WELD COLLAPSE is the amount of distance the actuator traveled after the trigger switch was activated and before the hold time. TOTAL COLLAPSE is the amount of distance the actuator traveled after the trigger switch was activated and after the hold time. ABSOLUTE DISTANCE or FINAL POSITION is the total amount of distance traveled by the actuator after leaving the upper limit switch. By monitoring these process variables, we hope to minimize unwanted causes of variation and improve control of the ultrasonic welding process.
There are two types of variation evident in all processes. Natural or random variation and variation caused by special or assignable causes. A process is said to statistically be in control when the only source of variation is coming from natural or random causes. But as Deming said, “a state of statistical control is not a natural state for a manufacturing process. It is instead an achievement, arrived at by elimination, one by one, by determined effort, of special causes of excessive variation.” Types of special causes that can be found in the use of ultrasonic equipment include variation evident in differences in equipment, tooling, setup, operators, material, molding conditions and the environment. In order to use the statistical tools available in “CompuWeld© 2000″ for monitoring a stable process, the welding process must first be brought into statistical control by eliminating causes of variation created by special or assignable causes.
On the CompuWeld© 2000 run view screen, one can see (6) XBar, RBar charts on the screen at one time. The XBar chart is drawn with the XBar above the RBar chart. The values of XBar and RBar are displayed on the vertical scale and the sequence of subgroups are displayed through time on the horizontal scale. The variable being monitored is shown on the top left corner of the XBar chart. XDouble Bar and RBar are shown with dotted lines running horizontally. Control limits are shown with solid lines running horizontally. The last subgroup average measurement and the last subgroup range or variation is shown on the bottom left corner below the RBar chart. The process capability measurements known as the CP ratio and CPK are shown on the bottom right hand corner below the RBar chart. The picture above is illustrative of the data available from the Compuweld© software.
Ultrasonically Bonded Swimsuit
The Speedo LZR Racer swimsuit was used by gold medalists at the olympic games and was bonded with ultrasonic welding to reduce drag. Ultrasonic bonding helped provide the LZR Racer swimsuit with the necessary competitive edge.
Routine Ultrasonic Plastic Welder Maintenance
The routine maintenance items for an ultrasonic welder are simple unless there is a machine failure. The following are recommended maintenance procedures.
1. Make sure that you have a Mylar washer between the converter and booster and between the booster and horn. We recommend replacing these washers every 6 months. A bag of (10) Mylar washers can be purchased for $12.00 from Plastic Assembly Technologies, Inc. at www.patsonics.com.
The Plastic Assembly Technologies part numbers are as follows:
· MW-12 for a bag of (10) Mylar washers for use on ultrasonic horns and boosters with ½-20 studs
· MW-38 for a bag of (10) washers for use on ultrasonic horns and boosters with 3/8-24 studs.
2. When disassembling the stack (ultrasonic converter, booster & horn) for replacing the Mylar washers, check the studs to make sure they are properly torqued. The stud torque is 290 inch/lb for 3/8-24 studs and 450 inch/lb for ½-20 studs. When reassembling the stack, the torque specification of the converter to the booster and the horn to the booster is 220 inch/lb.
3. Drain the air filter as required if it has collected moisture. Dry air is recommended as air with moisture will eventually impact the functionality of the pneumatic system.
4. Once a year or more in dirty environments, use light air pressure or vacuum to clean the inside of the power supply. Make sure the power is disconnected before following this procedure.
