Plastic Assembly News

Criss-Cross Energy Director Design For Ultrasonic Welding

CRISS- CROSS ENERGY DIRECTOR DESIGN

 

 

The use of the criss-cross energy director design has proven to be beneficial for many ultrasonic plastic welding applications. This weld joint has been used in the medical, electronic and cc-energy-directorautomotive industries and has been a good joint design for achieving strong bonds with hermetic seals.  Essentially, the criss-cross energy director design utilizes the standard energy director shape where a triangular shaped bead of material is molded into the plastic wall. This molded-in triangular ridge of plastic is very effective at reducing the cycle time to achieve a weld and in compensating for non-uniform wall surfaces.  Depending upon the wall thickness and the application, the energy director typically varies in height in a range from .010 to .035 of an inch.  The peak of the energy director should be sharp with a triangular shape formed from a 60º or 90º included angle.  The energy director design has been used for years as a means of focusing the energy to improve weld strength and reduce cycle time.  The energy director has typically been placed only on one half of the part and runs along the surface to be welded. Without this energy director the weld quality would be suspect for many applications. The criss-cross design adds additional energy directors to the mating part, which increases the amount of material interaction.  On the mating surface opposite the perimeter energy director, a series of perpendicular energy directors are molded-in to mate with the perimeter energy director.  When a hermetic seal is desired these additional energy directors should take on a saw tooth pattern with each energy director repeating from the base of the proceeding energy director.  Because there are energy directors on both mating surfaces, the energy director height on each half should be reduced to prevent excessive material flash during welding.  Typically, it is recommended that the criss-cross energy director height be approximately 60% of the standard design. 

The joint is particularly effective when used in combination with a tongue and groove joint design.  The tongue and groove design provides the alignment necessary for a good ultrasonic weld joint and the groove serves as an excellent reservoir for the melted material. This pooling of plastic material helps contain the material and reduces the likelihood of a leak path.

tongue-and-grooveBecause of the increased material flow with the criss-cross energy director design, it is recommended that a tongue and groove joint be used to capture the additional plastic and contain the flash.

This criss-cross design certainly increases the mold cost, but we’ve seen applications where the weld strength has far exceeded expectations and hermetic bonds have been achieved that might not have been achieved using the standard energy director on one mating surface.   As always, each application is unique and should be evaluated thoroughly before implementing a joint design.       

Setup of Ultrasonic Plastic Welding Applications – Part 1

Obtaining the optimum setup conditions for a given application using ultrasonic weld equipment usually requires a series of tests to determine the welding parameters that provide the best results. Although there is no solution for circumventing this testing procedure, a proper understanding of the principles and components of an ultrasonic assembly system will help to expedite the process of reaching the optimum setup conditions for a given application. To begin our understanding of the optimizing process, it is necessary to understand the basic components that make up an ultrasonic welding system. The system usually consists of six basic components.

1) The POWER SUPPLY changes 50/60 hz electrical energy into high frequency electrical energy and the power supply is rated in watts of available output power. Frequencies are typically available in 15, 20, 30 and 40KHz. Power output ranges from 150 watts to 4000 watts of power. It is important to remember that just because a unit is rated at a certain output power capacity does not mean that an application will require full power from the supply or that the power supply will have adequate power for a given application. The power supply provides power on a demand basis depending upon the amount of power required for the application and under a given set of conditions. The size and shape of the part, the material being welded, the joint design and welder setup variables that control the ultrasonic process can alter the amount of power drawn from a power supply.
2) The CONVERTER is the motor of the ultrasonic system. This component produces a motion via a piezoelectric effect, which is to say that the converter expands and contracts when electrically excited by the power supply. At 20Khz, this expansion and contraction is approximately .001 of an inch at the face of the converter and the movement is in an axial motion. The converter is matched to the output frequency of the power supply.

3) The BOOSTER is a machined piece of aluminum or titanium metal, which is tuned to resonate at the desired frequency and designed to increase or decrease the motion that is produced at the face of the converter. The following decreases/increases can be obtained depending upon the booster used. The booster is sometimes color coded to ease identification of the booster amplification. The booster is attached to the face of the converter via a mechanical stud.

Converter Amplitude (.001) * Booster Gain = Output at Booster

Purple Booster .001 * 0.6: 1 = .0006 output at Booster

Green Booster .001 * 1:1 = .001 output at Booster

Gold Booster .001 * 1.5:1 = .0015 output at Booster

Silver Booster .001 * 2.0:1 = .002 output at Booster

Black Booster .001 * 2.5:1 = .0025 output at Booster

The booster is an important element in determining the output amplitude of the converter/booster/horn assembly. The correct booster and horn combination is very important because amplitude is a critical variable in achieving a successful results with a given application.

4) The ultrasonic HORN is a machined tool that resonates at the desired frequency. The horn is usually manufactured of titanium, aluminum or steel. The primary purpose of the horn is to uniformly transfer the ultrasonic energy to the work piece. The horn is usually made to match the shape of the part to most efficiently transfer this energy. Depending upon the size of the horn, there are various shapes of horn designs to increase the amplitude of the converter/booster/horn assembly. The horn is attached to the booster through the use of a mechanical stud. You can find more information about ultrasonic horns at www.patsonics.com.

5) The ACTUATOR is the pneumatic delivery system of the ultrasonic power. Its components consist of:

  • Solenoid valve
  • Cylinder
  • Flow control valve
  • Pressure regulator
  • Air gauge
  • Carriage to hold the converter/booster/horn assembly
  • Slide mechanism to deliver the carriage with the horn to the part
  • Triggering mechanism to determine the amount of pressure delivered to the part before the ultrasonic energy is activated or turned on.
  • Communication ports to interface to the power supply

Ultrasonic welding systems can be as simple as on/off or more intelligent units providing greater control and monitoring. Both deliver ultrasonic power to the work, but the intelligent systems provide the ability to weld to distance, monitor distances during welding, digitally monitor the pressure gauge, electronically set the trigger force, monitor the trigger switch distance range and control and monitor the forces seen by the system during welding. Pressure is a key variable in the delivery of ultrasonic power to the work. The dynamics and control of this variable are not well understood by many manufacturers that use the ultrasonic welding process.

6) The POWER SUPPLY PROGRAMMER is the control component that determines the sequencing, the duration, the delivery and monitoring of ultrasonic power applied to the work. On less sophisticated ultrasonic units, the duration of ultrasonic vibration is controlled primarily by time. Specifically, once ultrasonics is turned on or activated by a trigger switch, ultrasonic power continues to stay on until the duration of time is complete as established by the preset programmed time. With this type of programmer, there is limited control and monitoring of other system variables.With the advent of technology, the capability to expand the modes of welding and the monitoring of the welding process has grown dramatically. The more sophisticated units allow the ultrasonic welding machines to weld by distance, weld by energy, weld by peak power, weld by compensation modes and to monitor time, energy, force, power, down speed, amplitude, trigger distances, etc. It is the power supply programmer coupled with other system devices that provides all the modern day control and information about the process.

These six basic components provide the basis upon which we will continue our exploration of the dynamics of ultrasonic welding. It is important to understand the function of the basic components before continuing with our discussion of optimizing the ultrasonic setup.

Ultrasonic Boosters

Monday, March 24, 2008

Ultrasonic Boosters

Plastic Assembly Technologies, Inc., www.patsonics.com, has recently introduced a new line of aluminum rigid mount ultrasonic boosters without spanner wrench holes. Boosters without spanner wrench holes result in a reduction of stress location points on the booster and provide for aluminum 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 effecting 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 aluminum boosters, but provide a host of additional benefits. The ratio 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.

This category will discuss ultrasonic welding of plastics.

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