Ultrasonic welding involves converting high-frequency electrical energy (generally 15-40 kHz) to high frequency mechanical energy in the form of reciprocating vertical motion. The welding tool, the horn, delivers pressure and high-frequency vibrational energy to the joint interface. The point under highest stress melts and flows across the interface of the mating parts, bonding the two surfaces.
With ultrasonic welding there are many requirements that must be met to be successful. The finished part as well as the joint, the weldablility of the material, alignment and support of the parts, and horn placement are all important factors, While all of these things play a major role in successful welding, design is probably the most crucial. In designing for ultrasonic welding, the first consideration must be the specifications of the part. These requirements include:
• The load bearing strength of the seam.
• Whether or not it must be a hermetic seal.
• Cosmetic appearance.
• Minimization of plastic waste.
• Preventing migration of plastic fragments to the interior of the part.
These factors will dictate the design of the joint, the fit tolerances, and the position of the ultrasonic horn.
Designing the Part
There are many possible joint designs and variations which are used in ultrasonic assembly. The most widely used are the energy director, the shear joint and the tongue and groove joint. An important point with joint design is reproducibility. A properly designed considerations are:
• The initial contact area between the mating surfaces should be small to concentrate and
decrease the total energy time needed to start and complete melting.
• A means for aligning the mating parts should be provided
• Mating surfaces around the entire joint interface should be uniform and in intimate contact with each other
Three crucial elements of successful welding are:
• The part and joint design.
• Welding equipment.
• The “nest” or fixture that holds the parts to be welded.
There are many possible joint designs and variations which are used in ultrasonic assembly. The most widely used are the energy director, the shear joint and the tongue and groove joint. An important point with joint design is reproducibility. A properly designed considerations are:
• The initial contact area between the mating surfaces should be small to concentrate and
decrease the total energy time needed to start and complete melting.
• A means for aligning the mating parts should be provided
• Mating surfaces around the entire joint interface should be uniform and in intimate contact with each other
Three crucial elements of successful welding are:
• The part and joint design.
• Welding equipment.
• The “nest” or fixture that holds the parts to be welded.
Material Factors
Factors such as the molecular structure, melt temperature, modulus of elasticity and chemical makeup will affect the weldability of the material. Amorphous resins are characterized by a random molecular structure and the tendency to melt and recrystallize gradually. Amorphous materials are efficient at transmitting ultrasonic vibrations and can be welded with many force/amplitude combinations. The height of the energy director should be 50% of the width of the base.
Crystalline materials have an orderly molecular structure and a sharp melting and resolidification point. These materials do not transmit vibrational energy as well and usually require a higher amplitude (greater energy imput) to reach a melt point. The height is over 60% the energy director could bend under pressure. The width of the ase for both amorphous and crystalline materials should be 20% to 25% of the total joint wall thickness.
Butt Joints
The butt joints is one of the simplest joint designs. By adding an “energy director”, excellent results can be seen. The energy director is a raised triangular ridge of material molded on one of the joint surfaces. The
energy director with be the first spot to start melting because it is under the greatest stress. The size and
angle of the energy director is usually dictated by the type of material.
Tongue and Groove Joints
The tongue and groove joint is considered to be much stronger than the butt joint because, themelt is enclosed, increasing strength and reproducibility. It is also self-aligning and visually perfect. Here again, the depth and width of the groove is slightly greater than the impinging tongue so that themelt is captured within the joint. The downside of the design is that the close tolerances make parts harder to mold and large wall thickness are required.
Shear Joints
The shear joint is best when welding crystalline materials which have a sharp and narrow melting point. Energy directors are not used for crystalline because melting and re-solidification can occur so quickly that fusion of the joint surfaces is not achieved. Because of the small contact areas the surfaces begin to melt the parts telescoping them together and continue along the vertical walls. The molten interface never comes in contact with the surrounding aire, and yields a strong structural seal. The vertical dimension of meltdown of the joint can be adjusted for the part. The shear joint requires the following:
• Rigid side wall support to prevent deflection during welding
• The walls of the bottom section must be supported at the joint bu the holding fixture.
• The top part should be able to withstand internal deflection.
• The top part should be as shallow as possinle.
• The design should allow for a clearance fit.
• A minimum lead-in should be incorporated.
Shear joints and crystalline materials require more energy. This means the weld time must be 4 times longer or input power must be greater than 2,000 watts with high amplitude output. Shear joints work extremely well for cylindrical parts.
Weldability Factors
Weldability depends on the compatibility of the materials. Crystalline materials can only be welded to themselves. Amorphous materials are limited when bonding to other amorphous resins and blends. Other factors include:
• The melt temperature must be within 30-40°F, for all like resins.
• For best results, resins of the same greade should be used.
• Moisture will have an adverse effect on the quality of the weld. Hygroscopic resins may absorb moisture from the air and start to bubble at the joint surface during welding, affecting bond strength and appearance. Parts shouldbe kept in a polyethylene bag.
• Fillers can increase the weldability of thermoplastics to a point.
• Mold release agents can affect the heat generation at the part interface during welding.
• Lubricants weken the weld bu reducing the intermolecular friction.
• Plasticizers can interfere with the resin’s ability to transmit the vibratory energy.
• Pigments’ particularly oil-based colorants can adversely affect the welding process.
• Design flaws or loose tolerances can result in dimensional and weight fluctuations, surface
defects and material stress and poor welding.
Other Considerations
• The location of the joint and its position relative to the surface of horm contact is critical.
• Joints less than 0.25″ from the horn are near field and, joints more than 0.25″ are far-field.
• Crystalline materials don’t transmit vibration energy well, they should be welded neer-field.
• Far-field welding is not recommended for amorphous resins especially with distances greater than 0.25″
• Horn-to-part contact area should be larger thant the total weld area.
• Design the part so that the energy travel is the same distance through the material to get from the horn to the joint.
Conclusion
Any success of ultrasonic welding will only be as good as the part design, type joint and equipment used. Prototyping and testing consistency are very instrumental when proving any given application. It is relatively inexpensive to build prototype tools to prove the design. As we know, prototype tools can be altered fairly easy which in turn favors joint design and part function. One should get a good statistical sample with long prototype runs. Consistent testing is importnant becuase even if the weld looks perfect and you can’t pull it apart it must still perform out in the field. Take advantage of technical resources such as ultrasonic equipment suppliers. They are ready, will ing and able to offer help in part and joint design.
Dallas Cada is a highly trained plastics engineer with over 20 years of sales support experience. Owner of a plastic consulting business (DDC Consulting), his experience includes technical service, application development, market engineering, injection molding, design, tooling, material suggestions and problem solving for plastic manufacturing companies.
For more information with troubleshooting plastic problems or helping with new plastic applications, contact Dallas Cada by e-mail at dallascada@charter.net. Contact Dallas by phone (507) 458-5785 or (507) 452-1584.
