Effect of additives on ultrasonic welding

Update:09 Mar 2019

Additives, although improving the overall properties or […]

Additives, although improving the overall properties or formability of the base metal, often increase the difficulty of obtaining a high quality welded joint. Typical additives include: plasticizers, impact modifiers, flame retardants, colorants, lubricants, blowing agents, regrinds. Plasticizers, high temperature organic liquids or low temperature melts increase the flexibility and softness of the plastic and reduce its rigidity. They reduce the intermolecular attraction within the polymer and affect the transmission of vibrational energy. Highly plasticized materials such as vinyl are very poor ultrasonic energy transfer media. Plasticizers are often considered as internal additives, but they migrate to the surface over time, making ultrasonic welding almost impossible. Metal-containing plasticizers are more harmful than plasticizers approved by the US Food and Drug Administration (FDA). Impact modifiers such as rubber reduce the ability of materials to transmit ultrasonic vibrations, requiring higher amplitudes to produce melting. Impact modifiers reduce the amount of thermoplastic material at the bonding surface and thus the weldability of the material. Flame retardants, inorganic oxides or halogenated organic elements such as aluminum, lanthanum, boron, chlorine, bromine, sulfur, nitrogen or phosphorus are added to the resin to inhibit combustion or to alter the burning properties of the material. In most cases, they are not solderable.
Flame retardants may account for 50% or more of the total weight of the material, reducing the amount of weldable material in the part. When welding these materials, it is necessary to use high-power equipment, higher than normal amplitude and change joint design to increase the amount of weldable material at the joint surface. The effect of pigments on ultrasonic welding can be quite large. Most pigments are inorganic compounds and are typically used at concentrations ranging from 0.5% to 2%. Most colorants do not inhibit ultrasonic energy transfer. However, they cause a reduction in the amount of effective weldable material at the joint surface. Titanium dioxide in white pigments is inorganic and chemically inert. It acts as a lubricant and, if used in excess of 5%, reduces solderability. Carbon black also hinders the ultrasonic energy transfer of the material. Process parameters may need to be changed when the colorant is included in the plastic. If the welding equipment uses standard unwelded parts to produce high-quality welds, the weld quality of the colored parts may be significantly lower (wear strength and brittleness). The mechanism by which pigments affect ultrasonic welding has not been confirmed so far. The presence of the pigment appears to affect the way the heat is generated at the joint. Usually the colored parts are welded longer than the expected time of the undyed parts, thereby solving the problem of low weld quality. Welding time may have to be increased by 50% or more.
However, longer weld times can have undesirable effects such as excessive weld burrs and damage under the weld head. When it is intended to ultrasonically weld a coloring material that must be molded, it is recommended to test the molded sample to determine its feasibility. Weld strength and toughness are not critical requirements in many commercial applications. The use of pigments that do not significantly affect ultrasonic welding may be an alternative solution. Internal lubricants (wax, zinc stearate, stearic acid, fatty acid esters) reduce the coefficient of friction between polymer molecules, resulting in reduced heat generation. However, because their concentration is very low and diffusely distributed in the plastic rather than concentrated at the bonding surface, the impact is usually minimal. The blowing agent reduces the ability of the resin to transfer energy. Depending on the density, the voids in the microporous structure interrupt energy flow to varying degrees, reducing the energy reaching the joint region. Materials to be welded containing higher or different levels of recycled materials should be carefully evaluated. In order to achieve optimal welding, it is necessary to control the quality and quantity of the regrind in the parts to be welded. In some cases, 100% pure raw materials may be required.