The performance of plastics affects the effect of ultrasonic welding. The ultrasonic weldability of plastics depends on the plastic's attenuation ability to ultrasonic vibration and the level of melting temperature, as well as physical properties such as elastic modulus, impact resistance, coefficient of friction and thermal conductivity. Generally speaking, the greater the hardness of plastic products, the easier it is to weld. Hard plastics can easily transmit ultrasonic energy, while soft plastics often attenuate energy before it reaches the joint area. The hardness of plastic products also changes with the environmental temperature and humidity. Pigments, mold release agents, glass fillers and reinforcing fibers have a greater impact on it.

1. The influence of fillers and reinforcements on ultrasonic welding

Fillers (glass, talc, minerals) in thermoplastics can improve or inhibit ultrasonic welding. Materials such as calcium carbonate, kaolin, talc, aluminum hydroxide, organic fillers, silica, glass balls (wollastonite) and mica can increase the rigidity of the resin. When the content reaches 20%, it can increase the entire material (especially semi-crystalline plastics). ) Ultrasonic energy transfer performance. When the content reaches 35%, there may be insufficient thermoplastic resin at the joints that need to be reliably hermetically sealed. When the filler reaches 40%, the fibers gather at the joint surface, and the thermoplastic material is insufficient to form a firm connection. The long fibers may aggregate during the molding process, causing the energy guiding ribs to contain a higher percentage of glass than the matrix material. This problem can be solved by using short fiber glass fillers. When the filler content exceeds 10%, many abrasive particles in the filler cause wear of the welding head. It is recommended to use hardened steel or hard alloy coated titanium welding heads. It may also require a higher power ultrasonic device to generate enough heat at the joint.

2. The influence of additives on ultrasonic welding

Although additives can improve the overall performance or formability of the base metal, they usually increase the difficulty of obtaining high-quality welded joints. Typical additives include: plasticizers, impact modifiers, flame retardants, colorants, lubricants, foaming agents, and recycled materials. Plasticizers, high-temperature organic liquids or low-temperature melts increase the flexibility and softness of plastics and reduce their rigidity. They reduce the intermolecular attraction force in the polymer and affect the transmission of vibrational energy. Highly plasticized materials such as vinyl are very poor ultrasonic energy transfer media. Plasticizers are generally regarded as internal additives, but over time they will migrate to the surface, making ultrasonic welding almost impossible. Metal-containing plasticizers have more harmful effects than plasticizers approved by the U.S. Food and Drug Administration (FDA). Impact modifiers such as rubber reduce the material's ability to transmit ultrasonic vibrations and require higher amplitudes to produce melting. Impact modifiers reduce the amount of thermoplastic material at the joint surface, which also affects the weldability of the material. Flame retardants, inorganic oxides or halogenated organic elements such as aluminum, antimony, boron, chlorine, bromine, sulfur, nitrogen, or phosphorus are added to the resin to inhibit combustion or change the combustion 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 solderable material in the part. When welding these materials, it is necessary to use high-power equipment, higher than usual amplitude and change the joint design to increase the amount of weldable material at the joint surface. The effect of pigments on ultrasonic welding can be considerable. Most pigments are inorganic compounds, and the concentration used is generally 0.5% to 2%. Most colorants do not inhibit the transmission of ultrasonic energy. However, they cause a reduction in the amount of effective solderable material at the joint surface. The titanium dioxide in the white pigment is inorganic and chemically inert. It can act as a lubricant, if the content exceeds 5%, it will reduce the weldability. Carbon black also hinders the ultrasonic energy transfer of the material. When plastics contain colorants, process parameters may need to be changed. If the welding equipment uses undyed parts to produce high-quality welds to weld colored parts, the weld quality of colored parts may be significantly lower (low weld strength and high brittleness). The mechanism by which pigments affect ultrasonic welding has not been confirmed so far. The presence of pigments seems to affect the way the joints heat up. Usually, the welding time of colored parts is longer than the expected time of undyed parts, which can solve the problem of low weld quality. The welding time may have to be increased by 50% or more. However, a longer welding time may have adverse effects such as excessive welding flash formation and damage under the welding head. When intending to use ultrasonic welding of coloring materials that must be molded, it is recommended to conduct trial welding on molded samples to determine its feasibility. In many commercial applications, weld strength and toughness are not critical requirements. 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 a decrease in heat generation. However, because their concentration is very low and they are dispersed in the plastic rather than concentrated on the joint surface, they usually have the least impact. Blowing agents reduce the ability of the resin to transfer energy. Depending on the density, the voids in the microporous structure interrupt the energy flow to varying degrees, reducing the energy reaching the joint area. The materials to be welded containing higher or different content of recycled materials should be carefully evaluated. In order to achieve the best welding, it is necessary to control the quality and quantity of recycled materials in the parts to be welded. In some cases, 100% pure raw materials may be required.

3. The influence of release agent on ultrasonic welding

External mold release agents (zinc stearate, aluminum stearate, fluorocarbon, silicone) are usually sprayed on the surface of the mold cavity to form a separate coating that facilitates the removal of parts. The release agent may transfer to the joint surface, reduce the friction coefficient of the material to be welded, affect the heat generation of the joint surface, and hinder the fusion of the molten surface and the formation of a suitable connection. Silicone has the most harmful effects. The external release agent can sometimes be removed with a solvent. If it is necessary to use an external release agent, the coatable and printable release agent does not transfer to the molded part, but prevents the resin from wetting the mold surface. These levels of release agent have the least harmful effect on ultrasonic welding.