ULTRASONIC WELDING AND INSERTION

Rebling Plastics utilizes ultrasonics for assembly operations that include: welding of plastic materials; installing a metal insert into a plastic part; and staking of plastic parts. Ultrasonics is a vibratory mechanical energy above the range of human hearing, typically 20,000 cps (cycles per second) although some units are designed to operate at 40,000 cps. This vibratory energy is transmitted to the plastic injection molded part which causes localized melting at the interface of the metallic insert and molded plastic part. After the preset weld time is reached or the insert is installed to the correct position, this vibratory energy ceases and the plastic solidifies locking in the insert or molecularly welding the two plastic parts together. This molecular weld can reach up to 90% of the material's strength if done properly.

For the ultrasonic operation to be successful, the plastic part must be supported by a fixture that limits the motion of the plastic part. This fixture must also provide support directly below the contact area of the Horn (titanium or D2 hardened steel piece which actually transmits the vibration to the part), otherwise a phase shift resulting in a poor weld will occur.

Plastic Molding Material Selection for Ultrasonics

Thermosets are not applicable to ultrasonics since they cannot be melted once cured. The thermoplastics can be divided into amorphous and crystalline types. Amorphous materials are the easiest to weld while the crystalline materials require a higher melt temperature and have a narrower processing window. Although dissimilar plastics can be welded together, their melt temperatures should be within 40°F of each other.

Moisture absorption of the plastic injection molding materials can impact the weld and is of particular concern for nylons, polycarbonates and polystyrenes and other hygroscopic materials. During welding, trapped moisture is boiled off by the ultrasonic energy. For applications requiring a sealed joint, the plastic should be dried prior to ultrasonic assembly. Glass and mineral fillers formulated in the plastic resin should be less than 45% PBW as higher concentrations are difficult to weld.

Ultrasonic Design Parameters for Plastic Moldings

The plastic part to be ultrasonically welded requires a small initial contact area that forces the ultrasonic energy into a small surface, commonly referred to as an "energy director". This energy director can take the shape of a "v' shaped protrusion of the plastic. For amorphous materials, the included angle of the "v"shaped protrusion should be 90 degrees, while 60 degrees is appropriate for crystalline materials.

The strongest joint achievable using ultrasonics is the shear joint. Parts designed for a shear joint weld require interferences of .010 to .020 per side depending on the part size and geometry.

Gate location of the molded part should be positioned so that the weld or insertion does not occur at the "knit line".

Rebling Plastics engineering staff can assist you in specifying the correct geometry of the molded part in order to insure a successful ultrasonic assembly operation.