Assembly Considerations

An energy director is a triangular-shaped rib located on one of the parts to be welded. The angle formed by the sides of the energy director is typically 60° or 90°. The sharper 60° angle tends to be more practical for thin-walled parts. In butt joints, the width of the base of the energy director is typically about 1/4 of the wall thickness, but experience shows that energy directors need to be at least 0.020" tall to generate consistent, continuous seals. This may force the base width to be greater than 1/4 the wall thickness. The energy director tapers up to a sharp point which acts to concentrate the energy of the vibratory motion at a single point of contact and generates significant heating of the plastic. As the heat builds, the energy director tip melts rapidly, filling the joint with molten resin and melting the surrounding areas slightly. A second force is applied to the two parts pushing them together forcing the melted plastic to spread into the closing gap which welds the two parts together.

A flat butt joint, like the one shown here, may result in melt ooze-out from the sides making it look unsightly. This is often called weld flash and is avoided by using "Step" or "Tongue and groove" design to catch the melted plastic.

Energy director type joints yield a strong mechanical bond if designed and welded properly. However, voids in the welded surface can occur causing discontinuity of weld and a failure to provide a leak-proof joint. A shear joint, also known as an interference joint, provides a more reliable hermetic seal.

Frequency	15 KHz	20 KHz	30 KHz	40 - 50 KHz
Max. linear weld size*	Large (40")	Medium (25")	Delicate & small parts (16")	Delicate & small parts (10")
Energy in watts	to 6000	to 3300	to 1500	150 to 800
Sound enclosure	Necessary**	May be needed	Not needed	Not needed

* Approximate numbers - based on present state of technology
** 15 KHz is within human hearing range

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