Materials and Ultrasonic Welding

Polymer materials consist of long-chain molecules. These molecules are many hundreds if not thousands of times longer than their width. They can be visualized as human hairs, and are usually illustrated by lines. Thermoplatic materials come in two general groups: amorphous or semi-crystalline. Amorphous materials have the same molecular structure when solid as when they are liquid, that is, no particular structure at all. The molecules randomly orient, flow easily, and may or may not have a predominant orientation depending on flow conditions and speed of cooling. The most common non-polymer amorphous material is glass. When glass is blown, the artisan heats parts of the object and applies force internally through air pressure and externally using tools to produce the particular shape desired. The glass flows and is formed but whether in the “liquid” or “solid” state it retains clarity. This is because there is no internal structure to inhibit the passage of light (or sound). Semi-crystalline materials behave (and appear) more like candle wax. They are brittle when frozen, that is, below the glass transition temperature, increase in toughness as temperature rises to the melt range, at which point they become semi-flowable, though they retain shape memory of their crystal structure when last frozen, and can tend to return to their frozen form if vibrated or heated. They flow freely when truly melted. An amorphous material has no true melt temperature, and as such has no true “solid” state, it is simply less and less flowable as temperature drops until it is experienced as a solid. Semi-crystalline materials have portions of almost all of the molecules locked into crystal structures in an overall amorphous matrix, giving them a true melt temperature, with a corresponding phase change energy spike in specific heat. A semi-crystalline material will run like water when hot enough (and appear clear), like the pool of liquid wax around the burning wick of a candle, mold like putty when not fully melted but near melt temperature, like the wax surrounding the molten pool, and exhibit true solid behavior when cooler, like the body of the candle not near the flame. Just like candle wax dripped on a cool candle, melted material up against cool solidified material will not intermingle to produce a homogeneous weld area, so semi-crystalline materials always require near-identical resin chemistry and melt characteristics to join well. Amorphous materials, since they will flow and intermingle over a relatively broad range of temperature, are much more tolerant of resin chemistry or melt charaterstic mismatches or variations.