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PLASTICS WITH MEMORY EFFECT
Thermoplastics can be bent, pulled, or squeezed into various useful shapes. But eventually — especially if you add heat — they return to their original form. This is known as plastic memory. Plastic memory offers some interesting design possibilities.
Thermoplastics never forget. You deform them; and after a while, depending on temperature, they move back toward their original shape.
When most materials are bent, stretched, or compressed, they somehow alter their molecular structure or grain orientation to accommodate the deformation — permanently. Not so with polymers. Polymers temporarily assume the deformed shape but always maintain internal stresses that want to force the material back to its original shape. Usually, this desire to change shape is called plastic memory.
This so-called memory is often undesirable. Sometimes people prefer that thermoplastic parts forget their original shape and stay put — especially when the parts must be formed, machined, or rapidly cooled. However, this memory, or instability, can be used advantageously.
The time/temperature-dependent change in mechanical properties results from stress relaxation and other viscoelastic phenomena typical of polymers. When the change is an unwanted limitation, it is called creep. When the change is skilfully adapted to the overall design, it is called plastic memory.
Most plastic parts can be produced with a built-in memory. That is, the tendency to move into a new shape is included as an integral part of the design. So then, after the parts are assembled in place, a small amount of heat can make them change shape. Seals, gaskets and seamless covers for tubing and wiring are typical examples.
In other applications, plastic parts can be deformed during assembly, and then allowed to return to their original shape. In this case, parts can be stretched around obstacles without permanent damage.
Potential memory exists in all thermoplastics. Polyolefin, neoprene, silicone, and some other polymers can be given a memory either by radiation or by a chemical change.
Memory can be exploited in four ways:
The component is deformed at room temperature. Upon heating, the component recovers its original dimensions.
The component is deformed at an elevated temperature and — while held in the strained condition — it is cooled to room temperature so that the deformation is “frozen in”. Upon reheating, the component returns to its original dimensions.
The component is used in a confined situation under constant stress. The deformed sections try to return to their original dimensions or form. Since the part is restrained from doing this, a stress — in addition to the normal elasticity— is produced which is most often used for sealing.
The component is deformed for a short interval, and then the stress is removed. After a time, at room temperature, most of the deformation is recovered. This condition is often used for installation of parts over obstructions.
2. Find in the text English equivalents to the following words and word combinations:
можливості, термопластики, деформувати, первинна форма, змінювати молекулярний склад, полімери, внутрішня напруга, труднощі, прокладка, безшовна оболонка, первинні розміри.