Plastic Parts That Remember

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 skillfully 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, that allowed to return to their original shape. In this case, parts can be stretched around obstacles without permanent damage.

VI. Переведите и ответьте на письмо по-английски.

Dear Sirs,

Six members of our staff will be coming to London on busi­ness from 28 th of May to 1st of June. Would you therefore please reserve the following accommodations for five nights:

2 single rooms

2 double rooms for bed and breakfast

As my wife will be joining me in London on Saturday, 2 nd June, I shall need

1 double room with bed and breakfast for this date. Please bill this accomodation to me at my private address as above. Please confirm this booking.

Yours faithfully.

Вариант 4

I. Перепишите и письменно переведите следующие предложения, принимая во внимание, что инфинитив в функции определения и, особенно, объектный и субъект­ный инфинитивные обороты большей частью соответст­вуют придаточным предложениям (см. образец выполне­ния 1).

1. Students must be taught not to totally rely on computers.

2. Computers will allow people to reach every part of the world in only a few minutes or seconds.

3. Wild animals need to be examined by a vet with special knowledge and training in wildlife medicine.

4. Doctors managed to force Retton not to participate in the Olympics after the knee surgery.