If a heavy object is attached to one end of an elongated plate made of a thermoplastic resin, after a long period of time, it is found that the length of the plate increases little by little with time, and even if When the weight is removed, the length of the board will not be restored. We call this phenomenon creep.

If tensile stress or compressive stress is applied to both ends of the plate and kept at a certain length, and after a long period of time is placed, it will be found that the applied stress is gradually becoming smaller or even zero. Call this phenomenon stress relaxation.

Of course, this phenomenon is not seen when the force is applied for a short period of time, and can only occur under long-term stress. This phenomenon is not seen in steel, ceramic or thermosetting resins, thermoplastic resin. This phenomenon occurs in connection with the linear giant molecules slipping along the direction of stress over time.

Therefore, in order to prevent this from happening, it is necessary to increase the lateral binding force between macromolecules and macromolecules. One of the methods is to increase the crystallinity, such as nylon, polyester and polypropylene, which would otherwise crystallize. The degree is higher, but in order to better prevent the above phenomenon, heat treatment or stretching must be used to maximize the crystallinity of the fiber processing. Another method is to use certain chemicals to make it The formation of a three-dimensional network structure, such as a thermosetting resin, belongs to this type, and the restraint of the three-dimensional network structure can effectively prevent slippage between macromolecules.

Creep or stress relaxation of plastics often causes some accidents. For example, if two products are concave and one is convex, when forced into a form of "sub-fitting", due to the forced pressing, the two products in the joint must be subjected to compressive stress and The effect of tensile stress, the joint will loosen for a long time and affect the continued use of the assembly. In addition, during the storage or handling of the product, care must be taken not to subject it to great stress, otherwise it will cause deformation of the product. Damage and other issues.

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