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Stiffness and strength for toolbox moulding,safety helmet moulding
Temperature and time are two factors which affect the deformational characteristics of plastics like toolbox moulding,safety helmet，usually described in terms of ‘creep, Creep is defined as deformation of a material under load over a period of time. In this connection, one of the most important parameters for plastics is the operating ambient temperature of the material, since relative creep increases with increase in temperature. Most creep data are obtained under constant load，which is usually tensile; thus as creep increases the stress increases by reason of the corresponding reduction in area. If during a creep test a component is loaded for a period of time.
Most creep data are obtained under constant load，which is usually tensile; thus as creep increases the stress increases by reason of the corresponding reduction in area. If during a creep test a component is loaded for a period of time, the fracture can occur following an initial period of creep. This is known as creep rupture. It has been found that the lower the stress at which creep rupture occurs，the longer the time taken to fracture. Creep rupture is extremely important and has a significant effect on，for example (1) the design of plastics water-pipe which is subjected to internal pressure for long periods of time; (2) covers which, although possibly purely decorative，are manufactured for interference fits on metal bases under constant load; and (3) significant loads which may be encountered in assembly either from metal screws or rivets fastened on to a plastics component or from plastics fastening devices. The following are significant factors affecting the stiffness and strength of plastics materials.
- 1. Temperature. The stiffness of many plastics is reduced very significantly by an increase in temperature. Table Al.l gives values of tensile modulus at room temperature and at 115°C.
- 2. Time. A plastics component will creep under constant load, that is the deformation will increase continuously over the period of time that the load is applied. Table Al.l gives the value of the tensile modulus after a short time under load and also after 500 h and 1500 h under load.
- 3. Other factors. The stiffness of a plastics material is also affected by other factors such as moulding and post-moulding storage conditions，and the presence of liquids (for example nylon 66 is appreciably softened by atmospheric moisture even at normal humidity). If a plastics component is likely to be operating at close to its limit of capability then these factors should be given careful consideration.