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The creep of plastic

by:CSSSLD     2021-01-23
For a given material, rigid filler or enhance fiber is added in the feed composition or changing internal physical structure makes it have a rigid material, helps to reduce material creep rate and creep limit increased. For product designers, however, the key is to obtain practical creep data. Because it is often difficult to test and match the actual situation, so it's best to creep experiment according to the actual conditions, the creep parameters under the conditions of use, if not to test for a long time, the plastic creep rate and initial modulus value in engineering calculation, to appraise several years after injection processing parts of creep condition and the modulus attenuation. In estimating the long-term creep condition, we should not ignore the influence of temperature change on the creep, it also should be fully expected.

creep process can be in a few hours to tens of thousands of hours, length of time depends on the size of the working load and working environment. For designers, design of plastic parts, should control its effective life at this stage. Creep development in the end, is a failure process, namely material creep rupture or yield. Under constant temperature and loading, display the curve of the relationship between deformation and time is called the creep curve.

if to determine a time, stress and to make all kinds of plastic materials, etc Strain curve, and gathers in the same coordinate system, would be easy to compare the creep properties of various materials. Different types of plastic, creep behavior are obviously different. This difference due to different types of polymers, under the effect of external force, the chain orientation of macromolecular thermal motion and rearrangement showed different results. Usually, in stress - Strain curve on rigid plastic good creep resistance.

usually characterization of creep behavior of the creep curves under different load consists of a set time Deformation curve. Under the same temperature, load had a great influence on creep behavior, high stress creep time short, material faster development by creep rupture strain values. Under low stress creep time is long. Load decreases to a certain degree, the creep rate close to zero, such as the variable to maintain a constant value for a long time, also namely material creep damage will not occur, the maximum stress appears creep damage not become creep limit.

in the design of plastic parts, life design is an important content, because the plastic strength and stiffness are related to time. In order to get the design in the fracture strength and modulus, stress and can be based on suppliers Creep curve and modulus - time Time creep curve for engineering calculation. Using single logarithmic coordinate or double logarithmic coordinate, can curve into a straight line. So, when a long service life, because most of the measured data is insufficient, can be extrapolated to data.

in the engineering, sometimes should proper creep variable is permitted, as long as to the specific material in the creep law of specific working conditions expected, namely know not happen damage during the required working life, or will not affect the use by constant creep. Injection molding processing parts creep is a slow and long term deformation development process. Initial stage, the rate of creep is larger, but gradually reduce the rate of instability, this process is usually the longest do not exceed a few hours. Followed by creep gradually stabilised, creep rate is almost in a certain value.

if extrapolation is too large, the data reliability is reduced, so only the extrapolation single digits, that is, outside the 100 h creep should not push more than 1000 h, 1000 h creep extrapolation should not exceed 10000 h. So, use for a point on the creep strength and service life of creep modulus, can be calculated by corresponding formula products under the condition of known load and the service life of the proper size. According to the stress and etc Strain curve, we can find out within the expected service life, in the role of to determine the stress, strain of material value or son allows maximum allowable stress deformation value range.

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