The physical and mechanical properties of plastics are closely related to temperature. As temperature changes, the properties of plastics change, exhibiting different physical states and displaying phased mechanical properties. The physical state and mechanical properties of plastics under heat are of great significance to the molding and processing of plastics.
Due to the influence of the main component of plastics, polymers, plastics often exist in the following physical states when heated: glassy state (also known as crystalline state for crystalline polymers), elastic state, and viscous flow state. The curve showing the relationship between the degree of deformation of plastics and temperature when heated is called a thermodynamic curve, as shown in Figure 1-1.
(1) Glass Transition State
When plastic is at a certain temperature θg, as the hardness of the solid gradually increases with decreasing temperature, the multi-component parts using this plastic will also gradually increase in hardness. This is a gradual increase of hardness from the softened state. When θg is the various allowable temperatures, below θg is a certain temperature, the plastic will undergo brittle fracture. This temperature value is called the glass transition temperature, which is the lower limit of plastic usage temperature.
Plastics at the glass transition state - which are not suitable for processing that requires significant deformation - can undergo processing such as bending, drilling, cutting, etc.
(2) High Elastic State
When plastic is heated to a temperature higher than θg, it will exhibit a highly elastic rubber-like high elastic state. The higher the temperature from θg, the better the high elastic state. For plastics in a highly elastic state, if external forces cannot be applied for large increases, they can easily deform. Under constant stress, creep and stress relaxation will occur in the elastic state. If demolding is performed too early, the molding will remain at the temperature that is higher than the immediate demolding temperature θg.
(3) Viscous Flow State
When plastic continues to be heated to a temperature higher than θf, it will exhibit significant viscous flow characteristics. The plastic in the viscous flow state becomes a liquid. In the plastic molten state, its deformation is no longer reversible. After maintaining a constant state and unloading, it cannot return to its original state. θf is the lower limit temperature for molding, which is the minimum processing temperature. The shape stability of materials from liquid state changes to an elastic state (or elastic state changes to viscous state).
When plastic continues to be heated, the temperature reaches θf and increases. The plastic begins to decompose and discolor. The electrical resistance of the plastic decreases sharply. θf is the upper limit of the decomposition temperature. It is the reasonable temperature limit for molding processing at high temperatures. Therefore, θf and θg are the upper and lower temperature limits that must be considered when selecting molding processes. θf - θg is the available range of molding processing temperatures.