Characteristics of Plastics
Plastics mainly have the following characteristics:
- Most plastics are light, chemically stable, and will not rust.
- Plastics have good impact resistance.
- Plastics have good transparency and wear resistance.
- Plastics have good insulation and low thermal conductivity.
- Plastics have good general formability, good colorability and low processing costs.
- Most plastics have poor heat resistance, high thermal expansion rates, and are easy to burn.
- Plastics have poor dimensional stability, and are easy to deform.
- Most plastics have poor low-temperature resistance and become brittle at low temperatures.
- Plastics are easy to age.
- Some plastics are easily soluble in solvents.
Plastics can be divided into thermoset and thermoplastic. The former cannot be reshaped and used, and the latter can be repeatedly produced. There are two types of structures of plastic polymers. The first is a linear structure, and the polymer compound with this structure is called a linear polymer compound; the second is a body structure, and the polymer compound with this structure is called a body polymer compound. Some polymers have branched chains and are called branched polymers, which belong to a linear structure. Some polymers have cross-links between molecules, but fewer cross-links, which are called net structure.
Important properties of plastics
Fluidity
We can analyze the fluidity of thermoplastics from melt indexes, apparent viscosity, sizes of molecular weights, Archimedes spiral flow length and flow ratio.
Thermal performance and cooling rates
Different types of plastics have different thermal properties such as distortion temperature, specific heat capacity and thermal conductivity. The cooling time of plastics with high heat distortion temperatures can be shortened and they can be demolded early. However cooling deformation should be prevented after demoulding. Plastics with high specific heat capacity require a large amount of heat when plasticizing, and an injection molding machine with a large plasticizing capacity should be selected. Plastics with low thermal conductivity have a slow cooling rate, so they must be fully cooled to enhance the cooling effect of the mold.
Crystallinity
Crystallization is that when the plastic changes from the molten state to the condensation state, the molecules move independently and then stop moving freely completely in a disordered state according to a slightly fixed position. There is a tendency to make the molecular arrangement a regular model. Thermoplastics can be classified into crystalline plastics and amorphous plastics according to whether there is crystallization in the condensation process.
Hygroscopicity
Plastics can be divided into hygroscopic plastics and non-hygroscopic plastics. The water content in the plastic must be controlled within the allowable range. Under high temperatures and high pressure, the water turns into gases or hydrolyzes, resulting in deterioration of fluidity, poor appearance and mechanical properties.
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