Physical Properties of Plastics（3）
Physical Properties of Plastics（3）
The specific heat capacity is the heat required when the temperature of the material per unit weight rises by 1 degree [J / kg. K].
The specific heat capacity of different polymers is different, and the crystalline type is higher than the non surface type. When heating the polymer, the supplementary heat energy must be consumed not only in the temperature rise, but also in the change of polymer structure. The crystalline type must supplement the amount of hot tears required for the latent heat of melting in order to melt the material.
During injection molding, the heating or cooling characteristics of plastics are determined by the heat content and temperature difference of polymers. The heat transfer rate is proportional to the temperature difference between the heated material and the heat source.
Generally, cooling is faster than melting, because generally, the temperature difference between barrel and material is small, and the temperature difference between molten material and mold is large. The heating time depends on the temperature difference between the inner wall of the barrel and the material layer and the material layer thickness
Thermal diffusion coefficient
The thermal diffusion coefficient refers to the speed at which the temperature is transferred in the heated material, also known as the thermal conductivity. Its value is determined by the heat required when the temperature of the material per unit mass increases by 1 degree (specific heat capacity) and the speed at which the material absorbs heat (thermal conductivity).
Pressure has little effect on thermal diffusion coefficient, but temperature has a great effect on it.
thermal conductivity
The thermal conductivity reflects the speed at which the material transmits heat. The higher the thermal conductivity, the faster the heat transfer in the material. Because the thermal conductivity of polymer is very low, it takes some time to heat in the barrel or cool its melt in the mold. In order to improve the heating and cooling efficiency, some technical measures need to be taken.
For example, the heating material barrel requires a certain thickness, which is not only to consider the strength, but also to increase the thermal inertia and ensure the good and stable heat transfer of the material. Sometimes, the hot runner mold is adopted by taking advantage of the low thermal conductivity of the polymer. The thermal conductivity of polymer increases with the increase of temperature. The dependence of thermal conductivity of crystalline plastics on temperature is more significant than that of amorphous plastics.
Density and specific volume
The increase of density will reduce the gas and solvent permeability in the product, but increase the tensile strength, breaking elongation, stiffness, hardness and softening temperature of the product; The compressibility, impact strength, fluidity and creep resistance can be reduced.
In the process of injection molding, the polymer experiences the repeated thermal process of cooling heating cooling. The temperature, gradient and polymer morphology change greatly, so the density is also changing constantly, which plays an important role in the quality of injection molded products.
Specific volume reflects the volume occupied by a unit substance. This is a very important parameter to measure the space occupied by polymer structure under different process conditions, expansion and compression under various states, size shrinkage of products, etc.
