What do you remember about the physical properties of plastic?

The performance of the material is closely related to the injection molding conditions and the quality of the product. Most of the injection materials are granular, these solid materials into the hopper, generally after preheating, remove moisture, and then through the compression of the screw transport and plasticization, in the barrel need to go through a long thermal process before being pushed into the mold cavity by the screw, after the pressure maintenance stage and then cooling.

The main factors that affect this process are material, temperature, cylinder temperature, mold filling pressure, speed. The processing properties of polymer materials, the internal structure of the molecular chain, the size and distribution of molecular weight, and also depend on the external structure of the polymer. The manufacturability of injection molding is closely related to the relative density of polymer materials, thermal conductivity, specific heat capacity, glass transition and crystallization temperature, melting, decomposition temperature, and mechanical properties and rheological properties displayed in processing.

General physical properties

1. Total heat capacity

The total heat capacity refers to the total heat capacity of the injection molding material at the injection molding process temperature.

2, melting heat

Heat of fusion, also known as latent heat of fusion, is the energy required by crystalline polymers to form or melt crystals. This part of the energy is used to melt the polymer crystalline structure, so the injection molding crystallized polymer than the injection molding amorphous material to achieve the specified melting temperature required more energy.

No latent heat of melting is required for amorphous polymers. It takes about 452/g(100.8cal/g) to make the POM reach the injection molding temperature, and PS only needs 375J/g to melt.

3. Specific heat capacity

Specific heat capacity is the heat required when the temperature per unit weight of the material rises by 1 degree [J/kg.k]. The specific heat capacity of different polymers is different, and the crystallization type is higher than the non-opposite type. Because when heating the polymer, the added heat energy must not only be consumed in the temperature rise, but also in the change of the polymer structure, the crystallization must supplement the tearful amount of latent heat required to melt the material.
During injection molding, the heating or cooling characteristics of the plastic are determined by the heat content of the polymer and the temperature difference. The rate of heat transfer is proportional to the temperature difference between the material being heated and the heat source. Generally, cooling is faster than melting, because the temperature difference between the cylinder and the material is generally small, and the temperature difference between the melt and the mold is large. The heating time depends on the temperature difference between the inner wall of the cylinder and the material layer and the thickness of the material layer.

4. Thermal diffusion coefficient

Thermal diffusion coefficient refers to the speed at which the temperature is transferred in the heating material, also known as the thermal conductivity coefficient. Its value is determined by the heat required when the temperature of the material is increased by 1 degree per unit mass (specific heat capacity) and the speed at which the material absorbs heat (thermal conductivity). The influence of pressure on thermal diffusivity is small, while the influence of temperature is large.

5. Thermal conductivity

Thermal conductivity reflects the rate at which a material transmits heat. The higher the thermal conductivity, the faster the heat transfer in the material. Due to the low thermal conductivity of the polymer, it takes some time to heat either in the cylinder or to cool its melt in the mold. In order to improve heating and cooling efficiency, some technical measures need to be taken. For example, the heating cylinder requires a certain thickness, which is not only to consider the strength, but also to increase the thermal inertia, to ensure that the material can transfer heat in a good and stable manner, sometimes using the low thermal conductivity of the polymer, the use of hot runner mold. The thermal conductivity of polymer increases with increasing temperature. The thermal conductivity of crystalline plastics is more temperature dependent than that of non-crystalline plastics.

6. Density and specific volume

The increase of density will reduce the permeability of gas and solvent in the product, but increase the tensile strength, fracture elongation, stiffness hardness and softening temperature of the product. The compressibility, impact strength, fluidity and creep resistance are reduced.

In the injection molding process, the polymer undergoes the cooling - heating - cooling repeated thermal process temperature, gradient and polymer shape are very large changes, so the density is also constantly changing, which plays an important role in the quality of injection molding products. The specific volume reflects the volume occupied per unit of material. It is a very important parameter to measure the space occupied by the polymer structure under different process conditions, the expansion and compression in various states, and the size contraction of the product.

7, expansion coefficient and compression coefficient

The change of specific volume caused by temperature at constant pressure is the expansion coefficient. From high temperature to low temperature, the specific volume of the polymer decreases gradually. The specific volume of the polymer depends not only on temperature but also on pressure. The specific volume of polymer changes with pressure at different temperatures, the specific volume decreases with pressure increasing and the density increases. This property is of great significance for using pressure to control the quality and dimensional accuracy of the product.