Plasticizing process of injection molding is the process of mixing and dispersing materials (for multiple components) and heating, mainly by rotating and stirring of the screw. There are two sources of heat in the heating process. 

 

The first method is heated by an electric heating element outside the barrel. Materials in the barrel can produce a great temperature gradient in the direction of the cross section and the length of the material. The second method is that the material depends on the rotation of the screw and generates heat by shearing mechanism and friction mechanism (the plunger injection system does not have this process).

 

Heating Mechanism of Material Barrel

The heating of material by material barrel is mainly realized by the heat conduction between the material and the material barrel. As the premise of heat conduction is the temperature difference, the temperature of the barrel is usually higher than that of the melt. The quantization of this process can be calculated based on the fundamental equation of conduction heat transfer ---- Fourier's Law. As follows:

 

dQ=-λdA•dt/dn …… (1)

Where Q ----- heat conduction rate，w；

             A ----- heat conduction area，m²；

            dt/dn ----- temperature gradient，K/m；

            λ ----- scale coefficient(thermal conductivity)，w/m•K.

 

Heating Control Factors:

Seeing from the above formula, there are three main factors affecting the absorption of heat per unit time:

 

1, Heat Conduction Area. that is, contact area. For materials in cylindrical barrel, the more accurate term is the specific surface area of the material. The greater the specific surface area of the material (and the contact with the barrel) is, the faster the heating rate is. So, the influence of the diameter of the material barrel on the heating process should be taken into full consideration in the design of the barrel.

 

 

2, Temperature Gradient. It mainly depends on the temperature at the beginning and the end and the diameter of the barrel. When the temperature difference between the beginning and the end is large, the heating speed is fast. Therefore, in reasonable temperature range, the appropriate increase of the cylinder temperature is beneficial to the enhancement of heating efficiency, which will increase energy consumption as well. When the diameter is large, the temperature difference between the inner and the outer layers will be greater and the unevenness of heating will be greater. This unevenness can be eliminated by mechanical rotation of the screw.

 

3, Thermal Conductivity. The greater the coefficient of thermal conductivity is, the better the thermal conductivity is. For a multi component melt, the higher the ratio of liquid (or flow) is, the greater the thermal conductivity is. When designing polymer systems, the proportion of high and low melting point components should be taken into full consideration.