Injection molding is a process of mixing and dispersing material (for multiple components) and heating. And the mixing and dispersing process is mainly realized 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).

 

 

For multi component materials, the shear action of screw on material is mainly reflected in three aspects:

1, Further mixing and dispersing

2. Extruding and discharging the gas

3. Heating the material

 

Source of Shearing Heat Effect:

 

The shearing heat effect of screw is one of the most important functions of screw (the shear model is shown above), which directly influences the plasticizing quality of the material. Seeing from the mechanism of production, the rotation of the screw leads to the movement of the melt, and the friction between the melt and the surface of the screw leads to heat build-up, while the friction of the molecular chains in different flow layers is exothermic.

 

For a common powder injection polymer system, the polymer system is usually a non-Newton fluid whose shear stress is not in direct proportion to the shear rate. The calculation is complicated and is usually simplified as a Newton fluid calculation. The inference formula for shear heat is as follows:

1. heat calculation, unit time, unit volume, melting material heat generated by the stress,

Q=τγ (1)

2. shear strain calculations are performed for simplified calculations according to Newton fluid treatment

γ=τ/η (2)

3. synthesis (1) (2) can get the formula of shearing heat

Q=ηγ2 (3)

4. for screw injection machine,

γ=πDn/60h (4)

 

where: τ...shearing stress;

γ...gamma shear strain;

η... shear viscosity of the fluid;

D... Screw diameter;

N... Screw speed;

H... Groove depth.

 

In actual production, the calculation is more complicated because of the non-Newtonian fluid, but the laws are basically the same. The heat absorbed by the melt is proportional to the diameter of the screw and the square of the screw speed, inversely proportional to the depth of the screw, and the greater the viscosity of the melt, the more heat it absorbs. When the screw speed is higher, the plasticizing ability is stronger. The melting of plastics is generally due to the heat generated by the screw rotation, so the screw speed is too fast, there are the following effects: 1. thermal cracking of plastic; 2, the screw or screw cylinder wear accelerated.