In a hot runner mold, usually a manifold is used for diverting the molten plastic resin injected from the nozzle of the injection molding machine to the hot runner. A manifold is a plate for branching to the runner, has a heater incorporated in it, and hence the plastic resin inside the manifold is maintained in the molten state and is maintained in a state in which it can flow.

A large amount of heat will be accumulated in the manifold in order to maintain the plastic in the molten state, and as a result, the steel material itself of the manifold undergoes thermal expansion.

Since the fact that the steel material undergoes thermal expansion implies that its external dimensions expand, this in turn generates compression stresses. Such stress generated due to heat is called "thermal stress".

Unless we know the size of the thermal stress, a large compression stress will act on the mold, particularly on the mold plate on the fixed side and on the mold base components, which can lead to deformations of the cavity and core, burrs, and can also cause breakage of the cavity in extreme cases.

In view of this, in this lesson we learn the method of calculating the thermal stress acting on the manifold. The thermal stress σ (sigma) acting on the manifold can be calculated using the following equation.

Where,
ε (epsilon): strain (%), and
E: Young's modulus of the manifold steel material (MPa or kgf/mm²).
In the case of carbon steel, E = 20,000 to 21,000 kgf/mm².
Δl (delta) = Amount of thermal expansion of length (mm)
L: Overall length of the manifold (mm)

Here, Δl can be calculated as follows.

Where, α (alpha): Linear thermal expansion coefficient of the manifold steel material (mm/°C)
In the case of carbon steel, α is about 11.5 to 12.8×10-6 (mm/°C)).
Δt: Temperature change from room temperature up to the set temperature of the manifold (°C)
The thermal stress can be obtained from these equations.

In the next lesson, we will practice some actual examples of calculations.