The Flow Ratio (L/t) of Plastic

In order to fill the plastic inside the cavity, it is necessary to push it inside the cavity while applying pressure from the injection cylinder. When the plastic is in the heated and molten state, it is a fluid with some viscosity. However, the viscosity starts decreasing as the plastic reaches the cavity while flowing through the sprue and the runner because it loses heat to the surface of the mold. If the viscosity decreases beyond a certain limit, the leading part solidifies due to cooling, and flowing becomes impossible thereafter.

Up to what distance the leading part can flow without cooling and solidifying? By knowing this it is possible to consider at the time of designing the mold the number and placement of gates, the placement of the runner, etc.

An index that becomes a guideline for that is the flow ratio (L/t). The flow ratio is an experimental index indicating the distance to which the leading edge of the flow can reach when a specific plastic is made to flow inside a cavity with a fixed plate thickness and at a fixed pressure.

The flow ratio is expressed, for example, as "the flow ratio (L/t) is 450 to 530 mm when POM plastic is made to flow inside a cavity with a wall thickness of 1 mm and with an injection pressure of 900 kgf/cm2".

In general, the following trends are shown by the flow ratio.

(1) The flow ratio value increases as the injection pressure increases.
(2) The value decreases as the cavity plate thickness decreases.
(3) The value increases as the cavity surface temperature increases.
(4) The value shows some fluctuation depending on the condition of the molding machine and of the mold.
(5) The flow state of a partially thin plate thickness part cannot be the target of prediction.

The flow ratios of major plastics are given below.

Plastic name (kgf／cm2) Injection pressure (mm) Flow ratio (mm) Cavity thickness (L／t) POM Natural 900 1 450〜530 ABS Natural 900 1 270〜310 PBT with 30% glass fibe 1000 1 110〜130