Flow length ratio is ratio of actual flow distance (L) of plastic in mold cavity from gate (injection point) to the end of flow path (melt front) to average cavity thickness (T) or minimum thickness along that flow path.

 

Core Formula: Flow Length Ratio (L/T) = Flow Distance (L) / Cavity Thickness (T)
Unit: Typically a dimensionless ratio (e.g., 150:1), but units of L and T must be consistent (usually mm).
Essence: It is a key indicator of plastic melt's ability to overcome flow resistance and maintain sufficient fluidity to complete filling within a channel of a specified thickness.

1. Determine the Longest Flow Path (L):
Identify path from main gate or each sub-gate to the farthest end of mold cavity.
Path length L refers to actual distance melt flows, typically along a curve rather than a straight line along cavity surface. In complex shapes, possible expansion path of melt front must be carefully tracked.
For multi-point gating (multiple gates), calculate flow distance from each gate to the farthest point of its controlled area and take maximum value as critical path L.
2. Determine cavity thickness (T):
Average thickness method: Calculate weighted average of cavity thicknesses along flow path L. This is the most common method, but requires estimating or measuring thickness at different locations along path.
Conservative method (recommended for critical designs): Use minimum cavity thickness along flow path L. This is safer because the thinnest point creates the greatest flow resistance.
Critical point method: If there are significant narrow areas along path (such as ribs or thin-walled areas), use thickness of that area as T.
3. Calculate the ratio: Divide selected L value by selected T value to obtain flow length ratio L/T.

Flow length ratio is a core factor that must be considered during plastic product design phase, directly impacting following design decisions:
1. Wall Thickness Design:
Core Constraint: Flow length ratio is one of primary factors determining minimum viable wall thickness for a product. For a given plastic material and flow distance L, there exists a minimum T value (i.e., maximum L/T ratio). Below this value, filling becomes difficult or requires extremely high process pressures.
Uniformity Requirements: Designs should strive for uniform wall thickness. A sudden decrease in wall thickness significantly increases local flow resistance, effectively resetting flow path (calculating a new L/T ratio from point of decrease), potentially leading to short shots or high stresses.
Thickening Design: When a product is large (L is long) and additional gates are not possible, it may be necessary to locally thicken wall (increase T) along critical flow paths to reduce actual L/T ratio and ensure fillability.
2. Gating System Design (Number and Location of Gating Points):
Key Determinant: Flow length ratio directly determines minimum number of gates required and their optimal locations.
Reducing L/T: Increasing number of gates essentially splits a long flow path into multiple shorter ones, significantly reducing L value of each flow path and, consequently, L/T ratio.
Balanced Flow: Gate placement should ensure that flow path length (L) and L/T ratio from each gate are as balanced as possible to achieve simultaneous filling and minimize problems such as weld lines, air entrapment, and warpage.
Large/Thin-Walled Products: For large products or products with very thin walls, additional gates are necessary to meet material's maximum allowable flow length ratio.
3. Material Selection:
Maximum allowable flow length ratio varies significantly among different plastics. High-flow materials (such as PA, PP, and PE) allow for larger L/T ratios (up to 200:1 or even higher), while low-flow materials (such as PC, PVC, and PSU) allow for smaller L/T ratios (perhaps 70:1 to 150:1).
When designing a product, if structure (long L, thin T) dictates a large L/T ratio, it is necessary to prioritize high-flow materials (allowing for a high L/T ratio) or modify design (thickening or adding gates).

Flow length ratio is an important factor in determining and optimizing injection molding process parameters:
1. Filling Pressure:
Core Impact: Higher L/T ratios generally require higher injection pressure (P). When melt flows through long, thin channels, frictional resistance (shear stress) increases dramatically, requiring higher pressure to propel melt front forward.
Equipment Limitations: Excessively high L/T ratios may exceed maximum injection pressure capability of injection molding machine, resulting in an inability to fill cavity (short shot).
2. Melt and Mold Temperature:
Increasing Temperature: To fill products with a high L/T ratio, it is generally necessary to increase melt temperature (Tm) and mold temperature (Tw). Higher temperatures can reduce melt viscosity, improve flowability, and offset cooling and viscosity increase effects of long flow paths.
Temperature Balance: However, temperature cannot be increased indefinitely, limited by issues such as material's thermal degradation temperature, extended cooling time, and increased energy consumption.
3. Injection Speed:
Increasing Speed: A faster injection speed (V) helps complete filling of long flow paths and thin-walled areas before melt significantly cools and thickens. Frictional heat generated by high shear also helps maintain melt temperature.
Potential Issues: Excessively high speeds can lead to jet flow, surface defects (flow marks, gas marks), excessive molecular orientation, and increased internal stress.
4. Packing Pressure Transfer
Challenges of High L/T: At the ends of long, thin flow paths, melt cools and solidifies rapidly, resulting in extremely high viscosity. This makes it difficult to effectively transfer packing pressure to these areas.
Consequences: Insufficient shrinkage in the end region can easily lead to noticeable sink marks and shrinkage cavities. Insufficient shrinkage can also lead to uneven dimensional shrinkage, high internal stresses, and reduced strength.
5. Residual Stress and Warpage:
High L/T ratios are often accompanied by high injection pressures, high filling speeds, and significant melt front cooling. This can lead to greater differences in molecular orientation, crystallinity, and shrinkage in different regions along flow path (especially near gate and at the end).
Result: High internal residual stress in part makes warpage more likely after demolding.