Flow length to wall thickness ratio (L/s) in injection moulding
The ratio of flow length to wall thickness shows early whether a part can be filled, or only with high pressure, high melt temperature and several gates. Here are the formula, the assessment and three worked examples; the calculator itself is in Resinovo Basic.
The flow length to wall thickness formula
Ratio L/s
R = Σ (L_i / s_i)
L is the longest flow path from the gate to the end of flow in mm, s the wall thickness in mm. If the part has sections with different wall thicknesses, the ratios of the sections are added up.
Assessment against the material’s guideline values
R ≤ R_min · R_min < R ≤ middle · middle < R ≤ R_max · R > R_max
Up to the lower guideline value the part can be filled without problems. Up to the middle of the band it can be filled but the process window is narrow. Up to the upper guideline value it is near the limit, above that it is outside the range of experience: a short shot is very likely.
Worked examples
ABS: 180 / 2 = 90 · PC: 200 / 1.5 ≈ 133 · PA6: 300 / 0.8 = 375
With the guideline values from the app: ABS 170 to 220, flow length 180 mm at 2 mm wall gives 90, so uncritical. PC 100 to 150, flow length 200 mm at 1.5 mm wall gives 133, so near the limit. PA6 240 to 300, flow length 300 mm at 0.8 mm wall gives 375, so outside the range of experience: the flow length would have to be 20% shorter or the wall 25% thicker for R to reach the upper guideline value. Two sections, 100 mm at 2 mm and 60 mm at 1.5 mm, give 50 + 40 = 90.
What to do when the ratio is too large
- Move the gate towards the middle of the flow path or add a gate: this shortens L directly.
- Thicken the wall locally (flow leader, about 10 to 25% thicker).
- Raise mould temperature and melt temperature, observing the material’s maximum residence time.
- Inject faster or choose a material with a higher MFR.
What the calculation does not cover
The ratio is a quick check without simulation. It is neither a flow model nor a pressure calculation. The guideline values apply to usual wall thicknesses of about 1.5 to 3 mm; for walls under 1 mm the result should be judged more strictly. The datasheet, filling simulation and sampling take precedence.
Frequently asked questions
What is the flow length to wall thickness ratio?
The quotient of the melt’s longest flow path and the wall thickness, L/s for short. The larger it is, the harder the part is to fill.
Which L/s can still be filled in injection moulding?
That depends on the material. Easy-flowing grades such as PP or PA tolerate much more than tough ones such as PC or PMMA. The app lists guideline values per material; the examples above show three of them.
How do you handle several wall thicknesses?
Each section gets its own L/s and the ratios are added up.
Does this replace a filling simulation?
No. It is a quick check for quotation and mould design. The filling simulation and sampling decide.
Every calculator in your pocket.
The flow length to wall thickness ratio is part of Resinovo Basic: try it free for a week, then from €2.99 a month. More calculators follow. On iPhone, iPad and Android.