Last updated: 2026-09-05 · 5 min read
Mold Flow Analysis — What Does the Simulation Predict Before the Mold Is Built, What Inputs Does It Need, and How Do You Read the Results?
Mold flow analysis (MFA) simulates how melt fills the cavity — it predicts fill pattern, weld line location, air traps, shrinkage, warpage and clamp force before the mold is cut. The simulation uses the part CAD, resin data sheet and gate location as inputs. Cost: $500–$3,000 per study. It catches design errors that would otherwise require $5,000–$20,000 in mold rework.
- Mold flow analysis solves the Navier-Stokes equations for non-Newtonian fluid flow through the cavity geometry. The software (Autodesk Moldflow, Moldex3D, Sigmasoft) divides the part into a mesh of 50,000–500,000 elements. For each element, it calculates: melt pressure, temperature, velocity and shear rate at every time step during filling, packing and cooling. The result: a color-mapped animation showing how the melt front advances through the cavity, where it converges (weld lines), where air is trapped, and how the part shrinks and warps during cooling.
- Five key outputs: (1) Fill pattern — melt front advancing; identifies short shots and flow hesitation. (2) Weld line map — where two melt fronts converge; predicts weld strength (typically 50–80% of bulk). (3) Air trap map — where gas is compressed; predicts burn marks. (4) Shrinkage map — volumetric shrinkage distribution; predicts sink marks and voids. (5) Warpage prediction — final part deformation after ejection; predicts whether the part meets tolerance. Each output is a color map — red = worst, blue = best.
- The simulation quality depends on input accuracy. The resin data sheet must include viscosity curve (shear rate vs. viscosity), PVT data (pressure-volume-temperature), and thermal properties (conductivity, specific heat). The mesh must be fine enough to capture thin features — for a 0.5mm wall: element size 0.25mm (2 elements through thickness). The gate location and process parameters (melt temp, mold temp, injection speed, packing pressure) must match the intended production process. Garbage in, garbage out — inaccurate inputs produce misleading results.
What Mold Flow Studies Are Most Useful for Different Part Types?
| Part Type | Key Study | What It Predicts | Cost | Value |
|---|---|---|---|---|
| Multi-cavity mold | Fill + balance | Cavity-to-cavity flow balance | $1,000–$2,000 | Ensures all cavities fill equally |
| Large flat panel | Fill + warpage | Weld lines, warpage direction | $1,500–$3,000 | Prevents sink and warp |
| Glass-filled part | Fiber orientation | Fiber alignment, anisotropic warp | $2,000–$3,000 | Predicts fiber warpage |
| Thin-wall container | Fill + packing | Fill pressure, packing window | $1,000–$2,000 | Ensures fillability |
| Optical lens | Cooling + warpage | Cooling uniformity, birefringence | $2,000–$3,000 | Prevents optical distortion |
Not every part needs mold flow analysis. Simple brackets and small housings (under 100mm, single cavity, non-cosmetic) can be molded without simulation — the mold maker’s experience is sufficient. Complex parts (multi-cavity, thin-wall, glass-filled, optical, large panels) benefit from simulation — the $1,000–$3,000 study cost is small compared to the $10,000–$50,000 mold rework cost if the mold does not fill correctly on T1.
Air trap — a pocket of compressed gas where two melt fronts converge or where flow hesitates. The gas compresses to high pressure and temperature, causing a burn mark (black or brown discoloration) on the part surface. Air traps are predicted by mold flow and fixed with vent pins or vent slots at the predicted location. The weld line strength guide covers how melt front convergence also affects joint integrity.
What Three Questions Should You Ask When Reviewing Mold Flow Results?
- Where are the weld lines, and are they in a non-critical area? The weld line map shows where two melt fronts converge. Check: are weld lines at high-stress areas (snap fits, screw bosses, load-bearing ribs)? If yes, weld strength drops 20–50% — move the gate or add an overflow well to push the weld to a low-stress zone. Check: is the weld line temperature above the resin’s minimum fusion temperature? If below, the weld will be weak — raise mold temperature or move the gate.
- What is the maximum injection pressure, and does it exceed the press capacity? The fill study shows peak injection pressure to fill the cavity. Check: is peak pressure below 80% of press maximum? If above 80%, the press runs at its limit — process variation may cause short shots. Options: raise melt temperature (reduces viscosity), enlarge the gate (reduces pressure drop), or move to a larger press. For a 500-tonne press: max pressure is typically 1500 bar — if simulation shows 1400 bar, only 100 bar margin.
- What is the predicted warpage, and does it fit within the part tolerance? The warpage study shows final part deformation after ejection. Check: does maximum warpage fall within drawing tolerance? For a 100mm part at ±0.2mm: warpage must be below 0.4mm total. If warpage exceeds tolerance: adjust packing pressure (higher packing reduces shrinkage), change gate location (balanced flow reduces asymmetric shrinkage), or add ribs for stiffness. The simulation can iterate — change process, re-run, check new warpage.
PVT data — pressure-volume-temperature curves that describe how a resin shrinks as it cools under pressure. Semi-crystalline resins (PP, nylon, POM) shrink 1.5–3.0% — PVT data captures the sharp volume change at the crystallization temperature. Amorphous resins (ABS, PC, PMMA) shrink 0.4–0.8% — more gradual volume change. Without accurate PVT data, the shrinkage and warpage prediction is unreliable. The packing pressure guide covers how pressure profiles compensate for shrinkage during production.
Mold flow: simulates fill, weld lines, air traps, shrinkage, warpage before mold is cut. Inputs: part CAD, resin data, gate location. 5 outputs: fill pattern, weld map, air traps, shrinkage, warpage. Cost $500–$3,000. Catches errors that cost $5k–$20k in rework.
- Weld lines in non-critical zones — away from snap fits, screw bosses, load-bearing ribs.
- Peak pressure <80% press max — above 80% leaves no margin for process variation.
- Warpage within drawing tolerance — check max deformation vs. ±tolerance band.
- Resin data sheet matches production lot — viscosity and PVT must be current.
- Mesh fine enough for thin features — ≥2 elements through minimum wall thickness.
Frequently Asked Questions
How accurate is mold flow analysis compared to actual molding results?
For fill pattern and weld line location: 90–95% accurate — simulation correctly predicts where melt flows and weld lines form. For warpage: 70–85% accurate — direction is correct but magnitude may differ 10–30% from actual parts. Accuracy depends on input quality: if resin data sheet matches the actual lot and process parameters match production, simulation is within ±10% of measured values. For shrinkage: 80–90% accurate for amorphous resins, 60–80% for semi-crystalline (crystallization is harder to model).
Can mold flow analysis replace T1 sampling?
No — mold flow analysis predicts what will happen, T1 sampling shows what actually happens. The simulation uses idealized conditions (perfect mold temperature, exact resin properties, no machine variation). T1 sampling reveals real-world effects: cooling circuit imbalances, resin lot variation, machine response lag. Use mold flow to optimize the design before the mold is built. Use T1 to confirm the simulation predictions and fine-tune the process. The two are complementary — simulation reduces the number of T1 iterations from 5–10 to 1–3.
How long does a mold flow study take?
For a standard part (single cavity, 1–2 gates): 3–5 business days. For complex parts (multi-cavity, hot runner, fiber-filled): 5–10 business days. The study includes: mesh generation (1–2 days), simulation run (0.5–1 day), results analysis and report (1–2 days). Rush studies (24–48 hours) are available from most service providers at a 50–100% surcharge. For ongoing programs: some mold makers have in-house simulation capability and can run studies in parallel with mold design.
Key Takeaways
- Mold flow: simulates fill, weld, air traps, shrinkage, warpage.
- Inputs: part CAD, resin data, gate location. Garbage in, garbage out.
- Check: weld lines in safe zones? Pressure <80% press max?
- Cost $500–$3k. Catches errors worth $5k–$20k in rework.
Send the part CAD (STEP or IGES), resin grade with data sheet, and gate location preference — the mold flow study predicts fill pattern, weld line location, air traps and warpage before the mold is built, and specifies the design changes needed to ensure first-time mold success.
Sources: Plastics Technology — Mold Flow Analysis: Principles and Applications · Plastics Technology Online — Simulation-Driven Mold Design · Society of Plastics Engineers (SPE) — Mold Flow Simulation Technical Papers · Autodesk Moldflow — Injection Molding Simulation Software · Moldex3D — Advanced Injection Molding Simulation
— Arthur, Tooling Engineer at aCatMould