What does mould flow simulation actually predict?

Fill pattern and fill time, weld and meld line positions, air traps, required injection pressure, clamp force, packing behaviour, sink mark risk, cooling efficiency and part warpage. Modern software chains these from filling through cooling to final shrinkage and deformation.

How accurate is mould flow simulation?

For filling patterns, weld line positions and pressure predictions, well-built models are typically accurate within 5-15% of measured reality. Warpage prediction is directional and comparative rather than absolute: it reliably ranks design options but absolute deformation values carry wider uncertainty.

How much does mould flow analysis cost?

A standard fill-plus-pack analysis typically costs €300-800; adding cooling and warpage analysis takes it to €800-2,000 depending on part complexity and mesh size. That is trivial against a tooling modification cycle of €2,000-10,000 or a mould that fills badly.

When is mould flow simulation worth doing?

For large parts, thin walls, critical cosmetic surfaces, multi-cavity tools, tight tolerances, fibre-filled materials and any design where weld lines or warpage carry functional risk. For simple thick-walled commodity parts, experienced DFM review is usually sufficient.

What do you need to provide for a simulation?

A 3D CAD model of the part (STEP is fine), the intended material grade or a near equivalent, target wall thickness, and any constraints on gate positions or cosmetic surfaces. Proposed gate locations and cooling layouts improve the value of cooling analysis.

Mould Flow Simulation Explained: What It Predicts, What It Costs and When to Use It

Mould flow simulation is a computer model of what happens inside the mould: how the melt front advances, where weld lines form, how the part packs, cools and warps. It has quietly become one of the highest-leverage tools in injection moulding, because it moves expensive surprises from hardened steel to a screen where they cost almost nothing to fix. This guide explains what simulation predicts, how accurate it really is, and when you should insist on it.

What does a mould flow simulation predict?

A full analysis chains four stages:

Fill: the advancing melt front over time. Outputs include fill pattern, fill time, injection pressure, clamp force, temperature at flow front, and the positions of weld lines and air traps. This answers: will it fill, from where, at what pressure, and where do the cosmetic risks land?
Pack: pressure holding as the part densifies. Predicts volumetric shrinkage distribution and sink mark risk, especially behind ribs and thick sections.
Cool: heat flow through the mould and cooling circuit. Predicts mould surface temperatures, cooling time and hot spots, which drive cycle time and warpage.
Warp: the final deformed shape after ejection, from shrinkage anisotropy, cooling imbalance and fibre orientation. Predicts flatness, bow and twist in magnified form.

Fibre-filled materials add fibre orientation prediction, which affects both warpage and local strength. Gas-assist and two-shot processes have their own specialised analyses.

How accurate is it, honestly?

Fill predictions are the strongest: fill patterns, weld line locations and pressure requirements on a well-built model typically land within 5-15% of measured reality, and engineers routinely validate models against short-shot studies. Cooling analysis is solid for identifying hot spots and comparing circuit layouts. Warpage is the honest caveat: simulation reliably tells you which way a part will move and which option warps less, but absolute deformation values carry wider uncertainty because they depend on material data quality, packing reality and mould temperature uniformity that the model idealises.

Treat simulation as a decision tool, not a prophecy. Comparing gate option A versus B, or rib thickness 50% versus 60% of wall, is exactly where it excels.

What inputs does a good simulation need?

Part geometry: 3D CAD, with final wall thickness; midplane or 3D mesh depending on part complexity
Material data: the actual grade is best; rheological, thermal, shrinkage and (for warp) mechanical data. Generic family data is acceptable for screening, risky for warp-critical parts
Process intent: target melt and mould temperatures, and proposed gate positions. Multiple gate options are cheap to compare at this stage
Cooling layout (for cool/warp): proposed channel positions; even a schematic layout beats none

What does it cost and how long does it take?

Scope	Typical cost	Typical duration
Fill + pack	€300-800	1-3 days
Fill + pack + cool	€600-1,500	2-4 days
Full chain incl. warp	€800-2,000	3-7 days

Set this against what it protects: a single tooling modification round on a medium mould runs €2,000-10,000 and two to four weeks; a mould that fills with unbalanced pressure or welds across a stressed feature can be a five-figure problem. Simulation is among the cheapest insurance in the industry.

When should you insist on simulation?

Thin-wall parts: flow-length-to-thickness ratios above ~150 deserve a fill check; see our thin-wall guide
Large parts: fill balance, clamp force and warp all scale with size
Cosmetic class-A surfaces: weld lines and sink marks must be predicted and designed away, not discovered
Multi-cavity and family tools: runner balance determines whether cavity 8 fills like cavity 1
Fibre-filled materials: orientation-driven warp is unpredictable by intuition
Tight tolerances and assemblies: if flatness or hole positions are critical, warp analysis is the only preview available; our tolerance guide explains the stakes
Special processes: gas-assist, two-shot and insert moulding all have failure modes that only simulation shows early

For simple, thick, small commodity parts in unfilled commodity resin, an experienced DFM review usually suffices.

How does simulation fit the development process?

The efficient sequence is: DFM review first (fix wall thickness, draft and ribs against the design rules), then simulation on the DFM-cleaned geometry, then gate and cooling optimisation inside the model, then tool design with results in hand. Running simulation on a part that violates basic rules wastes the run; skipping it on a risky part gambles the tool.

Nordmould commissions mould flow analysis on programmes where the risk justifies it and reviews results with you in plain language: what will happen, what we changed, and what residual risk remains.

Related guides

Have a risky part? Ask Nordmould for a DFM and simulation review before tooling is cut.