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How do I read a mould flow analysis report?
Start with the fill time plot to see whether the part fills evenly and completely, then check the injection pressure against the machine's capacity, then look at where weld lines and air traps fall and whether they sit on cosmetic or load-bearing areas. Autodesk's Moldflow guidance suggests keeping the pressure needed for the part itself to about 70 MPa (10,000 psi) and for the whole mould to about 100 MPa (14,500 psi) when the machine's capacity is unknown.
A mould flow analysis simulates how plastic fills, packs and cools in the mould before any steel is cut. Toolmakers often run one during design review, especially for parts with thin walls, long flow paths, tight flatness or several gates. The report is a set of colour plots; knowing what each plot means lets you ask useful questions.
The main results
| Result | What it shows | Warning signs |
|---|---|---|
| Fill time | The position of the flow front over time, from first (blue) to last (red) to fill | Translucent areas (short shot); closely spaced contours (hesitation); one flow path finishing early (over-packing) |
| Pressure at V/P switchover | Pressure along the flow path when the machine switches from speed control to pressure control | Very high peak pressure; tightly bunched contours; pressure not falling to zero at the flow ends |
| Confidence of fill | Likelihood that each area fills under normal conditions | Any area marked unlikely to fill |
| Weld and meld lines | Where flow fronts meet | Lines on cosmetic faces, at snap roots, across holes under load |
| Air traps | Where air may be caught between flow fronts or against a wall | Traps with no vent nearby; risk of burns, voids or blemishes |
| Sink marks estimate | Predicted depth of sink on the surface | Sink on cosmetic faces over ribs and bosses |
| Warp (if run) | Predicted deflection after ejection | Deflection larger than the flatness tolerance |
Descriptions follow Autodesk's Moldflow help and Xcentric Mold's guide to mould flow results.
Pressure: the published guide values
Autodesk's help for the switchover pressure result notes that a typical machine delivers around 110 to 210 MPa at the nozzle, with 140 MPa (20,000 psi) as an average. It recommends designing to no more than about 100 MPa for the whole mould and 70 MPa for the part; if the machine's capacity is known, to use no more than about 75 percent of it for the mould and 50 percent for the part. A part that needs more than that has little margin, and small changes in resin or temperature may cause short shots.
Weld lines: quality as well as position
Weld lines cannot always be avoided; they form wherever flow splits around a hole or comes from more than one gate. Autodesk's help says a good weld usually forms where the melt is no more than about 20 °C below the injection temperature, and that weld lines can be moved by changing gate positions or wall thickness, and improved with higher melt temperature, speed or packing pressure, larger gates and runners, or a vent at the weld.
Questions to ask the analyst
- Which exact resin grade and process settings were used? Results depend heavily on both.
- Was the runner and cooling system modelled, or only the part?
- Which gate positions were compared, and why was this one chosen?
- Where are the weld lines relative to cosmetic faces and loaded features?
- What is the predicted pressure as a share of the press's capacity?
- If warp was simulated, how does it compare with the drawing's flatness tolerance?
Simulation is a prediction. Guide values here are published by the software maker; real moulding trials confirm or correct them.
Questions buyers ask
Do I need a mould flow analysis for every part?
Not always. Simple, small, uniform-wall parts often mould well without one. It is most useful for large, thin, cosmetic, multi-gate or tight-tolerance parts.
Can simulation replace a mould trial?
No. It reduces risk and helps choose gates and cooling, but the first trials still confirm shrinkage, warp and appearance.
What is the difference between a weld line and a meld line?
Both form where flows meet. Autodesk describes meld lines as forming from roughly parallel flows; they are usually stronger and less visible than weld lines, which form where fronts meet head-on.
Why does the report change when the resin changes?
Each grade has its own viscosity and thermal behaviour, so fill pressure, weld quality and shrinkage all change. Always ask that the actual production grade is used.
Sources
- Autodesk, Pressure at velocity/pressure switchover result
- Autodesk, Weld and meld lines result
- Autodesk, Viewing Fill+Pack results (tutorial)
- Xcentric Mold, Leveraging mold flow analysis for injection molded parts