Category: Mold Manufacturing Reading time: 6 min Meta description: What is mold flow analysis, why it matters for your injection mold, and how to read the results. Includes interpretation guide for non-experts. URL: /blog/mold-flow-analysis-guide/ Tags: mold-flow, mold-analysis, simulation, injection-molding, DFM
If you're investing $10,000 or more in an injection mold, mold flow analysis is not optional. It's the cheapest insurance you can buy — catching design issues before any steel is cut.
Yet many buyers skip it to save $500-1,500, then pay ten times that to fix a mold that doesn't fill properly.
Mold flow analysis uses computer simulation to predict how molten plastic will behave inside a mold cavity. It models the entire injection cycle:
The simulation outputs visual heat maps and data that tell you exactly where problems will occur — before a single cutting tool touches the steel.
The most fundamental output. It shows:
Look for: Fill should be balanced — the last-filled areas should all finish within 10% of each other's fill time. Unbalanced fill means overpacking in some areas and short shots in others.
Shows how much injection pressure is lost as plastic flows through the cavity.
Good: Pressure drop < 40% of available machine pressure Warning: Pressure drop > 60% — consider larger gates, additional gates, or higher-flow material
Plastic degrades if it flows too fast through narrow channels.
| Material | Maximum Shear Rate |
|---|---|
| ABS | 40,000 1/s |
| PC | 60,000 1/s |
| PA6 | 50,000 1/s |
| PP | 100,000 1/s |
If shear rate exceeds these values, you need to increase gate size or modify the flow path.
The simulation predicts how long the part takes to cool — typically the longest part of the cycle.
Rule of thumb: Cooling time ≈ wall thickness² × 3-4 seconds (for standard materials)
The simulation predicts exactly where weld lines will form. A good analysis tells you:
Predicts how much the part will deform after ejection, due to:
Output: Displacement magnitude in millimeters, shown as a color contour on the part.
| Mold Cost | Mold Flow Cost | Coverage | Expected Savings |
|---|---|---|---|
| $5,000 | $500 | Simple analysis | 2-5x ROI — catches gate issues |
| $15,000 | $800 | Full analysis | 5-10x ROI — catches cooling issues |
| $30,000 | $1,200 | Full + warpage | 10-20x ROI — prevents rework |
| $80,000+ | $2,000 | Full + optimization | 20-50x ROI — critical for multi-cavity |
Real-world case: A medical device manufacturer had a 16-cavity mold for syringe components. Without mold flow, the cavities filled unevenly — cavity #1 filled 0.3 seconds before cavity #16, causing inconsistent weight and dimensional variation. The mold flow analysis revealed the issue. A gate size adjustment costing $300 to implement saved $12,000 in rejected parts in the first production month.
Most mold flow software uses a standard color scale:
| Color | Meaning — Fill Time | Meaning — Temperature | Meaning — Pressure |
|---|---|---|---|
| Red | Fills last | Too hot | Overpacked |
| Yellow/Orange | Mid-fill | Acceptable | Moderate pressure |
| Green | Good fill time | Optimal | Good packing |
| Blue | Fills early | Too cold | Underpacked |
| Gray/Dark | Doesn't fill | — | Short shot |
| Metric | Good | Needs Attention | Problem |
|---|---|---|---|
| Fill time balance | <10% variation | 10-20% | >20% |
| Injection pressure | <60% of machine | 60-80% | >80% |
| Shear rate (ABS) | <20,000 1/s | 20,000-40,000 | >40,000 |
| Weld line strength | >80% of base | 60-80% | <60% |
| Warpage (100mm part) | <0.2mm | 0.2-0.5mm | >0.5mm |
| Air traps | None | 1-2 small | Multiple |
| Problem Found | Typical Fix | Cost Impact |
|---|---|---|
| Unbalanced fill | Adjust gate sizes or add gates | $200-500 (steel-safe modification) |
| High shear rate | Enlarge gate or round flow path edges | $0-300 |
| Weld line on cosmetic surface | Move gate, add overflow well | $200-1,000 |
| Air traps (burn marks) | Add vents or change fill pattern | $0-500 |
| Cooling too slow | Add or reposition cooling channels | $500-2,000 |
| Excessive warpage | Adjust cooling or add ribs | $500-3,000 |
You should require mold flow analysis for:
"If I don't accept mold flow analysis, what could go wrong?" If they can't give specific examples, they may not be experienced enough to handle your project.
At app.moldkey.com/quote, all mold projects over $10,000 include a mold flow analysis as part of the DFM process. It's not an add-on — it's the standard.