Moldflow Analysis Explained: Fill, Pack, Warp, Cool

Moldflow analysis (and equivalent CAE tools such as Moldex3D and Sigmasoft) simulates the injection molding process before steel is cut. Used well, it removes the most expensive class of tooling rework: the problems that are caused by the geometry itself and cannot be fixed by adjusting the press.

Figure 1. Moldflow Analysis Explained: Fill, Pack, Warp, Cool.

Used badly, it produces colorful pictures that nobody acts on. This guide is about the difference.

1. What Simulation Actually Predicts

2. When to Run Simulation — and When Not To

Run it when the cost of being wrong is high:

• A new tool for a large or cosmetic part, where a re-cut of a gate or a channel is expensive.
• Warpage is a requirement (a flat panel, an optical part, a sealing surface).
• A glass-filled material is specified, because anisotropic shrinkage is not intuitive.
• The part has a structural function and short glass fiber is being used to replace metal.
• The tool is multi-cavity and balancing matters.
• The part wall thickness varies significantly, or has thin ribs on a thick base.

It is usually not worth it when:

• The part is a simple, low-volume item on a pre-hardened tool where a tryout adjustment is cheap.
• You have a validated, very similar part and the same material — but even then, confirm the gate.
• Simulation would replace a DFM review rather than supplement it. Simulation cannot tell you that a boss is too far from a wall or that a draft angle is missing.

3. How to Read Each Result

### 3.1 Fill

• Fill pattern: check that the melt front reaches the extremities of the part evenly, and that it does not race through thick sections and leave thin sections to fill last. Hesitation at thin-to-thick transitions causes sinks and short shots.
• Pressure at end of fill: should be a comfortable fraction of machine capacity — commonly quoted as 50–80%. A part that needs 95% of available pressure has no process window.
• Melt front temperature: the temperature spread across the part at the end of fill should stay within roughly 10–20 °C for amorphous resins and tighter for semi-crystalline ones. A large spread means the material will shrink differently in different regions.
• Shear stress and shear rate: check that the values stay within the material supplier's limits. Excessive shear stress at the gate causes residual stress and warpage; excessive shear rate causes material degradation and splay.
• Weld lines: identify them and decide whether they land on a cosmetic surface or a structural region. Move the gate rather than accept a weld line across a visible face.
• Air traps: these become burn marks. Find them and provide venting — often a simple fix that a simulation catches for free.

### 3.2 Pack

• Volumetric shrinkage: ideally uniform across the part. Where shrinkage differs region to region, you will get sink, warp, or dimensional drift.
• Gate freeze time: the packing profile should hold pressure until the gate freezes; anything after that is wasted cycle time.
• Pressure at end of packing: a well-packed part has an even pressure distribution and the gate frozen. A part that stays pressurized everywhere is over-packed and will be difficult to eject and prone to stress.

### 3.3 Cool

• Coolant temperature rise: typically keep the temperature rise between inlet and outlet under 2–3 °C.
• Part temperature at ejection: should be uniform. The maximum temperature drives cycle time; the spread drives warpage. A part with a 30 °C spread across it will not come out flat.
• Cooling time breakdown: use it to find the region that sets the cycle. That region — usually the thickest section or a hot core — is the target for a bubbler, a baffle, or a conformal channel.

### 3.4 Warp

The warp result is the most valuable and the most misread. Do not just look at the total deformation — look at the breakdown:

If differential cooling dominates, no amount of process tuning will fix the warp — the tool has to be changed. If orientation dominates, the answer is usually the gate location or the material.

4. Material Data Quality Decides the Value of the Result

Simulation is only as good as the material model:

• Generic data (bundled with the software) gives you a directional answer — good enough to choose a gate location or compare wall thicknesses.
• Supplier-measured data (a genuine material file from the resin producer) gives you a result you can design a tool around, particularly for shrinkage and warpage.
• Actual-process-measured data is for critical programs where you can afford to characterize the specific lot.

For any program where warpage is a requirement, insist on supplier-measured material data. It is the single largest improvement in result reliability per unit of cost.

5. Setting Up a Study That People Will Act On

1. Define the question first. "Where should the gate be?" and "will the part warp past 0.3 mm?" are useful questions. "Run a simulation" is not.
2. Use the real geometry, with drafts and fillets that will be in the tool. Stripped geometry gives misleading flow paths.
3. Use the real gate and runner design, not an idealized point gate, if you want the runner balance result to mean anything.
4. Use the intended machine and material.
5. Report the process window, not one point. Run the simulation at the limits of the acceptable process range and show whether the part is still good.
6. Convert results into decisions. Every result should end in a recommendation: move this gate, thicken this rib, add a bubbler here.
7. Validate at tryout. Record the actual fill pattern from a short-shot study and compare it with the simulation. This is how you learn to trust (or distrust) the model for the next program.

6. What Simulation Cannot Do

• It cannot replace a DFM review of draft angles, wall uniformity, undercuts and tolerances.
• It cannot predict surface appearance defects reliably — splay, silver streaks and flow marks depend on moisture, venting and volatiles.
• It cannot fix a bad material choice.
• It cannot tell you the tool will not wear out.
• It will not be accurate if the geometry, gates or material data are wrong.

Figure 2. Quick reference: the key checks and reference values from this guide.

Conclusion

Simulation is cheapest at the concept stage and most expensive to ignore late. The highest-value use is a 20-minute fill study that moves a gate before the steel is cut — and the lowest-value use is a beautiful report delivered after the design is frozen.

Run the study early, define the question narrowly, use real material data, and turn every result into a decision. If you would like a simulation-backed DFM review on a new part, our engineering team can run the fill, pack, cool and warp studies alongside the tooling design.