Fill, Pack, Warp, Cool: Moldflow Simulations Explained

This is the practitioner’s guide to which simulation predicts which failure mode and how to scope a Moldflow engagement.

Fill Simulation: The Foundation

Fill simulation models the molten plastic flowing into the cavity from gate-open to switchover. It is the simplest, fastest, and cheapest Moldflow run, and it should be done on every project that gets any simulation at all.

What fill simulation predicts

• Flow front progression. Where the plastic reaches first, where last, and the time to fill each region.
• Weld-line locations. Wherever two flow fronts meet, a weld line forms. The animation makes them visible.
• Short shot risk. If the flow front stalls or fails to reach a region, that region won’t fill in production.
• Injection pressure requirement. The peak pressure needed to drive the flow front to fill completion.
• Air traps. Regions where air can’t vent before the flow front arrives, leading to burn marks or short fill.

What fill simulation can’t predict

Fill alone won’t tell you about part dimensions, warp, sink marks, residual stress, or cycle time. Those need pack and cool.

When fill simulation alone is enough

Cosmetic-only projects where weld-line location is the primary concern. Single-cavity tools with no dimensional callouts tighter than ±0.5 mm. Geometry-driven flow problems where you suspect a short shot but aren’t worried about dimensional stability.

Pack Simulation: Where Part Dimensions Come From

Pack simulation models the post-fill phase where additional plastic is forced into the cavity to compensate for cooling shrinkage. This is the phase that determines part dimensions, density, and sink-mark formation.

What pack simulation predicts

• Volumetric shrinkage. How much each region of the part shrinks during cooling.
• Pack pressure distribution. Whether pack pressure reaches the far regions of the part before gate freeze-off.
• Sink-mark formation. Thick-section regions that don’t get adequately packed before gate freeze.
• Density gradients. Variations in part density that can cause warpage and mechanical-property variation.
• Gate freeze-off time. When the gate solidifies and pack pressure can no longer reach the cavity.

When pack simulation matters

Any part with controlled dimensions tighter than DIN 16901-2 commercial tolerance. Parts with thick sections (over 4 mm in unfilled resins, over 3 mm in fiber-filled). Cosmetic parts where sink marks would be visible. Multi-cavity tools where pack consistency cavity-to-cavity matters.

Cool Simulation: Cycle Time and Surface Temperature

Cool simulation models the heat transfer from molten plastic to the steel mold and through the cooling channels to the temperature-controlled fluid. It’s the most computationally expensive simulation type because it requires solving thermal flow equations across the full cycle.

What cool simulation predicts

• Time to ejection temperature. How long the part needs in the mold before it can be ejected without distortion. This sets the minimum cycle time.
• Mold surface temperature distribution. Cool-side vs hot-side temperature delta across the cavity.
• Cooling channel effectiveness. Whether existing channels remove heat fast enough or whether more are needed.
• Conformal cooling opportunity. Where conventional straight-bore channels can’t get close enough to the part and conformal (3D-printed) cooling would dramatically improve cycle time.

When cool simulation matters

High-volume production where cycle-time reduction has clear ROI. Parts with cosmetic requirements that demand uniform mold surface temperature. Multi-cavity tools where cavity-to-cavity dimensional consistency depends on consistent cooling. Any project considering conformal cooling — cool simulation is the only way to justify the conformal cooling investment.

Warp Simulation: The Hardest, Most Valuable Output

Warp simulation combines fill, pack, and cool data and adds post-mold cooling and stress relaxation. It predicts the final part shape after the part has been ejected and equilibrated to ambient temperature.

What warp simulation predicts

• Total displacement field. A 3D map of how much each point of the part has moved from nominal CAD geometry.
• Warp source decomposition. How much of the warp is from cooling, how much from molecular/fiber orientation, how much from geometric effects.
• Dimensional pass/fail. Whether controlled dimensions on the drawing will be within tolerance.

When warp simulation matters

Any project with fiber-filled or reinforced material — fiber orientation drives anisotropic shrinkage that is impossible to predict without simulation. Parts with flat panels, large area-to-thickness ratios, or asymmetric features. Parts with dimensional callouts that affect downstream assembly or function.

How to Scope Your Moldflow Engagement

The right scope depends on project risk. Here is the practitioner’s tiering:

Tier 1: Fill only ($1,000-$2,500)

Single-cavity cosmetic tool, unfilled resin, no dimensional callouts tighter than ±0.5 mm. Primary risk is weld-line placement on visible surface.

Tier 2: Fill + Pack ($2,500-$4,500)

Multi-cavity production tool, dimensional callouts within DIN 16901-2 commercial tolerance, no aggressive cosmetic requirements. Primary risk is dimensional consistency and sink marks.

Tier 3: Fill + Pack + Cool ($3,500-$6,000)

High-volume production, cycle-time-sensitive, cooling design needs validation. Primary risk is cycle time and steady-state thermal balance.

Tier 4: Full Fill + Pack + Cool + Warp ($4,500-$8,500)

Fiber-filled material, tight dimensional callouts, complex geometry, automotive or medical end-use. Primary risk is warpage and dimensional drift.

FAQ

Can I add warp analysis after the fact if pack already shows problems?

Yes. Most Moldflow service providers will price a warp add-on at $2,000-$3,500 if they already have the fill-pack-cool model built. Cheaper than commissioning a full warp engagement from scratch.

Does pack simulation require fill simulation as input?

Yes. Pack analysis uses the temperature and pressure field from the end of fill as its starting condition. You can’t run pack without fill.

Are there cheaper alternatives to full Moldflow?

Yes, for fill-only analysis. SolidWorks Plastics, Autodesk Fusion 360’s molding tools, and some lower-cost simulation suites cover basic fill analysis at much lower cost. They lack the validation history of Moldflow Insight for pack and warp but are adequate for early-stage flow checks.

How do I know if the simulation is set up correctly?

Ask for the input file (the .sdy or .udm model). A senior tooling engineer can review the mesh density, gate location, and boundary conditions to validate the model is built correctly. Garbage in, garbage out applies to Moldflow as much as any simulation.