Mold flow analysis (injection molding simulation) uses computer-aided engineering (CAE) software to simulate how molten plastic flows inside an injection mold cavity. The software performs finite element analysis (FEA) on the 3D model to predict: fill pattern, melt front temperature, pressure drop, weld lines, air traps, cooling time, part warpage, sink marks, and shrinkage. By identifying potential defects before the steel is cut, mold flow analysis saves weeks of mold trial iterations and thousands of dollars in rework.
The most widely used software globally is Autodesk Moldflow (Synergy + Insight) and Moldex3D (from Taiwan — very popular in China because of its Chinese interface and compatibility with Chinese plastic material databases). In China, Moldex3D has roughly 40% market share, followed by Moldflow (30%), and domestic software like Huaxiang CAE, Dymax, and Z-Mold. Many Chinese mold shops now offer basic mold flow as a standard first-article pre-check, but the depth and quality vary drastically — from a quick 2D midplane fill simulation to a full 3D warpage analysis with cooling circuit optimization.
| Analysis Type | Outputs | Typical Mesh Type | Time to Run | Cost Premium |
|---|---|---|---|---|
| Fill analysis (basic) | Fill time, melt front temperature, injection pressure, flow front velocity | Midplane or dual domain (2.5D) | 10–30 min | Included (standard) |
| Fill + Pack + Cool | + Volumetric shrinkage, sink marks, clamp force, molded-in stress | 3D tetrahedral or prismatic | 30–90 min | +$100–300 |
| Warpage analysis | + Predicted deformation (mm), warpage mode, residual stress map | 3D (tetrahedral — need 3+ layers through thickness) | 1–6 hours | +$200–500 |
| Cooling circuit analysis | + Temperature distribution in mold, cooling time, thermal gradient | 3D (flow channels + mold block) | 2–8 hours | +$200–400 |
| Fiber orientation analysis | + Fiber alignment, anisotropic shrinkage, warpage due to fiber orientation | 3D (10+ layers) | 2–12 hours | +$300–600 |
| Gas-assisted / water-assisted | + Gas/water penetration length, wall thickness, hollow core position | 3D | 2–8 hours | +$400–800 |
| Multi-shot / co-injection | + Interface between materials, bond line strength, relative filling | 3D | 3–12 hours | +$500–1,000 |
Most mid-to-large Chinese mold factories (100+ employees) have at least one mold flow engineer and a Moldex3D or Moldflow license. They run mold flow as a "free" pre-production service (cost embedded in the mold price). Quality: highly variable — best-case engineers are experienced and can predict real-world behavior; worst-case the analysis is run as a checkbox, with inaccurate material data or mesh that's too coarse (primarily 2.5D simulation that misses 3D effects like corner overpacking or jetting). The key distinction: the mold flow is usually free, but the engineer's assignment of material data bank defaults is often the biggest source of discrepancy from actual. For 80% of simple parts (2 mm wall, PP/ABS/HDPE), it's adequate. For complex parts (thin-wall, glass-filled, high-gloss surfaces), it's often insufficient — you may need a dedicated CAE service.
Independent CAE bureaus in Shenzhen, Shanghai, Kunshan, and Dongguan that offer mold flow analysis as a standalone service. They typically use Moldex3D (most common) or Autodesk Moldflow, with fully licensed 3D modules (fill+pack+cool+warpage + fiber + gas-assist). They have extensive material databases calibrated to Asian and international resin brands. A full 3D analysis with warpage prediction costs ¥3,000–10,000 ($450–1,500). Lead time: 2–5 days. They often include: comprehensive report with color-coded plots, flow leader/follower path identification, and specific gate location, runner diameter, and cooling channel optimization recommendations.
Major resin suppliers (BASF, SABIC, DuPont, Covestro, LG Chem, Kingfa in China) offer free mold flow analysis as a technical service to customers buying their materials. This is often the highest-quality analysis because the material supplier has the most accurate rheological data for their specific resin. The limitation: the analysis is tied to using their material in production. For initial sourcing evaluation, this can be leveraged at zero cost while getting excellent material-specific recommendations. However, the analysis turnaround time depends on the supplier's CAE team workload (typically 1–3 weeks).
Classic mold flow mistake: The most common error we see in Chinese mold flow reports is materials database mismatch. A shop uses the default "ABS (generic)" material from Moldex3D's library for an analysis, but your production will use Chi Mei PA-765 or LG HF-380 — two very different ABS grades with different melt flow indices (MFI of PA-765: 5.0 g/10min @ 220°C/10kg vs generic ABS: ~15 g/10min). The analysis will predict adequate filling at a given temperature and pressure, but the actual part will be 15° hotter at fill-end than the simulation (because the actual material has higher viscosity and shear heating). Always insist on using the specific material grade from the supplier's calibrated database — not a generic substitute. The extra effort to source the correct .udb (Moldflow material file) or .mda (Moldex3D) for your specific resin is small but the impact on accuracy is large.
Mold flow accuracy is fundamentally limited by mesh quality. A poor mesh with highly skewed elements, aspect ratio > 20, or insufficient element count through the thickness (< 3 layers for thin-wall parts) will produce inaccurate results regardless of how good the analyst is. Key requirements: minimum 3 layers of elements through the part thickness for 3D warpage analysis; for fiber orientation, 6–10 layers. Aspect ratio < 10 for 90% of elements. Thickness direction ratio < 5. Always request the mesh quality statistics from the provider (commonly Moldex3D or Moldflow reports element quality metrics). If the provider can't or won't share mesh statistics, the analysis is likely only good enough for placeholding — consider it a routing sketch, not a production simulation.
Cooling accounts for 60–80% of injection molding cycle time, yet many Chinese mold flow analyses skip or simplify it. A proper cooling analysis requires modeling the mold block (not just the cavity) with cooling channels. Key inaccuracies: (1) assuming uniform 25°C cooling water temperature across all channels — in reality, the water temperature rises 3–8°C from inlet to outlet, creating a temperature gradient across the mold. (2) Not modeling cooling channel baffles and bubblers (which significantly affect local cooling). (3) Using the wrong heat transfer coefficient (HTC) between part and mold. For better accuracy: specify that the analysis use a transient cooling simulation (not steady-state) that accounts for the cyclic nature of molding, and that you want the "mold temperature distribution map" that shows hot spots.
The gate must freeze AFTER the part is packed. If the gate freezes too early (before full packing), the part doesn't reach the correct packing pressure, causing sink marks, low crystallinity in semi-crystalline materials, and excessive shrinkage. A common error: the mold flow analysis shows adequate fill but doesn't report gate freeze time vs packing time. If gate freeze time is less than packing time + holding time, the analysis is incomplete. Request a "gate freeze time vs packing profile" chart that shows gate diameter impact on the packing performance — this single parameter often drives the gate design decision.
| Analysis Package | Cost (USD) | Lead Time | Typical Deliverables |
|---|---|---|---|
| Basic fill analysis (2.5D midplane/dual domain) | $150–400 | 1–2 days | Fill time, pressure, temperature, weld line maps PDF |
| Fill + Pack + Cool (3D, single gate, no fiber) | $400–800 | 2–4 days | + Volumetric shrinkage, cooling time optimization report |
| Full 3D: Fill + Pack + Cool + Warpage (no fiber) | $600–1,500 | 3–7 days | + Warpage plot, residual stress, gate freeze, sinking analysis |
| Full 3D with fiber orientation | $800–2,000 | 3–7 days | + Fiber orientation tensor, anisotropic shrinkage plan, warpage correction |
| Multi-cavity / family mold analysis | $800–2,500 | 5–10 days | + Runner balance check, cavity-to-cavity flow variation, corrected runner dimensions |
| Gate location optimization (carry out + analysis) | $400–1,000 | 2–4 days | + 3+ candidate gate location comparisons with fill, pressure, and warp rankings |
As a rule, Chinese dedicated CAE providers charge 50–65% less than equivalent European/US bureaus. The major differentiator is problem-solving depth in the report — a US bureau might provide a comprehensive 40-page analysis with far more iterative runner refinement than a Chinese provider. For 90% of production parts, the Chinese analysis is still very good value, but the scope of the analysis needs to be clearly agreed to avoid a lightweight result.
Recommended for any mold that costs > $5,000, involves glass-filled materials, thin-wall geometry (< 1.5 mm wall), multiple cavities, or complex 3D shapes. For simple 2D parts (flat parts, thick-walled commodity products under 500 g, single cavity, no structural requirement), a basic fill analysis (30–60 minutes setup) provides 80% of the value (confirming gate location and fill pattern) at 20% of the cost. The break-even point: one mold repair to fix a flow-induced defect typically costs $500–3,000 (steel modification, no lost production time). A full 3D analysis costs about the same. The analysis also prevents downstream scrap and customer quality rejections. The decision: if your mold has a 10%+ chance of a flow-related defect (which most first-shot molds do), it's cheaper to analyze than to correct.
Depends on part complexity and analysis scope. Typical timelines from Chinese CAE bureaus: Simple 2D midplane fill analysis: 1–2 days. Full 3D analysis (fill+pack+cool+warpage): 3–7 days. Complex multi-cavity or fiber-filled analysis: 5–14 days. The bottleneck is usually mesh generation — a complex 3D part with fine features may take 1–2 DAYS just to mesh with adequate quality. Computational solving time for a full 3D warpage run on a decent workstation (32 GB RAM, 8-core CPU) takes 2–8 hours. Cooling analysis is the most computationally expensive (often overnight runs).
Yes — but only if the analysis includes packing AND a properly calibrated PVT (pressure-volume-temperature) equation of state for the specific material. Sink marks form when volumetric shrinkage is not compensated by packing pressure. A good mold flow analysis will show: the volumetric shrinkage distribution, the "sink index" (a derived metric), and the actual sink depth predicted at a given location. However, the absolute sink depth prediction has about ±30% accuracy — use it as a relative ranking (which gate/process configuration reduces sink at a given rib location?) rather than a precise sink depth value. For visual parts with cosmetic standards, the analysis should be used to identify zones at risk, not to certify part appearance.
The simulation predicts required injection pressure at the nozzle. A useful rule: if the analysis shows injection pressure > 80% of the machine's maximum injection pressure, there is significant risk that the mold won't fill completely (short shot) under production conditions. If > 1,200 bar is needed for a standard machine (1,500 bar maximum), consider: increasing melt temperature (10 °C reduces viscosity by 20–30%), increasing mold temperature (20 °C reduces needed pressure by 10–15%), or redesigning the runner/gate system. For comparison, a typical well-designed mold uses 500–900 bar cavity pressure to fill a balanced flow.
The ideal workflow: (1) Send the CAE provider the nominal part CAD (STEP/IGS) + molding machine specification (clamp tonnage, injection unit, nozzle type). (2) The CAE provider runs the analysis and delivers the report WITH the recommended mold design changes — gate location, runner dimensions, cooling channel layout, venting location, and suggested mold surface temperature. (3) Pass these recommendations to your mold maker BEFORE they cut steel. (4) The mold maker quotes mold modifications (if any) based on the analysis. (5) If the mold maker disagrees with any recommendation, they should explain in writing with their rationale. We recommend photo-documenting the mold after it's cut with the exact runner dimensions and gate position — then a "mold-as-built" inspection can confirm whether the mold matches the analysis assumptions. A mismatch between 6 mm runner in analysis and 8 mm runner in the mold is a common finding that accounts for fill discrepancy.