Run a structured mold flow analysis checklist before tooling to catch fill, pack, weld, and cooling issues early. This guide provides a numbered audit of design checks, red flags, and validation steps to reduce trial-and-error.
- Run mold flow analysis at the concept, draft, and final geometry stages.
- Check gate strategy, cooling balance, and weld line location before committing to steel.
- Treat simulation results as a risk filter, not a guaranteed outcome.
- Keep a design checklist document linked to the part number for auditability.
Why run a mold flow analysis before finalizing the design?
Mold flow analysis is a simulation of how polymer flows, cools, and solidifies inside a mold cavity. It helps identify fill problems, pack deficits, weld lines, and cooling imbalance before expensive tooling is cut.
This checklist is built for mold design and tooling reviews. It is not a substitute for trial shots, but it is a structured way to reduce risk.
1. Verify part geometry and material data first
- Confirm the final CAD geometry is exported in a format the simulation software accepts.
- Check that wall thickness, fillets, and draft angles are consistent across the model.
- Select the correct material grade and processing conditions from the supplier’s datasheet.
- If the part contains multiple materials or recycled content, define the blend or grade explicitly.
Red flags to watch for
- Wall thickness variation beyond what the selected process can handle.
- Sharp internal corners or thin ribs without adequate draft.
- Missing material properties such as melt flow rate or shrinkage data.
- Using generic material values instead of the specific grade intended for production.
2. Check gate strategy and fill pattern
The gate location determines the fill sequence. A well-placed gate helps the cavity fill evenly and reduces air traps.
- Evaluate gate type: single, multiple, fan, or edge gates.
- Confirm the fill path is predictable and not blocked by geometry.
- Check whether the gate location matches the part’s functional orientation.
- Review gate size and location for potential sink marks or flash risk.
Red flags to watch for
- Fill reaching the far corner late or with high pressure.
- Multiple gates causing uneven pack or weld line migration.
- Gate placement directly over a feature that must be flat or strong.
- A gate that is too small for the selected resin and production speed.
3. Evaluate weld line location and quality
Weld lines form where two melt fronts meet. They can be a weak point or a visible defect if placed poorly.
- Locate the weld line on the part drawing.
- Check whether the weld line is hidden by a rib, vent, or cosmetic area.
- Review the temperature and pressure at the weld point.
- Confirm the weld line is not near a high-stress or high-visibility zone.
Red flags to watch for
- Weld line on a visible surface or near a mating interface.
- Cold or incomplete weld due to low temperature or pressure.
- Weld line crossing a thin wall or a feature that requires strength.
- Unpredictable weld line shift when the gate is moved.
4. Analyze pack, pressure, and shrinkage
Pack pressure keeps the cavity full during solidification. Without adequate pack, the part can shrink or develop sink marks.
- Review the pack pressure profile at critical locations.
- Check whether the part is overpacked or underpacked.
- Compare simulated shrinkage to the part’s dimensional tolerance.
- Look for pressure drop in the far corner or thin sections.
Red flags to watch for
- Pressure falling below the resin’s recommended pack range.
- Sink marks above the gate or on thick features.
- Dimensional variation outside tolerance due to shrinkage.
- Cavity pressure too low during the final seconds of the cycle.
5. Check cooling balance and cycle time
Cooling controls cycle time and part quality. An uneven mold temperature can create warpage or inconsistent cycle times.
- Map mold temperature across the cavity.
- Check that cooling channels are balanced on both sides of the cavity.
- Review cycle time for the target production rate.
- Confirm that cooling does not create a temperature gradient that causes warpage.
Red flags to watch for
- One side of the mold running significantly hotter than the other.
- Cycle time longer than the target without a justified reason.
- Cooling channels blocked by thick features or rib intersections.
- Temperature gradients that may cause uneven shrinkage or warpage.
6. Validate with a design checklist table
Use the table below as a quick audit during the design review. Mark each item as pass, review, or fail.
| Check Area | Key Question | Pass Criteria |
|---|---|---|
| Fill | Does the cavity fill evenly? | No trapped air, no late fill beyond a set threshold |
| Pack | Is the cavity full at solidification? | Pressure and volume meet the resin’s pack window |
| Weld | Is the weld line acceptable? | Hidden or acceptable, with no cold or incomplete weld |
| Cooling | Is the mold temperature balanced? | No hot spots or cold spots beyond tolerance |
| Shrinkage | Is dimensional variation within tolerance? | Simulated shrinkage fits the part’s tolerance |
| Gate | Is the gate size and location suitable? | No flash, no sink, and gate is accessible |
7. Document the simulation and decision
A simulation is only useful if the team can reference it later. Keep the output tied to the part number and revision.
- Save the simulation file with the part number and revision.
- Record the material grade, mold temperature, and cycle time used.
- Note any known limitations of the model.
- Attach the checklist table to the design review package.
- Review the simulation again after any major CAD change.
Red flags to watch for
- No documentation of the simulation setup.
- Simulation run with outdated geometry or material data.
- Decisions made without reviewing the checklist.
- Simulation results ignored when a defect appears on the first trial.
When to run mold flow analysis again
Run mold flow analysis at three key points:
- During concept design, when major geometry is still flexible.
- At draft stage, when the geometry is close to final.
- After any significant change to gate, cooling, or material.
If the simulation shows a problem, fix the CAD first, then re-run the analysis. Do not try to solve a mold flow issue by changing machine settings only.
Final check before tooling
Before committing to steel, confirm that the simulation supports the design. If the simulation shows a fill, pack, weld, or cooling problem, address it in the CAD before tooling. If the simulation is clean and the checklist is signed off, you have a strong basis for moving forward.
Frequently asked questions
How accurate is mold flow analysis?
Mold flow analysis is a strong predictive tool, but it is not a guarantee. Accuracy depends on the quality of the CAD geometry, material data, and process assumptions used.
Do I need mold flow analysis for every part?
Not necessarily, but it is highly recommended for parts with thin walls, complex geometry, tight tolerances, or visible surface features. It is less critical for simple, thick-walled parts with forgiving tolerances.
Can mold flow analysis replace trial shots?
No. Trial shots are still needed to validate the process and confirm the part meets requirements. Mold flow analysis reduces the number and cost of trial shots by catching major issues early.
What if the simulation shows a weld line in a bad location?
Review the gate strategy and part geometry. Moving the gate, adding a second gate, or modifying the part shape can redirect the weld line to a less visible or less critical area.
How often should I re-run the simulation?
Re-run it after any major CAD change, material substitution, or gate and cooling modification. A small geometry change can significantly affect fill, pack, and cooling.
