Key takeaways
- A mould flow study models fill, pack, cooling and warp, in that order, because each stage feeds the next.
- Its value is in comparing options early: gate position, wall change, cooling layout or material.
- The result is only as good as the mesh, the material data and the process settings behind it.
- Ask for the assumptions and the comparison, not just a colourful fill picture.
What a mould flow study actually does
Mould flow analysis simulates the injection process before a tool is cut. The part is meshed, the material is described by a viscosity model and a pressure, volume and temperature relationship, and the software solves the flow through the cavity shot by shot. The output is a set of predictions: fill pattern, pressure, temperature, clamp force, shrinkage and warp.
It is not a guarantee. It is a comparison tool. A study that says option A fills at 55 MPa and option B at 80 MPa is useful even if the absolute number is out by 10%, because the ranking is usually more reliable than the value. That is why the best studies compare two or three designs rather than reporting one result.
Fill: short shots, weld lines and air traps
The fill stage shows how the melt reaches the last point in the cavity and what pressure it needs to get there. It flags a short shot risk, an unbalanced flow in a multi-cavity tool, and a flow front that races down one wall and then doubles back. It also predicts where weld lines form and where air will be trapped.
Weld lines are not automatically a defect, but they are a weakness. A weld line on a cosmetic face, at a boss or in a pressure-tight wall is a decision, and the fill result lets you move the gate or change the wall before the tool exists. Air traps need a vent or a change in flow, because a trapped pocket burns or short fills.
Pack: pressure at the gate and volumetric shrinkage
After fill, the study holds pressure while the gate freezes. This is where the part is packed and where the difference between a full and an under-packed region appears. The result shows the pressure at the gate, the time the gate stays open and the volumetric shrinkage across the part.
A gate that freezes early cuts off the pack to the far end, and the last region to fill becomes the first region to sink. That is why a thick section far from the gate is a sink risk even when the fill looks balanced. The pack result also sets the clamp force requirement, which decides the machine size and therefore the hourly rate.
Cooling: cycle time and hot spots
The cooling stage predicts how quickly each region reaches ejection temperature. A thick rib, an isolated boss or a heavy boss pad stays hot after the rest of the part is ready, and the cycle is set by that hot spot. The study can propose a baffle, a bubbler or a change in the circuit, and it can show the cycle time penalty of a wall that is too thick.
Cooling is usually the largest share of the cycle, so a small change here moves the part cost. For illustration, cutting cooling from 22 seconds to 16 seconds takes a 30 second cycle to 24 seconds, which cuts machine cost per part by 20%. The exact number depends on the machine rate and the part.
Warp: the prediction that changes the drawing
The warp stage takes the temperature and pressure history from the earlier stages and calculates the shrinkage that follows. It shows which way the part will bow, where the corners will lift and how flatness will change after ejection. For a glass-filled grade, the fibre orientation model is the part of the study most likely to be wrong, so treat warp as a direction rather than a dimension.
The value of the warp result is in the comparison. If moving the gate from the end to the centre halves the bow, that is a design decision worth making before the tool. If a rib change makes it worse, the study has paid for itself. The absolute millimetre figure should be checked against a trial shot, not quoted to a customer as a promise.
| Stage | Main output | Decision it informs |
|---|---|---|
| Fill | Fill pattern, pressure, weld lines | Gate position, wall thickness, venting |
| Pack | Gate freeze, shrinkage, clamp force | Machine size, holding profile, wall balance |
| Cooling | Ejection time, hot spots | Circuit layout, baffles, cycle estimate |
| Warp | Bow and flatness direction | Gate, rib and cooling changes, tool allowance |
| Material | Viscosity and shrinkage data | Grade choice between options |
| Process window | Pressure and temperature limits | Stable setting range for production |
A study is a prediction. Confirm the critical outputs on a trial shot before accepting a tool or a cycle time.
What the study cannot tell you
The study cannot model a dirty tool, a worn check ring, a blocked vent or a moulder who runs the machine outside the agreed window. It also cannot predict cosmetic defects that depend on surface texture, and it will not settle a colour or a gloss question. Those need a trial and a sample.
Material data is the other limit. A generic grade from a library is not the same as the grade in the bag, and glass-filled materials are especially sensitive to fibre orientation and breakage. Ask for the grade to be confirmed, and for the fibre model to be stated if the part is glass-filled.
How to commission a useful study
Give the analyst the part model at the correct revision, the material grade, the wall sections, the proposed gate and the machine size. Say what decision the study has to support. A study asked to compare two gate positions will be more useful than one asked to confirm a design that is already fixed.
- The part model and the drawing revision
- Material grade, filler and colour
- Gate options and any fixed gate position
- Machine size and available clamp force
- The critical dimensions: flatness, warp or a seal face
- The decision the study must support, stated in one line
- A request for assumptions and a comparison, not only pictures
Frequently asked questions
What does mould flow analysis predict?
It predicts how the cavity fills, the pressure needed, the weld line and air trap positions, how the part packs and shrinks, how long it takes to cool and which way it will warp. Each stage builds on the previous one, so the material data and process settings matter.
Can mould flow analysis prevent warpage?
It can show the likely direction and help you compare gate, wall and cooling options before the tool is cut. It cannot remove every source of warpage, and a glass-filled part is hard to predict precisely. Use it to choose between designs, then confirm on a trial shot.
Is a mould flow study worth it for a simple part?
Often not for a small, thin, unfilled part with a single gate. It earns its cost when the part is large, thin-walled, glass-filled, multi-gated or has a tight flatness or seal requirement, or when the tool cost and lead time make a trial loop expensive.
Who should run the study?
Usually the moulder or the toolmaker, often with the material supplier's support. The buyer should ask for the assumptions, the material data source and a comparison of at least two options, then keep the report with the tool record.
Sources and further reading
- ISO 294-1:2017, Plastics, injection moulding of test specimens of thermoplastic materials
- ISO 20457:2018, Plastics moulded parts, tolerances and acceptance conditions
- ISO 11403-1:2021, Plastics, acquisition and presentation of comparable multipoint data
Qumek Engineering Desk
Engineering team
The engineering team behind Qumek drawings, DFM notes, tolerances and inspection standards. Every guide is reviewed against the drawings and processes used on live programmes.
About to cut a tool on an untested gate?
Send the part model, the material and the flatness or seal requirement. The engineering desk will set out the questions a flow study should answer.
Send a drawing