Autodesk Moldflow is simulation software that predicts how molten plastic will fill, pack and cool inside an injection mould. It also predicts how the part will shrink and warp after it is ejected. It is run before any steel is cut, so problems with gates, wall thickness and cooling are found on a computer instead of on a tool trial.
Its value is that the tool design questions get asked early, when changing the answer costs a few hours on a computer rather than a tool modification.
What is Autodesk Moldflow?
Moldflow is a computer-aided engineering (CAE) tool built specifically for plastic injection moulding. Autodesk acquired Moldflow Corporation in 2008. The software takes the 3D geometry of a part, a material from its database and a set of process conditions, then calculates what happens in each phase of the moulding cycle.
- Filling. How the melt front advances and whether the cavity fills completely. Where flow fronts meet to form weld lines and where air will be trapped.
- Packing. How much material is pushed in after fill to make up for shrinkage. Where the part is under or over packed.
- Cooling. How heat leaves the part through the mould steel and the cooling channels. Where hot spots will form.
- Warpage. How uneven shrinkage will distort the part once it is out of the tool. In filled grades, fibre orientation adds to it.
The output is a set of results plots and numbers that the tool designer and the moulder can act on.
What it helps you find
1. Material and waste
Moldflow shows where walls are thicker than they need to be to fill and pack. It also shows how much of each shot goes into the runner rather than the part. Thinning a wall that has no reason to be thick cuts the material used per shot. So does balancing and reducing a cold runner. On small parts the runner can be a significant share of the shot weight, so this matters more than it looks.
2. Defects before they happen
The defects Moldflow predicts well are the ones driven by flow, pressure and heat.
- Warpage. Distortion from uneven cooling, uneven packing or fibre orientation.
- Sink marks. Surface depressions over thick sections, ribs and bosses, where the core keeps shrinking after the skin has frozen.
- Air traps. Gas caught where flow fronts meet or at the last point to fill. If it cannot vent it causes burns, short shots or weak weld lines.
- Weld lines. Where two flow fronts meet. Moldflow shows where they will be. It also shows how hot the fronts are and at what angle they meet, which indicates how strong the weld will be. Moving the gate moves the weld line away from load bearing or cosmetic areas.
- Short shots and hesitation. Areas that freeze before they fill, usually thin sections a long way from the gate.
3. Cooling and cycle time
Cooling is usually the longest part of the moulding cycle, often more than half of it. Moldflow shows the mould surface temperature and how long each area of the part takes to freeze. A cooling layout that leaves a hot spot in one corner sets the cycle time for the whole part. It will usually warp it as well. Fixing that at the design stage is much cheaper than adding cooling to a finished tool.
How a Moldflow study is run
A study follows the same five steps whatever the part.
Step 1: Import and mesh the part
The part geometry comes in from CAD, usually as a STEP file. Moldflow then meshes it, breaking the part into small elements it can calculate on. The mesh type matters. A midplane or dual domain mesh is quick and suits thin walled parts. A full 3D mesh is slower but is needed for thick or chunky geometry.
Step 2: Choose the material
The exact grade is picked from the Moldflow material database. This is the step that most affects how far the results can be trusted. A grade with full characterisation data, including viscosity, pvT and shrinkage data, gives reliable warpage predictions. A generic or poorly characterised grade gives fill results you can use and warpage results you should treat with caution.
Step 3: Define the tool and the process
The gate positions, runner system and cooling channels are modelled, along with the process settings: melt temperature, mould temperature, injection time or flow rate, packing pressure and packing time. Injection pressure is a result of the study, not an input. Moldflow tells you what pressure the part needs, which is how you check it will fill on the machine you intend to run it on.
Step 4: Run the analysis
The analysis sequence is chosen to answer the question being asked. Fill alone shows whether the part fills and where the weld lines are. Fill, pack, cool and warp together predict the final shape. Run times range from minutes for a simple fill to hours for a full 3D warp study.
Step 5: Read the results and change something
The results are only useful if they lead to a decision: move a gate, add a rib, core out a thick section, change the grade or add cooling. The study is then run again to confirm the change did what was expected. It usually takes more than one round.
What Moldflow does not do
- It is a process simulation, not a structural one. It predicts how the part is made, not whether it survives its load in service. That is finite element analysis (FEA), which can take Moldflow's fibre orientation results as an input for filled materials.
- It is only as good as its inputs. Wrong material data, a mesh that is too coarse or process settings the machine cannot actually achieve all give confident looking results that are wrong.
- It does not replace the tool trial. It reduces the number of surprises at first shot. It does not remove the need to sample the tool and set the process properly on the press.
Where it is used
Moldflow is worth running wherever the cost of getting a tool wrong is high.
- Medical devices. Small, precise parts where weld lines, flatness and dimensional stability are tightly specified.
- Automotive components. Large or complex parts, often glass filled, where warpage and fibre orientation decide whether the part fits.
- Consumer electronics. Thin walled housings where sink marks, weld lines and gloss differences are visible to the customer.
How Plastic-IT offers Moldflow analysis
We run Autodesk Moldflow in two ways. As part of our own moulding jobs, before a tool is ordered. And as a standalone service on tooling designed or built by someone else, where the customer receives the report and makes their own decisions with it.
What is included is on the Moldflow analysis service page. There is also a short history of Moldflow if you want to know where the software came from.
FAQs on Autodesk Moldflow
How does Autodesk Moldflow improve manufacturing efficiency?
It finds gate, wall thickness and cooling problems before the tool is made, so fewer tool modifications and trial rounds are needed. The cooling layout can also be designed for a shorter cycle from the start.
Can Moldflow analysis prevent defects in injection moulding?
It predicts the defects driven by flow, pressure and heat, such as warpage, sink marks, air traps, weld lines and short shots, so they can be designed out. It cannot predict defects caused by a damaged tool, contaminated material or a process that is set up badly on the day.
Is Moldflow analysis suitable for all types of plastic parts?
It suits most thermoplastic injection moulded parts. The results are weakest where good material data does not exist for the grade. It is not the right tool for structural analysis of the finished part.
What do I need to send for a Moldflow study?
A 3D CAD file, ideally STEP, the material grade you intend to use and a short note on what worries you about the part. That might be a cosmetic face, a flatness requirement, a weld line position or a thin section that may not fill.
To discuss a part, get in touch.