Injection Mold Flow Analysis: What It Predicts and How to Read It

Mold flow analysis is the closest thing injection molding has to a
preview of what will happen inside your tool. Before a single piece of steel is cut, it
takes the 3D file and the resin you intend to use and answers the questions that
otherwise get answered expensively: will the cavity fill, where will the melt fronts
meet, how much pressure will it take, which region will be thinner than the drawing,
where will air get trapped, and how much will the part shrink. For most projects it
takes an hour of engineering time, and it routinely saves a cavity rework worth ten
times that. This guide explains what the simulation actually produces, how to read
it, and the specific defects that can be designed out before tooling rather than
chased on the shop floor.

Short answer

Mold flow analysis predicts how molten plastic will fill, pack and cool inside your
cavity. It gives you a fill time, a pressure and temperature history, a thickness
distribution, the position of weld lines and air traps, and a first estimate of
shrinkage. Use it to compare design options before the tool exists,
because after the tool exists the same problems cost a cavity rework.

The three questions a simulation answers that a drawing cannot are:
Will it fill? (fill time, short shots, trapped air),
Will it hold its dimensions? (packing, shrinkage, sink) and
Where will it look bad? (weld lines on show surfaces, air traps
producing visible marks). If your part has thin walls, a long flow path, tight
tolerances or a cosmetic surface, one of those three questions is the one that will
cost you money.

What you need before a simulation runs

Simulation quality is bounded by input quality. Five inputs determine whether the
result is a decision tool or a decoration.

Two details are worth insisting on because they are routinely guessed. First, the
resin grade: a simulation run on “generic ABS” will not reflect the shrinkage or the
fill behaviour of the actual ABS you buy, and those two properties are the entire
point of the exercise. Second, the critical dimensions: if you tell us
which features must hold a tolerance and which faces are visible, we can concentrate
the mesh and the reporting on those areas. A simulation with priorities produces a
decision; a simulation without priorities produces a report nobody reads. The
materials page covers the resin
families we run and the handling each one needs.

Reading the fill phase

Fill is the first phase, from the moment the melt enters the gate until the cavity
is 99 percent full. Everything that goes wrong in filling shows up here first.

Fill time. For most engineering thermoplastics the working range is
1 to 4 seconds wall to wall. Faster than about 1 second raises melt shear, which can
degrade the polymer and makes glass-filled grades harder to fill consistently. Slower
than 4 seconds usually points to high pressure drop over a long flow path, a
restricted gate, or poor venting at the end of flow — and it adds directly to cycle
time.

Pressure drop along the flow path. The relevant number is the
difference between the pressure at the gate and the pressure at the last point to
fill. If that difference approaches the machine’s available injection pressure, the
part will not fill reliably on a cold tool, and cold tools are the normal starting
condition. High pressure drop is also where you start seeing shear heating and
degradation of the melt.

Thickness distribution. This is the output buyers most often
underrate. Material diverted into a rib, a boss or a snap consumes flow, and the
wall opposite it comes out thinner than nominal. A design that reads 2.0 mm on the
drawing can produce a 1.4 mm wall in production because 0.6 mm of material went
somewhere else. If your tolerance is 2 percent on a wall thickness, this is the
result that will fail your first article.

Gate and runner balance. In a multi-cavity tool, flow must reach
every cavity in the same time. When one cavity is slower, it runs at a lower
temperature and a lower packing pressure, and it will have different shrink from its
neighbours. That produces parts from the same tool that do not match each other —
a problem that is invisible on a single-cavity prototype and expensive on a
production tool. See the multi-shot molding
page for how cavity count and runner design interact.

The mold design guide covers the geometric
side of the same decisions — draft, ribs, parting line and ejection — and the two
documents are meant to be read together, because the fill analysis will tell you
where a design feature is costing you material and the design guide tells you how to
change it.

Reading the packing phase

Once the cavity is 99 percent full, the machine switches from filling to packing,
and this phase is what actually determines the part’s final dimension. The gate is
sealed by the freeze layer that formed at the end of filling, and melt is pushed in
behind it under high pressure to compensate for the material that has cooled and
contracted.

Two numbers matter here, and they are different from each other.

• Pack pressure and pack time. Together these determine how much
  material goes into the thick regions to offset shrinkage. Packing pressure that is
  too low relative to the gate means the part is light and the thick sections sink.
  Packing time that is too short means the gate freezes before the cavity is fully
  compensated. The right value is a function of the material’s shrinkage rate and the
  gate size, not a fixed number from a handbook.
• Gate freeze and gate size. A bigger gate stays open longer and
  packs more effectively, but leaves a larger mark and needs a longer cooling time. A
  small gate gives a better mark and a faster cycle, but freezes early and cannot
  compensate shrinkage in thick sections. This trade-off is one of the most common
  reasons a tool needs a second gate added, and it is far cheaper to decide it in
  simulation than after the trial.

A useful discipline is to record the packing conditions alongside the first
article report, so that the process window you validate today is the window you
quote to the customer in two years. Our injection
molding process page describes the full production sequence around these
decisions.

The defects flow analysis catches

The defects below are the ones that reliably show up in analysis and reliably
cause problems in production. Being able to predict them before tooling is the
entire commercial argument for running the simulation.

The pattern worth remembering is that every one of these is cheap in the file and
expensive in the steel. A cavity rework on a multi-cavity tool is not a small
event: it involves new steel, re-machining, re-polishing, new trial time, and a
schedule that has already been promised to a customer. The injection
molding defects page covers what each defect looks like in the finished part, and
this table covers where it comes from in the process.

Weld lines and air traps

These two get a separate section because they are the two results buyers are most
likely to be told about too late, and because both are decided by the gate position
rather than by the steel.

Weld lines form where two melt fronts meet and fuse imperfectly.
They are weaker than the surrounding material and often visible as a faint line. They
are unavoidable in some geometries — a part with a central hole filled from one side
will always have a weld line across the hole — so the design question is not “can we
remove it” but “can we move it somewhere that does not matter”. The analysis gives
you the location as a function of the gate position, which makes it a cheap
optimisation: run the same geometry with three or four gate positions and compare
where the line lands.

Air traps form where the flow front closes around a pocket of
air before the air can escape. The trapped air compresses, and on a visible surface it
shows as a bubble or a matte spot. The fix is a vent at the trapped location, but a
vent only helps if the vent itself has somewhere to go — a vent that discharges into
a blind pocket simply moves the problem. Analysis shows the trapped region as an
enclosed area, which tells you exactly where the vent belongs.

None of these are exotic. They are the ordinary consequences of where you put the
gate, and they are exactly what a simulation is for.

Shrinkage and sink marks

Shrinkage is where simulation is most useful and where it is most often
over-interpreted. Being clear about the limits is what makes it trustworthy.

What simulation predicts well. The overall volumetric shrinkage
of a given resin at a given packing condition, and therefore the general direction
of the effect: which regions will be under-packed and therefore sink, and which
will be adequately packed and therefore hold dimension. It also reliably identifies
the thick regions where material volume is disproportionate — the boss that is
four times the nominal wall thickness, the corner where three walls meet.

What simulation predicts less well. The precise final dimension
on a specific feature, particularly with fibre-filled grades. Glass and carbon fibre
align with the flow, so a material that simulates at 1.8 percent volumetric shrink
may behave very differently in a wall running along the flow versus a wall across
it. Treat fibre-filled shrinkage as a directional tendency rather than a number.

What you should do with the output. Use it to find the thick
sections and correct the design, not to set a nominal dimension. The standard
engineering correction for a sink mark is to core out the thick section or add
ribs that thin it locally — the injection
mold design checklist covers rib geometry in detail. Packing harder against a
sink mark treats the symptom and adds stress to the mould, and the mark returns as
the tool wears and the packing window drifts.

Sink marks are also the defect most sensitive to the choice of mould steel, because
a more stable steel holds its dimensions longer as the tool ages. Our mold
steel selection page covers how P20, H13, S136 and pre-hardened steels differ in
wear resistance and dimensional stability, and which ones suit a cosmetic tool.

Turning simulation into a process window

A simulation is only as valuable as the decision it leads to. The useful output is
not a PDF; it is a set of starting conditions that get confirmed at trial and then
become the documented production window.

1. Fix the geometry first. Do not compensate for a bad design with
   process settings. If the analysis shows a wall starved by a neighbouring boss, change
   the file. Process changes cannot create material that the design already diverted
   elsewhere.
2. Choose a starting point that is centred. Do not set the first
   process window at the edge of what is acceptable. Fill time, melt temperature and
   pack pressure should each start near the middle of the range the analysis indicates
   is achievable, so that the trial has room in both directions.
3. Plan the trial as a designed experiment. The first article
   should sample the window deliberately — typically a short shot, a nominal shot and a
   long shot, plus a pack pressure sweep — rather than running one setting and
   accepting whatever comes out. Our molding
   process page describes the trial sequence we use.
4. Record what you learn. Write the confirmed window, the first
   article results and the measured dimensions into the tool documentation. A tool
   whose validated process is recorded will be maintained correctly in three years; a
   tool whose process lives only in the memory of one setter will be run differently by
   every operator.
5. Re-simulate before any revision. When a tool is modified for
   wear, a cavity insert is replaced, or the part is resourced to a different grade of
   resin, the analysis should be repeated. Changes that look small in the steel can
   move the fill balance.

When to simulate and what it costs

The honest answer is that a simulation is inexpensive and a cavity rework is not,
so the threshold for running one is low. That said, the value is concentrated in
particular situations.

On cost: the analysis is normally a few hours of engineering time, and at our
rates it is a small fraction of the tool price. The reason to publish that number
plainly is that buyers often compare it against the tool cost and conclude the
simulation is unnecessary. The correct comparison is against one cavity rework,
which in a multi-cavity tool involves new steel, new machining, new polish, a new
trial and a schedule slip — routinely more than the entire original tool.

The injection molding cost page
breaks down where tool price actually goes, which is useful context when you are
deciding how much engineering attention a tool deserves before it is quoted.