Mold Flow Analysis Fill Pack Cool: What Each Phase Actually Predicts

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  Fill: Where the Melt Goes and Where Flow Fronts Meet
• 2
  Pack: Shrinkage Compensation and Gate Freeze-Off
• 3
  Cool: Cycle Time, Warpage, and the Balance Problem
• 4
  Where Mold Flow Analysis Stops Being Reliable
• 5
  From Predicted Gate Size to Machined Gate Land
• 6
  What Each Phase Predicts and How It Is Controlled

Mold flow analysis fill pack cool starts with fill because everything downstream inherits its errors. The solver fills the cavity node by node, tracking pressure, temperature, and shear rate at each time step until the volume is nominally full. The output engineers care about most is the weld line map: every place two flow fronts meet leaves a knit line, and knit line strength is usually a fraction of bulk material strength.

A weld line behind a core pin sits in a high-stress region, the part may crack there in service. The fix is rarely more pressure. It is a gate move, a flow leader, or a wall thickness change that keeps the fronts merging outside the loaded area. If the weld line lands on a cosmetic surface, texture and color can hide it, but structural knit lines cannot be polished away.

Fill analysis also flags short shots and air traps. Air traps form in the last-filled regions where gas has nowhere to vent. The software predicts the location; the toolmaker adds a vent, a vent insert, or an ejector pin that doubles as a vent. We have seen 0.02 mm of extra vent depth turn a burning defect into a clean part.

Hesitation is the quieter fill problem. When melt enters a thin rib before the main wall is packed, the rib fills slowly and freezes early. The result is a short rib that no amount of packing pressure can recover. Simulation shows the hesitation; the design fix is a thicker rib root or a flow leader that feeds the rib from the thick section.

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  Weld linesRelocate to non-cosmetic, low-stress zones by moving gates or adding flow leaders.
• 2
  Air trapsVent the last-filled region; 0.02–0.03 mm vent depth is a common starting point.
• 3
  HesitationThin ribs fed from thick walls fill late and freeze early.

Once the cavity is volumetrically full, the pack phase pushes additional melt in to offset shrinkage as the polymer cools from melt temperature to ejection temperature. Semi-crystalline materials such as PP and PA shrink far more than amorphous ones like ABS or PC, so the pack profile matters more for them. The simulation quantifies volumetric shrinkage and shows how pressure decays from the gate inward.

The single most useful number from pack analysis is gate freeze-off time. After the gate solidifies, no more material can enter the cavity, and any remaining shrinkage becomes a sink mark or an internal void. A gate that freezes too early starves the part. A gate that freezes too late wastes cycle time and can leave a protruding gate vestige.

Packing pressure should be high enough to compensate shrinkage but low enough to avoid overpacking near the gate. Overpacked regions carry molded-in stress, and that stress releases later as warpage or stress cracking. The solver shows the pressure gradient; a flat gradient across the part is the goal, not a high number at the gate.

Hold time is tied directly to gate freeze-off. Setting hold time shorter than freeze-off leaves sink marks. Setting it far longer adds cycle time with no benefit because the gate is already solid. Simulation gives the freeze-off time; the molder then sets hold time to that value plus a small margin.

• 1
  Sink marksCaused by insufficient packing or a gate that freezes before the part is packed out.
• 2
  VoidsInternal shrinkage cavities that appear when the skin solidifies before the core.
• 3
  OverpackingHigh pressure near the gate builds molded-in stress and later warpage.

Cooling is the longest of the three phases in most cycles, often 50–70% of total cycle time. The analysis predicts temperature distribution at ejection, the time for the part to reach a safe ejection temperature, and the differential shrinkage that drives warpage. Uneven cooling on two sides of a wall is the most common cause of warp.

A wall cooled faster on one side shrinks first and pulls the part toward that side. The solver shows the temperature difference across the wall. Keeping that difference small matters more than cooling the whole tool faster. A balanced cooling layout with well-placed baffles and bubblers usually beats a dense but uneven circuit.

Cooling analysis also sizes the circuit. Coolant flow should be turbulent, not laminar, because turbulent flow transfers heat far better. Reynolds number above roughly 10,000 is the usual target. A circuit that looks adequate on paper can run laminar if the channel diameter is too large for the available flow rate.

Ejection temperature is a boundary condition, not a suggestion. Ejecting a part before it is stiff enough causes warpage, ejector pin push marks, and sometimes part deformation that never recovers. Simulation tells you when the part is ready; the molder matches the timer to that value.

• 1
  Differential coolingA temperature difference across a wall bends the part toward the cooler side.
• 2
  Turbulent flowReynolds number above about 10,000 for effective heat transfer.
• 3
  Ejection temperatureEject only after the part reaches a temperature where it holds its shape.

Simulation is only as good as its inputs. Material data cards are measured on specific grades, and a generic PP card may not match the actual pellet you buy. If the melt flow rate of the production resin differs from the card, fill pressure and freeze-off time shift. Ask for the resin supplier's data card for the exact grade.

Mesh quality sets the ceiling on accuracy. A coarse 3D mesh smooths over thin ribs and sharp corners. Results look clean but miss hesitation and hot spots. For parts with walls under 1 mm or high aspect ratio features, a finer mesh is not optional. It costs solve time and it is still cheaper than a cut mold.

Simulation assumes a clean, well-maintained tool. Flash, worn gates, and cooling channels scaled with mineral deposits change the real process. The analysis predicts a new tool on day one. If the tool is five years old, treat the results as a direction, not a setpoint.

The biggest limitation is that simulation does not machine the tool. A predicted gate land of 0.8 mm is useless if the electrode or cutter leaves 0.95 mm. Mold flow analysis fill pack cool tells you the target; the CNC work decides whether you hit it. That gap is where most warpage problems actually live.

• 1
  Material card mismatchUse the data card for the exact production grade, not a generic family.
• 2
  Mesh resolutionThin walls and small features need a finer mesh than the default.
• 3
  Tool conditionWorn gates and scaled cooling lines shift the real process away from the model.

The handoff from simulation to machining is where predictions become parts. A gate land dimension of 0.8 mm with a tolerance of ±0.02 mm is a normal requirement. On a mold insert, that means the electrode or cutter path has to hold that tolerance in hardened steel, often after heat treatment. This is where ±0.005 mm machining capability is not a marketing number but a process requirement.

Cooling channels are the second handoff. A predicted 8 mm channel with a 1.5 mm wall distance has to be drilled or milled to that position. If the channel drifts 0.3 mm closer to the cavity, the wall cools faster than the model assumed. The warpage prediction is now wrong even though the simulation was right.

Complex cores and sliders often need 5-axis work because the cooling circuit and the part geometry are not orthogonal. A conformal cooling channel that follows the part surface cannot be drilled on a 3-axis machine. Our 16 simultaneous 5-axis centers handle these geometries, and the same setup machines the core, cavity, and slider details so the stack closes correctly.

Verification closes the loop. We measure gate land, channel position, and cavity dimensions before the tool is assembled, and we report the numbers. If a dimension drifts, it is corrected before trial, not after the first short shot. That discipline is what makes the simulation worth running in the first place.

• 1
  Gate land toleranceTypically ±0.02 mm; requires stable machining on hardened inserts.
• 2
  Cooling channel positionA 0.3 mm drift changes the cooling balance the model assumed.
• 3
  Conformal circuitsNon-orthogonal channels need 5-axis milling, not drilling.