Injection Mould Flow Analysis and Result Interpretation

Overview

Mold flow analysis is a simulation-based technique used to predict how molten plastic behaves during injection molding. It evaluates filling patterns, temperature distribution, pressure requirements, shrinkage, and potential defects — enabling engineers to identify and resolve manufacturing issues during the design phase rather than after tooling.

Key Concepts

• Fill Time — duration for the melt front to travel from the injection point to the last-filled region
• Weld Lines — weak zones formed where two or more melt fronts converge
• Air Traps — pockets of air unable to escape during cavity filling
• Shear Rate / Stress — velocity gradient and force per unit area experienced by the polymer melt against cavity walls
• Volumetric Shrinkage — reduction in material volume as the polymer cools and solidifies
• Sink Marks — surface depressions caused by insufficient packing in thicker sections
• Ease of Fill — qualitative indicator of whether the cavity can be filled within safe pressure limits

Fill Time

• Shows how the melt front progresses through the cavity over time
• Blue regions = earliest filled (near the injection point); Red regions = last filled (farthest extremities)
• Fill duration depends on flow length, wall thickness, and material viscosity
• Corners and thin features farthest from the gate fill last
• A uniform fill-time gradient across the part indicates balanced flow

Injection Location

• The point where molten plastic enters the cavity
• Central placement reduces maximum flow length, lowers pressure requirements, and promotes uniform filling
• Off-centre or end placement causes one side to fill before the other → uneven packing → potential warpage
• Best practice: review the fill-time plot and confirm all extremities fill simultaneously
• Non-uniform filling leads to differential volumetric shrinkage and post-mold dimensional issues

Air Traps

• Occur when the melt front encloses a pocket of air that cannot escape
• Consequences:
  • Incomplete filling (short shots)
  • Compressed air can ignite → burn marks on the part surface or damage to the mold
• Mitigation strategies:
  • Parting line vents
  • Ejector pin venting
  • Cavity inserts
  • Porous metal inserts at trap locations
• Best approach: redesign flow path to eliminate traps entirely

Weld Lines

• Form where two or more melt fronts meet and merge
• Causes: mold shut-offs, core features, multiple gates, wall thickness variations
• Effects:
  • Mechanically weaker than surrounding material
  • Visible surface defects
  • Act as stress concentrators
• Typically form 180° opposite the point where melt splits around a core or shut-off
• Cannot be fully eliminated in parts with through-holes or multiple gates — only repositioned by changing the gate location

Velocity Vector at End of Fill

• Displays molecular orientation of the polymer as it flows through the cavity
• Spherical fillers → more isotropic (uniform) mechanical properties in all directions
• High-aspect-ratio fillers (e.g., glass fibres) → anisotropic properties:
  • Stronger in the flow direction
  • Weaker perpendicular to flow
• Understanding orientation helps predict mechanical performance and potential weak zones

Pressure at End of Fill

• Injection pressure is controlled via screw forward velocity
• Pressure drop occurs along the flow length due to viscous resistance
• Factors affecting pressure drop:
  • Flow length (longer = higher drop)
  • Wall thickness (thinner = higher resistance)
  • Melt viscosity
• Thin-walled parts demand higher injection pressures
• Short shot detected? → move the gate to the centre to halve the flow length and reduce pressure requirements
• Central gating forces flow in two directions but significantly lowers peak pressure

Temperature at End of Fill

• A thin frozen layer forms on the cavity wall as the melt contacts the cooled mold surface
• Frozen layer thickness depends on:
  • Temperature differential between melt and mold
  • Thermal conductivity of the polymer
• Not significantly affected by part wall thickness

Bulk Temperature at End of Fill

• Represents the average melt temperature across the wall thickness at the moment filling completes
• Blue regions = stagnant material that has cooled significantly
• Red regions = material that retained heat due to recent flow velocity
• Variations indicate uneven thermal history across the part

Temperature Growth at End of Fill

• During filling, the polymer experiences shear heating — friction from flow raises the temperature above the set melt temperature
• Causes of excessive temperature growth:
  • Very short fill times
  • Small gate cross-sections
  • Material-specific flow characteristics
• Risk: if temperature rise is extreme, the polymer may degrade (thermal degradation)

Shear Stress at End of Fill

• Shear stress (τ) = Force / Area applied parallel to the flow plane
• Formula: τ = F / A
• In a mold cavity:
  • The cavity wall is stationary; the melt moves along it
  • Material near the wall experiences higher shear stress (greater flow resistance)
  • Material at the centre of flow experiences minimal shear stress
• Analogy: similar to a moving wall dragging fluid — highest stress at the contact surface, least stress farthest away

Shear Rate at End of Fill

• Shear rate = speed at which one fluid layer moves over an adjacent layer at a different velocity
• Profile across the cavity wall thickness:

• The velocity profile across the cross-section creates a parabolic shear-rate distribution with two maxima near each wall and a minimum at the centre

Volumetric Shrinkage at End of Fill

• High shrinkage in thick sections indicates insufficient packing
• Without an adequate packing stage → elevated shrinkage shown in yellow/red on the plot
• Vacuum voids:
  • Not air bubbles — they form when a rigid outer surface maintains shape while the molten core separates inward
  • Visible in transparent parts as internal bubbles
  • In opaque parts, only detectable by sectioning the part
  • Common at thickness transitions (e.g., rib-to-wall junctions, boss bases)

Freezing Time at End of Fill

• Time for the melt to cool to its glass transition temperature (Tg)
• Depends on:
  • Melt-to-mold temperature difference
  • Thermal conductivity of the polymer and mold material
• Ejection temperature ≠ Tg — parts can be ejected at the deflection temperature under load (HDT), typically around ⅔ of Tg or Tm (in Kelvin)

Cooling Time

• Time to reduce material temperature to the ejection temperature (HDT)
• Typically accounts for ~70% of total cycle time
• Key influencing factors:
  • Melt temperature (higher → longer cooling)
  • Mold temperature (higher → longer cooling)
• Plastics are poor thermal conductors → slow heat dissipation
• Critical relationship: cooling time is proportional to the square of wall thickness
  • Doubling wall thickness → 4× longer cooling time
• Design rule: keep wall thickness uniform and as thin as safely possible

Temperature at End of Cooling

• Measured when 90% of part volume is below the HDT
• Thick regions with varying temperatures → risk of:
  • Sink marks
  • Internal voids
  • Warpage
• Mitigation: uniform wall thickness design

Sink Marks

• Surface depressions caused by insufficient polymer packing to compensate for shrinkage
• Thicker sections cool slower → shrink more → pull the surface inward
• Plastics' low thermal conductivity slows core cooling, amplifying differential shrinkage
• Design rules to minimize sink marks:
  • Design with uniform wall thickness
  • Place gates at thicker sections for better packing pressure transmission
  • Avoid undersized gates that freeze off before packing is complete
  • Keep ribs and bosses at 60–80% of the nominal wall thickness

Injection Location Filling Contribution

• With a single gate, 100% of the cavity is filled from that location
• With multiple gates, each gate fills a portion of the cavity
• Significant weld lines form at the interface where material from different gates meets

Ease of Fill

• Qualitative traffic-light indicator of filling feasibility:

• If yellow/red appears (simulating cavity only, no runners), consider:
  • Increasing wall thickness
  • Repositioning or adding gates
  • Changing material grade
  • Adjusting process parameters (melt temp, injection speed)

Key Relationships & Design Rules

flowchart TD
    A[Part Design] --> B[Wall Thickness]
    A --> C[Gate Location]
    A --> D[Feature Design<br>Ribs / Bosses]

    B -->|Uniform & thin| E[Shorter Cooling Time]
    B -->|Uniform & thin| F[Reduced Sink Marks]
    B -->|Uniform & thin| G[Lower Shrinkage Variation]

    C -->|Central placement| H[Balanced Fill Pattern]
    C -->|Central placement| I[Lower Injection Pressure]
    C -->|At thick sections| J[Better Packing]

    D -->|60-80% of wall| K[Minimised Sink Marks]

Injection Molding Analysis Workflow

flowchart LR
    A[Define Material<br>& Geometry] --> B[Set Gate<br>Location]
    B --> C[Run Fill<br>Analysis]
    C --> D{Check Results}
    D -->|Short Shot / High Pressure| E[Adjust Gate /<br>Wall Thickness]
    D -->|Air Traps / Burn Marks| F[Add Venting /<br>Redesign Flow Path]
    D -->|Weld Lines in<br>Critical Areas| G[Reposition Gate]
    D -->|High Shrinkage /<br>Sink Marks| H[Improve Packing /<br>Uniform Walls]
    D -->|Acceptable| I[Proceed to<br>Tooling]
    E --> C
    F --> C
    G --> C
    H --> C

Defect Cause–Effect Summary

Shear Distribution Across Cavity Cross-Section

flowchart LR
    subgraph Cross-Section Profile
        W1[Cavity Wall<br>Shear Rate = 0] --> M1[Max Shear Rate<br>Just Inside Wall]
        M1 --> C1[Centre of Flow<br>Shear Rate ≈ 0]
        C1 --> M2[Max Shear Rate<br>Just Inside Wall]
        M2 --> W2[Cavity Wall<br>Shear Rate = 0]
    end

Cooling Time Relationship

flowchart TD
    A[Cooling Time] --> B[Proportional to<br>Wall Thickness²]
    A --> C[Influenced by<br>Melt Temperature]
    A --> D[Influenced by<br>Mold Temperature]
    A --> E[~70% of<br>Total Cycle Time]
    B --> F[2× thickness =<br>4× cooling time]

Key Terms

• Fill Time — duration for the melt front to travel from gate to the last-filled region of the cavity
• Flow Front — the leading edge of molten plastic advancing through the cavity
• Gate / Injection Location — the point where molten polymer enters the mold cavity
• Short Shot — incomplete cavity filling due to insufficient pressure or material
• Weld Line — a weak boundary formed where two or more melt fronts converge
• Air Trap — a pocket of air enclosed by converging melt fronts with no escape path
• Shear Stress (τ) — force per unit area applied parallel to the flow direction (τ = F/A)
• Shear Rate — velocity gradient measuring how fast one fluid layer slides over another
• Volumetric Shrinkage — percentage reduction in volume as polymer cools and solidifies
• Vacuum Void — internal cavity formed when a rigid outer skin holds shape while the molten core contracts
• Sink Mark — a surface depression caused by differential shrinkage in thick sections
• Glass Transition Temperature (Tg) — temperature below which the polymer transitions from rubbery to glassy state
• Heat Deflection Temperature (HDT) — temperature at which the polymer deforms under a specified load; determines ejection timing
• Ejection Temperature — the temperature at which the part is rigid enough to be removed from the mold (~⅔ of Tg or Tm in Kelvin)
• Bulk Temperature — average melt temperature across the wall thickness at a given moment
• Shear Heating — temperature rise in the melt caused by viscous friction during flow
• Packing Stage — post-fill phase where additional material is forced in under pressure to compensate for shrinkage
• Ease of Fill — a qualitative plot indicating whether the cavity can be filled within safe pressure limits (green / yellow / red)

Quick Revision

• Mold flow analysis predicts filling behaviour, temperature, pressure, shrinkage, and defects before tooling is built
• Fill time shows how the melt front progresses; blue = first filled, red = last filled
• Central gate placement halves flow length, reduces pressure, and promotes balanced fill
• Air traps cause short shots or burn marks — mitigate with venting or flow path redesign
• Weld lines are unavoidable with through-holes or multiple gates; they are mechanically weak and act as stress concentrators
• Shear rate peaks just inside the cavity wall and drops to near zero at the flow centre (parabolic profile)
• Cooling time ∝ wall thickness² — doubling thickness quadruples cooling time; cooling is ~70% of cycle time
• Sink marks result from insufficient packing in thick sections — keep ribs/bosses at 60–80% of nominal wall thickness
• Vacuum voids form internally at thickness transitions when the rigid outer surface holds shape while the core contracts
• Ease of fill uses a green/yellow/red scale: green = normal pressure, yellow = >70% machine max, red = >85% machine max