Mold Flow Analysis

## 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 --- ## Detailed Notes ### 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: | Location | Shear Rate | Reason | |---|---|---| | **Cavity wall (frozen layer)** | Zero (0.0 1/sec) | Frozen material does not move relative to the wall | | **Just inside the wall** | **Maximum** | Molten polymer slides rapidly past the frozen layer | | **Centre of flow** | Local minimum (~0.0 1/sec) | All polymer chains move at the same speed — no relative motion | - 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: | Colour | Meaning | |---|---| | **Green** | Cavity fills under normal injection pressure | | **Yellow** | Injection pressure exceeds **70%** of machine maximum | | **Red** | Injection pressure exceeds **85%** of machine maximum | - 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 ```mermaid flowchart TD A[Part Design] --> B[Wall Thickness] A --> C[Gate Location] A --> D[Feature Design 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 ```mermaid flowchart LR A[Define Material & Geometry] --> B[Set Gate Location] B --> C[Run Fill Analysis] C --> D{Check Results} D -->|Short Shot / High Pressure| E[Adjust Gate / Wall Thickness] D -->|Air Traps / Burn Marks| F[Add Venting / Redesign Flow Path] D -->|Weld Lines in Critical Areas| G[Reposition Gate] D -->|High Shrinkage / Sink Marks| H[Improve Packing / Uniform Walls] D -->|Acceptable| I[Proceed to Tooling] E --> C F --> C G --> C H --> C ``` --- ## Defect Cause–Effect Summary | Defect | Root Cause | Mitigation | |---|---|---| | **Short shot** | Insufficient pressure / flow length too long | Move gate centrally, increase wall thickness | | **Air traps** | Entrapped air with no vent path | Add vents, inserts, or porous metals at trap locations | | **Burn marks** | Compressed trapped air ignites | Improve venting; redesign flow to eliminate air pockets | | **Weld lines** | Melt fronts converge around cores or from multiple gates | Reposition gate; cannot be fully eliminated with through-holes | | **Sink marks** | Insufficient packing in thick sections | Uniform walls, gate at thick sections, ribs at 60–80% wall | | **Vacuum voids** | Rigid skin + molten core separation | Uniform wall thickness; avoid abrupt thickness changes | | **Warpage** | Non-uniform shrinkage / uneven filling | Balance fill pattern; uniform cooling; central gate | | **Material degradation** | Excessive shear heating | Increase gate size, lengthen fill time, check material limits | --- ## Shear Distribution Across Cavity Cross-Section ```mermaid flowchart LR subgraph Cross-Section Profile W1[Cavity Wall Shear Rate = 0] --> M1[Max Shear Rate Just Inside Wall] M1 --> C1[Centre of Flow Shear Rate ≈ 0] C1 --> M2[Max Shear Rate Just Inside Wall] M2 --> W2[Cavity Wall Shear Rate = 0] end ``` --- ## Cooling Time Relationship ```mermaid flowchart TD A[Cooling Time] --> B[Proportional to Wall Thickness²] A --> C[Influenced by Melt Temperature] A --> D[Influenced by Mold Temperature] A --> E[~70% of Total Cycle Time] B --> F[2× thickness = 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