The Flow Ratio: The plastic must flow from the gate to the farthest point without freezing. The flow length divided by wall thickness should be ≤ 150:1 for most materials. If you need a long rib, keep it at 0.6x the nominal wall thickness.
To eject the part, it must be tapered.
The mold can only be as good as the part design. Verify these first:
| Requirement | Why it matters | |-------------|----------------| | Uniform wall thickness (0.5–4 mm typical) | Prevents sink, warpage, and fill imbalances | | Draft angle – 0.5° to 2° (per side) | Allows part release without drag marks | | Radii at corners – 0.25–0.5 × wall thickness | Reduces stress concentration and improves flow | | Nominal wall progression – gradual changes only | Avoids flow hesitation and freeze-off | injection mold design guide
❌ Avoid: Thick-to-thin sudden steps, sharp internal corners, zero-draft vertical walls.
Mold design is constrained by the physics of the part design. To design a good mold, you must enforce these rules on the part:
Not all steel is created equal. The choice of material affects the mold's durability and cost. The Flow Ratio: The plastic must flow from
The runner system delivers the plastic from the machine nozzle to the mold cavities.
Design Guide: Full round runners offer the lowest pressure drop and best flow. If using a trapezoidal runner (easier machining), ensure the depth is sufficient to prevent premature freezing.
Location: End of flow path, last 10–20% of fill, behind bosses, ribs. To eject the part, it must be tapered
❌ Never vent into ejector pin holes unless pin has a flat.
| Parameter | Formula / rule | |-----------|----------------| | Projected area | Part + runner area on parting line | | Clamp force (tons) | (Projected area in cm² × injection pressure in tons/cm²) × safety factor (1.2–1.5) | | Ejector pin stress | F_push / (n × pin_cross_section) < 300 MPa for steel | | Cooling time estimate | (thickness² × constant) / thermal diffusivity (empirical) | | Shrinkage compensation | Multiply part dims by (1 + shrinkage %) – use moldflow to refine |