Cycle time is the single biggest factor affecting the unit cost of injection molded parts. A part that cycles in 20 seconds costs roughly half as much to produce as the same part in 40 seconds — assuming the same machine rate. Yet many importers accept the cycle time their mold maker quotes without question.
This guide explains how cycle time is determined, what the molder can control, and what buyers should negotiate before production begins.
| Phase | Typical % of Cycle | Controllable by Mold Design? | Controllable by Process? |
|---|---|---|---|
| Mold close | 3-5% | Partial (parting line complexity) | Machine speed setting |
| Injection + Fill | 5-10% | Yes (gate size, runner design) | Injection speed profile |
| Packing / Hold | 10-20% | Partial (gate freeze-off time) | Hold pressure & duration |
| Cooling | 50-70% | Yes (cooling channels) | Mold temp controller settings |
| Mold open + Ejection | 3-5% | Yes (ejector stroke, lifter travel) | Machine speed setting |
Traditional straight-drilled cooling channels are limited to straight-line paths, so they often miss the hot spots around cores and ribs. Conformal cooling uses 3D-printed inserts or 5-axis-machined channels that follow the part contour precisely. The result: cooling time drops from 25 seconds to 12 seconds on a typical 2mm PP part.
A gate that freezes too slowly extends the hold phase unnecessarily. Optimizing gate land length and cross-section so the gate "seals" at the exact moment packing completes can shave 2-5 seconds off the cycle. Hot runner systems eliminate runner cooling entirely — no waiting for the cold runner to solidify before ejection.
Faster injection reduces fill time and causes shear heating that lowers the effective melt viscosity, allowing the part to be ejected sooner. However, too fast causes burn marks from trapped air. A profiled injection speed — slow at start, fast in middle, slow at end — optimizes fill without defects.
Hold time should be calculated based on when the gate solidifies, not a fixed guess. Run a gate-freeze study: increase hold time in 0.5-second increments and weigh the parts. When part weight stops increasing, the gate is frozen — hold time is sufficient. Every 0.5 second beyond that adds cost without value.
Running the mold at the upper end of the recommended range reduces the time needed for the part to reach its heat deflection temperature — the point at which it can be ejected without deformation. This seems counterintuitive (hotter mold = longer cooldown?), but a warmer mold allows part crystallization to complete faster in semi-crystalline materials.
| Volume (parts/yr) | Machine rate ($/hr) | Cost saved/year (5 sec reduction) |
|---|---|---|
| 100,000 | $80 | $11,111 |
| 500,000 | $80 | $55,555 |
| 1,000,000 | $80 | $111,111 |
| 5,000,000 | $60 | $416,666 |