Injection Molding Cycle Time Matters

If you want predictable cost and delivery, you need a stable injection molding cycle time. Cycle time is the total time to complete one shot fill, pack, hold, cooling, and part ejection. Every second affects press availability, per-part cost, and your delivery schedule, so understanding the drivers helps you tune results without guesswork.

What makes up the cycle

• Fill: Moves molten resin from the nozzle into the cavity.
• Pack & hold: Compensate for shrinkage as the gate freezes.
• Cooling: Removes heat until the part can be ejected without deformation.
• Ejection: Clears the cavity so the next shot can begin.
  Any bottleneck—slow fill, long hold, warm ejection—extends cycle time and raises cost.

Wall thickness and cooling dominate

The cooling time increases roughly in proportion to the square of the wall thickness, which is why uniform walls are important. Thick sections hold heat, create differential shrinkage, and force longer cooling to prevent warp. Use consistent wall thickness with ribs for stiffness, choose radii over sharp corners, and avoid isolated mass. These choices let heat leave the part evenly so you can shorten cooling without risking deformation.

Mold temperature and steel design

Mold temperatures play a vital role in the injection molding cycle time process. Mold temperature sets the starting conditions for cooling and surface quality. Higher mold temperatures improve flow, reduce molded-in stress, and enhance cosmetics, but they increase cooling time unless channels are well designed. Baffled and conformal cooling channels pull heat out efficiently, so you can run a balanced temperature without paying a cycle-time penalty. Where possible, align channels near thick regions and around cores, then confirm with thermal checks.

Gate strategy & freeze time

Gate position should feed the thickest region first and keep flow lengths reasonable. If the gate freezes too early, you lose pack (sink/void risk). If it stays open too long, you waste hold time. Tune freeze to the part’s needs, not a fixed rule.

Ejection, draft & surface finish

Clean release = faster, safer cycles. Add adequate draft, match steel finish to the texture/gloss you want, and use air assists/smart ejector layouts to prevent scuffing.

Process levers (fast wins)

• Normalize drying → stable viscosity, repeatable fill.
• Set shot size and back pressure for consistent melt.
• Ramp hold time/pressure down to the point where dimensions or cosmetics just start to drift, then step back.
• Trim cooling in small increments while checking warp and eject marks.
• Document a setup sheet (melt, mold temp, fill time, hold, cooling, shot size).

Design & tooling levers (bigger but durable wins)

• Replace thick bosses with ribs + core pins; keep rib-to-wall ~40–60%.
• Add draft where parts drag; avoid sharp corners that trap heat.
• Upgrade cooling: more uniform channel spacing, better contact, or conformal inserts in heat-dense zones.

Real-world math (why seconds matter)

• Press rate: $65/hr, 1-cavity, 2% scrap
• Before: 32 s cycle → 3,600 ÷ 32 = 112.5 parts/hr → good parts/hr = 110.25 → machine cost per good part ≈ $65 ÷ 110.25 ≈ $0.589
• After: 24 s cycle → 3,600 ÷ 24 = 150 parts/hr → good parts/hr = 147 → machine cost per good part ≈ $65 ÷ 147 ≈ $0.442
  Savings ≈ $0.147/part. On 50,000 parts, that’s ~$7,350 in press time alone (before resin, labor, overhead).

5-step cycle-time trial (use this on T1/T2)

1. Establish a conservative baseline (stable but not optimized).
2. Reduce hold gradually until dimension/cosmetic risk appears; step back one notch.
3. Trim cooling in small steps; watch warp, knit appearance, and eject scuffs.
4. Validate gate freeze timing vs. pack needs; adjust gate or temperature if needed.
5. Lock the recipe and publish the setup sheet + inspection points.

SPC & monitoring

Track fill time, peak pressure, cushion, mold/melt temps and correlate with critical dimensions. Light SPC (X-bar/R) on cycle time catches drift early. Tie alarms to cushion or fill time to detect shorts or venting issues before scrap builds.

When NOT to chase speed

If faster cycles introduce warp, sinks, knit failures, or gloss mismatch, quality wins. Record the limit, note the driver (e.g., thick hub cooling), and log a design/tooling improvement for the next revision.

Takeaway

Cycle time isn’t guesswork — it’s an outcome you design. Balanced wall sections, effective cooling, smart gates, and clean ejection deliver faster shots without sacrificing quality, giving you predictable cost and dependable schedules.