Most of the price of a moulded plastic part is bought and sold in seconds. Machine time is billed by the hour, and a cycle that runs 60 seconds versus 40 halves the output of that machine. Understanding the cycle — especially the cooling phase — tells you why parts cost what they cost, and which design changes genuinely save money.
The four phases of a cycle
| Phase | What happens | Share of cycle |
|---|---|---|
| Injection | Molten resin fills the cavity | 10–20% |
| Packing/holding | Pressure compensates shrinkage | 5–10% |
| Cooling | Part solidifies in the mould | 50–70% |
| Ejection | Part removed, mould closes | 5–15% |
For a large part like a pallet or crate, total cycles run from under a minute (thin-walled small bins) to several minutes (thick structural pallets). Multiply by thousands of parts and the seconds become the price.
The square rule: thickness is destiny
Cooling time scales roughly with the square of wall thickness. Double the thickness, quadruple the cooling. This single relationship explains most of moulding economics:
- Thin-walled crate at 3 mm → fast cycle → low machine-time cost per part
- Thick-walled bin at 6 mm → ~4× cooling → machine-time dominates material cost
- Ribbed thin wall → same stiffness as thick plain wall, fraction of the cycle
This is why structural design advice always says thin walls with ribs — the engineering is covered in our wall thickness and rib design guide. The buyer’s version: ask any supplier quoting a thick part whether a ribbed design could do the same job at lower thickness.
Other levers that shorten cycles
- Resin flow grade — faster-flowing grades fill thin walls at lower pressure and temperature, shortening both injection and cooling.
- Mould cooling design — well-placed cooling channels near thick sections remove heat faster than brute-force longer cycles. Tooling quality shows up here.
- Melt temperature discipline — running hotter than necessary lengthens cooling; running too cold lengthens injection. Experienced process setters hold the window.
- Batch consistency — interrupted short runs waste machine hours on startup scrap and re-stabilisation. Steady long runs are cheaper per part.
What shortening must never mean
There is a fraudulent way to shorten a cycle: eject the part hot and let it cool in air. The seconds saved are paid back as warping, sink marks and dimensional drift — defects that pass a quick visual check and fail in service or assembly. This connects directly to incoming quality control: our quality inspection guide covers what to check (flatness on a surface plate, dimensional sampling) so this shortcut is caught at goods-in, not in the field.
Reading a quote with cycle eyes
| Quote signal | Cycle-time interpretation |
|---|---|
| Thin-walled ribbed design proposed | Supplier is engineering cost out — good sign |
| Quote per part far below market | Check for hot-ejection shortcuts or recycled resin |
| Cavity count offered (2-cavity, 4-cavity) | More parts per cycle — real economies for small parts |
| Long tooling lead time quoted | Often pays back via better cooling channel design |
The OEM process from tooling to first article has its own timeline — see the custom injection molding OEM process guide — but the economics of every part made after tooling are decided by the cycle. Buyers who understand the square rule ask better questions and get better parts.