More than half of the cycle time goes on cooling; and the fate of cooling is decided by holes inside the mold that you cannot see. Where the channel runs, how fast the water flows, whether the circuits are balanced — these three questions determine whether the same mold will cycle in 28 seconds or 40, and whether the part will come out straight or warped. This article brings together the design rules of mold cooling and the mistakes most often made on the shop floor.
The journey of the heat: from melt to water
Every gram of melt entering the mold gives up its heat — until it has fallen to the ejection temperature — first to the mold steel, from the steel to the channel wall, and from there to the cooling water. Cooling waits for whichever link in this chain is slowest. If the channel is far from the cavity, the path through the steel is longer; if the water flows slowly, the heat at the channel wall cannot be removed; if one of the circuits is blocked, that area stays hot and the part warps on that side.
How much heat has to be carried is set by the production rate — the material processed per hour multiplied by the enthalpy of the material. You can calculate it with your own values in our mold cooling calculator; the subject of this article is how to lay out the channels that will carry that heat.
The basic rules of channel layout
In channel design, two distances determine everything, and both are expressed as multiples of the channel diameter:
| Dimension | Guideline value | If smaller | If larger |
|---|---|---|---|
| Channel diameter (d) | 8 - 12 mm (according to part size) | Flow and turbulence become harder | Fewer channels, greater local difference |
| Distance from channel to cavity | 1.5 - 2.5 × d | Cold marks on the surface, strength risk | Cooling slows, cycle lengthens |
| Distance between channels | 2.5 - 3.5 × d | Machining cost rises | Temperature fluctuation: cold in line with channels, hot between them |
The balance here is this: the closer and denser the channels are to the cavity, the faster the cooling and the flatter the surface temperature — but the strength of the mold and the machining cost set a limit. Deep ribs, bosses and cores are places a straight channel cannot reach; they are reached with baffles, bubblers (tube within a tube) or cores made of high-conductivity copper alloy. In recent years conformal channels produced by laser melting follow the part geometry exactly and remove this limit too; the cost is high, but they pay for themselves in parts with a large cycle gain.
Turbulence: three times the cooling with the same water
The least-known fact of cooling is the flow regime. Slowly flowing water moves through the channel in layers (laminar flow); the thin layer in contact with the wall heats up and behaves like an insulator. When the flow is speeded up and made turbulent, the water mixes as it moves through the channel and its ability to take heat from the wall increases many times over — the same channel, the same water temperature, three to five times more cooling.
The practical criterion is the Reynolds number: above 4,000 is regarded as turbulent, and the design target is usually above 10,000. On the shop floor this means: if the cooling is not enough, the first reflex should not be to cool the water but to increase the flow rate. Cooling the water seems to work until surface quality and cycle balance suffer; increasing the flow rate changes the regime and gives a lasting gain. The inlet-outlet temperature difference of a circuit is kept within a 2 - 5 °C band; if the difference has grown, that circuit is either flowing slowly or carrying too much load.
Series or parallel?
| Connection | Plus | Risk |
|---|---|---|
| Series (circuits one after another) | The same flow in all channels — turbulence guaranteed | The water heats up as it travels; the last channel is hotter than the first |
| Parallel (distribution via a manifold) | Water at the same temperature to all circuits | Flow escapes to the circuit with the lowest resistance; a blocked circuit goes unnoticed |
In small molds series connection wins through its simplicity. In large molds with many circuits, parallel distribution is unavoidable — but then you need to see the flow rate of each circuit separately. Distributors with flow indicators are exactly for this: when the flow of a circuit drops, the blockage becomes visible before the part goes bad. For multi-circuit distribution and flow monitoring solutions that communicate with the machine, see our mold temperature control page.
The invisible enemy: deposits inside the channel
A cooling channel is a system that ages. Lime in the water precipitates on the hot wall; corrosion products and biofilm are added. A one-millimeter layer of scale offers resistance to heat conduction close to that of half a meter of steel — and because the channel diameter also narrows, the flow falls. The result is insidious: over the years the mold turns into "it used to cycle in 30 seconds", and the blame is often looked for in the machine. The symptoms, diagnosis and cleaning program are explained in detail in our mold maintenance guide; the design lesson here is this: closed-circuit water treatment and periodic flow measurement are as important as channel design.
Verification on the shop floor: don't let the design stay on paper
When the mold is commissioned, three measurements show whether cooling really works as designed:
1. Flow measurement circuit by circuit — not the total flow; in a parallel system the total may be right while a single circuit is dead. 2. Inlet-outlet temperature difference — a circuit going outside the 2 - 5 °C band is flagged. 3. Surface temperature map — the mold surface is scanned with a contactless thermometer or thermal camera; a difference of more than a few degrees between the lines of the channels and the spaces between them reveals a layout problem.
The record of these three measurements is the reference line for tomorrow's question "why has the cycle got longer?". You can find the effect of cooling time on the cycle and the square rule of wall thickness in our cycle time guide; our cooling calculator calculates your theoretical cooling time.
Keeping the mold temperature at target starts with channel design but is completed by choosing the right temperature control unit — our water/oil selection guide and our technical team help with the questions of water or oil, how many circuits and what flow rate.