Injection mold cooling design: channels, cycle time and cost

The cycle has three terms and only one follows your wall

A molding cycle is normally written as three terms added together, where the dry cycle covers clamp open, part removal, insert loading and clamp close, the injection term covers filling the cavity and packing it out, and the cooling term covers the time the part spends in the tool until it is stiff enough to be pushed off by the ejector pins without distortion, which is exactly how a resin supplier's technical handbook frames it when it teaches cycle-time estimation.

Only one of those terms follows your geometry.

Wall thickness sits inside the cooling term squared, so a designer who thickens a 2 mm wall to 3 mm has multiplied the cooling portion of the cycle by roughly 2.25 before touching the cooling hardware, while the dry cycle and the injection time barely register the change; that asymmetry is the reason cooling design earns attention long after the part is drawn and quoted.

Cooling decides flatness as well as seconds.

An uneven cavity surface temperature is what warps a flat panel, pulls a long ribbed part away from its own ribs and walks a tight dimension out of tolerance, so a cooling change is often bought for quality first and reads as a cycle-time win second.

Estimate the cooling time before arguing about the cycle

The standard estimate treats the wall as a flat plate that loses heat through both faces, which returns a cooling time from four inputs: the wall thickness, the thermal diffusivity of the resin, and the three temperatures that matter, being the melt, the mold and the point at which the part can be ejected.

In symbols that reads t = s² / (π² × α) × ln[ (4 / π) × (T melt - T mold) / (T eject - T mold) ], where s is the thickest wall and alpha is the thermal diffusivity of the resin.

For a 2 mm ABS wall with melt at 240 C, a 60 C mold and the 98 C ejection temperature that Toray publishes alongside its diffusivity figure of 0.075 mm²/s, the calculation returns a cooling time near 10 seconds, and running the same numbers on a 3 mm wall gives about 22 seconds, which is the wall-thickness factor arriving as real press time.

Thin walls buy cycle. They also buy fill pressure.

The plate model has limits worth knowing: it ignores bosses, ribs, hot cores and the runner, and the supplier's own note says that once walls pass roughly 3 mm the measurement point should be moved off the centre line of the section, so the result is a floor for a simple part rather than a cycle forecast for a busy one.

Channel diameter, pitch and depth: the numbers a toolmaker works to

Design guidelines published with a conformal cooling study in Polymers size the channel diameter from the part thickness and then take the spacing from that diameter, which pairs a 2 mm wall with an 8 to 10 mm channel and a 6 mm wall with a 12 to 15 mm channel, spaces the centre lines 3d to 4d apart and holds the channel centre 2d to 3d below the cavity surface.

Ratios travel better than absolute millimetres.

Depth is the trade a toolmaker makes when a circuit has to clear structure, since the same guidance puts depth and pitch at about 2.5 times the channel diameter for a nearly uniform mold surface temperature, and notes that drilling deeper than the pitch allows reduces cooling efficiency even when the spacing looks correct.

Cores, deep boxes and thick bosses are where a straight drill runs out of road.

Baffles, bubblers and thermal pins exist for the pockets a drill cannot reach, and the Autodesk cooling guide makes two practical points: a baffle plate slightly larger than the channel keeps the bore fully blocked, and inserts in a high-conductivity material extend cooling into cores down to about 5 mm across.

Moldflow's published cooling-optimization work keeps optimized layouts at least one channel diameter clear of the part and of neighbouring channels, which is the practical floor on how close a circuit can be brought to a cavity.

Circuit layout carries its own rule, and it is not the one an untrained eye expects.

Parallel channels drilled between a supply and a collection manifold look tidy, but each leg has its own flow resistance and therefore its own velocity, while a single series loop holds a steadier velocity from inlet to outlet, which is why series circuits remain the default in production tools.

Even flow beats a symmetrical picture.

Flow matters as much as the drilling

A channel only carries heat away if the water inside it moves fast enough to stay turbulent, because turbulent flow transfers heat by convection as well as conduction, and the working target in Autodesk's cooling guide is a Reynolds number above 10,000.

As a practical anchor, that guide gives 2.35 GPM through 3/8 in NPT pipe as the flow needed to reach the target, adds that the figure doubles for a 50 percent glycol mix because the viscosity rises, and warns that once turbulence is achieved extra flow adds little heat transfer while pressure drop keeps climbing.

Turbulence has both a floor and a ceiling.

Pressure drop is the constraint that catches out designers who only checked the geometry: the same guide flags a circuit losing more than 5 psi as one that will fall below the target Reynolds number, and points to undersized fittings, long thin channels and manifold restrictions as the usual causes, so a cooling circuit is a plumbing calculation as much as a drilling one.

Coolant temperature rise belongs in the same check, since a circuit carrying too little water returns warm, and the cavities at the far end of that loop then run at a different mold temperature than the ones near the inlet, which is a familiar source of cavity-to-cavity dimension scatter on a multi-cavity tool.

Where cooling design shows up in the tool and part price

Cooling is paid for twice, once when the mold is cut and then on every part the press runs, and the split between a cheap circuit and an expensive one is usually a question of geometry rather than of technology.

Straight drilled channels are the baseline; a deep core or a slender feature that a straight hole cannot reach is what forces baffles, bubblers or a high-conductivity insert into the design, and each of those adds machining, fittings and a leak path to the maintenance list.

Conformal inserts built by additive manufacturing move the channel onto the contour of the part, and the Polymers study measured cooling time down by more than 30 percent against a straight channel on the geometry it tested, which is a real gain on the right part and an unnecessary bill on the wrong one.

The part-cost side runs through cycle seconds, and the arithmetic is worth doing before the tool is ordered.

A cycle that drops by 4 seconds on a 900,000-part program returns 1,000 hours of press time, and at a machine rate between 50 and 80 dollars an hour that is a five-figure difference, which is what makes a costly insert rational for a part with a genuine hot spot and wasteful for a part whose circuit already cools evenly.

Model the walls, then read the range

The estimator behind /quote reads the uploaded geometry and returns a first-pass range for tool and part cost with its assumptions listed, and since the tool prices the cycle from wall thickness and the cooling layout it can see, the numbers above are the ones that move the answer.

Four things are worth setting before the run:

• The nominal wall at the thickest section of the part, because that section sets the cooling time for the whole shot.
• Whether the design holds a deep core, box or boss that will need baffles or a bubbler rather than a straight hole.
• The resin family, since diffusivity and ejection temperature both move the calculation.
• Annual volume, which decides whether an expensive insert can pay for itself inside the program.