Tool Steel 3D Printing | H13 Conformal Cooling

Karl Gillihan

🔨 TOOL STEEL

Tool Steel 3D Printing: H13, Conformal Cooling, and the Crack-Prone Grades

Tool steel additive manufacturing is really one application wearing one alloy: H13 for conformally cooled mold and die inserts. That single use case — cooling channels that follow the part contour and slash cycle times — justifies the entire premium of printing hardened steel. The other tool steels buyers know, like A2, D2, and O1, mostly fight the process because of how readily they crack.

ISO 9001AS9100

Conformal Cooling: The Reason Tool Steel Gets Printed

In injection molding and die casting, cooling time dominates cycle time, and conventional drilled cooling lines are straight — they can't follow curved or deep features, leaving hot spots that slow the cycle and warp parts. Additive manufacturing builds cooling channels that conform to the cavity surface, removing heat uniformly and cutting cycle times often by 20-40%. Over a high-volume tool's life, that productivity gain dwarfs the cost of printing the insert. This is the single strongest commercial case in all of metal AM, and it runs almost entirely on one steel: H13. H13 hot-work tool steel prints to high density, tolerates the thermal cycling of repeated molding, and hardens to roughly 44-52 HRC for good wear and thermal-fatigue resistance. Shops typically print the complex-channel core in H13 and may machine simpler steel for the rest of the tool, or print the full insert. The printed insert is then machined on cavity surfaces, polished, and heat treated.

Why H13 Prints and A2, D2, O1 Mostly Don't

Tool steels are high-carbon and high-alloy, which means they form hard, brittle martensite the instant they cool from the melt pool — and that martensite cracks under the residual stress of layer-by-layer building. H13 is the most printable of the common tool steels because its chromium-molybdenum-vanadium chemistry and moderate carbon give it enough toughness and a forgiving transformation behavior, especially when printed on a heated build plate (often 200°C+) to slow cooling and preempt cracking. A2, D2, O1, and S7 are progressively harder to print. D2's very high carbon and chromium (it's nearly a 12% Cr semi-stainless) make it extremely crack-prone in LPBF; O1 is an oil-hardening grade designed for conventional heat treat, not melt-pool solidification; A2 and S7 sit in between. These grades exist to be machined in the annealed state and then hardened — the classic toolroom workflow — so there's rarely a reason to print them, and doing so demands aggressive preheat and immediate tempering to avoid quench cracking. When you need an air-hardening or high-wear tool not in H13, machining annealed stock and heat treating is almost always the right path.

Heat Treatment, Hardness, and Post-Processing

Printed H13 comes off the plate hard and stressed and must be handled carefully: stress relief before plate removal, then a proper austenitize-quench-and-double-temper cycle to reach the target hardness (commonly 44-52 HRC for mold inserts) with stable dimensions. HIP is sometimes specified to close porosity in fatigue-critical or pressure-sensitive tools. Because the part is hard, finish machining of cavity surfaces is done by hard milling or EDM, and high-gloss mold finishes require careful polishing of the printed-then-machined surface. A real consideration is that as-built tool steel surfaces (Ra 6-15 µm) are too rough for molding contact and must be machined and polished, so design the insert with machining stock on cavity faces. Internal cooling channels need self-draining design for powder removal and, for flow-critical tools, internal finishing. The combined heat-treat and finishing chain is what pushes tool-steel AM lead times out, but for a high-volume production tool the cycle-time payback still wins.

Frequently Asked Questions

Because cooling time dominates injection molding and die casting cycle times, and conformal channels attack it directly. Conventional cooling lines are gun-drilled and straight, so they can't follow curved cavities or reach deep cores, leaving hot spots that slow the cycle and warp parts. Printed conformal channels follow the cavity contour, pull heat out uniformly, and commonly cut cycle times 20-40%. On a high-volume tool running millions of shots, that productivity gain dwarfs the extra cost of printing the insert — which is why conformal cooling is the single strongest commercial case in all of metal additive. It runs almost entirely on H13 hot-work tool steel, which prints densely, handles repeated thermal cycling, and hardens to 44-52 HRC. Shops typically print the complex-channel insert and finish-machine and polish the cavity surfaces. If your tool has hot spots limiting throughput, a printed conformal insert often pays for itself quickly.

Technically yes, practically rarely, and it's difficult. These grades are high-carbon and high-alloy, so they form hard brittle martensite straight out of the melt pool and crack under the residual stress of layer-by-layer building. D2 is the worst offender — its very high carbon and ~12% chromium make it extremely crack-prone in LPBF. O1 is an oil-hardening grade designed for conventional heat treatment, not rapid melt-pool solidification, and A2 and S7 sit in between. Printing them requires an aggressively heated build plate (often 200°C or higher) and immediate tempering to dodge quench cracking, and the supplier base is thin. Since these steels exist specifically to be machined in the soft annealed state and then hardened — the standard toolroom workflow — there's seldom a reason to print them. For air-hardening or high-wear tools not in H13, machine annealed stock and heat treat; that's faster, cheaper, and lower-risk than fighting these grades in AM.

Printed H13 must be heat treated to reach useful tool hardness, and the typical target for mold and die inserts is 44-52 HRC. As-built H13 comes off the plate hard but stressed and non-uniform, so the process is: stress relieve before removing from the plate, then austenitize, quench, and double temper to develop stable hardness and dimensions. HIP may be added to close internal porosity for fatigue- or pressure-critical tools. After heat treatment the part is hard, so cavity surfaces are finished by hard milling or EDM and then polished — as-built roughness around Ra 6-15 µm is far too rough for molding contact. Always specify the target hardness and require certification, and design the insert with machining stock on the surfaces that need precision or polish. The full stress-relieve, harden, temper, machine, and polish chain is what drives tool-steel AM lead times, typically 3-5 weeks for a finished insert.

A printed H13 insert costs more up front than a conventionally machined one — often several thousand dollars for a complex insert given the powder, build time, heat treatment, hard machining, and polishing, with 3-5 week lead times. The justification isn't unit cost, it's production economics: if conformal cooling cuts cycle time 20-40% on a tool running hundreds of thousands or millions of parts, the throughput gain and reduced scrap recover the insert premium quickly, sometimes within weeks of production. So the comparison isn't insert-vs-insert price, it's total cost of ownership over the tool's life. For a low-volume or prototype tool, the payback may never materialize and conventional straight-drilled cooling is the economical choice. For a high-volume tool where cycle time is the bottleneck and there are hot spots conventional cooling can't reach, the printed conformal insert is usually the clear winner. Model the cycle-time savings against your production volume before deciding.

Last updated: July 2026

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