A Right Assistant for the Precision Machining of the Mold
A mold only holds tolerance if every cut before assembly was planned around shrink, steel hardness, and the parting line. This page explains how precision machining of the mold actually works, where the limits sit, and how to judge whether your tool should be milled, ground, or EDM'd.
In this article
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What matters most
How precision machining of the mold differs from part machining
A production part is checked against a drawing. A mold is checked against the part it will produce, and that part does not exist yet. So precision machining of the mold starts with a stack of allowances: shrink, draft, polish stock, and preload. Every one of those is a real number on the toolpath, and if any is missing, the cavity will be the wrong size no matter how good the machine is.
The second difference is that a mold is a closed system. Cavity and core must close on each other within a few microns at the parting line, or the press pushes plastic into the gap. That means the two halves are usually roughed separately, heat treated, then semi-finished and finished as a matched pair. Splitting them across suppliers is where most projects lose their tolerances.
Third, the mold has to survive hundreds of thousands of cycles. A die with sharp internal corners concentrates stress and cracks first. That is why we radius internal corners as a rule, not as an option, and why steel choice and heat treatment are decided before the first roughing cut, not after a failure.
- 1Cut oversize, verify on shotShrink allowance is applied to the 3D model, then trimmed once parts are measured.
- 2Machine halves as a pairCavity and core share datum faces so the parting line closes cleanly.
- 3Radius internal cornersSharp inside corners act as crack starters under repeated clamping load.
Cutting a cavity: rough, semi-finish, finish, and what each step holds
Roughing removes most of the volume with the goal of speed and tool life, not accuracy. We leave 0.3–0.8 mm of stock on the cavity walls and floor so the semi-finish pass has something to bite. On a hardened block this stock is smaller, typically 0.2–0.4 mm, because deflection grows fast once the steel passes 40 HRC.
Semi-finishing brings the surface to within 0.05 mm of nominal. At this point the cavity shape is basically fixed. If a shrink correction is still needed, this is the last cheap moment to make it, because everything after is finishing stock.
Finishing cuts determine the final tolerance and the surface the polisher will start from. A typical finished cavity wall holds ±0.005 mm on critical features and Ra 0.8–1.6 μm off the tool. Deeper ribs and thin cores deflect, so we reduce radial engagement and take more passes rather than push feed.
On a five-axis center the cavity walls, ribs, and the parting face can often be cut in one setup. Every extra setup adds a re-clamping error, and re-clamping is where a 0.005 mm job quietly turns into a 0.02 mm job.
- 1RoughLeave 0.3–0.8 mm stock on soft steel, 0.2–0.4 mm on hardened steel.
- 2Semi-finishBring walls to within 0.05 mm; last point for a shrink correction.
- 3FinishHold ±0.005 mm on critical features, Ra 0.8–1.6 μm off the tool.
Steel choice drives the machining of the mold
P20 and 1.2311-type pre-hardened steel sits around 30–34 HRC. It machines well with coated carbide, takes a good polish, and suits low-to-medium volume tools. Most of the cycle count in consumer electronics and general industrial parts lives here.
H13 and similar hot-work steels are used where the tool sees high temperature or abrasive filled resin. They arrive annealed, get machined to near-net shape, then are hardened to roughly 48–52 HRC. After that, milling is limited to light finishing passes; the bulk of the detail comes from EDM and grinding.
S136 and 420 stainless grades are chosen when the resin is corrosive, such as PVC or a halogen-filled compound. They polish to a mirror and resist pitting, but they are sticky to cut and demand sharp tooling and generous coolant.
Copper and graphite electrodes are machined on the same centers. Graphite is abrasive and dry-cut with dust extraction; copper cuts wet and holds finer detail in the corners. The electrode is a negative of the cavity, so it inherits the same tolerance discipline.
- 1Pre-hardened, ~30–34 HRCP20 / 1.2311. Good machining and polish, medium tool life.
- 2Hardened, ~48–52 HRCH13 and similar. Detail comes from EDM and grinding.
- 3Corrosion resistantS136 / 420 stainless for PVC and halogen-filled resins.
When milling is not the right assistant anymore
Deep ribs narrower than 1 mm in a hardened block will not be milled reliably. The tool is too slender to survive the cutting force, and it will chatter before it reaches depth. EDM with a machined electrode handles that geometry cleanly, at the cost of cycle time.
Sharp internal corners are a similar story. A rotating cutter always leaves a radius equal to its own radius. If the print calls for a true sharp corner, the corner has to be burned or ground after milling.
Mirror finishes are usually a polishing job, not a cutting job. Milling can reach roughly Ra 0.2–0.8 μm on a good day with a fresh tool, but a true optical surface on a lens or light guide is achieved by hand polishing after the last finish pass. The machining job is to leave uniform stock so the polisher does not chase waves.
Very large cavities are a different constraint. Our largest travel is 4,000 × 400 × 150 mm, so a single block beyond that has to be split or approached from multiple setups, and each setup must be planned into the datum scheme from the start.
- 1Deep thin ribsBelow about 1 mm width in hard steel, EDM is the safer route.
- 2Sharp internal cornersA cutter leaves its own radius; burn or grind the corner afterwards.
- 3Mirror surfacesMilling leaves uniform stock; the final gloss comes from polishing.
Inspection, fit-up, and the first shot correction
A mold is measured in pieces and then as an assembly. We check cavity depth, rib width, and parting-face flatness on the CMM, then close the tool and check the shutoff gap. If the halves do not close, the machine work is not finished, however good the individual dimensions look.
Fit-up is followed by spotting blue on the shutoff faces. Contact should be even around the perimeter. A high spot shows up as a bright patch and is stoned back by hand in small steps. This is slow, but it is the step that decides whether the tool flashes on the first shot.
The first shot is the real measurement. Parts are measured, shrink is back-calculated, and the correction is applied to the cavity. If the error is small, a re-cut or a weld-and-recut handles it. Large errors mean the block is scrapped, which is why the shrink number is worth arguing about before cutting starts.
Reports are available on request. We inspect 100% before shipment, covering raw material check, in-process monitoring, and final inspection. For tooling that goes into a regulated line, the documentation matters as much as the steel.
- 1Measure the assembly, not just the partsThe shutoff gap is the number that decides flash.
- 2Spot and stone shutoff facesEven contact around the perimeter, adjusted by hand in small steps.
- 3Correct from the first shotBack-calculate shrink from measured parts, then re-cut.
Milling, grinding, or EDM for mold detail
Pick by geometry, hardness, and surface requirement. Most molds use all three.
| Method | Typical tolerance | Best for | Watch out for |
|---|---|---|---|
| Three-axis milling | ±0.01 mm | Flat cavities, plates, pockets, bolt patterns | Multiple setups add re-clamp error |
| Five-axis milling | ±0.005 mm | Deep walls, ribs, undercuts, one-setup finishing | Tool deflection on long reach |
| Surface grinding | ±0.002 mm | Parting faces, datum faces, hardened blocks | Slow on complex 3D shapes |
| Sinker EDM | ±0.005 mm | Sharp corners, deep ribs, hardened steel detail | Electrode wear and burn time |
| Wire EDM | ±0.003 mm | Through features, inserts, shutoff edges | Conductive material only |
| Hand polishing | Dimensional change | Mirror and optical surfaces | Can round off shutoff edges |
The trade-off, stated plainly
If your mold is pre-hardened steel with open geometry and a normal cosmetic finish, mill it on a five-axis center and move on. If it is hardened past 45 HRC with deep ribs or true sharp corners, budget for EDM and grinding instead of trying to force the milling cut. Choosing the wrong one costs more than either process.
Questions engineers ask before cutting steel
How much shrink allowance should go into the cavity model?
It depends on the resin and the wall thickness, not on the mold. Unfilled grades typically run lower than glass-filled grades, and thick sections shrink more than thin ones.
We apply a starting allowance to the model, then verify it against first-shot measurements and correct the cavity. Treating shrink as a fixed number for the whole tool is the most common source of a second cut.
Can you machine a cavity after it has been hardened?
Yes, within limits. Above roughly 45 HRC we switch to hard milling with light radial engagement, plus grinding and EDM for the detail.
Cutting force has to stay low. Aggressive hard milling is how a block cracks or a thin core snaps.
What tolerance can be held on a deep rib?
On a rib with a reasonable width-to-depth ratio, we hold ±0.005 mm on the width and Ra 0.8–1.6 μm on the flanks.
As the rib gets taller and thinner, tool deflection grows and the achievable tolerance loosens. That geometry is usually better burned on EDM.
Do you machine electrodes as well as the mold?
Yes. Copper and graphite electrodes are machined on the same centers, to the same tolerance discipline, because the electrode error transfers directly into the cavity.
Electrode wear is tracked across the burn so the final cavity size stays inside tolerance.
How do you handle the parting line and shutoff faces?
They are machined as part of the matched pair, then spotted and stoned by hand until contact is even around the perimeter.
This is deliberate hand work. A parting face that looks flat on a CMM can still flash if the contact is uneven.
What is the smallest order you will take for mold work?
There is no minimum order quantity. We run from one prototype tool up to 10,000+ part runs.
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