P20 vs 718H Mold Steel: How to Choose the Right End Mill

Reading volume: 3

Release time :2026-06-18

Estimated reading time:

P20 and 718H are both commonly used for plastic molds, cavities, inserts, and other mold components. Although they belong to a similar group of pre-hardened mold steels, their machining behavior may not be exactly the same in every application.

Differences in supplied hardness, section size, material uniformity, heat-treatment history, mold geometry, and surface requirements can all influence tool life and machining stability. Instead of selecting an end mill by the material name alone, it is better to begin with the actual workpiece condition and the feature being machined.

What Is the Difference Between P20 and 718H Mold Steel?

P20 is a widely used pre-hardened plastic mold steel. It is commonly selected for injection molds, mold cavities, mold bases, and general mold components that require a practical balance of machinability, strength, and finishing performance.

718H is commonly regarded as an enhanced pre-hardened plastic mold steel in the P20 family. Depending on the steel supplier and grade specification, it may be selected for larger mold sections or applications where more consistent hardness through the block and stable polishing performance are important.

This does not mean that every 718H workpiece is automatically harder or more difficult to machine than every P20 workpiece. The supplied condition can vary, so the material certificate and measured hardness should be checked before the cutting tool and machining parameters are selected.

Actual Hardness Matters More Than the Material Name

The words “P20” or “718H” do not provide enough information to determine the correct cutter by themselves. Different suppliers may provide related grades in different hardness conditions, and a large mold block may not behave exactly like a small insert.

As actual hardness increases, cutting pressure, flank wear, corner wear, and sensitivity to runout generally become more important. The end mill needs stronger edge support, stable coating performance, and a rigid setup to maintain predictable tool life.

Before machining, check the material certificate and confirm whether the workpiece remains in its supplied pre-hardened condition or has received additional heat treatment. Our guide on how to choose an end mill for pre-hardened mold steel explains why actual hardness and machining stage should be considered together.

How P20 and 718H Behave During Milling

In a stable pre-hardened condition, both materials can usually be machined with coated carbide end mills designed for mold steel. However, tool performance can change with the size of the component, local cutting engagement, tool overhang, and the amount of material removed in each pass.

P20 machining often focuses on efficient cavity preparation, flat-surface milling, sidewall machining, and general mold finishing. When the material condition and setup are stable, predictable tool wear can usually be achieved with a suitable mold steel end mill.

In 718H machining, large mold sections and longer toolpaths may place more emphasis on consistency. Even small changes in runout, tool wear, or remaining allowance can become visible during long sidewall or cavity-finishing operations.

For either material, the cutter should be selected according to the actual operation rather than assuming that one set of cutting conditions will suit every P20 or 718H component.

Common Milling Problems in P20 and 718H

Choose the End Mill According to the Mold Feature

The difference between P20 and 718H is only one part of tool selection. The shape of the mold feature often has an even more direct effect on the cutter choice.

Square End Mills for Flat Bottoms and Sidewalls

Square end mills are suitable for flat bottoms, slots, shoulders, and vertical sidewalls. They can create a defined corner, but the sharp tool corner carries a concentrated load and should be monitored for wear or small edge damage.

For flat surfaces and sidewall machining in P20 or 718H, a UEX square end mill for mold steel can be used for semi-finishing and finishing within the recommended hardness range.

Corner Radius End Mills for More Stable Corners

A corner radius design strengthens the tool corner by spreading the cutting load over a curved edge. It is useful for semi-finishing, shoulders, sidewalls, and cavity transitions where a completely sharp internal corner is not required.

When corner wear or chipping is a concern, a UEX corner radius end mill for mold steel can provide better corner support and more predictable edge wear.

Ball Nose End Mills for Cavities and 3D Surfaces

Ball nose end mills are commonly used for curved cavities, free-form mold surfaces, and 3D contour finishing. Surface quality depends on the tool radius, contact point, step-over, toolpath direction, and condition of the ball edge.

For curved P20 or 718H mold surfaces, a UEX ball nose end mill for mold steel can support smooth contour transitions and consistent finishing when the machining strategy is stable.

Roughing, Semi-Finishing, and Finishing Need Different Priorities

The same mold may require different tools or cutting strategies at each stage. Using a worn roughing cutter for the final finishing pass may reduce tool cost in the short term, but it can also increase polishing work and dimensional variation.

Semi-finishing is especially important on large 718H mold components. Leaving a consistent allowance helps prevent the finishing cutter from experiencing sudden changes in cutting load across long sidewalls or cavity surfaces.

Surface Finish Requirements Can Change the Tool Choice

A mold that will receive extensive polishing does not always require the same milling strategy as a mold that needs a near-finished surface directly from the machine. The required finish affects cutter geometry, step-over, tool condition, and machining time.

For precision sidewalls and cavity surfaces, runout and tool wear should be checked before the finishing operation. A small difference in flute loading can leave visible lines that require additional polishing.

The relationship between cutter condition, finishing allowance, step-over, and surface quality is discussed in our guide on how to improve surface finish when milling pre-hardened mold steel.

Toolholding and Overhang Are Important for Both Materials

A suitable end mill cannot compensate for an unstable setup. Excessive runout causes one flute to carry more load than the others, leading to uneven wear, poor surface finish, and shorter tool life.

Long tool overhang increases deflection and makes chatter more likely. This is common in deep P20 or 718H cavities where access is limited. Use the shortest practical overhang and select a holder that provides enough rigidity for the tool diameter and cutting depth.

When long-reach machining cannot be avoided, reduce sudden engagement and use a toolpath that keeps radial load more consistent. Cutting data should be adjusted to the reduced rigidity of the setup.

When Should You Move from UEX to a High-Hardness Steel End Mill?

P20 and 718H are normally associated with pre-hardened mold applications, but the actual workpiece condition should still be verified. Additional heat treatment or a different supplied grade can change the machining requirement.

Dohre's UEX mold steel end mills for materials up to HRC60 are intended for mold steel and difficult-to-machine applications within the recommended hardness range.

If the measured workpiece hardness enters the HRC60–68 range, a dedicated high-hardness steel end mill for HRC60–68 materials should be considered. The decision should be based on actual hardness and machining condition, not only on whether the material is called P20 or 718H.

Practical Checklist for P20 and 718H End Mill Selection

• • Check the material certificate and actual supplied hardness.

• • Do not select the cutter only from the name P20 or 718H.

• • Consider the mold block size and length of the machining cycle.

• • Use a square end mill for flat bottoms, slots, shoulders, and defined sidewalls.

• • Use a corner radius end mill when stronger corner support is needed.

• • Use a ball nose end mill for curved cavities and 3D mold surfaces.

• • Separate roughing and finishing tools when dimensional accuracy or surface quality is critical.

• • Keep tool overhang as short as the mold feature allows.

• • Check holder accuracy and runout before finishing.

• • Leave a consistent allowance during semi-finishing.

• • Match feed, speed, step-over, and engagement to the tool and setup rigidity.

FAQ

Is 718H always harder to machine than P20?

Not always. Machinability depends on the supplied grade, actual hardness, section size, toolpath, cutter condition, and setup. The material certificate and measured hardness should be checked before selecting the end mill.

Can the same end mill be used for P20 and 718H?

A suitable mold steel end mill series may cover both materials, but the cutter size, shape, overhang, and cutting conditions should still be selected according to the actual hardness, mold feature, and machining stage.

Which end mill is best for P20 mold steel?

The best tool depends on the feature. Square end mills are used for flat surfaces and sidewalls, corner radius end mills improve corner support, and ball nose end mills are commonly used for curved cavities and 3D profiles.

Which end mill is suitable for 718H cavity finishing?

Ball nose end mills are commonly used for curved cavity surfaces, while square or corner radius tools may be used for flat areas, sidewalls, shoulders, and semi-finishing. Tool choice should follow the cavity geometry and finishing requirement.

Should P20 and 718H use different cutting parameters?

They may require different parameters if their actual hardness, block size, setup rigidity, or machining operations are different. Cutting data should be adjusted after confirming the material condition and tool supplier's recommended range.

Conclusion

P20 and 718H are both important pre-hardened plastic mold steels, but end mill selection should not be based on the material name alone. Actual hardness, mold size, feature geometry, machining stage, toolholding, and surface requirements all influence the correct choice.

Square end mills are suitable for flat surfaces and sidewalls, corner radius end mills provide stronger corner support, and ball nose end mills are used for curved cavities and 3D finishing. A consistent semi-finishing allowance and stable setup are especially important for long mold-finishing operations.

Dohre provides UEX mold steel end mills for materials up to HRC60, including square, corner radius, and ball nose designs for P20, 718H, and other mold steel applications. Contact us for standard end mill recommendations or custom mold steel cutting tool solutions.