Rolled vs Forged P20 Mold Steel Blocks | Which Is Better for Large Injection Molds

For most flat mold plates, rolled steel offers the best balance of price, availability and machining time. For a very thick block with a deep cavity, a forged block may reduce risk—but only when the supplier can prove how it was forged, heat treated and inspected.

For mirror-polished or finely textured cavities, the rolled-versus-forged question is not enough. Steel cleanliness, hardness uniformity and the melting route may matter more than the final forming process.

This comparison applies mainly to large plastic injection molds, including cavity blocks, core blocks, mold bases, backing plates and support plates. Die-casting dies and hot-forging dies need a different analysis because they also face high temperatures and repeated heating and cooling.

Start With the Mold Requirement

Rolled and forged products are both accepted forms of wrought tool steel. Neither process guarantees that the steel is suitable for a particular mold.^[1]

Choose the steel in this order:

1. Select the grade. The grade must suit the plastic resin, wear level, corrosion risk, mold life and required surface finish.
2. Check the required block size. A grade that performs well in a thin plate may not have the same center properties in a much thicker block.
3. Choose the steelmaking route. Conventional steel, vacuum-treated steel and ESR steel do not have the same cleanliness or uniformity.
4. Compare rolled and forged forms. Select the form that can provide the required size and properties with acceptable cost and lead time.
5. Define inspection requirements. A material name without hardness, ultrasonic testing and traceability requirements is not a complete specification.

ASTM A681 also makes clear that tool-steel selection depends on the design, service conditions and required properties—not simply on whether the steel is supplied as plate, bar or forging.^[2]

Buyers can first compare available mold steel grades and supply forms, then decide whether the selected grade should be purchased as rolled plate, a cut block or a forging.

Rolled and Forged Blocks: What Is the Difference?

Rolled steel

Rolled mold steel starts as an ingot, bloom or slab. The steel is heated and passed through rolls until it reaches the required thickness.

Rolling can:

• Break down part of the original cast structure
• Make the grains finer
• Close some suitable small internal voids
• Produce plate efficiently and consistently

Most rolled products have a main rolling direction. Inclusions and bands inside the steel may become stretched in that direction. As a result, properties can differ along the length, across the width and through the thickness.

Some thick plates are cross rolled. The plate is turned during production so that it is worked in both its length and width directions. This can improve the balance between those two directions, but it does not make the steel identical in every direction.

This directional difference may not matter in a well-supported mold-base plate. It matters more when the block contains deep pockets, thin walls, large cross-drilled cooling holes or loads acting through the thickness.

For a critical thick rolled plate, ask the supplier for:

• The starting slab or bloom size
• The final plate thickness
• The rolling direction
• Whether cross rolling was used
• Experience with the same grade at a similar thickness
• Hardness and ultrasonic-testing records for comparable material

Forged steel

Open-die forging uses a press or hammer to compress a hot ingot or bloom. The producer repeatedly presses, turns and reshapes the material until it reaches the required block size.

Forging gives the producer more freedom to apply heavy working to a thick section. Two common operations are:

• Upsetting: The material is pressed shorter and wider. This can place more compression into the original center.
• Drawing out: The section is reduced and made longer. The block may be turned between passes so that it is worked from different directions.

A good forging sequence can help close some internal voids and break down a coarse cast structure. However, void closure depends on how much strain reaches the defect, the defect's size and position, the temperature and the forging sequence. Outside dimensional change alone does not prove that the center was worked properly.^[3]

What a forging ratio does and does not prove

A forging reduction ratio may be calculated by comparing the original cross-sectional area with the final area.

For example:

• Original section: 1,200 × 1,000 mm
• Original area: 1,200,000 mm²
• Final section: 600 × 500 mm
• Final area: 300,000 mm²

1,200,000 ÷ 300,000 = 4:1

This is only a simple area-reduction example. Suppliers may calculate reduction differently depending on the forging route. The purchase document should therefore state how the ratio was calculated.

A stated 4:1 ratio does not prove that the block is free from defects. The result also depends on:

• Press capacity
• Die width
• Forging temperature
• Upsetting and drawing sequence
• How often the block was turned
• The condition of the original ingot

The forging ratio is process information. It does not replace hardness tests, ultrasonic examination or records from blocks of a similar size.

Why Cavity Position Matters

The outside dimensions do not show the full material risk. The important question is how far the final cavity will be machined into the original block.

Consider two example blocks, both 600 mm thick:

• Mold A: The cavity is 80 mm deep. The molding surface remains close to the original outside surface.
• Mold B: The cavity is 260 mm deep. The molding surface reaches much closer to the original center.

These dimensions are examples, not fixed industry limits. Mold B presents more risk because the center of a thick block is normally the most difficult area to make uniform.

Deep machining may expose:

• Centerline segregation
• Hardness differences
• Large inclusions
• Porosity or shrinkage-related indications
• Coarse structural bands

These conditions may not become visible until the mold maker has already added many hours of machining.

A practical design review should record:

• Original block thickness
• Maximum cavity depth
• Distance from the final cavity to the original centerline
• Minimum steel thickness below the cavity
• Location of deep cooling holes
• Amount removed from each face

The guide to calculating mold block size and machining allowance explains how to separate finishing allowance, clamping space and stock needed for distortion correction.

Keep the original direction and centerline marked

When a plate or forging is cut into smaller blocks, the original material direction can be lost. Critical blocks should remain marked with:

• Heat number
• Rolling or forging direction
• Original top and bottom surfaces
• Original centerline position
• Surfaces used for ultrasonic scanning

If the ultrasonic report shows an indication near one side, the mold maker must be able to compare that location with the planned cavity. An indication map is of little value when the block orientation is no longer known.

Hardness Is Not the Same as Hardenability

Many large plastic molds use prehardened P20-type steel. Examples commonly compared by buyers include 1.2311 prehardened mold steel and nickel-modified 1.2738 mold steel. These names should not be treated as interchangeable without checking the actual specification.

Hardness

Hardness is resistance to indentation at the point where the test is made. A surface reading does not prove that the entire block has the same hardness. ASTM E10 specifically warns that a Brinell result from one location may not represent the whole product.^[4]

Hardenability

Hardenability means how deeply the steel can develop the required hardened structure during quenching. It is not the same as the hardness measured on one surface. ASTM A255 uses the Jominy end-quench method to measure how the hardening response changes with distance from the quenched end.^[5]

In a thick block, the outside cools faster than the center. If the grade does not have enough hardenability for that section, the surface may meet the required hardness while the center develops a softer or different structure.

Center hardness depends mainly on:

• Chemical composition
• Block thickness
• Heating temperature and holding time
• Quenching method
• Furnace loading
• Tempering conditions

Forging does not create hardenability. It may improve the starting structure, but the grade and heat treatment still control how the block hardens.

What hardness variation looks like during machining

A softer area may cause:

• Changing cutting sound and spindle load
• Uneven tool wear
• Local indentation during molding
• Uneven polishing or texturing

A local hard area may cause:

• Tool chipping
• Unstable drilling or tapping
• Higher grinding heat
• Greater crack risk when combined with sharp corners, residual stress or surface damage

For a critical thick block, request:

• Hardness readings from several accessible faces
• The test method and test locations
• The permitted hardness range and variation
• Center or quarter-thickness data from a representative block or test section
• Records from the same grade at a similar size

HB-to-HRC conversions are approximate. ASTM E140 explains that conversions depend on the material group and condition and should not be treated as exact replacements for the specified test method.^[6]

Internal Quality and Properties in Different Directions

Porosity and shrinkage

Steel contracts while it solidifies. If liquid steel cannot feed a shrinking area, pores or cavities may remain.

Both rolling and forging can close some suitable voids when enough hot compression reaches them. Neither process guarantees that every original void has disappeared.

Segregation

Segregation means that alloying elements are not spread evenly through the original ingot.

Rolling or forging can stretch and break up segregated areas, but it cannot completely remove the chemical difference. Segregation may lead to:

• Hardness bands
• Different cutting response
• Uneven polishing
• Uneven chemical texturing
• Local differences in toughness

Non-metallic inclusions

Inclusions are particles such as oxides or sulfides formed during steelmaking. Rolling and forging can change their shape and direction, but cannot make them disappear.

Elongated sulfide inclusions can reduce transverse toughness and contribute to differences between the longitudinal and transverse properties of steel.^[7]

Manganese-sulfide inclusions can also help chips break during machining. Their benefit therefore comes with a trade-off: easier machining may be accompanied by lower transverse toughness or poorer high-level polishability.^[8]

This is why free-machining 1.2312 mold steel may suit support parts and easily machined mold components, but it should not automatically be chosen for a mirror-polished cavity.

Properties can change with direction

Rolled plate normally has three main directions:

• Longitudinal: Along the main rolling direction
• Transverse: Across the plate width
• Short transverse: Through the thickness

A forged block may have more balanced properties if it has been upset, drawn and turned correctly. It is still not perfectly uniform in every direction.

If a deep cooling hole or thin wall creates stress through the block thickness, ask for test data in the relevant direction. A longitudinal test result does not prove through-thickness performance.

Machining Movement and Residual Stress

A block may be flat when delivered and still bend or twist during machining.

Residual stress is stress locked inside the steel even when no external load is being applied. It may come from uneven cooling, heat treatment, straightening, sawing or previous machining.

Removing material releases part of this existing stress. At the same time, cutting heat, cutting force and heavy clamping can add new local stress. Machining-related residual stress and distortion are well-recognized manufacturing problems.^[9]

For example, removing 200 mm from one side and only 20 mm from the opposite side may disturb the original stress balance. The block may appear flat while clamped to the machine table and move after the clamps are released.

Machining a prehardened block

1. Confirm the heat number, material direction and original centerline.
2. Machine reliable datum faces.
3. Balance heavy stock removal where possible.
4. Leave enough material for later correction and finishing.
5. Release heavy clamping after rough machining.
6. Allow the whole block to return to a uniform temperature.
7. Measure flatness and twist while the block is free.
8. Adjust the remaining allowance before semi-finishing.

Do not apply a general stress-relief temperature to every prehardened P20-type block. An unsuitable treatment can further temper the steel and reduce its hardness. Follow the steel producer's instructions for the exact grade and delivery condition.

Machining an annealed block that will later be hardened

An annealed tool-steel block that will receive final hardening may use a different sequence:

1. Rough machining
2. Stress-relief treatment
3. Semi-finishing
4. Hardening and tempering
5. Final machining or grinding

This sequence should not be presented as the default route for an already prehardened block.

The six-sided mold block machining guide explains datum selection and face-machining order. The separate guide to stock allowance after duplex milling discusses how much material to leave for correction and finishing.

Polishing, Texturing and EDM Surfaces

A forged block is not automatically easier to polish. Final surface quality depends on the steel's cleanliness, microstructure, hardness uniformity and the polishing process itself.^[10]

Common polishing defects

Why ESR may matter more than forging

Electroslag remelting, or ESR, remelts steel under a controlled slag. The process combines refining with controlled solidification and is used to improve cleanliness and ingot structure.^[11]

At a similar grade and hardness, a clean ESR product may polish better than a conventionally melted forging. ESR still cannot correct:

• The wrong steel grade
• Poor heat treatment
• EDM surface damage
• Incorrect welding
• Contaminated polishing tools
• Excessive polishing pressure

Prepare the surface before texturing

Chemical etching reacts with the steel surface. Differences in chemistry, hardness, welding history or surface preparation may cause uneven texture depth or gloss.

Before chemical or laser texturing:

• Remove the EDM recast layer to the texture supplier's requirement.
• Prepare a consistent surface across the cavity.
• Record all welded areas.
• Confirm the steel grade and hardness.
• Identify inserts made from different heats.
• Use a test sample for an appearance-critical texture.

EDM melts and resolidifies a thin surface layer. The thickness and condition of this recast layer depend on the discharge conditions, so it should not be left uncontrolled before polishing or texturing.^[12]

For high-gloss or finely textured molds, retain a sample from the same heat and delivery condition for trial polishing, EDM finishing, welding or texturing.

Ultrasonic Testing: What the Report Must Show

Ultrasonic testing sends sound through the block and records returning signals. It can find many internal discontinuities, but the words “UT passed” are not a complete result.

Use the correct standard

ASTM A578/A578M covers straight-beam ultrasonic examination of rolled carbon and alloy steel plate. It mainly targets discontinuities that lie roughly parallel to the rolled surfaces and provides three acceptance levels.^[13]

ASTM A388/A388M covers ultrasonic examination procedures for steel forgings using straight- and angle-beam methods. It does not give one universal acceptance class for every forging. The purchase order or another applicable specification must state the reporting and acceptance limits.^[14]

What the report should include

• Heat number and block identification
• Whether the product is rolled or forged
• Standard and edition used
• Acceptance level or agreed acceptance limits
• Scan coverage
• Surfaces used for scanning
• Probe type and frequency
• Calibration method
• Reporting threshold
• Back-wall signal loss, where applicable
• A location map for reportable indications
• Inspector identification and qualification

What ordinary UT cannot prove by itself

UT is an inspection tool, not proof that the steel is chemically uniform or free from every microscopic defect.

The guide to reading a mold steel mill test certificate explains how to match the heat number, chemistry, hardness results and UT report to the delivered block.

When Rolled Steel Is the Better Choice

Rolled steel is normally the better starting option when:

• The component is flat or plate shaped.
• The thickness is within the producer's proven range.
• The final cavity does not reach close to the original centerline.
• The surface requirement is conventional rather than optical.
• Standard stock reduces lead time and waste.
• The component mainly supports other mold parts.

Common applications include:

• Mold-base plates
• Backing plates
• Support plates
• Ejector plates
• Clamping plates
• Shallow cavity plates
• Low-volume molds with moderate cavity depth and normal surface requirements

Rolled steel still needs traceability, an agreed hardness condition and suitable internal inspection.

When Forged Steel Is the Better Choice

A qualified forged block deserves stronger consideration when:

• The required thickness is outside normal rolled-plate availability.
• The final cavity reaches close to the original centerline.
• Thin remaining walls or large cooling holes make through-thickness properties important.
• A custom starting size can remove a large amount of machining waste.
• A material failure would stop an expensive production program.
• The required center properties cannot be proven in the available rolled product.

A high production volume or heavy clamping load alone does not prove that forging is necessary. The decision must also consider the steel grade, hardness, toughness, wall thickness, hole position and load direction.

Before accepting a forged block, ask for:

• Starting ingot or bloom information
• Final block dimensions
• Forging sequence
• Reduction ratio and calculation method
• Heat-treatment condition
• Hardness data from similar sections
• Test direction for mechanical properties
• Ultrasonic-testing procedure and acceptance limits

Compare Total Cost, Not Price per Kilogram

A rolled plate may cost less per kilogram but require more purchased weight and rough machining. A custom forging may cost more per kilogram but arrive closer to the required size.

Total material cost = steel price + transport + handling + machining + tools + inspection + heat treatment + rework + schedule risk

Compare:

• Incoming block weight
• Finished block weight
• Sawing and six-side machining
• Rough-machining hours
• Deep-hole drilling time
• Cutting-tool use
• Hardness and ultrasonic inspection
• Heat treatment, where required
• Transport and lifting
• Delivery time
• Value already added when a defect might be found

For example, an oversized rolled plate can have the lower purchase price but create many tonnes of chips. A near-size forging may cost more initially but reduce machine time enough to narrow or remove the price difference. The saving must be calculated from actual dimensions and machining hours, not assumed from the product name.

Buyers can compare custom mold steel blocks with the cost of cutting and machining an oversized standard plate.

What to Put in the Purchase Order

Material requirements

• Exact steel grade and standard
• Permitted alternative grades
• Required delivery condition
• Required hardness range and scale
• Maximum permitted hardness variation
• Conventional, vacuum-treated or ESR-remelted route where needed
• Polishing, texturing, corrosion and wear requirements

Size and processing requirements

• Ordered stock dimensions
• Final part dimensions
• Machining allowance on each face
• Flatness, parallelism and squareness
• Saw-cut, milled or ground delivery condition
• Surface and edge requirements

Rolled or forged route

• Rolled or forged form, when the route is technically required
• Rolling direction or cross-rolling information
• Forging sequence and reduction information
• How the forging ratio was calculated
• Original ingot, bloom or slab traceability
• Direction and centerline markings

Inspection requirements

• Hardness test method and locations
• Representative center or quarter-thickness data where required
• Correct UT standard for the product form
• ASTM A578 acceptance level for rolled plate, where applicable
• Separately agreed acceptance limits for ASTM A388 forgings
• Scan coverage and scanning surfaces
• Location map for reportable indications
• Cleanliness or metallographic data when surface quality is critical

Documents and restrictions

• Material certificate
• Heat number
• Chemical analysis
• Heat-treatment condition
• Hardness report
• UT report
• Dimensional report
• No weld repair without written approval
• Transfer of identification after cutting
• Rules for reinspection, replacement or technical acceptance

Incoming inspection

1. Match the grade, heat number and certificate.
2. Check dimensions, weight and machining allowance.
3. Confirm material direction and centerline markings.
4. Confirm that the UT report belongs to the delivered block.
5. Check the UT standard and acceptance conditions.
6. Measure hardness at the agreed locations.
7. Inspect the surfaces for cracks, folds, heavy scale or signs of repair.
8. Check newly exposed surfaces after six-side machining.
9. Keep a same-heat sample when polishing or texturing is critical.
10. Stop high-value machining if a serious material problem is found.

For large or repeated orders, the mold steel supplier evaluation checklist can be used to check equipment, traceability, calibration and quality-control records.

Final Decision

The better block is not the one with the stronger marketing label. It is the one whose grade, size, center properties, hardness, surface quality and inspection records match the actual mold design.