P20 and 1045 sit on opposite sides of the same mold base decision: P20 is a pre-hardened Cr-Mo-Ni tool steel supplied at roughly 30-36 HRC (about 300-340 HB) so the mold base plates go straight to CNC after stress relief, while 1045 is a plain medium-carbon steel (about 0.45% C, 0.60-0.90% Mn) shipped in a soft annealed or as-rolled condition and then hardened by the tool shop [S1][S2].
The choice is rarely "either/or" inside one mold: a typical plastic injection tool mixes both. P20 is used for the cavity, core and any high-wear retainer features; 1045 / S45C / C45 is used for the frame, support plate, backing plate, ejector housing and clamp slots where the requirement is stiffness and weldability, not surface hardness [S2][S3].
What P20 actually is: chemistry, hardness, delivery condition
P20 is a chromium-molybdenum low-carbon tool steel, usually carrying about 0.28-0.40% C, 1.40-2.00% Cr, 0.30-0.55% Mo and, in the "P20+Ni" variant, roughly 0.80-1.20% Ni to clean up the microstructure and improve polishability [S1][S2]. The grade is sold pre-hardened to about 30-36 HRC, which is the single most important spec for a mold base buyer: it means heavy frame sections can be deep-cavity roughed, EDM'd and wire-cut without distortion from a post-machining quench [S4][S5].
Two practical consequences follow. First, P20 is usually machined in the hardened-and-tempered condition and then stress-relieved at roughly 480-540 degrees C before finish machining; it is not quench-hardened after the cavity is cut [S5]. Second, P20 can be case-hardened or nitrided later if a local wear surface needs it: nitriding can push the surface to about 60 HRC with case depths up to roughly 0.5 mm, and Cr plating is also an option with a low-temperature temper to avoid hydrogen embrittlement [S5].
What 1045 actually is: chemistry, hardness, delivery condition
1045 (AISI) is the North American designation for a medium-carbon steel with about 0.45% carbon and 0.60-0.90% Mn; it is the same chemistry sold as C45 / C45E (EN 1.1191) in Europe, S45C in Japan and 45 steel in the Chinese GB system [S2]. In the as-rolled or annealed condition it sits at roughly 170-200 HB (about 5-10 HRC equivalent), so it is soft enough to machine aggressively, mill deep pockets, drill large holes and weld on repair pads without cracking [S2][S3].
Through-hardening 1045 to about 55-60 HRC is possible by water or oil quenching from roughly 800-845 degrees C and tempering back, but sections above about 25 mm will not fully through-harden, and the distortion risk on large mold base plates is high enough that most shops flame-harden, induction-harden or nitride the wear surfaces locally and leave the core tough [S3]. For mold base applications the steel is almost always used in the as-rolled or normalized condition, acting as a stiff, weldable, low-cost structural member [S2][S3].
Decision matrix: P20 vs 1045 on the criteria that matter
On machinability, P20 in the pre-hardened 30-36 HRC range cuts at about 65-75% of the free-machining 160 HB B1112 reference steel, while annealed 1045 at about 170 HB cuts faster, but 1045 then needs downstream heat treatment if any wear surface is specified [S4][S2]. P20 has a clear cost premium of roughly 1.8-2.5x per kg over 1045 in most regional spot quotes, so specifying P20 for the entire frame is wasted money when the frame is hidden, unstressed and never sees a wear surface [S2][S3].
On weldability and repair, 1045 is the better base because its lower carbon equivalent and softer delivery condition make it tolerant of weld-repair, pad-build-up and frame modification on the shop floor, while P20 needs pre-heat (about 300-400 degrees C) and post-weld stress relief to avoid cracking [S2][S5]. On dimensional stability during EDM and wire cutting, pre-hardened P20 is the safer choice because there is no phase transformation to drive movement; 1045 in the as-rolled condition can move several tenths of a millimetre on large plates if the rolling fibre is not balanced [S5].
On wear and polish, P20 is the spec for any surface that touches the plastic: it polishes to SPI #1-#3 range and can be nitrided to roughly 60 HRC for glass-filled or abrasive resins, while 1045 is too soft (about 170-200 HB) to hold a polish or a textured finish in production and is not used for cavity or core surfaces [S4][S5]. On toughness and frame duty, 1045's higher carbon gives better as-rolled strength for clamp slots, ejector housings and large backing plates, and the soft delivery condition absorbs impact from ejection without chipping [S2][S3].
Where each grade is the right call in a real mold stack
P20 is the correct choice for the cavity retainer plate (A-plate), core retainer plate (B-plate), support plate behind the cavity, and any slide, lifter or insert that sees molding pressure on its wear face; it is also the default for 1-4 cavity prototype and short-run tools with cold runners, molding materials like PP, PE, PS, ABS and unfilled PC/PA [S4]. Pre-hardened P20 lets the shop go from stock plate to finished mold base without a heat-treat furnace in the loop, which is why it remains the most commonly named mold steel in buyer surveys [S5].
1045 is the correct choice for the bolster, the bottom clamp plate, the ejector box, the spacer blocks, the heel block and the support pillars where the requirement is to react the clamp force of the press, keep the assembly flat, and accept welded repairs; 1050 or A36 sits below 1045 for the largest, lowest-stress backing plates where stiffness and cost dominate [S2][S3]. Where the steel plate is a structural member that never touches plastic, 1045 / S45C / C45E is the economic default, and an upgrade to P20 only pays back if the plate also acts as a retainer or wear surface [S2].
Common failure modes and the limits of each grade
P20 fails by wear, not by fracture: at 30-36 HRC it cannot survive long runs of glass-filled PA, glass-filled PEEK, PPS or any resin with mineral filler above about 30% loading, and it should be stepped up to H13 (35-42 HRC supplied, 50-60 HRC after heat treat) for those applications, with stainless 420 specified where corrosion from PVC, flame retardants or humid storage is a concern [S4]. P20 also tolerates only modest nitriding case depth; a poorly heat-treated thick block can show a pronounced case-core effect, and if roughing strips the case the residual stress will distort the plate, forcing an extra stress-relief cycle before finish machining [S5].
1045 fails by deflection and by surface breakdown: in the as-rolled 170-200 HB condition it will not hold a wear surface under any abrasive resin, it will gall if used as a slide or stripper surface, and large unwelded sections can cup if the rolling direction is not randomized. Welding 1045 without proper pre-heat produces hard HAZ zones that crack in service, and any shop that is buying mold base 1045 plate needs a written weld-procedure covering pre-heat, filler choice and post-weld stress relief [S2][S3].
Selection rules that work on the shop floor
Use the following four rules, in order, when the drawing does not name the grade. First, any plate that forms a cavity, core, slide, lifter, gate insert or stripper surface is P20 (or H13 / 420 / S7 if a higher wear or corrosion class is justified) [S1][S4]. Second, any plate that is a structural frame, clamp, spacer, ejector housing or backing member, and is hidden, is 1045 / S45C / C45E in the as-rolled or normalized condition [S2][S3]. Third, if the tool is one of the mold base standard sets (DME, Hasco, Futaba, Misumi) the frame is almost always 1045 / S50C grade per the standard, and the A and B plates are either 1045 (for low-cost frames) or P20 (for pre-hardened frames) [S3]. Fourth, for prototype or bridge tooling under about 10 000 shots with cold runners and unfilled resin, P20 is the safest single-grade choice across the whole base because the procurement and machining path is the simplest [S4].
For foundries and die shops that also run sand casting mold work, the same logic applies in reverse: P20 / H13 for the working face, 1045 or 1020 for the flask and the backup plate, with no premium steel specified where the casting mold face is not in contact with the molten metal. Material selection is a tradeoff between desired properties, and the P20 vs 1045 question is really "where in the mold do you need pre-hardened wear steel, and where do you need soft, weldable, cheap structural steel" [S1][S2][S5].
Trackable signals for the next review cycle: ASTM A681 tool-steel specification revisions covering P20+Ni sub-grades, AISI re-classification work on the P-series, and any movement in 1045 / S45C spot premiums driven by the Chinese GB 45 export allocation. Buyers should also watch the H13 vs P20 crossover point on glass-filled resin, since that threshold is what forces a base-plate grade change at the next tool revision [S1][S4][S5].
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