Mold Steel Property Ranking Chart for P2 to P21 and Stainless Grades
A mold steel is bought for the job the cavity has to do, and the properties that decide the choice pull against one another. The grade that hobs most easily is not the grade with the most wear resistance, and the stainless grades that hold up against a corrosive plastic are the weakest of the group in core strength. The tables below rank the mold steels of the P group and three martensitic stainless grades against each other on seven properties, with the steels that rate equally bracketed together. This is a reference summary of published practice and not a specification of ours.
How the ranking table is arranged
The source groups the steels instead of scoring each one, so read the groups rather than the numbers alone. Under each heading the steels that rate equally are bracketed together, and the groups are then listed in decreasing order of that property, which puts the best group at the top of the column and the weakest at the bottom.
Two details matter before the data is used. The rank number restarts in every column, so a 5 in the wear resistance column has nothing to do with a 5 in the machinability column, and only the order inside one column carries meaning. The letter (a) after a grade means the rating is for the carburized condition, while the plain grade name is the steel as supplied and through hardened.
The three stainless grades are named by their composition as the source prints them, 13Cr-0.12C, 13Cr-0.35C and 17Cr-0.65C. They are the martensitic stainless mold steels, the class that carries 13 per cent chromium or more and is chosen where the plastic being molded is itself corrosive.
Performance properties
Wear resistance
Wear resistance is rated for the surface that meets the flowing plastic, which is why the carburized grades appear at the top and the two machined only grades at the bottom. Carburized P4 and the 17Cr-0.65C stainless share the highest rating of the group.
| Rank | Mold steels, highest first |
|---|---|
| 5 | P4 (a), 17Cr-0.65C |
| 4 | P5 (a), P6 (a) |
| 3 | P2 (a), P3 (a), P20 (a), 13Cr-0.35C |
| 2 | 13Cr-0.12C |
| 1 | P20, P21 |
Source, Tool Steels (G. A. Roberts and G. Krauss), Table 15-8, Properties and ranking of mold steels. Steels with an equal rating are grouped and the groups run from the highest rating on that property down to the lowest. The letter (a) marks the carburized condition. This is a reference summary of published practice and not an Aobo Steel specification.
Toughness
Toughness is read against a mold that has to survive opening and closing, the injection of plastic at high velocity and the thermal shock of the cycle. The whole P group sits above every stainless grade here, and the five low alloy grades share the top rating.
| Rank | Mold steels, highest first |
|---|---|
| 9 | P2, P3, P4, P5, P6 |
| 8 | P20, P21, 13Cr-0.12C |
| 6 | 13Cr-0.35C |
| 5 | 17Cr-0.65C |
Source, Tool Steels (G. A. Roberts and G. Krauss), Table 15-8, Properties and ranking of mold steels. Steels with an equal rating are grouped and the groups run from the highest rating on that property down to the lowest. The letter (a) marks the carburized condition. This is a reference summary of published practice and not an Aobo Steel specification.
Hot hardness
Hot hardness is the resistance of the steel to losing its hardness at the temperature the molding or casting cycle reaches. The stainless grades lead this column and the low carbon carburizing grades fall to the bottom, which is consistent with their low alloy content.
| Rank | Mold steels, highest first |
|---|---|
| 6 | 13Cr-0.35C, 17Cr-0.65C |
| 5 | P4 (a), 13Cr-0.12C |
| 4 | P6 (a), P21 |
| 3 | P5 (a) |
| 2 | P2 (a), P3 (a), P20 |
Source, Tool Steels (G. A. Roberts and G. Krauss), Table 15-8, Properties and ranking of mold steels. Steels with an equal rating are grouped and the groups run from the highest rating on that property down to the lowest. The letter (a) marks the carburized condition. This is a reference summary of published practice and not an Aobo Steel specification.
Mold making properties
Machinability
Machinability decides how the cavity is produced when the shape cannot be hobbed. P20 is rated highest of the whole set, ahead of the carburizing grades and well ahead of the stainless steels, and the three stainless grades share the lowest rating.
| Rank | Mold steels, highest first |
|---|---|
| 8 | P20 |
| 7 | P2, P3, P5 |
| 6 | P6 |
| 5 | P4, P21 |
| 4 | 13Cr-0.12C, 13Cr-0.35C, 17Cr-0.65C |
Source, Tool Steels (G. A. Roberts and G. Krauss), Table 15-8, Properties and ranking of mold steels. Steels with an equal rating are grouped and the groups run from the highest rating on that property down to the lowest. The letter (a) marks the carburized condition. This is a reference summary of published practice and not an Aobo Steel specification.
Hubbability
Hubbability is the ability of the steel to take a cavity by forcing a hardened master hub into the soft blank, which is the cheapest route for a multi cavity mold with identical impressions. It follows the annealed hardness, so the low carbon grades lead and the precipitation hardening P21 is last.
| Rank | Mold steels, highest first |
|---|---|
| 7 | P2, P5 |
| 6 | P3 |
| 5 | P4 |
| 4 | P20, 13Cr-0.12C |
| 3 | 13Cr-0.35C |
| 2 | P6, 17Cr-0.65C |
| 1 | P21 |
Source, Tool Steels (G. A. Roberts and G. Krauss), Table 15-8, Properties and ranking of mold steels. Steels with an equal rating are grouped and the groups run from the highest rating on that property down to the lowest. The letter (a) marks the carburized condition. This is a reference summary of published practice and not an Aobo Steel specification.
Service properties
Core strength
Core strength is what stops the cavity from sinking under injection pressure, and it is the column where the ranking runs the opposite way to hubbability. The high carbon stainless grades lead, P20 is next, and the low carbon carburizing grades fall to the bottom.
| Rank | Mold steels, highest first |
|---|---|
| 6 | 13Cr-0.35C, 17Cr-0.65C |
| 5 | P20 |
| 4 | P21, 13Cr-0.12C |
| 3 | P4, P6 |
| 2 | P2, P3, P5 |
Source, Tool Steels (G. A. Roberts and G. Krauss), Table 15-8, Properties and ranking of mold steels. Steels with an equal rating are grouped and the groups run from the highest rating on that property down to the lowest. The letter (a) marks the carburized condition. This is a reference summary of published practice and not an Aobo Steel specification.
Corrosion resistance
Corrosion resistance is the only column of the table that is not a ranking, because the split is absolute. The three martensitic stainless grades are rated high and every other mold steel in the table is rated low.
| Rating | Mold steels, highest first |
|---|---|
| High | 13Cr-0.12C, 13Cr-0.35C, 17Cr-0.65C |
| Low | All others |
Source, Tool Steels (G. A. Roberts and G. Krauss), Table 15-8, Properties and ranking of mold steels. Steels with an equal rating are grouped and the groups run from the highest rating on that property down to the lowest. The letter (a) marks the carburized condition. This is a reference summary of published practice and not an Aobo Steel specification.
A worked selection example
The source gives one worked example and it shows how the columns are meant to be used together. A mold machined from uncarburized P20 is wearing out well before the required number of parts has been produced, in the compression molding of an abrasive plastic, so the replacement has to raise wear resistance without losing the shape the cavity already has.
P21 can be set aside at once, because it rates no higher than P20 on wear resistance. P2 and P3 in the carburized condition do rate higher, but their core strength is low and a job with high compression would not suit them. That leaves carburized P20 and carburized P6, both of which rate above the steel that failed. The choice between them then falls to machinability, where P20 rates 8 and P6 rates 6, and on a mold that still has to be cut after the change that gap is usually the deciding factor.
Related reference data
The two grades most often named in this table are described with their forms and delivery conditions on P20 tool steel and on 420 stainless steel, which is the 13 per cent chromium class the three stainless mold steels belong to. The strength, ductility and hardness those mold steels reach in service are tabulated on mechanical properties of mold steels P4, P20 and P21, the surface finish those grades have to take is described on plastic mold steel polishing and texturing, and the wider rating of processing and service characteristics across all tool steel grades, taken from the ASM Handbook, is on processing and service characteristics of tool steels.
Source: Tool Steels (G. A. Roberts and G. Krauss).