General Design Guidelines
Uniform hardness (approximately 32 HRc) from surface to core assures that strength and hardness at the mold center are the same as at the surface. Exceptional toughness reduces cracking problems while increasing flexibility in mold design
PX5 is substantially more stable than common P20-type steels. Since it has a unique heat treating process, it does not have the stresses inherent in typical quenched and tempered steels. PX5 never needs stress relieving, even after heavy machining. It has excellent dimensional stability and consistency during the machining process, and during the heating and cooling cycles of injection or compression molding.
Use of PX5 assures the longest cutting tool life of any 30 HRc, P20-type material and an overall 20-30% improvement in machining efficiency.
Note: Positive, effective cutting rake angles are recommended, as are inserts with concave faces and chip breaking edges. TiAlN coatings work well.
A recent machining test was performed on PX5 material at a mold base manufacturer. Listed below are the tool settings for P20 and the results achieved with PX5.
Tool No. 1
Program time was reduced from 38 minutes to 15.2 minutes
Tool No. 2
Program time was reduced from 5.7 minutes to 2.85minutes
Tool No. 3
(Finish cutter) 1.000 Diameter Iscar Ballnose Endmill
Tool No. 4
Program time was reduced from 10 minutes to 4.85 minutes
There was no noticeable wear in any of the cutting tools using PX5.
A 40″ x 41″ x 87″ block of PX5 was forged and heat treated. The piece was cut through 61″ into the 87″ length. The following hardness readings were taken across the face of the test piece.
Sectional Hardness Rockwell C Scale:
The recast layer from EDM for PX5 is soft, approximately 70% of that produced with typical chrome-moly steels. Because the EDM white layer must be removed, the subsequent stoning or grinding of PX5 is much easier than with other steels. There is also a significant reduction in the incidence of problems involving the hardened layer, such as surface layer cracking or peeling. Consequently, no post EDM stress relieving is needed.
Hardness of EDM Affected Area
PX5’s exceptional cleanliness, uniform microstructure, and uniform through hardness facilitate excellent and consistent surface finishing characteristics. PX5 polishes faster and easier to a superior mirror finish than common P20-type steels. PX5 will polish to a 6000-7000 grit finish, while P20 polishes to only a 5000 grit finish.
PX5 is an excellent steel for photo etching. Low chemical segregation of the alloying elements results in a clean, homogenous steel. Absolutely no etch unevenness will occur due to chemical segregation. PX5’s uniform hardness and refined grain structure also provide a consistent surface condition for texturing. A reduction in work time and cost can be anticipated due to the elimination of etch unevenness problems.
Ion-nitriding increases wear resistance and creates a hard surface ideal for slides or molds which will be molding abrasive or mineral-filled thermoplastics. PX5 can be ion-nitrided to produce a surface hardness over 60 HRc without distortion or dimensional changes. This ion-nitrided surface also improves part release and corrosion resistance.
It is essential that no rod other than PX5 should be used in all welding situations. All other rods are incompatible with the base metal chemistry of PX5 and will produce unacceptable results.
In most cases, PX5 can be welded with no pre- or post-heating procedures. However, this is not true in all situations. We suggest that in the event a polished surface must be welded (such as a lens or chrome-plated parts), pre-heat the block to between 650-900°F. Weld with PX5 rod, then post-heat to between 1040-1050°F. Final draw temperature for PX5 mold steel is 1117°F.
DO NOT, under any circumstances, exceed final draw temperature.
On a textured surface, the need to pre- and post-heat is determined by the etching method used by the grain source. If the selected grainer uses the immersion process with a nitric acid-based solution, then there is generally no need for pre- or post-heating procedures. If the grainer uses a ferric chloride flow technique, it is recommended that a full pre- and post-heating procedure be performed prior to texturing as outlined above.
We recognize that many mold welders apply heat locally with torches as a means of pre-heating the welded area. While this is a commonly used procedure, and generally produces acceptable results on non-polished surfaces, it is not recommended for post-heating welded blocks. The result of localized heat of this type is an actual flame-hardening of the weld that will produce an inconsistent increase in hardness by as much as 10-12 points (Rockwell C).
1) Weld Cracking Sensitivity
Y-Split Cracking Test (determines cracking susceptibility)
PX5 exhibits no cracking in either the weld surface or interior.
Weld cracking will not occur as long as basic welding conditions and procedures are observed.
2) Weld Hardness PX5 has the lowest level of hardness in the heat-affected zone of any P20-type mold steel. This relatively low increase in hardness reduces post-weld cutting and grinding times (i.e., high-speed steel end mills can be used)
Hardness Distribution Around Weld
3) Photo Etchability
Photo Etch Quality Around Weld
5) Undercut Characteristics