STEELS FOR

MOLDS

Aspects related to Mold ManufacturingThe machining and polishing processes can take up to 80% of the total resources needed for the

manufacturing of molds and dies. And, for those molds that have long usage times and seldom get to

the end of their life cycle, the main items associated with the fi nal mold cost can be considered.

Amongst the conventional machining processes of a metal cavity, milling is the one responsible for

the manufacturing of the complex surfaces. Several aspects must be taken into consideration, such

as: cutting parameters, the material to be machined, the cutting tool, the cutting strategy and the

technological resources provided by the machine/tooling.

Another fundamental point in the manufacturing of molds are the fi nishing processes, especially

polishing and texturizing. The following document refers to some basic considerations so as to facilitate

the manufacturing operations of metal cavities.

MillingIn the milling process, the knowledge of techniques is necessary in order to minimize fl ank wear and to control vibrations, so as to avoid damages.

The cutting direction can be carried out both in concordance or in opposition. In the concordance mode milling, the cutting and travel forward

movements have the same direction, while in the opposition mode they have different directions (see Figure on the right).

• Cutting speed (Vc) – it infl uences the cutting tool wear, as it amplifi es the friction conditions, thus increasing the temperature at the cutting zone, resulting from diffusion related phenomena to problems associated

with thermal and/or mechanical origin shocks.

• Travel forward per tooth (fZ) – with the increase in travel forward per tooth, greater mechanical strains occur, increasing the tool defl ection. With low travel forward there happens an increase of the machined path

by the cutting edge, causing a high fl ank wear. One aims here at an intermediate condition.

• Axial depth of cut (ap) – it is the direct factor responsible for the increase in cutting power, thus limiting the roughing process.

• Radial depth of cut (ae) – large radial increments(>50% of the tool diameter) increase the machined path for each cutting edge. However, they do improve the impact characteristic, directing the stress towards the

tool inner part. The opposite case occurs for small engagement conditions (<50% of the tool diameter).

• Number of Teeth (z) – a coarse pitch milling cutter generates lower power than a fi ne pitch one, keeping up the same travel forward per turn, since in the former the travel forward per tooth is greater than it is in the

latter, for the same chip removal rate. Table 1 shows a summary of the applications depending on the tool pitch.

Concordance mode milling Opposition mode milling

The cutting parameters exert the following infl uences:

Type of Milling Cutter Application

Coarse Pitch – few teeth. Roughing and semi-finishing of steel or where there is a

vibration trend.

Fine Pitch – greater number of teeth and spaces between

small teeth.

Cast iron cutting, light roughing and steel finishing.

Extra Fine Pitch – many teeth and bags for storage of very

small chips.

Interrupted cutting of cast iron and titanium alloys steel

finishing.

Some Additional Practical Recommendations Parameter Calculation

Cutting speed (in/min)

Travel Forward Speed (mm/

min)

Chip Removal Rate (mm/min)

nfzv zf ⋅⋅= fpe vaaQ ⋅⋅=vcd n

1000Where d = tool diameter (mm); rotation (rpm); z = number of teeth of the tool; fz = travel forward per tooth (mm/tooth); ae = working penetration (mm); ap = depth or width of the machining (mm)

Example of Cutting ParametersRecommended cutting speeds for milling facing with coated hard metal.

Material VP20ISO VP20ISOF VP20ISOFS VH13IM VP420IM N2711M VP50IM

Condition Machined Machined Machined Annealed Annealed Machined Machined

Hardness 32 HRC 32 HRC 32 HRC 200 HB 200 HB 40 HRC 40 HRC

Roughing* 120 to 150 120 to 150 170 to 190 180 to 260 180 to 260 80 to 110 100 to 150

Finishing** 220 to 240 220 to 240 260 to 280 220 to 300 260 to 300 100 to 150 110 to 160

* fz = 0.15 to 0.3 mm/tooth and ap = 2 to 4 mm Class : P25 – P35** fz = 0.05 to 0.2 mm/tooth and ap = 0.5 to 1 mm Class P10 – P20

Table 1- Summary of the applications as a function of the tool pitch.

• Roughen as much as possible with tools with large sharp corner radius.

• A surface must be fi nished with the largest tool possible.

• Machining should have a continuous contact of the milling cutter with concordance cutting and with a minimum of travel variation in relation to the milling line.

• It is recommended to use smooth approximation movements and always towards one cutting direction in hard-to-machine materials.

• It is important to make the tool stay in contact with the work as long as possible (in order to increase the useful life).

• During fi nishing or super-fi nishing, it is recommended to use small cutting depths. The ratio axial depth of cut (ap) and radial depth

of cut (ae) must be equal to or less than 0.2 (ap/ae < 0.2).

• Quite often it is advantageous to use the travel forward per tooth (fz) equal to the radial depth of cut (ae), with advantages in terms of machining time and lesser roughness (better fi nish).

• Invest time in the interpolation method to be applied, so as to reduce the machining time and improve the fi nishing conditions of the machined surface.

STEELS FOR MOLDS USED IN THE PLASTIC

VILLARES METALS VP20ISO VP20ISOFS VP50IM VP100 N2711M

SIMILAR ALLOYS Wnr 1.2738 Wnr 1.2312 NO SIMILAR STEEL NO SIMILAR STEEL Wnr 1.2711 MOD.

CHEMICAL COMPOSITION (%)C 0.36 – Mn 1.60 - Cr 1.80 –

Mo 0.20 – Ni 0.70C 0.36 – Mn 1.60 – Cr 1.80 –

Mo 0.20 S 0.06C 0.15 - Cu 1.00 – Ni 3.00 – Mo 0.30 –

Al 1.00 – S 0.10Cr-Ni-Mn

C 0.56 – Mn 0.70 – Cr 0.70 –Mo 0.30 Ni 1.65 V 0.075

CHARACTERISTICS

Steel supplied as machined. It has good polishability and response on texturization. It has improved machinability throughspecial melt shop treatment. For improvement of the wear resistance it can be nitrided or cemented.

Steel supplied as machined. Excellent machinability. Good response to nitriding. Not recommended for parts that require texturization processing, chromium plat ing and higher polishability.

Tool steel for molds especially developed to be hardened via ageing heat treatment, with resistance higher than the VP20. It has excellent polishing and texturizing properties. It has an excellent weldability.

Steel made under vacuum degassing, having an improved machinability through Calcium inclusion treatment. The main benefits are: high uniformity in hardness with a less than 2 HRC variation along the whole part’s cross section. Excellent weldability. High polishability (except mirror finish) and high reproducibility ofperformance and manufacturing.

Steel supplied as machined. It has good polishability and response on texturization. For improvement of the wear resistance it can be nitrided or cemented. Due to the as supplied high hardness, special care in machining is necessary, particularly boring.

SUPPLYING STATEHardened and tempered

(30 - 34 HRC)Hardened and Tempered (30-34 HRC)

Solubilized and aged (40-42 HRC) or Solubilized (330 HB max)

Supplied as machined (285 to 321 HB) Hardened and Tempered (38-42HRC)

RECOMMENDED HEAT TREATMENT

- -Ageing (in case it is supplied in the

solubilized state)- -

MOLD HARDNESS TYPICAL RANGE

28 - 37 HRC 28-37 HRC 38-42 HRC 30-34 HRC 38 - 42 HRC

POLISHABILITY Medium to High Low High High Medium to High

CORROSION RESISTANCE Low Low Low to Medium Low Low

RESPONSE TO NITRIDING Medium Medium High Medium Medium

RESPONSE ON TEXTURIZATION Medium to High Low High Medium to High Medium to High

WELDABILITY Medium Medium High High Medium

WEAR RESISTANCE Low to Medium Low to Medium Medium Low to Medium Medium to High

STEEL IDENTIFICATION COLOR Lavender – Black – Lavender Green-Brown- Green Lavender-Gold-Lavender Red-Blue-Red Green-Yellow

APPLICATIONS

Molds for injection and extrusion of non-chlorinated thermo-plastics and little abrasive. Large dimension molds.

Molds for injection and extrusion ofnon-chlorinated and little abrasive thermo-plastics that have low polishability requirements. Bases and structures for plastic molds. Cores for injection molds.

Molds for injection and extrusion of non-chlorinated thermo-plastics. Molds for load reinforced thermo-plastics.

Mold carriers, molds for non-chlorinated plastic injection, dies for non-chlorinated plastic extrusion, molds for blowing, hot chambers, when a high corrosion resistance is not necessary and several applications in molds for plastics. Not recommended for applications in which toughness and mirror finish are design requirements.

Molds for injection of non-chlorinatedthermo-plastics. Molds for blowing.Extrusion dies for non-chlorinatedthermo-plastics.

The classification Low, Low to Medium, Medium to High and High are for guidance, and serve as a comparative guide just for this group of steels.* Due to the need to break the passive layer (stainless steels), it is recommended to use the ion plasma nitriding process.

C AND GLASS INDUSTRY

VP ATLAS VH13IM VP420IM V630 VIMCOR

- AISI H13 Wnr 1.2344AISI 420

Wnr 1.2083AISI 630

Wnr 1.4548NO SIMILAR STEEL

Cr – Mo – MnC 0.40 – Si 1.00 – Cr 5.20 – Mo 1.50 – V

0.90 – P 0.015 max - S 0.003 maxC 0.40 – Si 0.80 – Cr 13.5 – V 0.25

C 0.035 – Cr 15.40 – Ni 4.40 –Cu 3.50 – Nb+Ta 0.25

C 0.05 – Mn 2.5 Cr 12 –Si 0.40 – S 0.1

Steel supplied as machined. It has good polishability and response on texturization. For improvement of the wear resistance it can be nitrided or cemented. Due to the as supplied high hardness, special care in machining is necessary, particularly boring. However, it has machinability advantages when compared to the DIN 1.2711.

Tool steel used for applications in molds when hardness higher than VP20 and VP50 hardnesses are desired. It has an excellent polishing capacity.

Stainless steel after hardening and tempering. It has as its main advantage an elevated corrosion resistance, which allows working in humid environments..

Precipitation hardening stainless steel. It has the following excellent properties:- Dimensional and shape stability- Weldability- Corrosion resistance- Polishability and response on texturization

Resulfurized stainless steel, supplied as machined. It has an excellentperformance in machinability, mainly in deep boring. Properties:- Excellent machinability- Excellent weldability- Good corrosion resistance- Hardness homogeneity

Quenched and Tempered (38 – 42 HRC) Annealed (235 HB max)Annealed (200 HB max) or hardened

and tempered (30 – 34 HRC)Solubilized (38 HRC max) or solubilized

and aged (40 HRC max)Machined (290 – 330 HB)

- Hardening and TemperingHardening and Tempering(for the annealed material)

Ageing (in case it is supplied in the solubilized state)

-

38-42 HRC 42 - 52 HRC 48-54 HRC 24-40 HRC 29-34 HRC

Medium to High High High High Low

Low Low to Medium High High High

High High High * High* High*

Medium to High High High High Low

Medium to High Low to Medium Low to Medium High High

Medium to High Medium to High Medium to High Medium Low to Medium

Blue - White Green – Silver – Green Blue-Brown Silver-Red Black – White

Cr-Mo-Mn alloyed steel, with micro-additions, and already supplied in the machined state. Due t o its balanced chemical composition, this material has good machinability, weldability, polishability, good response on texturization and good response to nitriding. It is indicated for molds for injection of non-chlorinated plastics, especially for applications that need higher mechanical resistance and higher wear resistance than AISI P20 or DIN 1.2738 steels; dies for extrusion of non-chlorinated thermoplastics. Several applications in molds for plastic.

Molds for injection of non-chlorinated thermo-plastics for which a higher resistance to wear is required, coupled to good polishability. Molds for glass.

Molds for injection of chlorinatedthermo-plastics. Processing or storage in humid environment. Glass industry.

Tools for thermoplastic forming, including corrosive processing (chlorinated polymer forming). It can be worked and stored in humid places.

Hot chambers, Cooling Plates, Mold Carriers. It can be worked and stored in humid atmosphere.

Mold PolishingPolishing is used in molds to meet various requirements of the injected part:

• Aesthetic requirements: gloss and transparency.

• Mechanical: to avoid nicks and breaks by fatigue or overload.

• Functional: ex.: optical devices (lenses).

Polishing is a stage that takes up time and resources. The average time spent in the manual polishing of large size molds is

around 300 to 400 hours per mold. Two observations are important in the evaluation of the mold surface quality. First, the surface

must have the correct geometrical form, without any waving. These are derived from recent machining operations. Second, the

evaluation of the mirror polished condition of the metallic mold is often carried out by visual comparison of the mold or of the

surface of the injected part, based upon the operator’s experience.

The fi nal quality of the polished surface of a steel depends on factors such as: the polishing technique, the type of tool steel and

the heat treatment applied to the material. In general, one can say that the polishing technique is the most important factor. A

typical example is shown on the graph below, the excessive polishing known as over polishing, caused by a mechanical cold work

of very fi ne layers on the mold surface. In over polishing, roughness increases with the increase in polishing time. The problem

is only solved with the removal of part of the surface (tenths of mm) through machining and the application of a new polishing.

Visually, the phenomenon appears as “orange peel”.

50

Tempo (min)

VA2 - 60 HRC

P20 - 32 HRC

Ru

go

sid

ad

e R

A (

mm

)

45

40

35

30

25

20

15

10

5

0

0 5 10 15 20 25

Graph showing the effect of excessive polishing (over polishing) for two steels with different hardness levels.

Roug

hnes

s RA

(mm

)

Overpolishing

Roughness RA (mm)

a) Conventional ASTM G 2.0

b) ISOMAX, ASTM 1.0 F

Type D inclusions

a) Level 2.0 acceptable for Conventional materials

b) Level 1.0, Typical of material made via ESR (Source: ASTM E 45)

For a good polishing, the mold surface must be free from scratches, pores, orange peel effect, pitting and pinholes. In Brazil, the problem of pitting and pinholes are commonly called “porosity”. Although visually correct,

the term porosity is erroneously applied here, as it should be used only for pre-existing voids in the material. In the case of pinholes or pitting observed after polishing, the problem is normally caused by a process not

well carried out (for a given steel and hardness), using steels with an inadequate level of nonmetallic inclusions or even by the combination of these two factors. Other possible sources of problems are surfaces with

electro-erosion defects or excessive machining cold work.

As to the inclusions, they can be mechanically understood as particles on the steel surface with hardness and ductility rather different from the metal. All steels have inclusions. However, the quantity and distribution

depend on the manufacturing process. For high polishing applications, therefore, remelting via ESR is recommended (ISOMAX® process). As shown in the scheme below, there is a signifi cant reduction of the level of

inclusions.

TexturizationThe response on texturization measures how easy it is to apply a texture to the tool steel used in the mold. The texturizing treatment is normally carried out by photo-etching differentially applied on the mold surface,

thus generating the “negative” of the fi nal desired aspect of the injected part.

The process control, in terms of the acid medium used and of the applied procedure, is essential for a good texturization result. As to the quality of the steel, similar polishing requirements are necessary: homogeneity

of microstructure and hardness, besides a high level of cleanliness as to nonmetallic inclusions.

4 mm

4 mm

2 mm

2 mm

Several examples of texturized surfaces in VP 100 steel, observed in stereoscope.

Junho/2

012

0800 19 05770800 707 0577

55 19 3303.8160

arnaldo.zangueri@villaresmetals.com.br

www.villaresmetals.com.br

Rua Alfredo Dumont Villares, 155

Jardim Santa Carolina - Sumaré / SP

CEP 13178-902