OPTIMIZED STEEL SELECTION FOR APPLICATIONS
IN PLASTICS PROCESSING

C. Ernst

Edelstahl Witten-Krefeld GmbH

Research Tool Steels

Oberschlesienstraße 16

D-47807 Krefeld

W. Pannes

Edelstahl Witten-Krefeld GmbH

Q uality Department Tool Steels

Auestraße 4

D-58452 Witten

Abstract

A survey on the material properties and examples for the application of

some precipitation hardening tool steels in plastics processing are presented.
The new steel group "Thyroplast PH" offers an alternative to the conven-
tional hardened and tempered plastic mould steels for applications either as
high-volume tools with improved mechanical and corrosion properties or as
low-volume tools with improved machinability. After solution annealing and
aging the steels of this group reach a hardness of up to 42 HRC and show
an improved compressive strength. By modifying the chemical composition,
especially by lowering the amount of carbon, weldability and electrical dis-
charge machinability are significantly improved. Sulphur additions help to
advance machinability.

In detail, the new materials show the following specific properties: Thy-

roplast PH 42 FM combines good machinability and high hardness. It is rec-
ommended for building mould frames and backup plates with high demands
on strength as well as for hot runner systems. By remelting, Thyroplast PH 42
SUPRA obtains its high degree of purity which grants excellent polishability

321

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6TH INTERNATIONAL TOOLING CONFERENCE

and suitability for texturing. Examples for application are plastic injection
and compression moulds. The corrosion-resistant sulphurized steel Thyro-
plast PH X FM with improved machinability is used for the production of
mould frames, constructional parts and plastic moulds with low requirements
on polishability which have to show resistance to condensation and cool-
ing waters. Thyroplast PH X SUPRA with improved corrosion resistance
and toughness is favoured for building plastic mould inserts which are under
corrosive attack by the plastics to be processed.

Keywords:

Plastic mould steel, precipitation hardening, heat treatment, hardness, machin-
ability, toughness, corrosion resistance, weldability, typical application

INTRODUCTION

Plastics as innovative materials have been conquering more and more

fields of application and have successfully substituted many traditional metal-
lic materials. Worldwide in the year 1998 a total amount of more than 150
million tons of plastics was produced, a third of it in Europe Fig. 1. The
substitution of traditionally metallic compounds, especially in the vehicle
industry, is one of the most important reasons for this rapid growth. As an
example, the use of plastics in car building has increased from 7 % to 12
% during the past 25 years [1]. Besides of this application many household
articles, parts of electronic devices such as computer houses, furniture and
containers for food and beverages are made of plastics.

Plastic products are manufactured by mass production using thermoplas-

tic and duroplastic bulk materials which are processed by injection moulding,
blow moulding and compression moulding. The intensive increase in pro-
duction and consumption of plastics has also influenced the tool steel market
with its demand for increasing amounts and good availability of tool steels.
The described field of application therefore is an important market segment
for the producer of these steels. The tool steel suitable for the different ap-
plication in plastics processing have to show specific properties and need
continual research and development.

In the production of plastic items via extrusion, injection, blowing, com-

pressing and also via deep drawing, the mould is of main significance for the
efficiency of the process as the costs for the mould influence the production
costs in a fundamental way. Extremely important for the tool builder and the
tool user are an economic production of the tools and a high lifetime of the
mould. On selecting the right plastic mould steel grade from the wide variety

Optimized Steel Selection for Applications in Plastics Processing

323

Figure 1.

Production of plastics worldwide, in Europe and in Germany.

of steels available the different requirements on the tools which depend on
the type of application have to be known and respected. The most important
properties of plastic mould steels are wear resistance, hardness, corrosion
resistance, toughness, polishability, texturizing properties, weldability and
machinability. It is difficult to find these properties combined in one steel
grade as some of them counteract each other.

According to their heat treatment the tool steels suitable for plastic mould-

ing can be divided into four main groups which are listed in Table 1. Most
plastic mould steels are delivered in the soft annealed condition with a max-
imum hardness of 250 HB equivalent to 850 N/mm

2

. In this condition, the

best machinability is guarantied. A large disadvantage of these steels is the
additional heat treatment that has to be carried out by the tool builder after
the machining process. Today more and more large parts, e.g. components
of car bodies, are made of plastics. The use of case-hardening steels for large
tools is not recommended as they cannot provide the required properties over
the whole cross-section. Here the use of quenched and tempered steels is the
solution. They are completely heat treated by the steel producer and reveal
a strength of approximately 1000 N/mm

2

in as-delivered condition. The

great advantage is that tools made of quenched and tempered steels do not
bear the risk of distortion due to heat treatment after tool building but their

324

6TH INTERNATIONAL TOOLING CONFERENCE

machinability is decreased compared to the steels machined in soft-annealed
condition.

Table 1.

Steels for plastic moulding, an overview

Group

Example

Mat.-№.

Delivery condition and delivery

hardness

Heat treatment &

hardness

Quenched and

tempered steels

1.2311

Q + T, 280 - 325 HB

not necessary

1.2312

Q + T, 280 - 325 HB

not necessary

1.2711

Q + T, 280 - 325 HB

not necessary

1.2738

Q + T, 280 - 325 HB

not necessary

1.2316

Q + T, 280 - 325 HB

not necessary

1.2085

Q + T, 280 - 325 HB

not necessary

Through-

hardenable

steels

1.2343

soft annealed, < 230 HB

46 – 52 HRC

1.2379

soft annealed, < 250 HB

58 – 62 HRC

1.2767

soft annealed, < 260 HB

50 – 56 HRC

1.2842

soft annealed, < 220 HB

58 – 60 HRC

1.2083

soft annealed, < 230 HB

50 – 56 HRC

1.2361

soft annealed, < 265 HB

50 – 58 HRC

Case hardening

steels

1.2162

soft annealed, < 210 HB

surface 60 HRC

1.2764

soft annealed, < 250 HB

surface 60 HRC

Nitriding steels

1.8521

Q + T, 230 - 290 HB

surface 65 HRC

1.8550

Q + T, 230 - 290 HB

surface 68 HRC

Precipitation

hardening

steels

PH 42 SUPRA

sol. annealed + aged, 38 – 42 HRC

not necessary

PH 42 FM

sol. annealed + aged, 38 – 42 HRC

not necessary

PH X SUPRA

sol. annealed + aged, 38 – 42 HRC

not necessary

PH X FM

sol. annealed + aged, 32 – 38 HRC

not necessary

In this connection, the element sulphur has gained large importance in the

past. Sulphur is added to low alloyed steels and to corrosion resistant steels to
improve machinability as it has a chip breaking effect. If the tool to be build
needs a lot of machining, the sulphur alloyed grade 1.2312 (40CrMnMoS8-
6) for example is an appropriate steel with improved machinability. Due
to the addition of sulphur, inclusions of MnS are formed which on the one
hand improve machinability but on the other hand reduce the toughness of
the steel. Further in the nineties calcium treated, quenched and tempered
plastic mould steels without sulphur addition were developed that showed
good machinability and better isotropy of mechanical-technological proper-

Optimized Steel Selection for Applications in Plastics Processing

325

ties [2]. The favourable machinability of the calcium treated steels is based
on the formation of a layer consisting of calcium compounds on the cut-
ting edge. At high cutting speeds, the calcium compounds become soft and
semiliquid so that the cutting edge is protected with a lubricant, wear pro-
tecting layer. Nowadays, calcium treatment for machinability improvement
of plastic mould steels is state-of-the-art.

Constantly increasing demands in particular on the efficiency and dura-

bility of the plastic moulds applied have motivated Edelstahl Witten-Krefeld
GmbH (EWK) to include mould steels into the production programme which
have a higher hardness in the as-delivered condition compared to conven-
tional hardened and tempered steels and which at the same time still show
a good machinability Table 1. These materials are of a group of steels with
completely different heat treatment compared to the quenched and tempered
steels. The EWK short name "Thyroplast PH" characterizes this group of
precipitation hardened steels which is divided into corrosion-resistant and
non corrosion-resistant steels each with highest degree of purity (condition
SUPRA) or additionally improved machinability (condition FM).

QUENCHED AND TEMPERED STEELS VERSUS PH
STEELS

To meet the demands on the plastic mould steels a heat treatment proce-

dure before or after the machining of the tools is necessary. This procedure
involves changes in the microstructure to achieve certain properties which
are important either for the following manufacturing steps, such as polishing
or texturing, or for the application of the tools. In general the type of heat
treatment depends on the chemical composition of the steels Fig. 2. Carbon
containing, alloyed steels which are mainly used for making plastic moulds,
are quenched and tempered (heating up to hardening temperature, holding at
this temperature and quenching with sufficient speed to achieve a martensitic
transformation, followed by one or several tempering steps).

The less commonly known nickel alloyed steels with a low carbon content

need a precipitation hardening procedure, consisting of a solution annealing
to dissolve precipitated particles and of an aging procedure aiming at a
renewed precipitation of metallic or intermetallic particles with smaller size
and fine distribution. Besides of other changes in the material properties an

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6TH INTERNATIONAL TOOLING CONFERENCE

Figure 2.

Classification according to the heat treatment.

increase in tensile strength R

m

and in hardness are the results of this heat

treatment.

Of the group of quenched and tempered plastic mould steels which do not

need an additionally heat treatment by the tool builder especially the low al-
loyed steels with material №1.2311 (40CrMnMo7), 1.2312 (40CrMnMoS8-
6), 1.2711 (54NiCrMoV6) and 1.2738 (40CrMnNiMo8-6-4) have found
widespread use. These steels are usually applied in a highly tempered con-
dition with a hardness between 280 and 325 HB. Due to their low contents
of alloying additions they show a high thermal conductivity. A further ad-
vantage which helps to decrease the total production costs of a plastic mould
is the good machinability which is achieved either by a calcium treatment
in the steelworks or by additions of sulphur in a range of up to 0,10 %. To
obtain a hardness increase by the formation of martensite, these steels are
alloyed with carbon. The interstitial carbon is needed for the distortion of
the crystal lattice on changing from the austenitic to the martensitic phase.
It also forms carbides which are precipitated pre-eutectically and during the
tempering process. According to the necessity of repair welding or welding
operations resulting from changes in the design of the tool the hardened and
tempered tool steels are welded although the weldability is reduced by their
carbon content. Increasing contents of carbon are responsible for a decrease
in weldability as a distinct rise in hardness in the heat-affected zone due to

Optimized Steel Selection for Applications in Plastics Processing

327

martensitic transformation takes place during cooling from welding temper-
ature [3]. In these hardened areas, cracking of the welding seam is likely to
happen if no additional heat treatment is carried out after welding.

Compared to the described hardened and tempered tool steels, precip-

itation hardened plastic mould steels which are aged at temperatures be-
tween 480

◦

C and 550

◦

C show an increased as-delivered hardness of 38 to

42 HRC Fig. 3 and therefore a better wear resistance [4]. The high com-
pressive strength and the resistance against surface deformations leads to a
good stability of the clamping edges of the mould. Despite of their higher
as-delivered hardness precipitation hardened steels can still be machined
reasonably. Due to their low carbon contents they show improved weld-
ability. In contrast to the hardened and tempered steels the hardness of the
unaffected material and the welded area is not very different. The risk of
cracking after welding operations therefore is reduced.

Figure 3.

Comparison of hardness, quenched and tempered steel and PH-steel.

NON CORROSION-RESISTANT STEELS OF TYPE
THYROPLAST PH 42

The steels of type Thyroplast PH 42 are medium alloyed materials on

the basis of 3 % nickel, 1,5 % manganese, 1 % aluminium and 1 % copper.
Thyroplast PH 42 FM as an alternative to the well-known steel 1.2312 ad-
ditionally is alloyed with more sulphur to promote machinability. The steel
Thyroplast PH 42 SUPRA which always is delivered in remelted condition, is

328

6TH INTERNATIONAL TOOLING CONFERENCE

an alternative to the steels 1.2311 and 1.2738, combining a high as-delivered
hardness of 38 to 42 HRC with excellent polishability Table 2.

Table 2.

Steels of type Thyroplast PH 42

Standard grade

Alternative

∗

Chemical composition in %

C

Mn

Ni

Cu

Al

S

THYROPLAST 2311

PH 42 SUPRA

0.15

1.5

3.0

1.0

1.0

0.003

THYROPLAST 2312

PH 42 FM

0.15

1.5

3.0

1.0

1.0

0.100

∗

Higher hardness
Improved weldability
Improved polishability (SUPRA)
Improved machinability (FM)

To investigate one of the most important properties of a plastic mould steel

which is machinability, drilling tests with twist drills made of high speed
steel 1.3207 (HS10-4-3-10) without cooling were carried out. The tests
were based on Stahl-Eisen-Prüfblatt SEP 1161 ("Tool life test at elevated
temperatures"). As a criterion for tool life, the failure of the cutting edges
was employed. In Fig. 4 the tool life versus the cutting speed is shown.
For both steels without sulphur addition, steel 1.2311 and Thyroplast PH

Figure 4.

Machinability investigated in drilling test.

Optimized Steel Selection for Applications in Plastics Processing

329

42 SUPRA, a similar tool life was determined although the hardness of
Thyroplast PH 42 SUPRA is approximately 10 HRC higher. Drilling of the
steels with sulphur addition, steel 1.2312 and Thyroplast PH 42 FM, leads to
a longer tool life. Here especially the conduct of Thyroplast PH 42 FM has
to be emphasized as it also has a higher hardness compared to the quenched
and tempered steel 1.2312.

The ideal applications for the sulphur-alloyed steel Thyroplast PH 42 FM

are mould frames, base plates and tool components with an increased amount
of machining and at the same time high demands on stability as well as on
firmness of the edges. An example for such tools are manifold blocks for hot-
runner systems in injection moulding Fig. 5. On the other hand, Thyroplast
PH 42 SUPRA shows a minimal sulphur content and on principle is supplied
in remelted condition which grants an outstanding polishability. This plastic
mould steel therefore especially can be applied for small and medium size
plastic moulds and mould inserts with high demands on polishability.

Tools for the processing of plastics are occasionally repair welded or

welding operations are carried out which result from changes in the design
of the tool. The weldability of tool steels which are used for the produc-
tion of plastic moulds likewise is an important property. To investigate the
influences of a welding procedure on the hardness and the microstructure,
small flat specimens of the steels Thyroplast PH 42 SUPRA and 1.2311
were produced, their condition being precipitation hardened or quenched
and tempered, respectively. By melting a seam via the TIG-process with an
amperage of 150 A, a welding process without preheating of the specimens
was simulated. The hardness in the cross-section of the molten and heat-
affected zones was determined by Vickers hardness measurements Fig. 6.

In steel 1.2311 a new hardened zone can be detected which is caused by

the high temperatures and results from the formation of martensite. This
zone is followed by a tempered zone with a significantly decreased hardness
compared to the unaffected material. To avoid cracking and problems during
polishing and structural etching, a heat treatment is necessary by all means
to adjust the different hardnesses and microstructures. In contrast to that,
the steel Thyroplast PH 42 SUPRA only shows little variation in hardness
after welding. In the heat-affected zone, the welding temperatures lead to an
"overaging" effect which causes a slight decrease in hardness. Altogether,
the differences in hardness between the unaffected material, the heat-affected

330

6TH INTERNATIONAL TOOLING CONFERENCE

(a) Manifold block

(b) Injection mould

(c) Blowing mould

(d) Injection nozzle

Figure 5.

Typical applications of steels Thyroplast PH.

zone and the molten zone are clearly much lower compared to steel 1.2311,
so that a post-weld treatment is not necessary.

A concluding summary of the properties of the precipitation hardenend

plastic mould steels Thyroplast PH 42 SUPRA and Thyroplast PH 42 FM
compared to the hardened and tempered standard tool steels 1.2311 and
1.2312 is given in Table 3.

Optimized Steel Selection for Applications in Plastics Processing

331

Figure 6.

Simulated welding test, hardness after welding.

Table 3.

Comparison of properties

hardness

wear-

resistance

polish-

ability

photo-

etchability

machin-

ability

weld-

ability

Thyroplast 2311

+

+

+ +

+ +

+

+

Thyroplast PH 42 Supra

+ +

+ +

+ + +

+ + +

+

+ + +

Thyroplast 2312

+

+

+

O

+ +

+

Thyroplast PH 42 FM

+ +

+ +

+

O

+ + +

+ + +

CORROSION-RESISTANT STEELS OF TYPE
THYROPLAST PH X

Two further developments concerning corrosion-resistant plastic mould

steels are worth mentioning. As an alternative to the carbon alloyed tool
steels 1.2316 (X36CrMo17) and 1.2085 (X33CrS16) which are generally
used in their as-delivered condition with a hardness of around 300 HB two
precipitation hardened plastic mould steels are now available Table 4. Sim-
ilar to the non corrosion-resistant steels described above, these steels are a

332

6TH INTERNATIONAL TOOLING CONFERENCE

remelted steel with increased polishability named Thyroplast PH X SUPRA
and a sulphur-alloyed steel with improved machinability, Thyroplast PH X
FM.

Table 4.

Steels of type Thyroplast PH X

Standard grade

Alternative

∗

Chemical composition in %

C

Mn

Ni

Cr

Cu, Nb

S

THYROPLAST 2316

PH X SUPRA

0.05

0.5

4.5

15.0

+

0.003

THYROPLAST 2085

PH X FM

0.04

1.3

0.5

11.5

+

0.100

∗

Higher hardness
Improved corrosion resistance
Improved toughness (SUPRA)
Improved machinability (FM)

Thyroplast PH X SUPRA is a corrosion-resistant tool steel for plastic

moulds and mould inserts which have to meet highest demands on corrosion
resistance and hardness. Its chemical composition is based on the American
steels according to AISI 15-5 PH and AISI 17-4 PH, steels with a carbon
content below 0,07 % that originally were used in applications where high
strength, toughness and moderate corrosion resistance were required. These
steels especially were used for building components for the aerospace, nu-
clear and naval industry [5, 6]. Accordingly Thyroplast PH X SUPRA offers
some advantages compared to standard quenched and tempered steels such
as 1.2316: a higher hardness, an improved polishability, better weldability
and corrosion resistance as well as an excellent toughness.

Properly heat treated, that means solution annealed at 1020

◦

C and aged at

around 500

◦

C , Thyroplast PH X Supra consists of a lath martensite matrix

into which copper- rich epsilon-phase and fine Nb-containing carbides are
embedded [7]. The aging results in a marked hardness and strength increase
above that observed in the solution annealed condition. Values of 40 HRC
equivalent to 1260 N/mm

2

are reached. This strength increase is not accom-

panied by any significant decrease in the tensile ductility. Moreover, another
big advantage of Thyroplast PH X SUPRA compared to the standard tool
steel 1.2316 is its improved toughness at higher hardness levels [8]. Typi-
cal values that were determined in impact bending tests applying unnotched
specimens are distinctly higher than 250 J Fig. 7.

Optimized Steel Selection for Applications in Plastics Processing

333

Figure 7.

Toughness of steel Thyroplast PH X SUPRA.

Especially the corrosion resistance of Thyroplast PH X SUPRA is dis-

tinctly improved compared to steel 1.2316. The results from dipping tests in
different corrosive agents such as acetic acid, nitric acid, hydrochloric acid
and sulphuric acid are included in Fig. 8 which shows the gravimetrically
determined mass loss rate of the specimens. In dependence on the acid ap-
plied, the mass loss rate of Thyroplast PH X SUPRA is one or more powers
of ten lower than the rate of steel 1.2316. Two of the reasons for Thyroplast
PH X SUPRA’s improved corrosion resistance are its higher chromium plus
nickel content and the lack of coarse carbide particles. Also, a passivation
effect of copper has been reported in literature for 15-5 and 17-4 PH steels
[5], as copper dissolves as Cu

+

and Cu

++

-ions, forming stable corrosion

products like Cu

2

O on the steel surface. Thus the pitting corrosion resistance

is improved by inhibiting the anodic reaction.

Thyroplast PH X FM is a new sulphur-alloyed steel with resistance against

condensation and cooling waters which offers an alternative to the quenched
and tempered steel according to mat.-№. 1.2085. Typical for Thyroplast PH
X FM is its much lower carbon content of 0,04 % and lower chromium con-
tent of 11,5 % compared to steel 1.2085, resulting in a higher chromium/carbon

334

6TH INTERNATIONAL TOOLING CONFERENCE

Figure 8.

Corrosion resistance of Thyroplast PH X.

ratio. As can be seen from the results of dipping tests in artifical sea water,
this ratio has a positive effect on the corrosion resistance Fig. 9. Also, re-
sistance against humid conditions, as was tested in an atmospheric chamber
where dry and humid conditions were alternated with each other for two
weeks’ testing time, was found to be improved.

Figure 9.

Sample surfaces after test in artificial sea water.

Optimized Steel Selection for Applications in Plastics Processing

335

Due to its sulphur content, the steel shows improved machinability. This

property was investigated in drilling tests with twist drills made of high speed
steel 1.3343. In this test, holes with a depth of 24 mm were drilled (cutting
speed 12 m/min, feed 0,12 mm/r, under cooling). The wear at the cut-
ting edges as well as the flank wear were determined using a microscopical
measuring system. The steels 1.2085 in quenched and tempered condition
(hardness 325 HB) and Thyroplast PH X FM in precipitation hardened con-
dition (hardness 380 HB) were used as base materials for drilling. Figure 10
shows the highest and lowest wear that occurred at the edges and flanks. In
spite of its higher hardness, wear at the cutting edges on drilling steel Thy-
roplast PH X FM is comparable to the wear that appears when steel 1.2085
with lower hardness is drilled. The flank wear is significantly reduced. A
similar behaviour was found in milling tests where gravers made of steel
1.3207 were applied as tools. Here the wear of the cutting edges was twice
as high for the steel 1.2085 after a cutting length of 2,5 m compared to the
steel Thyroplast PH X FM.

Due to its properties, Thyroplast PH X FM in particular is suitable for

making mould frames for multicavity dies with increased demands on cor-
rosion resistance (marine climate, cooling water, condensation water) and
machinability. A comparison of the properties of the new precipitation hard-
ened steels is given in Table 5.

Figure 10.

Wear of tools in drilling tests.

336

6TH INTERNATIONAL TOOLING CONFERENCE

Table 5.

Comparison of properties

hardness

wear

resis-
tance

corrosion

resis-
tance

polish-

ability

machin-

ability

weld-

ability

Thyroplast 2316

+

+

+ +

+ +

O

+

Thyroplast PH X Supra

+ +

+ +

+ + +

+ + +

O

+ +

Thyroplast 2085

+

+

+

+

+ +

+

Thyroplast PH X FM

+ +

+ +

+

+

+ + +

+ +

SUMMARY

The new steel group "Thyroplast PH" which shows a higher tensile and

compressive strength in as-delivered condition offers an alternative to the
conventional quenched and tempered plastic mould steels. After solution
annealing and aging the four steels from this group reach a hardness of up
to 42 HRC, a heat treatment after machining by the tool builder is not nec-
essary. By modifying the chemical composition, especially by lowering the
amount of carbon, weldability and electrical discharge machinability were
significantly improved. Sulphur additions help to advance machinability.

In detail, the new materials show the following specific properties: Thy-

roplast PH 42 FM combines good machinability and high hardness. It is
recommended for building mould frames and backup plates with high de-
mands on strength. By remelting, Thyroplast PH 42 SUPRA obtains its
high degree of purity which grants excellent polishability and suitability for
texturing. Examples for application are plastic injection and compression
moulds. The corrosion-resistant sulphurized steel Thyroplast PH X FM
with improved machinability is used for the production of mould frames,
constructional parts and plastic moulds with low requirements on polisha-
bility which have to show resistance to condensation and cooling waters.
Thyroplast PH X SUPRA with improved corrosion resistance and tough-
ness is favoured for the production of plastic mould inserts which are under
corrosive attack by the plastics to be processed.

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Optimized Steel Selection for Applications in Plastics Processing

337

[2] H.-J. BECKER, E. HABERLING, P. OTTER, „Eigenschaften calciumbehandelter Kun-

ststoffformenstähle, Thyssen Edelstahl Techn. Ber. 15 (1989) 2, p. 95 – 101.

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ERNST,

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[6] H. KREBS, "Entwicklungsstand und Tendenzen bei ausscheidungshärtbaren Edel-

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