Ageing test of various plastic mold steels in 5% boiling acetic acid. © DEW

The most important factors in material selection are the degree of machining, polishability, operating environment and wear resistance requirements. Which of the three materials is used is therefore determined by the customer's individual application.

Polishable all-rounder: Formadur 2083 Superclean

Formadur 2083 Superclean offers very good wear resistance after hardening and tempering to the desired case hardness, so that this material is also used for knives. The ESR remelted (electroslag remelting) plastic mold steel is also suitable for high polishability requirements and can be used in plastics processing for injection molding, compression molding and extrusion molding. This all-rounder falls slightly short in terms of weldability and nitridability, but delivers good values in the areas of corrosion resistance, toughness and machinability. Despite its good corrosion properties, there is no need to do without graining.

Low-carbon acid blocker: Formadur PH X Superclean

Logarithmic representation of the corrosion current density at different pH values. © DEW

The highly corrosion-resistant Formadur PH X Superclean has the advantage that it does not need to be additionally hardened and tempered, as the material already has a case hardness of 38 to 42 HRC after hot forming and heat treatment. Formadur PH X Superclean is a precipitation-hardenable tool steel: it has very high strength combined with good toughness. The material is primarily used for molds that need to be highly resistant to condensation and cooling water. Due to its high resistance, Formadur PH X Superclean is particularly suitable for processing plastics such as PVC or aminoplastics, as the molds are often attacked by acid deposits. This is proven by corrosion tests and ageing tests (see Fig. 1 for results). Thanks to its homogeneous, almost carbide-free structure, the ESR-coated special steel is the first choice in terms of polishability. If the user attaches great importance to weldability, Formadur PH X Superclean also shows its strengths, as it contains very little carbon.

Aging test according to ASTM G48-03, method A for 72 h at 22 °C in 6 % FeCl solution. © DEW

With Corroplast, all the desired properties are set during heat treatment. It is characterized above all by good machinability and machinability with consistently good corrosion behaviour. The balance between these two properties is unique in Corroplast compared to other tool steels. Only polishability has to be compromised. However, the latter property is also less important, as the main areas of application for Corroplast are base plates, body parts and plastic molds. The only standard requirements here are polishability and resistance to condensation and cooling water. Mold maintenance is extremely low when using Corroplast, which is primarily due to its high corrosion resistance. It can be said that the more difficult the climatic conditions, the better Corroplast feels. Even contact corrosion, such as between the mold frame and mold frame insert, has no effect on the material.

In plastics processing, steel is used in an environment characterized by heat, condensation and coolant. In addition to corrosion resistance, good machinability is the be-all and end-all for the material. Alloying elements such as chromium, manganese and sulphur therefore play an important role in the production of Corroplast. They are added during steel production and, in the optimum ratio, play a key role in determining hardness, machinability and corrosion resistance. The exact adjustment of the properties of Corroplast is ultimately achieved through targeted metallurgical treatment for sulphur distribution and, of course, during heat treatment. Here, the steel is heated to over 800 °C in a controlled manner and quenched in water or polymer. This treatment promotes the characteristic, very homogeneous structure of the tool steel.

Tests under laboratory conditions

In order to estimate the corrosion resistance and determine the appropriate test method, the user must know what type of corrosion attack is present. Corrosion behavior is a system property. Corresponding tests only provide a classification of the steels in the same test setup. In order to be able to evaluate the corrosion behavior of Corroplast, for example, several tests were therefore carried out on behalf of DEW under laboratory conditions in chloride-containing media, condensates and under slightly acidic conditions:

• Electrochemical tests using current density-potential curves in HCl pH 3 and HCl pH 5 (electrolyte solution) at room temperature,
• Constant climate test (condensation water test) according to DIN EN ISO 6270 and evaluation according to DIN EN ISO 10289,
• Aging tests according to DIN 50905, analogous media as for the electrochemical tests,
• Testing of Corroplast at Cambridge Material Testing Ltd. by means of aging tests according to ASTM G48-03.

Comparison of machinability in percent. © DEW

The sulphur-alloyed materials Formadur 2085 and Formadur 2312 were also examined for comparison. Some of the test results are presented below. Figure 2 shows the evaluation of the corrosion current density, which can be used as a measure of the corrosion rate. This showed that the corrosion resistance of Corroplast is comparable to that of Formadur 2085, but is still considered to be significantly more resistant than the conventional Formadur 2312. In the ageing tests according to DIN 50905, the ranking of the steels was confirmed, with the corrosive attack being almost the same for Corroplast and Formadur 2085. The results of the aging tests according to ASTM G48-03, Method A are shown in Figure 3.

This test also shows the same sequence. As expected, Corroplast has a slightly higher mass loss rate under these test conditions, i.e. it is attacked more than Formadur 2085. In the constant climate test, the evaluation is carried out using point and spot cards with a rating scale of 1 to 10, where 10 describes the best surface condition after the test. Both Formadur 2085 and Corroplast were rated 9. In the condensation water test, only the slightest differences were found between Corroplast and Formadur 2085.

The application determines the material

This table compares the properties of plastic mold steels. © DEW

The good machinability of Corroplast compared to the material Formadur 2085 is also confirmed by the tests carried out in five different machining processes: roughing, finishing, grinding, drilling and milling.

Lower processing costs and more efficient processing are the advantages for mold makers and end users of using the special steel Corroplast.

As can be seen from the individual property descriptions and corresponding laboratory tests, the choice of material is very important in the plastics industry. Even modern plastic mould steels are not "all-rounders", but their performance properties are optimized for specific applications - and the user should be familiar with these.

Kathrin Eger, Daniel Kipp, Dr. Philipp Niederhofer, Dr. Hans-Günter Krull, all Deutsche Edelstahlwerke / ag