Many industries place strict requirements on the acceptable level of surface defects and imperfections that may appear on a plastic molded part. Such conditions often apply to critical components for the medical and pharmaceutical industries as well as for the manufacturers of lenses and other optical devices. However, for aesthetic reasons, many other consumer goods have similar restrictions. After all, any defect that appears on the surface of the mold steel is likely to be replicated onto the molded part.
Problems associated with the texturing or polishability of a mold cavity can often be traced back to the mold steelmaking process. The material properties that have shown the greatest influence on obtaining a good surface finish are the microcleanliness level, the severity of chemical segregation and the appearance of primary carbides.
The mold steel's chemical composition along with the manufacturing techniques used during its production, will determine its ability to perform well in a given service environment. State-of-the-art technologies such as specialized, remelting techniques, thermal diffusion treatments and high forging ratios all play a significant role in influencing the characteristics of mold steels.
The Limitations of "Off-the-Shelf" Mold Steels
Many grades of tool steel that are used for building molding components also are used for other industrial applications. For example, AISI S7 and H13 are commonly used for plastic injection molds; however, S7 also is used for metalforming operations and H13 for forging. The properties that are important for forging or stamping are quite different from the properties that are important to a moldmaker or plastics molder. Therefore, one must take precautions to ensure that they are using a mold quality steel. To do that one must consider the microcleanliness level of the mold steel, the degree of micro and macrosegregation and the restrictions on the number and size of large, primary carbides.
Standard Mold Steel Production
From the standpoint of the steel manufacturer, there are basically two means for improving existing steels used for molding applications:
(1) Adjusting the chemical composition. Adding specific alloying elements and balancing their levels can significantly influence the characteristics of the material.
(2) The actual steelmaking process. With the use of specialized melting techniques, mold steels can be produced which possess a very high microcleanliness level and homogeneous microstructure. These are two extremely important properties with regard to the steel's polishability and etching/texturing characteristics.
The Mold Steelmaking Process
In order to distinguish the different quality levels that are available for mold steels it is important to first have a fundamental understanding of the mold steelmaking process.
Mold steels are manufactured by melting starting material in an electric arc furnace (EAF). The starting material is comprised of carefully selected, low alloy scrap steel with the lowest possible level of impurities. Typically, the EAF units can melt as much as 50 tons of starting material per heat. Once the initial melting is complete, the molten steel is transferred to a ladle or refining vessel, where the composition of the steel is adjusted to provide the final chemistry. Additional steps such as slag treatments and degassing procedures also are involved to remove undesirable elements.
Following the refining stage, the molten steel is poured into large molds where it solidifies into a simple form called an ingot. These ingots will take several hours to completely solidify. This relatively long period of time will lead to significant amounts of chemical segregation, resulting in a variation in composition throughout the ingot's cross-section.
Segregation and Banding
During solidification of an ingot, the steel production process involves unavoidable segregation of the alloying elements. On a grain-size scale one talks of microsegregations; on an ingot-size scale they are referred to as macrosegregations. These inhomogeneities will exert a negative effect on the polishability and texturing characteristics of the mold, as previously discussed. In addition, the toughness properties will be degraded, particularly in the direction that is transverse to the primary hot forming direction.
The solidification process begins with the formation of crystals within the melt. These crystals have a tree-like, branching appearance and are referred to as dendrites. The first dendrites that form in the molten steel have a relatively low carbon content. As the freezing continues, these dendrites will become surrounded with the remaining liquid steel that is comprised of a higher carbon level. The melt that then freezes around the original dendrites will therefore have a different chemical composition.
If for example we examine an H13 steel - a material commonly used for molding applications - it is shown that segregation also occurs with regard to some of the other alloying elements. This variation in chemical composition will take place with respect to the chromium, molybdenum and vanadium additions. The variation in alloying element concentrations across a sample of steel can be measured with the help of an electron beam microprobe analyzer. This laboratory technique uses an electron beam to scan the surface of a sample of mold steel and determine the distribution of alloying elements.
However, it also is helpful to use a much simpler method to visually show these variations. For example, when polished and treated with an acidic solution samples of an H13 steel will reveal their relative levels of chemical segregation. These alloying inhomogeneities appear in the form of bandings. That is, the banded areas or zones within the ingot are comprised of various concentrations of the steel's alloying elements.
Another concern is the precipitation and accumulation of primary carbides in the ingot core. This cannot be avoided in the conventional mold steel manufacturing process. After the ingot goes through the forging and rolling process, these carbides also will exhibit a banded structure. These regions will greatly reduce the transverse toughness properties and as the level of banding increases, the impact toughness of the mold steel will suffer.
To some extent thermal treatments and mechanical working such as forging and rolling, will remove the pattern of chemical and carbide segregation. However, the carbide networks will have a tendency to align themselves as stringers or banded areas that run parallel to the primary hot working direction.
When the carbides form as long stringers they will act as a stress riser within the steel. These regions will be more susceptible to brittle failure and the overall toughness properties of the steel will be degraded. In addition, this inconsistency is actually a variation in the mold steel's chemical composition. Therefore, it will have a detrimental effect on the polishability and texturing characteristics of the material.
A more homogeneous microstructure can be obtained by performing an additional re-melting step during the production of the material. An electro-slag remelted (ESR) mold steel possesses a more homogeneous microstructure. The lack of primary carbides improves the material's resistance to cracking and also provides for superior polishability and texturing characteristics.
Polishing of Mold Steels
Polishing, the final step in the mold building process, is highly dependent on the experience and technique of the polisher. However, if the material is not a "mold quality steel," it may be virtually impossible to bring it up to the required finish.
For example, AISI materials specifications for S7, H13 and 420 stainless steel have relatively high limits with regard to elements such as sulfur. This leads to the formation of undesirable particles, referred to as nonmetallic inclusions. These foreign particles are formed during the conventional mold steelmaking process and their origins may be traced back to the tramp elements found in the original starting material. During primary melting these elements will react with dissolved gases in the melt to form these impurities.
Since these impurities are essentially insoluble, they will precipitate out of the molten steel during the solidification process and form as hard particles. These precipitates are comprised of oxides, sulfides, silicates or other constituents. Such particles are inherently harder than the surrounding steel matrix. If the steel contains a large percentage of nonmetallic inclusions, problems may arise during the polishing operation.
Polishing consists of using an abrasive medium (stone, diamond paste, etc.) to remove a thin surface layer of steel and create a series of finer-and-finer "scratches" on the mold's surface. Therefore, during the polishing operation the softer steel matrix that surrounds an inclusion will be removed at a greater rate. Under such conditions there is a tendency to lift the inclusion particle out of the steel matrix and leave behind a void. Several problems are associated with polishing a mold steel that contains large, non-metallic inclusions.
The number of inclusions found in the steel is directly proportional to the level of tramp elements such as sulfur. Therefore, the higher the sulfur level, the greater the amount of sulfide stringers will be in the mold steel.
The presence of large or numerous nonmetallic inclusions often leads to problems during the polishing operation. At this stage a large amount of time and money has already been invested into the machining and heat treatment of the mold. Typically, very little time is available for the polishing operation and any problems at this point will jeopardize the production schedule. Therefore, in order to ensure that the mold steel possesses good polishability characteristics, it is necessary to consider its microcleanliness level.
Microcleanliness is a metallurgical term that is used to define the level of nonmetallic inclusions in the steel. As previously mentioned inclusions are compounds, such as sulfides, oxides or silicates that are insoluble within the steel matrix. During the forging and rolling process the inclusions will become elongated, forming stringers that tend to align themselves parallel to the primary hot forming direction. Their presence also will significantly degrade the steel's toughness properties due to their notch effect.
Any sulfur within the steel is insoluble and therefore, will precipitate out as sulfide particles. The sulfides are a specific type of non-metallic inclusion that lead to problems during the polishing and texturing stages of the mold building process. A greater degree of cleanliness will improve the overall toughness levels and in particular, the polishability properties of the mold steel. Improvements in the melting processes (e.g. ladle treatments) and the use of remelting techniques (e.g. ESR) are responsible for improving the cleanliness level.
Electroslag Remelting Process
As mentioned previously, solidification of a large volume of liquid steel over long periods of time will lead to phase separation and chemical segregation. In addition, conventionally cast ingots are prone to unevenly distributed non-metallic inclusions and internal defects such as porosity.
Furthermore, when examining the cross-section of an ingot one can identify two clearly distinct zones: (1) outer zone - which results from high temperature gradients that promote a transgranular grain pattern with a preferred orientation and (2) central zone - which is the last area to freeze and contains a higher level of alloying elements.
This pattern that develops within the ingot can only be avoided by carefully controlling the solidification process. That is, when the solidification is carried out in such a manner that crystallization develops at a steady rate from the bottom toward the top of the ingot.
In addition, if the steel is held just below its solidification temperature for the proper length of time solid state diffusion is possible. This condition will give the steel enough time to reach equilibrium, resulting in a relatively consistent composition throughout the remelted cross-section.
Such a carefully controlled solidification process is accomplished by a technique known as electroslag remelting (ESR). With a basic understanding of the conventional mold steelmaking process one can appreciate the benefits of producing these materials as a remelted quality.
The ESR process begins with an ingot that is produced in the aforementioned manner; however, in the ESR process the ingot is referred to as an electrode. The process consists of remelting this consumable electrode through a molten slag bath, using the slag to provide the required heat The remelting is initiated when the end of the electrode is brought into contact with the molten slag bath. The slag is contained in a stationary mold or a mold that can be raised as the ingot solidifies. The heat of the slag will begin to melt the electrode, creating droplets of molten steel. Since the molten steel has a higher density than the slag, it will pass through it and collect at the bottom of the mold where it will quickly solidify.
During the ESR process the portion of the remelted ingot that is in a liquid state, at any given time, is relatively small. This results in shorter solidification times and subsequently low microsegregation levels. The chemical composition will be more consistent through-out the remelted ingot and the alloy carbides that form are relatively small and more evenly distributed.
An added benefit of the ESR process is that the slag bath will extract nonmetallic inclusions from the melt as each molten droplet of steel passes through it. Therefore, the ESR process ensures a very clean material with a low sulfur content (.003 max). The lack of brittle sulfides in the steel greatly improves toughness properties and the ability to polish the steel to a high surface finish.
The advantages of the ESR process are the pattern-free material, low microsegregation levels in the ingot center, removal of sulfur to extremely low levels and good surface quality on remelted ingots.
Thermal Diffusion Treatments and Hot Forming
The improvements in mold steel quality are not only due to better melting techniques, but also due to the optimization of hot forming and heat treatment operations used during steel production. Even after a significant amount of hot forming (e.g., a 4:1 forging ratio), a mold steel may still exhibit a considerable amount of banding. Therefore, one of the most important process modifications for mold steels is an additional thermal homogenization treatment of the ingot, prior to the hot forming operation. By applying such treatments, the local concentration of elements such as carbon, chromium, molybdenum and vanadium are reduced. This additional thermal treatment plays an important role in lowering the segregation level of the steel.
Throughout the last several years, the plastics molding industry has taken a closer look at the quality level of materials used to build molding components. Difficulties with inconsistent etching patterns or pitting on the mold surface are frequently attributed to the melting and forging practices that are used during the manufacturing of the mold steel. The material properties that have demonstrated the greatest influence on such defects are the microcleanliness level, severity of chemical segregation and the appearance of primary carbides.
Additional operations such as thermal diffusion treatments, high forging ratios and carefully controlled remelting procedures are required to manufacture "mold quality" materials. Steel manufacturers that produce mold steels require state-of-the-art technologies to yield high cleanliness levels, remove undesirable elements and create a homogeneous material. It is the use of these practices that allows for the production of materials that possess good polishability and texturing characteristics.