Multiple Steel Choices
Suppliers offer a range of products for selective materials specification. Chief among these are metals and coatings that resist the aggressive process conditions of resins with reinforcements and additives, whose abrasive characteristics affect mold finish and surface details. Other materials are designed for use in components like locks, slide bodies, core pins, gibs and nozzles, based on the wear conditions and life cycles they encounter. One important category is heat dissipation during molding, with beryllium copper alloys and porous steel being the main materials offered.
Porous steel can be used as an insert in areas where there’s no easy way to vent hot gases. IMS has a grade called Porcerax II with micron-sized honeycombs that vent gases during molding. When placed in a mold as an insert near a hot spot, gases pass directly through the metal. Inserts also can be used with channels and exhaust lines bored through surrounding metal to speed heat evacuation.
According to Britton, porosity is achieved by applying a sintered powdered metal to the steel after it is formed. The pores come in sizes of 7 and 20 microns. The porosity of Porcerax II is 25 percent air by volume. Benefits of the inserts include elimination of short shots due to the absence of gas from previous molding cycles, faster molding cycles and fewer quality rejects. The material also reduces or eliminates flow and knit lines.
Porcerax II has a pretreated 35-40 HRC, which can be increased to 50 to 52 with heat treatment. The steel is easily polished and machined, though it requires special handling and finishing. Machining, for example, crushes the topmost layer of pores, but this can be fixed with EDM, which burns off the top layer and restores the honeycomb structure to its full venting capability.
One technique for thermal management is placing inserts of copper alloy in mold cavities and on components like hot runner nozzles and manifolds. Among companies supplying this metal is Brush Wellman Engineered Materials (Cleveland, OH), whose grades include MoldMAX HH, an alloy of 98.2 percent copper and beryllium. The 40 HRC grade has thermal conductivity that’s claimed to be almost four times greater than P-20 steel, six times higher than 420 stainless and roughly equivalent to some grades of aluminum. The result, says Doug Veitch, Director of Global Industrial Products, is fast mold cooling and reduced cycle times.
Veitch says the grade can be textured and polished. It resists abrasion when treated with a hard coating like Armoloy, a chrome material from Armoloy Corp. (DeKalb, IL), grades of which range from 78-98 HRC. The MoldMAX HH alloy has been specified for one undisclosed manufacturer’s plasma TV frames, where its thermal properties reduce warp and sink marks in the corners and yield faster cycle times.
Cell phone styles change every few months. Mold design must be flexible enough to change with them. Photo courtesy of Motorola.
Among the materials moldmakers are using to offset the aggressive processing conditions of some resins is Elmax, a through-hardened, corrosion-resistant, powder-metallurgy stainless steel alloy from Bohler-Uddeholm Corp. (Rolling Meadows, IL). Smith of Fairway Injection Molding Systems says the grade’s main benefit is hardness—58 HRC—which, with the addition of chromium, vanadium carbide and molybdenum, makes it extremely wear resistant, and ideal for use in inserts.
Bohler-Uddeholm claims the grade outperforms 440C stainless steel, due to its uniformity of composition in all directions. The material also has a high degree of dimensional stability and compressive strength. It is formulated for high volume molding of engineering plastics containing high levels of reinforcements, additives and fillers. Among the electronic parts it is recommended for are integrated circuits, connectors, resistors and switches.
Another stainless steel Bohler-Uddeholm developed for chemically aggressive resins is M303 Extra. The martensitic grade is highly corrosion-resistant due to its chromium (14.5%) and molybdenum (1%) content, and is formulated to outperform 1.2316, another high chromium steel. The company says the homogeneity of M303’s chemistry yields a major advantage over 1.2316, in that no delta ferrite forms in the matrix. Mechanical properties like fracture resistance are thus unaffected. M303 is pre-hardened to 32-36 HRC. It is wear-resistant, easily machinable and can be polished. It is primarily for prototyping and for short and medium production runs.
When it comes to steels designed solely for mold bases, Edro Engineering (Walnut, CA) claims its RoyAlloy grade offers a number of benefits over 420F stainless steel. These include more consistent levels of uniformity and hardness, improved dimensional stability during machining and molding, higher ductility, reduced brittleness, and better plate surface and appearance. Repairs are simplified by the steel’s high degree of weldability. The pre-hardened steel comes with a 32-36 HRC that can be raised to 40.
The grade, also a martensitic, isn’t new—it’s been available for a decade, says Mike Guscott, Vice President. But its development reflected current efforts at upgrading the economics of mold fabrication through the use of select materials that meet the needs
of individual applications. In this case it was high cavitation molds. Guscott says Edro developed RoyAlloy to be a higher-performing alternative to 420F stainless grades in this area. Market acceptance has been positive, with the material accounting for a major share of the company’s stainless steel sales in the U.S.
Coatings Meet Many Needs
Moldmakers can upgrade the performance of their steels with coatings that resist wear and corrosion, enhance longevity, improve molding operations like injection and part ejection, and eliminate the need for topical application of mold-release agents.
These properties are among the advantages claimed for Balinit Arctic coatings from Oerlikon Balzers (Elgin, IL). The materials make up a line of PVD (physical vapor deposition) nitride coatings whose chief benefit is application at temperatures of 200°C, half that of conventional PVD processes. Oerlikon Balzers developed an arc evaporation technique that coats mold steels and copper alloys at low temperature. The normal temperature range is 400-450°C, says Dwayne Douglas, Product Manager for Molds and Die Casting.
There are three grades in the line: Balinit A, Balinit D and Balinit Futura Nano. Douglas believes that in addition to the benefits cited, these coatings can increase the quality and productivity of molds, and add to a moldmaker’s competitive advantage. “Molders are looking for quality and cost-savings,” he says. “Their costs are primarily due to downtime, defects and mold maintenance. If a moldmaker can offer a superior engineered product that slashes bottom-line losses, the battle is won.”
Coatings that increase hardness are ideal for thinwall mold components, textured components and almost any area of a mold exposed to aggressive resins or harsh processing conditions. Bales Mold Service (Downers Grove, IL) has been promoting the ability of its NIBORE electroless coating to upgrade the strength and performance of mold components. It can, in fact, be used to reduce wall stock with no loss of strength.
NIBORE is described as a nickel phosphorous coating with boron nitride particles in a co-deposited nickel matrix. What this means is that it achieves a 54 HRC after coating, which can be expanded to 67 HRC with heat treatment, near that of a hard chrome coating. The coating, usually applied as a 0.0005-inch deposit, enhances abrasion- and wear-resistance and lubricity, says Harry Raimondi, Technical Services Manager. This is due to its low coefficient of friction—0.05. The coating is applied at 85°C, and has an operating range of -184 to 677°C.
The electronics market has demanding engineering needs and heavy cost pressures. The shift of so much mold building offshore attests to the economics that drive the market. The rising price of manufacturing in China, however, coupled with persistent concerns about quality and time-to-market delays are creating some openings for North American moldmakers. Those looking for a share of the marginal increase in business that this could yield will find that a selective materials strategy is as important as engineering know-how and the latest fabrication equipment.
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