Machinable Mold Materials
Well-chosen alloys and better metalcutting methods support the quest for competitive tooling
By Michael Tolinski
Moldmakers cut tons of standard P-20 and H-13 mold steels, and they're familiar with these materials' machinability. These two grades carry the load for injection molds and other tooling, but the increasing use of abrasive, highly filled plastics—especially in automotive components—makes higher-alloy, wearresistant steel a reasonable choice for longer tooling life for certain components.
However, the issues go beyond just selecting and cutting steel. Moldmakers have had to deal with higher raw material prices over the last few years, pushing up costs—increases that automotive OEMs and others can't or won't accept. And when North American moldmakers can't offer an acceptable price, moldmakers in China and elsewhere stand ready to bid on every possible job (though many in manufacturing question their ability to make intricate, high-quality molds).
So maintaining competitiveness while providing equal or better quality requires a combination of at least two things: durable, value-added materials that moldmakers can learn to cut and polish economically, and machining techniques and tools that reduce costs.
Rising steel and alloy prices make material selection more challenging. For some moldmakers, raw material cost may now be over double the traditional, rule-of-thumb, 10–15% of total mold cost. A consultant for the American Mold Builders Association (Roselle, IL) estimates that this proportion has increased to 15–20%, depending on the materials, mold complexity, and machining methods used in the mold shop.
Price increases have been particularly noticeable over the last few years for high-alloy steels. The chaotic world metals market has led to the practice of steel suppliers adding itemized surcharges onto the base price of steel. These surcharges reflect each alloying element's market price at a given purchase time, and this market cost can vary wildly.
Examples of these price swings are offered by Crucible Service Centers (Camillus, NY), a provider of wear-resistant materials. The price of a pound of vanadium, an important alloying element in high-wear steels, jumped from $12.69 to $57.67, and then fell back to $19.81, in May 2004, 2005, and 2006, respectively. Changes like these are reflected by shifting surcharges for alloys, such as conventional H-13 steel. Here, the surcharge changed from $0.27/lb to $1.01 to $0.61 during the same 2004–2006 period—significant, considering a base steel price of about $2.00/lb for H-13.
Higher material prices complicate arguments about the importance of material selection. Mold steel suppliers argue that since the raw material's proportion of total cost is relatively low, choosing a more durable steel for mold inserts doesn't increase costs much, but does increase mold life and value for a mold shop's customers. Machine tool experts point out that improved techniques such as hard milling have cut costs for shops that use them, thereby increasing the materials-cost fraction, and making shops much more sensitive to steel price premiums.
Wear-resistant materials, for example, can be justified for some plastics tooling components, despite their higher costs and lower machinability. The most obvious example is the material used for the screws and barrels that mix, melt, and meter highly filled plastics. Here, a high-alloy material made with a powder-metal (PM) process can resist abrasive, glass-reinforced resins.
There's a reason that these materials have found a niche in screws and barrels, says Ed Tarney, senior metallurgist for Crucible. The company's CPM 9V or S90V (stainless) alloys may cost significantly more than a common lower-end alternative, like a nitrided alloy steel. However, "the people who are buying 9V are seeing screws that offer quite a bit longer wear life in abrasive applications, commonly many times the life of an alloy steel screw," says Tarney. So the alloy screw presents a noticeable cost penalty because of the need to shut down and change tools often, a principle that could also apply to high-wear tool inserts in a mold.
But PM materials' low machinability can be an obstacle, says Timothy Wise of Timken Latrobe Steel (Latrobe, PA), another supplier of PM steels. "Most of these [materials] don't machine particularly well, based on their high hardness and vanadium-carbide content." Vanadium carbides have a hardness equivalent of about RC 85, and they make up a volume fraction of over 20% in some RC 60 PM tool steels. But PM carbides aren't clustered together as in wrought steels—they're smaller and finer, allowing cutting tools to form a chip in-between carbides more often, Wise adds.
Ed Tarney says the machining characteristics of PM have more in common with those of high-alloy tool steels like D-2 or M-2. Generally speaking, no unconventional tooling is needed. Feeds and speeds are similar, though cutting-tool wear may increase in boring and drilling. (And, of course, EDM is always an option for machining conductive materials.)
"The tradeoff is that the cost of fabrication may go up in exchange for productivity gains that would come from longer tool life in service," Tarney admits. So some toolmakers may be tempted to avoid these issues (and alloy surcharges) by using lower-alloy steel for high-wear tool components. But this could be a mistake. "If the moldmaker elects to go to a less-expensive material to keep the cost of the mold down, then his customer will either lose productivity or be competing against another moldmaker who's offering a higher-performing mold."
Fortunately, P-20 and other less-costly materials satisfy the requirements of most plastics mold components. And for extra moldmaking productivity, steel producers have been improving these grades' machinability with variations in chemistry and processing.
For example, a modified P-20 that reportedly machines 30–50% faster than standard P-20 is offered by International Mold Steel Inc. (Florence, KY). Designated PX5, the RC 29–33 prehardened steel has a modified chemistry and more homogenous grain structure. This allows faster machining and lower tool wear, says the company's Paul Britton. "Even though you can run a cutter much faster in our material, we are seeing about double the cutter life than you would see in regular P-20." Other moldmaking operations like polishing, welding, and texturing are also improved. "All these areas can increase or decrease the total cost of the mold."
As conventional alloys are made harder, they start to suffer from machinability issues. Patricia Miller, senior technical manager of Bohler Uddeholm Corp. (Wood Dale, IL), points out that high through-hardness, wear resistance, and impact/corrosion resistance cause, respectively, longer machining times, high tool wear, and poor chip breaking and sticking.
In the raw material, these machinability properties are influenced by primary carbides, nonmetallic inclusions, and chemical composition (increased carbon lowers cutting speed, while sulfur improves it). The influences play out in Uddeholm's Ramax 2 mold holder-plate steel. Comparable in properties to 420 stainless, this stainless steel contains sulfur, which reportedly allows cutting speeds in high-speed machining that are about 80% those of free-cutting steel.
But, overall, qualities in mold steel that molders like tend to hurt its machinability. "This means that the end-use of the mold may dictate that a material with these capabilities must be used, and the toolmaker must work with the steel supplier and his insert/tool suppliers to come up with the best combination of conditions for these materials," says Miller.
At the lower end of the hardness scale, nonferrous mold materials are also affected by price volatility. But aluminum and copper alloys' high thermal conductivity makes them attractive for reducing cycle times in plastics molding. They're readily machinable, though they present their own issues in the mold shop.
For example, copper tool alloys typically contain beryllium for high hardness, and beryllium dust can be dangerous if inhaled. This isn't a concern in most machining operations for milling, drilling, turning, or sawing, says Bob Kusner. applications development engineer for Brush Wellman Alloy Products (Cleveland). Operations that generate fine particulate, however, like grinding or polishing, require a combination of safe work practices and controls.
Given the issue, the company offers MoldMax XL beryllium-free alloys, which can be machined at high speeds (above 3000 fpm or 914 m/min), Kusner says. "If a machining center has sufficient power, these alloys can be machined at rates seen with aluminum." Compared with P-20 steel, rough-machining is several times faster, finishing is faster, and overall machining time can be reduced by about half, he says.
More productive machining can offset rising alloy costs, asserts Craig McQueen, machining center application team leader for Makino Inc. (Mason, OH). For the moldmakers McQueen talks to, the focus isn't on material selection—it's on the methods and equipment required for cutting the materials.
Both McQueen and Makino's Bill Howard observe that steel cost is indeed a major issue with moldmakers around the world, especially as improved machining processes like hard milling make it more visible. "As you get into hard milling and things like that, you're really collapsing a lot of the cost structure and time and hand-working and finishing and polishing, so consequently material is getting to be the major cost, at least in die/mold operations," explains Howard.
Because it cuts out extra heat-treating and reworking steps, hard milling—machining steel in its fully hardened condition—helps compensate for increasing costs by bringing down labor content and lead times. McQueen estimates that about 70% of Makino's customers regularly use hard milling, and interest is high among those that don't. He says those who use it effectively have machine tools that supply the required stiffness, squareness, parallelism, thermal resistance, and spindle positioning.
The tool tip itself can be a source of productivity gains. Cutting tool suppliers are constantly developing new substrates and coatings to increase tool-life and performance, states William Fiorenza, die and mold product manager for Ingersoll Cutting Tools (Rockford, IL). Steel suppliers make things challenging by offering higher nickel/chromium/molybdenum-content steels, which tend to be abrasive on cutting tools. It's a never-ending battle: "Whenever steel manufacturers develop a better tool steel, a brand new challenge is presented to the tool manufacturer."
Of more specific interest to some moldmakers is increasing depth-of-cut, to avoid expensive EDM operations. "The object today in the mold world is to cut as much as you can and burn as little [as you have to]," says Fiorenza. Sometimes this requires milling with tools having length-to-diameter (L/D) ratios as great as 20 to 1, and stick-out distances of 18" (457 mm) from the spindle to the tool tip.
"People have to realize that there's a tradeoff when you begin to attack these deeper mold cavities and taller core areas. At these extended lengths, you're not high-speed machining," he says. It's difficult to determine the speeds and feeds that will work for a given stick-out distance, L/D, and depth of cut. At a recent seminar, a moldmaker asked Fiorenza for a formula to calculate these speeds and feeds. "There's no formula for that," he explains, only guidelines.
And new tools: At IMTS, Ingersoll announced the release of a new serrated insert, called Form Master Pro. "This particular cutter is well suited and specifically designed for machining at extended lengths." For finishing, the company suggests its Chip Surfer six-flute modular cutting tool with a backdraft geometry. These tools reportedly can help fight rising carbide tool prices with their indexable carbide tips on a reusable shank.
Polishing, the last step in the process, also affects total costs, especially when it's done by hand. The obvious ideal is to eliminate the need for polishing. Hard milling with the right equipment can create finished surfaces, assisted by the right control software for the machine tool, says Makino's Bill Howard.
But when polishing is needed, moldmakers in high-wage countries stand to benefit from making the process as automated as possible. One option is abrasive flow machining (AFM), says Bill Walch, AFM feasibility manager of Extrude Hone Corp. (Irwin, PA). "There has been a general trend from manual polishing to automated AFM polishing for high-quality polishing results with a higher level of process capability/repeatability." AFM, which has been around since 1966, uses abrasive media under high pressure to create uniformly polished surfaces.
AFM process settings can be changed to polish materials from the softest plastics to the hardest ceramics, Walch says. "In simple cases, the more wear-resistant materials require extended exposure to the AFM process, and in more complex cases, harder abrasives are used to accelerate the material removal rate." Softer materials like aluminum and copper require a less aggressive exposure. When changing between workpieces of different hardness, adjustments can be made to the process time, flow speeds, abrasive size, or even the abrasive hardness.
There are some part-size restrictions, however, and the company's Vector Series machines have been redesigned to accommodate bulky and heavy components. For example, "AFM has been used to deburr and polish a plate full of holes that is 12" [305 mm] in diam," says Walch. The redesign also includes operation and safety improvements that were based on endusers' suggestions.
This article was first published in the September 2006 edition of Manufacturing Engineering magazine.