MoldMaking Technology

SEP 2018

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Page 42 of 67 41 NAK-55 polishes better than P20, according to Dramco President Larry Patten, who says his polishers are taking their SMC tools up to a B2 finish using a 400 stone, which polishes very well up to that finish. NAK-55 allows for an effective substraight when coatings are applied at temperatures not exceeding 950°F, if machinists desire extra surface protection. cavity are removed because it comes from the mill with no stress in it. All of the extra steps are not needed." Paul Britton, president of International Mold Steel, says, "When machin- ing steel (hard spots or soft spots), the machinist releases the stresses and the steel moves. The degree to which it moves depends on what type of steel the machinist is using. In this type of application, whether it is P20 or H13, Dramco must do some sort of thermal cycle through the machining process. Dramco does not have that part of the equation with NAK-55. That is why a lot of companies use it when they must maintain tight tolerances, especially on long parts." Britton explains that it begins back at the mill. NAK-55 is manufactured using a double-melt or vacuum-arc re-melt (VAR) process. In other words, where P20 is a quench-and-tempered steel and H13 goes through a quench- and-temper cycle, NAK-55 is age-hardened, sitting at 950°F for a specific num- ber of hours so there is no stress as is typical in quenched and tempered steels, only uniform hardness. Uniform Hardness Improves Wear Properties In addition to eliminating stresses and providing better stability, choosing NAK-55 over hardened tool steels also gave Dramco a more-uniform hardness to protect against wear from running abrasive materials in this mold. "The problem with running carbon-fiber reinforced epoxy SMC when we are com- pression molding is that we have to run at an elevated cure temperature, and the material contains up to 40 percent carbon-fiber reinforced glass, which is very abrasive," Patten says. "Since NAK-55 has a 40-42 Rockwell hardness rating, we can machine it, and yet it is hard enough to hold up to the abrasion of run- ning the tool." NAK-55 works very well with coatings for those who desire further surface protection, Britton says. The even distribution of NAK-55's hardness allows for an effective substrate when coatings are applied given that the temperature does not exceed the age- hardening point of 950°F. Patten says that NAK-55 conducts heat better than other tool steels, too. "Thermal conductivity properties are probably more significant in injection molding, but they do benefit Dramco with this job in that NAK-55 produces a more evenly cured part more quickly," Britton says. "All thermal conductivity is based on chemistry. The less alloy element that there is in the steel, the better thermal conductivity the machinist can achieve." For example, chrome has a high thermal-conductivity quotient, so the more chrome the mold material contains, the more the mold will retain heat during the processing cycle. NAK-55 has zero chrome in it. The material Dramco is molding must stay in the mold and cure at temperature, then cool outside of For Sales and Support in North America contact: Call: 909.941.0600 V500 CZ The AuTomoTiVe RACehoRse • Soft Braking • Extremely Long Stroke Lengths Available • SpeedPorts One Meter Per Second Travel • Compact w/ Multiple Mounting Options Get an extremely compact cylinder with the same speed as a much bigger one.

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