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Plastic Mold Steels NAK55 General Design Guidelines Compressive Strength NAK55 possesses excellent resistance to surface deformation by compressive force. Its resistance to surface deformation equals that of H-13 steel at like hardness.   Stability NAK55 is not quenched to achieve hardness. Therefore, it does not have the stresses inherent in quenched and tempered steels. NAK55 never needs stress relieving due to heavy machining because of uniform hardness throughout! It has excellent dimensional stability during mold construction and while in service. Even after long mold runs, cool-down, and subsequent re-heating, the material maintains dimensional stability.   Operating Temperature Ranges NAK55 is a precipitation hardening steel. The formation of the precipitates occurs at a temperature range between 932° and 968°F. Exposure to temperatures below the original precipitation hardening temperature has no effect on the grain structure of the steel. Therefore, even molds in service at high operating temperatures maintain dimensional stability. Similarly, nitriding or Physical Vapor Deposition (PVD) treatments done below the precipitation hardening temperature range do not cause distortion. This is a tremendous advantage over typical mold steels that have been tempered at low temperatures to maintain hardness.   Hardness The precipitates, which account for the steel’s hardness, begin to grow in size if the original hardening temperature range for NAK55 (932°–950°F) is exceeded for an extended period of time. This results in a loss of hardness and toughness, and accompanying dimensional change. Re-solution heat-treating and subsequent precipitation hardening can recover hardness and toughness, but dimensional changes will have occurred.   Wear Properties NAK55 is a low carbon steel that acquires hardness by precipitation hardening. Very small carbide structures and precipitates are formed during the hardening process. Care is required when NAK55 steel slides against itself in molding situations. The use of dissimilar metals, with a 10 HRc point difference on the mating surfaces, is advisable in slide situations. Alternatively, you may change the hardness of the surface of similar metals by plating, nitriding, or applying other types of coatings.   Shut-Off Tolerances The tolerance between mating surfaces on angle shut-offs is very important, and proper setting of the mold is required. The recommended tolerance at molding temperature is .0008" per side for ABS, Polypropylene, Polyethylene, etc. A .0004"(.01mm) per side clearance is required for Polyacetals or nylons, due to their lesser viscosity.   Ejector Pin Tolerances   Dia. of Ejector Pin .200" and below .201" to .500" Recommended Clearance .0008" .0012" Note: All clearance values are at molding temperature. Sharp Corners Sharp or square corners serve as focal points for concentrating stresses that build up in molds and dies during operating conditions. A .100"– .120" radius to all corners is recommended, especially large box-type molds. To avoid these stresses, the thickness of the material backing up a sharp corner should be increased by 50% when compared with softer steels.   Thin Sections Due to the toughness limitations of NAK55, it is suggested that other types of steel be inserted at thin rising sections (4-to-1 ratio or more) and that a radius be put on the base of rising sections from the initial design stages.   Cavity Depth Cavity depth should be limited to no more than 50 – 55% of the block thickness in high-pressure injection molds.   Cooling/Heater Water Lines Do not locate cooling or heater lines directly below the corner of a mold cavity. The ideal location for heating or cooling lines is three (3) times the hole diameter from the molding surface. 1.5 times the hole diameter from the molding surface is the minimum. Example: a 1/4"diameter waterline should be at least 3/8" away from mold surface and preferably 3/4". The distance between lines should be five (5) times the line diameter minimum. All technical and engineering data and suggested procedures, specifications and applications contained in this publication are for general information only. Daido Steel, International Mold Steel and/or their distributors disclaim any and all express or implied warranties of merchantability, suitability for any particular purpose or use, or freedom from infringement of any patent, trademark or copyright. NAK80® is a registered trademark of International Mold Steel, Inc. and Daido Steel Limited.

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