Rubber Mold Steels
- 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
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.
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
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"
|Note: All clearance values are at
- 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.