Plastic Mold Steels
NAK80
General Design Guidelines:
Compressive Strength
NAK80 possesses excellent resistance to surface deformation
by compressive force. Its resistance to surface deformation equals that
of H-13 steel at like hardness.
Stability
NAK80 is not quenched to achieve hardness. Therefore,
it does not have the stresses inherent in quenched and tempered steels.
NAK80 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
NAK80 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 NAK80
(932°-950°F) is exceeded for an extended period of time. This results
in a loss of hardness and toughness, accompanied by dimensional change.
Re-solution heat-treating and subsequent precipitation hardening can recover hardness and toughness, but dimensional changes will have occurred.
Wear Properties
NAK80 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 NAK80 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 Recommended Clearance
.200'' and below .0008"
.201" to .500" .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% compared with softer
steels.
Thin Sections
Due to the toughness limitations of NAK80, 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 located 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 ¾". The distance between lines should be five (5)
times the line diameter minimum.