Manufacturing and Operational Advantages
DIE MANUFACTURING ADVANTAGES
Cheaper material costs
DMI chromium plating allows the use of cheaper grades of flake grey cast
iron and eliminates the need for exotic alloys of nodular cast iron.
Cost savings of up to 30% on material are possible. Typical materials
used in Japan are FC25 and FC30 which are equivalent ot GG25 and PS11.
Flake grey cast iron is more easily machined, gives longer life to cutter
heads and allows more efficient use of die cutting machinery.
Hardness and wear resistance
DMI chromium plated dies have a surface hardness greater than 1000HV (71
HRC), four to five times that of cast iron, and have a high resistance to
wear and scoring (see graph below).
Low friction - High Lubricity
Low friction properties enable the reduction or elimination of lubricant
and removes the environmental problems of oil spray. The graph below
shows a simulated test of male and female draw bead conditions which was
created under laboratory conditions. Draw bead sections were produced in
different base materials and surface treatments as indicated. Zinc coated
material was drawn through the test pieces. The blank holder pressure
required to stop material movement was measured. Dry tool steel was the
worst performer and was used as the base pressure. It can be seen that
chromium, unlike other materials, performs equally as well without as with
lubricant. This property enables a chromium plated die to lose the oil
film locally without scoring the panel and damage to the die surface.
CHROMIUM AND ZINC COATED STEEL
Almost all manufacturers now use coated steels for corrosion resistance and
double sided zinc coated steel is becoming the standard specification.
The "non-stick" properties of chromium and the ability of chromium to "seal" the
open grain structure provide dramatic solutions to zinc build up on die
Resistance to pimples
The open grain structure of cast iron encourages any loose particles of zinc to
bond to the surface of the punch. During the course of a press run the zinc
particles build up on the surface of the punch.
Pimples are formed on the panel when the particles of zinc are compressed
between the surface of the punch and the surface of the blank resulting in
unacceptable panel quality.
The polished layer of chromium on the surface of the punch provides a "non-
stick" condition which discourages the build up of zinc on the punch and
transfers the zinc particle to the surface of the blank which then leaves the
press. The result is an improvement in panel quality, less rework and
significant reduction in press downtime since the surface of the punch no longer
requires stoning to remove bonded zinc particles from the surface of the cast
Any zinc particles which do find their way onto the surface of the punch can
quickly be removed by wiping over with a cloth.
Reduced press down time
Draw related press down time reduction of up to 90% has been reported.
Press down time
Reduced rework and scrap
Zinc build up on the blank holder faces is eliminated resulting in reduction in
scoring and thus reduction in rework or scrapping of the panels.
Reduced die maintenance
Damage to the die surface through scoring and seizure is also eliminated.
Choice of materials
Chromium tool surfaces allow a wide range of choice of coated material,
often resulting in significant savings in material cost.
CHROMIUM - A WEAR INDICATOR
A 25 micron chromium layer will act as a wear indicator. As a tool
wears, the chromium thickness can be easily monitored using an electronic
thickness meter. When the chromium has worn it can quickly be replaced by
de-chroming and re-chroming the die. Because the die retains its original
profile to within 25 µm throughout its working life the panel also
maintains a constant profile.
DMI technicians carry out basic preparation of the die surface including
local welding of small defects, stoning of repaired areas and polishing of
draw beads and radii prior to chrome plating if required.