An introduction to Rotary Hot Stamping
An Introduction to Rotary Hot Stamping David Polkinghorne, link: DMS Inc.
Hot stamping is a process for transferring a dry film to a
substrate through heat and pressure. This requires the following
basic components:
Main Components of the Hot Stamping Process:
ANVIL Usually made of a hard rubber or phenolic. The anvil must
be hard enough to resist cupping under the load of the die, soft
enough to protect the die from irregularities in the substrate,
and resilient enough not to take a set through repeated
pounding.
SUBSTRATE A wide variety of substrates are suitable for the hot
stamping process. Surface tension of the substrate has the same
affect on hot stamping as it does on printing. In addition to
surface tension, the porosity of the material has a profound
affect on stamping speed. A dry material will tend to cool the
foil before it gets a chance to adhere properly. The resulting
stamp will show a mottled surface similar to how ink looks on
such a material. As a rule of thumb, a material that prints well
will also stamp well.
ROLL LEAF Also known as Foil Roll leaf is the ink of a hot
stamping operation. It is made up of a carrier with a series of
coatings to be transferred. The carrier is made from polyester
film that can range from just under 0.0005" to about 0.0015".
The thinner the carrier, all other things being equal, the
faster the foil will transfer.
The first layer applied to the carrier is a thin release coat.
This layer affects whether or not a foil is suitable for fine
copy or heavy coverage and affects the "cutting" properties of
the foil.
The second layer called the "tie" or "color" coat serves dual
purposes in a roll leaf product. The first function of this
layer is to provide a hard abrasion resistant surface to the
foil when applied. At 1.2-1.5 microns the tie coat makes up
about 30% of the transferred product. Since the metallic layer
is always made from aluminum, a tint is required to make a gold
or other colored metallic . This tint is the second function of
the tie coat.
Under the tie coat is a vacuum deposited aluminum layer. This is
the most opaque layer in the construction even though it is only
0.02-0.05 microns thick. This layer has no structural integrity
of its own.
The final layer in the foil construction is the sizing. This is
a heat-activated adhesive that glues the foil image to the
substrate. At 1.5-3 microns this layer makes up 50-70% of the
transferred product. A thin layer of sizing cuts cleanest and
tends to work best on films or high gloss materials. Heavily
sized products do a better job of filling a more porous
substrate.
HOT STAMPING DIE The hot stamp die is an engraved plate where
the non-printing areas are relieved. It is important that the
material used holds as much thermal energy as possible at the
surface. Copper alloys excel in this area and are used almost
exclusively in rotary applications where dwell times are
extremely short. There are two primary methods of engraving hot
stamp tooling.
The original method is photoengraving. In this process the image
is transferred via a photographic negative. The unwanted metal
is then etched away by a strong acid. The process is very
similar to printed circuit board production. The advantage of
photoengraving is that as the complexity and scale of the image
increases the cost remains more or less fixed. The disadvantage
is that as image complexity decreases the cost remains more or
less fixed. Photoengraving is a high overhead process that
excels in producing complex images.
A more recent method of producing hot stamp tooling is CNC
engraving. In this process the image is taken in an electronic
format and specialized software generates tool paths that remove
the unwanted material. The advantage of CNC engraving is that
simple images can be produced very efficiently. As the quantity
and complexity of the image increases the cost increases
proportionately. CNC engraving is a low overhead operation that
most efficiently produces less complex images.
HEAT + PRESURE + TIME = FOIL TRANSFER.
Foil is transferred to the substrate when a heated die forces
the foil onto the substrate against the anvil. The die must be
hot enough to activate the adhesive (sizing) on the back of the
roll leaf and adhere it to the substrate. The die also performs
the very important function of cutting the foil all around the
perimeter of the detail being transferred. Without this cutting
action the foil will bridge across any small reverses in the
copy.
ROTARY HOT STAMPING The information provided up to this point is
general and applies to both rotary and flat bed hot stamping. In
rotary hot stamping the time element in our transfer formula
becomes an inverse function of press speed. Since the contact
point between two rollers is theoretically a line this time
function becomes very short. A good point of reference is that
at 100 FPM 0.020" of material goes by in one millisecond, it
takes about 75-100 millisecond to blink your eye.
This extremely short dwell time explains why porous materials
prove difficult to process rotary. As the top fibers cool the
sizing, the foil solidifies on top of the material before it can
creep down into the fiber. The dwell time that allows a flat bed
system to force the foil down into the substrate also allows
heat to move laterally through the film. This causes the foil to
release or "bridge" across fine reverse details. Thus, short
dwell time is also the reason rotary hot stamping is able to
produce a greater level of detail than the best flat bed
equipment.
ROTARY HOT STAMP EQUIPMENT For a rotary hot stamping system to
operate it must perform two basic functions. First it must
maintain the desired temperature of the die. Second it must hold
the die securely in position to produce even depth of impression
through heavy and light coverage regions of the die.
TEMPERATURE CONTROL Temperature control of rotary hot stamp
systems is achieved by one of two methods. Most common is
internal electric. This can be used with or without cooling. The
second is hot oil. While inherently more accurate, hot oil also
adds a dangerous component.
IMPRESSION CONTROL The second task of any rotary hot stamping
system is that it maintains an even depth of impression. The
main problem with this is that the heated die must also be taken
off of impression when the web is stationary. The original hot
stamp units used a pneumatic bridge to achieve this. After time
the shortcomings of this system became obvious. First, the
amount of lift-off was limited by the gearing. This was a
particular problem on 32dp gearing where the small teeth limited
throw off to approximately 0.030". Second, any variation in line
pressure would affect the impression.
The next generation of impression control utilized a cam over
center type mechanism where the die remained stationary and the
impression roll rotates up to meet it. This system uses air
pressure to make and break the impression but will hold its
impression regardless of available air pressure. While this
approach was a great improvement it is still limited to webs 10"
and less. The problem with going to wider webs is that as solid
as the base roll may be, by applying pressure to the bearing
blocks rather than die bearers allows to much flexing in the die
shaft assembly to achieve good speeds on wider webs.
The key to achieving high speeds on wider webs is in making the
hot stamping system follow the same principles of operation as a
cutting die. It has been a long time since applying pressure to
the bearing blocks in a die cutting operation was an accepted
practice. The latest designs have addressed this issue by adding
steel bearers to the hot stamp die. With this design the die and
anvil are sandwiched between the lower anvil roll and an assist
roll where pressure can be set and monitored. With the die
locked tightly in the station the anvil is adjusted
independently to meet the die.