by Brenda Denny, United Silicone
Hot stamping is “the combination of three central components (temperature, pressure, and dwell) to transfer an image from a carrier to a part.” These three components are interdependent; a change or variation in one may affect/impact the other.
Temperature is the amount of heat required to soften the foil or heat transfer pigments and the part surface to insure thermal bonding of foil/transfer to the part.
Pressure is the clamping force required to insure full contact of die to the part. The size of the machine is identified by the size and location of artwork, as well as the size of the part being decorated. The pressure required for an application can be determined using the following guidelines:
Foil and Silicone Rubber Dies approximately 350 pounds per square inch of die contact area (artwork size to be stamped).
Heat Transfers and Silicone Rubber Dies approximately 500 pounds per square inch of the die contact area (image size of transfer).
Foil and Metal Dies approximately 1,000 pounds per square inch of die contact area (artwork size to be stamped).
Example: Artwork size = 3 x 7 or 21 square inches
21 sq. in. x 500 PSI (HT w/rubber die) = 10,500 pounds
10,500 pounds / 2000 (pounds per ton) = 5.25 tons
5.25 tons of pressure required for a 3x 7 heat transfer
3.6 tons required for hot stamp application with rubber dies
10.5 tons required for same application with metal dies
Dwell is the time that the die or roller is in contact with the foil/transfer and the part. Tooling
A structure built on a shaky foundation will not withstand the test of time; neither will the quality of your decoration! Tooling is the foundation of any hot stamp/heat transfer decorating application. The nesting fixture is required to secure/hold the plastic part in place during the decorating process, support the stamping area, and locate/position the part for repeatability. Nesting fixtures should be manufactured to be user-friendly, and easily adapted for automation or an operator to load and unload parts. Nesting fixtures are manufactured of various materials as dictated by the part shape, substrate, size and support requirements:
Aluminum Lighter than steel, easier to handle, easy to machine, will not rust, durable.
Cast Urethane Ideal for contoured applications, provides tighter fit than aluminum, PVC, nylon or Teflon, not as durable as aluminum.
PVC Machined to part configuration, wont scuff clear plastics or a high gloss finish, not as durable as aluminum.
Nylon or Teflon Ideal for automatic part discharge because of low friction, wont scuff plastics, not as durable as aluminum.
Steel Offers the greatest amount of strength and durability, ideal for high tonnage applications.
Additional features are often added to a tooling set to make set-up easier and provide consistent, quality parts more quickly on a tooling changeover. Jackscrews can be added to the base plate to facilitate proper alignment of the nest to the die. Alignment pins will assist in aligning the part to the die on contoured parts or flat parts where position of the graphics is critical. Fixtures also can be designed so they can only be mounted one way, preventing a mismatch from die to part or stamping the graphic out of orientation. In addition, part sensors can be interconnected with equipment controls to detect part presence and identify proper positioning prior to stamping. These devices are ideal in automated systems, greatly reducing scrap rates.
Having the correct type of fixture in place will help provide consistent, repeatable part placement with proper support for the decorating process that is easy for the operator to use. Keep in mind, these basic requirements are essential for a successful hot stamp/heat transfer application.
Hot Stamping/Heat Transfer Dies
Part holding fixtures make up only one half the requirements of a tooling set. The other half, the die, is equally important. Hot stamping/heat transfer dies are produced to mate to the part while fixtured. These components should be manufactured together to guarantee a matched set.
Silicone rubber dies conform to surface variations inherent in plastic molded parts. Rubber dies are available in flat, multi-level, and contoured configurations for diverse applications. Rubber dies are manufactured from machine-engraved and/or photo-etched molds.
Durometer – The measure of rubber hardness as expressed on the Shore A Scale. Low durometers exhibit high-elongation properties and improve coverage on irregular surfaces. Rubber in higher durometers are more tear resistant and will withstand pressure with minimum distortion.
Dual Durometer Two different rubber hardnesses molded together in levels. Usually a harder material is used in the face of the die and a softer rubber in the background. This construction provides a pliable belly beneath the stamping surface to help absorb variations in part, wall thickness, etc.
Texture A texture in the rubber face helps redistribute trapped air. Ideal for vertical applications with large surface areas.
Crown A convex crown (as little as .010) in the rubber face of a die will assist in eliminating air entrapment. Ideal for vertical applications with large surface areas.
Metal dies (magnesium, copper, brass, and steel) offer excellent heat recovery properties and are ideal for high speed, vertical applications. Metal dies offer a debossed appearance to your finished part as image is actually pressed into the plastic. This can be important for outdoor applications where the hot stamping foil may wear off over time. A metal die stamped image will still be readable even if the foil is removed. Metal dies are recommended for softer plastics and for image areas that have a flat, non-contoured, surface.
Die Mounting Blocks
Die mounting blocks act as a connector in the tooling set, mating the decorating equipment to the stamping die. Mounting blocks provide the support behind the die. Without this support, a die may deflect under the force of the machine, causing a lack of pressure in this area. Traditional die blocks are drilled with mounting holes to match your equipment and tooling set. Variations of the standard die block (dovetail, type holders, and magnetic) offer alternatives for mounting dies.
Quick-Change die technology vastly reduces time related to die changeover and set-up. A magnetic die block mounts to the machine allowing dies to be affixed easily, thereby eliminating the need for screws to secure the hot stamp die. Using pin registration, quick-change dies improve repeatability from die-to-die, offering increased production and less down time.
New Innovations with Tooling
In the not-so-distant past, hot stamp and heat transfer tooling was manufactured only from actual parts and delivered 2-3 weeks after receipt of injection molds, which often resulted in production delays. Prototype parts were sometimes used in an effort to have hot stamp tooling and molds delivered at the same time. However, in many cases, the prototypes ended up not matching the actual molded parts, resulting in fixtures that did not fit the part.
With the development of new software packages and advances in CNC-controlled equipment, hot stamp and heat transfer tooling sets now can be constructed simultaneously with injection molds. An electronic drawing file is all that is required to initiate the manufacture of required decorating tools nesting fixtures, die mounting blocks, and dies. Completed tooling will be matched to the first-piece run utilizing production molds. Any tweaking required will be made at this time, avoiding previous long delays in new product launches. Electronic files are kept indefinitely, thus allowing for quick and easy duplication when artwork or part changes dictate it.
When venturing into a new hot stamp/heat transfer project, remember that tooling is the foundation of all decorating applications. Many factors impact a decorating project. Too often, critical tooling required for quality, finished parts is overlooked. A structure built on a shaky foundation will not withstand the test of time. It’s worth repeating!